Solar refrigeration

RENEWABLE ENERGY (Solar Refrigeration)

RENEWABLE ENERGY
(Solar Refrigeration)

Abstract:

As the population increases there is increase in need for energy. So many Country’s stated producing electrical energy. Since, it is easy to transport for long distance and also to control. Electricity generation is the leading cause of industrial air pollution in India. Most of our electricity comes from coal, nuclear, diesel and other non-renewable power plants. Producing energy from these resources takes a severe toll on our environment, polluting our air, land, and water. And growing fuel scarcity also raising an alarm to GO GREEN to ensure and preserve a green environment for our future generation. To keep pollution under control and to produce the required energy the only way is to utilize the Renewable energy recourse which is available in abundance.

Renewable energy sources like solar electric, wind, geothermal, biomass, and small and low-impact hydro can be used to produce electricity with fewer environmental impacts. It is possible to make electricity from renewable energy sources without producing CO₂, the leading cause of global climate change. Resent statistical report says that only 7% of renewable energy has been utilized in U.S. for the year 2007 which is 6.8 quads (quadrillion Btu). Only half of renewable energy goes to producing electricity. The next largest use is the production of heat and steam for industrial purposes. Renewable fuels, such as ethanol, are also used for transportation and to provide heat for homes and businesses. Renewable energy plays an important role in the supply of energy. When renewable energy sources are used, the demand for fossil fuels is reduced. Unlike fossil fuels, non-biomass renewable sources of energy (hydropower, geothermal, wind, and solar) do not directly emit greenhouse gases.

Energy from the Sun

Solar energy is a very large , inexhaustible source of energy . The power from the sun intercepted by the earth is approximately 1.8 x 10 ¹¹ MW , which is many thousands of times larger than the present consumption rate on the earth of all commercial energy sources. The sun has produced energy for billions of years. Solar energy is the sun’s rays (solar radiation) that reach the earth. Solar energy can be converted into other forms of energy, such as heat and electricity. In the 1830s, the British astronomer John Herschel used a solar thermal collector box (a device that absorbs sunlight to collect heat) to cook food during an expedition to Africa. Today, people use the sun's energy for lots of things.

W will now survey a number of thermal applications. These are,

(1)Water heating

(2) Space cooling and refrigeration

(3) Power generation

(4) Space heating and drying

Solar Refrigeration System:

Three types of solar Refrigeration systems are possible:

(1) Conventional Vapour Compression system with electricity generated using solar

photovoltaic cells.

(2) Conventional Vapour Compression system with electricity generated by Solar thermal

power cycle.

(3) Thermally powered Refrigeration cycle using direct solar heat

Vapor Compression Refrigeration:

Prior to discussing how solar energy could potentially provide refrigeration, it is appropriate to review the basic principles of operation for vapor compression refrigeration cycles that form the foundation for nearly all conventional refrigeration. A schematic of the vapor compression cycle is shown in Figure 1. In the vapor compression cycle, cooling is provided in the evaporator as low temperature refrigerant entering the evaporator as a mixture of liquid and vapor at State 4 is vaporized by thermal input from the load. The remaining equipment in the system reclaims the refrigerant and restores it to a condition in which it can be used again to provide cooling. The vapor exiting the evaporator at State 1 in a saturated or slightly superheated condition enters a compressor that raises the pressure and, consequently, the temperature of the refrigerant. The high pressure hot refrigerant at State 2 enters a condenser heat exchanger that uses ambient air or water to cool the refrigerant to its saturation temperature prior to fully condensing to a liquid at State 3. The high-pressure liquid is then throttled to a lower pressure, which causes some of the refrigerant to vaporize as its temperature is reduced. The low temperature liquid that remains is available to produce useful refrigeration. The major energy input to a vapor compression refrigeration system is the mechanical power needed to drive the compressor. The minimum compressor power is given in Equation 1. The compressor power requirement is substantial because the specific volume of the refrigerant vapor, v, is large. Additional power is needed to operate the fans or pumps to move the external fluids. --------------- 1

The figure of merit for a vapor compression refrigeration system is its coefficient of performance (COP) defined as the ratio of the cooling capacity to the total electrical power required. The COP for a system providing refrigeration at –10°C (14°F) while rejecting heat to a temperature at 30°C (86°F) is approximately 3. The system COP diminishes from that level when the electrical power required for moving the external fl uids is accounted for in the coefficient of performance. Two of the solar refrigeration systems considered here rely on the vapor compression refrigeration cycle in some form. The third solar refrigeration system uses thermal energy as the primary input to the cycle. Open cycle systems that use water as the refrigerant, such as the solar desiccant cycle, could be used to provide cooling at temperatures above freezing but these alternatives are not considered here.

Photovoltaic Operated Refrigeration Cycle:

Photovoltaics (PV) involve the direct conversion of solar radiation to direct current (DC) electricity using semi conducting 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 (10 800 W/ft2) and a module temperature of 25°C (77°F). Unfortunately, PV modules will operate over a wide range of conditions that are rarely as favorable as the rating condition. In addition, the power produced by a PV array is as variable as the solar resource from which it is derived. The performance of a PV module, expressed in terms of its current voltage and power-voltage characteristics, principally depends on the solar radiation and module temperature. At any level of solar radiation and module temperature, a single operating voltage will result in maximum electrical power production from the module.

The efficiency of the solar panels, defined as the ratio of the electrical power produced to the incident radiation is between 8% to 10% at maximum power conditions for the PV array represented in Figure. If the PV refrigeration system is to operate at high efficiency, it is essential that the voltage imposed on the PV array be close to the voltage that provides maximum power.

This requirement can be met in several ways. First, a maximum power tracker can be used which, in effect, continuously transforms the voltage required by the load to the maximum power voltage. If the system includes a battery, the battery voltage will control the operating voltage of the PV module. PV panels can then be chosen so that their maximum power voltage is close to the voltage for the battery system.

The battery also provides electrical storage so that the system can operate at times when solar radiation is unavailable. However, the addition of a battery increases the weight of the system and reduces its steady-state efficiency. Electrical storage may not be needed in a solar refrigeration system as thermal storage, e.g., ice or other low temperature phase storage medium, may be more efficient and less expensive.

A final option for systems that do not use a maximum power tracker or a battery is to select an electric motor having current-voltage characteristics closely matched to the maximum power output of the module. Figure superimposes the current-voltage characteristics of a series dc motor and separately excited motor on the photovoltaic module. In this case, the separately excited motor would provide more efficient operation because it more closely matches the maximum power curve for the photovoltaic module. However, neither motor type represented in Figure 3 is well-matched to the characteristics of the PV module over the entire range of incident solar radiation. Studies of solar-powered motors have shown that permanent magnet or separately excited dc motors are always a better choice than series excited dc motors in direct-coupled systems that are not equipped with a maximum power tracker.

Solar 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. Absorption cooling systems that use lithium bromide-water absorption-refrigerant working fluids can not 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 minimum mechanical power needed to operate the pump is given by Equation 1, the same equation that applies to the minimum power needed by a compressor. However, the power requirement for the pump is much smaller than that for the compressor since v, 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 fl at-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 efficiency of an evacuated tubular collector for different levels of solar radiation and energy delivery temperatures is given in Figure . 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.

Conclusion

The major advantage of solar refrigeration is that it can be designed to operate independent of a utility grid. Applications exist in which this capability is essential, such as storing medicines in remote areas. Of the three solar refrigeration concepts presented here, the photovoltaic system is most appropriate for small capacity portable systems located in areas not near conventional energy sources (electricity or gas). Absorption and solar mechanical systems are necessarily larger and bulkier and require extensive plumbing as well as electrical connections. In situations where the cost of thermal energy is high, absorption systems may be viable for larger stationary refrigeration systems. The solar mechanical refrigeration systems would require tracking solar collectors to produce high temperatures at which the heat power cycle efficiency becomes competitive. If the capital cost and efficiency of tracking solar collectors can be significantly reduced, this refrigeration system option could be effective in larger scale refrigeration applications.