Development of an energy efficient stand-alone solar photovoltaic dc refrigerator for off-grid communities
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Date
October, 2015
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Abstract
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.
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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 %.
Description
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
Keywords
Variable Speed, Direct Current, Compressor, Stand-Alone Solar PV, Refrigeration, Energy Savings,