7.2.5 Gas Engine Driven Heat Pumps

These systems typically are conventional DX packaged systems that utilize a natural gas or propane engine in place of an electric motor. During cooling operation, the gas engine will operate the cooling compressor to provide cooling to the DX coil. During heating operation, the same effect will be utilized. In both cases, the manufacturer may include the option for heat recovery from the gas engine for use in building hot water loads, such as domestic hot water or heating coils.

Given the unique nature of these systems, the energy modeling will need to utilize a system selection in the modeling software specific to gas engine driven heat pumps. In addition, user defined operating curves will need to be input to reflect the manufacturer specific operation during heating and cooling modes. Should the system utilize heat recovery, the heat recovery efficiency will need to be determined and assigned to the system. Two possible types of heat recovery include jacket based heat recovery, where heat is extracted from the engine jacket used to cool the engine, and exhaust based heat recovery, where heat is recovered from the engine exhaust. In some cases, heat recovery may come from both sources. The heat recovery energy will then need to be assigned to a hot water or other demand in the building. This might include supplying domestic hot water needs for the occupants or perhaps reheat coils used downstream in the HVAC system.

The baseline building is not modeled with gas engine driven heat pumps.