The use of PV powered refrigerator/freezers (R/F) is increasing because of the high reliability they provide. Also, the high efficiency dc refrigerators are being used more in residential applications because their cost is decreasing as larger numbers are manufactured. In most instances, the efficiency of the dc refrigerator is higher than an equivalent size ac unit. This reduces the power requirements accordingly. For PV powered refrigerators, the mode of operation directly affects the total system cost. Factors such as number of users, door opening habits, seasonal use variations, time of loading, temperature of incoming material, and physical location of the unit will significantly affect the amount of power required. User training in proper operation and maintenance should be a part of any project to install R/F systems in remote areas.
The World Health Organization has specified PV power be used for medical vaccine refrigerators in numerous countries around the world. Many dollars worth of vaccine can be ruined if not maintained at the recommended temperature. This requires a reliable power source for the refrigerator.
Agricultural goods and milk cooling refrigeration
Solar PV is a promising alternative solution for cooling and preserving a variety of perishable goods, especially agricultural and dairy products as well, in areas with difficult access to the electricity supply, as is the case in developing countries or isolated rural areas. The simple technology applied to the refrigeration facility and the fact that it lacks complex electronics makes it possible to use and repair the system without external help.
Traditionally solar-powered refrigerators and vaccine coolers use a combination of solar panels and lead batteries to store energy for cloudy days and at night in the absence of sunlight to keep their contents cool. These fridges are expensive and require heavy lead-acid batteries which tend to deteriorate, especially in hot climates, or are misused for other purposes. In addition, the batteries require maintenance, must be replaced approximately every three years, and must be disposed of as hazardous wastes possibly resulting in lead pollution. These problems and the resulting higher costs have been an obstacle for the use of solar powered refrigerators in developing areas.
Of late, some companies have come up with more efficient and low cost solar power refrigerators. One such company is Promethean Power Systems, Cambridge, MA.
Promethean Power Systems has developed a solar-powered refrigeration system for commercial cold-storage applications in off-grid and partially electrified areas of developing countries. The technology enables food suppliers to store and preserve perishable food items—such as milk, fruits and vegetables—without the need for expensive diesel-powered generators.
The hybrid solar & grid-powered refrigeration system is designed for mission critical refrigeration applications that must operate 24 hours a day, 365 days a year. The combination of solar and grid power provides a necessary backup mechanism and eliminates the need for a dedicated diesel generator. The company’s first product, Antarctica 250, is a milk chiller with cooling capacity 250 liters per day and storage capacity of 500 liters.
According to the company, since the system use solar energy for part of energy input, operating cost is 50% lower than the operating cost of an equivalent grid-powered unit and nearly 80% lower than that of a diesel-powered unit.
Technology: The refrigeration system uses a thermal battery system to store cold energy during daytime using solar power and nightime using grid power. A “black-box” electronic control unit runs the entire system, minimizes energy consumption and optimizes the use of available solar and grid power. The hybrid-power optimizer uses proprietary algorithms to efficiently transfer solar and grid power to the thermal battery and electrical loads.
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