U.S. patent application number 12/897685 was filed with the patent office on 2012-04-05 for thermal storage system.
Invention is credited to Edmund Joseph KELLY.
Application Number | 20120080161 12/897685 |
Document ID | / |
Family ID | 45888782 |
Filed Date | 2012-04-05 |
United States Patent
Application |
20120080161 |
Kind Code |
A1 |
KELLY; Edmund Joseph |
April 5, 2012 |
Thermal storage system
Abstract
Apparatus and method for efficiently storing thermal energy, in
particular thermal energy transferred from a concentrated solar
energy receiver. The apparatus consists of a collection of modular
thin walled tube pressure vessels enclosing solid sensible heat
thermal storage elements. The high temperature solid is separated
from the thin wall by thermal insulation and two thin gaps that
carry the cold in and cold out high-pressure working fluids. Hot in
and hot out working fluids circulate in counter flow channels
within the solid sensible heat thermal storage elements. The solar
receiver can also be modular to match the thermal storage elements,
facilitating manufacturing and on site assembly. The design enables
scalable thermal storage and concurrent thermal charging and
discharging with no circuit switching between charge and
discharge.
Inventors: |
KELLY; Edmund Joseph; (San
Jose, CA) |
Family ID: |
45888782 |
Appl. No.: |
12/897685 |
Filed: |
October 4, 2010 |
Current U.S.
Class: |
165/10 |
Current CPC
Class: |
F28D 20/00 20130101;
F24S 23/12 20180501; Y02E 10/47 20130101; F24S 23/71 20180501; Y02E
70/30 20130101; Y02P 80/20 20151101; Y02E 60/14 20130101; F24S
60/00 20180501; F24S 20/20 20180501; G02B 6/0008 20130101; Y02E
10/40 20130101; G02B 6/0006 20130101; F03G 6/067 20130101; F24S
25/13 20180501; F24S 25/617 20180501; Y02E 10/46 20130101 |
Class at
Publication: |
165/10 |
International
Class: |
F28D 20/00 20060101
F28D020/00 |
Claims
1. A method of efficiently storing thermal energy using sensible
heat storage in solid material comprising: a) providing multiple
thin walled pressure vessels containing pressurized working fluids,
b) providing solid thermal storage material contained within said
pressure vessels, c) providing thermal transfer channels within
said solid thermal storage material, d) providing means to couple
said thermal storage material to a thermal charging source, e)
providing means to couple said thermal storage material to a
thermal discharging sink, f) transmitting the concentrated thermal
energy from the thermal source, to said solid thermal storage
material using a working fluid circuit, g) transmitting the
concentrated thermal energy from the solid thermal storage
material, to said thermal sink using a second working fluid
circuit, whereby concentrated thermal energy can be modularly
stored and some used and some stored for later use.
2. The method of claim 1, wherein: The thin walled pressure vessels
are made of steel.
3. The method of claim 1, wherein: The solid thermal storage
material is graphite.
4. The method of claim 1, wherein: The working fluids are
pressurized helium gas.
5. The method of claim 1, wherein: The working fluids are
pressurized argon gas.
6. The method of claim 1, wherein: the thermal charging source is a
solar receiver.
7. The method of claim 1, wherein: the thermal charging sink is a
power block.
8. The method of claim 1, wherein: the thermal charging sink uses
process heat.
9. The method of claim 1, wherein: the thermal charging source is
multiple modular solar receivers, each charging one or more
pressure vessel thermal storage modules.
10. The method of claim 1, further comprising: a) providing thermal
insulation between the thermal storage material and the pressure
vessel wall, b) providing cooling channels between the thermal
insulation and the pressure vessel wall, c) coupling the cold in
and cold out working gas connections to said cooling channels,
whereby the pressure vessel wall is cooled by the working
fluids.
11. A thermal storage apparatus for efficiently storing thermal
energy using sensible heat storage in solid material comprising: a)
multiple thin walled pressure vessels containing pressurized
working fluids, b) solid thermal storage material contained within
said pressure vessels, c) thermal transfer channels within said
solid thermal storage material, d) means to couple said thermal
storage material via said heat transfer channels to a thermal
charging source, e) means to couple said thermal storage via said
heat transfer channels to a thermal discharging sink, whereby
concentrated thermal energy can be modularly stored and some used
and some stored for later use.
12. The apparatus of claim 11, wherein: The thin walled pressure
vessels are made of steel.
13. The apparatus of claim 11, wherein: The solid thermal storage
material is graphite.
14. The apparatus of claim 11, wherein: The working fluids are
pressurized helium gas.
15. The apparatus of claim 11, wherein: The working fluids are
pressurized argon gas.
16. The apparatus of claim 11, wherein: the thermal charging source
is a solar receiver.
17. The apparatus of claim 11, wherein: the thermal charging sink
is a power block.
18. The apparatus of claim 11, wherein: the thermal charging sink
uses process heat.
19. The apparatus of claim 11, wherein: the thermal charging source
is modular solar receivers, each charging one or more pressure
vessel thermal storage modules.
20. The apparatus of claim 11, further comprising a) thermal
insulation between the thermal storage material and the pressure
vessel wall, b) cooling channels between the thermal insulation and
the pressure vessel wall, c) cold in and cold out working gas
connections to said cooling channels, whereby the pressure vessel
wall is cooled by the working fluids.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] Non-provisional application Ser. No. 12/430,869, filed on
Apr. 27, 2009. Non-provisional application Ser. No. 12/488,852,
filed on Jun. 22, 2009. Non-provisional application Ser. No.
12/579,849, filed on Oct. 15, 2009.
FEDERALLY SPONSORED RESEARCH
[0002] Not Applicable
SEQUENCE LISTING OR PROGRAM
[0003] Not Applicable
BACKGROUND
[0004] 1. Field
[0005] This invention generally relates to thermal storage systems,
and more particularly to high temperature, high efficiency,
sensible heat thermal storage systems.
[0006] 2. Prior Art
[0007] Concentrating Solar energy systems use optical components
such as lenses and mirrors to collect and concentrate the sun's
radiation and then absorb it for practical use. The main practical
use is to provide high temperature working fluids to drive heat
engines that in turn drive electricity generators. Other uses for
concentrated sunlight include high intensity photovoltaic
electricity generation, direct high temperature "clean" process
heat, and indirect high temperature process heat.
[0008] A wide variety of designs have been developed to accomplish
these goals. The following references provide a good overview of
this technology. "Solar Engineering of Thermal Processes" by John A
Duffle and William A Beckman, chapter seven and "Solar Energy" by
G. N. Tiwari, chapter eight.
[0009] All current concentrating solar energy designs include two
major elements: [0010] a) optical concentrators that accept and
concentrate the incoming solar radiation [0011] b) receivers that
absorb the solar energy and heat a working fluid.
[0012] Concentrators can use some of the following structural
arrangements: [0013] a) single lenses with attached receivers as
one moveable structure, [0014] b) rigid arrays of lenses or arrays
of lens segments with an attached receiver, all on a common
moveable structure, [0015] c) arrays of independently moving lenses
on a stationary base (like the ground) with a central stationary
receiver, such as heliostat arrays.
[0016] The lenses can use imaging optical elements and non-imaging
optical elements. The lenses can use reflective optical elements
and/or refractive optical elements. Regardless of their
construction, concentrators are characterized by their entrance
aperture and their exit aperture. In the case of multiple lens
arrays or segments, the input aperture is the sum of the apertures
of the elements of the array. The ratio of the area of the input
aperture divided by the area of the exit aperture is the
concentration ratio.
[0017] Receivers absorb the concentrated radiation from the
concentrator and transfer this absorbed energy to a working fluid.
This hot fluid is then either used to directly power a heat engine
(such as a steam turbine), or is used to transfer heat via a heat
exchanger to a second working fluid which is then used to power a
heat engine. The heat engine then drives a generator which produces
electricity. Some systems first transfer heat from the working
fluid to thermal storage, and then from thermal storage to a second
working fluid in order to decouple when electrical energy is
generated from when solar energy input is captured by a
receiver.
[0018] Some hypothetical space based systems have been proposed
that generate electricity via these processes in space and then
convert the electrical energy to microwave energy to be beamed to
the earth's surface and collected via large microwave antennae
arrays.
[0019] Another unrelated area of prior art is light pipes. Glass
and plastic versions of these have long been used in the area of
telecommunications to transmit low power light signals over long
distances. Light pipes using hollow tubes with various highly
reflective inner surfaces are used to guide sunlight or artificial
light over short distances for lighting purposes within buildings.
A particularly efficient method used for lighting is described in
U.S. Pat. No. 4,260,220, "Prism light guide having surfaces which
are in octature" issued to Lorne A. Whitehead on Apr. 7, 1981.
Another method used for lighting is described in U.S. Pat. No.
4,895,420, "High reflectance Light Guide" issued to John F.
Waymouth on Jan. 23, 1990.
[0020] Light pipes are characterized by their aperture, acceptance
angle, and attenuation. Light pipes accept light travelling in the
direction of the light pipe within their acceptance angle.
Generally light from point or small area concentrated light sources
needs to pass through a collimator in order to satisfy the
acceptance angle criteria and reduce attenuation. Collimators are
common optical elements and are effectively the reverse of optical
concentrators, with a smaller entrance aperture than larger exit
aperture. As well as conditioning light for light pipes or guides,
optical collimators are used for a variety of purposes. These
include projector condensor lenses, parabolic reflector light
bulbs, and telescope objective lenses.
[0021] Current Concentrating Solar systems suffer from several
problems that have limited their success. Their high capital costs
make the cost of the energy they produce uncompetitive without
subsidy. They also have high ancillary costs to compensate for the
unpredictability of their energy output and the long transmission
distance from the system to the average power user.
[0022] Concentrating Solar systems make use of direct sunlight,
i.e. light directly from the sun that is not scattered or absorbed
in earth's atmosphere. Current systems are severely negatively
affected by effects of weather such as rain, clouds, moisture and
dust in the atmosphere. This restricts their geographical location
to hot dry desert areas which are relatively scarce and far from
consumers of electricity. In addition, even in deserts, bad weather
sometimes restricts electric power output availability,
necessitating the provision of alternate sources of supply.
[0023] Solar concentrators need to have large entry apertures to
produce meaningful amounts of power. Utility scale systems have
apertures measured in millions of square meters. Current systems
consequently consume large areas of land and significant quantities
of construction materials like glass and steel needed to fabricate
this large aperture collector. Also weather in the form of dust,
wind, rain, hail frost and snow require that structures be strong
and durable which adds significantly to their cost.
[0024] Current large scale systems use large arrays of individually
steered collecting elements. Robust motors, gears, electrical
equipment etc are needed for each collector element, contributing
significantly to overall cost.
[0025] The cost problem is compounded by the generally low overall
energy conversion efficiency of current systems, which
consequentially requires a larger surface area and more material to
produce a given power output compared to higher conversion
efficiency systems.
[0026] Thermal storage is a necessary part of a solution to deliver
electricity 24/7. The current Research & Development focus in
thermal storage for concentrating solar power (CSP) plants uses
nitrate and fluoride salt mixtures to both store and transfer heat.
However, the molten salts have several serious shortcomings. They
have a relatively low range of temperatures where they are liquid,
and they solidify at room temperature. Considerable engineering is
required to keep them molten under all circumstances. Corrosion and
chemical reactions from impurities cloud the long-term reliability
picture.
SUMMARY
[0027] The present invention is realized by suspending a solar
energy concentrator at a high altitude in the earth's atmosphere,
above clouds, moisture, dust, and wind. This is accomplished using
a light-weight, rigid, buoyant, structure. The concentrated solar
energy output from the concentrator is then (optionally) collimated
with a collimator and coupled to a light pipe. The solar energy is
then transmitted through the light pipe to the earth's surface
where it is (optionally) further concentrated in order to better
achieve high temperatures. It is then coupled to a receiver which
heats a working fluid, which in turn transfers heat to a thermal
storage system for use in generating electricity and/or as process
heat. There are many uses for this process heat. Examples include
thermal desalination, thermally augmented hydrogen or methanol
generation, along with a myriad of conventional chemical
processes.
[0028] These and other objects and features of the invention will
be better understood by reference to the detailed description which
follows taken together with the drawings in which like elements are
referred to by like designations throughout the several views.
DRAWINGS
Figures
[0029] In the drawings, closely related figures have the same
number but different alphabetic suffixes.
[0030] FIG. 1A is a perspective view of an initial position of a
first embodiment of the invention.
[0031] FIG. 1B is a perspective view of a second position of the
embodiment shown in FIG. 1A.
[0032] FIG. 2A is a perspective view of the ground based final
stage optical concentrator, cavity absorber, and thermal storage
unit of a first embodiment.
[0033] FIG. 2B is a vertical cross section of FIG. 2A.
[0034] FIG. 3A is a perspective view of the Optical Concentrator
and collimator assembly portion of a first embodiment.
[0035] FIG. 3B is a close up perspective view of part of FIG.
3A
[0036] FIG. 4A is a perspective view of the structure of the mirror
surface of the Optical Concentrator of a first embodiment.
[0037] FIG. 4B is a perspective view of a hexagonal mirror segment
of a first embodiment.
[0038] FIG. 4C is a perspective view of the back of the structure
of the mirror surface of the Optical Concentrator shown in FIG.
4A.
[0039] FIG. 5A is a perspective front view of a section of the
structure of the mirror surface of a concentrator embodiment using
circular mirror segments in a Fresnel fashion.
[0040] FIG. 5B is a perspective side view of a section of the
structure of the mirror surface of a concentrator embodiment using
circular mirror segments in a Fresnel fashion showing the circular
mirror tilt.
[0041] FIG. 5C is a perspective view of the Optical Concentrator
and collimator assembly portion of an embodiment using circular
mirror segments in a Fresnel fashion.
[0042] FIG. 6 is a schematic of the optical elements of a
concentrating solar energy system according to one embodiment.
[0043] FIG. 7A is a schematic view of the thermal elements of a
system for generating electrical power from solar energy according
to one embodiment.
[0044] FIG. 7B is a schematic view of the thermal elements of a
system for generating electrical power from solar energy according
to one embodiment.
[0045] FIG. 8 is a schematic view of the optical and thermal
elements of an embodiment that directly couples a receiver to a
concentrator.
[0046] FIG. 9 is a cross section view of a portion of a light pipe
wall of one embodiment.
[0047] FIG. 10 is a schematic view of a desalination system
according to one embodiment.
[0048] FIG. 11 is a schematic view of a combined electricity and
desalination system according to one embodiment.
[0049] FIG. 12 is a perspective view of an embodiment of a
concentrator in the form of a heliostat array attached to a static
structure.
[0050] FIG. 13 is a perspective view of an embodiment of a receiver
and thermal storage system connected to a power block and a cooling
block.
[0051] FIGS. 14A and 14B are perspective views of an embodiment of
a modular receiver and thermal storage element.
REFERENCE NUMERALS
TABLE-US-00001 [0052] 20 light pipe 21 light pipe segment 22
foundation anchor ring 23 buoyancy section 24 foundation anchor leg
25 cable stays 26 second concentrator 27 positioning beam 28 cavity
absorber cover 30 thermal storage unit 32 cavity absorber surface
34 transparent membrane 36 concentrator mirror 38 collimator mirror
40 connecting beam 42 mirror shading line 44 alt-azimuth mount 46
alt pivot 48 truss structure strut 50 truss structure joint 52
mirror segment frame 54 mirror segment surface 56 mirror to truss
attachment 60 light pipe wall 62 reflective layer 64 refractive
layer 66 reinforcement cables 80 solar receiver 82 high voltage
transmission line 84 heat engine 86 electricity generator 88
ambient heat exchanger 90 compressor 92 regenerator heat exchanger
94 boiler heat exchanger 96 steam turbine 98 second electricity
generator 100 condensor 102 water pump 104 concentrator input
aperture 106 concentrator output aperture 108 collimator input
aperture 110 collimator output aperture 112 light pipe input
aperture 114 light pipe output aperture 116 second concentrator
input aperture 118 second concentrator exit aperture 120 receiver
input aperture 122 salt water inlet 124 concentrated brine output
126 fresh water output 128 thermal desalination unit 130 thermal
storage element 132 power block 134 cooling block 136 receiver
element 138 cold fluid out pipe 140 hot fluid in pipe 142 cold
fluid in pipe 144 hot fluid out pipe 146 pressure vessel wall 148
thermal insulator 150 top manifold 152 thermal storage block 154
bottom manifold 156 hot out plenum 158 heat transfer channel 160
cold in plenum 162 hot in plenum 164 cold out plenum
DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS
[0053] The description that follows is divided into separate
sections with unique headings to help clarify the exposition. The
system is first described with respect to schematic diagrams for
the optical system and the solar energy conversion system, and then
with more detailed perspective drawings, mostly for optical system
elements of various embodiments.
Detailed Description--FIG. 6 Optical System
[0054] FIG. 6 is a schematic view of the optical elements of a
solar concentrator energy system. It consists of a solar
concentrator 36 with input aperture 104 and output aperture 106.
Solar concentrator 36 is a rigid buoyant structure suspended high
in the earth's atmosphere. Input aperture 104 is pointed
continuously at the sun. Output aperture 106 is a physical aperture
for some non imaging concentrators, but is the focal plane image of
the sun for imaging concentrators using reflective or refractive
lenses or arrays of lens segments. These lenses will mostly be
formed from multiple segments attached to a rigid structure, and
can be Fresnel or Fresnel like in construction.
[0055] The output aperture 106 of concentrator 36 is directly
coupled to the input aperture 108 of collimator 38. The output
aperture 110 of collimator 38 is directly coupled to the input
aperture 112 of light pipe 20. Collimator 38 can be an imaging,
reflective or refractive lens or a non imaging compound parabolic
collimator or similar. Compound parabolic non imaging optical
elements are well suited to this application as they are easy to
couple to light pipes and can be constructed using the same
materials and processes.
[0056] The size of output aperture 106 is approximately two times
the product of the focal length of concentrator 36 and the 4.653
mrad half angle of the Suns radius seen from earth. Collimator 38
is necessary when the focal length of concentrator 36 is short,
which may be driven by structural goals. Embodiments that either
desire a high concentration ratio, or can accommodate the
structural requirements of a longer focal length for concentrator
36 can eliminate the need for collimator 38. Means to couple light
from concentrator 36 to light pipe input aperture 112 can therefore
be a collimator or an appropriate focal length for concentrator 36
without a collimator.
[0057] Light pipe 20 is a flexible, buoyant, thin walled tube
structure that stretches from the earth's surface to the
concentrator 36 and collimator 38 assemblies high in the earth's
atmosphere. Its length is typically in the region of 20 km, and its
length to width aspect ratio is in the region of 100 for
concentration ratios in the region of 200. Its thin structural skin
is airtight, maintains a circular cross section through an internal
gauge pressure, and is stabilized against high wind and gravity
forces. It excludes weather effects like rain, snow, moisture and
dust from the transparent gas in the pipe interior. The primary
stabilizing force that counteracts wind forces and gravity is
buoyancy from hydrogen or helium within high altitude sections of
the pipe. Stability is augmented with stiffness in the structural
wall, and cable stays when appropriate. Its interior wall surface
is highly reflective and provides a low loss transmission path for
the concentrated collimated solar energy input through aperture
112.
[0058] Second stage concentrator 26 has input aperture 116,
directly coupled to light pipe 20 output aperture 114. Concentrator
26 can be an imaging reflective or refractive lens or a non imaging
compound parabolic concentrator or similar. Compound parabolic non
imaging optical elements are well suited to this application as
they are easy to couple to light pipes and can be constructed using
the same materials and processes.
Detailed Description--FIG. 7 Solar Energy Conversion System
[0059] FIG. 7A is a schematic view of one embodiment of a solar
energy conversion system that receives the solar energy delivered
by concentrator 26 shown in perspective drawings FIG. 2A and FIG.
2B, and schematic drawing FIG. 6. It consists of receiver 80 with
input aperture 120 coupled to the exit aperture 118 of second
concentrator 26. Other elements are thermal store 30, heat engine
84, and electricity generator 86. Heat engine 84 might typically be
a Brayton cycle gas turbine but other types of expansion engines,
including Stirling cycle heat engines are possible.
[0060] The thermal store 30 decouples the arrival of solar energy
from the use of electrical energy, and allows for 24 hour delivery
of electricity despite the much shorter duration of daylight. One
embodiment of thermal store 30 uses large graphite blocks with
integral cooling channels. Graphite has long been used in this form
in a variety of nuclear reactor cores. In that application
graphite's advantages as a thermal storage medium were secondary to
its benefits as a neutron moderator. Graphite has good thermal
capacity, and can tolerate higher temperatures than almost any
material. It maintains structural integrity at high temperature and
can withstand severe thermal cycling. It is abundant and relatively
inexpensive. In this embodiment the cooling channels within the
graphite are depicted as two separate heat transfer circuits, one
transferring heat from receiver 80 to the graphite store 30 and the
second independently and concurrently transferring heat from
thermal store 30 to heat engine 84. These two circuits are
configured as a counter flow heat exchanger. Other arrangements
using more piping and valves would operate the thermal store with
one thermal circuit that alternately charges the thermal store with
heat energy while also running heat engine 84, and subsequently
discharges the thermal store, switching the working fluid output to
run heat engine 84.
[0061] Other elements of this electricity generation system are
ambient cooler heat exchanger 88, compressor 90 and regenerator
heat exchanger 92. A common working fluid for high temperature
Brayton cycles is helium at high pressure and temperature.
[0062] FIG. 7B is a schematic of a second embodiment of a solar
energy conversion system that absorbs the solar energy delivered by
concentrator 26 from exit aperture 118 shown in FIG. 6 and converts
it into electricity. This embodiment is a combined cycle
electricity generation system which uses both a Brayton gas turbine
cycle, and a Clausius-Rankine water/steam cycle. This embodiment is
more efficient at energy conversion but adds additional cost for
the steam cycle elements. The additional elements are boiler heat
exchanger 94 that is heated with the exhaust gas from heat engine
84, steam turbine 96, second electricity generator 98, condensor
heat exchanger 100, and water pump 102. Ambient heat exchanger 88
is probably not needed or can be much simpler for this combined
cycle system.
[0063] A particular advantage of the use of a combined cycle system
with a high temperature concentrating solar system is the improved
thermal capacity of sensible heat storage system 82. The thermal
storage capacity of thermal store 82 is dependent on the average
temperature difference between the high temperature gas delivered
to heat engine 84 from high temperature storage and the low
temperature gas returned to thermal store 82. For the Brayton cycle
with regeneration shown in FIG. 6A this temperature delta is
approximately 500 degrees Celsius. For the combined cycle of FIG.
6B this temperature delta approaches 1000 degrees Celsius. This
approximately doubles the thermal capacity of thermal store 82,
which effectively halves the cost of thermal storage. This comes at
the cost of using a thermal fluid that accommodates this large
temperature range, which in one embodiment might entail the use of
high pressure helium gas as opposed to low pressure liquid fluoride
salts.
[0064] FIG. 8 is a schematic of a solar concentrator and solar
energy conversion system where all solar energy conversion system
elements are suspended in the stratosphere with the concentrator
and electrical power is transmitted to the earth's surface using
buoyant high voltage transmission lines. As with the other
embodiments, light is concentrated by buoyant concentrator 36. In
this embodiment the output aperture 106 of concentrator 36 is
directly coupled to input aperture 120 of receiver 80. The other
elements of this solar energy conversion system are similar to
those of the embodiment shown in FIG. 7A, with the removal of
thermal store 30, and the addition of buoyant high voltage
transmission lines 82. Heat engine 84 could be a Stirling engine in
this application.
[0065] Another embodiment not shown in FIG. 8, but similar in
function, employs an array of photovoltaic solar energy convertors
electrically connected in series to convert the concentrated solar
energy from aperture 106 of concentrator 36 directly into high
voltage electricity. The high voltage electrical power is
transmitted to the earth's surface using buoyant high voltage
transmission lines.
[0066] Due to its large mass, it would be difficult to suspend a
thermal store in the stratosphere with these embodiments. These
embodiments would need a ground based energy storage system to
provide continuous electrical power.
Detailed Description--FIGS. 1, 2 Ground Elements
[0067] FIGS. 1A and 1B are perspective views of two positions of a
solar concentrator energy system of a first embodiment. It consists
of the following:
1) A large, buoyant, segmented, reflecting, parabolic, mirror
concentrator, and collimator assembly detailed in FIG. 3B. 2) A
flexible hollow buoyant light pipe 20. 3) A ground based
foundation, anchor, optical concentrator and receiver assembly
detailed in FIG. 2A.
[0068] FIG. 2A and FIG. 2B show the ground structures of a first
embodiment in more detail. Light pipe 20 is attached to anchor ring
22 which is supported by foundation legs 24. Light pipe 20 is a
hollow buoyant tube which exerts a considerable vertical upward
force on this anchor structure. Transparent membrane 34 is
fabricated from polyethylene terephthalate (PET) or other similar
transparent film and contains the pressurized gas within the light
pipe 20. As is typical for inflated structures the gauge pressure
is quite low, only a small fraction of an atmosphere. The section
of the light pipe at this bottom end is filled with dry air.
Buoyancy gas such as hydrogen and Helium are contained within the
top section of light pipe 20.
[0069] Light pipe 20 connects to the entry aperture on the top of
Compound Parabolic Concentrator (CPC) 26 shown in cross section in
FIG. 2B. The reflective surface of the upper portion of CPC 26 can
be constructed from the same reflective materials as the inner
surface of light pipe 20. The bottom portion of CPC 26 is
fabricated from heat resistant material with a highly reflective
inner surface and has provision for air or water cooling. One
embodiment of the exit portion of a CPC is fabricated using carbon
composite replica mirror panels. CPC 26 is a small part of the
overall system and easily replaced. This allows the concentration
factor of the whole system to be adjusted by using different
versions of CPC 26. This is flexibility not present in prior art
solar concentrating systems where the concentration factor is a
fixed feature of the whole system.
[0070] In one embodiment a Cavity absorber receiver assembly is
attached to the bottom of CPC 26. This is shown as receiver 80 in
FIG. 7. The embodiment shown in FIG. 2 blends the receiver and
thermal storage functions, and so the receiver is not explicitly
diagrammed. The cavity absorber of the receiver assembly consists
of thermal insulating top cover 28 and absorber surface 32. Top
cover 28 has receiver input aperture 120 which admits the light
from the exit aperture of CPC 26 into the cavity absorber. The
inner surface of lid 28 is highly reflective, and constructed from
high temperature resisting ceramic material such as fire brick.
[0071] In one embodiment, absorber surface 32 is coupled via a heat
transfer working fluid circuit to thermal storage graphite blocks
that form thermal store 30. Graphite is particularly suited to high
temperature sensible heat thermal storage, as its melting point is
extremely high (3652-3697 degrees Celsius) and it maintains its
structural integrity close to its melting point. Large graphite
blocks with integral cooling channels have long been used in the
core of a variety of nuclear reactors. Integrating cooling channels
and heat exchangers into the graphite storage blocks eliminates
large amounts of high temperature piping and enables higher
temperature operation than is feasible with current high
temperature piping materials. The high temperature working fluid
can be a low-pressure liquid such as liquid fluoride salts, or a
high-pressure inert gas such as helium or argon.
[0072] As an example to illustrate the scale, 50,000 cubic meters
of solid graphite using a thermal delta of 500 degrees Celsius can
store over 8 million kilo-Watt hours of thermal energy
(28,800,000,000 kJ), and provides sufficient thermal storage
capacity to average out the day to night fluctuation in input solar
energy for the system described in the first embodiment at mid
latitudes. This provides a 24 hour continuous source of thermal
energy with considerable flexibility in when during the 24 hour
period the stored thermal energy can be withdrawn for use in
generating electricity.
[0073] The thermal storage capacity required varies with latitude,
and the electrical demand curve it is desired to satisfy. The most
thermal storage is needed for systems at high latitude. Continuous
constant electrical power requires more storage than a system with
more electrical output during daylight hours. While thermal storage
is a technology that helps make solar energy a practical
alternative for all electric power generation, it is also a
considerable expense, perhaps half the total system cost, and
perhaps the single biggest cost element.
[0074] In one embodiment, absorber surface 32 transfers heat to
pressurized steam which is used to directly drive Rankine cycle
steam turbines. This embodiment is particularly useful when built
beside an existing fossil fueled power plant and uses the existing
turbines, generators, cooling, and distribution facilities etc.
This embodiment acts as a supplement to the existing power plant,
reducing its operation to non daylight hours and thus reducing fuel
consumption and carbon generation by large percentages. This is
potentially a very economic alternative as it re-uses existing
infrastructure and avoids the costs of a thermal storage unit.
[0075] It should be noted that FIG. 2 illustrates the only part of
the system that consumes any land on the earth's surface. The land
area needed to house elements found at conventionally fuelled power
plants, such as turbines, generators, heat exchangers and
electricity distribution equipment which are not shown in FIG. 2
easily fits in the area within the foundation and anchor
structure.
Detailed Description--FIGS. 3, 4, 5 High Altitude Elements
[0076] FIG. 3A and FIG. 3B are perspective views of the solar
concentrator and collimator assembly shown as part of the entire
system in FIG. 1. Offset parabolic mirror concentrator 36 is
rigidly attached to offset parabolic mirror collimator 38 with beam
structure 40. The assembly is neutrally buoyant. Curved shading
lines 42 illustrate the offset parabolic curved surface of mirror
36. Beam 40 serves two main purposes. It accurately positions
mirrors 36 and 38 at a common optical focus, and it acts as a
structural support for light pipe segment 21, enabling its entry
aperture to be positioned very close to mirror 38. As the
concentrated light reflected off mirror 38 has a wide divergence
angle, positioning the light pipe entry close to mirror 38 ensures
the capture of most of the collected solar energy.
[0077] In one embodiment, mirror 36 is approximately 2.3 km in
diameter. The mirror 38 is approximately 180 m in diameter. The
light tube 20 is also approximately 180 m in diameter and
approximately 20 km long. These dimensions are in the range
appropriate for a one Giga Watt (GW), continuous output electricity
generation system. The mirrors 36 and 38 are rigid, lightweight,
buoyant, gas filled, structures. The light tube 20 is a flexible,
buoyant, gas filled, pressurized, structure. The buoyancy gas is
either helium or hydrogen. In one embodiment the light tube stores
the buoyancy gas within a portion of the light tube to avoid the
need for additional buoyancy elements attached external to the
light tube.
[0078] Alt-azimuth mount 44 is used to enable the solar receiver
assembly to accurately track the sun while compensating for wind
forces on the concentrator and collimator assembly, and motion in
the base of the mount 44 attached to the top of light pipe 20.
Altitude and azimuth motors within mount 44 are controlled by a
closed loop feedback sun tracking system. The structure consisting
of concentrator 36, collimator 38 and beam 40 pivots in altitude
around altitude pivot 46. The structure pivots in azimuth around
the axis of alt azimuth base attached to the top of light pipe
20.
[0079] In addition to, or as a replacement for motors in the alt
azimuth mount, fan thrusters on the solar concentrator structure
can provide the forces that move the concentrator assembly in order
to track the sun.
[0080] Light pipe segment 21 is flexible and is slightly narrower
in diameter than light pipe 20. The end of light pipe segment 21
attached to the mount 44 telescopes within light pipe 20. This
allows it to lengthen as the angle of altitude is increased and the
amount of bend is reduced, and also to rotate freely within light
pipe 20 as the angle of azimuth is adjusted.
[0081] In a first embodiment, parabolic concentrator 36 is designed
to have 200 times the aperture of parabolic collimator 38 in order
to achieve a concentration factor of 200. The concentration factor
used in the first embodiment is an engineering design choice
constrained by several factors, primarily the reflection efficiency
and acceptance angle of the material on the inner surface of the
light tube 20, and the light tube aspect ratio, and is not a fixed
feature of the invention. As the concentration factor is increased
the dispersion angle of the concentrated light beam also increases.
This results in more light absorption in the light tube as more
reflections occur in the path down the tube to the earth's
surface.
[0082] The other variable that affects the transmission
effectiveness of the light pipe is the length to width ratio, or
aspect ratio, of the light pipe. The larger this number is, the
higher the energy losses are, as more reflections occur on average
as light traverses the light pipe. Numbers for this aspect ratio
are usually in the region of 100 for light pipes with efficient
reflective surfaces.
[0083] So in summary, a combination of the optical design, the
concentration factor, the light pipe wall reflectivity, and the
light pipe aspect ratio, all influence the overall transmission
efficiency of the light pipe.
[0084] Very high reflectivity light pipe wall material with
reflective efficiency exceeding 99.9% enables embodiments that do
not need collimator 38. The acceptance angle of the light pipe then
sets the limits for concentrator 36.
[0085] FIG. 4A and FIG. 4C show one method for constructing
concentrator mirror 36 and collimator mirror 38. FIG. 4A shows a
small section of the surface structure of mirror 36, the rest of
which is identical in form. It consist of a rigid tetrahedral truss
made from struts 48 and connecting nodes 50, which support
hexagonal mirror segments shown in FIG. 4B. FIG. 4C shows a back
view of FIG. 4A and more clearly illustrates the attachment of
mirror segments 4B to the truss at three surface nodes 56. As can
be seen from FIG. 4A and FIG. 4C there are nine struts 48 and two
nodes 56 for each hexagonal mirror segment.
[0086] The struts 48 are lightweight hollow tubes. Nodes 50 in one
embodiment are spherical and have connectors to attach struts 48.
Struts 48 and nodes 50 are standardized elements and mass
producible at low cost. The parabolic curvature of the truss is
created by slightly increasing the length of the inner layer of
struts. The degree of lengthening required is small and is
accomplished in one embodiment with spacers. Another embodiment
uses linear actuators to adjust strut lengths and more accurately
control the structure's shape. This helps compensate for factors
that affect the structure's accuracy, such as wind load and thermal
expansion and contraction.
[0087] Mirrors 36 and 38 are shown in FIG. 3 as approximately 150
meter thick structures. The structural elements shown in FIG. 4A
are used to fashion the surface skins of mirrors 36 and 38. The
same structural elements without the mirror segments fashion the
back skin of mirrors 36 and 38. Clearly the struts and nodes used
to fashion the structures for mirror 36 are longer than those used
to fashion the structures for mirror 38.
[0088] The back and front skins of mirrors 36 and 38 are connected
via long struts arranged in the same tetrahedral fashion used to
form the skin layers. These long struts are fashioned from the same
small truss elements consisting of struts 48 and nodes 50. Thus the
overall structure of mirrors 36 and 38 can be considered as a
"sandwich" of two stiff thin skins separated by a substantially
hollow core with interior tetrahedral and octahedral spaces formed
within a tetrahedral truss. This is the same structure used to
fashion the mirror skins repeated at a larger scale.
[0089] The large interior octahedral and tetrahedral spaces within
the tetrahedral truss are filled with like shaped gas bags
fashioned from thin impermeable film material such as PET film, and
filled with buoyancy gas such as helium or hydrogen maintained at
ambient pressure and temperature. For mirror 36, these gasbags will
have edge lengths measured in hundreds of meters. The edges are
attached to the nodes on the interior struts and the surface skin
and provide the buoyancy that supports the overall structure.
[0090] In one embodiment, the mirror segments 4B are made from an
air tight, lightweight, reflective, pressure tensioned membrane 54
attached to a light rigid airtight frame 52. In one embodiment the
membrane is fashioned from aluminized PET film. This relatively
fragile material is commonly used in space based inflated
structures, and it and similar thin film materials can be used in
this application because of the benign "space like" weather free
environment in the stratosphere.
[0091] The curvature of the mirror 4B surface membrane is
controlled by providing reduced air pressure within the frame,
thereby stretching the membrane into an approximate spherical
curvature. This can be accomplished to a high degree of accuracy
because the degree of curvature required is very small. A common
measure of the degree of curvature of an optical element is its
focal length to aperture ratio, (f/D). For example a 20 meter
aperture mirror segment with a focal length of one kilometer has an
f/D of 500, which is very large. At such large ratios, spherical
surfaces are practically indistinguishable from parabolic surfaces,
and are easily formed to high accuracy.
[0092] An optical feedback system within the mirror segment
measures the mirror depth of curvature on the back of the mirror
membrane via an accurate distance measuring device, and adjusts the
internal air pressure via a feedback control system to establish
and maintain the desired depth of curvature for the mirror segment
4B. In this way a common mirror segment can be manufactured, and
then adjusted during operation to a particular curvature depending
on its position within the larger mirror assembly. The feedback
control system also compensates for manufacturing variability in
film thickness etc. It can maintain the desired curvature despite
varying conditions such as atmospheric temperature and pressure,
segment wall and membrane surface expansion and contraction under
heating and cooling cycles, and gas leakage through the surface
membrane.
[0093] In another embodiment mirror segment 4B is fabricated using
very thin carbon composite replica mirror technology that has been
developed for large space based optical telescopes. This requires
many unique fabrication molds, but does not need active control of
the membrane and is more durable. It is a suitably lightweight
fabrication technology.
[0094] Each mirror segments 4B is attached to the tetrahedral truss
at three surface connecting nodes 56 using linear actuators. Each
surface node thus acts as an attachment point for three different
mirrors and thus three linear actuators. An optical measurement
system measures the precise orientation of each mirror segment with
respect to the common focus, and the sun. Each mirror segment
orientation is then controlled via feedback control adjustment of
the length of its three attached actuators, to reflect light to the
common focus with great accuracy. This type of system is used in
large segmented mirror optical astronomical telescopes to position
mirror segments to extremely high accuracy. The mirror segments and
linear actuators described in this embodiment are much larger and
less accurate than those used in astronomical telescopes, but the
positioning method is the same, and pointing accuracy in the order
of 0.1 mrad is easily achievable using simple stepper motor linear
actuators.
[0095] The use of linear actuators to position mirror segments
serves additional purposes:
1) The actuators can be adjusted to ensure that light from some
mirror segments is not directed from mirror 36 to mirror 38 and is
"dumped". This is useful when it is desired to reduce the amount of
light collected. 2) With sophisticated control software the
actuators can be used as a fine-grained sun tracking mechanism for
each mirror segment. This is like the systems used in heliostat
arrays, but for a much smaller range of motion. This mechanism can
compensate for inaccuracies or slow response in the alt azimuth
control of the overall mirror structure 36.
[0096] The rigid truss framework forms the stiff and accurate
reaction structure that enables the mirror segments to maintain
their position with sufficient accuracy. It also provides the
framework to hold the ambient pressure, buoyancy gas filled bags.
At 20 km altitude, atmospheric pressure is low and substantial
volume is needed to provide meaningful lift. The design of both
struts and mirror segments emphasizes lightweight. For example for
an average structural weight of 1 kg/m.sup.2 of mirror aperture
area, a volume of approximately 10 m.sup.3 of Helium is needed to
provide neutral buoyancy at an altitude of 20 km. A mirror
structure 100 meters thick can thus provide lift for an average
structural mass of approximately 10 kg/m.sup.2.
[0097] Lightweight rigid structures are practical for this
application because the structural loads are very light. Wind speed
is very low and steady at 20 km, and buoyancy removes much of the
stress of supporting gravitational forces. At 20 km altitude,
mirrors 36 and 38 are safely in the stratosphere and above all
weather in the troposphere, including clouds, moisture, dust, wind,
and the jet stream.
[0098] The 20 km height of the mirrors in the atmosphere is
illustrative. The actual height of individual systems may vary with
geographic location. The height of the boundary between the
troposphere and the stratosphere varies with latitude, season of
the year and local weather conditions. The boundary height is
generally lower at higher latitudes and in the winter.
[0099] FIG. 5 Illustrates another embodiment similar to FIG. 4, but
using circular mirror segments 55 arranged in a Fresnel fashion.
The supporting structure 36 is a flat thin circular cylinder and
the attached circular mirror segments 55 are each mounted tilted
from structure 36 to reflect light to a common focus. The
structural elements, struts 48, nodes 50 and 56, and linear
actuators are identical to those in embodiment shown in FIG. 4. The
method of construction of circular mirror segments 55 is similar to
that described for hexagonal mirror segments 4B.
[0100] Circular segments are easier to fabricate than hexagonal
segments, but they cover less area due to their non space filling
shape. The Fresnel approach simplifies the supporting structure,
but is slightly less efficient due to shading or extra spacing
requiring a larger structure with more mirrors and struts than the
hexagonal segments approach.
Detailed Description--FIG. 9 Light Pipe
[0101] FIG. 9 is a perspective cut-out view that illustrates the
structure of a portion of the wall of the light pipe tube 20. The
first embodiment uses a fiber-reinforced fabric as the main wall
structural element 60. Structural fabrics have long been used to
construct large inflatable structures. Among the most widely used
materials are polyester fabric laminated or coated with polyvinyl
chloride (PVC), and woven fiberglass coated with
polytetrafluoroethylene (PTFE). The light tube is an inflated
structure filled with gases that provide buoyancy and light
transparency. Dry air, nitrogen, and either helium or hydrogen are
some viable options. Several transparent membranes 34 fabricated
from polyethylene terephthalate (PET) or other similar transparent
film, like that depicted in FIG. 2B are positioned at points along
the length of the light pipe. They contain the pressurization of
the light pipe 20 and act as separators between buoyancy gas filled
regions and air-filled regions.
[0102] In one embodiment, vertically oriented steel cables 66
attached to the outer side of the structural fabric carry the
vertical pre-tension load provided by the buoyancy of the light
pipe. Cables from other materials or stronger fabrics without
external reinforcement are viable alternatives.
[0103] The vertical force acts to stabilize gravity forces and the
horizontal displacement of the light pipe caused by wind forces.
These horizontal forces are very large, especially at the higher
altitudes of the jet stream. In effect this section of the light
pipe behaves approximately like a hanging Catenary cable.
[0104] The diameter of the steel cables can be reduced as the
tension load diminishes with height in buoyant regions of the light
pipe. Ultimately, at the top of the light pipe the tension is close
to zero.
[0105] At the bottom section of the light pipe 20 that connects to
the second concentrator 26 and anchor structure it is beneficial to
add additional structural reinforcement in a manner that enhances
the stiffness of the light pipe. In one embodiment this is done
using steel cable hoop reinforcement attached to the vertical steel
reinforcement. This section of the light pipe 20 behaves more like
a cantilever beam attached to the foundation structure, than the
Catenary cable of the upper portion of light pipe 20. This
stiffness helps to keep the light pipe vertical at the CPC entry
which optically aligns the light pipe with the CPC without resort
to complex mechanical methods. This takes advantage of the light
pipes ability to accommodate gradual bends while still maintaining
high transmission efficiency.
[0106] A reflective layer is attached to the inner surface of
structural fabric wall 62. In one embodiment this is a layer of
aluminized PET film. Mounted to the inner wall 62 is a refracting
layer 64. In one embodiment layer 64 is made of prismatic
transparent reflecting film, as used in lighting system light
tubes. One such commercially available material is 3M Optical
Lighting Film.TM.. This film uses the principal of total internal
reflection as taught by Whitehead in U.S. Pat. No. 4,260,220,
"Prism light guide having surfaces which are in octature" and can
be 99% efficient in reflecting light within the acceptance
angle.
[0107] A very small amount of light is absorbed by the film
material, but the bulk of the losses are due to light leaking out
at the prism apexes due to the finite radius of the apexes, and to
a lesser extent due to diffraction at the apexes. A portion of this
small amount of light that leaks through the prismatic film 64 is
reflected by the metallic specular reflective film 62, further
enhancing overall light pipe wall surface reflection
efficiency.
[0108] Another embodiment uses multiple layers of transparent film
such as PET film for layer 64 and exploits the principle of multi
layer reflectance as taught by Waymouth in U.S. Pat. No. 4,895,420,
"High reflectance Light Guide". This approach also only works for a
limited acceptance angle. The overall reflectivity is dependent on
the number of layers of transparent film and the major loss is
through light leaking out. This approach can also be enhanced by
using a specular reflective layer 62. These various techniques,
exploiting both specular reflection from layer 62 and refraction
within layer 64 can exceed 99.5% overall reflectivity.
[0109] It should be noted that this combination of a conventional
metallic specular reflective layer in combination with high
reflectance, or prismatic light guides to enhance the overall
reflectivity is new art.
Detailed Description--FIGS. 10, 11 Process Heat Thermal
Desalination Plants.
[0110] FIG. 10 shows a schematic view of a solar process heat
desalination plant. Large scale desalination plants are becoming
increasingly common. The largest plants burn fuel to produce steam
for use as process heat. This steam is used as the primary energy
input into the desalination process. Two common thermal
desalination techniques are flash evaporation, and multi effect
distillation.
[0111] In FIG. 10, aperture 120 in receiver 80 receives solar
energy from the exit aperture of a solar concentration system, such
as that illustrated in FIG. 6. Receiver 80 heats cold water
provided by desalination plant 128 as the working fluid and returns
it as steam. Desalination plant 128 receives brackish or salt water
via inlet 122 and produces fresh water, a portion of which provides
the water for the receiver and the majority of which is provided
from outlet 126. Concentrated brine is output from outlet 124.
[0112] In conventional thermal desalination plants, the major cost
is the fuel to operate the plant, which also puts carbon dioxide
into the atmosphere. In this embodiment, all the solar energy is
being utilized for desalination. There is no thermal storage or
electricity generation, which reduces the capital cost. There is no
fuel cost and no carbon dioxide emissions. The plant can be
positioned where the salt or brackish water supply is located.
Clean water can be provided at lower cost than current systems
without putting carbon dioxide into the atmosphere.
[0113] FIG. 11 shows a schematic view of a combined electricity
generation and thermal desalination plant. This uses heat that
would otherwise be rejected to the environment as process heat for
desalination. Receiver 80 receives solar energy through aperture
120 and transfers heat via a working fluid to thermal store 30. A
second working fluid transfers heat from thermal store 30 to heat
engine 84 which expands it producing work which drives generator 86
producing electricity. Hot exhaust from heat engine 84 is passed
through boiler 94, producing steam for desalination plant 128.
Desalination plant 128 accepts salt or brackish water through inlet
122 and produces fresh water through outlet 126, and concentrated
brine discharge through outlet 124.
Detailed Description--FIG. 12 Heliostat Array.
[0114] FIG. 12 is a perspective view illustrating an embodiment
using a heliostat array. Concentrator 36 is a static buoyant
structure held in place with cable stays 25. Each individual mirror
segment attached to concentrator 36 tracks the sun and positions
itself to reflect sunlight to a common focus at the entry aperture
of light pipe 20. Beam 27 helps maintain the position of the entry
aperture. Buoyancy collar 23 provides buoyancy for embodiments
where the light pipe diameter is too small to provide sufficient
internal buoyancy. The embodiment shown has a high concentration
factor and hence a narrow light pipe, and does not need a
collimator at the light pipe entry aperture. For embodiments with a
lower concentration factor and larger diameter light pipe where a
short focal length is desired, a Compound Parabolic Collimator is
well suited to these heliostat embodiments, as opposed to the
parabolic mirror collimator shown in the first embodiment.
Detailed Description--FIG. 13, FIG. 14A, FIG. 14B and FIG. 15
Modular Thermal Storage System.
[0115] FIG. 13 is a perspective view of the ground-based elements
of one embodiment of a solar energy system. Concentrated sunlight
exits the bottom part of concentrator 26 where it impinges on the
surface of solar receiver 80. A partial cavity absorber cover 28 is
shown to provide visibility of the receiver elements. Heat absorbed
by the receiver is transferred to the thermal storage unit 30 via
the pipes shown. The thermal storage unit is composed of multiple
thermal storage elements 130. Multiple thin walled pressure vessels
are easier and cheaper to fabricate and transport than large thick
walled pressure vessels. Hot fluid from the thermal storage unit
flows to the power block which contains the heat engine, compressor
and generator. Cooling block 134 cools the exhaust fluid from the
power block prior to it being compressed in power block 132 and
returned to the thermal storage unit 30.
[0116] FIG. 14A is a perspective view of a hexagonal cylinder
thermal accumulator element 130 and its attached receiver element
136. Hexagonal accumulators as shown in FIG. 14A allow for closer
packing, but circular cylinders are simpler to fabricate and may be
preferred. Thermal storage unit 30 and solar receiver 80 are
constructed from a collection of these modular elements. In a
preferred embodiment, each thermal accumulator element 130 is
constructed from solid graphite contained within a pressurized thin
walled steel container. The graphite is separated from the steel
wall by a layer of thermal insulation and narrow cooling channel
gaps through which the cold in and cold out working fluids
circulate. Hot in and hot out working fluids circulate within
cooling channels within the graphite. At high temperatures graphite
ignites in an oxygen rich environment, so inert gases like helium
and argon are preferred for high pressure working fluids. For
systems using hot helium as the working fluid, the pressure is in
the region of 10 MPa. Other solids that have reasonable heat
capacity, heat transfer and can sustain high temperatures are
alternatives to graphite. The relatively weak graphite and thermal
insulating materials are contained within the pressure vessel and
do not sustain forces due to the high pressure.
[0117] FIG. 14B is a perspective close up view of the attachments
between thermal accumulator element 130 and receiver element 136.
From the perspective of thermal storage element 130, cold fluid out
pipe 138 and hot fluid in pipe 140 connect it to the receiver
element 136. Cold fluid in pipe 142 delivers cold pressurized fluid
from the power block 132 and hot fluid out pipe 144 returns hot
pressurized fluid to the power block 132. Not shown are the
multiple narrow channels within the solid graphite inside thermal
storage element 130 which form the two counter flow heat transfer
circuits.
[0118] FIG. 15 is a vertical cross sectional view of a thermal
storage element 130. Arrows in spaces occupied by working fluids
indicate the direction of fluid flow. The thin pressure vessel wall
146 contains the pressurized working fluids. Cold out pipe 138, hot
in pipe 140, cold in pipe 142 and hot out pipe 144 penetrate the
pressure vessel wall 146 to connect the working fluid circuits. Top
manifold block 150 provides narrow channels 158 that connect hot in
plenum 162 to channels in thermal storage block 152. Manifold block
150 also provides narrow channels (not shown) that connect hot out
plenum 156 to channels in thermal storage block 152. Only one
thermal storage block 152 is shown, but many stacked thermal
storage blocks 152 with aligned heat transfer channels are also
possible. Only one heat transfer channel 158 is shown, but there
are many channels not shown that form a vertical grid of close
spaced channels in thermal storage blocks 152. Bottom manifold
block 154 is similar to top manifold block 150, connecting cold out
plenum 164 and cold in plenum 160 to the heat transfer channels in
thermal storage block 152. Thermal insulation layer 148 protects
the pressure vessel wall from high heat and provides a channel
between the pressure vessel wall and the insulation through which
the cold in and cold out working fluids can flow. This enables all
pipe connectivity to be on the top of the thermal storage
element.
Operation--FIGS. 13, 14A, 14B, 15
[0119] In operation of the modular thermal storage element, hot
pressurized working fluid from a solar receiver or other thermal
source enters the hot in plenum 162 through insulated pipe 140. It
then flows through channels in top manifold block 150, and through
connecting channels in thermal storage blocks 152. As it flows down
through thermal storage blocks 152 it initially encounters channel
walls that are already at the hot inlet temperature. Eventually it
encounters cold channel walls and heat is transferred to the
thermal storage block 152. The flow rates and the number and size
of the heat transfer channels in storage block 152 are arranged so
that heat is transferred in a short distance, ensuring a sharp
vertical temperature gradient and efficient thermal storage. The
now cold working fluid continues through cold channels in thermal
storage block 152, bottom manifold block 154 and out into cold out
plenum 164. It then flows up through the narrow cold wall channel
and out through cold out pipe 138. This describes the thermal
charging circuit.
[0120] The thermal discharge circuit operates similarly,
transferring heat to pressurized cold in working fluid that enters
through cold inlet 142, flows down through the narrow cold wall
channel into cold in plenum 160, through bottom manifold block 154,
thermal storage block 152 and top manifold block 150 into hot out
plenum 156 and out hot out pipe 144.
Operation--FIGS. 1, 2, 3, 4, 5, 6, 7
[0121] FIG. 1A and FIG. 1B shows two operational positions of a
first embodiment. FIG. 1A shows the solar concentrator assembly
pointing at the horizon, or approximately 0 degrees of altitude as
it is at dawn and dusk. The light pipe 20 is shown straight and
vertical, as is the case when no wind is blowing.
[0122] As the sun rises, light is reflected from concentrator
mirror 36 onto collimating mirror 38, and then reflected from
collimator mirror 38 to the entrance aperture of light pipe segment
21. As the light reflects off mirror 36 and 38 and is concentrated,
its dispersion half-angle increases from the 4.653 mrad of the
incident sunlight to a larger number approaching 60 mrad at the
entrance to light pipe segment 21. The light then travels down
light pipe segment 21 reflecting occasionally off the reflective
walls and around the bend in the pipe at the alt azimuth mount 44
that connects to the main light pipe 20. Light then travels down
light pipe 20 reflecting occasionally off the reflective wall which
has gradual bends as the light pipe adjusts to the force of the
wind it experiences in the troposphere.
[0123] At the bottom of light pipe 20 at ground level the light
passes through the final optical concentrator element CPC 26,
exiting the optical system and entering the thermal system.
[0124] In one embodiment of a solar energy conversion system shown
in FIG. 7A receiver 80 accepts the concentrated solar energy
through input aperture 120, absorbs it, and transfers it as thermal
energy to the working fluid. The working fluid is used to transfer
the thermal energy to thermal store 30 via a closed loop. A second
closed loop transfers heat from thermal store 30 to a high
pressure, high temperature working fluid. This gas is expanded
through heat engine 84, converting the heat to mechanical work.
This work is used to drive electricity generator 86, which converts
the mechanical work to electricity.
[0125] Exhaust gas from heat engine 84 passes through regenerator
92, transferring heat to the cooler high-pressure gas output from
compressor 90. After exiting regenerator 92, the low pressure, low
temperature gas passes through ambient cooler heat exchanger 88,
where it is further cooled before entering compressor 90.
High-pressure pre-heated gas from the regenerator 92 enters thermal
store 30, completing the thermal circuit.
[0126] In the combined cycle solar energy conversion system
embodiment shown in FIG. 7B, the exhaust gas from heat engine 84 is
used to heat high pressure, low temperature water in boiler heat
exchanger 94. This superheated water then expands as steam through
steam turbine 96, converting the heat to mechanical work. This work
is used to drive electricity generator 98, which converts the
mechanical work to electricity. Condensor heat exchanger 100 cools
turbine 96 exhaust gas which is then pressurized by water pump 102.
Ambient heat exchanger 88 can be much simpler for this combined
cycle system.
[0127] As the day progresses, the solar concentrator and collimator
tracks the sun, rotating the entire mirror and alt-azimuth mount
structure in azimuth and tilting the structural axis (which is
parallel to beam 40) in azimuth. The motion is driven by motors in
the alt-azimuth mount 44 which are controlled by a sun tracking
control system. As is shown in FIG. 1B, the light pipe 20 bends in
response to winds in the lower atmosphere. The upper approximately
5 km portion of light pipe 20 is always kept vertical. There are no
significant winds acting on this section and it is filled with
buoyancy gas. This minimizes the adjustments necessary to maintain
sun tracking as the base of the alt-azimuth mount moves with light
pipe 20.
ADVANTAGES
[0128] Unlike prior art concentrating solar energy systems, the
geographic location of these embodiments is not constrained to
desert areas. This is of particular benefit to normally cloudy mid
latitude locations where most large urban areas are located.
[0129] The combination of geographic flexibility and power
generation without the need for any fuel provides a secure and
clean energy system.
[0130] Power in the form of concentrated solar energy or
electricity can be provided at any point on the earth's surface,
where the definition of surface includes the entire surface,
including all land and oceans.
[0131] Offshore platforms could be a particularly convenient in
some locations. Concentrated direct solar energy and or electricity
could be provided near mines, allowing convenient high temperature
processing without transportation of bulk ores.
[0132] The very small amount of land area needed means that systems
can be located very near existing power plants, or existing
transmission and distribution networks, which reduces or eliminates
the need for new electricity transmission infrastructure.
[0133] With the thermal storage described in the embodiments a
reliable and flexible power generation system that can continuously
supply all energy requirements without augmentation with other
power generation sources, as is required with prior art solar power
and other alternative energy systems.
[0134] Energy systems that do not put carbon dioxide into the
atmosphere are highly desirable. Currently all alternative energy
systems suffer from major problems:
1) They are very costly to build 2) They are unreliable providers
of electricity due to intermittent weather effects, and so need
backup generation using alternate energy sources such as natural
gas. 3) They need large additional energy storage and transmission
infrastructure investments. 4) The energy is located far from
users, again requiring large transmission infrastructure
investments. 5) They require large areas of land which increases
their environmental impact and limits their use to areas where both
energy and land are available.
[0135] This new system has the benefit of not producing carbon
dioxide and has none of these problems. The bottom line is clean
secure energy can be provided at much lower cost and minimal
environmental impact.
[0136] The benefits of suspending a collector in the stratosphere
are the reliability of the energy source, the higher incident
energy density, and the benign stable calm low wind weather free
environment. These benefits come at the price of lower atmospheric
density, which means less buoyant lift and a consequent need for a
very lightweight structure. This complicates the use of heavy
energy absorbing and/or power conversion equipment at the collector
in the stratosphere, and explains the benefit of embodiments using
a light pipe to transport the light energy directly to the earth's
surface, without conversion.
[0137] It is envisaged that the large buoyant optical structures,
particularly mirror structures 36 and 38 will be assembled at one
or more centralized factory locations in a highly automated
fashion. The assembled structures will then be raised by their own
buoyancy into the stratosphere and "flown" to their destination
anywhere on the planet. There they will be connected to the light
pipe and ground based elements. This manufacturing method greatly
reduces cost, improves quality, and speeds construction. It is
envisaged that when production is mature, complete utility size
electricity generating facilities could be operational in less than
a year from breaking ground. This compares with current
technologies which require three to five or more years to
construct.
[0138] Although the present invention has been described in terms
of a first embodiment, it will be appreciated that those skilled in
the art might make various modifications and alterations without
departing from the spirit and scope of the invention. Though the
first embodiment is described using offset parabolic mirror arrays
for the concentrator and collimator elements, other forms of
concentrators and collimators using a myriad of different optical
geometries are possible. Also refractive optics elements such as
lightweight arrays of thin prismatic elements, or inflatable
reflective or refractive optical elements are also possible.
[0139] All that is required of the light tube is a sound gas tight
structure filled with transparent gas, and having a high inner
surface reflective efficiency, both of which can be met with many
alternative materials and structural approaches. As described in
the specification, there are various structural techniques to
stabilize the light pipe against atmospheric wind forces including:
[0140] a) pre-tensioned hanging cables using excess buoyancy to
provide the tensioning force, [0141] b) cable stays, [0142] c)
rigid light pipe walls.
[0143] Each of these techniques or a combination of some or all is
possible.
[0144] The invention should therefore be measured in terms of the
claims which follow.
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