Design and Construction of a Geothermal CO_2 Air Conditioner

摘要

A major portion of energy (～48%) in residences is consumed for air-conditioning and heating. Geothermal (water-to-air) heat pumps have proved to be more efficient than air-to-air heat pumps, however, they still use refrigerants with high global warming potential (GWP). Carbon dioxide (CO_2) is a low-GWP (GWP=1) natural refrigerant and can be a viable option for application in heat pumps. Since the heat is rejected to circulating water at temperatures ranging from 18℃ to 30℃ (64.4℉ to 86.0℉), the CO_2 thermodynamic cycle often operates in the subcritical region, as opposed to being supercritical if used in an air-to-air system. The efficiency of a CO_2 system is expected to be comparable to a conventional system if it is operated in the subcritical region. In this project, a geothermal air conditioner with a 7.9 kW (26.6 kBtu/r) cooling capacity was designed and built. The expected EER of the air conditioner was 4.94 based on the test conditions stated in the ISO 13256-1 standard. The effects of the components on the unit capacity and EER were analyzed. A reciprocating semi-hermetic compressor was selected to compress refrigerant. A steady-state heat exchanger simulation model was applied to design and select an evaporator with 5 mm copper tubes, and a brazed-plate heat exchanger was used for the condenser. A suction line brazed-plate heat exchanger was added to the unit to achieve a higher EER CuFe2P alloy tubes were used as refrigerant lines to accommodate the high-side operating pressure. Performance testing is currently underway at the National Institute of Standards and Technology (NIST). This paper reviews the design process, modeling results, and the unique manufacturing challenges required to produce a geothermal air conditioner with CO_2 as the working fluid.