U.S. patent application number 13/502767 was filed with the patent office on 2012-08-09 for downhole heat exchanger for a geothermal heat pump.
This patent application is currently assigned to Evonik Degussa GmbH. Invention is credited to Andreas Dowe, Rainer Goring, Markus Hartmann, Andreas Pawlik.
Application Number | 20120199317 13/502767 |
Document ID | / |
Family ID | 43763568 |
Filed Date | 2012-08-09 |
United States Patent
Application |
20120199317 |
Kind Code |
A1 |
Hartmann; Markus ; et
al. |
August 9, 2012 |
DOWNHOLE HEAT EXCHANGER FOR A GEOTHERMAL HEAT PUMP
Abstract
The invention relates to a downhole heat exchanger for
extracting geothermal energy from a borehole, wherein the inner
surface of the exchanger tube comprises the following roughness
values: a) an arithmetic mean roughness Ra according to DIN EN ISO
4287 in the range of 1 to 15 .mu.m, b) an averaged roughness Rz
according to DIN EN ISO 4287 in the range of 8 to 80 .mu.m, and c)
a maximum roughness depth Rz1 max according to DIN EN ISO 4287 in
the range of 10 to 500 .mu.m, comprising an improved precipitation
film during operation, such that the entire surface of the
exchanger tube is uniformly wetted.
Inventors: |
Hartmann; Markus;
(Sendenhorst, DE) ; Dowe; Andreas; (Borken,
DE) ; Goring; Rainer; (Borken, DE) ; Pawlik;
Andreas; (Recklinghausen, DE) |
Assignee: |
Evonik Degussa GmbH
Essen
DE
|
Family ID: |
43763568 |
Appl. No.: |
13/502767 |
Filed: |
October 11, 2010 |
PCT Filed: |
October 11, 2010 |
PCT NO: |
PCT/EP2010/065162 |
371 Date: |
April 23, 2012 |
Current U.S.
Class: |
165/45 |
Current CPC
Class: |
F24T 10/13 20180501;
Y02E 10/125 20130101; Y02E 10/10 20130101 |
Class at
Publication: |
165/45 |
International
Class: |
F24J 3/08 20060101
F24J003/08 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 21, 2009 |
DE |
10 2009 045 882.4 |
Claims
1. A downhole heat exchanger, comprising a heat exchanger pipe,
wherein an interior surface of the heat exchanger pipe has the
following roughness values: a) an arithmetic mean roughness Ra in
accordance with DIN EN ISO 4287 in the range from 1 to 15 .mu.m; b)
an average peak-to-valley height Rz in accordance with DIN EN ISO
4287 in the range from 8 to 80 .mu.m; and c) a maximum
peak-to-valley height Rz1max in accordance with DIN EN ISO 4287 in
the range from 10 to 500 .mu.m, wherein the roughness measurement
is carried out in accordance with DIN EN ISO 4288.
2. The downhole heat exchanger of claim 1, wherein: Ra is in the
range from 2 to 12 .mu.m; Rz is in the range from 10 to 60 .mu.m;
and Rz1max is in the range from 15 to 150 .mu.m.
3. The downhole heat exchanger of claim 1, wherein: Ra is in the
range from 3 to 7 .mu.m; Rz is in the range from 15 to 40 .mu.m;
and Rz1max is in the range from 25 to 65 .mu.m.
4. The downhole heat exchanger any of claim 1, wherein the heat
exchanger pipe comprises a layer comprising a thermoplastic molding
composition.
5. The downhole heat exchanger of claim 4, wherein the heat
exchanger pipe or an innermost layer of the heat exchanger pipe
comprises a molding composition whose matrix comprises a
fluoropolymer, a polyarylene ether ketone, a polyolefin, or a
polyamide.
6. The downhole heat exchanger of claim 4, wherein the heat
exchanger pipe or an innermost layer of the heat exchanger pipe
comprises a molding composition comprising from 0.1 to 50% by
weight of at least one selected from the group consisting of
reinforcing fibers and reinforcing fillers.
7. The downhole heat exchanger of claim 1, wherein the heat
exchanger pipe comprises a metal comprising a rough coating on an
interior surface.
8. A process for recovering geothermal energy from a borehole, the
process comprising: vaporizing a vaporizable refrigerant in the
downhole heat exchanger of calim 1; compressing the refrigerant
vapor which has ascended in the heat exchanger pipe in a
compressor, to liquefy the vapor and remove heat obtained from the
heat of condensation; and then feeding the cooled liquid
refrigerant back to the downhole heat exchanger as falling film
which is conveyed downward.
9. The downhole heat exchanger of claim 1, in the form of a direct
boiling heat exchanger, which is suitable for recovering geothermal
energy from a borehole.
10. The downhole heat exchanger of claim 1, wherein the internal
diameter of the heat exchanger pipe is in the range from 15 to 80
mm.
11. The downhole heat exchanger of claim 1, wherein the internal
diameter of the heat exchanger pipe is in the range from 20 to 55
mm.
12. The downhole heat exchanger of claim 1, wherein the internal
diameter of the heat exchanger pipe is in the range from 26 mm to
32 mm.
13. The downhole heat exchanger of claim 1, having a length in a
range from 60 to 200 m.
14. The downhole heat exchanger of claim 1, having a length in a
range from 80 to 120 m.
15. The downhole heat exchanger of claim 6, wherein the molding
composition comprises from 0.5 to 20% by weight of at least one
selected from the group consisting of reinforcing fibers and
reinforcing fillers.
16. The downhole heat exchanger of claim 6, wherein the molding
composition comprises from 3 to 10% by weight of at least one
selected from the group consisting of reinforcing fibers and
reinforcing fillers.
17. The downhole heat exchanger of claim 6, wherein the molding
composition further comprises at least one selected from the group
consisting of an impact modifier, a plasticizer, a stabilizer, and
a processing aid.
18. The downhole heat exchanger of claim 5, wherein the heat
exchanger pipe consist of a single layer comprising the molding
composition.
19. The downhole heat exchanger of claim 5, wherein the heat
exchanger pipe comprises a plurality of layers.
20. The downhole heat exchanger of claim 7, wherein the metal is
selected from the group consisting of aluminum, an aluminum alloy,
and steel.
Description
[0001] The invention relates to a downhole heat exchanger for
recovering geothermal energy from a borehole.
[0002] The recovery of geothermal energy from boreholes is carried
out by extraction of thermal water from opened-up aquifers or by
cooling of the earth along a borehole. Cooling of the earth is
effected by means of various downhole heat exchangers. To extract
heat from the earth, it is possible to use vaporizable refrigerants
which recover the energy by boiling. Such direct boiling heat
exchangers are being used to an increasing extent. Compared to
brine heat exchangers, they offer a significantly higher degree of
efficiency and in technical circles are considered to be the
technology of the future. There are, for example, systems based on
propane (R290), butane, ammonia (R717) or carbon dioxide (R744),
with propane being preferred. A distinction is made between
near-surface geothermal energy for direct utilization, for instance
for heating and cooling, usually as heat pump heating, and deep
geothermal energy for direct utilization in the heat energy method
or indirectly for generation of electric power. Deep downhole heat
exchangers with direct boilers are also referred to as heat
pipes.
[0003] DE 42 11 576 Al and DE 298 24 676 U1 describe arrangements
of heat pipes in which the heating zone of the heat pipe and thus
the boiling of the liquid refrigerant are located in the lower part
of the pipe. The vapor is generated by boiling of the liquid
refrigerant; it is then conveyed upward in a pipe and releases its
energy at the top by condensation. This is utilized directly or
with the aid of a heat pump.
[0004] In WO 01/04550, the refrigerant is conveyed upward through a
channel into the heat exchanger and through a second channel. Film
vaporization is sought by means of a spiral track which has to be
produced in a complicated manner. However, vaporization of the
refrigerant over the entire length of the borehole and thus heat
exchanger cannot be achieved using the arrangement described there,
so that complete extraction of heat is not made possible.
[0005] The utility model DE 20 2004 018 559 U1 describes a heat
generator for recovering geothermal energy from a borehole, in
which a condensate stream distributor is incorporated in a heat
exchanger pipe. Although wetting on all sides is likewise said to
be achieved, film vaporization cannot be realized.
[0006] Finally, DE 10 2007 005 270 Al describes a downhole heat
exchanger which contains a condensate stream distributor having
condensate conveying devices arranged radially and/or tangentially
to the wall of the heat exchanger pipe. A radially distributed
condensate film is said to be produced in this way.
[0007] EP 1 450 142 A2 describes a heat exchanger pipe consisting
of a filler-containing polymer material. The pipe serves to convey
air as heat transfer medium.
[0008] Finally, WO 2008/113569 discloses a pipe arrangement for
downhole heat exchangers, in which the pipes have at least one
layer of a polymer molding composition which contains a filler or
reinforcing material which increases the mechanical strength.
Damage to the outer surface during installation and subsequent
crack growth are said to be prevented in this way. The pipe
arrangement is intended for transport of a liquid heat transfer
medium.
[0009] It is an object of the invention to produce a complete
falling film in a downhole heat exchanger by simple means, so that
the entire interior surface of the heat exchanger pipe is uniformly
wetted.
[0010] This object is achieved by a downhole heat exchanger
designed as direct boiling heat exchanger for recovering geothermal
energy from a borehole, in which the interior surface of the heat
exchanger pipe has the following roughness parameters: [0011] a) an
arithmetic mean roughness Ra in accordance with DIN EN ISO 4287 in
the range from 1 to 15 .mu.m, preferably in the range from 2 to 12
.mu.m and particularly preferably in the range from 3 to 7 .mu.m,
[0012] b) an average peak-to-valley height Rz in accordance with
DIN EN ISO 4287 in the range from 8 to 80 .mu.m, preferably in the
range from 10 to 60 .mu.m and particularly preferably in the range
from 15 to 40 .mu.m, and [0013] c) a maximum peak-to-valley height
Rz1max in accordance with DIN EN ISO 4287 in the range from 10 to
500 .mu.m, preferably in the range from 15 to 150 .mu.m and
particularly preferably in the range from 25 to 65 .mu.m.
[0014] The roughness measurement is carried out by the tracer
method in accordance with DIN EN ISO 4288. In the roughness
measurement using a mechanical tracer instrument, a tracer tip made
of diamond is moved at constant speed over the surface of a
specimen. The measurement profile is given by the vertical
displacement of the tracer tip, which is generally measured by
means of an inductive displacement measurement system. To describe
a surface technically, standardized roughness parameters are
obtained from the measured profile.
[0015] Ra is the arithmetic mean roughness from the absolute values
of all profile values.
[0016] Rz is the average of the five peak-to-valley heights from
the five individual measurements.
[0017] Rz1max is the greatest peak-to-valley height from the five
individual measurements.
[0018] The downhole heat exchanger comprises a heat exchanger pipe
which is connected to the earth via a packing material, for example
bentonite. The vaporization of the refrigerant condensate occurs on
the interior surface of the heat exchanger pipe. The upward
transport of the vapor formed occurs in the center of the pipe.
[0019] The internal diameter of the heat exchanger pipe is
generally in the range from 15 to 80 mm, preferably in the range
from 20 to 55 mm and particularly preferably in the range from 26
mm to 32 mm.
[0020] The heat exchanger length is generally from 60 to 200 m,
with greater or smaller lengths also being possible in individual
cases. The heat exchanger is preferably from 80 to 120 m long.
[0021] As refrigerant, use is made of, for example, propane (R290),
butane, ammonia (R717) or carbon dioxide (R744). Further suitable
refrigerants are, for example, propene (R1270), tetrafluoroethane
(R134a), difluoromethane (R32), pentafluoroethane (R125), a mixture
of R32, R125 and R134a in a ratio of 23/25/52 (R407C) or a mixture
of R32 and R125 in a ratio of 50:50 (R410A). According to physical
laws, the interior of the heat exchanger is therefore under
relatively high pressure. The refrigerant vapor which has ascended
is compressed in a compressor and thus liquefied. Compression
liberates heat of condensation which is discharged as useful heat.
The cooled liquid refrigerant is fed via an expansion unit back to
the heat exchanger and conveyed downward as falling film. The
refrigerant here vaporizes again with uptake of the geothermal
energy. As regards the details of the technical procedure,
reference is made to the abovementioned prior art.
[0022] The heat exchanger pipe can, for example, consist of metal.
In this case, the interior surface bears a rough coating. Of
course, the exterior surface can also be coated here, for example
for reasons of corrosion protection. The metal can be aluminum, an
aluminum alloy, steel, for example stainless steel, or any other
metal. Coating can be effected by powder coating or by coating with
the melt of a further molding composition as described below, for
example by means of extrusion coating.
[0023] However, the pipe preferably consists of plastic and
particularly preferably of a thermoplastic molding composition.
Such pipes can be rolled up so that it is not necessary to join
comparatively short pieces to one another, e.g. by welding, during
installation.
[0024] The molding composition used has to have sufficient
stiffness for the wall thickness to be made thin for reasons of
heat transfer. In addition, the plastic which forms the matrix of
the molding composition has to be sufficiently resistant to the
refrigerant and to the moisture in the earth. This means that the
wall must not swell since this would be associated with undesirable
length changes.
[0025] Suitable plastics are, for example, fluoropolymers such as
PVDF, PTFE or ETFE, polyarylene ether ketones such as PEEK,
polyolefins such as polyethylene or polypropylene and
polyamides.
[0026] Among polyamides, particular preference is given to those
whose monomer units contain an arithmetic mean of at least 8, at
least 9 or at least 10 carbon atoms. The monomer units can be
derived from lactams or w-aminocarboxylic acids. When the monomer
units are derived from a combination of diamine and dicarboxylic
acid, the arithmetic mean of the carbon atoms of diamine and
dicarboxylic acid has to be at least 8, at least 9 or at least 10.
Suitable polyamides are, for example: PA610 (which can be prepared
from hexamethylenediamine [6 carbon atoms] and sebacic acid [10
carbon atoms], and the mean number of carbon atoms in the monomer
units is thus 8), PA88 (which can be prepared from
octamethylenediamine and 1.8-octanedioic acid), PA8 (which can be
prepared from caprylic lactam), PA612, PA810, PA108, PA9, PA613,
PA614, PA812, PA128, PA1010, PA10, PA814, PA148, PA1012, PA11,
PA1014, PA1212 and PA12. The preparation of the polyamides is prior
art.
[0027] Of course, it is also possible to use copolyamides based
thereon, with monomers such as caprolactam also being used if
desired.
[0028] It is likewise possible to use mixtures of various
polyamides, provided the compatibility is sufficient. Compatible
polyamide combinations are known to those skilled in the art;
mention may here be made by way of example of the combinations of
PA12/PA1012, PA12/PA1212, PA612/PA12, PA613/PA12, PA1014/PA12 and
PA610/PA12 and also corresponding combinations with PA11. In the
case of doubt, compatible combinations can be determined by means
of routine tests.
[0029] The thermoplastic molding composition can be filled with
reinforcing fibers and/or fillers. The fibers or filler particles
which project at the surface in this way produce the required
roughness. For this purpose, the molding composition contains from
0.1 to 50% by weight, preferably from 0.5 to 20% by weight and
particularly preferably from 3 to 10% by weight, of fillers and/or
fibers. In one embodiment, the molding composition contains only
fibers. In another embodiment, the molding composition contains
only fillers. In a further embodiment, the molding composition
contains a mixture of fibers and fillers.
[0030] Suitable reinforcing fibers are, for example, glass fibers,
basalt fibers, carbon fibers, aramid fibers and potassium titanate
whiskers and also fibers composed of relatively high-melting
polymers.
[0031] Suitable fillers are, for example, titanium dioxide, zinc
sulfide, silicates, chalk, aluminum oxide and glass spheres.
[0032] The thermal conductivity of the heat exchanger walls can be
increased by means of suitable reinforcing fibers or fillers. For
this purpose, metal fibers can be used as fiber material or metal
powders, carbon black, graphite, CNTs (carbon nanotubes), hexagonal
boron nitride or combinations or mixtures of the various materials
can be used as filler.
[0033] The molding composition can additionally contain the
customary auxiliaries and additives, for example impact modifiers,
plasticizers, stabilizers and/or processing aids.
[0034] In a further embodiment, the surface roughness is generated
by compounding in a second polymer which is incompatible or only
slightly compatible with the matrix polymer and is therefore
dispersed only relatively coarsely. Suitable combinations of
materials are, for example, polyamide/polypropylene and
polyamide/ethylene-acrylic ester-acrylic acid
copolymer/polypropylene.
[0035] The heat exchanger pipe can, in one embodiment, be made up
of a single layer and thus consist of one of the above-described
molding compositions over the entire wall thickness. In a further
embodiment, the heat exchanger pipe is made up of a plurality of
layers, with the inner layer consisting of one of the
above-described molding compositions and the other layers having
functions which are not performed sufficiently by the layer of
molding composition having a rough surface, for example
flexibility, impact toughness or barrier action toward the
refrigerant or the moisture in the earth. If the layers do not
adhere to one another sufficiently well, bonding agents can be used
as described in the prior art.
[0036] Suitable layer sequences from the inside outward are, for
example: [0037] polyamide (for example PA12)/bonding
agent/polypropylene or polyethylene; [0038] polyamide (for example
PA12)/bonding agent/ethylene-vinyl alcohol copolymer (EVOH)/bonding
agent/polyamide; [0039] polyamide/bonding agent/EVOH/bonding
agent/polypropylene or polyethylene; [0040] polyamide/bonding
agent/fluoropolymer (for example PVDF or ETFE); [0041]
polyamide/adhesion-modified fluoropolymer; [0042] polyamide/bonding
agent/polybutylene-2,6-naphthalate/bonding agent/polyamide.
[0043] Suitable bonding agents for the bonding of polyamide and
polyolefins are, for example, polyolefins functionalized with
maleic anhydride.
[0044] Polyamides such as PA12 and EVOH can, for example, be joined
to one another with the aid of polyolefins functionalized with
maleic acid or by means of polyamide blends corresponding to EP 1
216 826 A2.
[0045] Polyolefins functionalized with maleic acid, for example,
are suitable as bonding agents for forming the bond between EVOH
and polyolefins.
[0046] Bonding agents for joining polyamides and fluoropolymers are
known, for example, from EP 0 618 390 A1, while adhesion-modified
fluoropolymers can be prepared, for example, by mixing in small
amounts of polyglutarimide as described in EP 0 637 511 A1, by
functionalization with maleic anhydride or by incorporation of
carbonate groups as described in EP 0 992 518 A1.
[0047] To support the effect of the surface roughness, the heat
exchanger pipe can additionally contain internals as are known from
the prior art, for example DE 10 2007 005 270 A1.
[0048] The invention results in the falling film having a uniform
layer thickness over the circumference of the heat exchanger;
streaming or separation of the film is prevented. Owing to the
increased surface area, better heat exchange is made possible; at
the same time, the flow velocity is decreased, which counters
flooding of the lowermost part of the heat exchanger.
* * * * *