Note: Descriptions are shown in the official language in which they were submitted.
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Downhole heat exchanger for a geothermal heat pump
The invention relates to a downhole heat exchanger for recovering
geothermal energy from a borehole.
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.
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.
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.
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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.
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.
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.
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.
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.
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:
a) an arithmetic mean roughness Ra in accordance with DIN EN ISO 4287 in
the range from 1 to 15 pm, preferably in the range from 2 to 12 pm and
particularly preferably in the range from 3 to 7 pm,
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b) an average peak-to-valley height Rz in accordance with DIN EN ISO 4287
in the range from 8 to 80 pm, preferably in the range from 10 to 60 pm and
particularly preferably in the range from 15 to 40 pm, and
c) a maximum peak-to-valley height Rzlmax in accordance with DIN EN ISO
4287 in the range from 10 to 500 pm, preferably in the range from 15 to
150 pm and particularly preferably in the range from 25 to 65 pm.
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.
Ra is the arithmetic mean roughness from the absolute values of all profile
values.
Rz is the average of the five peak-to-valley heights from the five individual
measurements.
Rz1 max is the greatest peak-to-valley height from the five individual
measurements.
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.
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.
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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.
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
(R41 OA). 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.
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.
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.
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
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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.
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.
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 co-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.
Of course, it is also possible to use copolyamides based thereon, with
monomers such as caprolactam also being used if desired.
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.
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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.
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.
Suitable fillers are, for example, titanium dioxide, zinc sulfide, silicates,
chalk,
aluminum oxide and glass spheres.
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.
The molding composition can additionally contain the customary auxiliaries
and additives, for example impact modifiers, plasticizers, stabilizers and/or
processing aids.
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.
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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.
Suitable layer sequences from the inside outward are, for example:
- polyamide (for example PA12)/bonding agent/polypropylene or
polyethylene;
- polyamide (for example PA12)/bonding agent/ethylene-vinyl alcohol
copolymer (EVOH)/bonding agent/polyamide;
- polyamide/bonding agent/EVOH/bonding agent/polypropylene or
polyethylene;
- polyamide/bonding agent/fluoropolymer (for example PVDF or ETFE);
- polyamide/adhesion-modified fluoropolymer;
- polyamide/bonding agent/polybutylene-2,6-naphthaIate/bonding
agent/polyamide.
Suitable bonding agents for the bonding of polyamide and polyolefins are, for
example, polyolefins functionalized with maleic anhydride.
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.
Polyolefins functionalized with maleic acid, for example, are suitable as
bonding agents for forming the bond between EVOH and polyolefins.
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Bonding agents for joining polyamides and fluoropolymers are known, for
example, from EP 0 618 390 Al, while adhesion-modified fluoropolymers can
be prepared, for example, by mixing in small amounts of polyglutarimide as
described in EP 0 637 511 Al, by functionalization with maleic anhydride or
by incorporation of carbonate groups as described in EP 0 992 518 Al.
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
2007 005 270 Al.
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.
