U.S. patent application number 13/090530 was filed with the patent office on 2011-10-27 for encapsulated phase change apparatus for thermal energy storage.
This patent application is currently assigned to LEHIGH UNIVERSITY. Invention is credited to John C. Chen, Wojciech Z. Misiolek, Sudhakar Neti, Alparslan Oztekin, Kemal Tuzla.
Application Number | 20110259544 13/090530 |
Document ID | / |
Family ID | 44814785 |
Filed Date | 2011-10-27 |
United States Patent
Application |
20110259544 |
Kind Code |
A1 |
Neti; Sudhakar ; et
al. |
October 27, 2011 |
ENCAPSULATED PHASE CHANGE APPARATUS FOR THERMAL ENERGY STORAGE
Abstract
Provided are apparatus and methods for storing thermal energy.
For example, an apparatus including comprising at least one phase
change material, and a capsule containing the at least one phase
change material. The capsule may be permanently or temporarily
sealed to contain the encapsulation material. The encapsulation
material includes at least one material that is chemically and
physically distinct from the phase change material. The
encapsulation material and phase change material are selected to
store and discharge thermal energy at temperatures of greater than
400.degree. C. without capsule failure.
Inventors: |
Neti; Sudhakar; (Bethlehem,
PA) ; Chen; John C.; (Bethlehem, PA) ;
Misiolek; Wojciech Z.; (Hatfield, PA) ; Oztekin;
Alparslan; (Allentown, PA) ; Tuzla; Kemal;
(Allentown, PA) |
Assignee: |
LEHIGH UNIVERSITY
Bethlehem
PA
|
Family ID: |
44814785 |
Appl. No.: |
13/090530 |
Filed: |
April 20, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61326412 |
Apr 21, 2010 |
|
|
|
Current U.S.
Class: |
165/10 |
Current CPC
Class: |
F24S 60/10 20180501;
Y02E 60/14 20130101; Y02E 10/40 20130101; F28D 20/023 20130101 |
Class at
Publication: |
165/10 |
International
Class: |
F28D 20/00 20060101
F28D020/00 |
Goverment Interests
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0003] This invention was developed with financial support from the
Department of Energy, Government Grant Award Number
DE-FG36-08G018150. The United States government may have certain
rights to the invention.
Claims
1. An apparatus for storing thermal energy, the apparatus
comprising: at least one phase change material; a capsule
containing the at least one phase change material, wherein the
capsule comprises an encapsulation material, and wherein the
encapsulation material comprises at least one material that is
chemically and physically distinct from the phase change material,
and wherein the encapsulation material and phase change material
are selected to store and discharge thermal energy at temperatures
of greater than 450.degree. C. without capsule failure.
2. The apparatus of claim 1, wherein the capsule comprises an inner
chamber for containing the at least one phase change material.
3. The apparatus of claim 1, wherein the capsule comprises a layer
of encapsulation material deposited onto the surface of the phase
change material.
4. The apparatus of claim 2, wherein the inner chamber is
substantially completely filled with the phase change material when
the phase change material reaches a first preselected operating
temperature.
5. The apparatus of claim 4, wherein the inner chamber is filled to
less than about 90 percent of its volume by the phase change
material at the first preselected temperature.
6. The apparatus of claim 5, wherein the preselected temperature is
at least about 308.degree. C.
7. The apparatus of claim 5, wherein the preselected temperature is
at least about 444.degree. C.
8. The apparatus of claim 5, wherein the preselected temperature is
about 725.degree. C.
9. The apparatus of claim 5, wherein the inner chamber is not
substantially completely filled with the phase change material when
the phase change material reaches a second preselected temperature,
the second preselected temperature being lower than the first
preselected temperature.
10. The apparatus of claim 1, wherein the phase change material
comprises Zinc, and wherein the encapsulation material comprises
Nickel.
11. The apparatus of claim 1, wherein the phase change material
comprises at least one of stainless steel and MgCl.sub.2--NaCl
eutectic salts, and wherein the encapsulation material comprises at
least one of carbon steel or stainless steel.
12. The apparatus of claim 1, wherein the phase change material
comprises at least one of NaNO.sub.3, KNO.sub.3,
NaNO.sub.3--KNO.sub.3, MgCl.sub.2, MgCl.sub.2--NaCl,
MgCl.sub.2--KCl, NaCl-KCl, and combinations thereof.
13. The apparatus of claim 9, wherein a plurality of capsules are
provided to form a thermal energy storage system.
14. The apparatus of claim 13, wherein the thermal energy storage
system further comprises at least one fluid in thermal contact with
the plurality of capsules, wherein the fluid serves conduct heat
from an energy source to the capsules, thereby heating the phase
change material to at least the first preselected temperature.
15. The apparatus of claim 14, wherein the fluid is selected from
the group consisting of air, silicones, biphenyls, eutectic salt
mixtures such as NaNO.sub.3--KNO.sub.3,
NaNO.sub.3--KNO.sub.3--LiNO.sub.3, and combinations thereof.
16. The apparatus of claim 15, wherein the phase change material,
encapsulation material, and heat transfer fluid are selected and
arranged to enable storage of thermal energy for at least 6 hours
at temperatures of at least about 450.degree. C.
17. The apparatus of claim 16, wherein the thermal energy comprises
energy generated from at least one energy source, the energy source
comprising at least one of solar, wind, geothermal, and nuclear,
and combinations thereof.
18. The apparatus of claim 17, wherein the efficiency of the system
permits storage of thermal energy at less than about $40 kWh.
19. The apparatus of claim 1, wherein the capsule comprises an
opening that can be operated to adjust the amount of phase change
material contained therein.
20. The apparatus of claim 1, wherein the capsule includes a void
space that is determined based upon the thermal and physical
properties of the phase change material, the thermal and physical
properties of the encapsulation material, and the geometric shape
of the capsule.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the priority of U.S. Provisional
Patent Application No. 61/326,412, filed on Apr. 21, 2010, which is
incorporated herein by reference in its entirety.
[0002] Not Applicable
BACKGROUND
[0004] The inventors have developed new thermal energy storage
devices and methods, such as for use in conjunction with energy
generation systems, such as solar energy panels and related
devices.
[0005] Solar power generation has been proven to be an excellent
source of renewable energy. The program entitled "Advanced Heat
Transfer Fluids and Novel Thermal Storage Concepts for
Concentrating Solar Power Generation" belongs to Renewable Energy
Research and Development Division of U.S. DOE Golden Field Office,
Golden, Colo. Concentrating Solar Power (CSP) technology is
recognized as an attractive option for solar power for plants of
100 MW or greater capacity. Two key needs for this technology are
the development of improved heat transfer fluids (HTF) and the
development of improved methods for thermal energy storage (TES).
This proposal addresses the second need, with the objective of
developing a novel TES technology that utilizes phase-change latent
heat to increase thermal capacity, and offers energy storage at
preselected high temperatures (e.g., such as about 400.degree. C.)
to improve thermodynamic efficiency.
SUMMARY
[0006] Provided herein are apparatus and methods for storing
thermal energy.
[0007] For example, in one embodiment, an apparatus is provided
including at least one phase change material and a capsule
containing the at least one phase change material. The capsule may
be permanently or temporarily sealed to contain the encapsulation
material. The encapsulation material includes at least one material
that is chemically and physically distinct from the phase change
material. The encapsulation material and phase change material are
selected to store and discharge thermal energy at temperatures of
greater than 400 degrees without capsule failure.
[0008] In another embodiment, an apparatus is provided for storing
thermal energy, the apparatus comprising at least one phase change
material, and a capsule containing the at least one phase change
material. The capsule comprises a preselected encapsulation
material, and the encapsulation material comprises at least one
material that is chemically and physically distinct from the phase
change material.
[0009] In some embodiments, the capsule comprises an inner chamber
for containing the at least one phase change material. The inner
chamber may include a void space at a first preselected
temperature. That void space may be substantially completely filled
with the phase change material when the phase change material
reaches a second preselected temperature.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] The patent or application file contains at least one
photograph or drawing executed in color. Copies of this patent or
patent application publication with color photograph(s) or
drawing(s) will be provided by the U.S. Patent and Trademark Office
upon request and payment of the necessary fee.
[0011] FIG. 1 illustrates a cross sectional view of a cylindrical
capsule in accordance herewith.
[0012] FIG. 2 illustrates a schematic of a cross section of a
spherical capsule in accordance herewith.
[0013] FIG. 3 illustrates a graph of temperature distribution of an
exemplary capsule in accordance herewith.
[0014] FIG. 4 illustrates a graph of temperature distribution at
the end of a PCM melting process in accordance herewith.
[0015] FIG. 5 illustrates a graph of temperature variations during
the charging heat transfer process for a 100 mm diameter cylinder
with sodium nitrate PCM and with air as HTF in accordance
herewith.
[0016] FIG. 6 illustrates a graph of locations of interface between
solid state PCM and liquid state PCM during melting process for a
100 mm diameter cylinder with sodium nitrate PCM and with air as
HTF in accordance herewith.
[0017] FIG. 7 illustrates a graph of temperature variations during
the discharging heat transfer process for a 100 mm diameter
cylinder with sodium nitrate PCM and with air as HTF in accordance
herewith.
[0018] FIG. 8 illustrates a graph of locations of interface between
solid state PCM and liquid state PCM during discharging (freezing)
for a 100 mm diameter cylinder with sodium nitrate PCM and air as
HTF in accordance herewith.
[0019] FIG. 9 illustrates a graph of stress-strain relationship
assumed with strain hardening in accordance herewith.
[0020] FIG. 10 illustrates stresses, strains and displacements of a
50 micron thick shell encapsulation coating in elastic deformation
in accordance herewith.
[0021] FIG. 11 illustrates stresses, strains and displacements of a
100 micron thick shell encapsulation coating in elastic deformation
in accordance herewith.
[0022] FIG. 12 illustrates stresses, strains and displacements of a
150 micron thick shell encapsulation coating in elastic deformation
in accordance herewith.
[0023] FIG. 13 illustrates stresses, strains and displacements of a
200 micron thick shell encapsulation coating in elastic deformation
in accordance herewith.
[0024] FIG. 14 illustrates stresses, strains and displacements of a
250 micron thick shell encapsulation coating in elastic deformation
in accordance herewith.
[0025] FIG. 15 illustrates elastic-plastic stresses for spherical
shell with pressure for 3 percent strain in accordance
herewith.
[0026] FIG. 16 illustrates elastic-plastic strain for spherical
shell with pressure in accordance herewith.
[0027] FIG. 17 illustrates a phase diagram of a Ni--Zn binary
system in accordance herewith.
[0028] FIG. 18 illustrates alumina silicate ceramic crucibles with
test specimens for exposure to liquid zinc at 450.degree. C. in
accordance herewith.
[0029] FIG. 19 is an optical photomicrograph that illustrates
Differential Interference Contrast (DIC) of a Ni/Zn system as
polished using differential interference contrast (The Ni is in the
upper left of the picture and the Zn is in the bottom right) in
accordance herewith.
[0030] FIG. 20 illustrates stresses for a pressure-only model with
a thickness of 250 micron in accordance herewith.
[0031] FIG. 21 illustrates stresses due to a triangular crack (top
left), a straight dent (top right), and a thinned part of the
nickel shell (bottom) in a 250 micron thick nickel shell in
accordance herewith.
[0032] FIG. 22 is an isometric view of maximum principal stresses
on crimped cylinder due to 70% initial zinc content loading case
(left), view of stresses inside of crimped cylinder due to 70%
initial zinc content loading case (right), in accordance
herewith.
[0033] FIG. 23 illustrates stress distribution of the 70% initial
zinc content pressure only case (left), stress distribution of the
85% (middle), and 86% (right) PCM (zinc) content cases with point
loads of 100 particle weights in accordance herewith.
[0034] FIG. 24 illustrates stress distribution on cylinder of
aspect ratio 1 for the 70% initial PCM (zinc) content case; (left)
stresses on the outside of the cylinder, (right) stresses inside of
the cylinder in accordance herewith.
[0035] FIG. 25 are phase diagrams of a binary NaCl--MgCl.sub.2
system from two separate sources in accordance herewith.
[0036] FIG. 26 illustrates the comparison of two separate DSC runs
to determine the melting point of the 55 wt % MgCl.sub.2-45 wt %
NaCl eutectic salt. The melting point is determined to be
444.degree. C. in accordance herewith.
[0037] FIG. 27 illustrates stainless steel capsules for calorimetry
tests--with and without the NPT pipe plug in accordance
herewith.
[0038] FIG. 28 illustrates carbon steel (1018) capsule for use with
the eutectic salt in accordance herewith.
[0039] FIG. 29 illustrates sections of a MgCl.sub.2--NaCl eutectic
EPCM encapsulated with Stainless Steel-304 in accordance
herewith.
[0040] FIG. 30 is a schematic of a precision calorimeter designed
and built for use in the present work--all units are inches in
accordance herewith.
[0041] FIG. 31 illustrates a graph of temperature profiles of
sample, silicon oil, and air in accordance herewith.
[0042] FIG. 32 illustrates the geometry of an exemplary capsule in
accordance herewith.
[0043] FIG. 33 illustrates a graph of the cost of storage units
($/kWh) as a function of the diameter of the cylindrical shaped
capsule in accordance herewith.
[0044] FIG. 34 illustrates a graph of the cost of storage unit
($/kWh) as a function of length of the capsule in accordance
herewith.
DETAILED DESCRIPTION
[0045] The methods and apparatus described herein focuses on
encapsulated phase-change material (EPCM), such as in either
particulate (near spherical) and/or tubular forms, either of which
can be assembled into heat exchangers for thermal exchange with
HTFs. This work builds upon the team's unique experience for
encapsulation of PCM, using an electrochemical coating technique.
There are many PCMs suitable for use herein, including Zn,
NaNO.sub.3, MgCl.sub.2, and other materials as well as eutectic
mixtures of MgCl.sub.2--NaCl, and other materials, with our primary
focus on salts. Nickel, carbon steels and stainless steels are
among the materials suitable as encapsulation materials. In some
embodiments, Nickel is used with Zn. Scoping analysis has shown
that nominal dimensions of 1-100 cm are suitable for the EPCM to
provide for ease of fabrication and assembly into heat
exchangers.
[0046] The inventors have conceived of various thermal energy
storage apparatus and methods. They have further established the
technical requirements and demonstrated feasibility, along with
bench scale testing, of various embodiments. The examples herein
prove the usefulness of thermal energy storage systems
incorporating encapsulated phase change materials (EPCM) at
elevated temperatures (such as up to about .about.750.degree. C.,
for example). Additionally, the testing and engineering results
described herein prove the scalability of the technology, and
thereby, enable the development of large scale thermal storage
systems. To the knowledge of the inventors, the apparatus and
methods described herein have not previously been described or
demonstrated by others.
[0047] For the encapsulated phase change apparatus described
herein, several methodologies and geometries (spherical and
cylindrical) are demonstrated having different designs and
materials. For example, Zinc (Zn) encapsulated in Nickel (Ni);
stainless steel and NaNO.sub.3, MgCl.sub.2, MgCl.sub.2--NaCl
eutectic salts encapsulated in carbon steels or stainless steel;
and other appropriate material combinations. The technologies
developed in this work are designed to enable storage of thermal
energy in 100 MW.sub.e solar energy plants for 6 hours or more, at
temperatures of up to about 750.degree. C.
[0048] Experimental activities conducted include modeling of the
transient heat transfer and associated analysis involving zinc, the
zinc being used as a phase change material. Examples included
spherical and cylindrical geometry for particles of encapsulated
phase change material (EPCM) in capsules. Since the heating of
encapsulated PCM can exert significant stress on the encapsulating
material, significant effort was put into FEM (Finite Element
Analysis), which is a well-established numerical technique for
solving differential equations computations using Abaqus.TM. brand
of FEM software to quantify these stresses. Commercializable
embodiments of EPCM materials, including the encapsulating of zinc
using nickel, were explored in detail. A calorimeter was designed,
built, and utilized to evaluate the energy storage properties of
the various EPCM embodiments, as well as the surrounding
enclosures/tanks holding a plurality of EPCM capsules therein.
"Capsule" as used herein is the closed coating or other container
that immediately surrounds and/or holds the phase change material
("PCM"). The "encapsulating material" as used herein is the
material that makes up the capsule. The capsule may be permanently
closed (whether a continuous wall of encapsulating material) or
re-closable (e.g. a cap or other removable closure over each
opening in the encapsulation material wall.
[0049] The thermal energy performance characteristics, namely
energy absorption, energy retention, and energy release, and
operational temperature vary from one embodiment of EPCM to the
next. Those performance criteria are primarily determined by the
selection of the encapsulated material and the selection of the
material for the capsule. In some embodiments, suitable phase
change materials include NaNO.sub.3, KNO.sub.3,
NaNO.sub.3--KNO.sub.3, MgCl.sub.2, MgCl.sub.2--NaCl,
MgCl.sub.2--KCl, NaCl--KCl, inorganic salts, and combinations
thereof that exhibit phase change at desirable temperatures
compatible with particular heat exchange systems and associated
power sources (solar, nuclear, steam, geothermal, etc.). Further,
the geometry and relative sizes of the encapsulated materials of
the EPCM and the capsule material impact the thermal performance
characteristics. Similarly, various thermally induced stresses are
associated with the EPCM and any surrounding enclosure system.
Several exemplary EPCM have been manufactured and tested by the
inventors, each tested using a calorimeter to prove the charging
and discharging of energy into and out of the EPCM.
[0050] In one example, eutectic mixtures of MgCl.sub.2--NaCl (55-45
w/o) were prepared and the melting point of 444.degree. C.
confirmed using DSC. EPCM capsules with that Chloride salt mixture,
as well as NaNO.sub.3, have been prepared and tested separately and
independently in the calorimeter described herein. Calorimetry
results confirm the storage capability of the salts and prove the
feasibility of charging and discharging of energy into and out of
the EPCM capsules.
[0051] Additionally, an economic analysis of the thermal energy
storage systems described herein (using the EPCM capsules described
above) was conducted. The analysis assumes a 100 MW.sub.e plant
storing energy for 6.3 hours. All of the other assumptions used in
the analysis are the same as those used in a recent (National
Renewable Energy Lab ("NREL") report for a trough plant. The
economic analysis confirms that the EPCM systems, based on current
designs, can store thermal energy in salts at about $20.29/kWh.
That cost is well below the publicly stated Department of Energy
(hereinafter, "DOE") goal of $40/kWh for new, alternative energy
storage systems.
[0052] The following accomplishments and examples have been
demonstrated by the inventors. First, the inventors have formulated
details of desirable heat transfer in various EPCM, including phase
change characteristics. Potential stresses in the encapsulating
material for various geometries have been quantified, including
those associated with the stacking of EPCM pellets in a bed for
heat storage. Encapsulation capabilities of the commercial Nickel
forming houses have been evaluated. Further, encapsulation of Zinc
using Nickel and stainless steel tubes has been evaluated for
stresses as well as phase formations. A calorimeter to quantify the
energy storage in an EPCM pellet has been designed, analyzed and
built. Measurements of energy stored and retrieved using precision
calorimetry of the EPCM have been completed and prove that thermal
energy can be stored. Additionally, heat transfer fluids (HTF)
compatible with the EPCM materials herein include silicones,
biphenyls, eutectic salt mixtures such as NaNO.sub.3--KNO.sub.3,
NaNO.sub.3--KNO.sub.3--LiNO.sub.3, and combinations thereof. Other
HTF will be evident based upon their compatibility with operating
temperatures in particular heat transfer and storage systems and
use (charging, discharging of thermal energy) requirements.
[0053] The following examples are exemplary of the various
embodiments of thermal transfer and storage apparatus and
methods.
EXAMPLE 1
[0054] We have been successful in preparing MgCl.sub.2--NaCl
eutectic PCM with a melting point of 444.degree. C. We have
manufactured cylindrical EPCM capsules with MgCl.sub.2--NaCl
eutectic salt mixture encapsulated in stainless steel (304) and
carbon steel (1018) cylinders. FIG. 1 shows a section of the EPCM
MgCl.sub.2--NaCl eutectic with stainless steel encapsulation. As
shown in FIG. 1, which is a cross-sectional view of an exemplary
EPCM apparatus, a void space is purposefully provided within the
encapsulated space for managing stresses related to expansion of
the encapsulated material and any air or gas therein.
EXAMPLE 2
[0055] Thermal energy storage capabilities of MgCl.sub.2--NaCl
eutectic mixture has been proven by conducting calorimetry of
heated EPCM cylinders to temperatures above the melting
temperatures of the PCM and recovering the energy by quenching it
in silicon fluid.
EXAMPLE 3
[0056] We have manufactured cylindrical EPCM capsules with
NaNO.sub.3 salt as PCM with a melting point of 308.degree. C.
encapsulated in stainless steel (304). Thermal energy storage
capabilities of NaNO.sub.3 has been proven by conducting
calorimetry of heated EPCM NaNO.sub.3 cylinders to temperatures
above the melting temperatures of the PCM and recovering the energy
by quenching it in silicon fluid.
TABLE-US-00001 TABLE A Thermal Energy Storage Calorimetry Results
for MgCl.sub.2--NaCl Eutectic- Carbon Steel EPCM (1''Dia .times.
2'' EPCM Capsule) Energy Fraction of Transferred Reference
Reference Cycling to Silicone Energy from Energy from Energy from
Energy from Number Oil (kJ) Capsule (kJ) Eutectic (kJ) Eutectic
(kJ) Eutectic Expt 1 41.6 21.4 20.2 23.8 0.85 Expt 2 41.8 21.7 20.1
24.4 0.82 Expt 3 41.1 21.2 19.9 23.4 0.85 Expt 4 40.9 21.1 19.8
23.2 0.85 Expt 5 (8 hrs) 42.6 21.6 21.0 24.1 0.87 Expt 6 38.7 21.5
17.2 19.8 0.87 Expt 7 38.9 21.7 17.2 20.1 0.86 Expt 8 (8 hrs) 39.0
21.8 17.2 20.2 0.85
TABLE-US-00002 TABLE B Thermal Energy Storage Calorimetry Results
for NaNO3-Stainless Steel and Carbon steel EPCM (2''Dia .times. 5''
EPCM Capsule) Energy Fraction of Transferred Reference Reference
Cycling to Silicone Energy from Energy from Energy from Energy from
Number Oil (kJ) Capsule (kJ) NaNO3 (kJ) NaNO3 (kJ) Eutectic Expt 1
215 63.5 152 167 0.91 Expt 2 222 64.5 158 168 0.94 Expt 3 222 65.3
156 169 0.92 Expt 4 214 60.0 154 160 0.96
[0057] The repeatability of the energy stored by the PCM
(percentages .about.85% and .about.92%) is surprising and
remarkable. The percentages are not 100% only because the thermal
properties of these materials are not well established in the
temperature ranges of interest. Establishment of thermal properties
is an ongoing study.
[0058] A cost analysis of thermal energy storage systems that use
Zinc, MgCl.sub.2--NaCl eutectics and NaNO.sub.3 as PCM has been
conducted. The cost analysis is for a 100 MW.sub.e plant and for
6.3 hours of energy storage. The assumptions used for this analysis
are the same as those used by NREL and presented in an NREL report.
The Thermal Energy Storage system that uses salts in EPCM
combinations are well under the DOE goal of $40/kWh at about $27
and $20/kWh.sub.e. The storage system using zinc-stainless steel
will cost a little over the DOE goal.
TABLE-US-00003 TABLE C Cost of the storage unit ($/kWh) for
different combinations of PCM and encapsulation materials for D =
75 mm and H/D = 15 and t = 1.5 mm NaNO.sub.3- MgCl2/NaCl PCM-Shell
Zn-Stainless Stainless Eutectic - Combination Zn--Ni Steel Steel
Carbon Steel Cost ($/kWh) $93.08 $53.55 $26.62 $20.29
[0059] A preferred EPCM geometry, based on heat transfer
considerations, stress analysis, large-scale fabrication (millions
of EPCM capsules) and cost analysis, incorporates a large aspect
ratio cylinder (L/D of cylinder .about.15) of 75 mm diameter
containing a salt as the PCM. By way of further example, it is
expected that thermal energy storage for any of the PCM materials
proposed herein will be adequate when using an appropriate
encapsulation consisting of a long cylinder-like structure, with
management of hot air flow in proximity to the capsule and/or its
surrounding tanks or container(s).
Modeling of Heat Transfer
[0060] To store energy in the EPCM material, the convective heat
transfer from the solar heat transfer fluid (HTF) to the EPCM
through the encapsulation needs to be efficient. HTF at known flow,
temperature and thus heat transfer coefficient is assumed to be
available. Conduction heat transfer in spherical pellets is
considered. Though closed form solutions are available for this
transient heat transfer, since melting of the Zinc PCM and front
tracking are essential for the problem on hand, a numerical
formulation is provided in spherical coordinates. Diffusion of heat
through the encapsulation is included in the formulation.
[0061] Conduction heat transfer in spherical and cylindrical
pellets is considered herein. Numerical results for transient heat
transfer for spherical geometry are presented below. The
formulation includes conduction as well as phase change. Initially,
to estimate the order of magnitudes, lumped mass analysis was
conducted. Space-dependent and time dependent results are briefly
discussed herein.
[0062] As shown in FIG. 2, the heat transfer in a spherical shell
with multiple materials is a moving boundary problem, and it is
nonlinear in nature.
Example Calculations of FIG. 2. Spherical Geometry of Particle.
[0063] The equation governing the unsteady heat diffusion in each
layer of the exemplary sphere of FIG. 2 is of the form:
.rho. j c j .differential. T j .differential. t = k j 1 r 2
.differential. .differential. r ( r 2 .differential. T j
.differential. r ) , j = 1 , 2 , and 3 ##EQU00001##
where .rho..sub.j is density; c.sub.j is specific heat; k.sub.j is
thermal conductivity; T.sub.j is temperature distribution in each
layer; r is the radial distance; t is the time. Furthermore, the
suffixes j equals 1, 2, or 3 for the thin layered shell (nickel or
stainless steel), liquid phase, or solid phase of the phase change
material, respectively. Appropriate boundary and initial conditions
are used. The solid temperature is scaled with the inlet fluid
temperature (.about.450.degree. C.).
[0064] Boundary Conditions for the heat transfer into or out of the
spherical or cylindrical pellet are:
- k 1 .differential. T 1 .differential. r = h ( T s - T f ) , at r
= R 1 ##EQU00002## k 1 .differential. T 1 .differential. r = k 2
.differential. T 2 .differential. r , and T 1 = T 2 , at r = R 2 ;
- k 2 .differential. T 2 .differential. n = - k 3 .differential. T
3 .differential. n + L .rho. 2 V n , and T 2 = T 3 = T m ;
##EQU00002.2## .differential. T 3 .differential. r = 0 , at r = 0.
##EQU00002.3##
where h is convective heat transfer coefficient; R.sub.1 is the
radius of the whole particle; R.sub.2 is the radius of the phase
change material part; T.sub.s is the temperature at r=R.sub.1;
T.sub.f is the temperature of heat transfer fluid; T.sub.m is the
melting temperature of phase change material; L is latent heat of
phase change materials; V.sub.n is the velocity of the interface
movement in the normal direction; n is the normal direction of the
interface movement. Initially, T(r,0)=T.sub.0, which is
constant.
[0065] Dimensions of an exemplary EPCM pellet. 25 mm (1'') diameter
Zinc-Ni (or Stainless Steel) cylindrical and spherical pellets are
considered. The size has been treated as an independent variable.
1.6 mm ( 1/16'') Thickness for Stainless Steel Layer is used in the
encapsulating material. Two Heat Transfer Fluids (hereinafter,
"HTF") are considered--air and VP-1 (though VP-1 cannot be used at
the temperatures of interest here, a VP-1 like liquid capable of
550.degree. C., designated as VP-1 is used for the current
calculations).
[0066] Examples of normalized zinc temperature as a function of
time and space are indicated in the graph of FIG. 3, which shows
temperature distribution at about 9 seconds. FIG. 4 illustrates
temperature distribution at the end of melting process (t=15
s).
[0067] Extensive computational results have been generated to be
able to generalize the heat transfer of interest. The goal here is
to be able to get energy into the zinc in about 8 hours with the
potential to give up the energy to the heat transfer fluid in
approximately 16 hours. The overall times for the melting of zinc
and to store the latent heat of melting depends on the size of the
zinc EPCM pellet and conduction and convective heat transfer. To
get an estimate of the heat transfer times for the charging process
(based on numerical simulations), the melting times for the zinc
PCM for 25 mm pellets are given below.
TABLE-US-00004 TABLE D Diffusion and Melting Times of zinc in the
EPCM Capsule 25 mm in Dia. Heat Transfer Process Fluid Geometry
Time Diffusion and Air Sphere 21.6 minutes Melting Cylinder 41
minutes Process VP-1 like fluid Sphere 2.1 minutes capable of
Cylinder 3.9 minutes 550.degree. C.
Modeling Heat Transfer Related to Use of Salts as the PCM.
[0068] Exemplary salt phase change materials (PCM) include
MgCl.sub.2--NaCl eutectic mixture, as well as NaNO.sub.3. The
results of these exemplary salt PCMs can be compared to the
Zinc-Stainless Steel EPCM. However, the charging times are
functions of material properties as well as inlet HTF temperature
used as the boundary condition for the charging process.
[0069] Extensive heat transfer computations based on first
principles (with equations and boundary conditions) as described
above in salts being used as phase change materials have been
conducted. The heat transfer fluid, HTF (Air or VP-1 like fluid
capable of 550.degree. C.) boundary conditions establish the nature
of energy input to the PCM. The tables below present the total
times for phase change in the PCM with radial heat transfer into
the cylindrical EPCM capsule. For each PCM, an appropriate
encapsulating material has been used with appropriate thermal
properties forming the EPCM. The times for the charging (energy
into PCM) and discharging (energy out of the PCM and into the HTF)
are presented. The times for the phase change are presented as
functions of EPCM capsule radius, PCM in use for Air and VP-1 like
fluid capable of 550.degree. C. Data for air presented here, as for
reference, though air is not likely to be used as a HTF in the
field. For all the cases shown below, an overall 150 K temperature
difference is chosen between the inlet and outlet for the HTF.
[0070] Radial and temporal temperature variations for the capsule
are presented in the figures herein. The governing equations and
boundary conditions are very similar to the ones described above.
Since the 100 mm diameter cylinder EPCMs are likely to have the
longest times for charging (energy stored into PCM) and discharging
(energy to HTF), the temperature profiles are presented for these
in the graphs of FIGS. 5-8.
[0071] The time it takes for the radial diffusion through and
melting of all the PCM is presented in the Tables below as
functions of EPCM capsule diameter during the charging (energy into
the PCM) process. The heat transfer fluid that brings thermal
energy to the PCM can be any suitable HTF for the operating
temperature of the system. In some embodiments, the HTF is air. In
other embodiments, it is a liquid such as a VP-1 like fluid liquid.
"VP-1" is a brand of heat transfer fluid that is a Biphenyl Heat
Transfer Fluid currently used in Trough solar plants but is limited
to 390.degree. C. As used herein, "VP-1 like" means the HTF
possesses a high capacity for absorbing and transferring thermal
energy, without degradation of its chemical structure and desirable
chemical and physical properties. By way of non-limiting example,
such desirable properties include chemical inertness, flowability,
and viscosity of 750.degree. C. Air is compatible and will be used
in future thermal energy storage system experiments. Another
compatible liquid-state HTF is XL-1.
[0072] A preferred EPCM configuration is a long (L/D .about.15)
aspect ratio cylinder, such as a 75 mm diameter and 1125 mm long
EPCM module filled with PCM. The charging time for the
MgCl.sub.2--NaCl eutectic is longer than those for NaNO.sub.3
primarily because the HTF temperature is much closer to the phase
change temperature (444.degree. C.) of the eutectic temperature.
These times as well as the axial progression of the freeze front
(and energy retrieval rate from PCM) can be controlled with
judicious choices of HTF flow parameters.
Charging Process:
TABLE-US-00005 [0073] TABLE E Total Heat Transfer Times for
Charging Process (Minutes) in a Cylindrical EPCM Capsule Using Air
as Heat Transfer Fluid Eutectic Salt/Carbon Sodium Nitrate/
Diameter Steel Capsule Stainless Steel Capsule 10 mm 7.1 min 5.8
min 25 mm 34.5 min 25.7 min 50 mm 109 min 78.3 min 75 mm 214 min
152 min 100 mm 350 min 245 min
TABLE-US-00006 TABLE F Total Heat Transfer Times for Charging
Process (Minutes) in a Cylindrical EPCM Capsule Using VP-1 like
fluid as HTF capable of 550.degree. C. Eutectic Salt/Carbon Sodium
Nitrate/ Diameter Steel Capsule Stainless Steel Capsule 10 mm 1.1
min 0.7 min 25 mm 9.4 min 5.1 min 50 mm 40.5 min 22.5 min 75 mm
93.3 min 52.3 min 100 mm 167 min 94.5 min
Discharging Process:
TABLE-US-00007 [0074] TABLE G Total Heat Transfer Times for
Discharging Process (Minutes) in a Cylindrical EPCM Capsule Using
Air as Heat Transfer Fluid Eutectic Salt/Carbon Sodium Nitrate/
Diameter Steel Capsule Stainless Steel Capsule 10 mm 4.8 min 8.2
min 25 mm 16.8 min 41.8 min 50 mm 44.6 min 145 min 75 mm 79.6 min
300 min 100 mm 120 min 502 min
TABLE-US-00008 TABLE H Total Heat Transfer Times for Discharging
Process (Minutes) in a Cylindrical EPCM Capsule Using VP-1 like
fluid HTF capable of 550.degree. C. Eutectic Salt/Carbon Sodium
Nitrate/ Diameter Steel Capsule Stainless Steel Capsule 10 mm 0.3
min 1.8 min 25 mm 1.2 min 16.6 min 50 mm 4.5 min 73.7 min 75 mm 9.8
min 171 min 100 mm 17.1 min 309 min
[0075] From the times presented, it is seen that the discharge time
of 309 minutes is approaching the 6.3-hour storage anticipated,
although these results can be engineered to be different with a
judicious choice of flow rates and inlet temperatures. Though these
times as well as the axial progression of the freeze front (and
energy retrieval rate from PCM) can be controlled with a judicious
choice of HTF flow parameters, 75 diameter capsules are currently
considered the more conservative design.
[0076] Modeling of Heat Transfer. Heat transfer modeling of the
EPCM for various PCM and encapsulation materials has been completed
for phase change and melt front propagation in the capsules. The
analysis yielded results as well as tools that can be used for
optimization of heat transfer in the future. It is seen that for
all anticipated solar thermal energy storage scenarios considered
here, heat transfer and melting or freezing of the EPCM are not
likely to be the limiting criteria. The heat transfer modeling and
methods are important tools for predicting the anticipated behavior
of PCM in the EPCM in the context of desirable HTF. The examples
chosen here in no way limit the methods and techniques to the much
higher 750.degree. C. that the current storage methods can be
applied to with the use of MgCl.sub.2 filled EPCM.
[0077] Selection/Fabrication of Exemplary EPCM. The encapsulating
material of EPCM pellet is required to withstand stresses
associated with thermal expansion, volume change by phase change of
the PCM-Zinc, MgCl.sub.2--NaCl eutectic, or NaNO.sub.3 salts. Cost
analysis will play a big role in selecting the PCM to be used and
the choice of encapsulating material will depend on compatibility
between the phase change material and the encapsulating material.
Based on material compatibility considerations, it is anticipated
zinc can be encapsulated with 316 stainless steel or nickel, the
chloride salts are best encapsulated with 1018 carbon steel, and
sodium nitrate does well with 316 stainless steel. The stress
analysis and shape of the encapsulated phase change material (EPCM)
capsules are scalable and thus the present discussion related to
stress analysis and encapsulation is done with regard to nickel
with the resulting conclusions having broader implications.
[0078] The EPCM capsules will have to withstand the stresses due to
volume increase of PCM as well as external forces such as those due
to stacking, local crimping or denting of the encapsulation
material. As the zinc is heated and melts, there will be volume
increase subjecting the encapsulating zinc to elastic as well as
plastic stresses. Elastic stress analysis has been conducted using
closed form solutions and Elastic/Plastic stress analysis using
Abaqus has been conducted. This stress analysis determines the
thickness of the encapsulating material.
[0079] The material selections for the proposed EPCM systems were
done after an exhaustive search of literature from purely
theoretical considerations. The validity and practicality of those
considerations are tested by shape the encapsulating material for
the EPCM. The actual form/shape of the EPCM will be determined
after further research--for example, the size and wall thickness of
the encapsulation from stress analysis as well as what is feasible
in the industry. Actually, large-scale manufacturability will be a
key criterion in making this choice. We started with the analysis
of the near spherical Zn--Ni system and then applied the findings
to the more complex shapes as necessary, but always keeping cost as
an important deciding parameter.
[0080] Material Properties and Assumptions. The material properties
of the nickel shell are shown in the Table below.
TABLE-US-00009 TABLE I Properties of Nickel Density 8880
kg/m{circumflex over ( )}3 Modulus of Elasticity 207 GPa Yield
Strength 59 MPa Poisson's Ratio 0.31 CTE 13.1 .mu.m/m-.degree. C.
Specific Heat 460 J/kg-.degree. C. Capacity Thermal 60.7 W/m-K
Conductivity
[0081] Strain hardening was assumed at 30.degree. from the
perfectly plastic case; therefore, the stress-strain relationship
looks as shown below. This assumption was made because reliable
data could not be found for the plastic deformation of nickel. The
present assumption hopefully is more conservative than the
perfectly plastic case and, thus, should be safer and
appropriate.
[0082] Stresses for Ni thicknesses of 50 to 250 micron spherical
shells are considered with elastic as well as plastic strain. The
results (e.g., FIGS. 14-16, et al.) are for elastic case of a
spherical shell. The results are obtained using FEM analysis using
Abaqus program.
[0083] As the zinc liquefies and expands by about ten percent in
volume, the expansion of the zinc inside the nickel shell could
have estimated 3% strain due to (.about.10% volumetric expansion)
the thermal expansion of the zinc. This involves elastic-plastic
deformation and is analyzed using Abaqus. Since this strain was
initiated with equivalent forces inside the sphere, the model could
be simplified by simply exerting an internal pressure inside the
nickel shell until it reaches a value of 3% strain. The
axisymmetric case was used for this model. The differences in the
stresses at 3% strain for the different shell thicknesses were
minimal, showing that the shell will ideally be able to stretch to
3% strain regardless of thickness. The figures show the maximum
principal stresses and strains from the 250-micron thickness case
of the spherical shell, the deformation of the figures is
multiplied by a factor of 2.209 and to exaggerate the effect of the
internal pressure. Results for these plastic deformations are given
in FIGS. 15 and 16.
[0084] The above results indicate that 250-micron Ni shells could
withstand the expansion forces involved. Preliminary indications
are that we will need at least 250 microns of Ni thickness. The
ability of the commercial manufacturers to generate 250 microns or
thicker shells for encapsulation has been investigated in detail
and it appears that at present commercial manufacturers cannot
generate such 250-micron thick encapsulations. Additionally,
encapsulation of Zn with stainless steel or Nickel may have
considerations dealing with the formation of intermetallic
compounds between the PCM and encapsulating material and issues
related to that are discussed in the next sub-section.
[0085] The encapsulation details are discussed in more detail in
the next section with an alternative and more economical approach
to fabricating the Zn--Ni EPCM capsules or rods. Encapsulation of
PCM in the form cylinders with thickness sufficient to withstand
anticipated stresses could be the most economical way of making
EPCM. The preferred encapsulating material is stainless steel for
Chloride and Nitrate salts, and carbon steel for chloride
eutectics.
[0086] Intermetallic phase formations and related solutions. The
melting and solidification of the zinc inside the metal
encapsulation material is somewhat analogous to galvanizing,
wherein a metal is exposed to molten zinc and coated with it as the
zinc freezes. The galvanization process results in the formation of
binary phases with the substrates exposed to diffusion in the
liquid zinc [1-5]. A couple of relevant papers that describe the
interaction of liquid zinc and stainless steel and nickel provide
us some guidance in this regard. Unlike in an ordinary galvanizing,
the phase change material (PCM) in the thermal energy storage
system will undergo multiple melting/solidification cycles. A
measurement system to better understand the phase formations is
described below.
Solutions
[0087] The formation of these phases will depend on the amount of
metal that can be diffused across the layers of interest. If a
phase that does not melt at the operating temperature is formed,
the diffusion through that solid phase will be several orders of
magnitude smaller. This is the approach that this work is taking to
form phases that have much higher melting points than the operating
temperature of 450.degree. C. Thus, we would like to condition each
of the cylinders so that there is a very small layer of
intermetallics that will severely slow or stop further diffusion
between the liquid zinc and substrate. Conditioning will involve
heating complete cylinders (containing zinc and completely sealed)
to the system's operating temperature of 450.degree. C. and holding
for a designated time, then cooling the cylinders to room
temperature. A series of small intermetallic layers should form
that will not melt upon reheating the cylinders. The outermost
layer should be comprised of a large amount of zinc and a much
smaller amount of some alloying element coming from the containing
material. This layer should limit further diffusion upon reheating
and re-melting.
[0088] The Ni--Zn binary phase (FIG. 17) indicates just such
feasibility. For Zn percentages of about 85 and 90 percent, g and d
phase that can be formed have melting temperatures of about
490.degree. C. and 881.degree. C. We hope to take advantage of this
feature by the formation of the g and d phases inside the EPCM
pellet with a brief exposure of molten zinc to the nickel
encapsulation.
Material Selection--(Ni, SS316L, Ti-6-4, 1018, Cu, Grey Cast
Iron)
[0089] Six different types of material were selected to encapsulate
the zinc. The materials are nickel, stainless steel 316L,
titanium-6 aluminum-4 vanadium, 1018 plain carbon steel, copper,
and grey cast iron. These materials were selected based upon
literature, materials that are galvanized, common engineering
materials, general availability, and their high melting points.
Aluminum alloys were also considered, but were discounted because
of their relatively low melting temperatures
(.about.600-660.degree. C.) and their strength at 450.degree. C.
decreases significantly. Inconel type-high temperature Ni based
alloys are also being considered.
Material Testing (Rods in Liquid Zn)
[0090] The interactions of the six encapsulating materials with
liquid zinc were tested. A custom nine-well crucible was fabricated
to hold all of the test specimens. 99.997% pure zinc inserts were
machined to fit in each well of the crucible and one, two inch
long, 0.5 inch in diameter rod of each material was fit into the
zinc insert as shown in FIG. 18. The test apparatus was placed in
an L&L Special Furnace and heated to 450.degree. C. where it
was held for eight hours then cooled to room temperature in the
furnace.
Analysis of Phase Formation Tests
[0091] Each of the six specimens were sectioned, mounted, and
metallographically prepared for light optical and electron
microscopy. Examination by light optical microscopy showed that
intermetallic layers formed in each specimen between the rod and
the zinc (FIG. 15 for Zn--Ni). The layer thicknesses ranged from
200 um-300 um in the nickel/Zn specimen to 3000 um in the grey cast
iron/Zn specimen. The iron based specimens showed signs of in
homogeneous and non-uniform layer growth. Stainless steel 316L
demonstrated the least uniform layer growth. The nickel/Zn,
copper/Zn, and Ti-6-4/Zn specimens demonstrated the most layer
growth. These specimens also formed the most amounts of layers.
[0092] Energy Dispersive Spectroscopy (EDS) was performed on the
copper/Zn and nickel/Zn specimens using a Hitachi 4300 Scanning
Electron Microscope equipped with an EDAX system. Line scans were
made across the intermetallic layers of each specimen extending
from the copper or nickel rod into the zinc. The plots of K alpha
peak intensity versus distance show that there are discreet peak
intensity steps at the interfaces of each layer. This suggests that
there are corresponding compositional changes at the
interfaces.
[0093] Electron Probe Microanalysis (EPMA) was then performed on
the SS316L/Zn and Ni/Zn specimens to determine the compositions in
weight percent of the intermetallic layers and this confirmed the
presence of the thin layers of various phases. It was found that
the composition across an individual layer was relatively
constant.
[0094] Three to four intermetallic layers were present in the Ni/Zn
specimen as evident in DIC Figures and EPMA measurement confirmed
this observation. Only very small changes in concentration were
observed traversing across each layer. Significant changes in
concentration were present on either side of an interface as seen
in FIG. 19. As mentioned above, take advantage of the thin layers
of phases formed that have much higher temperature than an
anticipated operating temperature of a heat storage or heat
transfer system.
[0095] Selection of Materials for Salt-based EPCM. Salts are
suspected of being quite corrosive in the presence of air/Oxygen
under normal circumstances and even more so at high temperatures if
proper material choices are not made. The choice of material to
encapsulate MgCl.sub.2--NaCl eutectic and NaNO.sub.3 is based on
past experiences. Stainless steel-304 is known to have good
compatibility with Nitrate salts. This is known from its use for
Thermocline tanks that contain NaNO.sub.3--KNO.sub.3 eutectics in
solar industry. Stainless steel-304 as well as carbon steel-1018
have been used for the encapsulation of MgCl.sub.2--NaCl eutectic.
Stainless steel-304 appears to work well (with no air/Oxygen) and
we expect carbon steel-1018 also to be compatible if care is taken
to avoid air/Oxygen inside the capsule where the chlorides are in
contact with the encapsulating materials. The inventors contemplate
studying the long-term corrosion and compatibility of the salts
with the corresponding encapsulating materials.
[0096] Selection of Materials for Fabrication of EPCM. Methods for
the encapsulation of Zinc using Nickel were analyzed based on
state-of-the-art of the electroless process and manufacturing
capabilities of the industry. Encapsulation shape and thickness
were optimized and cylindrical capsules (25 to 100 mm dia.) with
long lengths (L/D>10) were determined to be the best. Current
industrial state-of-the-art does not permit large-scale economical
means of generating electroless layers of the desired thickness
(>250 to 500 microns). Encapsulation of PCM in welded
cylindrical shells has been determined to be the most economical
way of manufacturing EPCM. The thickness of the encapsulation
(.about.1.5 mm) is determined from weldability and stress criteria.
This will generate safe encapsulation of PCM and economical EPCM
for thermal energy storage particularly in large quantities.
Stainless for NaNO.sub.3 and stainless steel and carbon steels are
seen to be acceptable as encapsulation materials for the NaNO.sub.3
and MgCl.sub.2--NaCl eutectic PCM.
[0097] Fabrication of Sample EPCM. As the PCM is heated and melts,
there will be significant volume increase subjecting the
encapsulating PCM to elastic as well as plastic stresses. These
stresses depend on the geometry, amount of air in the pellet and
material properties. The actual form/shape of the EPCM is
determined using potential stresses in the nickel encapsulation
based on stress analysis described in an earlier section, e.g., the
size and wall thickness of the encapsulation based on FEM stress
analysis. The phase formations for the PCM-Encapsulation
combination (e.g., Zn--Ni) need to be taken into account.
Large-scale manufacturability will be an additional key criterion
in making the choices for the EPCM system for thermal energy
storage. The current emphasis is on the development of cylindrical
Zn--Ni EPCM pellets keeping cost as an important deciding
parameter.
[0098] Material Properties and Assumptions. The below discussion
related to stresses in and the strength of the encapsulation is
given with Nickel as the example material. The resulting discussion
and results are applicable to other encapsulating materials just as
well in a broad sense.
[0099] The material properties for the nickel, shell or
encapsulation material, used herein have been discussed in a
previous section. Strain hardening was assumed at 30.degree. off
the perfectly plastic case for stress-strain relationship. This
assumption enables a more conservative design. Three different
designs for the Ni--Zn ECPM containment vessels encapsulating the
PCM have been investigated. The investigated shapes are spherical
nickel shell, stainless steel cylindrical shell with crimped and
welded ends, and a stainless steel cylinder with caps welded on the
ends.
[0100] The spherical nickel shell design enables the largest
possible surface area-to-volume ratio and maximizes heat transfer.
In the electroless process the nickel shell is formed by
electrically covering zinc spheres with a nickel. Very little void
space (also referred to as "air gap") is provided or assumed in the
EPCM. Zinc would substantially completely fill the nickel shell and
the thermal expansion of the zinc over the melting process would
cause the nickel sphere to experience a strain value of 3% for an
overall 10% expansion of the PCM. The stresses in the nickel shell
resulting from the expansion of the zinc, along with the effects of
any imperfections in the nickel shell and external point loads have
been investigated. Stress distributions for some of the cases
investigated are shown in FIGS. 20 and 21.
[0101] The 250 micron thick nickel shell, the thickest shell
simulated in these examples, does not seem to be sufficient because
the stresses resulting from point forces, dents, and thinning in
the shell reach values of approximately 80 MPa. Furthermore, a
crack in the outside of the shell could cause stresses surpassing
100 MPa. In each of the simulations only one of these problems is
taken into account, whereas in the actual part, multiple
inconsistencies and forces will be present. Modeling only a single
crack, dent, thinned area, or point force into account does not
show what will happen if all of these effects are acting together.
If the spherical shell has a crack in it, then thinning of the
shell as it plastically deforms while a point force is pushing on
the shell at another point, the shell will definitely fail. Also
the shell cannot be guaranteed to be manufactured perfectly;
therefore, it is likely that imperfections such as thinner sections
in the shell are a possibility. Even if the shell is manufactured
perfectly, dents, cracks, and scratches could be formed on the
shell when the balls are poured into the container for the actual
system.
[0102] Electroless Coating Manufacturing of EPCM. The electroless
process is usually a batch process used mainly for small quantities
of the substrate material to be coated. Though Zinc is expensive,
it is currently our primary material of choice. A continuous
electroless technology is envisioned here. The proposed process
will use a mesh type belt which will agitate Zn balls in
electrolyte allowing them the most uniform coating possible. The
speed of the electroless process is an extremely important process
parameter and will be controlled by the speed of the belt (closed
loop process control). Chemical composition of the electrolyte used
in the electroless process needs to be controlled, and during the
process, electrolyte needs to be replenished to maintain constant
composition. The belt will be made of a material that does not
impede the electroless deposition and does not accept Nickel
coating. These requirements are not very complicated in principal,
but on the other hand, it needs attention to detail and cost
considerations as the systems are scaled up for the uniform Nickel
deposition on Zn balls. The 229 million kg of Zinc in the form of 8
million 0.01 m balls will need to be coated with about 10 microns
thickness of Nickel layer. This will entail the use of five 10
m.times.1 m.times.0.25 m tanks; the process control system to
complete this process in about 2 months. The cost of the tanks,
controls and the Nickel in the form of electrolyte to coat the
balls will cost approximately $200K, which is not prohibitively
expensive for the benefits anticipated here.
[0103] The secondary PCM of choice are the eutectic salt mixtures
(MgCl.sub.2--NaCl 38 wt %-62 wt %) that are likely to be quite
inexpensive but may have a short service life due to potential
segregation of the two media. They will be encapsulated in 2.5 m
long, 0.02 m (2 to 5 cm) diameter tubes made of metallic corrosion
resistant material such as stainless steel. The total 33,000 m3 of
salt mixtures needed (for the 26 Tera J) will be melted and
injected into 2.5 m long tubing, which will be first flattened and
joined by the homogenous welding process. In order to make this
approach economically attractive the rolled 0.001 m thick sheet of
316 stainless steel will be split into tapes, which will then be
roll formed and welded to produce the welded tube of 0.02 m
diameter. After molten salt injection, the top end of the tube will
be flattened and welded to ensure encapsulation of the salt
mixture. The custom made robotic welding equipment, cost of salts
and welding tube fabrication is estimated to be about $4 million
(not part of current project budget) and thus this project will
only look into the design of such systems.
[0104] Crimped Cylinder. The crimped and welded stainless steel
cylinder design is considered as a potential geometry for
encapsulating the PCM. It enables easy large-scale manufacture and
provides large surface area-to-volume ratio. The encapsulation was
formed by crimping and welding one end of a cylinder, filling
approximately 80% of the cylinder with zinc powder, and then
crimping and welding the other end of the cylinder. The stresses on
the inside of the vessel at the point where the weld is located
were investigated because this is where the stress magnitude is
largest. The stresses are highest at the weld line because the
sharp corner created by the crimping process creates an area of
point loading and a stress concentration. See FIG. 22.
[0105] The stresses in the crimped cylinder will be in the elastic
range for the cases of 70%, 80%, and 85% initial zinc content at a
temperature of 450.degree. C., with the compressive stresses shown
on the inside of the crimped cylinder and tensile stresses shown on
the outside of the crimped cylinder. The stresses calculated using
the pressures found with the combined gas law for the 70%, 80%, and
85% initial zinc content cases are 0.37, 0.58 and 1.31 MPa,
respectively. The stresses found on the outside of the crimped
cylinder at the seam at the end of the crimped and welded section
of the crimped cylinder are compressive because the crimped and
welded section does not expand outwards due to the internal
pressure like the rest of the crimped cylinder does; therefore, as
the crimped cylinder expands, it pushes on that seam, pulling on it
on the inside and pushing on it on the outside.
[0106] At 86% initial zinc filling the internal pressure is 2.03
MPa, and the stresses in the crimped cylinder will be higher than
the yield strength of the type 316L stainless steel, which is 125.8
MPa at 450.degree. C. When plastic deformation was taken into
account, the difference between the elastic and plastic models was
found to be extremely small because at such a small amount of
plastic strain, the stresses will always be relatively the
same.
[0107] The maximum stress values occur on the inside of the weld
lines of the crimped cylinder because the internal pressure becomes
a line load on the seam of this section. The stresses in the rest
of the crimped cylinder are well below the stresses at these
maximum points and the maximum stresses are also much lower than
the failure stress, which is 423 MPa.
[0108] The crimped cylinder would be the perfect container to use
in the thermal energy storage system; provided the stainless steel
container can be filled with zinc easily. This could be done by
filling the cylinder crimped on one side with molten PCM and then
welding the other end after the PCM has solidified. Another way to
fill the crimped cylinder would be with small (5 mm or 0.25'')
solid pieces of zinc. The crimped cylinder geometry is a viable
encapsulation method.
[0109] Capped Cylinder (welded cylinder). The capped stainless
steel cylinder design made use of traditional machining processes
and the strength of the cylinder in the hoop-wise direction. One
cap is initially welded onto the cylinder, and then a rod of zinc
is inserted into the cylinder, which is followed by the welding of
the second cap. Void space was left between the zinc rod and the
steel cylinder so that air pressures inside of the part could be
controlled and do not reach levels that will cause failure in the
part. The stresses were investigated on the inside of the cylinder
around the corner where the weld line of the cap is located. The
stress concentrations caused by the point loading on the outside of
the cylinder when the cylinders are stacked in the full system were
also investigated.
[0110] A two-dimensional model of the cylinder showed that an
infinitely long cylinder would not be subject to failure by the
stresses caused by each of the different loading cases. The
addition of point loads shows that the stack size could be over
1,500 cylinders without causing the hoop stresses to reach the
ultimate stress of the material. The actual forces will be less
since real loading will be non-point type and precautions can be
taken to ensure that there are no more than about 1,000 cylinders
exert weight onto the lower cylinders. The side forces caused by
the expansion of the cylinders adjacent to the cylinder in question
or the edge of the system could be ignored because they will not be
very large compared to the points loads caused by the weight of the
balls. Also, in the actual system, the cylinders will not be
perfectly stacked as shown in the model; rather, they will be
poured into the system and will arrange themselves randomly. An
aspect ratio close to one would ensure that the distribution is
random. Other considerations may also govern the aspect ratio of
the EPCM cylinders. As explained later, manufacturing cost
considerations could favor long aspect ratio EPCM cylinders proving
for a rod bundle geometry for the thermocline as opposed to a
packed bed.
[0111] A three-dimensional model of the cylinder shows that the
stainless steel cylinder will be able to handle the stresses caused
by the expansion of the zinc in the axial direction, the tangential
direction and the area directly surrounding the inside corner of
the welded cap. The weld at the cap of the cylinder was assumed to
be perfect in this model, where the weld creates a continuous part,
without any change of material properties. Any zinc that
contaminates the weld would weaken the weld. During the welding
process, melted zinc can possibly form cyanide; therefore, safety
precautions must be taken. The capped cylindrical geometry and
container made of 304L stainless steel for the encapsulation of
zinc as the phase change material appears to be the most viable
option for thermal energy storage. It can withstand the stresses
caused by the internal pressure and also the external loads applied
on it by the surrounding cylinders in the system.
Fabrication of Sample EPCM.
[0112] MgCl.sub.2--NaCl Eutectic and NaNO.sub.3 Salts. The melting
temperatures of NaCl or MgCl.sub.2 are 804.degree. C. and
714.degree. C., respectively. These temperatures are well beyond
the current range of interest for thermal energy storage.
Fortunately, the eutectic composition of 55 wt % MgCl.sub.2-45 wt %
NaCl has a melting point (444.degree. C.) that is very near the
temperatures of interest here. The phase diagrams for these
chloride salts from two references are included in the figures
below. Thermal energy storage and retrieval at 444.degree. C. with
MgCl.sub.2--NaCl eutectic as phase change material (PCM) can enable
very good efficiency for the Rankine cycle.
[0113] We have completed a significant amount of work related to
the manufacture of NaNO.sub.3 EPCM and proof of their ability for
thermal energy storage. NaNO.sub.3 has been used in the solar
industry for thermal energy storage but usually using sensible heat
only. NaNO.sub.3 has a significant heat of fusion and thus in the
present work, NaNO.sub.3 is used as a phase change material
enabling significant savings in the overall cost of thermal energy
storage systems. The steps for the successful use of the salts for
thermal energy storage will include the preparation of the
MgCl.sub.2--NaCl eutectic or preparation of NaNO.sub.3, manufacture
of the salt based EPCM with appropriate encapsulating materials,
and the testing of the EPCM for storage and retrieval of thermal
energy. These steps are described below.
[0114] Initial eutectic salt synthesis. To prepare the chloride
eutectic, 99.0% GR ACS grade NaCl and 99% Anhydrous MgCl.sub.2
(manufacturers: EMD and Alfa Aesar, respectively), two solid
crystalline powders were mixed in an alumina crucible with an Ar
blanket. The powder mixture was heated to 500.degree. C. (well
below the respective melting points of 804.degree. C. and
714.degree. C.) and allowed to soak at this temperature for an
hour, above the eutectic temperature (445.degree.
C.<T.sub.e<450.degree. C.--based on the phase diagrams). The
eutectic was formed under these conditions resulting in crystalline
powder. The resulting eutectic has a melting point of 444.degree.
C. as proven below.
[0115] Proof of Eutectic Composition. Differential Scanning
calorimetry (DSC) was used to determine the melting point of the
synthesized MgCl.sub.2--NaCl eutectic salt. The results of the DSC
measurements are quite consistent with the data in the phase
diagrams shown above. The melting point, determined by peak
analysis was found to be approximately 444.degree. C. which is
within 1.degree. C. of the published/reported value of 445.degree.
C. The DSC scans for the chloride eutectic are shown in FIG.
26.
[0116] Manufacture of EPCM Cylinders for the MgCl.sub.2--NaCl
Eutectic and NaNO.sub.3 Salts. Several cylindrical capsules to
contain the PCM salts have been made. Stainless steel capsules with
dimensions: D1''.times.H2'' and 0.065'' wall thickness were made
with two different caps welded to the open ends of a SS304 tube. In
another design, one cap was made with a (tapered) threaded hole to
allow for a 1/16 NPT pipe plug. This opening allows the PCM salt
(in powder form) to be poured into the capsule. This was done to
avoid heating NaNO.sub.3 to weld temperatures, which is considered
potentially dangerous.
[0117] NaCl and MgCl.sub.2 solid crystalline powders were mixed
together in the eutectic ratio (55 wt % MgCl.sub.2 to 45 wt % NaCl)
and heated to 550.degree. C. and allowed to completely melt. The
molten salt eutectic was poured into the carbon steel cylinder. The
cylinders were filled only to a predetermined `fill-line` to ensure
sufficient space for the expansion of the PCM in the capsules.
[0118] MgCl.sub.2--NaCl eutectic encapsulated cylindrical capsules
were made with stainless steel (304) as well as carbon steel (1018)
encapsulation to look into compatibility issues. To increase the
proportion of the mass of PCM compared to the encapsulation, and to
test EPCM cylinders that are likely to be used in the field, some
larger diameter (3'' dia.) and longer capsules were also made and
tested.
[0119] The sealed EPCM capsules made with the procedures described
above and containing known amount of one of the salts was used for
calorimetry to prove the energy storage capabilities of the
salts.
[0120] FIG. 29 shows sections of a MgCl.sub.2--NaCl eutectic EPCM
cylinder with stainless steel encapsulation. The void in the EPCM
is purposeful for managing stresses related to expansion. The cross
section of the MgCl.sub.2--NaCl eutectic EPCM cylinder indicates
that the chloride eutectic is good status and so is the stainless
steel encapsulation. The sectioning of this cylinder was done after
nearly 50 hours of repeated heating and cooling (thermal energy
charging into and discharging out) of the cylinder and contents.
Further study of this issue is needed.
[0121] Fabrication of Sample EPCM. Elongated cylinders have been
determined to be the best shape for EPCM based on several competing
criteria. Several cylindrical EPCM capsules of various PCM (Zinc,
MgCl.sub.2--NaCl eutectic and NaNO.sub.3) with appropriate
encapsulation materials have been made with diameters of 25 to 100
mm and with lengths up to 125 mm. Thermal energy storage
capabilities of the various PCM are tested using the sample
EPCM.
[0122] Calorimetric Measurements. The first thermodynamic
experiments to evaluate the encapsulated phase change material
consist of calorimetric measurements to establish the amount of
energy storage that can be actually realized in a single unit of
the encapsulated assembly.
[0123] These calorimetric experiments will determine the enthalpy
change of the EPCM when it is cooled from .about.450.degree. C.
(above the melting point of PCM) to ambient room temperature. Thus,
energy storage will include sensible heat of solid PCM as well as
latent heat of PCM, and sensible heat of liquid PCM as well as
sensible heating of encapsulation material. A search for
calorimeter suitable for this purpose did not reveal any off-shelf
instrument with sufficient accuracy. Therefore, we have designed a
calorimeter, of "drop" type, which should provide the desired level
of accuracy. FIG. 29 presents a sketch of this calorimeter system.
A thin metal container with various layers of thermal insulation is
loaded with silicon oil (Siltherm 800 from Dynalene, Inc.), serving
as the heat sink. A test unit of the encapsulated PCM is preheated
in an oven to the starting temperature of 450.degree. C. , then
immersed in the silicon oil. Measurement of the subsequent change
in oil temperature permits determination of the enthalpy change of
the EPCM sample. Judicious care is required to guard against
parasitic heat loss and to correct for mixing power input. A
schematic of the precision calorimeter is shown in FIG. 30.
TABLE-US-00010 Calorimeter parts are indicated as -- 1- Stainless
steel container 2- Sample 3- Silicon oil 4- Mixer 5, 6- Thermistor
7- Thermocouple 8- shielding cylinder#1 9- shielding cylinder
#2
[0124] We used about 1.5 gallons of silicon oil (Dynalene Siltherm
800) in the conical stainless steel flask as the working fluid to
absorb the heat from the sample. The silicon oil is an excellent
liquid with high conductivity coefficient, and its flash point is
315.degree. C., which contributes to ignorable evaporation mass
loss in the experiment. A mixer is used to stir the oil when sample
is dipped into it in order to get a uniform temperature quickly.
Outside of the container, we attach shielding to control heat
losses. A PC-DAQ system is used to record the temperature profiles
of the sample, silicon oil and ambient air as a function of
time.
[0125] The initial sample tested is a cylindrical stainless steel
with dimensions of D=0.998''.times.H=1.828''. Samples of EPCM are
also tested. The sample is heated to 500.degree. C., and quickly
immersed into the silicon oil. FIG. 31 shows the temperature
history of the sample, silicon oil and air in the experiment.
[0126] From FIG. 32, we can see that it just takes a few minutes
for the silicon oil and sample to reach equilibrium temperature and
decrease together because of the heat loss to the environment.
Using an overall energy balance including all energy inputs (mixer
power input, etc.) and outputs (heat losses etc.), the actual
energy given up by the solid or EPCM is to the silicon oil
evaluated. The calorimetry proves the thermal energy storage
capability of a single EPCM pellet, and the packed bed experiment
to be done in phase 2 will prove the thermal energy storage
capabilities of a large mass of phase change material.
[0127] Thermal Energy Storage capabilities of MgCl.sub.2--NaCl and
NaNO.sub.3. Encapsulated phase change material (EPCM) capsules were
tested using calorimetry to quantify the energy storage
capabilities of the PCM. The calorimeter used for this purpose has
been designed and built specifically for this purpose and has been
described in the previous section.
[0128] The test procedure consisted of heating the EPCM cylinders
in a furnace with tight temperature controls. The cylinders were
heated to a temperature well above the melting temperature (thus
storing sensible and latent heat) and held for a significant amount
of time for steady temperature measurements. The cylinders were
quenched in the calorimeter silicon fluid and the energy stored was
evaluated based on the temperature increase of the fluid and
well-established heat loss data. The overall heat balance is
verified to be within 2%.
[0129] Lack of reliable and consistent set of thermal properties of
the phase change materials as functions of temperature turned out
to be a major difficulty in evaluating the calorimeter data. The
most consistent data available has been used here. It is important
to note the consistency of the data presented below and its
repeatability rather than delve on the fact that the fraction of
energy from the PCM calculated is not unity.
TABLE-US-00011 TABLE J Thermal Energy Storage Calorimetry Results
for MgCl.sub.2--NaCl Eutectic-Stainless Steel and Carbon Steel EPCM
Energy Fraction of Transferred Theoretical Theoretical Cycling to
Silicone Energy from Energy from Energy from Energy from Number Oil
(kJ) Capsule (kJ) Eutectic (kJ) Eutectic (kJ) Eutectic Expt 1 41.6
21.4 20.2 23.8 0.85 Expt 2 41.8 21.7 20.1 24.4 0.82 Expt 3 41.1
21.2 19.9 23.4 0.85 Expt 4 40.9 21.1 19.8 23.2 0.85 Expt 5 (8 hrs)
42.6 21.6 21.0 24.1 0.87 Expt 6 38.7 21.5 17.2 19.8 0.87 Expt 7
38.9 21.7 17.2 20.1 0.86 Expt 8 (8 hrs) 39.0 21.8 17.2 20.2 0.85
1''Dia .times. 2'' EPCM Capsule
TABLE-US-00012 TABLE K Thermal Energy Storage Calorimetry Results
for NaNO.sub.3-Stailess Steel EPCM (2''Dia .times. 5'' EPCM
Capsule) Energy Fraction of Transferred Theoretical Theoretical
Cycling to Silicone Energy from Energy from Energy from Energy from
Number Oil (kJ) Capsule (kJ) NaNO3 (kJ) NaNO3 (kJ) Eutectic Expt 1
215 63.5 152 167 0.91 Expt 2 222 64.5 158 168 0.94 Expt 3 222 65.3
156 169 0.92 Expt 4 214 60.0 154 160 0.96 2''Dia .times. 5'' EPCM
Capsule
[0130] The repeatability of the energy stored by the PCM
(percentages .about.85% and .about.92%) is remarkable and the
uncertainties in overall energy balance for these experiments are
about 2%. To repeat a predicament related to material properties
mentioned earlier, the percentages reported are not 100% only
because the properties of these materials are not well established
in the temperature ranges of interest.
[0131] Calorimetric Measurements. An inexpensive precision
calorimeter has been designed and built for testing thermal storage
capabilities of the EPCM since none were available for purchase.
Encapsulated phase change material (EPCM) cylindrical capsules
consisting of Zn, MgCl.sub.2--NaCl eutectic and NaNO.sub.3 have
been manufactured. The encapsulation materials used are stainless
steel or carbon steel as appropriate. Calorimetry of Zn,
MgCl.sub.2--NaCl eutectic and NaNO.sub.3 EPCM cylinders have proven
the thermal storage and retrieval capabilities of these materials
successfully and with repeatability.
[0132] Cost Analysis of Thermal Energy Storage System. Cost
analysis of thermal energy storage is described here. The cost
analysis has been performed for a 100 MWe plant to have 6.3 hours
of thermal storage capability to determine the viability of EPCM
based thermal storage systems. The cost analysis has used exactly
the same assumptions made by NREL scientists and includes separate
cost analyses for Zn, MgCl.sub.2--NaCl eutectic and NaNO.sub.3 EPCM
cylinders used in a large thermal energy storage system
(Thermocline). Solar power plant data used here are from two NREL
Reports--Nexant NREL/SR-550-40163, July, 2006 and "Parabolic Trough
Reference Plant for Cost Modeling with the Solar Advisor Model,"
NREL Reference subcontractor Report, DRAFT May 11, 2010.
[0133] Cost analysis is based EPCM cylindrical shaped capsules that
can be manufactured in large-scale with welding. As described
earlier in the report, cylindrical EPCM capsules are the most
practical based on stress analysis, also. As a result, the
discussion below is related to the cost analysis of cylindrical
capsules. Some of the details of the cost analysis are described
below followed by the variation of cost of thermal energy storage
($/kWh).
[0134] The analysis for various phase change materials and
encapsulation materials is presented here for various diameters and
lengths of the capsules. Manufacturing of cylindrical EPCM capsules
with large length to diameter ratio turns out to be more cost
effective since that involves smaller manufacturing costs. The cost
analysis is presented for wall thickness t=1.5 mm. The geometry of
the capsule is shown in the figure below.
[0135] Costs are calculated on what it costs to make the EPCM
capsules and how many are needed to be in a tank with an
appropriate pump etc. (fixed costs).
[0136] Volume of individual capsule,
V cap = .pi. D 2 H 4 ##EQU00003##
[0137] D is the diameter of the capsule, H is the length of the
capsule. With t as the wall thickness of the shell the volume
occupied by the PCM is
V enc = .pi. 4 ( D - 2 t ) 2 ( H - 2 t ) ##EQU00004##
[0138] The cap that is welded at each end of the capsule is
considered to have a wall thickness of t as well. The result of the
cost analysis is presented for the diameter of the capsule varying
from 25 mm to 100 mm and the length of the capsule varying from
1.times.D to 20.times.D.
[0139] The thermal energy stored by the individual capsule,
Q.sub.cap, can be calculated as
Q.sub.cap=Q.sub.pcm+Q.sub.encl
[0140] Where the energy stored by phase change material is
Q.sub.pcm and the energy stored by encapsulation material is
Q.sub.enc
Q.sub.pcm=.rho..sub.pcmV.sub.pcm(cp.sub.pcm,s(T.sub.m-T.sub.min)+cp.sub.-
pcm,l(T.sub.max-T.sub.m)+L)
and
Q.sub.enc=.rho..sub.encV.sub.enccp.sub.enc(T.sub.m-T.sub.min).
[0141] Here .rho..sub.pcm and .rho..sub.enc are the density of the
PCM and encapsulation materials, cp.sub.pcm and cp.sub.enc are the
specific heat capacities of the PCM and encapsulation materials and
L is the latent heat of fusion. For PCM, the heat capacity of the
solid and liquid phase may be different. T.sub.m is the melting
temperature of the PCM and T.sub.max and T.sub.min are the maximum
and minimum operating temperature of the storage unit. The material
properties of PCM and encapsulation materials considered in the
present study are given in the Table below.
TABLE-US-00013 TABLE L Physical properties of PCM and encapsulation
materials Zinc Eutectic-Salt NaNO.sub.3 Stainless Carbon Materials
Solid Liquid Solid Liquid Solid Liquid Steel Steel Nickel Density
7140 2160 2257 8003 7860 8908 (kg/m.sup.3) Specific Heat 0.48 0.39
0.86 0.86 1.67 1.78 0.48 0.48 0.44 (kJ/kgK) Latent heat 112 329 173
(kj/kg) Melting 419 459 308 Temperature (.degree. C.)
[0142] The number of EPCM capsules, N, needed to store 300
MW.sub.th (.about.100 MW.sub.e) for 6.3 hours can then be
determined
N = Q total Q cap , with Q total = 300 , 000 * 6.3 * 3 , 600
##EQU00005##
[0143] The total cost of the thermal energy storage system
(Thermocline unit) is determined from the cost of material making
up the capsule (PCM and encapsulation), the cost of manufacturing
of the capsule and fixed cost. The fixed cost includes the cost of
tanks, heat exchanger, pumps, pipes and controller. The cost of
material is calculated as
Cost.sub.mat=mass.sub.pcm*price.sub.pcm+mass.sub.enc*price.sub.enc
[0144] Here mass.sub.pcm and mass.sub.enc are the total mass of the
PCM and encapsulation in the storage unit and price.sub.pcm and
price.sub.enc are the price of PCM and encapsulation material. The
prices of PCM and encapsulation material are given in the Table
below.
TABLE-US-00014 TABLE M Material and tubing costs Eutectic Stainless
Carbon Materials Zinc Salt NaNO.sub.3 Steel Steel Nickel Material
$2.13 $0.35 $0.62 $3.69 $0.74 $24.21 Price ($/kg) Tubing $0.50
$0.34 $1.00 Price ($/kg)
[0145] The cost of manufacturing of capsules is calculated as
Cost.sub.man=N*price.sub.cap+mass.sub.enc*price.sub.tubing
[0146] where N is the number of capsule, price.sub.cap is the cost
of welding caps of each capsule and price.sub.tubing is the price
of encapsulation tubes. The price of tubing for encapsulation
materials used in this work is listed in the Table above.
[0147] The fixed cost is determined as the cost of pipes, pumps,
controllers, heat exchangers and tanks. The cost of such components
excluding the cost of tanks is $2,638,000 (as given in the Nexant
NREL report). The cost of tank is determined from the total volume
as
Cost ta n k = V total V existing Cost existing ##EQU00006##
[0148] It is assumed that the cost of tank is proportional to its
volume. V.sub.existing, the volume of the tank of the existing
solar power system, and Cost.sub.existing, the cost of tank in the
existing solar power system, are taken from the reference.
V total = N * V cap 1 - void ##EQU00007##
[0149] where `void` is determined from the arrangement of the
capsules in the vessel. The center-to-center spacing between the
capsules is taken to be 1.4 D.
[0150] The result of cost analysis is presented in FIG. 33 as a
function of the diameter of the cylindrical shaped capsule for H/D
of 15 for various combinations of PCM and encapsulation material.
The cost ($/kWh) decreases significantly as the size of capsules
increases. The eutectic salt and NaNO.sub.3 are cost-effective PCM
material; they easily met the DOE's target of the $40/kWh. Zinc in
a large size capsule with stainless steel encapsulation material is
slightly above the cost target.
[0151] The EPCM thermal energy storage system cost is shown in
Figure above as a function of length/diameter (H/D) for different
combinations of PCM and encapsulation material for D=75 mm. The
longer capsules provide more cost-effective storage unit for any
combinations of PCM and encapsulation material.
[0152] The cost of the thermal energy storage unit with the costs
broken down are shown in the Table below--for material costs,
manufacturing costs and fixed costs which include the cost of
pipes, pumps, controllers, heat exchangers, tanks, etc. The cost of
such components excluding the cost of tanks ($2,638,000) are the
same as those used in the Nexant NREL report (reference listed at
end).
TABLE-US-00015 TABLE N The breakup of the total cost of the storage
unit ($/kWh) for different combinations of EPCM and for D = 75 mm
and H/D = 15. The costs categorized as material, manufacturing and
fixed (heat exchanger/pumps/tanks etc). NaNO3- MgCl2/NaCl PCM-Shell
Zn-Stainless Stainless Eutectic- Combination Zn--Ni Steel Steel
Carbon Steel Material Cost 80.50 41.97 11.17 4.13 ($/kWhth)
Manufacturing 2.87 1.86 2.20 2.38 Cost ($/kWhth) Tank, Pump and
9.70 9.70 10.82 11.92 Balancing Cost ($/kWhth)
[0153] The total cost of EPCM thermal storage system for D=75 mm
and H/D=15 is shown in the Table below for the material,
manufacturing and fixed costs indicated in the Table above.
TABLE-US-00016 TABLE O The cost of the storage unit ($/kWh) for
different combinations of PCM and encapsulation materials for D =
75 mm and Length of capsule 15 D. NaNO.sub.3- MgCl.sub.2/NaCl
PCM-Shell Zn-Stainless Stainless Eutectic- Combination Zn--Ni Steel
Steel Carbon Steel Cost ($/kWh) $93.08 $53.55 $26.62 $20.29
[0154] The Thermal Energy Storage system that uses salts in EPCM
combinations are well under the DOE goal of $40/kWh at $27 and
$20/kWh.sub.e. The storage system using zinc-stainless steel will
cost a little over the DOE goal.
[0155] Manufacture Cost of TES. A cost analysis has been performed
for a 100 MW.sub.e plant to have 6.3 hours of thermal storage
capability to determine the viability of EPCM based thermal storage
systems. The cost analysis has used exactly the same assumptions
made by NREL scientists. A separate cost evaluation will be
conducted for both the Zinc and salt systems. A summary of the
analysis is given below.
[0156] Preliminary analysis indicates that the EPCM cost decreases
by a factor of 2 for L/D from 1 to 10. Ideal L/D for EPCM capsules
is likely to be between 10 and 15.
[0157] The total cost of the thermal energy storage system that
includes the cost of the materials, manufacturing costs and costs
associated with pumps, tanks, heat exchanger, etc. are shown in the
Table below for capsule diameter D=75 mm and length/diameter,
H/D=15.
TABLE-US-00017 TABLE P The cost of the storage unit ($/kWh) for
different combinations of PCM and encapsulation materials for D =
75 mm and Length of capsule 15 D. NaNO.sub.3- MgCl.sub.2/NaCl
PCM-Shell Zn-Stainless Stainless Eutectic- Combination Zn--Ni Steel
Steel Carbon Steel Cost ($/kWh) $93.08 $53.55 $26.62 $20.29
[0158] The above cost analysis indicates the advantages of large
diameters and long aspect ratios (L/D of cylinder) of EPCM. It is
apparent salts are likely to make better PCM and cost per kWh to
use Zinc may be high. The use of salts as PCM for thermal energy
storage can be quite economical and could cost much less expensive
than currently used two tank thermoclines that use sensible heat
only.
[0159] Determination of the optimum thickness of the Nickel to
withstand the anticipated stresses is a critical step. This in turn
depends on the diameter of the Zinc pellets which will be based on
heat transfer considerations. Additionally, the eutectic behavior
of the NaCl and MgCl.sub.2 salts will also determine the path for
further action. The EPCM based thermal storage system(s) must have
potential to cost less than $40/kWh.sub.th in order to be
considered viable.
[0160] The best EPCM geometry for economical and effective thermal
energy storage is a long cylinder (with L/D .about.10 or larger).
Cylindrical encapsulated phase change material (EPCM) capsules with
Zinc, MgCl.sub.2--NaCl eutectic and NaNO.sub.3 as phase change
material (PCM) have been manufactured. Zinc, MgCl.sub.2--NaCl
eutectic and NaNO.sub.3 EPCM capsules have been tested in a
precision calorimeter made for this purpose to test their efficacy
for thermal energy storage. All the three PCM (Zn, MgCl.sub.2--NaCl
eutectic and NaNO.sub.3) tested in this project have been shown to
store thermal energy well though with different caveats, advantages
and disadvantages. Salts used as PCM can have costs significantly
below DOE goal of $40/kWh.sub.th and can be much better than
current thermoclines that use sensible heat only for thermal energy
storage.
[0161] Preliminary designs for testing the PCM currently under
consideration are flexible enough to accommodate any of the three
PCM described considered above.
[0162] Manufacture Cost of TES. A cost analysis will be performed
to determine the viability of EPCM based thermal storage systems. A
separate cost evaluation will be conducted for both the Zinc and
salt systems.
[0163] The electroless process is usually a batch process used
mainly for small quantities of the substrate material to be coated.
Though Zinc is expensive, it is currently our primary material of
choice. A continuous electroless technology is envisioned here. The
proposed process will use a mesh type belt which will agitate Zn
balls in electrolyte allowing them the most uniform coating
possible. The speed of the electroless process is an extremely
important process parameter and will be controlled by the speed of
the belt (closed loop process control). Chemical composition of the
electrolyte used in the electroless process needs to be controlled
and during the process electrolyte needs to be replenished to
maintain constant composition. The belt will be made of a material
that does not impede the electroless deposition and does not accept
Nickel coating. These requirements are not very complicated in
principal but, on the other hand, it needs attention to detail and
cost considerations as the systems are scaled up for the uniform
Nickel deposition on Zn balls. The 229 million kg of Zinc in the
form of 8 million 0.01 m balls will need to be coated with about 10
microns thickness of Nickel layer. This will entail the use of five
10 m.times.1 m.times.0.25 m tanks, the process control system to
complete this process in about 2 months. The cost of the tanks,
controls and the Nickel in the form of electrolyte to coat the
balls will cost approximately $200K, which is not prohibitively
expensive for the benefits anticipated here.
[0164] The secondary PCM of choice are the eutectic salt mixtures
(MgCl.sub.2--NaCl 38 w %-62 w %) that are likely to be quite
inexpensive but may have a short service life due to potential
segregation of the two media. They will be encapsulated in 2.5 m
long, 0.02 m (2 to 5 cm) diameter tubes made of metallic corrosion
resistant material such as stainless steel. The total 33,000 m3 of
salt mixtures needed (for the 26 Tera J) will be melted and
injected into 2.5 m long tubing, which will be first flattened and
joined by the homogenous welding process. In order to make this
approach economically attractive, the rolled 0.001 m thick sheet of
316 stainless steel will be split into tapes, which will then be
roll formed and welded to produce the welded tube of 0.02 m
diameter. After molten salt injection, the top end of the tube will
be flattened and welded to ensure encapsulation of the salt
mixture. The custom made robotic welding equipment, cost of salts
and welding tube fabrication is estimated to be about $4 million
(not part of current project budget) and thus this project will
only look into the design of such systems. In some embodiments, a
plurality of sealed (or resealable) EPCM units are provided to form
a thermal energy storage system to operate at multiple temperatures
leading to higher exergy capabilities and lower entropy production.
Such multiple sealed EPCM units are particularly desirable for
partial load operations, and also for sudden changes from energy
storage mode to a discharge mode, and vice-versa.
[0165] EPCM Heat Transfer Experiment. An experiment for to
quantitatively demonstrate the feasibility of the use of EPCM for
the storage of thermal energy will be conducted. The experimental
arrangements will consist of three tanks each approximately 0.2 m
dia and 0.1 m height. All the tanks will be insulated and the
middle tank will be capable of housing different EPCM, Zinc or
MgCl2--NaCl with appropriate encapsulating material (Ni or
stainless steel, respectively). The total amount of PCM in the
middle tank will be about 1.5 Kg (.about.375 balls) as a packed bed
corresponding to around 170 kJ of heat of fusion. Including the
sensible heat to be supplied for a charge/discharge experiment that
lasts about 60 minutes.
[0166] Air mass fluxes of about 6 Kg/hr (.about.3.5 SCFM) will be
heated to 450.degree. C. in the bottom tank and used to charge the
EPCM with thermal energy. During the charge, the PCM changes into
the liquid phase thus storing thermal energy. During the discharge
process, air through the top tank will be used to remove the energy
from the EPCM. The air flow rate and temperatures of the fluid and
EPCM bed (or tubes) at several points in the experiment will be
recorded to quantify the energy flow rates and storage precisely
using a LabView DAQ system. The transient data for such bed
behavior will characterize the bed behavior and may lead to design
changes.
[0167] Scale up Heat Transfer Design. The decision process for this
proposal is very much based on the potential for the scale up of
technologies developed here for large scale implementation of
thermal energy storage--for a 100 MWe plant with storage for 24
hours. A review of the past efforts in this area reveals failures
in two arena; not being able to store energy at low entropy high
temperature (exergy) conditions and not being able to scale up to
power plant size applications due to various technical
problems.
[0168] The efficiency of the large (26 Tera J) storage system can
be discussed in the context of the percentage of energy available
during the recovery (discharge) process. The overall efficiency of
the storage system is limited by the amount and percentage of heat
losses from the outer surfaces of the storage systems to the
ambient. The storage systems under consideration will be huge to be
able to store 26 Tera J of energy (100 MWe for 24 h or more hours).
The amount (mass and volume) of Zinc or salts needed for such
storage is huge of the order of 200 million tons. The storage heat
exchangers are also going to be quite large of the order of several
30 m.times.30 m devices. Though the surface areas for the heat
losses will be large, with insulation, the surface temperatures
will not be high. The storage system at this cost enables the use
of solar energy 24 hours a day. The high temperatures (420.degree.
C.) that the proposed EPCM operates will enable the generation of
superheated steam at 420.degree. C. instead of 380.degree. C. and
that in turn will improve plant thermodynamic efficiency by 0.41%
with a corresponding decrease in the cost of electricity of
$0.0012/kWh.
[0169] Scale up Design. During this stage of the work, processes
and procedures for scale up of manufacturing EPCMs will be
developed. Some basic ideas for such designs are presented here.
This will lead to the design of full size fabrication process for
the selected material. Fabrication processes such as tube and
extruded can, powder consolidation and laser welding will be
evaluated for this application.
[0170] Future Experiments and Activities. The heat transfer into
the zinc and the stresses in the encapsulating material will be
taken into account in finalizing the EPCM design. Methods described
for the encapsulation of zinc will be used to manufacture EPCM in
the shops at Lehigh University in sufficient quantities to do heat
transfer measurements in a packed bed full of EPCM with hot air
(450.degree. C.) to charge the bed material and cooler (300.degree.
C.) air to discharge the bed energy. A one dimensional analysis of
heat transfer in a bed will be conducted to establish the operating
regimes of the bed and ensure that the experiments at Lehigh
represent actual field experience. An air flow experiment with
packed bed of EPCM pellets will be conducted to establish proof of
the concept proposed for this project. The cost analysis of the
EPCM based thermal storage system(s) performed in Phase 1 will be
updated based on the Phase 2 results and current commodity prices.
Performance of EPCM in the bed will prove/disprove the proposed
hypothesis and may require design changes to EPCM. The EPCM based
thermal storage system(s) must have potential to cost less than
$40/kWhth in order to be considered viable. The cost analysis of
the EPCM based thermal storage system(s) performed is somewhat
dependent upon then-existing material and energy capture and
generation costs and prices.
[0171] For sake of completeness, the figures attached and made part
hereof are described herein. FIG. 1 illustrates a cross-sectional
view of a cylindrical capsule in accordance herewith. FIG. 2
illustrates a schematic of a cross section of a spherical capsule
in accordance herewith. FIG. 3 illustrates a graph of temperature
distribution of an exemplary capsule in accordance herewith. FIG. 4
illustrates a graph of temperature distribution at the end of a PCM
melting process in accordance herewith. FIG. 5 illustrates a graph
of temperature variations during the charging heat transfer process
for a 100 mm diameter cylinder with sodium nitrate PCM and with air
as HTF in accordance herewith. FIG. 6 illustrates a graph of
locations of interface between solid state PCM and liquid state PCM
during melting process for a 100 mm diameter cylinder with sodium
nitrate PCM and with air as HTF in accordance herewith. FIG. 7
illustrates a graph of temperature variations during the
discharging heat transfer process for a 100 mm diameter cylinder
with sodium nitrate PCM and with air as HTF in accordance herewith.
FIG. 8 illustrates a graph of locations of interface between solid
state PCM and liquid state PCM during discharging (freezing) for a
100 mm diameter cylinder with sodium nitrate PCM and air as HTF in
accordance herewith. FIG. 9 illustrates a graph of stress-strain
relationship assumed with strain hardening in accordance herewith.
FIG. 10 illustrates stresses, strains and displacements of a 50
micron thick shell encapsulation coating in elastic deformation in
accordance herewith. FIG. 11 illustrates stresses, strains and
displacements of a 100 micron thick shell encapsulation coating in
elastic deformation in accordance herewith. FIG. 12 illustrates
stresses, strains and displacements of a 150 micron thick shell
encapsulation coating in elastic deformation in accordance
herewith. FIG. 13 illustrates stresses, strains and displacements
of a 200 micron thick shell encapsulation coating in elastic
deformation in accordance herewith. FIG. 14 illustrates stresses,
strains and displacements of a 250 micron thick shell encapsulation
coating in elastic deformation in accordance herewith. FIG. 15
illustrates elastic-plastic stresses for spherical shell with
pressure for 3 percent strain in accordance herewith. FIG. 16
illustrates elastic-plastic strain for spherical shell with
pressure in accordance herewith. FIG. 17 illustrates a phase
diagram of a Ni--Zn binary system in accordance herewith. FIG. 18
illustrates alumina silicate ceramic crucibles with test specimens
for exposure to liquid zinc at 450.degree. C. in accordance
herewith. FIG. 19 is an optical photomicrograph that illustrates
Differential Interference Contrast (DIC) of a Ni/Zn system as
polished using differential interference contrast (The Ni is in the
upper left of the picture and the Zn is in the bottom right) in
accordance herewith. FIG. 20 illustrates stresses for a
pressure-only model with a thickness of 250 micron in accordance
herewith. FIG. 21 illustrates stresses due to a triangular crack
(top), a straight dent (middle), and a thinned part of the nickel
shell (bottom) in a 250 micron thick nickel shell in accordance
herewith. FIG. 22 is an isometric view of maximum principal
stresses on crimped cylinder due to 70% initial zinc content
loading case (left), view of stresses inside of crimped cylinder
due to 70% initial zinc content loading case (right), in accordance
herewith. FIG. 23 illustrates stress distribution of the 70%
initial zinc content pressure only case (left), stress distribution
of the 85% (middle), and 86% (right) PCM (zinc) content cases with
point loads of 100 particle weights in accordance herewith. FIG. 24
illustrates stress distribution on cylinder of aspect ratio 1 for
the 70% initial PCM (zinc) content case; (left) stresses on the
outside of the cylinder, (right) stresses inside of the cylinder in
accordance herewith. FIG. 25 are phase diagrams of a binary
NaCl--MgCl.sub.2 system from two separate sources in accordance
herewith. FIG. 26 illustrates the comparison of two separate DSC
runs to determine the melting point of the 55 wt % MgCl.sub.2-45 wt
% NaCl eutectic salt. The melting point is determined to be
444.degree. C. in accordance herewith. FIG. 27 illustrates
stainless steel capsules for calorimetry tests--with and without
the NPT pipe plug in accordance herewith. FIG. 28 illustrates
carbon steel (1018) capsule for use with the eutectic salt in
accordance herewith. FIG. 29 illustrates sections of a
MgCl.sub.2--NaCl eutectic EPCM encapsulated with Stainless
Steel-304 in accordance herewith. FIG. 30 is a schematic of a
precision calorimeter designed and built for use in the present
work--all units are inches in accordance herewith. FIG. 31
illustrates a graph of temperature profiles of sample, silicon oil,
and air in accordance herewith. FIG. 32 illustrates the geometry of
an exemplary capsule in accordance herewith. FIG. 33 illustrates a
graph of the cost of storage units ($/kWh) as a function of the
diameter of the cylindrical shaped capsule in accordance herewith.
FIG. 34 illustrates a graph of the cost of storage unit ($/kWh) as
a function of length of the capsule in accordance herewith.
[0172] While this description is made with reference to exemplary
embodiments, it will be understood by those skilled in the art that
various changes may be made and equivalents may be substituted for
elements thereof without departing from the scope. In addition,
many modifications may be made to adapt a particular situation or
material to the teachings hereof without departing from the
essential scope. Also, in the description there have been disclosed
exemplary embodiments and, although specific terms may have been
employed, they are unless otherwise stated used in a generic and
descriptive sense only and not for purposes of limitation, the
scope of the claims therefore not being so limited. Moreover, one
skilled in the art will appreciate that certain steps of the
methods discussed herein may be sequenced in alternative order or
steps may be combined. Therefore, it is intended that the appended
claims not be limited to the particular embodiment disclosed
herein.
* * * * *