ASHRAE Journal - January 2009 - (Page 29) and consulting engineers. They are general, rather than exhaustive, and focus on basic application questions, such as loads and hot water temperatures. More detailed design guidelines, such as control systems and operation procedures, are the subject of a future article. Load Profile. The ideal facility for this application would have a load profile similar to Figure 1, which illustrates a typical 24-hour day in each of the four seasons. The heat available from a heat pump (green line) is approximately 35% greater than the cooling load (blue line) because the heat of compression is included. From Figure 1, it can be seen that during most of the operating season, the heat available in the chilled water loop is more than adequate to satisfy all or most of the hot water requirement (red line). Only during the periods represented by the patterned portion of the graph is the heat pump not able to produce 100% of the facility’s hot water requirements. During these times, the heat pump can significantly supplement traditional sources of heat. The heat pump should be the first heat source to be used, because it is most likely the low-cost heat source. That can be verified by the following calculations, which use energy costs of $0.068 per kWh of electricity and $11.21 per 1,000 ft3 ($0.40 per m3) of natural gas: Energy Cost of Natural Gas Boiler 30,000 25,000 20,000 kBtu/h 15,000 10,000 5,000 – 12a 6a 12p 6p 12a 6a 12p 6p 12a 6a 12p 6p 12a 6a 12p 6p Typical Winter Day Typical Spring Day Typical Summer Day Typical Fall Day Heating Cooling Heat Pump Figure 1: Sample load proἀle for facility with simultaneous heating and cooling loads. 100,000 Btu ÷ 0.85 COP ÷ 1,000 Btu/ft3 × $11.21/1000 ft3 = $1.32 per 100,000 Btu (105,000 kJ ÷ 0.85 COP ÷ 37,500 kJ/m3 × $0.40/m3 = $1.32 per 105,000 kJ) Energy Cost of Electric Water-to-Water Heat Pump 100,000 Btu ÷ 3.83 COP ÷ 3,415 Btu/kW × $0.068/kWh = $0.52 per 100,000 Btu (105,000 kJ ÷ 3.83 COP ÷ 3600 kJ/kWh × $0.068/kWh = $0.52 per 105,000 kJ) Location of Heat Pump. As illustrated in Figure 2, the most popular location for the heat pump is in a side-stream arrangement, between both water streams. The heat pump is situated in this location for two reasons: 1) so it can be preferentially loaded before any of the chillers are brought on-line, and 2) so that the warmest return water is cooled in the heat pump, which slightly improves its COP. Hot Water Temperature. One difference between the traditional boiler system and a heat pump system is that the hot water temperature supplied by the heat pump may be significantly lower. This is due to the necessity of balancing the economics of operating the heat pump, versus the capital and operating costs of the connected heating system. Since the COP of a noncondensing boiler varies little over a large range of supply temperatures, traditional hot water heating designs standardized on a supply temperature of 180°F (82°C) January 2009 with a rise of 30°F – 40°F (17°C – 22°C). This offered the benefit of low water flow rates, smaller piping, lower pumping costs, and less expensive heating coils. On the other hand, the COP of a heat pump decreases significantly as the hot water supply temperature increases, as shown in Figure 3. Using the COP values in this graph can assist the system designer in selecting the optimum heating design temperature. The COP values in Figure 3 are based on a chiller manufacturer’s rating data for a heat pump system with a constant leaving chilled water temperature of 42°F (6°C), which is representative of many chilled water systems. For systems in which the leaving chilled water temperature is significantly different, heat pump manufacturers can supply revised data. Specific utility rates, labor rates, system design, and material costs all contribute to the determination of the hot water supply temperature in a heat pump system. However, most system designers are finding that the optimum temperature is between 120°F and 150°F (49°C and 66°C). Service Hot Water. Sometimes, the hot water temperature may be the minimum temperature required to ensure a bacteria-free service water supply. If the service hot water represents a relatively small percentage of the total hot water heating load, then consideration should be given to elevating the temperature of the service water with a separate fossil fuel or electric boiler. Booster Heating. It is also possible that a specific heating system requirement, or an existing heating system design, may dictate a hot water supply temperature greater than the heat pump can supply. In that case, the heat pump condenser can be piped in series with a hot water boiler or steam converter, which can supply the final hot water temperature (Note: If this is done with a boiler, it is important to consult with the boiler manufacturer to ensure that the boiler can operate with a reduced hot water temperature difference and/or increased hot water flow rates). In fact, effective operation of the heat pump dictates its capacity be less than the design chilled water and hot water ASHRAE Journal 29
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