Current performance of ground source heat pumps for space conditioning and for water heating under simulated occupancy conditions.
Buildings account for about 40% of primary energy use in the U.S. (EIA 2009). Greenhouse-gas (GHG) emissions from the building sector exceed those of both the industrial and transportation sectors in the U.S (Metz, Davidson et al. 2007). Buildings also represent approximately 40% of the European Union's (EU's) energy consumption and carbon dioxide production (Baden, Waide et al. 2006). Clearly, building energy efficiency plays a vital part in addressing the environmental challenges confronting industrialized and developing nations of the world. The 2012 International Energy Conservation Code (IECC) offers an opportunity to achieve 50% increase in energy savings in new U.S. buildings by 2015 (DOE 2010). To achieve these objectives, data from buildings with cost-effective novel designs, and efficient methods to utilize renewable energy resources are needed to elevate existing technology to higher levels of performance.
Although Ground Source Heat Pumps (GSHPs) are recognized (IGSHPA 2010) as the most efficient technology to harvest thermal energy from the ground during winter, or to use the ground as a thermal sink during the summer, market penetration has lagged its technical potential (Ally 2006). Thus, the goal of this 11-month (January-November 2010) field test was to measure the performance of two GSHPs; one for space conditioning, and the other exclusively for water heating in a low-energy residential test house built with structural insulated panel (SIP) technology. Since ground loops are a major cost component of GSHPs, we utilized the house foundation and utility trenches for as much of the ground loop as possible to reduce the amount of additional trenching required. Quantifying efficiency and energy consumption under simulated occupancy conditions as per Building America Benchmark (Hendron and Engebrecht 2010) provides useful data to assess the feasibility and practicality of GSHPs in residential applications and their potential to achieve source energy and greenhouse gases reduction targets set under the IECC 2012 Standard.
Since subsurface ground temperatures are generally warmer in winter and cooler in summer than the ambient air, GSHPs should theoretically operate more efficiently than air source heat pumps (ASHPs), especially in colder climates. Although deep vertical bores offer the advantage of a fairly constant ground temperature, the same is not true of shallow horizontal trenches where the sub-surface ground temperature varies with the surface air temperature. However, shallow horizontal trenches may be easier to install and are immune from the vagaries of karstic geology experienced in vertical wells. Our field study focuses on the performance achieved with a shallow horizontal ground loop utilizing building utility trenches, and high efficiency water-source heat pumps currently on the market.
Building Envelope, Ground loop, and GSHP systems
The test house is a two-story building with a walk-out basement and a total conditioned space of 345 [m.sup.2] (3700 ft2), a SIP-based thermal envelope design, and weather resistive barriers to limit moisture infiltration. The house roof is a standing seam metal with an infrared reflective (IRR) paint yielding a solar reflectance of 0.30 and a thermal emittance of 0.85. The roof assembly has an overall thermal resistance of about RSI-8.8 (RUS-50). The walls are 0.14 m (5.5-in.) thick and have a thermal resistance of RSI-3.7 (RUS-21). Windows are triple pane with a U-factor of 1.64 W/m2*K (0.29 Btu/h*ft2*oF) and a solar heat gain coefficient (SHGC) of 0.25. One feature of SIP structures is that they can be built to have low air leakage rates - blower door tests yielded 0.74 air changes per hour (ACH) at 50 Pa for the house. The building has a Home Energy Rating System (HERS) Index of 44. In comparison, an EPA Energy Star home would have a HERS score of 85, and a Net Zero Energy Home would have a HERS Index of 0 (where lower HERS Index values indicate better energy efficiency). The thermostat set point temperatures are 21.7[degrees]C (71[degrees]F) for heating and 24.4[degrees]C (76[degrees]F) for cooling and were maintained within 1[degrees]C (1.8[degrees]F) in each of four zones (top floor, master bedroom, living space, and basement). The monthly relative humidity (RH) (averaged for all 4 zones) was maintained around 47% with the lowest and highest values of 36% and 59% in January and July, respectively. The temperatures and RH are well within comfort levels as defined by (ASHRAE 2001).
The ground heat exchanger (GHX) loop consisted of three parallel circuits (6 pipes, 1.9 cm (3/4 in.) ID) made of high density polyethylene (HDPE) with a total length of 559 m (1834 ft). A mixture of 20% polypropylene glycol and 80% water, by volume was used as the GHX loop fluid (brine). A nominal 2-ton (7 kW), two-capacity water-to-air heat pump was used for the space conditioning system. It had a rated high-stage heating COP of 4.0 and a rated high-stage cooling COP of 5.4 (ground loop rating conditions per ANSI/AHRI/ISO 13256-1). A separate 1.5-ton (5.3 kW) water-to-water GSHP (WW-GSHP) connected to the same GHX loop serviced a 303 L (80-gallon) water tank providing approximately 220 L/day ([begin strikethrough]~[end strikethrough]58 gallons/day) of hot water at 49[degrees]C (120[degrees]F).
Equipment, Sensors and Data Acquisition Hardware
Compressor discharge and suction pressure transducer accuracy were [+ or -]0.25% FS. Brine flow through the space conditioning heat pump was determined by measuring pressure drop across the brine/water-refrigerant HX (with a differential pressure transducer with [+ or -]0.25% FS accuracy) and a detailed 'P vs. flow factory calibration. Brine and domestic hot water flow through the water heating heat pump were measured with flowmeters having [+ or -]1.5% accuracy full-scale and [+ or -]0.5% repeatability. Compressor, fan+controls/pump, and total heat pump power were measured with watt/watt-hour transducers with split-core current transformers (CTs) having a minimum accuracy of [+ or -]0.45% of reading plus 0.05% of FS. Pressure drops in connecting lines were estimated at 1-1.5% of the upstream pressure, based on manufacturer's data. The pressure drop across the expansion valve included the pressure drop in the connecting lines between the indoor and refrigerant-to-water/brine heat exchanger coils.
A total of 20 thermistors, 4 refrigerant line pressure transducers, 2 differential pressure transducers measuring brine flow (not shown), 2 flow meters and 8 watt transducers were used for the measurements on the two heat pumps. A data logger was programmed to collect data every 15 seconds for the WW-GSHP and every 30 seconds for the GSHP. The data was then averaged over a 15 minute interval, and dispatched to a server via an internet connection for subsequent analysis. A schematic of the GSHP indicating state points is shown in Figure 1. Data are reported for 11 continuous months from January-November, 2010. The time-averaged values of key state variables are tabulated for each month, and several measures of performance are reported to highlight areas of potential efficiency improvement.
[FIGURE 1 OMITTED]
Some practical trade-offs were necessary in taking our measurements. Although in situ measurements of refrigerant temperatures would have a quicker response time and be more accurate, the potential for leaks would not be worth taking. Thus surface-mounted thermistors with conductive paste to enhance contact were used for refrigerant temperatures. But pressure measurements were made with in situ pressure transducers. Steady-state conditions were assumed in our mass and energy balances and this assumption is valid when run times exceed five minutes. State points used in this paper refer to the refrigerant side only. The expansion valve was assumed to be perfectly adiabatic. Pressure drops across each segment of the GSHPs were according to the manufacturer-supplied data, except for the compressor whose suction and discharge side pressures were directly measured. State points were calculated using the National Institute of Standards and Technology (NIST) Reference Fluid Thermodynamic and Transport Properties Database (REFPROP) Version 8.0 (NIST)
For purposes of our analyses, the heat pump cycle is assumed to operate under steady-state, steady flow conditions (SSSF) with negligible kinetic and potential energy effects. The steady state, steady flow assumption holds true except during compressor start-up or shut down or when cycling times are less than 5 minutes. The expansion valve is assumed to be adiabatic; however the compressor is not. Its heat loss is assumed to be with the surrounding temperature in the basement set at 24.4[degrees]C (76[degrees]F) during the summer and 21.7[degrees]C (71[degrees]F) during the winter by the thermostat whose accuracy is [+ or -]1[degrees]C (1.8[degrees]F).
Space Conditioning with WA-GSHP
The general energy balances for the WA-GSHP (space conditioning equipment) for any given month is obtained by integrating the SSSF equation (Wylen, Sonntag et al. 1994; Seader, Henley et al. 2011) over the duration of the respective month to yield
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1)
In component from, Eq (1) applied to the space conditioning GSHP becomes
[[Q.sub.BrineHX] + [W.sub.Comp] + [W.sub.fan + Control] + [W.sub.BrinePump] + [W.sub.e,Aux]] - [[Q.sub.Indoor HX] + [Q.sub.Comp] + [Q.sub.e,Aux] + [W.sub.fan + controls]] = 0 (2)
The Heating Coefficient of Performance is given by
[COP.sub.h] = ([Q.sub.Brine HX] + [W.sub.omp] + [W.sub.fan + controls] + [W.sub.BrinePump] + [W.sub.e,Aux])/([W.sub.Comp] + [W.sub.fan + controls] + [W.sub.BrinePump] + [W.sub.e,Aux]) (3)
The Cooling Coefficient of Performance is given by
[COP.sub.c] = ([Q.sub.Indoor HX] - [W.sub.fan + controls] - [Q.sub.Comp])/([W.sub.Comp] + [W.sub.fan + controls] + [W.sub.Brine Pump]) (4)
The monthly energy balances for space conditioning are shown in Tables 1 through 3. The accuracy of accounting of energy inflows to energy outflows is characterized by [DELTA]% (Table 3).
Table 1. Measured Energy Input to WA-GSHP Months [W.sub.Comop.] [W.sub.fan+controls] [W.sub.Brine (yr. (kWh) (kWh) Pump] 2010) (kWh) Jan 647.7 102.2 95.14 Feb 633.9 102.0 88.04 Mar 431.7 72.4 61.27 Apr 47.1 7.6 6.653 (heat) Apr 24.3 6.4 5.587 (cool) May 0.353 0.059 0.053 (heat) May 111.3 24.4 22.65 (cool) Jun 282.0 49.1 47.14 Jul 413.4 63.6 62.366 Aug 504.2 67.5 70.154 Sept 301.3 43.0 43.437 Oct 2.223 0.4 0.306 (heat) Oct 34.9 6.1 5.896 (cool) Nov 99.0 11.6 14.564 Months [Q.sub.Brine [W.sub.e, Energy (yr. HX] Aux.] In, Ei 2010) (kWh) (kWh) (kWh) Jan 2200.2 9.8 3055.0 Feb 2026.8 0.0 2850.8 Mar 1436.8 0.0 2002.2 Apr 197.2 0.0 258.6 (heat) Apr -248.3 0.0 -212.0 (cool) May 1.7 0.0 2.2 (heat) May -988.4 0.0 -830.1 (cool) Jun -2047.5 0.0 -1669.3 Jul -2647.9 0.0 -2108.6 Aug -2955.5 0.0 -2313.6 Sept -1844.5 0.0 -1456.7 Oct 11.6 0.0 14.5 (heat) Oct -252.0 0.0 -205.1 (cool) Nov 418.6 0.0 543.7 Table 2. Measuredm Energy Output of WA-GSHP Months [Q.sub.Indoor] [W.sub.fan+controls] [Q.sub.Comp.] (Yr. 2010) (kWh) (kWh) HX (kWh) Jan -2685.9 -102.2 -105.8 Feb -2501.5 -102.0 -101.6 Mar -1731.0 -72.4 -102.7 Apr (heat) -227.6 -7.6 -14.1 Apr (cool) 228.8 -6.4 -4.8 May (heat) -1.9 -0.1 -0.1 May(cool) 898.7 -24.4 -21.2 Jun 1816.2 -49.1 -46.6 Jul 2307.3 -63.6 -65.8 Aug 2531.8 -67.5 -73.1 Sept 1594.1 -43.0 -47.0 Oct (heat) -13.2 -0.4 -0.5 Oct (cool) 224.8 -6.1 -7.5 Nov -486.3 -11.6 -17.1 Months [Q.sub.e, Energy Out, (Yr. 2010) Aux.] [E.sub.o] (kWh) (kWh) Jan -9.78 -2903.7 Feb -0.009 -2705.1 Mar -0.005 -1906.0 Apr (heat) 0.0 -249.3 Apr (cool) 0.0 217.6 May (heat) 0.0 -2.1 May(cool) 0.0 853.2 Jun 0.0 1720.5 Jul 0.0 2177.9 Aug 0.0 2391.2 Sept 0.0 1504.0 Oct (heat) 0.0 -14.1 Oct (cool) 0.0 211.2 Nov 0.0 -515.0 Table 3. Satisfaction of Energy Balance Equation and Performance Metrics of Space Conditioning WA-GSHP Months EWT Energy Energy [DELTA]% = ([E.sub.in]- (Yr. ([degrees]C/ In Out, [E.sub.o)100/ 2010) [degrees]F) [E.sub.i] [E.sub.o] [E.sub.i] (kWh) (kWh) Jan 4.6/40.3 3055.0 -2903.7 5.0 Feb 2.8/37 2850.8 -2705.1 5.1 Mar 3.7/38.7 2002.2 -1906.0 4.8 Apr 10.1/50.2 258.6 -249.3 3.6 (heat) Apr 11.4/52.6 -212.0 217.6 -2.6 (cool) May 17.6/63.7 2.2 -2.1 3.3 (heat) May 14.7/58.4 -830.1 853.2 -2.8 (cool) Jun 24.3/75.8 -1669.3 1720.5 -3.1 Jul 28.8/83.8 -2108.6 2177.9 -3.3 Aug 31.7/89.0 -2313.6 2391.2 -3.4 Sept 30.1/86.2 -1456.7 1504.0 -3.2 Oct 19.7/67.5 14.5 -14.1 2.9 (heat) Oct 24.7/76.5 -205.1 211.2 -3.0 (cool) Nov 16.1/60.9 543.7 -515.0 5.3 Months [COP.sub.h] [COP.sub.c] (Yr. 2010) Jan 3.57 Feb 3.46 Mar 3.54 Apr 4.21 (heat) Apr 5.99 (cool) May 4.73 (heat) May 5.39 (cool) Jun 4.55 Jul 4.04 Aug 3.73 Sept 3.88 Oct 5.02 (heat) Oct 4.51 (cool) Nov 4.34
Comparison of the last column in Tables 1 and 2 is a measure of the degree to which the energy balance Eq. (2) holds true. The minor discrepancy in the energy balance, due to errors in measurement, the heating and cooling COPs and the entering water temperatures averaged for the month, are all summarized in Table (3). Details of the WA-GSHP are discussed in (Ally, Munk et al. 2012).
Water Heating with WW-GSHP
The same ground loop that services the space conditioning GSHP also serves the WW-GSHP for water heating. The general energy balance equation Eq. (1) also applies to the WW-GSHP. In component form, the application of Eq (1) to the WW-GSHP yields,
[Q.sub.Brine HX] + [W.sub.Brine Pump] + [W.sub.DHW Pump] + [W.sub.Comp] - ([Q.sub.HW Tank] + [Q.sub.Comp]) = 0 (5)
The COP for the WW-GSHP is
[COP.sub.WW-GSHP] = [Q.sub.HW Tank]/([W.sub.Brine Pump] + [W.sub.DHW Pump] + [W.sub.Comp]) (6)
The monthly energy balances for water heating are shown in Tables 4 and 5. The accuracy of accounting of energy inflows to energy outflows is characterized by [DELTA]% (Table 6)
Table 4. Measured Energy Input to WW-GSHP Month [W.sub.Brine [W.sub.DHW [Q.sub.BrineHX] [W.sub.Comp] (Yr. Pump] Pump] (kWh) (kWh) 2010) (kWh) (kWh) Jan 12.2 2.9 289.0 132.9 Feb 10.2 2.5 254.8 114.4 Mar 11.0 2.6 282.2 122.9 April 7.6 1.8 235.1 89.2 May 7.0 1.7 250.0 87.3 Jim 5.8 1.5 244.1 77.3 Jul 4.6 1.3 216.6 67.5 Aug 5.9 2.9 264.2 84.9 Sept 5.9 2.9 292.1 85.3 Oct 7.3 3.2 306.6 94.8 Nov 8.0 3.3 291.3 95.6 Month Energy (Yr. Input, 2010) [E.sub.i] (kWh) Jan 437.0 Feb 381.9 Mar 418.7 April 333.8 May 345.9 Jim 328.7 Jul 289.9 Aug 357.9 Sept 386.2 Oct 412.0 Nov 398.1 Table 5. Measured Energy Output from WW-GSHP Month [Q.sub.HW [Q.sub.Comp] Energy (Yr. Tank] (kWh) Output, 2010) (kWh) [E.sub.o] (kWh) Jan -374.0 -37.9 -411.9 Feb -335.8 -33.3 -369.1 Mar -361.1 -44.0 -405.0 April -288.4 -36.0 -324.3 May -298.8 -38.4 -337.2 Jun -286.3 -35.1 -321.4 Jul -251.2 -32.8 -284.0 Aug -307.4 -36.9 -344.3 Sept -335.4 -40.4 -375.8 Oct -356.4 -42.2 -398.6 Nov -344.5 -40.3 -384.8 Table 6. Energy Balance and Performance Metrics of WW-GSHP Months(Yr. EWT Energy In, Energy [DETAL]% = 2010) Out, ([degrees]C/ [E.sub.i] [E.sub.o] ([E.sub.in, [degrees]F) [E.sub.out]) 100/[E.sib.i] (kWh) (kWh) Jan 4.5/40.2 437.0 -411.9 5.7 Feb 2.7/36.9 381.9 -369.1 3.3 Mar 4.0/39.2 418.7 -405.0 3.3 Apr 10.651.0 333.8 -324.3 2.8 May 16.6/61.8 345.9 -337.2 2.5 Jun 23.2/73.8 328.7 -321.4 2.2 Jul 28.1/82.5 289.9 -284.0 2.0 Aug 30.9/87.6 357.9 -344.3 3.8 Sept 28.9/84.0 386.2 -375.8 2.7 Oct 23.1/73.6 412.0 -398.6 3.3 Nov 17.3/63.1 398.1 -384.8 3.3 Months(Yr. [COP.sub.W 2010) W-GSHP] Jan 2.5 Feb 2.6 Mar 2.6 Apr 2.9 May 3.1 Jun 3.4 Jul 3.4 Aug 3.3 Sept 3.6 Oct 3.4 Nov 3.2
Comparison of the last columns in Tables 4 and 5 is a measure of the degree to which the energy balance for the WW-GSHP, Eq. (5) holds true. The minor discrepancy in the energy balance, due to errors in measurement, the coefficient of performance of the WW-GSHP and the entering water temperatures averaged for the month, are all summarized in Table (6).
The highest and lowest monthly average space heating COP achieved was 5.02 (October) and 3.46 (February), respectively. The highest and lowest monthly average space cooling COP achieved was 5.99 (April) and 3.73 (August). The highest and lowest monthly average water heating COP was 3.6 (September) and 2.5 (January). COPs were calculated on the basis of total power input (including duct, pumps, and control board power consumptions). Building insulation and air-tightness significantly reduce loads. A judicious and cost-effective step towards energy conservation is to increase the level of insulation to reduce thermal loads. In this study, winter auxiliary heating was virtually eliminated allowing a more efficient use of electricity than would otherwise have been the case.. Brine pumps should be sized such that it operates in the optimum range for a majority of the time, thus reducing its energy input and increasing the COP. Variable speed drive technology in which the compressor speed can be modulated, further improves compressor performance. Ground loops with phase change materials(Ally, Tomlinson et al. 2010) are another novelty for reducing the size of the loop and improving its heat transfer characteristics.
The authors acknowledge sponsorship of this work by the Tennessee Valley Authority and by the DOE Building Technologies Program under contract DE-AC05-00OR22725 with UT-Battelle, LLC.
[DELTA]% = per cent difference
COP = coefficient of performance
EWT = entering water temperature
Q = heat transfer
[R.sub.US] = thermal insulation in English units
[R.sub.SI] = thermal insulation in SI units
T = temperature
W = work
Aux. = auxiliary heat, same as resistance heat
Brine HX = Brine heat exchanger
c = cooling cycle
Comp. = compressor
DHW = domestic hot water
HW = hot water
h = heating
i = input
Indoor HX = indoor heat exchanger o = output
WW-GSHP = water-to-water ground source heat pump
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Notice: This manuscript has been authored by UT-Battelle, LLC, under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes.
Moonis R. Ally, PhD
Jeffrey D. Munk
Van D. Baxter
Anthony C. Gehl
Moonis R. Ally, Jeffrey | D. Munk, Van D. Baxter, and Anthony C. Gehl are researchers at the Oak Ridge National Laboratory, Oak Ridge, Tennessee.
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|Author:||Ally, Moonis R.; Munk, Jeffrey D.; Van D. Baxter; Gehl, Anthony C.|
|Date:||Jul 1, 2012|
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