Closed Water Loop Water-Source Heat Pump (WLHP) Systems

General

Many buildings have large internal zones that require cooling only during occupied hours, or cooling, year round due to internal heat gains. The chiller condenser water heat is recovered, instead of being rejected to the outside, and used as a heat source for a water-source heat pump system serving the perimeter spaces.

The closed water loop provides condenser water to the interior zone cooling units, picking up the heat normally rejected to a cooling tower, and transferring it as the heat source (evaporator load) for the perimeter zone heat pumps. This substantially increases the coefficient of performance (COP) and lowers the operating cost of the heat pumps over conventional air-source heat pumps. The water loop temperature is typically maintained between about 65 and 90°F and therefore piping can be uninsulated. Loop circulation is typically between 2 to 3 gpm per ton of cooling capacity.

Closed Loop Water-Source Heat Pump

During peak winter conditions when the heat pumps cool the loop below 65°F, a boiler (electric, gas, oil) is used to maintain the 65°F temperature. During peak cooling conditions when all or most of the units are rejecting heat to the loop and its temperature tends to rise above 90°F, a closed-circuit evaporative cooler is used to reject the unneeded heat. As the loop temperature rises, the dampers on the evap cooler open to allow gravity circulation over the water coil. If the loop continues to rise, the water spray circulator is turned on. If the loop still rises, then the fans are turned on for full capacity performance.

Advantages:

  • Immediate all-year cooling and heating availability in every zone with only 2 water pipes
  • Maximum operating diversity at all times as units are only on when needed
  • Small mechanical room since no central refrigeration plant is needed
  • No seasonal changeover required
  • No skilled operators required
  • No piping insulation required
  • Architectural flexibility since terminal units are available in a wide number of configurations
  • Odd-hour flexibility without running a central refrigeration plant
  • Recovery of waste heat lowers operating costs
  • Failure of a unit does not affect others
  • Minimum initial investment on a speculative building project since water loop can be designed and installed and terminal units purchased and installed later as space is leased
  • Relatively constant electrical demand year round (i.e., better annual load factor)
  • Usable space is increased since ductwork is minimal
  • One system suitable for both interior and perimeter spaces
  • Individual tenant heat pumps can be metered and tenants billed for their use

Disadvantages:

  • Circulating pump must operate continuously and these pumping costs can be high
  • If core areas do not require many hours of cooling during the heating season, (such as apartment houses, or hotels with infrequently used public spaces), excessive boiler use can negate the high COP
  • Maintenance is required in tenant spaces

Another variation that recovers even more of the otherwise rejected heat involves adding a water-to-water heat pump in the loop. This heat pump sends hot water to a thermal storage tank. This stored hot water is then re-injected back into the loop to heat the loop when heat is required, thereby delaying or avoiding the use of any purchased energy in the boiler. Another alternative is to preheat water for use as service (domestic) hot water.

Links to Related Topics

Heat Pump Designs
Closed Water Loop Heat Pump