ASHRAE Journal - January 2009 - (Page 30) 6.00 130°F P2 Hot-Water Main 145°F 150°F 5.50 Boilers 5.00 150°F COP Chillers 58°F P1 44°F 42°F Chilled Water Main 3.50 4.00 Heat Pump 4.50 Figure 2: Heat pump location. 3.00 100 110 120 130 140 150 160 Leaving Hot Water Temperature (°F) loads. This is because any reduction of the cooling load handled by the heat pump will also reduce its heat output. So, it is desirable to keep the heat pump as close to fully loaded as possible. Therefore, one or more chillers, and one or more boilers, will be required to supplement the capacity of the heat pump. Potable Water Isolation. Most building codes require two layers of separation between the oil and refrigerant in a heat pump condenser and any potable water supply. In that case, an isolation heat exchanger will be required. The additional heat transfer loss will slightly reduce the heat pump COP. Energy Consumption Calculations Figure 3: Heat pump COP versus hot-water supply temperature, with 42°F (6°C) chilled water supply temperature. 4,500 4,000 3,500 Load (Tons) 3,000 2,500 2,000 1,500 1,000 500 – 0 20 40 60 80 100 120 140 Outdoor Dry-Bulb Temperature (°F) Total Chilled Water Load (Tons) Total Heating Water Load (Tons) Chiller/Heat Pump Cooling (Tons) Chiller/Heat Pump Heating (Tons) How much energy can a heat pump save in this type of application, and is it a good investment? An energy analysis of an actual facility demonstrates the economic potential of a heat pump. It also shows how these applications can be analyzed. This analysis was recently performed for an Arizona hospital. It compares a conventional plant, which includes variable speed, centrifugal chillers and natural gas boilers, to an alternate plant that adds a heat pump to the conventional plant. Several analysis methods are available. The two most popular are bin analysis and hour-by-hour analysis, and a close correlation has been found when both methods are used. For simplicity of presentation in this article, the bin method will be demonstrated. Conventional System. The summertime design ambient temperatures are 120°F (49°C) dry bulb/72°F (22°C) wet bulb, and 27°F (–3°C) dry bulb/24°F (–4°C) wet bulb in the winter. The cooling towers produce 81°F (27°C) water in the summer, and are limited to 55°F (13°C) water in the winter. The design hot water load is 27,000 kBtu/h (7913 kW) and the efficiency of the boilers is 85%. A base hot water load of approximately 7,000 kBtu/h (2051 kW) exists at all times. The design cooling load is 4,200 tons (14 770 kW), and the variable speed chillers’ efficiency values are drawn from the chiller ratings. The facility uses airside economizers below 30 ASHRAE Journal Figure 4: Loads versus outdoor temperature. 55°F (13°C) dry-bulb ambient, but a base chilled water load of 400 tons (1,407 kW) exists at all times. Figure 4 shows the loads versus outdoor temperature. The electrical rates from this particular utility vary by season and time-of-day, and these variations are approximated in the bin analysis varying the rates in each bin. For comparison purposes, the weighted average used in this analysis is $0.068/ kWh. The average natural gas rate used in the analysis is $11.21/ Mcf ($0.40/m3). Both rates are representative of rates found in North America. The chillers and boilers will be analyzed separately and combined to establish a total cost. The analyses are shown in Tables 1 and 2 respectively. The calculations used in Tables 1 and 2 (as well as Tables 3 through 5) are not complicated, but an explanation of the computations in one bin should ensure there is no confusion. All weather data comes from Engineering Weather Data , which was complied by the Air Force Combat Climatology Center and was published in July 1978. Looking at the ashrae.org January 2009 http://www.ashrae.org
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