Solar Refrigeration System

Assignment onSolar Refrigeration System: Theory & Application

Submitted By:

Mohammad Shakil KhanMS in Renewable Energy Technology

Institute of Energy3rd 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/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 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.html4. http://www.nasa.gov/centers/johnson/home/solarfridge.html

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

6. http://www.backwoodshome.com/articles2/yago102.html 7. Solar Refrigeration by M. Devakumar8. Solar Refrigeration by Sanford A. Klein, Ph.D., Fellow ASHRAE and Douglas T. Reindl,

Ph.D., Member ASHRAE