Since the temperature of the compressed refrigerant gas is higher than that of the motor, heat loss from the motor cools, rather than heats, the refrigerant gas, resulting in a loss of radiant heat. The reasons are believed to be low motor efficiency (as with the first-stage compressor) as well as low mechanical efficiency due to operating the second-stage compressor under a low load. As was the case with the first-stage compressor, total efficiency was lower than the 0.59 assumed when efficiency estimates were made. Since the energy lost due to the low motor efficiency is used for heating the refrigerant gas, the “heat absorbed by the refrigerant” (denominator in η 1 and numerator in η 2) increases, and as a result, the value of η 1 decreases ana that of η 2 increases. (Experimental results led motor efficiency to be anticipated at 0.6.) The main reason is believed to be the extremely low motor efficiency. Total efficiency was thus notably lower than the 0.59 assumed when efficiency estimates were made. As the compressor efficiency varies at higher compressor efficiencies, the specific enthalpy at state 3 (compressor exit) for Systems 1, 2, and 3 are identical.įirst-stage compressor efficiencies were (ν 1 = 0.56 to 0.59, (ν 2 = 0.82 to 0.84, and (ν total = 0.45 to 0.49. The GSHP in System 3 is more sensitive to variations in compressor efficiency than that in Systems 1 and 2, primarily due to the design and operation of System 3 and its two stages of compression. The differences in heat pump COPs range from as low as 3.81 to as high as 5.32 (or by 1.51) for Systems 1 and 2, and from 3.80 to 5.42 (or by 1.62) for Systems 1 and 3. The COP increases more rapidly for System 3 with compressor efficiency. The results indicate that the heat pump COP increases almost linearly with compressor efficiency. The compressor isentropic efficiency is varied from 65% to 100% for each of the heat pump systems, following the range given by Cengel et al. 9, based on the results of previous investigations. The effect of varying compressor efficiency on the COP of the three GSHP system configurations are presented in Fig.
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