A water-source heat pump plant adds heat to, or rejects heat from, the closed-loop heat pump water that is circulated through water-source heat pumps located within the building. Water-source heat pumps, also known as water-to-air heat pumps, are water- cooled, self-contained units with a refrigeration system capable of reversing its cycle to provide either cooling or heating to the spaces in the building that they serve. A water- source heat pump differs from a heating, chilled, or dual-temperature water unit in that a water-source heat pump contains a refrigeration system that heats and cools the con- ditioned air through a direct expansion refrigerant coil instead of a heating water, chilled water, or dual-temperature water coil. During cooling operation, a water-source heat pump rejects heat to the heat pump water; during heating operation, a water- source heat pump absorbs heat from the heat pump water.
The temperature of the heat pump water must be maintained within certain limits, usually between 60 and 90°F, in order to enable the heat pump units to either reject or absorb heat from the heat pump water, depending upon their mode of operation. The
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water flow rate for a water-source heat pump system is typically 3 gpm per ton of installed cooling capacity. The range (the design temperature difference between the water leaving the individual water-source heat pump heat exchangers and the water entering the heat exchangers) is typically 10°F. Typical temperatures for the heat pump water are as follows:
• Summer: 90°F heat pump water supply and 100°F heat pump water return • Winter: 60°F heat pump water supply and 50°F heat pump water return The advantage of utilizing a water-source heat pump system to condition a build- ing is that some units can be operating in the cooling mode while other units are operat- ing in the heating mode. In fact, a water-source heat pump system is quite energy efficient when simultaneous heating and cooling within the building are required. For example, during the winter, the heat that is rejected to the heat pump water by the units operating in the cooling mode contributes to the warming of the heat pump water, thereby reducing the heat that must be added to the heat pump water by the heat source (which is typically a hot water boiler). The converse is true for units that operate in the heating mode during the summer. This exchange of heat from some areas of the build- ing to others through the heat pump water system reduces the use of purchased energy for the building.
However, when more heat is rejected by the units in the cooling mode than is absorbed by the units in the heating mode, it is necessary for the heat pump water sys- tem to have a means to reject the excess heat to the outdoors (referred to as a heat sink). Similarly, when more heat is absorbed by the units in the heating mode than is rejected by the units in the cooling mode, it is necessary for the heat pump water system to have a means to add heat to the system (referred to as a heat source).
Excess heat is normally rejected from the heat pump water either through a plate and frame heat exchanger to an open cooling tower, or through a closed-circuit cool- ing tower. Heat is added to the heat pump water typically through a hot water boiler. The ground can also be used as a heat sink and heat source for a water-source heat pump system. This type of system is called a ground-source (or geothermal) heat pump system.
Figure 4-39 is a schematic diagram for a typical water-source heat pump plant uti- lizing a plate and frame heat exchanger, an open cooling tower as the heat sink, and two high-efficiency hot water boilers as the heat source. In the heat addition mode, the 3-way diverting valve diverts the heat pump water to the boilers when the return water temperature drops below 65°F. The lead boiler and its associated primary pump start when the return water temperature drops below 60°F. The lag boiler and its associated primary pump start when the return water temperature drops below 50°F. The firing rate of each boiler burner is modulated to maintain a constant (boiler) outlet water tem- perature of 90°F. The 3-way mixing valve maintains the heat pump water supply tem- perature at 60°F. Upon a rise in return water temperature above 65°F, the reverse occurs. In the heat rejection mode, the 3-way diverting valve diverts the heat pump water to the plate and frame heat exchanger when the return water temperature rises above 65°F. When the heat pump return water temperature rises above 90°F, the tower water pump is started and the speed of the cooling tower fan is controlled to maintain the heat pump water supply temperature at 90°F. Upon a drop in return water temperature below 90°F, the reverse occurs.
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Figure 4-39 Water-source heat pump plant schematic piping diagram.
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Design Considerations
Design considerations for water-source heat pump plants are as follows:
1. Full redundancy should be designed for the hot water boilers and closed-loop heat pump water system pumps because these boilers and pumps serve the heating needs of the building.
2. Each of the two boilers in a water-source heat pump system should be sized for the sum of the heat absorbed by all the heat pump units operating at full heating capacity. This is done because all of the heat pump units could be operating at full heating capacity during morning warm-up. If a single boiler is not able to maintain the minimum loop temperature, the heat pump units could shut off due to a low refrigerant pressure safety. However, the heat absorbed by the heat pump units from the heat pump water system represents only about 75% of the total heat output of the heat pump units, assuming an average coefficient of performance (COP)52 for the heat pump units of 4.0. Therefore, the output of
each boiler serving a water-source heat pump system does not need to be sized for 100% of the peak heating load of the building, but only for approximately 75% of the peak heating load of the building.
3. The same design considerations regarding constant-speed, primary-only pumping, primary-secondary pumping, pipe sizing criteria, makeup water assembly, and the location of the boiler in the system are the same for heat pump water plants as they are for 4-pipe and 2-pipe heating and cooling plants.
4. Redundancy is not required in the heat rejection equipment if the system provides comfort cooling only (see the discussion of redundancy in the cooling equipment for 4-pipe and 2-pipe heating and cooling plants above).
5. A supply water temperature reset schedule based on outdoor air is not used for heat pump water plants. Rather, the heat pump water supply temperature is allowed to float between 60 and 90°F. For this reason, it is not necessary to add heat to, or reject heat from, the heat pump water system during certain periods of operation when both heating and cooling are required in the building and the loop temperature remains between 60 and 90°F.