Solar-powered refrigeration system

A solar powered vapor compression refrigeration system is made practicable with thermal storage and novel control techniques. In one embodiment, the refrigeration system includes a photovoltaic panel, a variable speed compressor, an insulated enclosure, and a thermal reservoir. The photo voltaic (PV) panel converts sunlight into DC (direct current) electrical power. The DC electrical power drives a compressor that circulates refrigerant through a vapor compression refrigeration loop to extract heat from the insulated enclosure. The thermal reservoir is situated inside the insulated enclosure and includes a phase change material. As heat is extracted from the insulated enclosure, the phase change material is frozen, and thereafter is able to act as a heat sink to maintain the temperature of the insulated enclosure in the absence of sunlight. The conversion of solar power into stored thermal energy is optimized by a compressor control method that effectively maximizes the compressor's usage of available energy. A capacitor is provided to smooth the power voltage and to provide additional current during compressor start-up. A controller monitors the rate of change of the smoothed power voltage to determine if the compressor is operating below or above the available power maximum, and adjusts the compressor speed accordingly. In this manner, the compressor operation is adjusted to convert substantially all available solar power into stored thermal energy.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 This invention relates generally to solar power control systems, and in
 particular, to an efficient system and method for applying solar-generated
 power to refrigeration.
 2. Description of the Related Art
 Two billion people live without electricity. They represent a market for
 various solar powered systems such as stand-alone power systems and small
 capacity solar refrigerators. Efforts have been made to develop
 stand-alone photo voltaic (PV) power systems that provide lighting and
 power for small devices such as radios and small televisions. For example,
 such systems may include a solar panel, a battery, and a low wattage
 fluorescent light. Solar refrigerators, however, represent a bigger
 challenge.
 Previous attempts to produce a marketable solar refrigerator have been
 largely unsuccessful. For example, consider the following patents:
 In U.S. Pat. No. 4,126,014, Thomas Kay discloses an absorption
 refrigeration system powered by a heated fluid from a solar panel.
 In U.S. Pat. No. 5,501,083, Tae Kim discloses an AC-powered air conditioner
 having a solar panel for backup electrical power.
 In U.S. Pat. No. 5,497,629, Alexander Rafalovich discloses the use of solar
 power in an air conditioning system to pump heat from an indoor space to a
 thermal store.
 In U.S. Pat. No. 5,685,152, Jeffrey Sterling discloses using a heated
 medium from solar collectors to produce a cold thermal store and
 mechanical energy to pump heat from an indoor space to the cold thermal
 store.
 Kay's refrigeration system provides no means to maintain refrigerator
 operation in the absence of sunlight (e.g. at nighttime or on overcast
 days). As the air conditioning systems are largely unsuited for even small
 capacity refrigerators or freezers, no attempt has been made to scale
 these systems to produce a commercializable solar refrigerator.
 Accordingly, it is desirable to provide an efficient, inexpensive,
 commercializable small capacity solar refrigerator which can operate for
 several days in the absence of sunlight. As batteries are often expensive
 and require regular maintenance, it would further be desirable to provide
 such a solar refrigerator which does not require batteries.
 SUMMARY OF THE INVENTION
 A solar powered vapor compression refrigeration system is made practicable
 with thermal storage and novel control techniques. In one embodiment, the
 refrigeration system includes a photovoltaic panel, a capacitor, a
 compressor, an insulated enclosure, and a thermal reservoir. The
 photovoltaic (PV) panel converts sunlight into DC (direct current)
 electrical power, some of which is stored in the capacitor. The capacitor
 provides additional current during compressor start-up, and thereafter
 acts to smooth out variations in the power voltage. The power from the
 PVT. Panel and capacitor drives the compressor to circulate refrigerant
 through a vapor compression refrigeration loop, thereby extracting heat
 from the insulated enclosure. The thermal reservoir is situated inside the
 insulated enclosure and includes a phase change material. As heat is
 extracted from the insulated enclosure, the phase change material is
 frozen. Thereafter the thermal reservoir is able to act as a heat sink to
 maintain the temperature of the insulated enclosure for an extended period
 in the absence of sunlight.
 This conversion of solar power into stored thermal energy is optimized by a
 compressor control method that effectively maximizes the compressor's
 usage of available energy. A controller monitors the rate of change of the
 smoothed power voltage to determine if the compressor is operating below
 or above the maximum available power, and adjusts the compressor speed
 accordingly. In this manner, the compressor operation is continuously
 adjusted to convert substantially all available solar power into stored
 thermal energy.

Whle the invention is susceptible to various modifications and alternative
 forms, specific embodiments thereof are shown by way of example in the
 drawings and will herein be described in detail. It should be understood,
 however, that the drawings and detailed description thereto are not
 intended to limit the invention to the particular form disclosed, but on
 the contrary, the intention is to cover all modifications, equivalents and
 alternatives falling within the spirit and scope of the present invention
 as defined by the appended claims.
 DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
 Turning now to the figures, FIG. 1 shows a first embodiment of a solar
 refrigeration system which includes a solar panel 102 connected to a power
 bus 103. Although a wide variety of solar panel types and styles may be
 employed, one suitable example is a 12 volt nominal PV panel that is
 capable of a peak power output of approximately 120 watts at approximately
 15 volts under full solar insolation.
 A capacitor 104 is connected to power bus 103 in parallel with solar panel
 102. Capacitor 104 operates to provide temporary storage of electrical
 charge in order to smooth any voltage variations on power bus 103 and to
 provide extra current during demand periods. The voltage variations may be
 caused by a variety of sources including changes in light intensity on the
 solar panel and changes in the electrical load driven by the solar panel
 102. The capacitor 104 may be varied in size and type, but a preferred
 example is a 0.2 Farad electrolytic capacitor.
 A variable speed compressor 108 with a load controller 106 is directly
 coupled to the solar panel 102 by power bus 103. In this context,
 "directly coupled" is defined to mean that no power converters are
 provided between the compressor 108 and solar panel 102. Although other
 embodiments are also contemplated, this embodiment advantageously exhibits
 relatively high efficiency due to the direct powering of the compressor
 108 by a PV panel. It is noted that systems which use batteries typically
 force the solar panel to operate below its peak power point to match the
 battery charging voltage. Powering the compressor directly from the solar
 panel allows the solar panel to be operated at the maximum power point.
 The variable speed compressor 108 is preferably a direct current compressor
 such as a Danfoss.RTM. BD35F direct current compressor with refrigerant
 134a. Persons of skill in the art will recognize that other suitable
 compressors and refrigerants can be employed. The BD35F includes a
 "brushless" DC (direct current) motor in that provides permanent magnets
 on the rotor. Electronics in the BD35F switch the DC input to provide a
 3-phase input to fixed coils that drive the rotor. The electronics improve
 the motor's efficiency by sensing the back-EMF in the coils to determine
 the rotor position. This compressor implementation is believed to exhibit
 efficiency and longevity advantages over typical DC compressors. As
 discussed in further detail below, load controller 106 senses the voltage
 on power bus 103 and regulates the speed of compressor 108 in response to
 variations in this voltage.
 Compressor 108 circulates refrigerant through a vapor compression
 refrigeration loop that preferably includes a first heat exchanger (a.k.a.
 a condenser) 110, a capillary tube 112, a second heat exchanger (a.k.a an
 evaporator) 114 internal to an insulated enclosure 120, and a third heat
 exchanger (sometimes referred to as SLLL HX, or the suction line/liquid
 line heat exchanger) 116 associated with the capillary tube 112. As
 refrigerant is circulated through the loop, it is compressed by compressor
 108, cooled to a liquid state by ambient air in condenser 110,
 flash-cooled by heat exchanger 116 in capillary tube 112, evaporated to a
 gaseous state in evaporator 114, warmed by heat exchanger 116, and
 recompressed and re-circulated by compressor 108. This circulation results
 in a net transfer of heat from the evaporator 114 to the condenser 110,
 thereby cooling the interior of the insulated enclosure 120 by heating
 ambient air. One of skill in the art will readily recognize that this
 refrigerant loop may be constructed in various suitable manners, and that
 other refrigerant loops may also be employed to achieve a net transfer of
 heat energy away from the insulated enclosure 120 without departing from
 the scope of the invention. For example, one specific alternate
 implementation uses an expansion valve in place of the capillary tube 112.
 Similarly, many types of insulated enclosures are well known and may be
 employed, but a preferred construction for the insulated enclosure 120
 uses fiberglass-reinforced plastic shells for the cabinet with vacuum
 panels between the inner and outer shells for insulation. A bezel
 interface is preferably provided between the cabinet and the door to
 minimize thermal conductance and convection through the seal. With this
 preferred construction, a cabinet having a composite R value (thermal
 resistance in units of hr.multidot.ft.sup.2.multidot..degree.F./BTU) of 26
 has been achieved. (Most conventional refrigerators have a composite R
 value of 5.)
 Referring still to FIG. 1, the load controller 106 senses the voltage on
 power bus 103 and provides a speed control signal 107 to variable speed
 compressor 108. By controlling the compressor speed, the load controller
 106 effectively maximizes the power extracted from the solar panel. It
 inexpensively implements an advantageous optimization method as described
 in further detail below. While it can take various forms, the load
 controller 106 is preferably implemented in the form of a microcontroller
 that implements a software algorithm. The microcontroller may also be
 designed to perform other system functions such as: monitoring internal
 temperature of the insulated enclosure, monitoring the compressor for
 error conditions, and initiating compressor start-ups and shut-downs in a
 manner designed to extend the life of the compressor. In alternate
 embodiments, the load controller 106 may also control power source
 switching to access alternate power sources, if available and when
 necessary, or to provide redundancy (in the case of multiple solar
 panels).
 A thermal reservoir 118 is preferably provided in the insulated enclosure
 120. Thermal reservoir 118 preferably comprises a phase-change material
 that has a phase-change temperature at or slightly below the target
 interior temperature for the insulated enclosure. Particularly desirable
 phase-change materials are those having a solid-liquid phase change with a
 high heat of fusion, and which are inexpensive and relatively non-toxic.
 Water and water solutions are examples of suitable phase change materials.
 A water solution of approximately 3-5% propylene glycol may be
 particularly desirable, as it exhibits a reduced tendency to rupture
 closed containers when freezing. The size and phase change material of the
 thermal reservoir is preferably chosen to maintain the target interior
 temperature for several days in the absence of solar power (or at least 36
 hours). One of skill in the art will recognize that thermal reservoir 118
 may be implemented in a variety of suitable configurations.
 In the embodiment of FIG. 1, the thermal reservoir 118 is contemplated as
 being adjacent to evaporator 114, and/or as being a part of evaporator
 114. As refrigerant circulates through the evaporator 114 to cool the
 interior of the insulated enclosure 120, a direct transfer of heat energy
 occurs to evaporator 114 from thermal reservoir 118 to cool the thermal
 reservoir and induce a phase change of the phase-change material. In other
 words, if the phase-change material is water, the flow of refrigerant
 through the evaporator cools and freezes the water.
 In operation, the solar panel 102 delivers power to power bus 103 during
 the day when the sun is shining. The load controller 106 runs the
 compressor 108 at a speed that maximizes the power extracted from the
 solar panel. The compressor 108 circulates refrigerant through a
 refrigerant loop to cool the insulated enclosure and to cool and induce a
 phase change of the material in the thermal reservoir. At night and during
 adverse weather conditions, no power is delivered to the power bus 103,
 and the compressor 108 is inactive. The temperature in the insulated
 enclosure is maintained by the thawing of the material in the thermal
 reservoir. Advantageously, no fluid circulation or active heat pumping is
 required to maintain the enclosure temperature during these inactive time
 periods.
 Referring now to FIG. 2, a second solar refrigeration system embodiment is
 shown. In this embodiment, an alternate power source 205 is coupled to
 power bus 103. The alternate power source 205 may take many forms
 including, e.g. a supplemental battery, a fuel cell, a generator, or an
 AC/DC converter connected to a commercial AC power grid. The load
 controller 106 turns the alternate power source 205 on or off by means of
 an enable signal 206. The load controller 106 preferably minimizes the use
 of alternate power source 206 to the greatest extent possible, using it
 only when solar power is unavailable and the temperature of the insulated
 enclosure exceeds a predetermined threshold. The load controller 106
 monitors the interior temperature of insulated enclosure 120 by means of a
 temperature signal 207 from a temperature sensor (not shown) in insulated
 enclosure 120.
 The solar refrigeration system embodiment of FIG. 2 also employs an
 alternate configuration for the evaporator 114 and thermal reservoir 118.
 In this configuration, the refrigerant passing through evaporator 114
 cools a second fluid that is pumped through the evaporator 114 by a pump
 209. Many fluids may be used, but currently a propylene glycol and water
 mixture is preferred. The cooled second fluid is then circulated through a
 heat exchanger in the thermal reservoir 118 to cool and induce a phase
 change in the phase change material. The load controller 106 may be
 configured to turn pump 209 on and off by means of a signal 208. Pump 209
 is preferably activated only when compressor 108 is operating. A fan may
 be provided to improve air circulation, and may also be controlled by
 signal 208.
 In one particular implementation of the alternate configuration shown by
 FIG. 2, the cooling of the insulated enclosure 120 is accomplished
 primarily by the thermal reservoir 118 and the heat exchanger therein.
 This implementation may prove advantageous relative to the configuration
 shown in FIG. 1 for several reasons. A first feature of this
 implementation is that the refrigerant volume is reduced, which may
 provide reduced cost and increased system longevity. A second feature of
 this implementation is that thermal leakage to the interior of the
 insulated enclosure during and after compressor shut-down is reduced. A
 third feature is that mechanical design of the thermal reservoir may be
 simplified due to a larger and more favorably distributed heat exchange
 area with the phase change material. It is noted that the solar
 refrigeration system embodiment of FIG. 1 may be modified to use this
 thermal reservoir configuration.
 The load controller 106 may be designed to monitor the temperature of the
 insulated enclosure and respond to temperature excursions above or below
 predetermined thresholds. As mentioned previously, the load controller 106
 may activate alternate power source 205 in response to a detected
 temperature above an upper temperature limit. Also, the load controller
 106 may halt the variable speed compressor 108 in response to a detected
 temperature below a lower temperature limit. Once the temperature returns
 to the desired range, the load controller 106 may then resume normal
 solar-powered operation. One of skill in the art will recognize the
 desirability of providing some hysteresis in any such temperature
 regulation strategy. It is noted that the upper temperature limit is
 preferably slightly above the phase change temperature, and the lower
 temperature limit is preferably slightly below the phase change
 temperature.
 As previously mentioned, load controller 106 operates to maximize the power
 drawn from the solar panel 102. Various methods which may be implemented
 by the load controller are now described with reference to FIGS. 3 and 4.
 FIG. 3 shows an I-V curve 302 representing the voltage V provided by solar
 panel 102 as a function of current I drawn from the solar panel, assuming
 maximum insolation (sunlight intensity). The voltage varies from V.sub.OC
 when no current is drawn to 0 when the short circuit current I.sub.SC is
 drawn. A typical example of an open circuit voltage V.sub.OC for a nominal
 12 volt panel is 20 volts, and a typical example of a short circuit
 current is 8 amperes. On the curve between these two points is a maximum
 power point (I.sub.MP, V.sub.MP) where the maximum power is extracted from
 the solar panel. This point occurs where the slope of the curve is
 dV/dI=-V/I.
 The load controller 106 preferably locates this maximum power point by an
 iterative search process. At an initial time t=0, the compressor 108 is
 not running, and no current is drawn. The load controller determines that
 a sufficient start-up voltage exists and starts the compressor at a
 minimum startup speed. Note that the current drawn by the compressor
 increases as the speed of the compressor increases. At a subsequent time
 t=1, the compressor is drawing a current and the voltage provided by the
 solar panel has been slightly reduced. The load controller 106 then begins
 gradually increasing the speed of the compressor 108, detecting the power
 bus voltage at regular intervals and adjusting the speed of the compressor
 in response to some criterion based on the detected voltage. The time
 progression of operating points has been exaggerated for illustration. In
 a preferred embodiment, the increments in speed are digital and are much
 smaller, so that 255 or more operating points on the curve are possible.
 Various adjustment criteria may be used. For example, referring momentarily
 to FIG. 4, a second I-V curve 402 is shown for reduced insolation. The
 maximum power point on curve 402 has shifted relative to the maximum power
 point on curve 302. It is noted that while the current I.sub.MP at the
 maximum power point is particularly sensitive to the amount of insolation,
 the voltage V.sub.MP at the maximum power point is relatively insensitive
 to the amount of insolation. Consequently, the load controller 106 may
 increase or decrease the compressor speed as needed to maintain the power
 bus voltage close to a predetermined voltage target, e.g. the maximum
 power voltage for full solar insolation.
 While simple, this criterion is suboptimal since the maximum power voltage
 varies with temperature, and in any case, this criterion does not provide
 for full power extraction during reduced insolation. Referring again to
 FIG. 3, it is noted that at all operating point voltages on curve 302
 above the maximum power point voltage, the power provided by the solar
 panel increases as the current increases, whereas for all operating point
 voltages on the curve below the maximum power point voltage, the power
 provided by the solar panel DECREASES as the current increases. When this
 observation is combined with the observation that the power required by
 the compressor always increases as the speed increases, an improved
 control method can be developed for the load controller 106.
 Referring simultaneously to FIGS. 1 and 3, it is noted that when the
 compressor 108 is run at a speed requiring less power than the solar panel
 102 can provide, an increase in compressor speed will result in a matching
 increase in power extracted from the solar panel. Due to the capacitor
 104, the power bus voltage will decrease smoothly and stabilize. In other
 words, the magnitude of the time derivative of the voltage decreases as a
 function of time. When the compressor 108 is run at a speed requiring more
 power than the solar panel 102 can provide, the charge on capacitor 104
 provides the extra power required. Since only a limited amount of charge
 exists on capacitor 104, the capacitor 104 is increasingly depleted as
 time goes on, and the compressor attempts to draw more current from solar
 panel 102. This in turn causes the solar panel to provide less power as
 the voltage drops, causing further depletion of the capacitor and even
 more current draw from the solar panel 102. The power bus voltage rapidly
 decays, and in fact, the rate of voltage decay increases as a function of
 time. Expressed in calculus terms, when the second derivative of the
 voltage with respect to time is greater than or equal to zero, the system
 is operating on the curve above the maximum power point voltage. When the
 second derivative of the voltage with respect to time is less than zero,
 the system is operating on the curve below the maximum power point
 voltage.
 FIG. 5 shows a first improved control method which may be implemented by
 load controller 106. After the load controller has started the compressor
 and allowed some small amount of time for the voltage on the power bus to
 settle into a steady state, the load controller begins sampling the
 voltage at regularly spaced time intervals. One of skill in the art will
 recognize that the sampling intervals may be allowed to vary if this is
 determined to be desirable, and appropriate adjustments can be made to the
 method. Additionally, the power bus voltage signal may be mildly
 conditioned to remove high frequency noise before being sampled by the
 load controller.
 In step 502 an initial voltage sample is taken before the load controller
 enters a loop consisting of steps 504-516. For each iteration of the loop,
 two additional voltage samples are taken. In step 504, a first voltage
 sample is taken, and in step 506 a first change in the voltage is
 calculated by subtracting the previous voltage sample from the first
 voltage sample. In step 508, a second voltage sample is taken, and in step
 510 a second voltage change is calculated by subtracting the first voltage
 sample from the second voltage sample.
 In step 512, the two calculated voltage changes are compared. If the
 magnitude of the second voltage change is less than or equal to the
 magnitude of the first voltage change, then in step 514, the loop
 controller increments the speed of the compressor by one step. On the
 other hand, if the magnitude of the second voltage change is larger than
 the magnitude of the second voltage change, then in step 516, the loop
 controller decrements the speed of the compressor by two or more steps.
 While various implementations of decrement step 516 are contemplated, it
 is currently preferred to make the number of decrement steps a
 predetermined constant based on the system embodiment. It is further
 contemplated to make the increment step sizes adaptive in nature. The
 adaptation may be based on the size of the calculated first voltage
 change, so that smaller voltage changes result in smaller step sizes. In
 this manner, the load controller may more quickly and accurately locate
 the maximum power point. The nature of the adaptation may be changed after
 the first time the speed is decremented to provide for a smaller range of
 variation about the optimal operating point. For example, the step size
 may become based proportionally on the size of the second calculated
 voltage change, so that larger voltage changes result in larger step
 sizes.
 FIG. 6 shows a second improved control method which may be implemented by
 load controller 106. When the system is operating on the portion of the
 solar panel curve below the maximum power point, the calculated voltage
 changes continually grow if the compressor speed is not adjusted. Hence
 the method of FIG. 5 may be simplified by eliminating steps 508 and 510,
 and replacing step 512 with step 612, in which the calculated voltage
 change is compared with a predetermined threshold. No matter where the
 system is operating on the lower part of the curve, eventually the
 calculated voltage change will exceed the threshold, and the compressor
 speed will be reduced accordingly. When the voltage change is less than
 the threshold, the system is assumed to be on the upper part of the curve,
 and the compressor speed is increased. The threshold is preferably
 adjusted to allow for only a small range of variation around the maximum
 power point.
 It is noted that the disclosed refrigeration systems and power control
 methods may have many varied embodiments. For example, one refrigeration
 system embodiment may employ an insulated enclosure with divided
 compartments that are maintained at different temperatures such as might
 be suitable for storing fresh and frozen foods. Another embodiment may
 employ the structure and stored contents of the insulated enclosure as the
 thermal reservoir. This latter approach may prove particularly suitable
 for refrigeration systems that are configured to produce the stored
 contents, such as would be the case for an ice maker. Some embodiments may
 include alternate energy sources such as batteries, a generator, or a
 commercial power grid, the use of which is may be minimized by using the
 solar panel as much as possible. These embodiments could use a smaller
 thermal reservoir due to availability of an alternate power source to
 maintain the temperature. In some embodiments, the refrigeration system
 may be applied to cool poorly insulated enclosures that are often exposed
 to substantial amounts of sunlight. In this vein, one refrigeration
 embodiment is an air conditioning system for vehicles that cools the
 interior when the vehicle is exposed to the sun. Such a system may or may
 not include some form of phase change material as a thermal reservoir.
 Numerous such variations and modifications will become apparent to those
 skilled in the art once the above disclosure is fully appreciated. It is
 intended that the following claims be interpreted to embrace all such
 variations and modifications.