1、218 2008 ASHRAE ABSTRACT The design of this 1280 square foot, 3-bedroom Denverzero energy home (ZEH) carefully combines envelope effi-ciency, efficient equipment, appliances and lighting, a photo-voltaic (PV) system, and passive and active solar thermalfeatures to exceed the net zero energy goal. In
2、 January, 2006a data acquisition system was installed in the home to monitorits performance over the course of a year. This paper presentsfull year of energy performance data on the home. From April 2006 through March 2007 the homes 4kW PVsystem produced 5127 kWh of AC electricity. Only 3585 kWhof e
3、lectricity and 57 therms of natural gas were used in thehome during this period. On a source energy basis, the homeproduced 24% more energy than it used. The energy used forspace heating, water heating, and lighting have been dramat-ically reduced through superinsulation, passive solar temper-ing, s
4、olar water heating, compact florescent lights and otherefficiency measures. The energy used in the home is now domi-nated by appliance and plug loads determined by occupantchoices and behavior. These loads constitute 58% of all thesource energy used in the home. Because these loads are gener-ally ou
5、tside of the control of the home designer and varyconsiderably with different occupants, sizing a PV system toachieve zero net energy performance is challenging. This case studies demonstrates that it is possible to buildefficient affordable zero energy homes in cold climates withstandard building t
6、echniques and materials, simple mechani-cal systems, and off-the-shelf equipment.INTRODUCTIONHow clean is clean enough? How efficient is efficientenough? These will be among the defining questions of the21stcentury. As the human population pushes beyond 6.5billion on the way to 9.2 billion by 2050 (
7、UNPD 2007) we arefaced with increasing environmental consequences . massspecies extinction, toxic air, water, and land pollution, andglobal warming to name a few. Many of these consequencesare related to our energy use and choices. It is clear that we willneed to reduce our per capita environmental
8、impact at least inrelation to our population growth (and likely beyond) if wewish to stabilize or reduce environmental degradation. Homes account for 37% of all U.S. electricity consump-tion and 22% of all U.S. primary energy consumption (EIA2005). This represents a huge opportunity to reduce our en
9、ergyconsumption and make cleaner choices for the energy weconsume. The U.S. Department of Energys Building America(BA) program is working to increase the energy efficiency ofnew and existing homes while increasing comfort, durabilityand resource use. As part of this program we pursue opportu-nities
10、to research highly efficient homes with the goal ofunderstanding what works, what doesnt work, and what arethe most economic ways to reach very high efficiency targets.The program aims to create cost neutral zero energy homes by2020. In pursuit of this goal, this home and other researchhomes around
11、the country designed to approach or achieve thezero energy goal are being built and studied.The zero energy home (ZEH) presented here was a resultof collaboration between the National Renewable EnergyLaboratory (NREL) and Habitat for Humanity of MetroDenver. A previous paper details the construction
12、 of the home(Norton and Christensen 2006). This paper will briefly reviewthe design then focus on the first year energy performance ofthe home. Performance Results from aCold Climate Case Study forAffordable Zero Energy HomesPaul Norton Craig ChristensenAssociate Member ASHRAEPaul Norton is a senior
13、 engineer and Craig Christensen is a principal engineer at the National Renewable Energy Laboratory, Golden CO.NY-08-0282008, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions, Volume 114, Part 1. For personal use only.
14、 Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAEs prior written permission.ASHRAE Transactions 219In general, a zero energy home is designed to produce asmuch energy as it consumes over the course of a full year. TheBA program de
15、finition is more specific: A zero energy homeis designed to offset as much source energy as it consumesover a typical year (based on TMY2 data) using BA Bench-mark assumptions for typical occupant behavior. To achievezero energy the home exchanges energy with the utility powergrid. It delivers energ
16、y to the grid when the photovoltaic (PV)system is producing more energy than is being used in thehome and draws from the grid when the PV system is produc-ing less energy than needed in the home. This project is a casestudy in reaching the zero energy goal within the affordablehousing sector in cold
17、 climates. Zero energy is especiallyimportant in this sector where increasing energy cost can takea high toll on homeowners with limited economic resources.A zero energy home guarantees long term energy cost stabilityfor the homeowner. HOME DESIGNThe home, shown in Figure 1, was designed using an ea
18、rlyversion of the BEOpt building optimization software (Chris-tensen, et. al. 2006) with additional analysis using DOE-2(LBNL 2004) and TRNSYS (Klein, et. al. 1996) separately.This engineering approach was tempered by regular discus-sions with Habitat construction staff and volunteers. Thesediscussi
19、ons weighed the applicability of the optimized solu-tions to the special needs and economics of a Habitat house - moving the design towards simple, easily maintainedmechanical systems and volunteer-friendly construction tech-niques. We chose solutions that avoided interconnected equip-ment with comp
20、lex control systems. The home specificationsare summarized in Table 1. Further details on the designprocess and the final design of the home is presented in andearlier paper (Norton and Christensen 2006).The envelope of the home is a double stud wall designwith the outer load-bearing walls of the ho
21、me constructed of2x4s on 16” centers. On the inside of the load-bearing wall weconstructed a second wall of 2x4s on 24” centers. There is a3 ” gap between these two stud walls. The finished doublestud wall construction allows for three layers of R-13 fiber-glass batts: two laid vertically in the cav
22、ities of the outer andinner stud walls and a third stacked horizontally betweenthem. This leads to a nominal R-40 wall with very few thermalbreaks since the studs do not continue through the entire wallthickness. Two foot raised heel trusses were used to accom-modate R-60 blown-in fiberglass insulat
23、ion. Fiberglass battsrated R-30 were used in the floor. All mechanical equipmentis contained within this thermal envelope. The crawlspace isvented and uninsulated. An energy recovery ventilationsystem is used to supply fresh air to the home. Ducting for thissystem is contained in a drop ceiling in t
24、he hallway.The home is designed with large southern glazing for solargain. The southern windows are double-glazed low-e with a“high” SHGC of 0.58. Three foot overhangs provide windowshading when solar gain is not needed. Double-glazed, low-emissivity, low solar heat gain coefficient (SHGC) windowswe
25、re used on the north, east, and west of the home.With these shell efficiency features, the peak design heat-ing load for the home is very small about 15,000 Btu/hr (4.4kW). This load was met using a single point sealed combustionfurnace located in the living room and small (750 Watt) elec-tric resis
26、tance baseboard heaters in the bedrooms. Heat distri-bution is enhanced by the energy recovery ventilation systemthat pulls stale air from the kitchen and bathroom and deliversfresh air to the living room and each bedroom. Water heatingis accomplished using a solar thermal system with a naturalgas t
27、ankless heater for back-up. The solar system has 96 sq. ftof collector area and 200 gallons of water for thermal storage.The system is sized to provide a high solar saving fraction yearround, and a drainback configuration is used to avoid potentialglycol overheating problems during summer stagnation
28、.Active solar space heating is not used to keep the systemsimple and because the combination of passive solar andsuperinsulation are already predicted to meet the space heatingloads on sunny winter days.DATA ACQUISITION SYSTEM DESIGNA data acquisition system was installed to determine if thehome met
29、 its energy design goal of zero energy. The systemwas designed to allow disaggregation of the PV energyproduction and some end uses. A summary of the datacollected and the equipment used is given in Table 2. Data were collected on 1-minute and 1-hour intervals.Most of the analysis of the home perfor
30、mance was done usingthe 1-hour data. The 1-minute data was used for troubleshoot-ing and for investigating transient behavior of the solar waterheating system. An Excel spreadsheet using array formulaswas created to aggregate daily and monthly averages and sumsand to create graphics on the performan
31、ce of the home. Allelectrical end use measurements were in place by February2006. However, the water flow and natural gas end use moni-toring was not complete until April 2006. Unless otherwisestated, all annual figures in this report include the period fromApril 2006 through March 2007.Figure 1 The
32、 NREL/habitat zero energy home.220 ASHRAE TransactionsHOME ENERGY PERFORMANCEThe home is located in Wheat Ridge, Colorado which ispart of the Denver metropolitan area. Wheat Ridge has 5988heating degree days (65oF base) and 496 cooling degree days(65oF base) (NOAA 2007). We measured a total of 1607
33、fullsun hours during the 12 month monitoring period. The home received a Colorado E-star rating of 95. The E-star rater tested the envelope air tightness using a calibrateddoor fan to measure the air flow required to depressurize thehome by 50 Pascals. This result was then used to calculate thenatur
34、al ventilation rate of 0.15 ACH indicating that theconstruction crew did an excellent air sealing job. The homes net source energy performance exceededexpectations. The PV system was sized to achieve net zeroannual source energy using TMY2 weather data for Boulder,Colorado (Marion and Urban 1995) an
35、d BA Benchmarkassumptions for occupant effects such as temperaturesetpoints and miscellaneous energy use (Hendron, et. al.2004). The BA Benchmark represents U.S. average occu-pancy choices and behavior. It turns out that the owner/occu-pants of the NREL/Habitat ZEH use less energy than the BABenchma
36、rk occupants average energy users. Therefore thehome performed beyond zero and was a net source energyproducer. A summary of the overall home performance isgiven in Table 3. The monthly site electricity and natural gas consumptionby end uses are shown in Figures 2 and 3. The monthly sourceenergy con
37、sumption by end use is shown in Figure 4. Thesefigures are consumption only they do not include the elec-tricity generated by the PV system. The natural gas energydivisions by end use for February and March 2006 in Figures3 and 4 are estimates. Although the total gas use for thesemonths is know from
38、 utility data, the natural gas end use moni-toring equipment was not installed until April 2006. Ratherthan being separately monitored for the entire year, the aver-Table 1. Summary of NREL/Habitat ZEH AttributesSquare footage 1280 sq. ft.Number of bedrooms 3Number of occupants 3Design heating load
39、15,000 Btu/hrWallsDouble stud wallFiberglass batt insulationNominal R-value = 40 hr ft2F/BtuCeiling2-foot raised heel trussesBlown-in fiberglass insulationNominal R-value = 60 hr ft2F/BtuFloorFiberglass batt insulationNominal R-value = 30 hr ft2F/BtuSouth windowsLow-e, high SHGCU = 0.30 Btu/hr ft2F,
40、 SHGC = 0.58North, west, and east windowsLow-e heat mirrorU = 0.23 Btu/hr ft2F, SHGC = 0.27Solar tempered96 ft2 of south facing windows3 ft overhangs for summer shadingWater heatingDrainback solar system96 ft2collectors with 200 gallon storage tankNatural gas tankless water heater for backup Ventila
41、tion Energy recovery ventilation system with ECMsSpace heatingDirect vent ductless natural gas heater in living roomElectric baseboard heaters (750W each) in bedroomsLighting Compact fluorescent throughout the houseAppliances Energy star clothes washer and refrigeratorSolar Electric Nominal 4 kWpDC
42、photovoltaic systemOther featuresAll mechanical equipment is within conditioned spaceLight colored roof shinglesIncreased attic ventilationASHRAE Transactions 221age refrigerator energy use over an 84 day period wasmeasured and applied to each day of the year. The ventilation energy use in the home
43、was lower thanexpected. When we investigated we found that the adjustmentfor the continuous ventilation rate installed in the mechanicalroom actually turned off the ventilation system when set to the“low” setting. The ventilation system was often off during theyear of monitoring. Therefore the home
44、was underventilatedduring this time and much of the monitored ventilation datarepresents only the standby power draw. The homeowner wasinformed of this issue so she can maintain proper ventilation.A stop on the adjustment that maintains the minimum venti-lation rate at ASHRAE 62.2 recommendations wo
45、uld solvethe problem in future installations. Table 2. Measurements and Components of the Data Acquisition SystemMeasurements ComponentElectrical Energy MeasurementsPV energy production Baseboard electric heaters Hard-wired lights Pulse output Kitchen range watt-hour transducersVentilation system So
46、lar pump Space and water heating controls All other loads Natural Gas Measurements Space heater Diaphragm gas meters Back-up water heater with pulse outputIndoor and Water Temperatures Living room North bedroom Southeast bedroom Cold water supply Solar tank Type T thermocouplesSolar - water to colle
47、ctors Solar - water from collectors Solar - water to back-up heater Hot water supply to house Water flowHot water use Water MeterWeather Related MeasurementsOutdoor temperature and RH T site-to-source multiplier for natural gas = 1.02Figure 2 Monthly site electricity consumption by end use.Figure 3
48、Monthly site natural gas consumption by enduse.ASHRAE Transactions 223ally speaking, the end uses within the control of the buildingdesigner include the space conditioning, water heating, venti-lation, and lighting. If we sum all other loads (often referred toas “appliance and plug loads”) they acco
49、unt for 58% of thetotal source energy consumption. These loads are primarilythe result of occupant choices and behavior. They varysubstantially with homeowner and time. This presents a chal-lenge for ZEH home designers. The PV system output must besized to match all energy consumption to reach the ZEH goalbut the energy consumption is dominated by loads that are outof the designers control, vary substantially with differenthomeowners, and are unknowable in advance for a spec home. PV ProductionA free PV performance calculator, called PVWatts, isavailable on NRELs Renewable Res
