Oxygen chemisorption cryogenic refrigerator

A chemisorption refrigeration system includes dual containers containing a material such as silver which is alternately heated and cooled to chemically desorb and reabsorb oxygen gas. The gas is desorbed at high temperature and pressure and is pre-cooled and then passed through a Joule-Thomson valve where it is expanded and partially liquefied to provide cooling at 60.degree.-100.degree. K. The liquefied oxygen is then boiled and returned to a cooled container where it is reabsorbed. By alternately heating and cooling the containers, a continuous source of high pressure high temperature oxygen can be provided.

2. Field of the Invention 
This invention relates to cryogenic refrigeration systems with minimal 
moving parts for use in aerospace applications such as the cooling of 
infrared sensors. Mechanical refrigeration systems are unsuited for 
aerospace long-term operation not only due to servicing difficulties and 
limited lifetime, but more importantly because the compressors and related 
components introduce vibration into the system. 
3. Description of the Prior Art 
Refrigeration systems employing non-mechanical compressors have been 
developed in the past. One type of system employs a charcoal/nitrogen 
physical adsorption system in which nitrogen is physically adsorbed onto 
charcoal and then heated to release high pressure nitrogen which is 
subsequently reduced in pressure to lower the temperature to provide 
refrigeration. Such systems have not found wide use due to very 
inefficient operation. Typically, about 200 watts of heat per watt of 
cooling may be required in such a system. 
Hydride chemical absorption (chemisorption) cryogenic refrigeration systems 
have also been developed. Such systems make use of well-known reversible 
chemical reactions between hydrogen and various materials. Upon cooling of 
the material, hydrogen is chemically absorbed. When heated, the hydrogen 
gas is released at a high temperature and high pressure. The high pressure 
gas is then passed through a Joule-Thomson expansion valve to reduce the 
pressure and temperature of the gas. 
A major drawback of hydride refrigeration systems is that their operating 
temperatures are extremely low and the hydrogen gas must be pre-cooled 
prior to being applied to the expansion valve. Above certain temperatures, 
expanding the hydrogen gas will actually cause it to increase in 
temperature rather than decrease. The necessary pre-cooling may be 
accomplished by the addition of a mechanical refrigeration system with the 
attendant problems of vibration and servicing, or by employing a coolant 
such as liquid nitrogen. The latter approach suffers from the disadvantage 
of employing an expendable material, which has obvious disadvantages in an 
aerospace environment where long life is desired. 
A hydride chemical absorption refrigeration system is described in Design, 
Life Testing And Future Designs Of Cryogenic Hydride Refrigeration 
Systems, J. A. Jones and P. M. Golben, Cryogenics 1985, Vol. 25 April. The 
system described in this article includes a liquid nitrogen source for 
providing pre-cooling of high pressure gas which is to be provided to a 
Joule-Thomson valve for expansion and further cooling. The article also 
mentions that chemical absorption using nitrogen or oxygen might be 
feasible for the upper stage of refrigeration. 
A cryogenic adsorption refrigerator employing a Joule-Thomson valve is 
disclosed in U.S. Pat. No. 4,366,680 to Tward. The system disclosed in 
this patent employs a gaseous refrigerant which is connected by means of 
heat switches to a thermal load in order to provide the desired cooling. 
U.S. Pat. No. 4,346,563 to Hood discloses a cryogenic refrigeration system 
employing a mechanical compressor for compressing helium gas, which gas is 
subsequently pre-cooled and then cooled by expansion to partially liquefy 
the helium. Gaseous helium is drawn off from the liquid bath and recycled 
through heat exchangers back to the compressor. A cryogenic adsorption 
refrigeration system is described in U.S. Pat. No. 4,183,227 to Bouvin et 
al. A cryogenic liquid absorption system is disclosed in U.S. Pat. No. 
3,854,301 to Cytryn. 
SUMMARY OF THE INVENTION 
The present invention is directed to a cryogenic chemisorption 
refrigeration system which may be used as the upper stage of a hydride 
refrigeration or which may be used independently to provide cooling in the 
temperature range of approximately 55.degree.-100.degree. K. The system 
includes at least two containers for containing a material which 
reversibly chemically reacts with oxygen to absorb oxygen at relatively 
low temperatures and pressures and release oxygen at high pressure when 
heated. The containers are alternately heated and cooled to provide a 
continuous source of high pressure, high temperature oxygen. This oxygen 
is then subjected to pre-cooling and passed through a Joule Thomson 
expansion valve to decrease the pressure of the oxygen and cause it to 
partially liquefy. A collection vessel is provided for the liquid oxygen. 
The liquid oxygen is used for cooling such as for the cooling of an 
infrared sensor. The oxygen will then boil and be returned to the 
containers for chemical absorption onto cooled absorbent material. 
The present invention takes advantage of substantial research in which it 
has been determined that oxygen will reversibly react with several 
materials in temperature and pressure ranges which renders it suitable for 
use in a cryogenic refrigeration system. Specifically, it has been 
determined that oxygen will chemically absorb to several materials at a 
temperature in excess of 0.degree. C. (273.degree. K.) at pressures less 
than five atmospheres, and will be rejected at a temperature of less than 
about 700.degree. C. (973.degree. K.) at a pressure of 10-100 atm. 
Previously, it had been thought that oxygen reacted in an irreversible 
manner, i.e., once it combined with an element or compound the reaction 
could not be reversed to release gaseous oxygen. However, research has 
indicated that oxygen will indeed react in a reversible fashion with 
certain materials and will do so at temperatures and pressures which make 
it practical for incorporation into a cryogenic refrigeration system.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The following description is of the best presently contemplated mode of 
carrying out the invention. This description is made for the purpose of 
illustrating the general principles of the invention and is not to be 
taken in a limiting sense. The scope of the invention is best determined 
by reference to the appended claims. 
Referring to FIG. 1, a refrigeration system according to the present 
invention includes first and second containers 10 and 12 made of a high 
melting point material, for example, titanium. The containers 10 and 12 
function together to form a compressor to provide high pressure oxygen 
gas. Each container includes a high surface area substrate material 14 
such as zeolite coated with a very thin layer 16 of material which will 
chemically absorb and desorb oxygen. Preferably, the layer of absorbant 
material is of unimolecular thickness to minimize weight and maximize 
efficiency. 
The requireient for the absorbent material 16 is that it be a solid which 
chemically absorbs oxygen at low pressures at approximately 0.degree. C. 
(273.degree. K.) or above, and secondly, when the material is heated, gas 
must be liberated or desorbed at higher pressures from the solid. This 
fully reversible wear-free chemical reaction cycle is termed a 
chemisorption compressor cryogenic refrigeration cycle. For practical 
applications, the cycle must be fully reversible and capable of continuous 
operation for a minimum period of ten years. It is preferable that the 
full cycle occur in less than one hour to minimize mass of the material 
Thorough studies were made of all commercially available gases with boiling 
points between 60.degree. K. and 100.degree. K., including oxygen, 
nitrogen, carbon monoxide, fluorene and argon to determine if any of them 
had suitable characteristics for a chemisorption refrigeration system. It 
has been generally accepted that reactions with these gases are 
irreversible, i e., once the gas has reacted to combine with another 
material it cannot be liberated from the material. Indeed, studies by the 
inventor have indicated that the vast majority of reactions with these 
gases are irreversible. However, it has been determined that oxygen has 
relatively efficient and reversible chemisorption in the temperature and 
pressure range of interest when combined with several different materials. 
Suitable material combinations with oxygen include: 
(a) K.sub.2 O.sub.2 (potassium oxide) which reacts to form K.sub.2 O.sub.3 
(b) Ag (silver metal) which reacts to form Ag.sub.2 O 
(c) SrO (strontium oxide) which reacts to form SrO.sub.2 
(d) PbO (lead oxide) which reacts to form PbO.sub.2 
(e) Li.sub.2 O (lithium oxide) which reacts to form Li.sub.2 O.sub.2 
In the system shown in FIG. 1, silver (Ag) is indicated as being the 
material contained in the containers 10 and 12. A heater 18 is provided to 
heat the material in the container 10, and a heater 20 is provided to heat 
the material in the container 12. Heat may be provided by electric heat, 
solar heat, waste heat or by other means. In order to cool the containers 
after heating, radiators 22 and 24 are provided. 
The refrigeration system of the invention operates so that as one of the 
containers 10 and 12 is being heated the other container is being cooled. 
Upon cooling of the material 16 to approximately room temperature 
(300.degree. K.), significant amounts of oxygen are chemically absorbed at 
low pressure (1 atm or below). When heated, the oxygen is liberated at a 
higher pressure (100 atm or above) and temperature (typically 
400.degree.-800.degree. K.). The heated gas is then pre-cooled to about 
room temperature and passes through a Joule-Thomson expansion valve 26. 
The expansion of the oxygen results in cooling which partially liquefies 
the gas at cryogenic temperature (90.degree. K. for 1 atm pressure or 
80.degree. K. for 0.3 atm pressure). The liquid oxygen is collected in a 
vessel 28, which is in turn coupled to an external infrared 
sensor/detector system or other system requiring cooling. Heat from the 
system being cooled passes internally causing the oxygen to boil from the 
cryogenic liquid state. The cold, low pressure evaporated oxygen gas then 
passes back to the containers 10 and 12 where it is reabsorbed by the 
cooled material in one of the containers. 
Details of the refrigeration cycle will now be described. Initially, it is 
assumed that the material in the container 10 is being heated and the 
material in the container 12 being cooled. A check valve 30 allows high 
pressure heated oxygen to escape from the container 10, where it is 
pre-cooled from, e.g., approximately 675.degree. K. to 450.degree. K. by 
means of a first radiator 40. The pre-coole-d gas then flows through a 
counterflow heat exchanger 42 where it is cooled to about 255.degree. K. 
Additional pre-cooling by a radiator 44 lowers the temperature to 
250.degree. K. before the gas enters a second heat exchanger 46. 
A small amount of additional heat is rejected to a thermoelectric cooler 48 
having a cold junction maintained at 185.degree. K. and a hot side at 
250.degree. K. The thermoelectric cooler helps lower the oxygen 
temperature to improve overall cooling performance. The oxygen is then 
passed through a third heat exchanger 50 and the Joule-Thomson expansion 
valve 26. When the oxygen expands to a pressure of just above 0.3 atm, it 
partially liquefies and produces cooling at 80.degree. K. Lower pressures 
result in lower temperatures, but heat exchanger efficiency becomes 
greatly diminished. 
External heat from a cryogenic sensor or other system being cooled passes 
inward to boil the liquid oxygen in the vessel 28, and the resulting cold, 
low pressure vapor travels back through the counterflow heat exchangers 
50, 46 and 42 via conduit 52, thus pre-cooling the entering high pressure 
oxygen gas passing in the opposite direction through each heat exchanger. 
The gas then passes through a check valve 36 and is reabsorbed onto the 
cool 450.degree. K. (177.degree. C.) silver in the container 12. 
Once the desorbing and reabsorbing cycle is completed, the heating and 
cooling operation on the containers 10 and 12 is reversed so that the 
material in the container 12 is heated and the material in the container 
10 is cooled. As a result, high pressure gas will pass through a check 
valve 34 to exit the container 12 and low pressure gas will enter the 
container 10 via a check valve 32. By alternately heating and cooling the 
material in each of the containers 10 and 12, a continuous source of high 
temperature, high pressure oxygen can be provided. 
It has been calculated that the above-described system requires about 78 
watts of heat to the silver sorbents and about 12 watts of heat to the 
thermoelectric cooler for the generation of 1 watt of cooling at 
80.degree. K. Although this is generally not as efficient as some of the 
more efficient mechanical systems (with total efficiency as high as 40 
W/W) the waste heat from the present system can be used to power a lower 
temperature hydride or other refrigeration system. Heating of the 
containers 10 and 12 can be supplied by efficient solar concentrators or 
by a radioactive power source waste heat. 
Thus, the present invention provides relatively efficient cooling in the 
range of 60.degree.-100.degree. K., with the system being usable by itself 
or as an upper stage to provide pre-cooling for a hydride refrigeration 
system. The system provides vibration-free, long life operation by 
employing reversible chemical reactions of oxygen to obtain a continuous 
source of high pessure, high temperature oxygen.