U.S. patent application number 11/912135 was filed with the patent office on 2009-05-21 for heat pump.
Invention is credited to Paul David Bernard Bujac, Derek William Edwards, Frederick Thomas Murphy, Richard Powell, Andrew Wilson.
Application Number | 20090126371 11/912135 |
Document ID | / |
Family ID | 34630967 |
Filed Date | 2009-05-21 |
United States Patent
Application |
20090126371 |
Kind Code |
A1 |
Powell; Richard ; et
al. |
May 21, 2009 |
Heat Pump
Abstract
A heat pump device in which a temperature difference is
established between two heat exchangers by inducing cyclical
expansion and compression pulses in a working fluid vapour or gas
which passes through an adsorbent porous solid located between the
heat exchangers.
Inventors: |
Powell; Richard; (Bunbury,
GB) ; Edwards; Derek William; (Runcorn, GB) ;
Wilson; Andrew; (Frodsham, GB) ; Murphy; Frederick
Thomas; (Frodsham, GB) ; Bujac; Paul David
Bernard; (Tarporley, GB) |
Correspondence
Address: |
Weaver Austin Villeneuve & Sampson LLP
P.O. BOX 70250
OAKLAND
CA
94612-0250
US
|
Family ID: |
34630967 |
Appl. No.: |
11/912135 |
Filed: |
April 21, 2006 |
PCT Filed: |
April 21, 2006 |
PCT NO: |
PCT/GB2006/001482 |
371 Date: |
May 5, 2008 |
Current U.S.
Class: |
62/6 ;
62/324.1 |
Current CPC
Class: |
F25B 2309/1425 20130101;
F25B 2309/1407 20130101; F25B 25/02 20130101; F25B 9/00 20130101;
F25B 9/145 20130101 |
Class at
Publication: |
62/6 ;
62/324.1 |
International
Class: |
F25B 9/00 20060101
F25B009/00; F25B 13/00 20060101 F25B013/00 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 21, 2005 |
GB |
050795308 |
Claims
1. A heat pump device comprising: at least one heat exchanger; a
body of porous absorbent material housing an inlet and an outlet
the body being disposed in thermal contact with the heat exchanger,
means for passing a working fluid through the body, and means for
inducing cyclical compression and expansion pulses in the working
fluid, to cause the working fluid to flow from the inlet to the
outlet to create a temperature gradient in the body between the
inlet and outlet.
2. A device as claimed in claim 1, further including a heat
transfer fluid, means for passing the heat transfer fluid in
thermal contact with the heat exchanger, arranged so that the flow
of heat transfer fluid removes or adds heat from or to the heat
exchanger.
3. A device as claimed in claim 2, wherein the direction of flow of
the heat transfer fluid changes with the compression and expansion
pulses of the working fluid.
4. A device as claimed in claim 3, wherein the direction of flow of
the heat transfer fluid reverses in concert with the compression
and expansion pulses of the working fluid.
5. A device as claimed in claim 4, wherein the reversal of
direction of flow of the heat transfer fluid is synchronized with
said pulses.
6. A device as claimed in claim 3, wherein the frequency of the
cyclical motion of the working fluid is the same as the frequency
of cyclical motion of the heat transfer fluid.
7. A device as claimed in claim 1, wherein the means for inducing
pulses in the working fluid is a positive displacement
compressor.
8. A device as claimed in claim 1, wherein the means for inducing
pulses in the working fluid comprises a valve switching system and
a compressor.
9. A device as claimed in claim 8, wherein the valve switching
system alternatively connects the body of absorbent material to
high and low pressure reservoirs of the working fluid.
10. A device as claimed in claim 1 comprising a further heat
exchanger adapted to remove heat of compression from the working
fluid before contacting the absorbent material.
11. A device as claimed in claim 1, wherein the temperature
gradient comprises a relatively high temperature at the inlet and a
relatively low temperature at the outlet.
12. A device as claimed in claim 1, wherein the working fluid is
selected from the group consisting of a vapour or gas or a mixture
thereof.
13. A device as claimed in claim 1 comprising a plurality of heat
exchangers.
14. A device as claimed in claim 1 in which the working fluid is a
single fluorocarbon or a mixture of fluorocarbons boiling between
-140.degree. C. and 40.degree. C.
15. A device as claimed in claim 14, wherein the working fluid has
a boiling point between -90.degree. C. and 0.degree. C.
16. A device as claimed in claim 15, wherein the working fluid has
a boiling point between -90.degree. C. and -20.degree. C.
17. A device as claimed in claim 1 in which the working fluid is a
hydrocarbon selected from the group consisting of methane, ethane,
propane, iso-butane and butane, and mixtures thereof.
18. A device as claimed in claim 1 in which the working fluid is
nitrogen.
19. A device as claimed in claim 1 in which the working fluid is
carbon dioxide.
20. A device as claimed in claim 1 in which the working fluid is
hydrogen.
21. A device as claimed in claim 1 in which the working fluid is a
noble gas.
22. A device as claimed in claim 1 in which the porous solid has at
least 10% of its void volume in the form of micropores with
diameters less than 2 nm.
23. A device as claimed in claim 22 in which the porous solid has
at least 10% of its void volume in the form of mesopores with
diameters less than 50 nm.
24. A device as claimed in claims 23 in which the porous solid has
less than 20% of its void volume in the form of macropores with
diameters greater than 50 nm.
25. A device as claimed in claim 1 in which the adsorbent porous
solid is a carbon-based material.
26. A device as claimed in claim 25 in which the adsorbent porous
solid is a charcoal material.
27. A device as claimed in claim 26 in which the adsorbent porous
solid is an activated carbon material.
28. A device as claimed in claim 25 in which the adsorbent porous
solid is an organic polymer-based material.
29. A device as claimed in claim 25 in which the adsorbent porous
solid is an essentially inorganic material.
30. A device as claimed in claim 29 in which the adsorbent porous
solid is the oxide of a metal or metalloid element, or combination
thereof.
31. A device as claimed in claim 29 in which the adsorbent porous
solid is a zeolite.
32. A device as claimed in claim 29 in which the adsorbent porous
solid is a sieve.
33. A device as claimed in claim 29 in which the adsorbent porous
solid is selected from the group consisting of silica, alumina,
titanium dioxide and mixtures thereof.
34. A device as claimed in claim 1 in which the adsorbent porous
solid is an aerogel.
35. A device as claimed in claim 1 in which the adsorbent porous
solid is impregnated with a low volatility solvent.
36. A device as claimed in claim 1 in which the adsorbent porous
solid includes an additive to enhance thermal conductivity.
37. A device as claimed in claim 1 in which the enthalpies of
adsorption and desorption in the heat exchanger are essentially the
same.
38. A device as claimed claim 1 wherein said pulses have duration
in the range of 1 second to 10 minutes.
39. A device as claimed in claim 36, wherein the duration is in the
range of 1 second to 30 seconds.
40. A device as claimed in claim 39, wherein the duration is in the
range of 1 second to 10 seconds.
41. A device as claimed in claim 1 wherein there is a pressure
gradient of working fluid between the inlet and outlet.
42. A device as claimed in claim 1 wherein the working fluid is a
mixture.
43. A device as claimed in claim 42, wherein the working fluid is a
mixture of a strongly adsorbed fluid and weakly adsorbed fluid.
44. A device as claimed in claim 42 wherein the working fluid is a
mixture of carbon dioxide and nitrogen.
45. A device as claimed in claim 42 wherein the working fluid is a
mixture of carbon dioxide and argon.
46. A device as claimed in claim 42 wherein the working fluid is a
mixture of carbon dioxide or ammonia and hydrogen, helium or a
mixture thereof.
47. A device as claimed in claim 42 wherein the working fluid is a
mixture of carbon dioxide and propane.
48. A heat pump device in which a temperature difference is
established between two heat exchangers by inducing cyclical
expansion and compression pulses in a working fluid vapour or gas
which passes through an adsorbent porous solid located between the
heat exchangers.
49. A device as claimed in claim 39 in which the temperature
difference between each heat exchanger and an external,
single-phase, heat transfer liquid is essentially constant.
Description
[0001] This invention relates to a heat pump device particularly
for air conditioning, refrigeration and heat pumping systems. The
device relates especially to systems that contain no fluids known
to have adverse effects on the stratospheric ozone layer or to have
high global warming potentials relative to carbon dioxide. The
device may provide a direct replacement for any apparatus that
currently employs a mechanical vapour recompression or
refrigerant/absorption solvent cooling or heat pumping system.
[0002] In this specification the term `heat pump` refers to any
powered device which moves heat from a source to a sink against a
thermal gradient. A refrigerator is a particular type of heat pump
where the lower temperature is required for an intended
application. The term `heat pump` may be also used in a more
limited sense than in this specification to describe a powered
device which moves heat from a source to a sink against a thermal
gradient where the higher temperature is required. The distinction
between a refrigerator and a narrowly-defined heat pump is merely
one of intended purpose, not operating principle. Indeed, many air
conditioning systems are designed to supply either heating or
cooling depending upon the user's need at a specific time.
[0003] Chlorofluorocarbons (CFCs e.g. CFC 11, CFC 12) and
hydrochlorofluorocarbons (HCFCs eg HCFC 22, HCFC 123) are stable,
of low toxicity and non-flammability providing low hazard working
conditions when used in refrigeration and air conditioning systems.
If released they permeate into the stratosphere and attack the
ozone layer which protects the environment from the damaging
effects of ultraviolet rays. The Montreal Protocol, an
international environmental agreement signed by over 160 countries,
mandates the phase-out of CFCs and HCFCs according to an agreed
timetable.
[0004] The CFCs and HCFCs have been superseded in new air
conditioning, refrigeration and heat pump equipment by
hydrofluorocarbons (HFCs eg HFC 134a, HFC 125, HFC 32) either as
pure fluids or as blends. To accelerate the phase-out of CFC and
HCFC existing units have also been retrofitted with appropriate HFC
blends. Although HFCs do not deplete stratospheric ozone they are
known to contribute to global warming. By the provisions of the
Kyoto Agreement governments have undertaken to limit or cease the
manufacture and release of these compounds. Some countries have
already decided that phase-out of HFCs should commence sometime
during the next decade and are actively promoting the development
of non-halogen containing fluids.
[0005] The fluids in devices intended to replace HFC-containing
units must have very low or preferably zero global warming
potentials. Preferably they should be compounds that are found
naturally and whose properties are well understood so that damage
to the environment from anthropogenic releases can be avoided.
Furthermore, devices should be at least as energy efficient as the
HFC containing units they are replacing to ensure that their
contributions to global warming due to fossil fuel power station
emissions are no greater. Preferably the devices should have better
energy efficiencies.
[0006] According to the present invention there is provided A heat
pump device comprising: [0007] at least one heat exchanger; [0008]
a body of porous adsorbent material the body having an inlet and an
outlet the body being disposed in thermal contact with the heat
exchanger, [0009] means for passing a working fluid through the
body, and [0010] means for inducing cyclical compression and
expansion pulses in the working fluid, to cause the working fluid
to flow from the inlet to the outlet to create a temperature
gradient in the body between the inlet and outlet.
[0011] The invention provides a heat pump device in which a
temperature difference is established by inducing cyclical
expansion and compression pulses in a working fluid vapour or gas
which passes through an adsorbent porous solid located in one or
more heat exchangers.
[0012] The device may be specifically arranged so that heat is
emitted at an elevated temperature at a first one location or end
of the heat exchanger and taken in at a lower temperature at a
second location or end of the heat exchanger.
[0013] In use of the device the working fluid enters the adsorbent
solid at one end under elevated pressure and is removed at lower
pressure or by suction at the second end.
[0014] The device may also include a heat transfer fluid.
[0015] In a preferred embodiment the device includes a heat
transfer fluid, means for passing the heat transfer fluid in
thermal contact with the heat exchanger, arranged so that the flow
of heat transfer fluid removes or adds heat from or to the heat
exchanger.
[0016] Preferably the direction of flow of the heat transfer fluid
changes with the compression and expansion pulses of the working
fluid.
[0017] More preferably the direction of flow of the heat transfer
fluid reverses in coordination with the compression and expansion
pulses of the working fluid.
[0018] In a particularly preferred embodiment the reversal of
direction of flow of the heat transfer fluid is synchronized with
said pulses.
[0019] The frequency of the cyclical motion of the working fluid
may be the same as the frequency of cyclical motion of the heat
transfer fluid.
[0020] The working fluid can be a vapour, gas or liquid preferably
a vapour or gas. In the first part of the cycle the heat transfer
fluid is caused to move in use from the low temperature end towards
the higher temperature end of the heat exchanger to remove heat
from the hot end of a heat exchanger and reject it into a suitable
heat sink. In a second part of the cycle the heat transfer fluid,
is caused to move in an opposite direction from the higher
temperature end towards the lower temperature end of the heat
exchanger where it is cooled before entering the refrigerated
volume. To achieve a cyclical operation the heat transfer fluid may
be caused to oscillate forwards and backwards along the heat
exchanger with same frequency as the cyclical compression and
expansion, that is pulsing, of the working fluid. When the working
fluid is being admitted to the porous solid under compression the
heat transfer fluid moves in the opposite direction to the working
fluid. When the working fluid is being removed from the porous
under suction the heat transfer fluid moves in the same direction
as the working fluid.
[0021] The means for inducing pulses in the working fluid may be a
positive displacement compressor.
[0022] Alternatively the means for inducing pulses in the working
fluid includes a valve switching system and a compressor.
Preferably the valve switching system alternately connects the body
of adsorbent material to high and low pressure reservoirs of the
working fluid.
[0023] Optionally but preferably the device comprises a further
heat exchanger adapted to remove heat of compression from the
working fluid vapour or gas before it contacts the adsorbent.
[0024] The hot and cold temperatures generated depend upon the
specific applications for which the embodiments of this invention
are used, in particular whether refrigeration or air conditioning
is required. In this context air conditioning is to be understood
to include both room cooling and heating. Devices that can provide
both heating and cooling depending upon the requirements of the
user are sometimes called reversible air conditioners.
[0025] Preferably the temperature gradient comprises a relatively
high temperature at the inlet and a relatively low temperature at
the outlet.
[0026] In an alternative embodiment the working fluid is a blend of
a relatively strongly adsorbed fluid and a relatively weakly
adsorbed fluid. These fluids do not strongly interact with each
other. The more weakly adsorbed fluid may serve to sweep the more
strongly adsorbed from the adsorbent during suction and carries it
to the adsorbent during compression. Examples of such combinations
are carbon dioxide/nitrogen and carbon dioxide/argon with activated
porous carbon as the adsorbent. A preferred embodiment the blend is
a combination of a relatively strongly adsorbed gas, especially
ammonia or carbon dioxide with one or both of the light gases
hydrogen and helium. The light gases have high thermal conductivies
compared to the heavier working fluid gases or vapours and thus
improve heat transfer to and from the adsorbent during the
adsorption and desorption of the heavier working fluid. A
combination of helium and carbon dioxide is especially preferred
because the blend is non-flammable.
[0027] In a further embodiment of this invention a blend of two or
more working fluids is selected such that one adsorbs more strongly
than the other. When the heat pump is operating under a relatively
light load the less strongly adsorbed fluid is at a higher
concentration in the circulating fluid than its concentration in
the blend originally introduced into the unit. Conversely the
concentration of the more strongly adsorbed component in the
mixture remaining on the adsorbent is greater than the original
blend concentration. When the heat pump is operating under a heavy
load the concentration of the circulating fluid contains a greater
proportion of the more strongly adsorbed fluid and the composition
of the circulating fluid approaches that of the loaded blend. By
using a blend with these properties the heat pump can adapt its
operation to changing loads and thus maximize its energy
efficiency. An example of such a blend is a combination of propane
and carbon dioxide used with a porous carbon adsorbent.
[0028] For single room air conditioning the heat transfer fluid
will be generally be air. In cooling mode the cold temperature will
be generally about 5 to about 15.degree. C. while hot temperatures
will be generally about 35 to about 60.degree. C. Cooling powers
will typically range from about 3 kW to about 100 kW. In heating
mode the output temperature to the room will be typically be about
20 to about 30.degree. C. and input temperatures from the outside
air typically about 2 to about 15.degree. C. Heating powers will
typically be about 4 to about 150 kW. Some devices may be designed
simply to provide heating and may generally be described as heat
pumps, although this is a more restricted use of the term than that
used in this specification.
[0029] For the air conditioning of large buildings with multiple
rooms such as hotels and office blocks the heat transfer fluid can
be water which maybe piped through each room where air will be
blown over the cold water piping to provide the required cooling.
This system is analogous to conventional chiller installations. The
temperature of the water fed into the system will be typically at
about 5-10.degree. C. and the water returning to the device will be
typically at about 10 to 15.degree. C. In heating mode the
temperature of the water leaving the device will be about 25 to
about 40.degree. C., while the return water will be typically about
15 to about 30.degree. C. Cooling powers typically range from 50 kW
to 10 MW. Chillers can also be used in process industries, for
example cooling condenser water in distillation equipment.
[0030] In one embodiment of the invention the device is used to
provide refrigeration typically at temperatures down to about
-30.degree. C. In this application it is preferable to use a
relatively low freezing point heat transfer liquid. In a further
embodiment of this invention equipment performance is optimised by
carrying out the heat pumping process over two or more stages. This
approach is especially advantageous for temperatures below about
-20.degree. C. Although carbon dioxide is a good refrigerant for
Rankine cycle heat pumps with condenser temperatures lower than
about 0.degree. C., it has a critical temperature of 31.degree. C.
and a high critical pressure of 72 bar. For this reason it is
unattractive for use in heat pumps where the heat output
temperature is above 0.degree. C.
[0031] An especially preferred embodiment comprises a conventional
heat Rankine cycle stage wherein carbon dioxide is the working
fluid and which pumps heat from a temperature in the range
generally about -55 to about -10.degree. C. and reject it at a
temperature generally in the range about -20 to about 0.degree. C.
to the low temperature side of a second stage device employing the
present invention. This second stage built according to this
invention rejects heat at temperatures of about 35 to about
70.degree. C. while operating at maximum pressures in the range
about 10 to about 30 bar, typical of current HFC based
refrigeration equipment.
[0032] The working fluid may be selected from any chemically stable
fluid that can be reversibly adsorbed onto and desorbed from a
suitable porous solid. Preferred fluids include carbon dioxide, air
and nitrogen. The working fluid may be a fluorocarbon or a mixture
of fluorocarbons boiling between -140.degree. C. and 40.degree. C.,
preferably between -90.degree. C. and 0.degree. C., more preferably
between -90.degree. C. and -20.degree. C. CFCs, HCFCs and EFCs are
acceptable in those territories where their use is permitted but
are not preferred because of their adverse environmental effects.
Preferred fluids are those that occur naturally. Hydrocarbons and
hydrogen can be used in applications where flammability is not an
issue. Ammonia is acceptable for applications where exposure to
humans and animals can be prevented. For applications where a
fluorinated fluid is preferred then HFCs, perfluoro-iodides and
unsaturated fluorinated compounds containing 2 to 6 carbon atoms
can be used with low global warming potentials relative to CO.sub.2
preferably less than 150, more preferably less than 100 and most
preferably less than 10. Preferred compounds are fluorinated
olefins. More preferred are fluoroolefins containing a
trifluorovinyl group. Even more preferred are fluoro-olefins
containing at least one hydrogen atom. Especially preferred are
fluoro-propenes and their blends. Where fluorinated compounds are
not acceptable, especially preferred working fluids are CO.sub.2
and N.sub.2 which combine low environmental impact with low
toxicity and non-flammability.
[0033] In the literature the term `porous solid` is used for
materials with a wide range of properties. Many solids have a very
limited porosity including the protective oxide layers found on
metals. In this specification the term, `porous solid` is used to
describe a material with a particular combination of
properties.
[0034] Firstly, the internal surface area of a preferred porous
solid is greater than about 10 m.sup.2g.sup.-1, more preferably
greater than about 100 m.sup.2g.sup.-1, most preferably greater
than about 1000 m.sup.2g.sup.-1.
[0035] Secondly, the void space in a preferred porous solid is
distributed between a combination of macro-, meso- and micro-pores.
The porous solid has at least 10% of its void volume in the form of
micropores with diameters less than about 2 nm, at least 5% of its
void volume in the form of mesopores with diameters less than about
50 nm.
[0036] Thirdly a preferred porous solid is capable of reducing the
pressure of the working fluid vapour or gas in contact with it,
i.e. it adsorbs the working fluid.
[0037] Fourthly, the adsorption process must be reversible, e.g. it
must be possible to desorb the working fluid by reducing its
pressure or by raising the temperature of the porous solid.
[0038] Fifthly, a preferred porous solid must be capable of
adsorbing the working fluid gas above its critical temperature.
[0039] A wide range of porous materials may be employed. Silica,
for example fumed silica, granular silica or aerogel silica,
including granular, monolith and flexible blanket aerogels may be
used. Natural or artificial glasses, ceramics or molecular sieves
may be used. Carbons which may be used include granules, monoliths,
fabrics, aerogels and membranes. Examples of porous carbons
suitable for this invention are described in PCT/GB01/04222 the
disclosure of which is incorporated into the specification by
reference. Various organic materials including
resorcinol-formaldehyde foams or aerogels, polyurethane,
polystyrene or other polymers in the form of foams and aerogels.
Polymers of intrinsic porosity in which the tailored pore sizes are
created by the 3-dimensional linking of appropriate precursor
molecules with constrained geometries are also suitable for this
invention. A range of composite materials are acceptable, including
silica-carbon composites.
[0040] Porous materials may be made by blowing polymeric foams, and
by sol-gel processes for manufacture of porous ceramics, silica or
other mineral aerogels or organic aerogels. Organic materials for
example coconut and coal, may be pyrolysed, and then further
processed for example by treating with steam, to produce activated
carbons. Polymer aerogels may be pyrolysed to produce carbon
aerogels. Hydrocarbons may be pyrolysed to produce carbon
membranes. Molecular sieves and carbon black or by plasma processes
such as the APNEP (Atmospheric Pressure Non-Equilibrium Plasma)
system developed by CTech Ltd. Carbon based materials, such as
activated carbons derived from biomass precursors, e.g. coconut
shell, are especially preferred since they are obtained from
sustainable resources, require minimal energy input in their
manufacture and effectively sequester atmospheric carbon dioxide as
carbon within heat pump devices. At the end of the working life of
a device such carbon adsorbents can be removed recovered and burnt
recovering their energy content, originally captured when the
biomass was formed, returning the carbon dioxide to the atmosphere.
Since the gas originated from this source the combustion is
CO.sub.2-neutral. Preferably the carbon adsorbent would be buried
in landfill, or in the subduction zones at boundaries of tectonic
plates, or recycled to new equipment thus ensuring that the carbon
is permanently removed from the atmosphere.
[0041] Inorganic porous materials may be obtained by thermolysis,
for example production of fumed silica from silicon tetrachloride
using an oxy-hydrogen flame or by plasma processes.
Organic-inorganic precursors may be processed by thermolysis to
produce molecular sieves. Natural mineral hydrates may be
thermalised, for example vermiculite and perlite.
[0042] In one embodiment of this invention a heat pumping device
comprises an adsorbent porous solid blend whose the properties vary
between the high and low temperature ends of the adsorbent tube
beds.
[0043] In a further embodiment of this invention the porous solid
is selected such that its permeability to gas flow along the tube
is sufficiently low to allow a pressure gradient to be generated
along the tube during compression and during suction. The
permeability of the adsorbent can be controlled in various ways.
For example the range of particle sizes can be selected according
to the length and cross-sectional area of the tube and the working
fluid flow rate to give the desired pressure gradient.
Alternatively the porous solid particles may be compressed into a
monolith with a suitable binder to give the required pressure
gradient.
[0044] The gas or vapour working fluid may be matched to preferred
porous solids such as: carbon (e.g. graphite, activated carbon,
charcoal, aerogel), silica (fumed, aerogel, alkylated aerogel),
alumina, alumino-silicates (molecular sieves), and organic polymers
(e.g. polystyrene, polyurethane, polyacrylate, polymethacrylate,
polyamines, polyamides, celluloses), metal sponges (e.g. Ni, Ti,
Fe), and metals or metal complexes supported on organic polymers or
carbon.
[0045] Not all these gases and their combinations with the
available porous solids may be appropriate. Although the CFCs and
the HCFCs continue to be manufactured and used in the developing
world their phase-out under the Montreal Protocol is already
occurring. In territories when continued use of these chlorinated
fluids is still legal then they can be used in combination with
activated carbon, silica, or an organic polymer. SO.sub.2 and HFCs
can be used with carbon, silica, alumina or an organic polymer,
especially those with "basic" atoms such as O and N or "acidic" H
atoms. In territories where phase-out of HFCs is not currently
being considered, then their use in the present invention is
acceptable, but not preferred because their global warming
potentials are much higher than some of the other gases listed
above. SO.sub.2 is not preferred because of its toxicity.
[0046] Hydrocarbons can be coupled with carbon, alkylated silica or
an organic polymer, especially a hydrocarbon polymer such as
polystyrene. Although more preferred than the halogenated fluids
and SO.sub.2, hydrocarbons are restricted to applications where the
appropriate precautions can be taken against their marked
flammability hazard, for example in large industrial applications
or low-inventory, hermetically-sealed systems such as domestic
refrigerators. A further disadvantage is that the enthalpy changes
associated with the sorption/desorption of hydrocarbons is less
than for more polar gases, notably CO.sub.2, SO.sub.2 and NH.sub.3.
In some territories hydrocarbons are also disliked because any leak
to the atmosphere where exposure to sunlight generates
"photo-chemical smog".
[0047] Hydrogen is readily sorbed and desorbed from various metal
alloys, notably those containing nickel. Hydrogen is preferred to
hydrocarbons because it will interact more strongly with metals
than hydrocarbons with the sorbents listed above. Like hydrocarbons
hydrogen reacts with atmospheric hydroxyl radicals that play a key
role in removing naturally-emitted hydrocarbons as well as man-made
pollutants such as HFCs. Increased hydrogen emissions can thus
indirectly increase globally warming.
[0048] Ammonia can be used with carbon, silica or with an organic
polymer. It is suitable for applications where its toxicity and
flammability can be controlled, for example large commercial and
industrial applications or low inventory, hermetically sealed
domestic applications.
[0049] The most preferred working fluid is carbon dioxide. Although
carbon dioxide derived from fossil fuel is the single largest
contributor to global warming the quantities required for this
invention would be very small. By obtaining carbon dioxide from a
natural source, such as biomass fermentation, any gas emitted from
the device would have zero contribution to global warming. Carbon
dioxide has low toxicity, is non-flammable and is readily adsorbed
by a variety of porous solids including carbon, silica and organic
polymers, especially those containing basic atoms such as O and
particularly N. The ability of porous solids to adsorb CO.sub.2 can
be enhanced by impregnating the solids with compounds containing
groups capable interacting with the fluid. Nitrogen and oxygen
containing substances can be employed. Amines, amides alcohols,
esters and ketones are preferred. More preferred are amines,
amides, and urethanes with high boiling points, preferably above
100.degree. C. Especially preferred are substances where molecular
mass per N atom is less than 200, preferably less than 100 and most
preferably less than 60. A particularly preferred substance is
poly-ethyleneimine.
[0050] For very low temperature refrigeration involving
temperatures below -55.degree. C. carbon dioxide is not practical
because its triple is -56.7.degree. C. For sub -55.degree. C.
temperatures N.sub.2 is preferred as a working fluid with
adsorbent, such as activated carbon. The preferred heat transfer
fluid is the atmosphere within refrigerated enclosure, which in
many cases will be air. This design will provide cooling in the
range -130.degree. to -40.degree. C. and will reject heat in the
range -55 to -25.degree. C. to a higher temperature stage.
[0051] In further embodiment of this invention blends of gases or
vapours can be employed provided that they do not chemical react.
Thus a hydrocarbon such as propane can be mixed with carbon
dioxide.
[0052] Preferably the temperature changes generated when the fluid
reversibly adsorbs and desorbs should be greater than 5.degree. C.
and more preferably greater than about 10.degree. C.
[0053] Seven important parameters may contribute to the temperature
change:
(a) the integrated heat of adsorption (IHA) measured between the
lowest and highest pressure between which the adsorbent operates;
(b) the heat capacity of the adsorbent (HCA); (c) the density of
the adsorbent (DA), (d) the internal surface area of the adsorbent
(SAA), (e) maximum operating pressure (MP), (f) the rates of
adsorption/desorption and (g) the thermal conductivity of the
adsorbent.
[0054] The integrated heat of adsorption (IHA) is a function the
interaction of the fluid with the porous solid and is defined as
the total heat generated when the fluid adsorbs onto the solid as
its pressure is raised from a lower pressure to a higher pressure.
The higher the IHA the stronger is the interaction of the fluid
with solid. Preferably IHA should be at least 50 kJ/kg and more
preferably greater than 100 kJ/kg. Most preferably the IHA,
expressed in units of kJ/mol, should be comparable with the latent
heats of condensation of existing refrigerants.
[0055] The higher the IHA the lower will be the pressure of the
fluid above the adsorbent. A maximum working pressure just below
approximately 2 bar at the heat rejection temperature is
advantageous in that it keeps the pressure at any point in the
device below the pressure at which pressure regulations apply. This
allows the device to be manufactured more cheaply. It does require
the use of high volume throughput compressor such as a centrifugal
compressor and this is especially suited to large water chillers
for example employed for air conditioning public buildings. An IHA
which reduces the fluid pressure over the adsorbent significantly
below 2 bar is not preferred since it increases the size of the
components, notably the compressor, without any economic
advantage.
[0056] In devices working with maximum operating pressures above
approximately 2 bar the IHA is preferably chosen such that the
pressure of the adsorbent at the lowest working pressure of the
device is not less than approximately 1 bar to prevent the ingress
of atmospheric gases which are not significantly adsorbed by the
porous solid. Preferably the IHA is chosen such that in a given
application the lowest operating pressure is not less than 1.5
bar.
[0057] A special advantage of the present invention is that it
allows even relatively small temperature changes below 5.degree. C.
induced by fluid adsorption and desorption to generate the required
substantial temperature differences between the ends of the
adsorbent tubes, for example a difference of 35.degree. which is
required to generate the cold and hot air temperatures of
10.degree. C. and 45.degree. C. typically required for air
conditioning applications. Despite this advantage larger
temperature changes facilitate heat exchange between the bed and
the external heat transfer fluid. Preferably changes on adsorption
and desorption are greater than 5.degree. C. and more preferably
greater than 10.degree. C. The higher the IHA the larger the
temperature change obtained. Lower adsorbent heat capacities (HCA)
also provide higher temperature changes. Preferably HCA is less
than 2.00 kJ/kgK and more preferably less than 1 kJ/kgK and most
preferably less than 0.8 kJ/kgK. Especially preferred are porous
carbon materials and metal adsorbents for hydrogen with HCAs less
than 0.75 kJ/kgK.
[0058] Although a group of adsorbents may have similar IHA their
adsorption capacities (CA) for a working fluid will depend upon the
numbers of active sites available per unit mass. The number of
active sites tends to be related to the internal surface area of
the porous solid (USA) accessible to the fluid molecules, thus the
higher ISA the greater the capacity of the solid per unit mass to
adsorb the fluid at a given pressure. ISAs of at least 1000
m.sup.3/g are most preferred.
[0059] Provided the IHAs, SHAs and ISAs for a series of adsorbents
with a specified fluid are similar the temperature changes will be
essentially independent of their densities (DA). But the
temperature changes also depend upon the heat capacities of the
materials from which the adsorbent tube is manufactured. The
quantities of these materials can be minimised by selecting porous
solids with high densities provided this does not affect the other
adsorbent physical properties discussed above. Furthermore the
quantity of the heat exchange fluid which removes heat from and
adds heat to the adsorbent tube can also determine the temperature
changes. Low inventories and high flow rates of the heat exchange
fluid are preferred.
[0060] High maximum adsorption pressures maximise the capacity of
the adsorbent for the working fluid. However as the pressure
increases the incremental capacity of the adsorbent diminishes
while the gauge of the pipe required to withstand the pressure
increases, with a consequent increase in mass and hence thermal
capacity of the heat exchanger. The latter reduces the magnitude of
the temperature changes obtained on adsorption and desorption. The
optimum maximum pressure depends upon the pressure/adsorption
properties of the porous solid. For the combination of
CO.sub.2/activated carbon the optimum working pressure is generally
around 20 bar.
[0061] The thermal conductivity of the adsorbent is important.
Porous solids, especially in particulate or granular form, have low
thermal conductivities consequently heat transfer during adsorption
and desorption limits the cycle time of the beds. In a preferred
embodiment the heat pump comprises one or more adsorbent tubes
which are long in comparison to their width or diameter and are
adapted for progressive removal or addition of heat from one end to
the other end. The ratio of tube length to diameter should be
preferably greater than about 5:1, more preferably greater than
about 10:1 and most preferably greater than about 20:1.
[0062] To improve the thermal conductivities of adsorbents they can
advantageously be composed partially or entirely from heat
conducting materials. The latter may include graphite, preferably
as flakes, fibres or foams; metal mesh, powder, wire or fibres,
preferably comprising high thermal conductivity metals such as
copper and aluminium; organic polymers with high thermal
conductivities, such as polyaniline and poly-pyrrolidine or
mixtures thereof. Such polymers, in at least one chemical form,
generally have good thermal and electrical conductivities.
[0063] In a preferred embodiment of this invention such thermally
conducting polymers containing basic nitrogen atoms and
constituting at least a proportion of the porous solid are used.
Such a porous solid may also contribute to the adsorption of carbon
dioxide. Compressing the porous solid into a monolith also improves
thermal conductivity.
[0064] Table 1 lists examples of various adsorbents and their
thermal conductivities. This demonstrates that the addition of a
heat conducting additive substantially improves the thermal
conductivity of an adsorbent.
TABLE-US-00001 TABLE 1 Thermal Conductivity Adsorbent W/(m K)
Consolidated zeolite 13X 0.58 Consolidated zeolite + expanded
graphite 5-15 Fused silica 1.3 Silica gel + 20-30% expanded
graphite 10-20 Monolithic carbon 0.27-0.60 Granular carbon 0.1
Monolithic carbon + aluminium laminate 20
[0065] Preferred adsorbents have thermal conductivities greater
than about 0.5 W/(m.K), more preferably greater than about 5
W/(m.K) and most preferably greater than about 50 W/(m.K).
[0066] The adsorbent heat exchanger configuration also influences
the ease with which heat is transferred to and from the porous
solid. An important requirement is to maximise the heat exchange
without increasing the thermal capacity of the metal components of
the heat exchanger so that temperature changes do not fall below
the preferred value of 5.degree. C.
[0067] The invention is further described by means of example but
not in any limitative sense with reference to the accompanying
drawings of which:
[0068] FIG. 1 is an illustration of a heat exchanger containing
adsorbent with an outer duct containing heat transfer fluid;
[0069] FIG. 2 is a cross-section through the heat exchanger
assembly shown in FIG. 1;
[0070] FIG. 3 is a cross-section through a multiple adsorbent tubes
contained within a cylindrical heat transfer liquid duct;
[0071] FIG. 4 is a cross-section through multiple adsorbent tubes
contained within an hexagonal heat transfer liquid duct;
[0072] FIG. 5 is a section of single adsorbent tubes showing
longitudinal heat transfer fins and spiral heat transfer fins;
[0073] FIG. 6 is an illustration of an adsorbent pipe heat
exchanger with an internal heat transfer fluid tube;
[0074] FIG. 7 is a cross-section of the heat exchanger illustrated
in FIG. 6;
[0075] FIG. 8 is a cross-section through an adsorbent tube
containing multiple heat transfer fluid tubes;
[0076] FIG. 9 illustrates a spiral wound heat adsorbent tube;
[0077] FIG. 10 is a cross-section a spiral wound heat exchanger
tube enclosed within a duct.
[0078] FIG. 11 is a schematic view of a first device in accordance
with the invention;
[0079] FIG. 12 is a schematic view of a second device in accordance
with the invention;
[0080] FIGS. 13a to 13g are views of a third device in accordance
with the invention; and
[0081] FIG. 14 is a schematic view of a fourth device in accordance
with the invention.
[0082] FIGS. 1 and 2 illustrate a first embodiment of the
invention.
[0083] In this embodiment the heat transfer fluid is a liquid
constrained to flow in an external duct concentric to the adsorbent
tube as shown in FIGS. 1 and 2. A heat transfer fluid duct 1
contains an adsorbent tube. The flow of working fluid through the
adsorbent tube is controlled by valves 1.2 and 1.3. When adsorbent
tube is under suction valve 1.6 is open and valve 1.7 is closed.
The working fluid flows out at 1.6 as shown in the diagram.
Simultaneously heat transfer fluid flows into the duct at 1.4 and
out at 1.5. When the unit is under compression 1.6 is closed and
1.7 is open. The heat transfer liquid flows in the reverse
direction, i.e. in at 1.5 and out at 1.4. In FIG. 2 adsorbent 2.1
containing tube 2.4, the heat transfer liquid 2.2 and the heat
transfer liquid duct 2.3 are shown. For a larger device a
multiplicity of absorber tubes can be contained within a single
liquid duct as shown in FIGS. 3 and 4. 3.1 and 4.1 are the
adsorbents, 3.2 and 4.2 the heat transfer fluids, 3.3 and 4.3, the
heat transfer fluid ducts, and 3.4 and 4.4 the adsorbent tubes. The
duct can have a circular, square, hexagonal or any other
cross-section that is appropriate for a specific application. The
external surfaces of the adsorbent tubes may have fins or other
projections to enhance heat transfer as shown in FIG. 5.
Longitudinal or spiral fins are preferred. In FIG. 5, components
5.1 and 5.4 are adsorbents, 5.2 and 5.5 are longitudinal and spiral
fins respectively, and 5.3 and 5.6 are the adsorbent tubes. Heat
transfer from the adsorbent to the walls of the internal wall of an
adsorbent tube may advantageously be promoted by incorporating
perforated metal plates, discs or other members composed of metal
mesh or fibre which are disposed perpendicular to the tube axis
within the adsorbent bed. For optimum heat transfer these should in
close contact with the inner wall.
[0084] In another embodiment of this invention the heat transfer
liquid flows through a tube within the absorption tube as shown in
FIGS. 6 and 7. Component 6.1 is the adsorbent containing tube. When
the adsorbent is under suction as shown in FIG. 6 valve 6.2 is open
and valve 6.3 is closed so that desorbed working fluid leaves at
6.6. Heat transfer fluid flows into the central tube at 6.4 and
leaves at 6.5. When the adsorbent is under compression 6.2 is
closed and 6.3 is open. Simultaneously heat transfer fluid flows in
at 6.5 and out at 6.4. FIG. 7 shows the adsorbent 7.1 surrounding
the heat tube 7.3 containing the heat transfer fluid 7.2. Heat
transfer between the adsorbent and the liquid tube may be enhanced
by heat transfer fins which are either perpendicular to the axis of
the tubes or are arranged in a spiral along the liquid tube.
Preferably the fins fit closely or are attached to the outer
surface of the liquid tube but are not in contact with the inner
surface of the adsorbent tube. This is shown in FIG. 7 where 7.3 is
in contact with heat transfer fin 7.5, shown partly cut away. 7.5
is perforated to allow the passage of working fluid. The adsorbent
is contained within the outer tube 7.4. The heat transfer fluid
tube can be circular in cross-section. Advantageously it can be
pressed into an elliptical cross-section which retains its surface
area but with reduced internal volume. This geometry provides a
higher linear velocity for a given volume flow of heat transfer
liquid resulting in an improved metal/liquid heat transfer
coefficient.
[0085] In one embodiment of this invention the gas flow is
advantageously constricted to generate a pressure gradient along
the adsorbent tube. This can be achieved by in various ways, used
alone or in various combinations. For example one method involves
the use of solid heat exchanger fins in the form of unperforated
discs perpendicular to the axes of the adsorbent and liquid tubes
providing small gaps between their edges and the inner wall of the
adsorbent tube through which the working fluid is constrained to
pass. In a second method gaps between the fins and the inner wall
are sealed by polymer gaskets, but the fins are perforated by small
holes which restrict the gas flow. By varying the numbers of fins
employed, the size of the gap between their edges and/or the
diameters and number of the perforations the desired pressure
gradient can be achieved.
[0086] In a further embodiment the inner wall of the adsorbent tube
is provided with a low thermal conductivity liner. This may inhibit
the flow of heat from the adsorbent to the wall of the adsorbent
tube. This arrangement has the advantage that during the thermal
cycling of the adsorbent the thermal capacity of the containing
tube does not significantly reduce the magnitude of the temperature
changes. The liner can also serve as container for the adsorbent
and heat transfer liquid tube allowing them to be assembled prior
to insertion in the adsorbent tube. A further advantage of this
design is that adsorbent tube, which is not required for heat
transfer, can be fabricated from materials such as mild or
stainless steels which are inherently stronger than copper or
aluminium, the metals generally favoured when high thermal
conductivities are preferred. Also, apart from cost and weight,
there is no constraint on the tube wall thickness selected which
can thus be chosen to resist high pressure. This is especially
advantageous when a multiplicity of heat transfer liquid pipes is
employed as shown in FIG. 8. Adsorbent 8.1 is contained within tube
8.4 from which is isolated by insulating material 8.6. 8.2 is heat
transfer fluid contained within tube 8.3. Heat transfer fin 8.5,
shown partly cut away, enhances heat transfer from the adsorbent to
the heat transfer fluid tubes. The liner is preferably fabricated
from a low thermal conductivity organic polymer. More preferred is
an open cell organic foam, such as polyurethane foam. Especially
preferred is an organic foam reinforced externally with a solid
polymer tube or surface layer to provide mechanical strength. In
one embodiment of this invention the containing tube for the bed is
fabricated from an engineering polymer such as
polyether-ether-ketone, preferably reinforced with a material such
as graphite fibre. Strong composites of this type are well known in
the aerospace industry and are preferred where light weight
construction is advantageous. In the context of this invention such
materials are preferred when light weight is required, for example
in vehicle cabin air conditioning devices. The tube carrying the
heat transfer liquid is preferably fabricated from a metal to
facilitate heat transfer. Preferably the tube is made from a high
thermal conductivity metal such as copper of aluminium. A further
advantage of this configuration is that the tube is exposed to an
external pressure of gas rather than an internal pressure. In this
mode copper and aluminium, which are less mechanically strong than
steel, are acceptable.
[0087] In another embodiment of this invention the adsorption tube
is fabricated in the form of a spiral and contained within the
annulus of two concentric tubes that form a liquid duct as shown in
FIGS. 9 and 10. Adsorbent 9.2 is contained within adsorbent tube
9.1. This design has the special advantage than it allows a long
effective length of adsorption bed to be contained within much
shorter actual length. In one embodiment the duct walls fit closely
to the adsorption tube as shown in FIG. 10. Adsorbent 10.1 is
contained within tube 10.2 which coiled around the inter duct wall
10.2 and enclosed by the outer duct wall 10.4. The heat transfer
liquid is constrained to flow through the spiral passageways formed
between the adsorption tube and the outer and inner ducts. This
configuration minimises the quantity of heat transfer liquid in the
heat exchanger and thus assists in keeping the temperature changes
as large as possible. To reduce heat gain or loss to the immediate
environment of the device the water duct can be advantageously
insulated, for example with a layer of polystyrene or polyurethane
foam or glass fibre, 10.5.
[0088] In a further embodiment of this invention the heat transfer
fluid is a gas, preferably air. This may be caused to flow along
the outer surface of the adsorbent tube. To provide good heat
transfer the outer surface of the tube is fitted with longitudinal
or spiral fins. Heat transfer from the adsorbent to internal wall
of an adsorbent tube may be advantageously promoted by perforated
metal plates or discs of metal mesh or fibre which are preferably
located perpendicular to the tube axis. For optimum heat transfer
these should in close contact with the inner wall. These discs can
also serve to constrict the working fluid gas flow to generate a
pressure gradient along adsorbent tube. For example if perforated
metal plates are used the number of plates and the diameter and
number of the perforations can be selected to give the desired
pressure gradient.
[0089] Good heat transfer between the adsorbent and the heat
transfer fluid is clearly desirable for optimum performance. Metal
adsorbents which can be used with hydrogen are especially
advantageous in that they have much higher thermal conductivities
than non-metallic materials such as carbon, zeolites and silica
gel.
[0090] The choice of the fluid/adsorbent combination may depend
upon a number of factors whose values must be selected to provide
an optimum performance for a given application and the design of
the adsorbent heat exchangers. The adsorbent can be contained in
tubes as described above. The adsorbent can be contained in sets of
tubes in parallel, each set simultaneously undergoing compression
or suction. Alternatively the adsorbent can be contained in sets of
tubes in series connected by pipes. While the pressure drop across
a single tube can be small a substantial pressure gradient can be
established across the tube series by incorporating restrictions to
the gas flow in the connecting pipes. In another embodiment of this
invention the adsorbent is contained within a pair of plates sealed
their edges and equipped with inlet and outlets at opposites ends
of the plates. Heat transfer fluid removes or adds heat by flowing
over the external faces of the plates. This mode of construction
produces an adsorption plate heat exchanger. Sets of these plate
heat exchangers can assembled in parallel into a module such that
the heat transfer fluid flows between each pair of heat
exchangers.
[0091] In a preferred embodiment of this invention a device
comprises a working fluid, a positive displacement compressor
driven by source of mechanical power, and a porous adsorbent solid
through which pulses of compressed working fluid are able to
expand. Cooling is produced by desorption of working fluid from the
porous solid by reducing the pressure of the gas in contact with
one end the solid while pressure induced adsorption of the working
fluid by the solid at the other end produces heating. Preferred
working fluid/adsorbent solid combinations are selected such that
the heats of adsorption and desorption are substantially greater
than the heat of compression. More preferably the heats of
adsorption and desorption should be comparable with the latent
heats of vaporisation of CFC, HCFC, HFC, hydrocarbon and ammonia
working fluids presently used for conventional Rankine Cycle based
devices which they are intended to replace.
[0092] One configuration for the device is shown in FIG. 11. The
compressor consists of piston 11.1, moving in cylinder 11.2, driven
by piston rod 11.3 attached to crank shaft 11.4, the latter being
powered by an electric motor or other motive source, which is not
shown. The compressor is fitted with two valves, 11.5 and 11.6. The
inlet or suction valve 11.5 opens when the swept volume between the
piston and the cylinder head is just below the pressure in heat
exchanger 11.7. This occurs when the piston is moving towards
bottom dead centre. Conversely the outlet or discharge valve 11.6
opens when the swept volume between the piston and the cylinder
head is just above the pressure in heat exchanger 11.9. This occurs
when the piston is moving towards top dead centre. Heat exchanger
11.8 rejects the heat of compression to atmosphere. Adsorbent
porous solid 11.10 interacts with the working fluid reducing its
pressure to a level for which the device is designed and providing
a resistance to its flow so that a pressure difference can be
maintained between 11.7 and 11.9. The device is charged with
sufficient working fluid to maximise the heat pumping capacity but
without compromising the pressure limitations of the design.
Typically the device will be able to withstand maximum operating
pressures of up to 30 barg. For lower cost devices the operating
pressure will preferably not exceed 20 barg.
[0093] The device operates in a cycle described by the following
steps starting from the state where the suction stroke of the
reciprocating compressor has just been completed and the
compression stroke is just about to start, i.e. bottom dead centre.
[0094] (a) The piston is driven into the cylinder simultaneously
raising the temperature and pressure of the gaseous working fluid
until the discharge valve 11.6 opens and compressed gas is
discharged into heat exchanger 11.8. [0095] (b) Heat exchanger 11.8
rejects the heat of compression to an air or water stream, or other
appropriate heat sink. [0096] (c) The cooled compressed gas then
enters the high temperature, porous solid-containing heat exchanger
11.9 where it is adsorbed and the heat of adsorption rejected to an
air stream which is thereby heated. [0097] (d) Working fluid
travels though the porous solid towards the low temperature heat
exchanger 11.7 under the influence of the pressure gradient
established across the solid by the compressor. [0098] (e) The
direction of the piston is reversed thus reducing the pressure in
the cylinder causing the suction valve 11.5 to open so that working
fluid is desorbs from the porous solid in the low temperature heat
exchanger 11.7. The heat of desorption is supplied by an external
air stream which is thereby cooled. [0099] (f) The direction of the
piston is again reversed, and starts to compress the gas causing
the suction valve to close thus completing the cycle.
[0100] For the device to operate successfully in the mode described
it is important that the pressure of the gas at any point in the
porous solid connecting 11.7 and 11.9 oscillates about a mean value
so that working fluid travels through the solid via a series of
adsorptions and desorptions induced by the compressor. This process
will provide the major contributions to the enthalpy changes in
11.7 and 11.9. To optimize the performance of the device the
external air stream should also oscillate along the heat exchanger
as shown in FIG. 11. These oscillations are phased such that during
desorption of CO.sub.2 by suction from a bed the air flows in the
same direction as the CO.sub.2. Conversely during adsorption of the
CO.sub.2 during compression the air flow is countercurrent to the
CO.sub.2 flow.
[0101] The reciprocating compressor shown in FIG. 11 could be
replaced by any positive displacement compressor, including a
rotary, sliding vane or diaphragm type. Compressors whose sliding
surfaces in the displacement volume are lubricated by a liquid
lubricant will require oil separators between the compressor and
the adsorption bed to prevent oil in fine droplet form fouling the
bed. Oil-free compressors are therefore preferred, i.e. compressors
whose moving surfaces in contact with the working fluid are not
lubricated in the displacement volume by a liquid lubricant.
Especially preferred are diaphragm compressors which operate nearer
to isothermal than isentropic conditions because of the effective
cooling of the working fluid through the large surface area of the
diaphragm and the compressor head augmented in some units by the
cooling of the circulating hydraulic oil that drives the diaphragm.
Diaphragm compressor energy efficiencies can be superior to those
of reciprocating compressors. Fluid leakage rate is much lower
because an excellent seal can be established between the diaphragm
and compressor case, while an oil-free reciprocating compressor
lacks the excellent sealing properties of the oil film of a
conventional reciprocating unit. For low duty applications, such as
domestic refrigerators and room air conditioning units, diaphragm
compressors have the advantage over conventional oil-filled
hermetic reciprocating and rotary units in the providing a
combination of an excellent gas seal with an external electrical
motor. The heat generated by the latter can be dissipated by a
simple cooling fan. In conventional hermetic systems motor cooling
is partly provided by the oil, which transfers heat to the casing,
and the refrigerant, which transfers heat to the condenser. By
removing requirement for internal cooling of the electric motor the
energy efficiency of the cycle can be improved.
[0102] Various compressor designs can be used, provided they are
configured to deliver pulses of compressed gas at the hot end of
the adsorption bed and remove pulses of expanded gas at the cold
end. FIG. 11 schematically illustrate one method for achieving this
which is especially suited to adsorbents which have high thermal
conductivities and thus good heat transfer from their bulk to the
external gas stream. Hydrogen/porous metal adsorbent combinations
in particular are suited for this purpose. The performance such
combinations can be enhanced by including periodic thermal breaks
in the adsorption bed, for example by including porous polymer
plugs along periodically along the bed.
[0103] FIG. 12 schematically shows a further embodiment of this
invention. A pressure difference is maintained between two vessels,
12.1 at low pressure and 12.2 at high pressure, by any type of
compressor 12.3 capable of achieving the desired pressure ratio.
This includes both pulsed positive displacement and continuous
delivery turbo/centrifugal types, if necessary multi-staged. Pulses
of compressed working fluid gas or vapour are delivered to the
hot/high pressure heat exchanger 12.4 of the adsorbent bed 12.5 by
periodically opening and closing powered valve 12.6. Pulses of
expanded working fluid are removed from the cold/low pressure heat
exchanger 12.7 by periodically opening and closing powered valve
12.8. The advantages of the design shown in FIG. 12 over that in
FIG. 11 include the independent phasing of the compression and
suction pulses and the ability to use continuous delivery
compressors. Heat exchanger 12.9 rejects the heat of compression to
atmosphere. The configuration shown in FIG. 12 allows the cycle
time of pressure/suction pulses applied to the bed time to be
significantly longer than the cycle time a positive displacement
compressor used to power the system. This design is especially
preferred when using adsorbents with lower thermal conductivities
than porous metals.
[0104] Porous solids, such as activated carbons, have excellent
adsorption capabilities for vapours and gases, but are poor thermal
conductors requiring long cycle times, e.g. >1 minute, to enable
heat to be taken in during the suction phase and heat to be
rejected during the compression phase. If operated in this manner
when the heat pump is being used for cooling it will only supply
cold during half of its operating cycle. This limitation is
overcome in a further preferred embodiment of this invention
containing two beds operating 1800 out of phase, such that as one
bed is under going suction/desorption while the other is undergoing
compression/adsorption. This embodiment, employing a pair of
adsorption tubes, is illustrated in FIGS. 13a to 13f. This device
is intended to air condition an enclosed space such as a room or
vehicle cabin. Cold air entering the enclosed space will typically
be 10 to 15.degree. C., while air exhausted to the outside
environment will typically be 35 to 60.degree. C. The adsorbent is
contained in two finned tubes, 13.1 and 13.2 enclosed in two air
ducts 13.12 and 13.13 shown in FIG. 13b. Air is driven through the
ducts by fans 13.5 and 13.9. Fan 13.9 sucks exhaust air from the
room being air-conditioned while fan 13.5 pulls in external air.
The flows from the two fans are periodically and simultaneously
alternated between 13.12 and 13.13 by operating the movable vanes
13.14 and 13.15 shown in FIG. 13c. In the latter the vanes are
shown such that the external air flow from 13.5 is being blown into
duct 13.13 while the exhaust room air is being directed through
duct 13.12. The dotted lines indicate the alternative positions of
the vanes. The working fluid is compressed by compressor 13.6 which
is driven by motor 13.16. The flow of the working fluid through the
equipment is controlled by the pressure equalisation valve 13.3 and
switching valves 13.7 and 13.8 which serve periodically to switch
the working fluid flow between 13.1 and 13.2, so that while one is
under suction the other is under compression. The heat of
compression is rejected via heat exchanger 13.10 over which air is
driven by fan 13.11. FIGS. 13d, 13e, and 13f, schematically showing
this design with multiple adsorption tubes arranged in two sets,
indicate its operation applied to a room air-conditioning device.
The working fluid is driven around the circuit by compressor 13.6.
To enhance heat transfer each bed consists of multiple parallel
tubes packed with adsorbent in order to maximise the surface area
exposed to the air stream. The operation of the cycle is described
by means of example, but not in any limitative sense, by starting
at point shown in FIG. 13d. Bed 13.1 is at the room exhaust air
temperature and contains working fluid adsorbed at the maximum
operating pressure, for example 20.degree. C. and 20 bar. 13.2 is
at the temperature of the external ambient air and contains working
fluid adsorbed at the minimum pressure in the system, for example
at 30.degree. C. and 1 bar. [0105] a. With no air flow over either
bed, with the compressor switched off and with valves 13.7 and 13.8
closed valve 13.3 is opened allowing working fluid to flow from
13.1 to 13.2. Alternatively the working fluid can be arranged to
pass through the compressor or another engine to equalise the
pressures with the advantage that useful work may be obtained from
the compressor or engine. In the resulting adiabatic process the
temperature of 13.1 falls below the exhaust air temperature.
Conversely the temperature of 13.1 rises above the external ambient
temperature as fluid is adsorbed. [0106] b. When the pressures have
essentially equalised 13.3 is closed. 13.7 and 13.8 are opened and
the compressor 13.6 is switched on so that working fluid is pumped
from 13.1 (desorption) to 13.2 (adsorption) (FIG. 13e). Ambient
external air flowing over 13.1 driven by fan 13.5 is thus cooled
and enters the room at the desired low temperature, for example
10.degree. C. Conversely an equal volume of exhaust room air is
removed from the room by fan 13.9 and flows over 13.2 where it is
heated to a temperature above that of the external ambient air
before being rejected to atmosphere. [0107] c. Because 13.1 and
13.2 are designed to have lengths substantially greater than their
diameters the temperatures of the air streams exiting from each
adsorbent containing heat exchanger will remain approximately
constant until 13.1 has essentially been heated to the ambient air
temperature along the whole of its length, while 13.2 has
essentially been cooled to the temperature of the exhaust air.
[0108] d. When this point has been reached the device will have
reached a condition similar to the initial condition (a) but with
13.1 now at low pressure and ambient temperature and with 13.2 at
high pressure and exhaust air temperature. In other words 13.1 and
13.2 have reversed their roles. [0109] e. The cycle continues as
described in (a) to (c) above until the device returns to its
initial condition. To achieve this result vanes 13.14 and 13.15 are
switched thus reversing the air flows through the ducts 13.12 and
13.13 in FIG. 13f. Simultaneously valves 13.7 and 13.8 are re-set
allowing the compressor to transfer the working fluid from 13.2 to
13.1.
[0110] In a further preferred embodiment of the device more than
one pair of adsorbent heat exchangers are used so that when the
pressure is being equalized between one pair of heat exchangers the
compressor continues moving working fluid between the two members
of a second pair of heat exchangers. This arrangement has the
advantage of providing effective continuous heat pumping.
[0111] In a further embodiment of this invention the adsorbent beds
are cooled and heated by a circulating liquid. FIG. 14 illustrates
one possible design for this device. The adsorbent tubes containing
internal heat transfer tubes are in a circuit with compressor 14.7,
pressure equalisation valve 14.11, flow switching valves 14.3 and
14.4, and heat exchanger 14.12 which rejects the heat of
compression. The liquid circuit incorporates eight fluid logic
diodes, arranged in two bridge rectifiers 14.5 and 14.6, a liquid
pump 14.10 whose input and output flows can be reversed
periodically, and two external heat exchangers 14.8 and 14.9. This
design allows liquid to pass through the external heat exchangers
14.8 and 14.9 in the same direction while allowing the liquid to
oscillate in the adsorbent heat exchangers. The heat transfer
liquid circuit is shown by solid lines in FIG. 14. The cycle
employed is essentially that described for the device in FIGS. 13d,
13e, and 13f. The working fluid is desorbed from one bed and
compressed on to the other by the action of compressor 14.7. The
dotted lines show the working fluid circuit. In one embodiment of
this invention the liquid passes through one or more tubes or pipes
within the adsorbent bed. Alternatively the liquid may flow through
a duct external to the adsorption containing tube. The device
represented in FIG. 14 is suited to a refrigerated enclosure, and
to a water chiller air conditioning system where significant
proportion of the air is recycled within the room or building.
[0112] A suitable heat transfer liquid requires a combination of
properties which are determined by its intended application. For
air conditioning air is an attractive option. When liquids are
employed they preferably have low viscosities to minimise pumping
energy. Preferably liquids should have dynamic viscosities less
than 0.025 Pas, preferably less than 0.01 Pas, and most preferably
less than 0.001 Pas. Provided the liquid circulation system is
suitably pressurized, liquids with a range of boiling points can be
considered. Preferably for operating convenience the liquid should
have a normal boiling point greater than highest temperature
reached by the adsorbent. The liquid must not freeze below the
lowest temperature generated within the device. Preferably the
liquid has a flash point greater that 100.degree. C., more
preferably greater than 130.degree. C. and even more preferably
greater than 200.degree. C. Most preferably the liquid should be
non-flammable. Preferred liquids include those already known to the
industry as secondary refrigerants. These materials include water,
brines, glycols, alcohols, hydrocarbon oils, silicone oils, and
halogenated compounds including partially fluorinated ethers,
perfluorinated ethers and chlorinated liquids. Where they are
mutually compatible these liquids may also be used in mixtures.
[0113] For low refrigeration temperatures down to -50.degree. C.
compositions with wide liquid ranges are require, while retaining
the desirable properties of flash points greater than 100.degree.
C. and normal boiling points greater than the highest temperature.
Preferred substances include esters and ethers containing 3 or more
carbon atoms which can be acyclic or cyclic. Preferred substances
include, but are not limited, to glycol- or polyol-cyclic
carbonates and cyclic ethers. Especially preferred are propylene
carbonate, ethylene carbonate and dimethylisosorbide. Blends
comprising esters, ethers, glycols with each other and with water
can also be used. The liquids may optionally contain additives
which enhance one or more desirable composition properties such as
lower freezing points, higher boiling points, lower viscosities or
higher flash points. If such additives were to be used alone they
would not be preferred, but are acceptable when used in mixtures
where they constitute less than 50% of composition by mass.
[0114] To avoid adverse environmental impacts compositions
containing fluorine or chlorinated substances these compounds
should preferably have very low vapour pressures or incorporate
reactive groups such as double or triple bonds that facilitate
their rapid destruction by reactive species in troposphere.
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