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Figure 5-7 140 120 100 80 60 40 20 00 105,000 90,000 75,000 60,000 45,000 30,000 15,000 HP output (53,000 Btu/hr) @ 92.5oF supply water 40 oF outdoor balance point HP output (46,200 Btu/hr) @ 110oF supply water 20 oF outdoor 140 oF 130 oF 120 oF 110 oF 100 oF 90 oF 80 oF HP output (41,500 Btu/hr) 128 oF supply water 0 oF outdoor design load 5.8% heating duration curve 94.2% A B design load C hours during which space heating load ≥ % of design load 0 70 60 50 40 30 20 10 070oF outdoor temperature (oF) no load point (65 oF outside, 70 oF suppl temp.) 1000 2000 3000 4000 5000 6000 7000 % of design load heat transfer rate (load or HP) supply water temperature (oF) outdoor temperature of -5oF would be about 132oF. The low- ambient heat pump used in this example would have a COP of about 1.7 under this condition. SPREADSHEET MODELING OF SYSTEM PERFORMANCE Previous discussions have shown the variability of heating capacity and COP based on changes in the temperature of water leaving the heat pump’s condenser and the outside air temperature. The building that the heat pump serves also experiences wide changes in heating load over the heating season. Hydronic distribution systems that use outdoor reset control to avoid “overheating” the water supplied to the distribution system will minimize the required supply water temperature, and thus, enhance the heat pump’s performance under partial load conditions. These systems will experience a wide range of supply water temperatures. The geographic location where the heat pump is installed will have specific average values for each temperature bin. All these variables make it impossible to provide an accurate “rule of thumb” for sizing an air-to-water heat pump on a given project. The preferred approach is to simulate a proposed system configuration by building a spreadsheet that includes reasonable mathematical models for the building load, performance of the heating distribution system, performance of the heat pump and outdoor air temperature for the location of the project. Once the simulation spreadsheet is built, the designer can experiment with various “what if” scenarios on equipment size, supply water temperatures, etc., to determine overall seasonal performance and make decisions on system design. The following mathematical models were used to build a simulation spreadsheet that merges the thermal performance of the previous discussed building, a nominal 4-ton low-ambient air-to-water heat pump, the building’s heating distribution system and climatic data for Boston. These calculations were performed for each bin of outdoor temperature. The outdoor temperature (Tamb) is taken as the average temperature of each bin (i.e., for the 15–20oF bin, Tamb would by 17.5oF). 42PDF Image | Heat Pump Systems 2020
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