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A COLD-CLIMATE CASE STUDY FOR AFFORDABLE ZERO ENERGY HOMESBy Paul Norton and Craig Christensen, National Renewable Energy Laboratory This project, supported by the U.S. Department of Energy’s Building America Program, is a case study in reaching zero energy within the affordable housing sector in cold climates. The design of the 1200 square foot, 3-bedroom Denver zero energy home carefully combines envelope efficiency, efficient equipment, appliances and lighting, and passive and active solar features to reach the zero energy goal. The home was designed using an early version of the BEOpt building optimization software 1. ZERO ENERGY HOME INTRODUCTIONIn October 2005, Amy Whalen and her two young boys moved into their new Habitat for Humanity 2. DESIGN CRITERIA AND PROCESSFrom its inception, the NREL/Habitat ZEH project focused on finding the balance between engineering ideals and real-world practicality. The team that designed the home included two NREL building energy researchers, two Habitat staff members (the Construction Manager and the Real Estate Development Manager), and two Habitat energy subcommittee volunteers. The NREL engineers made suggestions based on modeling results and analysis and presented them to the design team who then grounded the discussion with practical concerns and insights. This mix of perspectives led to a design that balances energy performance, ease of construction and low cost while maintaining the zero energy goal.2.1 Project Design Criteria A Habitat for Humanity house presents an unusual opportunity: thanks to volunteers, much of the labor comes at no cost and it is possible to get some of the equipment donated or at reduced cost. We established the following criteria for the home design: 1. Zero net energy. Zero net energy can be defined in terms of site energy (used at the building site) or source energy (sometimes called primary energy). For electricity purchased from a utility, the source energy used to produce and distribute the electricity is typically about three times as much as the delivered electricity. From a societal point of view, source energy better reflects the overall consequences of energy use. The home was designed to meet the definition of zero energy of the US Department of Energy’s Building America (BA) residential energy efficiency research program
2.2 The Use of Computer Simulation in the DesignThe home design process used a combination of computer simulations and heuristic judgment. Three simulation tools were used both sequentially and iteratively during the design process: TRNSYS Transient System Simulation software (4), DOE2 Building Energy Model (5), and BEOpt Building Energy Optimization Program (6,7).Fig.1 3. FINAL HOME DESIGNThe envelope design began by looking at Habitat of Metro Denver’s standard home plans. We sorted these plans for their applicability to the site and adaptability for a passive solar home design. A standard 3-bedroom 26’ by 46’ design with a crawlspace was chosen. The floorplan was mirrored from its original design to accommodate the site. Motivated by the BEOpt simulation recommendation for a superinsulated envelope, the design team considered a wide variety of approaches including structural insulated panels, insulated concrete forms, straw bale, and double stud wall. Structural insulated panels (SIPs) and Insulated Concrete Forms (ICFs) were eliminated because they tend to have high material costs and low labor costs – the opposite of what is needed to take advantage of Habitat volunteer labor. Straw bale construction was carefully considered because it has low material cost and high labor. However after reviewing the literature on straw bale construction and speaking with other Habitat affiliates who have built with straw we eliminated this option because of the lack of standard techniques and details and the low probability of replication by Habitat Metro Denver. We chose a double stud wall with fiberglass batt construction because it has low material costs, uses familiar volunteer-friendly construction techniques, and proven construction techniques and details are available from the National Affordable Housing Network (NAHN: http://NAHN.com).The walls consist of an outer 2x4 structural stud wall on 16” centers with R13 fiberglass batts in the cavities. Spaced 3 ½” inside this wall a second 2x4 studwall on 24” centers was built. Additional R13 fiberglass batts were placed horizontally in the space between the studwalls and vertically in the interior wall cavities. An outer vapor permeable housewrap and fibercement siding and an inner poly vapor barrier and drywall complete the nominal R40 assembly. The actual whole-wall R-value of this wall will be much closer to its nominal value than a single stud wall because the thermal shorting of the studs is broken by the insulation in the space between the double stud walls. Fig. 2: Details of the double stud wall design Raised heel trusses were designed to accommodate 2 feet of blown-in fiberglass in the attic giving the top of the thermal envelope an R-60 rating. The floors are insulated to a nominal R-30.The superinsulated shell dramatically reduces heating energy needs; “sun tempering” was used to reduce these needs further. We designed the home with increased glazing area on the long south side and reduced glazing area on other orientations. As a policy, Habitat Metro Denver does not equip their homes with air conditioning, so we were sensitive to overheating potential. Using DOE2 we evaluated different southern glazing areas and types and overhangs. We compared the simulated heating energy and simulated cooling energy (as if there were an air conditioning system) in the ZEH and the identical standard construction home for each window combination. We chose the design that maximized heating reduction without increasing cooling energy over the standard construction. Double glazed, low emissivity (U-value = 0.30 BTU/hr-F-ft2), high solar heat gain coefficient (SHGC=0.58) glass was chosen for the southern windows. Double glazed clear windows would have provided more solar heating, but also would have increased the overheating potential and would have lacked the thermal comfort and ultraviolet screening benefits. Double glazed low emissivity (U-value = 0.22 BTU/hr-F-ft2) low SHGC (0.27) were used for the east, west, and north windows. Fig. 3: Double stud wall framing Fig. 4: Raised heel trusses Fig. 5: Windows have been moved to the south wall to provide solar heating. 3.1 Ventilation SystemBecause we intended to build the home with very low air leakage, a mechanical ventilation system was required. To provide fresh air to the home while minimizing energy losses we chose to use a balanced energy recovery ventilation 3.2 Space HeatingHaving a very low design heating load is a blessing and a challenge. The blessing is obvious – it takes very little energy to keep this home warm! The challenge is that most commonly available heating systems are oversized for this home and the low heating energy needs cannot justify a complicated or expensive system. We considered a wide variety of heating systems for the home including:
The design team considered a solar “combisystem” that combines active solar thermal space heating and water heating. However, this approach requires a relatively large equipment investment. During the summer and during periods when the house is passively solar heated during the fall, winter and spring there is no need for active heating. Therefore the additional equipment investment for space heating is delivering no energy benefit for most of the year. Few integrated solar combisystems are commercially available in the U.S. so most of these systems are custom designed and built and can be quite complicated. We decided that the high first cost, low utilization, complexity and custom design of this approach was not consistent with our design criteria. For an all electric ZEH, using a ground coupled heat pump (GCHP) for heating has attractive benefits. The GCHP can deliver three to four units of heat for each unit of electricity used. In contrast, electric resistant heat delivers one unit of heat for every unit of electricity consumed. In addition the GCHP can deliver cooling in the summer. However, the ground loop for the GCHP and the heat pump itself are quite expensive and would require an air handler and duct system or a hydronic system to distribute the heating. The compact size and superinsulated shell of the Habitat ZEH reduced heating needs to so low a level that the cost of the GCHP was not justified.The use of natural gas for heating, cooking, and clothes drying within a ZEH is somewhat controversial. There are those who believe that since a ZEH exports only electricity, it must consume only electricity. However, in most of the U.S., the electricity consumed comes primarily from fossil fuels. So the home is consuming fossil fuels when it is using electricity and offsetting that consumption when it is producing excess photovoltaic electricity. This is similar for a ZEH that consumes natural gas. The photovoltaic system is sized to produce an excess of electricity to offset the natural gas used. The source energy use is net zero.The economics of the use of natural gas or all-electric differ. For an all-electric home too small and efficient to use a GCHP, the all-electric approach requires a larger PV system and is substantially more expensive. The all-electric approach has the advantage of eliminating the monthly fixed cost of having a natural gas hookup which is about $9.00/month in the Denver, CO area. However, the Habitat ZEH design team decided to use natural gas in the home, reducing the required PV array size by 1.1 kW and to take a hybrid approach to space heating.The space heating system combines a point-source direct vent natural gas furnace in the living/dining area of the home and small baseboard electric resistive heaters in the three bedrooms. This approach is relatively low cost, elegantly simple and provides zone heating because each appliance has its own independent thermostat. 3.3 Water Heating SystemAlthough we ruled-out a solar combisystem, the results of the early BEOpt runs convinced us to incorporate a high solar saving fraction solar water heating system into the home design. TRNSYS was used to do parametric studies to design the system. We found that mounting the collectors at the roof pitch rather than raising them to their optimal angle incurred only a small annual energy penalty (8). We found that a 96 sq. ft. collector area with 200 gallons of water storage would result in an 88% annual solar savings fraction using TMY2 weather data and BA Benchmark hot water use. This solar savings fraction includes pump energy use.We specified a natural gas tankless water heater as a backup to the solar system. Unlike tank water heaters, the tankless system uses no heating energy when the solar water tank is at or above the 115 degree F hot water delivery temperature, although it may use electricity in standby. The disadvantage of using the tankless system is the added cost compared to a tank system. We considered using the tankless water heater for space heating also, but ultimately decided to use separate systems to avoid the complexity of the combined system. 3.4 Photovoltaic System SizingOnce all possible energy loads in the house were significantly reduced, the photovoltaic system was sized to meet the remaining electricity needs and offset the expected natural gas use. In a similar home built to BA Benchmark standards, about one quarter of the energy in the home is consumed by lighting, appliances, and miscellaneous electric loads (“LAME” loads). We reduced the lighting load by using compact fluorescent lights throughout the home. We reduced the appliance load through the use of energy star appliances. This leaves the miscellaneous electric loads (MELs) that include everything plugged in by the occupants…. TV, hair dryer, toaster oven, computer, aquarium, etc. Because all other loads have been dramatically reduced, the MELs in the Habitat ZEH are expected to consume 57% of all energy used annually (see Fig. 6). Although the BA Program is pursuing research into ways to reduce these loads, they are currently out of the control of the home designer. Furthermore, these loads are highly unpredictable and vary substantially from household to household. So the ZEH designer is faced with sizing a PV system for a home where the largest load is really not known with any accuracy.
The BA Benchmark includes assumptions that we used to estimate the MELs and size the 4kW PV system. These assumptions are based on the best available nationwide studies of energy use. So the home’s PV system is sized with the assumption that it will be occupied by a “typical” American household. If the actual household and weather are typical, the home will achieve zero energy. If the household or weather is atypical, the home may not achieve zero energy or may be a net producer. 4. FOR MORE INFORMATION ON THE ZERO ENERGY PROJECTWe installed a data acquisition system in the home in January 2006 and will monitor the performance of the home for at least one year. Periodic email updates on the project are available from NREL. If you would like to receive these updates, send your email address to Paul_Norton@NREL.gov. A conference paper on the home’s performance will be available early in 2008 and a full technical report on the project will be available in mid-2008. These reports will be posted on the BA website at http://BuildingAmerica.gov 5. ACKNOWLEDGMENTSThe authors would like to thank the many individuals and organizations that supported this project including: homeowner Amy Whalen; The U.S. Department of Energy’s Building Technologies Program; Midwest Research Institute; Battelle; Energy Outreach Colorado; Xcel Energy; The Governors Office of Energy Management and Conservation; Secretary of Energy Samuel Bodman; Congressman Bob Beauprez; Lori Vaclavic, Bruce Carpenter, Andy Blackmun, Brady Nelson, Pandora Reagan, and Lynne Brown of Habitat of Metro Denver; Paul Kreischer of Lightly Treading; Pete Beverly; Paul Reeves of the Partnership for Resource Conservation; Ed Hancock and Greg Barker of Mountain Energy Partnership; Ren Anderson, Admiral Truly, Dan Arvizu, Jessie Harris, Byron Stafford, Kerry Masson, Lee Boughey, Sarah Barba, Jay Burch and Scott Horowitz of NREL; Betsy Petit of Building Science Corporation; Barbara Miller of the National Affordable Housing Network; and Rob Dumont of the Saskatchewan Research Council. 6. Zero Energy Home References(1) For more information on the Building Energy Program of the Department of Energy see http://www.buildingamerica.gov (2) Marion, W. and Urban, K. User's Manual for TMY2s Typical Meteorological Years, Derived from the 1961-1990 National Solar Radiation Data Base, National Renewable Energy Laboratory, 1995 (3) Hendron, B., et. al., Building America Benchmark Definition, Updated December 29, 2004, National Renewable Energy Laboratory, NREL/TP-550-37529, 2005 (4) Klein, S., et al., TRNSYS: A Transient System Simulation Program – Reference Manual, Solar Energy Laboratory, University of Wisconsin, 1996 (5) DOE2.2 Building Simulation Software, developed by Lawrence Berkeley National Laboratory, Berkeley, CA 94720, available at http://www.osti.gov/estsc/ or http://www.doe2.com (6) Christensen, C.; Barker, G.; Horowitz, S. A Sequential Search Technique for Identifying Optimal Building Designs on the Path to Zero Net Energy, Proceedings of the Solar 2004 Conference Including Proceedings of 33rd ASES Annual Conference and Proceedings of 29th National Passive Solar Conference, American Solar Energy Society, NREL/CP-550-36099, 2004 (7) Christensen, C.; Barker, G.; Tupper, K., Optimal Building Designs on the Path to Zero Net Energy, Proceedings of the Solar 2004 Conference Including Proceedings of 33rd ASES Annual Conference and Proceedings of 29th National Passive Solar Conference, American Solar Energy Society , NREL/CP-550-36100, 2004 (8) Christensen, C.; Barker, G., Effects of Tilt and Azimuth on Annual Incident Solar Radiation for United States Locations, Proceedings of Solar Forum 2001: Solar Energy: The Power to Choose, 2001 |
DOE-2 is a widely used and accepted freeware building energy analysis program that can predict the energy use and cost for all types of buildings. DOE-2 uses a description of the building layout, constructions, operating schedules, conditioning systems (lighting, HVAC, etc.) and utility rates provided by the user, along with weather data, to perform an hourly simulation of the building and to estimate utility bills. www.nrel.gov • • Solar Energy Resourseswww.solarenergy.org
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with additional analysis using 
