Water-source heat pumps (WSHP) are an alternative system for areas without natural gas available. Water-source heat pumps are also popular with public school districts because they’re dependable, have low upkeep, and staff maintenance personnel can work on them. Public schools are often pressed for funds, so there are three pluses in favor of WSHPs because WSHPs are typically the lowest-cost system for a sizable campus (as compared to DDX, air-cooled, or water-cooled chillers). The one flaw of the system is how to handle the heating load. If natural gas is available, a gas boiler in the loop can provide the needed heat with ease. If not, then an electric boiler is necessary—and the cost to operate it can be quite high. The only other alternative is to install ground-source heat pumps—which use the ground for the heat source as well as the heat sink. However, they’re prohibitively expensive.
A compromise solution is to install a hybrid arrangement: a cooling tower sized to handle the peak building cooling load and a ground loop field sized to manage the building heating load. (This is the approach for southern climates where the cooling load is predominant.) This hybrid system can provide heating at a cost competitive with natural gas service. The strategy applies well as a retrofit. A school with an existing WSHP/fluid cooler system and an electric boiler pays high use- and demand-electricity bills. There’s plenty of ground available at a school, for a single ground loop field, which can be sized for the entire campus heat load. The existing pumps do not have to be modified, or additional pumps added, so the incremental operating cost for the system is zero. The actual installation cost is comprised of a few wells and ground piping loops and some simple controls. The electric boiler remains in place, as backup heat in the event of prolonged cold weather. Furthermore, the wells are available for peak cooling-load reduction in the summertime.
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Excerpt from Energy Conservation Projects.