U.S. patent application number 09/838680 was filed with the patent office on 2002-02-21 for solar-powered refrigeration system.
This patent application is currently assigned to Government of the United States of America, National Aeronautics & Space Administration. Invention is credited to Bergeron, David J. III, Ewert, Michael K..
Application Number | 20020020181 09/838680 |
Document ID | / |
Family ID | 23319562 |
Filed Date | 2002-02-21 |
United States Patent
Application |
20020020181 |
Kind Code |
A1 |
Ewert, Michael K. ; et
al. |
February 21, 2002 |
Solar-powered refrigeration system
Abstract
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 photovoltaic (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.
Inventors: |
Ewert, Michael K.;
(Seabrook, TX) ; Bergeron, David J. III; (League
City, TX) |
Correspondence
Address: |
NASA JOHNSON SPACE CENTER
MAIL CODE HA
2101 NASA RD 1
HOUSTON
TX
77058
US
|
Assignee: |
Government of the United States of
America, National Aeronautics & Space Administration
|
Family ID: |
23319562 |
Appl. No.: |
09/838680 |
Filed: |
April 19, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09838680 |
Apr 19, 2001 |
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09337208 |
Jun 3, 1999 |
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6253563 |
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Current U.S.
Class: |
62/228.4 ;
62/235.1; 62/236 |
Current CPC
Class: |
F25D 11/006 20130101;
Y02E 10/566 20130101; Y10S 323/906 20130101; F25B 27/005 20130101;
F25B 2500/26 20130101; H02J 7/35 20130101; H02S 10/40 20141201;
Y02E 10/56 20130101; F25B 49/025 20130101; H01L 31/02021
20130101 |
Class at
Publication: |
62/228.4 ;
62/236; 62/235.1 |
International
Class: |
F25B 001/00; F25B
049/00; F25B 027/00 |
Goverment Interests
[0001] The invention described herein was made in the performance
of work under a NASA contract and is subject to Public Law 96-517
(35 U.S.C. .sctn.200 et seq.). The contractor has not elected to
retain title to the invention.
Claims
What is claimed is:
1. A refrigeration system which comprises: an enclosure having an
interior space; phase-change material situated in said interior
space; a photovoltaic panel configured to provide electrical power
having a DC voltage; and a compressor electrically connected to the
photovoltaic panel to receive electrical power at said DC voltage,
wherein the compressor is configured to circulate refrigerant
through a first heat exchanger to cool the refrigerant, through a
constriction configured to sustain a pressure drop, and through a
second heat exchanger to extract heat from said interior space.
2. The refrigeration system of claim 1, further comprising: a pump
configured to circulate a fluid through the second heat exchanger
to cool the fluid, and through a third heat exchanger to extract
heat from the phase change material.
3. The refrigeration system of claim 1, wherein at least a portion
of the second heat exchanger is situated adjacent to the phase
change material.
4. The refrigeration system of claim 1, wherein the phase-change
material mass, when solidified, is sufficient to maintain the
interior space of the enclosure substantially at a phase-change
temperature for more than 36 hours.
5. The refrigeration system of claim 1, wherein the phase-change
material comprises water.
6. The refrigeration system of claim 1, further comprising a
capacitor coupled to the compressor to smooth variations in said DC
voltage.
7. The refrigeration system of claim 6, wherein the compressor is a
variable speed compressor, and wherein the refrigeration system
further comprises a controller configured to monitor the DC voltage
and to regulate the compressor speed to run the compressor at a
substantially maximum available power.
8. The refrigeration system of claim 7, wherein the controller is
configured to receive a temperature signal indicative of a
temperature of the interior space of the enclosure, and wherein the
controller is configured to halt the compressor if the temperature
falls below a lower temperature limit.
9. The refrigeration system of claim 7, further comprising an
alternate energy source, wherein the controller is configured to
receive a temperature signal indicative of a temperature of the
interior space of the enclosure, and wherein the controller is
configured to enable the alternate energy source if the temperature
rises above an upper temperature limit.
10. The refrigeration system of claim 7, wherein the controller is
configured to calculate a voltage rate-of-change magnitude, wherein
the controller is configured to increment the compressor speed when
the voltage rate-of-change magnitude is below a predetermined
threshold, and wherein the controller is configured to decrement
the compressor speed when the voltage rate-of-change magnitude is
above a predetermined threshold.
11. The refrigeration system of claim 1, wherein the compressor has
a direct electrical connection to the photovoltaic panel.
12. The refrigeration system of claim 1, wherein the compressor is
a DC powered, variable speed compressor, the output of which is
responsive to an amount of received solar radiation.
13. A solar powered apparatus which comprises: a variable speed
motor; a power source which converts light into electrical power
having a DC voltage; a power bus coupled to the power source and
configured to provide electrical power at said DC voltage to the
variable speed motor; and a controller coupled to the power bus to
detect said DC voltage and coupled to the variable speed motor to
provide a speed control signal, wherein the controller is
configured to adjust the speed control signal to maximize usage of
the electrical power from the power source.
14. The solar powered apparatus of claim 13, wherein the variable
speed motor turns a compressor.
15. The solar powered apparatus of claim 14, wherein the compressor
is configured to circulate refrigerant through a vapor-compression
refrigeration cycle to store thermal energy by inducing a phase
change in a phase-change material included in a thermal
reservoir.
16. The solar powered apparatus of claim 13, wherein the controller
determines a time derivative of said DC voltage, wherein the
controller increases the motor speed if the derivative is above a
predetermined threshold, and wherein the controller decreases the
motor speed if the derivative is below the predetermined
threshold.
17. The solar powered apparatus of claim 13, wherein the controller
determines a second-order time derivative of said DC voltage, and
wherein the controller increases the motor speed if the
second-order derivative is above a predetermined threshold, and
wherein the controller decreases the motor speed if the
second-order derivative is below the predetermined threshold.
18. The solar powered apparatus of claim 17, further comprising a
capacitor coupled in parallel with the power source.
19. A method for extracting maximum power from a power-limited,
direct current (DC) power source having a capacitive
characteristic, wherein the method comprises: measuring a power
voltage; determining an unsigned rate of change for said power
voltage; incrementing a rate of power extraction from said power
source if said unsigned rate of change is less than a predetermined
threshold; and decrementing a rate of power extraction from said
power source if said unsigned rate of change is greater than the
predetermined threshold.
20. The method of claim 19, wherein incrementing the rate of power
extraction includes adjusting a compressor speed upward, and
wherein decrementing the rate of power extraction includes
adjusting the compressor speed downward.
21. The method of claim 20, wherein decrements to the compressor
speed are at least twice as large as increments to the compressor
speed.
22. The method of claim 20, wherein the compressor speed is
adjusted downward by an amount proportional to the unsigned rate of
change.
23. A solar powered compressor control method which comprises:
acquiring samples of a power voltage; determining a first unsigned
rate of change for said power voltage; determining a subsequent
unsigned rate of change for said power voltage; incrementing a
compressor speed if said first unsigned rate of change is greater
than the subsequent unsigned rate of change; and decrementing the
compressor speed if said first unsigned rate of change is less than
the subsequent unsigned rate of change.
24. The method of claim 23, wherein decrements to the compressor
speed are at least twice as large as increments in the compressor
speed.
25. The method of claim 23, wherein the compressor speed is
adjusted by an amount related to the subsequent unsigned rate of
change.
26. A solar powered refrigeration apparatus, comprising: a
compressor; a solar powered electrical power source electrically
connected to said compressor; a thermal energy storage device
having communication with said compressor; and control means,
associated with said power source, for accumulating electrical
power derived from said power source and for conducting accumulated
electrical power, and continuous electrical power derived from said
power source, to said compressor.
27. The solar powered refrigeration apparatus of claim 26, wherein
the thermal energy storage device comprises a phase change
material.
28. A control method for a solar powered refrigeration system
having a compressor motor, which obtains a first DC current from a
solar power source, and obtains a second DC current from a
capacitance storage source, and conducts the first and second DC
currents to said compressor motor for driving said motor during a
"start-up" phase of said compressor motor, said method comprising
the steps of: (A) monitoring system voltage of said compressor
system; (B) determining whether the power required to drive said
compressor system is greater than the power produced by said first
power source; (C) upon determining that additional power is
required to start said compressor system, drawing additional power
from said capacitance source for starting said compressor motor;
and, (D) subsequently, driving said compressor motor substantially
by said first DC current.
29. The control method according to claim 28, wherein step (D)
comprises driving said compressor at a decreased speed, relative to
nominal, during a start-up period, thereafter driving said
compressor at an increased speed.
30. A control method for a solar powered refrigeration system,
which system obtains a first DC current from a solar power
collector source, obtains a second DC current from a capacitive
storage source, adds the first and second DC currents, and conducts
the integrated currents to a load for operating said solar power
collector source at its maximum power point, comprising the steps
of: (A) operating said capacitance storage source as an energy
buffer; (B) balancing the electrical load of said solar powered
system with the power collecting capability of said solar
collector; and, (C) achieving load balance by monitoring said solar
powered system voltage and adjusting the system load in relation to
said voltage.
31. The control method according to claim 30, wherein step (B)
comprises the steps of: (a) determining if said system load
requires more power than said solar collecting source is producing;
(b) decreasing said load if said capacitor voltage is decreasing;
(c) determining if said system load requires less power than said
solar collecting source is producing; (d) increasing said load if
said capacitor voltage is increasing; and, (e) subsequently
maintaining the solar collecting source maximum operating and
collecting capacity by adjusting said voltage levels through
ongoing dithering of said load.
32. The control method of claim 31, wherein the steps (b) and (d)
are accomplished by adjusting the speed of said motor.
33. A solar powered refrigeration apparatus, comprising a vapor
compression refrigeration system; a solar powered electrical power
source electrically connected to said refrigeration system; an
alternate electrical power source connected to said refrigeration
system; a device which combines said power sources such that the
refrigeration system can run on any combination of the two sources;
and a controller which electrically connects the vapor compression
refrigeration system to both sources of electrical power and gives
preference to the solar power source.
34. The apparatus of claim 33, wherein the alternate source is an
alternating current (AC) power source.
35. A solar powered refrigeration apparatus which comprises: a
compressor system; a solar powered electrical power source
electrically connected to said compressor system; and a control
means, associated with said solar power source, for accumulating
electrical power derived from said solar power source, and for
conducting accumulated electrical power and continuous power
derived from said solar power source to said compressor system.
36. The apparatus of claim 35, wherein said control means includes
a capacitive power storage means.
37. The apparatus of claim 35, wherein said solar power source is
directly electrically connected to said compressor system during
start-up of said compressor system.
38. The apparatus of claim 35, wherein said control means further
comprises means for increasing a level of power applied to said
compressor during start-up of said compressor by applying both
continuous power and an increased level of accumulated power to
said compressor.
39. The apparatus of claim 35, wherein said control means comprises
means for repeatedly adjusting, by means of motor speed
adjustments, a load associated with said motor to enable said motor
to use substantially all of the available power.
40. A power control system for efficiently applying power received
from a solar collector array to an electric motor, wherein the
system comprises: buffer circuitry connected to receive power from
said solar collector array, the buffer circuitry including means
for accumulating electrical power derived from said solar collector
array; speed control means associated with said electric motor; and
control circuitry associated with said buffer circuitry and said
electric motor for repetitively monitoring the motor load and
available power available from both said solar collector array and
said buffer circuitry, and for adjusting said speed control means
to cause said motor to utilize substantially all of the available
power.
41. A solar powered apparatus which comprises: a variable speed
motor; a power source which converts light into electrical power
having a DC voltage; a power bus coupled to the power source and
configured to provide electrical power at said DC voltage directly,
without battery storage means, to the variable speed motor; and a
controller coupled to the power bus to detect said DC voltage and
coupled to the variable speed motor to provide a speed control
signal, wherein the controller is configured to adjust the speed
control signal to maximize usage of the electrical power from the
power source.
Description
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] 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.
[0004] 2. Description of the Related Art
[0005] 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 photovoltaic (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,
[0006] Previous attempts to produce a marketable solar refrigerator
have been largely unsuccessful. For example, consider the following
patents:
[0007] In U.S. Pat. No. 4,126,014, Thomas Kay discloses an
absorption refrigeration system powered by a heated fluid from a
solar panel.
[0008] In U.S. Pat. No. 5,501,083, Tae Kim discloses an AC-powered
air conditioner having a solar panel for backup electrical
power.
[0009] 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.
[0010] 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.
[0011] 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.
[0012] 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
[0013] 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 PV 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.
[0014] 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.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] A better understanding of the present invention can be
obtained when the following detailed description of preferred
embodiments is considered in conjunction with the following
drawings, in which:
[0016] FIG. 1 is a block diagram of a first solar refrigeration
system embodiment;
[0017] FIG. 2 is a block diagram of a second solar refrigeration
system embodiment;
[0018] FIG. 3 is a graph of an exemplary I-V curve for a
photovoltaic panel;
[0019] FIG. 4 is a graph of an exemplary I-V curve for a
photovoltaic panel in reduced light;
[0020] FIG. 5 is a flowchart of a first compressor speed control
method; and
[0021] FIG. 6 is a flowchart of a second compressor speed control
method.
[0022] While 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
[0023] 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.
[0024] 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.
[0025] 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.
[0026] 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.
[0027] 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.
[0028] 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.)
[0029] 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).
[0030] 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.
[0031] 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.
[0032] 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.
[0033] 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.
[0034] 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 208 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.
[0035] 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.
[0036] 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 is slightly below the phase
change temperature.
[0037] 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.
[0038] 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.
[0039] 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.
[0040] 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.
[0041] 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.
[0042] 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.
[0043] 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.
[0044] 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.
[0045] 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.
[0046] 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.
[0047] 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.
* * * * *