Solar Powered Refrigeration System
Introduction
As a source of abundant free energy from the sun solar energy has
vast
prospect
to
utilize
in
several
areas
to
mitigate
the
energy
demand of everyday use. Besides the conventional lighting purpose,
solar energy can be harnessed to use for refrigeration system mainly
in off-grid areas solar refrigeration can be expected a new dimension
in utilizing solar electricity use.
Refrigeration is a process in which work s done to move heat from
one place to another. In the process heat is removed from a material
or space, so that it’s temperature is lower than that of surrounding.
When
refrigerant
refrigerant’s
absorbs
temperature
the
unwanted
("Saturation
heat,
this
Temperature")
raises
so
that
the
it
changes from a liquid to a gas- it evaporates. The system then uses
condensation to release the heat and change the refrigerant back into
a liquid. This is called “Latent Heat". This cycle is based on the
physical principle, that a liquid extracts heat from the surrounding
area
as
it
expands
(boils)
into
a
as
to
accomplish
this,
the
refrigerant s pumped through a closed looped pipe system. The closed
looped pipe system stops the refrigerant from becoming contaminated
and controls its stream. The refrigerant will be both a vapor and a
liquid in the loop.
Solar Refrigeration: Theory
Refrigerator which runs on electricity provided by solar energy
is known as solar refrigeration. A solar powered refrigerator is a
refrigerator which runs on energy directly provided by sun, and may
include
photovoltaic
refrigerators
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operate
or
on
solar
the
thermal
same
energy.
principle
as
Solar
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Photovoltaic
compression
refrigerators but incorporate low voltage (12 or 24V) dc compressors
and
motors,
rather
refrigerator
has
compartments
to
than
higher
maximize
mains
level
voltage
of
energy
ac
types
insulation
efficiency,
A
around
a
photovoltaic
the
battery
storage
hank
for
electricity storage a buttery charge regulator and a controller which
converts
the
power
from
the
battery
to
a
form
required
by
the
compressor motor. A typical solar refrigerator layout is as shown
below (Figure 1). Most refrigerators include a freezer compartment for
ice pack freezing. Other systems have separate units to provide solely
for refrigeration or freezing. Available sizes range between 10 and 85
liters of vaccine storage capacity with ice production rates of up to
64 kg per 24 hours.
Figure 1: Solar Refrigeration System
How Solar Refrigeration Works
Solar-powered
refrigeration
system
employs
a
PV
panel,
vapor
compressor, thermal storage and reservoir and electronic controls. The
process that makes the refrigeration possible is the conversion of
sunlight into DC electrical power, achieved by the PV panel. The DC
electrical power drives the compressor to circulate refrigerant though
a
vapor compression refrigeration loop that extracts heat from an
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insulated enclosure. This enclosure includes the thermal reservoir and
a phase change material. This material freezes as heat is extracted
from the enclosure. This process effectively creates an "ice pack,"
enabling temperature maintenance inside the enclosure in the absence
of sunlight.
Proper
sizing
of
the
highly
insulated
cabinet,
phase
change
thermal storage, variable speed compressor and solar PV panel allow
the
refrigerator
to
stay
cold
all
year
long.
To
optimize
the
conversion of solar power into stored thermal energy, a compressor
cool
method
fully
exploits
the
available
energy.
Other
power
optimization measures include:

Smoothing the power voltage via capacitor., providing additional
current during compressor start-up

Monitoring the rate of change of the smoothed power voltage using
a controller to determine if the compressor is operating below or
above the available maximum power, enabling adjustment of the
compressor speed necessary

Replacing the capillary tube in the refrigerator system with an
expansion valve, improving energy efficiency in certain operating
conditions
These
adjustments
to
the
compressor
operation
contribute
to
the
conversion of the majority of the available solar power into stored
thermal energy. Applications may include a cold side water loop or
incorporation of the evaporator into the thermal storage.
Types of Refrigeration;
Refrigeration systems is broadly classified into two;
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1.Compression Refrigeration (VCR),uses mechanical energy
2. Vapour Absorption Refrigeration (VAR), uses thermal energy
a.
b.
Figure 2a. Refrigeration Cycle of (VCR) ; 2b. Refrigeration Cycle of (VAR)
Types of Solar Refrigeration
1. Photovoltaic Operated Refrigeration Cycle
Photovoltaic (PV) involve the direct converts ion of solar radiation
to direct current (dc) electricity using semiconducting materials. In
concept,
the
operation
of
a
PV-powered
solar
refrigeration
cycles
simple. Solar photovoltaic panel produce dc electrical power that can
be used to operate a de motor, which is coupled to the compressor of a
vapor compression refrigeration system. The major considerations in
designing a PV-refrigeration cycle involve appropriately matching the
electrical characteristics of the motor driving the compressor with
the available current and voltage being produced by the PV array. The
rate of electrical power capable of being generated by a PV system is
typically provided by manufacturers of PV modules for stand and rating
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conditions, ie., incident soar radiation of 1,000 W/m2
(10800 W/ft2)
and a module temperature of 25 °C (77 °F).
System Regarding Considerations:

Must
match
voltage
imposed
on
PV
array
to
the
motor
characteristics and power requirements of the refrigeration cycle

For given operating condition (solar radiation and module
temperature), single voltage provides minimum power output

Must find compressor motor closely
characteristics of the PV module
matched
to
the
electric
Figure 3: photovoltaic Operated Refrigeration Cycle
2. Solar Mechanical Refrigeration
Solar
mechanical
refrigeration
uses
a
conventional
vapor
compression system driven by mechanical power that is produced with a
solar-driven heat power cycle. The heat power cycle usually considered
for this application is a Rankine cycle in which a fluid is vaporized
at an elevated pressure by heat exchange with a fluid heated by solar
collectors. A storage tank can be included to provide some high
temperature thermal storage. The vapor flows through a turbine or
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piston expander to produce mechanical power, as shown in Figure 3. The
fluid exciting the expander condensed and pumped back to the boiler
pressure where it is again vaporized. The efficiency of the Rankine
cycle increases with increasing temperature of the vaporized fluid
entering the expander. The efficiency of a solar collector, however,
decreases with increasing temperature of the delivered energy. High
temperature can be obtained from concentrating solar collectors that
track the sun's position in one or two dimensions. Tracking systems
add cost, weight and complexity to the system. If tracking is to be
avoided, evacuated tubular, compound parabolic or advanced muti-cover
flat plate collectors can be used to produce fluid temperature s
ranging between 100°C -200°C (212°F -392°F). The efficiency of solar
collectors depends on both solar radiation and the difference in
temperature between the entering fluid and ambient. The overall
efficiency of solar mechanical refrigeration, defined as the ratio of
mechanical energy produce to the incident solar radiation, is the
product of the efficiencies of the solar collector and the power cycle.
Because of the competing effects with temperature, there is an optimum
efficiency at any solar radiation Solar mechanical systems are
competitive only at higher temperatures for which tracking solar
collectors are required. Because of its economy-of-scale, this option
would only be applicable for large refrigeration systems (eg. 1,000
tons or 3.517 KwT)
System Regarding Considerations:

Efficiency optimization based on delivery temperature

Efficiency of Rankine
exchanger temperature

Efficiency of
temperatures
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solar
cycle
increases
collector
with
decreases
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increased
with
heat
increase
in
Figure 4: Solar driven Mechanical Power Cycle for Refrigeration
3. Absorption Refrigeration
Absorption refrigeration is the least intuitive of the solar
refrigeration alternatives. Unlike the PV and solar mechanical
refrigeration
options,
the
absorption
refrigeration
system
is
considered a “heat driven” system that requires minimal mechanical
power for the compression process. It replaces the energy-intensive
compression in a vapor compression system with a heat activated
“thermal expansion system”. A schematic diagram of a single-stage
absorption system using ammonia-water as the absorbent is shown in
figure 4. Absorption cooling systems that use the lithium bromidewater absorption refrigerant working fluid cannot be used at a
temperature below 0°C (32°F). The condenser, throttle and the
evaporator operate in the exactly the same manners as for the vapor
compression system. In place of the compressor, however, the
absorption system uses a series of three heat exchanges (absorbers,
regenerating intermediate heat exchanger and a generator) and a small
solution pump. Ammonia vapor exiting the evaporator (State 6) is
absorbed in a liquid solution of water-ammonia in the absorber. The
absorption of ammonia vapor into the water-ammonia solution is
analogous to a condensation process. The process is exothermic and so
cooling water is requires carrying away the heat of absorption. The
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principles governing this phase of the operation that a vapor is more
readily absorbed into a liquid solution as the temperature of the
liquid solution are reduced. The ammonia-rich liquid solution leaving
the absorber (State 7) is pumped to a higher pressure, passed through
a heat exchanger and delivered to the generator (State 1). The power
requirement for the pump is much smaller than that for the compressor
since the specific volume of the liquid solution is much smaller than
the specific volume of a refrigerant vapor. It is in fact, possible to
design an absorption system that does not require any mechanical power
input relying instead on gravity.
However, grid-connected systems usually rely on the use of a
small pump. In the generator, the liquid solution is heated, which
promotes desorption of the refrigerant (ammonia) from the solution.
Unfortunately, some water also is desorbed with the ammonia, and it
must be separated from the ammonia using rectifiers. Without the use
of a rectifier, water exits at State 2 with the ammonia and travels to
the evaporator, where it increases the temperature at which
refrigeration can be provided. This solution temperature needed to be
drive the desorption process with ammonia-water is in the range
between 120°C to 130°C (248°F to 266°F.) Temperatures in this range
can be obtained using low cost non-tracking solar collectors. At these
temperatures, evacuated tubular collectors may be more suitable than
flat-plate collectors as their efficiency is less sensitive to
operating temperature. The overall efficiency of a solar refrigeration
system is the product of the solar collections efficiency and the
coefficient performance of the absorption system.
The COP for a
single-stage ammonia- water system depends on the evaporator and
condenser temperatures. The COP or providing refrigeration at -10°(14°)
with a 35°C (95°F) condensing temperatures approximately 0.50.
Advanced absorption cycle configurations have been developed that
could achieve higher COP values. The absorption cycle will operate
with lower temperature of thermal energy supplied from the solar
collectors with little penalty to the COP values, although the
capacity will be significantly reached.
System Regarding Considerations:

Minimal mechanical power input (pump instead of compressor

Absorption into water solutions allows it to be pumped
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
Desorbed in generator (rectifier required to separate out water)

Heat into generator provided by solar collectors

The pressurization is achieved by dissolving the refrigerant in
the absorbent, in the absorber section

Subsequently, the solution is pumped to a high pressure with an
ordinary liquid pump

In this system condenser, throttle, evaporator function exactly
the same way, replaces compressor with “thermal compression
system”.

Ammonia is working fluid

In this way the refrigerant
vapour is compressed without the
need of large amounts of mechanical energy that the vapourcompression air conditioning systems demand
Figure 5: Schematic vie of absorption refrigerator driven by heat from solar
radiation
Efficiency Measures
An overall system coefficient of performance (COPsys) can be de
fined as the ratio of refrigeration capacity to input solar energy.
The COPsys is low for all three types solar refrigeration systems.
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However, this definition of efficiency may not be the most relevant
metric for a solar refrigeration system because the fuel that drives
the system during operation solar energy is free. Other system metrics
that are more important are the specific size, weight, and, of course,
the cost.
COP= Refrigeration effect / Heat input in generator
Application of solar refrigeration

Household and Commercial indoor cooling purpose

Refrigeration in off-grid rural areas for preserving food and
vaccine

Use in cold storage system to preserve rotten-prone

Use to make ice cream and other related products
food
Importance of solar refrigeration

There is environmental concern regarding conventional refrigeration
technologies including contribution to ozone layer depletion and
global warming. Refrigerator which contains ozone depleting and
global
warming
substances
is
chlorofluorocarbons
(CFCs).
Hydrochlorofluorocarbons (HCFCs) in their insulation foams or their
refrigerants cycle are the most harmful. If a conventional
refrigerator is inefficient or used inefficiently, it will also
contribute more to global warming than a highly efficient
refrigerator. The use of solar energy to power refrigeration strives
to minimize the negative impacts refrigerators have on the
environment.

All vaccine has to be keeps within a limited temperature range
throughout
transportations
and
storage.
The
provision
of
refrigeration for this, known as the’ Vaccine Cold Chain’, is a
major logistical undertaking in areas where electricity supplies are
non-existent. The performance of refrigerators fuelled by kerosene
and bottled gas is often inadequate. Diesel powered system
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frequently suffer fuel supply problems. Solar power is therefore a
great hand to health care.
Conclusion
The World Health Organization (WHO) estimates that nearly two
billion people in the world are without access to electricity that is
essential for storage of vaccines and medicine. Using solar energy in
refrigeration system could save a huge energy demand as well as reduce
the GHG related impact in its entire life cycle. Solar powered
refrigerators and freezers are cost-effective and can be powered by
solar, wind, fuel cells and batteries as well with low energy
consumption, less expensive power systems and low operating expense,
excellent reliability and long life.
Reference
http://en .wikipedia.org/wiki/Solar-powered_refrigerator
http://www.neerg.cn/appliances-solar-refrigerator-freezer.htm
http://www.nasa.gov/centers/johnson/techtransfer/technology/MSC-229701_Solar-Refrigerator-TOP.html
http://www.nasa.gov/centers/johnson/home/solarfridge.html
http://www.slideshare.net/mobile/MohammadShakilKhan/solarrefrigeration-system-5210638
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