Air conditioning and refrigeration systems are designed to meet the load at some particular designconditions. At off-design conditions, the compressor is either cycled or a variable speed drive enables thecompressor to run at a speed that matches the load. However, cycling can incur efficiency losses as large as 10-20%of total compressor power. Similarly, the variable speed compressors suffer from an efficiency penalty as a result ofthe losses in the inverter drive (4-6%).Part 1 of this report focuses on the use of tandem compressors, that is, a combination of two or morecompressors running in parallel with each other, to more closely match the load at different conditions. Simulationmodels were run for various operating conditions and the results from the tandem simulations were compared withthe single speed and the variable speed compressor results, showing a significant COP over the single speed caseand a smaller improvement over the variable speed case. Temperature data from Pittsburgh and Dallas was thenincorporated, and the optimum compressor sizing and operating strategies were compared. Results again showed aCOP gain of 7.5% and 6.2% for the two cities, respectively, over the case of a single-speed compressor, and 2.2%and 2.0% compared to a variable-speed compressor.Part 2 focuses on vapor injection in case of scroll compressors used in air-conditioning and refrigerationapplications. Vapor injection alters the normal refrigeration cycle by adding an injection port to the scrollcompressor. After partial expansion upstream of the evaporator, refrigerant vapor is injected into the compressor. Itsaves compressor energy because that fraction of the flow needs only partial compression, and saves compressorvolume because less vapor enters the suction port and the refrigerating effect increases due to the lower inlet quality.Simulation models were run for different operating conditions and it was discovered that for systems sized to meetthe same design load, the VI systems had a higher COP of around 8-10%, for air-conditioning applications.Similarly for the refrigeration case, the VI system registered a COP increase of as much 16%. VI systems alsohelped in reducing the compressor displacement by around 25% in the air-conditioning and 30% in the refrigerationcase respectively.
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