Thermoelectric refrigeration apparatus

A solid state thermoelectric refrigeration apparatus suitable for cooling various types of optical radiation detectors to desired operating temperatures, without the need for augmented cooling from any supplementary refrigeration apparatus. The refrigeration apparatus contains several thermoelectric cooling arrays of successively smaller size arranged in a generally pyramidal configuration, with a separate thermally-conductive base plate disposed between each adjacent pair of cooling arrays. The cooling arrays each include Peltier cooling elements arranged in a planar configuration. Polished, thermally-conductive, cup-shaped heat shields are attached to the successive base plates and arranged in a nested relationship, with each heat shield and associated base plate enclosing all of the successively smaller shields and base plates. Each base plate is thermally driven by the preceding thermoelectric cooling array, which also cools the associated heat shield. A window is mounted on an outermost shield which is attached to the hot side of the largest thermoelectric cooling array, thus heating the window to prevent moisture condensation. To minimize parasitic heat flow, electrical leads to an optical radiation detector, mounted within the apparatus and aligned with the window, are attached to the periphery of each of the thermally-conductive base plates, and the interior of the apparatus is preferably evacuated or back filled with a high molecular weight inert gas.

BACKGROUND OF THE INVENTION 
The present invention is directed to refrigeration devices, and, more 
particularly, to solid state thermoelectric refrigeration devices suitable 
for cooling optical radiation detectors. 
Many electronic devices, such as optical radiation detectors, operate 
properly at temperatures substantially below ambient room temperature. 
Some types of very low light level charge-coupled device (CCD) detectors, 
for example, operate well only at temperatures of about -60.degree. 
Centigrade. Low light level CCD detectors are commonly used in connection 
with deep space observational astronomy. 
Because of this requirement for sub-zero temperature operation, a number of 
types of refrigeration devices have been developed for cooling electronic 
circuits. One type of refrigeration device suitable for this use is based 
on the thermoelectric or Peltier cooling effect, by which heat is 
transferred across a junction of two semiconductors of dissimilar 
conductivity by passing a current through the junction. Thermoelectric 
refrigeration devices of this type typically include one or more cooling 
stages, each stage including an array of small individual semiconducting 
cooling elements, electrically connected in series and arranged so as to 
form two generally flat surfaces. When an electrical current is passed 
through the array, one of these surfaces is cooled and the other is 
heated. The heated surface has a generally greater heat density than does 
the cooled surface. 
Inherent limitations in the heat pumping capacity and efficiency of 
thermoelectric refrigeration devices have previously limited the use of 
such devices to applications requiring only a modest amount of heat 
pumping, at temperatures just slightly below ambient. To achieve colder 
temperatures, thermoelectric refrigeration devices have typically been 
used along with some other type of refrigeration device to pre-cool the 
thermoelectric device and to dissipate waste heat. Thus, thermoelectric 
cooling devices have previously been employed to refrigerate optical 
detectors such as low light level CCD detectors, but only when used in 
connection with an additional refrigeration device, such as an ethylene 
glycol gas expansion type refrigerator. 
Combined refrigeration devices of this sort, however, suffer from a number 
of shortcomings. The addition of an ethylene-glycol refrigerator, for 
example, obviates many of the special advantages derived from employing an 
entirely solid state thermoelectric refrigeration device. Whereas the 
thermoelectric refrigerator device typically has a long operational life 
that is virtually free of maintenance, an ethylene glycol refrigerator 
typically requires repeated maintenance throughout a relatively shorter 
operational life span. In addition, the need to merge together two 
different types of refrigeration devices leads to undue complexity. 
Accordingly, there still exists a need for a thermoelectric refrigeration 
apparatus suitable for cooling electronic circuits, such as low light 
level CCD detectors without augmented cooling from another refrigeration 
device. The present invention fulfills this need. 
SUMMARY OF THE INVENTION 
In general terms, the present invention provides a solid state 
ethermoelectric refrigeration apparatus suitable for cooling a variety of 
optical radiation detectors without augmentation by a separate 
refrigeration device. More particularly, the thermoelectric refrigeration 
apparatus of the present invention includes a series of progressively 
smaller thermoelectric cooling arrays arranged in a generally pyramidal 
configuration, with a separate thermally conductive base plate, disposed 
between and contacting each pair of adjacent cooling arrays, and with a 
thermally-conductive shield attached to the periphery of each base plate 
and arranged in a nested relationship, to encase all of the successively 
smaller thermoelectric arrays, base plates and shields. Each of the 
shields is also provided with an aperture to provide an optical path to an 
optical radiation detector mounted on the smallest, and coldest, cooling 
array. 
In another aspect of the present invention, an optical window is mounted on 
the outermost shield of the refrigeration apparatus, which is attached to 
the hot side of the base plate adjacent the largest, and hottest, 
thermoelectric cooling array. This arrangement obviates a need to 
electrically heat the window to minimize dew formation on the window and 
on the apparatus' exterior surface. 
In yet another aspect of the invention, the thickness of the thermally 
conducting base plates is selected so that an imaginary line intersecting 
the outer edges of adjacent thermoelectric cooling arrays generally forms 
an agle approximately equal to the thermal divergence of the base plate 
material. In addition, adjacent base plates are bolted together to 
compress the intervening thermoelectric cooling array, thus enhancing 
thermal transfer between the array and base plates. The periphery of each 
base plate is also slotted to allow pasage of electrical leads attached to 
the optical radiation detector and to reduce the temperature of these 
leads by the successively lower temperatures of the base plates. 
The novel features which are believed to be characteristic of the present 
invention will be better understood from the following description of the 
preferred embodiment, considered in connection with the accompanying 
drawings, wherein like numbers designate like elements. It should be 
expressly understood, however, that the drawings are for purposes of 
illustration and description only and are not intended as a definition of 
the limits of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
With reference now to the figures, and particularly to FIG. 1, there is 
shown a side sectional view of a preferred embodiment of a thermoelectric 
refrigeration apparatus 10 in accordance with the present invention. The 
refrigeration device 10 includes a series of progressively smaller arrays 
12, 13, 14, 15 and 16 of thermoelectric cooling elements oriented in a 
generally pyramidal configuration. The successive arrays are sandwiched 
between a series of circular, thermally conductive base plates 18, 19, 20, 
21, 22 and 23. These base plates, likewise, have progressively smaller 
sizes and, thus, form a pyramidal shape. In the orientation of FIG. 1, the 
largest base plate 18 is located at the bottom and the smallest base plate 
23 at the top. A radiation detector 25, such as a CCD array, is mounted on 
the upper surface of the smallest base plate 23. 
In use, an electrical current is conducted through each of the cooling 
element arrays 12-16. This effectively pumps heat from the upper surface 
to the lower surface of each array. The placement of the arrays in their 
stacked arrangement, with intervening base plates 18-23, enables the 
uppermost base plate 23 to be cooled substantially below ambient 
temperature. 
Thin, thermally conductive shields 27, 28, 29, 30 and 31 are individually 
attached to the peripheries of the respective base plates 18, 19, 20, 21 
and 22. The shields are shaped like inverted cups, with the diameter of 
each corresponding to the diameter of its associated base plate. The 
successive shields, therefore, nest together, with the largest shield 27 
on the outside and the smallest shield 31 on the inside. The shields all 
include aligned apertures in their upper, circular walls, to define an 
optical path to the radiation detector 25, To control the environment and 
for enhanced thermal insulation, windows 33 and 35 cover the apertures of 
the outermost shield 27 and innermost shield 31, respectively. 
Additionally, an air cooled radiator 37 is attached to the lower side of 
the largest (and hottest) base plate 18. This radiates away heat extracted 
from the interior of the apparatus 10, as well as waste heat generated by 
the thermoelectric cooling arrays 12-16. 
The thermoelectric cooling arrays 12-16 are conventional single stage 
arrays of Peltier cooling elements and are available from a number of 
vendors, such as, for example, Melcor Corporation. Because of the 
nonlinear heat pumping capacity of Peltier cooling elements, the 
thermoelectric cooling arrays must have increased areas, in the downward 
direction of heat flow, to accommodate the increasing amount of waste heat 
being conducted away from the overlying cooling arrays. Thermal energy 
absorbed by each of the shields 28-31 also must be removed from the 
apparatus 10 by the successively larger thermoelectric cooling arrays. 
To enhance the flow of thermal energy at the interfaces between the 
interleaved cooling arrays 12-16 and base plates 18-23, layers of 
conventional thermally-conductive grease are preferably applied to their 
abutting surfaces. In addition, the thermal conductivity of these 
interfaces can be significantly enhanced by applying a substantial 
compressive force. The base plates 18-23 are therefore bolted together to 
compress the intervening thermoelectric cooling stages 12-16 and thus 
maximize the pressure at each interface. This arrangement additionally 
provides an advantageously high immunity to mechanical shock. As 
illustrated in FIG. 1, a plurality of bolts 40 interconnect each adjacent 
pair of base plates so as to compress the intervening thermoelectric 
cooling array. 
Preferably, the bolts 40 are made of stainless steel or some other material 
having high tensile strength and low thermal conductivity. Since the bolts 
unavoidably bridge the successive thermoelectric cooling arrays 12-16, 
they provide a thermal path for heat to flow from the heated side to the 
cooled side of each array. The use of bolts having low thermal 
conductivity, therefore, minimizes a parasitic heat flow that would 
otherwise reduce the arrays' net heat pumping capacity. To further 
minimize this parasitic heat flow, washers 44 made from low thermal 
conductivity materials such as nylon also are used, to insulate the bolts 
from the base plates. 
While the bolt material is selected to minimize heat flow, the material of 
the base plates 18-23 is selected so as to maximize heat flow. The base 
plates are, therefore, preferably made of highly thermally conducting 
materials such as silver, copper or aluminum. 
Additionally, the thickness of each of the successive base plates is 
selected to maximize the flow of thermal energy from the heated side of 
overlaying thermoelectric cooling array to the cooled side of the 
underlying thermoelectric cooling array. As illustrated in FIG. 4, an 
angle formed by a line 48 intersecting the outer edges of two exemplary 
thermoelectric cooling arrays 12 and 13, adjoining the base plate 18, 
preferably approximates the steady state thermal divergence of the 
particular material from which the base plate is formed. In the case of a 
base plate formed of cooper, for example, this angle is preferably about 
45.degree.. 
As illustrated in FIGS. 1 and 3, the base plates 18, 19, 20, 21 and 22 are 
provided with sets of electrical feedthrough terminals 56, 57, 58, 59 and 
60, respectively, to provide electrical connections for supplying current 
to the respective cooling arrays 12, 13, 14, 15 and 16. A second set of 
feedthrough terminals 61 provides electrical connections for leads (not 
shown) connected to the radiation detector 25. 
The peripheries of the intermediate base plates 19-22 are further provided 
with slots 65 adapted, to receive the electrical leads (not shown) 
connected to the radiation detector 25. These leads potentially can 
provide a significant parasitic heat flow from the exterior of the 
refrigeration apparatus 10 to the detector 25. Accordingly, the leads are 
disposed within the slots 65 to successively reduce the lead temperature 
and thus minimize the heat flow to the detector 25. This arrangement also 
reduces the thermal load on the smallest, coldest array 16, which has the 
lowest heat pumping capacity of all the arrays 12-16. 
As illustrated in FIG. 1 the heat shields 28, 29, 30 and 31 are attached to 
the outer edges of the thermally-conducting base plates 19, 20, 21 and 22, 
respectively. Since the base plates 19, 20, 21 and 22 are connected to the 
cold sides of the respective cooling arrays 12, 13, 14 and 15, the shields 
are thermally driven so that a portion of the thermal energy absorbed by 
each heat shield is removed through the underlying cooling stage. The 
shields are preferably made of a highly thermally-conductive material such 
as aluminum, copper or silver and are preferably very highly polished to 
minimize heat absorption. They can be attached to the edges of the base 
plates by any convenient means such as, for example, threaded bolts (not 
shown). A thermally-conductive grease may also be used at each interface 
between the heat shields and the base plates, to enhance the transfer of 
thermal energy. The use of heat shields in association with each of the 
base plates 19-22 provides for maximum cooling by the refrigeration 
apparatus 10; however, about 90 percent of that maximum cooling capacity 
can be achieved using only the two heat shields 28 and 31 and eliminating 
the intervening heat shields 29 and 30. 
The outermost heat shield 27 functions as an outer diameter shell for the 
refrigeration apparatus 10. It is attached to, and is in thermal contact 
with, the base plate 18, which is, in turn, in thermal contact with the 
heated side of the first cooling array. Since all of the thermal energy 
being extracted from the interior of the refrigeration apparatus 10, along 
with waste heat from the successive thermoelectric cooling arrays 12-16, 
flows through the base plate 18 to be dissipated by the air-cooled 
radiator 37, the outermost shield 27 will be at a higher temperature than 
the ambient air. This heats the window 33, mounted over the aperture in 
the outermost shield, and prevents moisture from condensing on the window, 
thus obviating the need for a low humidity environment. 
To maximize the cooling capacity of the refrigeration apparatus 10, and to 
prevent internal moisture condensation, the interior of the apparatus may 
further be back filled with a dry, inert gas. The insulating capacity of 
this inert gas improves with molecular weight. Thus, back filling the 
apparatus with xenon or argon gas provides greater insulation and a higher 
net cooling capacity for the apparatus than lighter gases such as nitrogen 
or helium. The shields 28-31 advantageously disrupt the formation of 
convection currents in this back fill gas. 
It should be appreciated from the foregoing that the present invention 
provides a reliable refrigeration apparatus suitable for cooling various 
radiation detectors to temperatures of less than about -70.degree. 
centrigrade. The apparatus needs little or no maintenance during its 
operational life and does not operation. It will, of course, be understood 
that modifications to the presently preferred embodiment will be apparent 
to those skilled in the art. Consequently, the scope of the present 
invention should not be limited by the particular embodiment discussed 
above, but should be defined only by the claims set forth below and 
equivalents thereof.