Development of an energy efficient stand-alone solar photovoltaic dc refrigerator for off-grid communities

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October, 2015
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A reliable and energy efficient approach to providing refrigeration needs (both grid connected and off-grid areas) is a major challenge in Ghana and most developing countries due to low energy access and unreliable power supply. Research and development of appropriate and sustainable stand-alone refrigeration technologies are therefore relevant and essential. In the current study, the methodology of converting an AC refrigerator to serve as a DC refrigerator has been explored. A 92 L stand-alone solar powered DC refrigerator operating with a Variable Speed Direct Current (VSDC) compressor has been developed. The design replaces the single speed alternating current (AC) compressor with a VSDC compressor. VSDC compressors are strong candidates to be employed in energy-efficient refrigeration systems, especially for renewable energy technologies, as these compressors do not require power inverters that an alternating current compressor would require. Secondly, these compressors present the option of matching the cooling requirements with refrigerator power consumption thereby reducing wastage of energy. In this study, the DC refrigerator is powered by an installed 170 Watt peak (Wp) solar PV panel, 20 A charge controller and a 100 AH battery bank without an inverter. By using the coefficient of performance and the pull down time as indicating factors, the developed refrigerator has been experimentally optimised with the correct amount of refrigerant charge using ISO test standard. The performance (in terms of evaporator temperature profile and energy consumption) of the DC refrigerator has been evaluated at various speeds of the VSDC compressor. Similarly, the performance of the solar powered DC refrigerator has been experimentally compared with an identical AC refrigerator operating on the national grid. vi The experimental results revealed that the 92 L DC refrigerator performed optimally when charged with 30 g of refrigerant 134a. This yielded an average coefficient of performance of 1.6 W/W and attained a pull down temperature of -12 o C in 20 minutes. Secondly, operating the refrigerator at lower speeds (2000RPM, 2500RPM and 3000RPM) of the VSDC compressor during high (positive) temperature settings presented the potential of saving 19 % to 44 % of the refrigerator’s energy consumption. It has also been observed that the AC refrigerator is associated with relatively high power consumption and power surges as compared to the developed DC refrigerator. Finally, an economic assessment conducted between the AC (with inverter) and the developed DC refrigerator (without an inverter) all powered by solar PV system indicates that converting an AC refrigerator to DC has the potential of reducing the overall system cost (refrigerator, battery, charge controller and solar panel) by 20 %.
A Thesis submitted to the School of Graduate Studies, Kwame Nkrumah University of Science and Technology, Kumasi, Ghana in partial fulfillment of the requirements for the Degree of Master of Science in Mechanical Engineering, 2015
Variable Speed, Direct Current, Compressor, Stand-Alone Solar PV, Refrigeration, Energy Savings,