Assignment on

Solar Refrigeration System:

Theory & Application

Submitted By:

Mohammad Shakil Khan

MS in Renewable Energy Technology

Institute of Energy

3rd

Batch, Roll: 01

3rd

Semester, Session: 2013-14

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

Refrigeration is a process in which work is done to move heat from one place to another. In this

process heat is removed from a material or space, so that it’s temperature is lower than that of

surroundings.

When refrigerant absorbs the unwanted heat, this raises the refrigerant’s temperature (―Saturation

Temperature‖) so that 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 gas. To accomplish this, the refrigerant is 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 or solar thermal energy. Solar Photovoltaic refrigerators operate on the

same principle as normal compression refrigerators but incorporate low voltage (12 or 24V) dc

compressors and motors, rather than mains voltage ac types. A photovoltaic refrigerator has

higher levels of insulation around the storage compartments to maximize energy efficiency, a

battery bank for electricity storage, a battery 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 litres of vaccine storage capacity with ice production

rates of up to 6.4 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 through a vapor compression refrigeration

loop that extracts heat from an 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 control method fully exploits

the available energy. Other power optimization measures include:

Smoothing the power voltage via a 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 if 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 Solar Refrigeration

Photovoltaic Operated Refrigeration Cycle

Solar Mechanical Refrigeration

Absorption Refrigeration

Photovoltaic Operated Refrigeration Cycle:

Photovoltaics (PV) involve the direct conversion of solar radiation to direct current (dc)

electricity using semiconducting materials. In concept, the operation of a PV-powered solar

refrigeration cycle is simple. Solar photovoltaic panels produce dc electrical power that can

be used to operate a dc 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 standard rating conditions, i.e., incident solar radiation of

1,000 W/m2 (10800 W/ft

2) 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 maximum power output.

Must find compressor motor closely matched to the electric characteristics of the PV

module.

Figure 2: Photovoltaic Operated Refrigeration Cycle

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 piston expander to produce mechanical power, as shown in Figure 3.

The fluid exiting the expander is 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 temperatures 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 multi-cover flat plate

collectors can be used to produce fluid temperatures 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 produced 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 (e.g., 1,000 tons or 3,517 kWT)

System Regarding Considerations:

Efficiency optimization based on delivery temperature

Efficiency of Rankine cycle increases with increased heat exchanger temperature

Efficiency of solar collector decreases with increase in temperature

Figure 3: Solar driven Mechanical Power Cycle for Refrigeration

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 compression system.‖ A schematic of a single-stage

absorption system using ammonia as the refrigerant and ammonia-water as the absorbent is

shown in Figure 4. Absorption cooling systems that use lithium bromide-water absorption-

refrigerant working fluids cannot be used at temperatures below 0°C (32°F). The condenser,

throttle and evaporator operate in the exactly the same manner as for the vapor compression

system. In place of the compressor, however, the absorption system uses a series of three heat

exchangers (absorber, 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 required to carry away the heat of absorption. The principle governing this

phase of the operation is that a vapor is more readily absorbed into a liquid solution as the

temperature of the liquid solution is 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 the rectifier. 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 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 collection efficiency and the

coefficient of performance of the absorption system. The COP for a single-stage ammonia-

water system depends on the evaporator and condenser temperatures. The COP for providing

refrigeration at –10°C (14°F) with a 35°C (95°F) condensing temperature is approximately

0.50. Advanced absorption cycle configurations have been developed that could achieve

higher COP values. The absorption cycle will operate with lower temperatures of thermal

energy supplied from the solar collectors with little penalty to the COP, although the capacity

will be significantly reduced.

System Regarding Considerations:

- Minimal mechanical power input (pump instead of compressor)

- Absorption into water solution allows it to be pumped

- Desorbed in generator (rectifier required to separate out water)

- Heat into generator provided by solar collectors

- The pressurisation 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 vapour-compression air conditioning systems demand.

Figure 4: Absorption Refrigeration

Efficiency Measures

An overall system coefficient of performance (COPsys) can be defined as the ratio of

refrigeration capacity to input solar energy. The COPsys is low for all three types of solar

refrigeration systems. 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.

Refrigeration Effect

COPsys =

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 food

Use to make ice cream and other related products

Cooling purpose for Transportation vehicle like bus, car etc

Importance of Solar Refrigeration

There is environmental concern regarding conventional refrigeration technologies including

contribution to ozone layer depletion and global warming. Refrigerators which contain ozone

depleting and global warming substances such as chlorofluorocarbons (CFCs),

hydrochlorofluorocarbons (HCFCs) in their insulation foam or their refrigerant 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 vaccines have to be kept within a limited temperature range throughout transportation

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 or erratic.

The performance of refrigerators fuelled by kerosene and bottled gas is often inadequate.

Diesel powered systems frequently suffer fuel supply problems. Solar power is therefore of

great importance 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

1. https://en.wikipedia.org/wiki/Solar-powered_refrigerator

2. http://www.neerg.cn/appliances-solar-refrigerator-freezer.htm

3. https://www.nasa.gov/centers/johnson/techtransfer/technology/MSC-22970-1_Solar-

Refrigerator-TOP.html

4. http://www.nasa.gov/centers/johnson/home/solarfridge.html

5. http://www.appropedia.org/The_Design_and_Development_of_a_Solar_Powered_Refrigerat

or

6. http://www.backwoodshome.com/articles2/yago102.html

7. Solar Refrigeration by M. Devakumar

8. Solar Refrigeration by Sanford A. Klein, Ph.D., Fellow ASHRAE and Douglas T. Reindl,

Ph.D., Member ASHRAE