ASHRAE Journal - July 2011 - 59

ing the house with a COP of nearly 1 when outdoor 1,200 ambient temperatures dropped below –18°C (0°F). As December shown in Figure 1, simulation predictions for Equinox 1,000 January House using average monthly weather conditions February for Urbana, Ill., with an assumed heating system March 800 April COP of 1.0 agree quite well with the actual house performance. December and January were quite cold 600 and cloudy compared to average winter conditions. December set a record for snow, with snow falling 17 days of the month. 400 Two simulation data points are shown in Figure 1 in which the actual December and January ambient 200 temperature and solar radiation data have been used to compare more directly with the actual data and to 0 show the variation from the long-term average data Solar 1 Solar 2 Heat Pump Subpanel A Subpanel B Energy Recovery performance. During December and January, daily energy requirements of 60 kWh per day are equivalent Figure 3: Monthly electric energy requirements for the major house circuits to 2,500 W of continuous heating. During some of our from December 2010 through April 2011. most bitter cold weather, we reached 80 kWh to 90 kWh per day (3,300 W to 3,800 W) with very little 70 modulation of the space heater operation. December 60 Our goal is to reach a minimum average heating January system COP of 2.0 (a heating seasonal performance February 50 factor [HSPF] = 6.9), with a performance as shown in March April Figure 1. A higher heating system COP of 4.0 (HSPF 40 = 13.8), achievable with ground source or geothermal heat pump systems, is also shown in Figure 1. 30 Heat pumps result in significant energy performance gains, as shown in Figure 2 where the annual energy 20 totals for Equinox House with heating system COPs of 1.0, 2.0, and 4.0 from Figure 1 are shown. Our 10 current heating system ($40 worth of space heaters) 0 can be assumed to be a “no cost” heating system with minimum energy efficiency (COP = 1.0). To justify the cost of an air source heat pump system with a nominal COP of 2.0, the system must cost less than the value of the annual 3,000 kWh savings. Solar energy costs $0.125/kWh, which on a simple basis results in energy cost savings of $375 per year. With a heat pump Figure 4: Monthly electric energy requirements for the minor house circuits system lifetime between 10 and 20 years, the heat from December 2010 through April 2011. pump system should cost less than $3,750 to $7,500. Likewise, a higher performing system with a heat pump observed, the amount collected is quite small compared to the system COP of 4.0 saves approximately 4,500 kWh per year three energy loads shown in Figure 3. February also shows relative to the electric resistance heated house, for a cost savings significantly higher major loads compared to solar energy colof $560 per year. The system cost on a simple basis should be lection. However beginning in March, solar energy collected less than $5,600 to $11,200 assuming the same lifetime. (Note, increases as major loads decrease. April is the month when solar the ground source heat pump installed at our laboratory in 1988 energy collection is significantly greater than the major energy operated 17 years before requiring replacement.) loads, and the house becomes heavily net positive in energy Figures 3 and 4 provide detailed electrical circuit data for balance, and will remain so through October. December 2010 through April 2011. Figure 3 shows energy The three major load circuits shown in Figure 3 are for the activities for five “major” circuits, while Figure 4 shows the heat pump energy recovery system previously discussed and energy for the remaining “minor” circuits. Two of the major Subpanel A and Subpanel B. The heat pump energy recovery circuits in Figure 3 are the two halves of the 8.2 kW PV solar system provides a level of heating varying from 400 W to 2,000 system. December and January solar energy collection was W depending on the outside ambient temperature, with an aververy low compared to the average expected collection, and as age power requirement of 400 W to 500 W.
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July 2011

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ASHRAE Journal

ASHRAE Journal - July 2011

Table of Contents for the Digital Edition of ASHRAE Journal - July 2011