ASHRAE Journal - May 2012 - 30

Simultaneous Heating and Cooling

Simultaneous heating and cooling systems are Water WTWHP Supply V4 more complicated, but offer higher COPs, which Chilled Condenser Water makes them worth considering. V1 Supply Hot Geothermal heating and cooling. Because V3 Water Evaporator V2 the Earth’s mass can be used as both a heat source Return Chilled and a heat sink, a geothermal system can provide Water simultaneous heating-and-cooling. Figure 5 ilReturn lustrates the system design. When the facility requires heating, valves V1 and V2 connect the heat pump to the well field, while valves V3 and V4 connect the heat pump to the hotwater system. The evaporator water picks up heat from the well field and uses it to heat the hot water in the condenser, as illustrated in Figure 5. If the Well Field well field is of sufficient size, the heat pump can be sized to handle the entire heating requirement; it is not dependent on a simultaneous cooling load. When the facility requires cooling (not illus- Figure 5: Geothermal system for simultaneous heating and cooling, operattrated), valves V1 and V2 connect the heat pump ing in heating-only mode. to the chilled water system, while valves V3 and V4 connect the heat pump to the well field. The Boiler Hot Water Loop 180°F evaporator water picks up heat from the facility 130°F 145°F and rejects it to the well field via the condenser Boiler water. Rejecting large quantities of heat to the well field can cause operational difficulties. Instead, there may be uses within the facility for the 130°F low-temperature hot water. 150°F Chiller Plant WTWHP There will be times when both heating and Condenser cooling are needed. The heat pump can handle Evaporator both functions simultaneously, and the well field 58°F can provide any additional heat supply that is not 42°F 42°F available from the cooling load, or heat rejection that the heating load cannot handle. In this system, the cost of the well field in58°F 50°F creases the capital investment in the system. Chilled Water Loop However, the conventional chillers and boilers can be eliminated, and the high COP of the heat Figure 6: Side-stream arrangement. pump generally leads to a reasonable payback.6 Heat pump side-stream to chiller plant. Figure 6 illustrates This system offers several advantages when compared to a the most common piping arrangement for a simultaneous heat- system that uses cooling-tower water as the heat source. The ing-and-cooling system. In this system, standard water chillers heat pump can increase plant chilled water capacity, satisfy reare used to supply chilled water to the building or process. dundancy requirements, and reduce the required cooling tower A portion of the return chilled water flows through the capacity. evaporator of the high-lift heat pump, where heat is extracted, Heat pump parallel to chiller plant. This type of instaland the chilled water leaves at a lower temperature. The heat lation (not illustrated) is less common, but can be very useabsorbed from the chilled water, along with the compressor ful if the cooling load is small enough that the heat pump is work, is transferred to the condenser, where the combined heat capable of handling all or most of it. The small cooling load is rejected to the heating loop. is insufficient to sustain the total heating requirement, and a If necessary, the hot-water temperature can be boosted by boiler must be used to supplement heating. All heat obtained the boilers. The heat pump reduces the loads on both the chill- from the chilled-water loop is transferred to the hot water, and ers and boilers. Although the boilers are still providing some beneficially used. However, the LHWT cannot be controlled of the water heating, producing some of the heat with the by the machine while operating in the cooling priority mode, high-COP heat pump is much more efficient than producing because the cooling load will vary, causing the heat producall the heat using the boilers. tion to vary.
30 ASHRAE Journal ashrae.org May 2012

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ASHRAE Journal - May 2012

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