ASHRAE Journal - January 2009 - (Page 33) Bin Weather Data Standard VSD Chiller Operating Cost Blended Chiller Chiller Electrical Energy Operating Use Rate (kWh) Cost ($/kWh) 4,280 60,409 342,100 640,092 783,076 818,612 685,257 757,852 489,351 257,844 170,133 202,378 166,902 0 0 0 0 0 0 0.073 0.073 0.073 0.073 0.073 0.073 0.073 0.067 0.067 0.067 0.067 0.067 0.067 0.067 0.067 0.059 0.059 0.059 0.059 $314 $4,434 $25,110 $46,983 $57,478 $60,086 $50,298 $50,397 $32,542 $17,147 $11,314 $13,458 $11,099 $0 $0 $0 $0 $0 $0 $380,659 Total Net Chiller/Heat Chiller Chiller Bin Chilled Conventional OADB (°F) MCWB (°F) ECWT (°F) Pump Cooling Efficiency Power Hours Water Load Chilled Water Capacity (Tons) (kW/ton) (kW) (Tons) Load (Tons) 117.5 112.5 107.5 102.5 97.5 92.5 87.5 82.5 77.5 72.5 67.5 62.5 57.5 52.5 47.5 42.5 37.5 32.5 27.5 72 71 71 70 69 67 65 62 59 55 52 49 47 44 41 37 33 28 24 80.1 78.9 78.6 77.5 76.3 74.3 72.3 69.5 66.6 62.9 59.5 57.0 55.0 55.0 55.0 55.0 55.0 55.0 55.0 2 30 184 353 473 628 680 963 827 711 638 872 791 677 486 249 165 25 6 4,045 3,864 3,612 3,538 3,297 3,009 2,686 2,457 2,062 1,745 1,721 1,721 1,721 400 400 400 400 400 400 412 410 406 406 403 397 390 386 376 436 567 699 800 400 400 400 400 400 400 3,633 3,454 3,206 3,132 2,894 2,612 2,296 2,071 1,686 1,309 1,154 1,022 921 0.589 0.583 0.580 0.579 0.572 0.499 0.439 0.380 0.351 0.277 0.231 0.227 0.229 0.229 0.229 0.229 0.229 0.229 0.229 2,140 2,014 1,859 1,813 1,656 1,304 1,008 787 592 363 267 232 211 0 0 0 0 0 0 Total Seasonal Chiller Operating Cost = Table 3: Chiller analysis. 120°F – 115°F (49°C–46°C) bin in the chiller analysis, the Blue Book says that Phoenix spends an average of two hours per year in that temperature range, and that the mean coincident wet-bulb (MCWB) temperature is 72°F (22°C). Cooling tower ratings show that, under those conditions, the towers will supply 80.1°F (27°C) to the condensers of the chillers. From Figure 4, the average chilled water load in this bin is 4,045 tons (14 226 kW) of refrigeration, and the chiller ratings peg their efficiency as 0.589 kW per ton at those conditions. The chiller power is a product of 4,045 tons × 0.589 kWelec/ kWcool = 2382 kW. During the two hours spent in this bin, the chiller energy consumption is two hours × 2,382 kW = 4,765 kWh. The blended electrical use rate for this bin is $0.0734 per kWh, so the chiller operating cost is 4,765 kWh × $0.0734 per kWh = $350 annually. All the numbers in Tables 1 through 5 are calculated in a similar fashion. Tables 1 and 2 contain the conventional system results, and show the annual operating cost as: $472,402 (Chillers) + $1,156,894 (Boilers) = $1,629,296 (Total Annual Cooling and Heating Cost) Alternate System. Next, a heat pump capable of 800 tons (2814 kW) of cooling and 13,500 MBH (3956 kW) of heating is added to the system. The heat pump is capable of handling January 2009 almost the entire hot water load above 60°F (16°C) dry-bulb ambient. It is also capable of handling the entire cooling load below 55°F (13°C) dry-bulb ambient, but its cooling capability above 55°F (13°C) dry-bulb ambient is limited by how much hot water can be used productively. The analysis is shown in Tables 3, 4, and 5. The summary of the annual operating cost for the heat pump plant is: $380,659 (Chillers) + $252,043 (Boilers) + $417,049 (Heat Pump) = $1,049,751 (Total Annual Cooling and Heating Cost) Therefore, adding the heat pump reduced the operating cost by: $1,629,296 – $1,049,751 = $579,545 The heat pump system, which had an additional capital cost of approximately $750,000, included the heat pump, piping, pumps, controls, and engineering design services. This investment resulted in a simple payback of 1.3 years, which is typical of the one- to two-year payback found in most hospital projects studied. Energy Analysis Comments. This analysis does not include evaluation of the many ancillary devices (pumps, towers, etc.), which could be included in a detailed analysis. The energy consumption of these devices is relatively small compared to the major components presented here. In addition, many of ASHRAE Journal 33
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