Design guidelines for direct expansion ground coils

Direct expansion ground coil coupled heat pump (DXGC) systems have certain energy efficiency advantages over conventional ground coupled heat pump (GCHP) systems. Principal among these are the elimination of the secondary heat transfer fluid heat exchanger and circulating pump. While the DXGCs concept can have higher efficiencies, it also has more system design and environmental problems (e.g., compressor starting, and oil return). General design guidelines for DXGC systems are not well documented. A three pronged approach was adopted for this study: (1) a literature survey; (2) visit to existing DXGC installations; and (3) a laboratory study of a DXGC heat pump system with R-22 as the refrigerant, for both heating and cooling mode tests done in parallel and series tube connections. The results of the literature survey and laboratory study are described in detail. It was found from the laboratory test results that a DXGC can both absorb from, or dissipate to, the ground at least twice as much heat as the conventional, plastic tube ground coil heat exchangers. A set of general design guidelines was derived from the test results and is presented. This information should be useful to those interested in DXGC designs. 59 refs., 24 figs., 5more » tabs.« less