Apparatus for controlling a thermostatic expansion valve

Apparatus for controlling a heating or cooling system of the type that includes an evaporator and a thermostatic expansion valve having a thermostatic bulb. The control apparatus comprises a thermoelectric heat pump device responsive to an electrical control signal for controlling transfer of thermal energy to and from the thermostatic bulb, at least one temperature sensing device associated with the heating or cooling system, and an electronic control circuit. The control circuit is responsive to the sensed temperature to provide the electrical control signal to the thermoelectric device for maintaining a desired operating condition of the evaporator. The thermoelectric device acts as a gate or heat pump for controlling flow of thermal energy to and from the thermostatic bulb with a relatively small amount of electrical energy input. Preferably, the thermoelectric heat pump device is positioned between the thermostatic bulb and the suction line of the evaporator so as to control transfer of thermal energy between the suction line and the thermostatic bulb. Preferably, the control apparatus senses the temperature difference between the suction line and the inlet line of the evaporator and maintains the temperature difference within prescribed limits.

FIELD OF THE INVENTION 
This invention relates generally to heating and cooling systems, including 
heat pumps, air conditioners and refrigeration systems, and, more 
particularly, to electronic apparatus for controlling thermostatic 
expansion valves that are utilized in such systems. 
BACKGROUND OF THE INVENTION 
Heating and cooling systems such as heat pumps, air conditioners and 
refrigeration systems normally include an evaporator having an inlet line 
for receiving a liquid refrigerant from a condenser and an outlet line, or 
suction line, for carrying the vaporized refrigerant to a compressor. As 
refrigerant passes through the evaporator, it is converted by heat 
absorbed from the surroundings from liquid form to a vapor. Devices 
utilized to meter flow of refrigerant through the inlet line into the 
evaporator include the thermostatic expansion valve, the short tube 
orifice and the capillary tube. The thermostatic expansion valve includes 
a flow control valve that is opened or closed by a diaphragm, and a 
thermostatic bulb connected to the valve by a capillary tube. The 
thermostatic bulb and the interconnecting tube contain a thermally 
sensitive charge. Many types of charges are used in thermostatic expansion 
valve bulbs. Examples of charges include liquid and liquid cross-charges, 
as and gas cross-charges and adsorption charges. When the thermostatic 
bulb is heated or cooled, the pressure of the charge acts on the diaphragm 
and opens or closes the valve. Further details regarding thermostatic 
expansion valves are provided in the ASHRAE 1988 Equipment Handbook, pages 
19.2-19.8. 
In a conventional system, the valve portion of the thermostatic expansion 
valve is located in the inlet line to the evaporator and the thermostatic 
bulb is in thermal contact with the suction line, so that the flow of 
refrigerant into the evaporator is controlled in response to the 
temperature of the refrigerant vapor in the suction line. Typically, for 
low pressure drop evaporators, the vapor in the suction line is several 
degrees warmer than the liquid refrigerant entering the evaporator through 
the inlet line. The term "superheat" means raising the temperature of the 
refrigerant vapor above the temperature required to change the refrigerant 
from a liquid to a vapor at a specified pressure level. For low pressure 
drop evaporators, the superheat is approximately equal to the difference 
in temperature between the vapor in the suction line and the refrigerant 
in the inlet line. Typically, a superheat on the order of 
8.degree.-20.degree. F. is required for proper operation of a thermostatic 
expansion valve. If the superheat drops below a prescribed value, 
indicating that the refrigerant is not being fully evaporated, the 
thermostatic expansion valve reduces the flow of refrigerant into the 
evaporator until the superheat returns to the prescribed value. 
Conversely, when the superheat exceeds the prescribed value, indicating 
that the refrigerant vapor is being overheated, the thermostatic expansion 
valve increases the flow of refrigerant into the evaporator. 
Various problems have been associated with systems wherein a thermostatic 
expansion valve is used to control the flow of refrigerant into an 
evaporator. The superheat required for operation of the thermostatic 
expansion valve is a source of inefficiency. In order to provide the 
prescribed value of superheat, a portion of the evaporator near the 
suction line contains refrigerant vapor rather than liquid refrigerant. 
This portion of the evaporator operates less efficiently than the portion 
which contains a liquid refrigerant, since the heat transfer coefficient 
to a vapor is lower than to a liquid. Ideally, the entire evaporator 
should contain liquid refrigerant, and the refrigerant leaving the 
evaporator through the suction line should be fully vaporized. Liquid 
refrigerant passing through the suction line can potentially damage the 
compressor. Therefore, in an optimized system, the superheat should be 
reduced as much as possible without permitting liquid refrigerant to reach 
the compressor. 
A further problem associated with thermostatic expansion valves is known as 
"hunting," which results from the time delay inherent in the control 
system. When the thermostatic expansion valve changes the rate of 
refrigerant flow, there is a time delay before the refrigerant is 
evaporated and causes a change in the sensed superheat. As a result, the 
system oscillates between a superheat above the desired value and a 
superheat below the desired value. This results in operating 
inefficiencies and inaccurate temperature control, and can potentially 
permit flow of liquid refrigerant to the compressor. 
The prior art contains various proposals for dealing with the 
above-described problems and other problems associated with thermostatic 
expansion valves. A thermal electric expansion valve is disclosed by 
Wirgau in "Development of A Thermal Electric Expansion Valve," Appliance 
Engineer, Aug. 1984, pp. 52-55. The valve is electrically controlled by a 
thermistor positioned on the evaporator suction line. Other 
electrically-controlled expansion valves are disclosed by Miller in 
"Electronic Expansion Valve Offers More Precise Control In A/C, 
Refrigeration Systems," Air Conditioning/Heating and Refrigeration News, 
Dec. 2, 1984, and in U.S. Pat. No. 4,651,535 issued Mar. 24, 1987 to 
Alsenz. A solenoid flow control valve is controlled by a pulsewidth 
modulated control signal in which the duty cycle determines the flow rate 
through the valve. The Miller article describes microprocessor control of 
the solenoid control valve. In such configurations, the thermostatic bulb 
is eliminated. While such configurations have certain advantages, they 
have not found widespread use. 
In U.S. Pat. No. 4,467,613 issued Aug. 28, 1984 to Behr et al, the 
superheat setting of a thermostatic expansion valve is automatically 
adjusted by an electric heater which biases the thermostatic bulb in 
response to a refrigeration parameter such as compressor lubricant 
temperature. While such configuration can reduce the superheat associated 
with the evaporator, significant energy is required to heat the 
thermostatic bulb with a resistance heater. U.S. Pat. No. 3,638,446 issued 
Feb. 1, 1972 to Palmer and U.S. Pat. No. 2,807,151 issued Sept. 24, 1957 
to Baker also disclose cooling systems wherein the thermostatic bulb of a 
thermostatic expansion valve is heated with a resistance heater. 
U.S. Pat. No. 4,441,329 issued Apr. 10, 1984 to Dawley discloses a 
refrigerator temperature control system including computer-controlled 
thermoelectric modules for heating or cooling temperature sensors during a 
sensor integrity test cycle. U.S. Pat. No. 3,237,415 issued Mar. 1, 1966 
to Newton discloses a zone-controlled refrigeration system wherein 
thermoelectric units are utilized in each zone to directly control 
temperature. 
It is a general object of the present invention to provide improved heat 
pump, air conditioning and refrigeration systems. 
It is another object of the present invention to provide improved apparatus 
for controlling a thermostatic expansion valve. 
It is a further object of the present invention to provide improved 
apparatus for controlling the flow of refrigerant to an evaporator in a 
heating or cooling system. 
It is a further object of the present invention to increase the energy 
efficiency of heat pumps, air conditioners and refrigeration systems. 
It is a further object of the present invention to provide apparatus for 
controlling a thermostatic expansion valve wherein hunting is eliminated. 
It is yet another object of the present invention to provide control 
apparatus capable of heating and cooling the thermostatic bulb of a 
thermostatic expansion valve at different times. 
It is another object of the present invention to provide apparatus 
including a thermoelectric heat pump device for controlling a thermostatic 
expansion valve. 
It is another object of the present invention to provide apparatus for 
controlling a thermostatic expansion valve, which is easily adaptable for 
use in a variety of heat pump, air conditioning and refrigeration systems. 
It is a further object of the present invention to provide apparatus for 
controlling a thermostatic expansion valve, which is low in cost and easy 
to manufacture. 
It is a further object of the present invention to provide apparatus for 
controlling a thermostatic expansion valve having the capability to 
rapidly close the valve by active cooling of the thermostatic bulb. 
It is a further object of the present invention to provide apparatus for 
controlling a thermostatic expansion valve having the capability to 
rapidly open the valve by active heating of the thermostatic bulb. 
It is yet another object of the present invention to provide apparatus for 
controlling a thermostatic expansion valve having a combination of the 
above features. 
SUMMARY OF THE INVENTION 
According to the present invention, these and other objects and advantages 
are achieved in apparatus for controlling a heating or cooling system of 
the type including an evaporator having an inlet line and a suction line, 
and a thermostatic expansion valve including a valve in the inlet line and 
a thermostatic bulb coupled to the valve. In accordance with the 
invention, the apparatus comprises a thermoelectric heat pump device 
responsive to an electrical control signal for controlling transfer of 
thermal energy to and from the thermostatic bulb, means for sensing at 
least one temperature associated with the heating or cooling system, and 
electronic control means responsive to the sensed temperature for 
providing the electrical control signal to the thermoelectric device for 
maintaining a desired operating condition of the evaporator. The 
thermoelectric device acts as a gate or heat pump for controlling flow of 
thermal energy to and from the thermostatic bulb with a relatively small 
amount of electrical energy input. 
In a preferred embodiment, the thermoelectric heat pump device is 
positioned between the thermostatic bulb and the suction line so as to 
control transfer of thermal energy between the suction line and the 
thermostatic bulb. Shaped thermal contact elements can be utilized to 
enhance thermal contact between the thermoelectric device and the suction 
line and between the thermoelectric device and the thermostatic bulb. The 
assembly is preferably surrounded with insulation so that the thermostatic 
bulb is not affected by ambient temperature variations. 
The sensing means, in a preferred embodiment, includes a first temperature 
sensor in thermal contact with the evaporator inlet line and a second 
temperature sensor in thermal contact with the evaporator suction line, 
and the electronic control means includes means responsive to the first 
and second temperature sensors for determining a temperature difference 
between the suction line and the inlet line. The electronic control means 
can include means for causing the thermoelectric device to cool the 
thermostatic bulb when the temperature difference is less than a first 
predetermined value, and for causing the thermoelectric device to heat the 
thermostatic bulb when the temperature difference is greater than a second 
predetermined value. The electronic control means can include means for 
causing the thermoelectric device to cool the thermostatic bulb, so that 
refrigerant flow is cut off when the evaporator is not in operation. 
Alternatively, the electronic control means can include means for causing 
the thermoelectric device to heat the thermostatic bulb, so that the 
thermostatic expansion valve is opened when the evaporator is not in 
operation. 
In another preferred embodiment, the sensing means comprises means for 
sensing a temperature in a zone being controlled by the heating or cooling 
system. The apparatus accurately controls temperature in the zone where 
the temperature is sensed. 
According to another aspect of the invention, the electronic control means 
includes a microprocessor responsive to the sensing means. The electronic 
control means preferably further includes a driver circuit responsive to 
an output from the microprocessor for providing the electrical control 
signal to the thermoelectric heat pump device. The driver circuit is 
responsive to a first state of the output from the microprocessor for 
supplying a current of one polarity to the thermoelectric device and is 
responsive to a second state of the output from the microprocessor for 
supplying a current of the opposite polarity to the thermoelectric device. 
The microprocessor permits the control apparatus to be easily adapted to 
different types and sizes of heating and cooling systems.

DETAILED DESCRIPTION OF THE INVENTION 
A block diagram of a heating or cooling system incorporating the control 
apparatus of the present invention is shown in FIG. 1. The system includes 
an evaporator 10, typically including an evaporator coil and means for 
directing warm air across the evaporator coil. The evaporator 10 has an 
inlet line 12 for receiving liquid refrigerant and an outlet line, or 
suction line, 14 through which refrigerant vapor is exhausted. The 
evaporator 10 receives liquid refrigerant through inlet line 12 from a 
condenser 16 and vaporizes the refrigerant. The vapor from evaporator 10 
is carried through suction line 14 to the inlet of a compressor 18. The 
outlet of the compressor 18 is coupled to the condenser 16. 
The system further includes a thermostatic expansion valve 20 for metering 
the flow of refrigerant through inlet line 12 to evaporator 10. The 
thermostatic expansion valve 20 includes a flow control valve 22, a 
thermostatic bulb 24 and a capillary tube 26 interconnecting the valve 22 
and the thermostatic bulb 24. Thermostatic expansion valves are 
conventionally used in heat pumps, air conditioners and water chillers of 
all sizes for residential, commercial and industrial use, and in 
refrigeration equipment such as refrigerated display cases, coolers, 
icemakers, freezers, transportation refrigeration devices and certain 
types of automobile air conditioners. 
In accordance with the present invention, apparatus is provided for 
controlling transfer of thermal energy to and from the thermostatic bulb 
24. The control apparatus of the invention can be utilized in any heating 
or cooling system that utilizes a thermostatic expansion valve. A 
thermoelectric heat pump device 30, also commonly known as a Peltier 
device, is positioned in thermal contact with thermostatic bulb 24. 
Preferably, but not necessarily, the thermoelectric device 30 is 
positioned between suction line 14 and thermostatic bulb 24 so as to 
control flow of thermal energy between suction line 14 and thermostatic 
bulb 24. Thermoelectric device 30 controls both the rate and direction of 
heat flow to the bulb 24. Small quantities of heat are supplied to or 
taken away from the bulb 24, thereby compensating for losses or gains to 
or from the ambient environment through insulation placed around the 
assembly, as described hereinafter. The thermoelectric device 30 is a 
solid state device, typically having two flat surfaces on opposite sides 
thereof and a pair of electrical terminals. When a d.c. voltage is applied 
to the terminals, one surface of the device gets cold while the other 
surface gets hot. Reversing the polarity of the applied voltage reverses 
the direction of heat flow. Such devices are commercially available from 
Melcor, 990 Spruce Street, Trenton, N.J. 08648. 
The thermoelectric device 30 receives energizing current from an electronic 
controller 34, which typically includes a microprocessor. Electronic 
controller 34 receives inputs from one or more temperature sensors 
associated with the heating or cooling system. In a preferred embodiment 
illustrated in FIG. 1, a temperature sensor 36 is located in thermal 
contact with inlet line 12, and a temperature sensor 38 is located in 
thermal contact with suction line 14. Each of the temperature sensors 36, 
38 is electrically coupled to electronic controller 34. The difference 
between the suction line 14 and inlet line 12 temperatures is calculated 
by electronic controller 34. The electronic controller 34, during normal 
operation, supplies to thermoelectric device 30 the current necessary to 
maintain a desired temperature difference. 
It will be understood that the control apparatus of the invention is not 
limited to the sensor configuration shown in FIG. 1. For example, any 
number of temperature sensors can be placed at desired positions on the 
inlet line 12, the suction line 14 or the evaporator 10 to sense operating 
temperatures. Also, a temperature sensor 40, shown by broken lines in FIG. 
1, can be positioned in a zone that is controlled by the heating or 
cooling system and where temperature control is critical. For example, the 
sensor 40 can be placed within a refrigerated display case to insure that 
a desired temperature is maintained. 
A cross-sectional view of a mounting assembly for the thermostatic bulb 24, 
the thermoelectric device 30 and the suction line 14 is shown in FIG. 2. 
Thermoelectric device 30 typically has flat surfaces, whereas the 
thermostatic bulb 24 and the suction line 14 have curved surfaces. Thermal 
contact elements are used to establish low thermal resistance contacts 
between these elements. A first thermal contact element 44 is positioned 
between thermoelectric element 30 and thermostatic bulb 24, and a second 
thermal contact element 46 is positioned between thermoelectric device 30 
and suction line 14. The element 44 has a flat surface for contacting 
thermoelectric element 30 and a curved surface shaped to match 
thermostatic bulb 24. Similarly, element 46 has a flat surface for contact 
with thermoelectric element 30 and a curved surface for contact with 
suction line 14. The thermal contact elements 44, 46 can, for example, be 
aluminum extrusions. The entire assembly is surrounded with insulation 48 
in order to minimize transfer of thermal energy between thermostatic bulb 
24 and the ambient environment. The assembly shown in FIG. 2 is 
conveniently made slightly longer than the thermostatic bulb 24. 
A schematic diagram of one example of the electronic controller 34 is shown 
in FIG. 3. The electronic controller 34 shown in FIG. 3 is suitable for a 
residential or light commercial heat pump. Different digital and analog 
inputs to the controller will be used for different applications. In a 
preferred embodiment, the electronic controller 34 includes a 
microprocessor so that a single circuit structure can be utilized in a 
wide variety of applications by reprogramming the microprocessor. A 
microprocessor 60 receives analog input signals from temperature sensors 
36 and 38 on lines 62 and 64, respectively. In a preferred embodiment, 
temperature sensors 36, 38 are type LM234, manufactured by National 
Semiconductor, and the microprocessor is a type .mu.PD8022, manufactured 
by NEC. The type .mu.PD8022 includes two built-in analog-to-digital 
converters, thereby permitting direct input of analog signals from 
temperature sensors 36 and 38. 
The microprocessor 60 also receives digital control signal inputs. A 
thermostat cooling contact 66, a thermostat heating contact 68 and a 
reversing valve solenoid 70 associated with a defrost cycle are coupled 
through optical isolators 72, 73, 74, respectively, to digital inputs of 
microprocessor 60. The optical isolators 72, 73, 74 prevent noise spikes, 
interference or high voltages picked up on the lines from the remote 
switch contacts from damaging the microprocessor 60. The thermostat 
contacts 66, 68 indicate whether the system is in an operating or standby 
mode and whether the system is heating or cooling, while the input from 
reversing valve solenoid 70 selects a routine suitable for a defrost 
cycle. 
Optional mode switches 76, 77, 78 are coupled to additional digital inputs 
of microprocessor 60. The mode switches 76, 77, 78 can be utilized to 
activate different operating routines. For example, different temperature 
set points can be selected at different times of day. Also, the 
microprocessor 60 can be programmed to respond to different temperature 
sensors in different modes. It will be understood that the inputs provided 
by mode switches 76, 77, 78 are not necessarily manual, but can be 
supplied from a remote computer for automatic control of the 
microprocessor 60. Additional conventional elements of the microprocessor 
circuit include decoupling capacitors 80, 81, 82 and 83, clock crystal 84 
and power supply 85. 
Digital outputs of the microprocessor 60 are supplied on lines 90 and 92 to 
a driver circuit 94. The driver circuit 94 supplies operating current to 
the thermoelectric device 30 on lines 95 and 96. In a preferred 
embodiment, the driver circuit 94 is a type ECG1619, manufactured by North 
American Philips. The preferred circuit accepts digital inputs on lines 90 
and 92 and provides d.c. output current of one polarity or of the opposite 
polarity on lines 95 and 96 to drive thermoelectric device 30. When lines 
90 and 92 are both at a low logic level, no current is supplied to 
thermoelectric device 30. When line 90 is at a high logic level, current 
is supplied to thermoelectric device 30 in one direction, and when line 92 
is at a high logic level, current is supplied to thermoelectric device 30 
in the opposite direction. In a preferred operating mode, pulse width 
modulated power is supplied to thermoelectric device 30. Pulses of one 
polarity are supplied to thermoelectric device 30 for heating, and pulses 
of the opposite polarity are supplied for cooling. The amount of heating 
or cooling is determined by the on-off duty cycle of the pulses. In a 
preferred embodiment, the thermoelectric device 30 is a type CP1.0-71-05L, 
manufactured by Melcor, and requires an operating current on the order of 
1.5 amps at 5 volts. 
The microprocessor circuit shown in FIG. 3 and described hereinabove is 
easily adapted to a variety of different configurations for control of a 
thermostatic expansion valve. As noted above, any desired number of 
temperature sensors can be utilized in selected locations. The readings 
from the temperature sensors can be processed according to any desired 
algorithm, and one or more threshold levels can be established. When a 
timer is utilized, different thresholds can be employed at different 
times, such as for day and night operation. The microprocessor can have 
selectable operating modes and can be controlled by a remote computer or 
other controller. 
A flow diagram of one example of a simple but effective control algorithm 
is shown in FIG. 4. The microprocessor 60 checks the inputs from 
thermostat contacts 66, 68 in step 102 to determine if the system is in 
operation. When the thermostat is off, the microprocessor 60 in step 104 
provides the necessary logic levels on lines 90 and 92 to cause driver 94 
to energize thermoelectric device 30 so as to chill the thermostatic bulb 
24. The chilling of bulb 24 closes valve 22 so that the flow of 
refrigerant to evaporator 10 is cut off. 
When the thermostat is on, the microprocessor 60 in step 106 obtains 
temperature readings T.sub.2 and T.sub.1 from sensors 36 and 38, 
respectively, and calculates the difference, T.sub.1 -T.sub.2, the 
evaporator superheat. In steps 108 and 110, the microprocessor 60 compares 
the superheat T.sub.1 -T.sub.2 with predetermined limits. In the present 
example, the superheat is required to be between an upper limit S.sub.max 
and a lower limit S.sub.min. When T.sub.1 -T.sub.2 is greater than 
S.sub.max, the microprocessor 60 causes driver 94 to supply current to 
thermoelectric device 30 so as to heat thermostatic bulb 24 in step 112. 
When the bulb 24 is heated, valve 22 increases the flow of refrigerant to 
evaporator 10 until the temperature at the suction line 14 is reduced. 
When the superheat T.sub.1 -T.sub.2 is below the lower limit S.sub.min, 
the microprocessor 60 causes driver 94 to supply current to thermoelectric 
device 30 so as to chill thermostatic bulb 24 in step 104. When bulb 24 is 
chilled, valve 22 reduces the flow of refrigerant into evaporator 10 until 
the evaporator 10 is able to fully evaporate all incoming refrigerant and 
reach the desired range of superheats. When the superheat T.sub.1 -T.sub.2 
is within the desired range between S.sub.max and S.sub.min, no current is 
supplied to the thermoelectric device 30 (step 114). The routine then 
returns for checking of the thermostat in step 102. 
In some systems with compressor motors that have a low starting torque, the 
thermostatic expansion valve 20 is opened when the evaporator 10 is not in 
operation. In this case, the flow diagram of FIG. 4 is suitably modified 
to produce heating of the thermostatic bulb when the thermostat is off. 
The control apparatus shown and described herein has numerous advantages 
over the prior art. Hunting is eliminated by implementing in 
microprocessor 60 a selected time delay between sensing and acting on the 
thermostatic bulb 24. Conventional thermostatic expansion valves have a 
tendency to hunt or control erratically when the time between sensing the 
bulb temperature change and valve actuation is outside the stable 
operating range. Since delay can easily be built into the microprocessor 
60, erratic control tendencies can be damped out of the system. 
The ability of the control apparatus to chill the thermostatic bulb 24 
below the temperature of suction line 14 is an important feature of the 
invention. Improvements resulting from the ability to chill the bulb 24 
include: 
(1) Better cycling performance is obtained due to refrigerant isolation 
during the off cycle. As the bulb 24 is chilled below suction line 14 
temperature in the off state, the thermostatic expansion valve closes, 
thereby isolating the refrigerant in the high and low pressure sides of 
the machine. Pressures do not equalize, and, since the bulb is chilled 
even after the suction line 14 has become warmed, the start of the 
subsequent cycle does not spill the entire quantity of liquid refrigerant 
into the evaporator 10. Instead, after starting, the bulb 24 is warmed 
slowly by the thermoelectric device 30, causing the low pressure side to 
rapidly reduce to standard operating pressure by means of the compressor 
18. Rapid evaporator temperature pull-down is a desirable energy-saving 
feature in refrigeration and air conditioning systems. 
(2) The same rapid pull-down obtained by chilling the bulb 24 below suction 
line 14 temperature also shortens the defrost cycle recovery time of a 
heat pump. Since defrosting is a major source of inefficiency in heat pump 
operation, any shortening of the defrost cycle improves system 
performance. 
(3) Flood-back of refrigerant to the compressor 18 during the off cycle and 
on startup is reduced or eliminated by keeping the bulb 24 chilled, 
thereby improving compressor life. 
Microprocessor control of the thermoelectric device 30 permits a variety of 
different operating modes, all of which represent improvements over the 
prior art. 
(1) Reduced operating superheats can be programmed over the entire 
operating range of the refrigeration system. Constant low superheat versus 
capacity can be programmed. This improvement is most desirable for heat 
pumps in the heating mode where the evaporator temperature must follow 
ambient over a wide range of temperatures. 
(2) A constant evaporator temperature or a constant low temperature can be 
maintained at nearly any system capacity. This improvement relates to 
refrigerated cases, ice makers and coolers where evaporator temperature 
and product temperature are important. 
(3) Many refrigeration systems, such as those in convenience stores, take 
advantage of cooler outside temperatures by allowing condenser pressures 
to fall with falling ambient temperatures instead of artificially keeping 
them high. This saves on energy to the compressor, but reduces the 
pressure at the inlet to the expansion valve. A constant high pressure is 
required for consistent control. The present invention permits the 
condenser pressure to fall and yet maintains full control at the expansion 
valve. The thermoelectric device can heat the bulb of the expansion valve 
to allow more valve opening, which thereby compensates for the lower 
upstream pressure. 
With the present invention, heat pumps can be sized larger to handle more 
of the winter heating load. Currently, heat pumps are sized based on 
summer cooling requirements in order to provide good dehumidification. 
Larger heat pumps would have short run times in the summer with high 
sensible heat ratios (low amounts of dehumidification). By using the 
control apparatus of the present invention in a single thermostatic 
expansion valve heat pump system, this problem can be avoided. In the 
summer, the control reduces the flow of refrigerant through the expansion 
valve, reducing both the evaporator temperature and system capacity. At 
the reduced capacity and lower evaporator temperature, the heat pump 
system has an acceptable sensible heat ratio and a capacity well matched 
to the house. For heating operation, the control yields a low superheat 
for full evaporator utilization and good ambient following 
characteristics. In this way, the heat pump can be made larger, and the 
winter balance point where electric resistance heat must begin will be 
lower, thereby yielding reduced energy consumption and operating costs. 
While there has been shown and described what is at present considered the 
preferred embodiments of the present invention, it will be obvious to 
those skilled in the art that various changes and modifications may be 
made therein without departing from the scope of the invention as defined 
by the appended claims.