Economics of greenhouse heating with a mine air-assisted heat pump

An analysis of the economics of greenhouse heating with a heat pump coupled with an inactive deep mine is presented. Several heat pump and conventional gas-fired heater combinations (hybrid systems) were evaluated using a computer model to perform thermal and economic analyses. A 10 x 30 m, double-polyethylene-covered greenhouse, located in Charleston, West Virginia, was assumed for this analysis. Heat pumps with sufficient capacity to maintain 21 degrees C inside the greenhouse for outside temperatures ranging from 0 to 20 degrees C (in 2 degrees degree increments) were modeled. For each heat pump capacity, it was assumed that the additional heat energy required to maintain the specified inside temperature (when outside temperature fell below the heat pump design temperature) was supplied by a natural gas-fired heater. Life-cycle cost analysis was employed to compare greenhouse heating alternatives. The hybrid system offers lower operating costs than a conventional system for any outside design temperature. However, when initial cost is also considered, the hybrid system has a higher life cycle cost for heat pump design temperatures in the range 0 to 18 degrees C and a coefficient of performance (COP) of 3. As the heat pump COP increases beyond 3, the hybrid system becomes economically feasible, showing a lower life cycle cost than a conventional natural gas-fired system.