Water Source Heat Rejection and Acquisition in ApacheHVAC
There are presently at least two distinct possibilities for rejecting heat to a water thermal sink, such as a river, lake, pond, ocean, or ground water, and a third possibility in the works (this FAQ will be updated when this additional option is added). The second of these actually supports two different types of water-source heat-pump equipment. These alternatives depend upon the type of cooling equipment to be used:
- When using a typical Chilled Water Loop (CHWL) with chilled-water coils, radiant panels, and similar demand-side loads, heat from return flow of the CHWL loop demand-side (i.e., before it reaches the chiller evaporator inlet) can be rejected via a Water-Source Heat Exchanger (WSHX) on the Pre-cooling tab of the CHWL dialog. The source water temperature for the WSHX can be set as either a constant value or given by a profile.
This Pre-cooling function is obviously not the same as rejecting heat from a condenser loop to a water-source as thermal sink; however, when there is adequate cooling delta-T, this is preferable, as it obviates the need for addressing that load with the chiller. In other words, this is not simply a low-energy alternative to a condenser loop cooling tower, it is a low-energy alternative to running a mechanical chiller.
Note, by the way, that presently the mechanical cooling that addresses any load not firstly addressed by the WSHX can include water-cooled chillers, air-cooled chillers, heat-driven absorption chillers, and/or a heat-recovery chiller that rejects heat to a HWL.
- When using zone-level or AHU-dedicated Water-Source Heat Pumps, also referred to as “water-to-air heat pumps” (WAHPs) a common Heat Transfer Loop (HTL) for the acquisition and rejection of heat by those WAHPs, excess heat on the HTL can be rejected by a WSHX. And, when the HTL temperature is less than the water-source temperature, that source will add heat to the HTL, thus potentially assisting in maintaining the floating temperature range of the HTL with respect to both heat rejection and acquisition.
This capability is found on the “Water-source heat exchanger” tab of the of the HTL dialog. The source water temperature for the WSHX can once again be either set as a constant value or given by a profile. Depending on the source temperature and the HTL floating temperature range, the WSHX will typically take the place of a cooling tower for heat rejection and could potentially displace some amount of the heat addition typically provided to the HTL by a boiler when the overall balance of building loads leans substantially toward heating.
The same capability on the HTL is available when using Water-Source VRF units as the means of heating & cooling air at the AHU or zone level. This is as described above, with the only difference being that multiple VRF “outdoor” units (rather than WAHPs) are coupled to the common HTL for heat rejection and acquisition.
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