Energy efficiency maximizes the value of photovoltaics (PV) in buildings systems. A fixed-sizre PV system will offset a much larger part of the electrical load in an energy-efficient building than in a building whose energy design has not been optimized

Hayter, S. J.

Torcellini, P. A.

UNT Digital Library

9Photovoltaics for Buildings:  Case Studies of High-Performance Buildings with PVSheila J. Hayter, P.E.Paul A. Torcellini, Ph.D., P.E.National Renewable Energy Laboratory1617 Cole Blvd.Golden, CO  80401ABSTRACTEnergy efficiency maximizes the value of photovoltaics(PV) in buildings systems.  A fixed-sized PV system willoffset a much larger part of the electrical load in an energy-efficient building than in a building whose energy designhas not been optimized.  Integrating an energy-design proc-ess when designing buildings helps to evaluate strategiessuch as daylighting, passive solar heating and cooling, prop-erly engineered heating, ventilating, and air-conditioning(HVAC) systems, improved envelope characteristics, andadvanced control systems.  These systems  working to-gether  result in substantial energy reduction, making itpossible for a reasonably sized PV system to meet a signifi-cant portion of the remaining electrical load.  This paperfocuses on three high-performance buildings whose elec-tricity loads are almost entirely met by modest-sized PVsystems.1.  Energy-Design ProcessDesigning and constructing low-energy buildings(buildings that consume 50% to 70% less energy than code-compliant buildings) require the design team to follow anenergy-design process that considers how the building en-velope and systems work together.  The process requires adesign team to set energy-efficiency goals at the beginningof the pre-design phase (Table 1) [1].  Detailed computersimulations used throughout the design and constructionphases ensure that the building is optimized for energy effi-ciency and that changes to the design do not adversely affectenergy performance.  Properly commissioning the buildingand educating the building operators are the final steps tosuccessfully constructing a low-energy building.2.  BigHorn CenterThe BigHorn Center is a collection of five retail spacesin Silverthorne, Colorado.  A design goal for the BigHornCenter was to create a sustainable retail environment.  Thecomplex was constructed in three phases.  Phase III, com-pleted in April 2000, houses a 17,000-ft2 (1579-m2) hard-ware store and support areas and a 22,000-ft2 (2044-m2)building materials warehouse.  This phase incorporates themost aggressive sustainable design strategies of the threephases.  These strategies include daylighting, advancedlighting technologies, natural ventilation for cooling, tran-spired solar collector, building-integrated PV, improvedenvelope features, and integrated controls.Photovoltaic modules laminated onto the standing-seammetal roof panels were installed on the south-facing roof ofthe hardware store clerestory and the warehouse dormer [2].The amorphous-silicon PV modules were wired into threearrays, each serving one phase of the three-phase buildingpower system.  The design capacity of the PV system is 8kW, and the array covers all the available Phase III, south-facing roof area.  The BigHorn Center developer/owner hasa net-metering agreement with the utility to receive fullcredit for power that the PV system exports back to the grid.The design team used the energy-design process to inte-grate daylighting opportunities, efficient mechanical sys-tems, and improved building envelope features into an op-timized building design.  It is estimated that the resultingdesign saves about 21 kW in demand, making it possible forthe 8 kW of PV power to meet a significant portion of theannual building electrical load.  Designers anticipate thatduring sunny summer days, it will be possible for thebuilding to export power to the grid during business hours.If daylighting and other design features had not been incor-porated, it would not have been feasible to purchase andinstall enough PV power to offset an equivalent portion ofthe base-case building load.The business plan for the project encompassed the abil-ity to sell green products in the retail environment.  Tothat end, the building became a statement to the sustainablemission, and the energy features were an integral part of theTable 1.  Energy-Design Process StepsPre-Design Design Construction/Occupation1. Simulate a base-case model, establishenergy-use targets2. Complete parametric analysis3. Brainstorm solutions4. Perform simulations on base-casevariants considering economic criteria5. Prepare preliminary architecturaldrawings6. Design the heating, ventilating, andair conditioning (HVAC) andlighting systems7. Finalize plans and specifications8. Rerun simulations before construc-tion design changes9. Commission all equipment andcontrols.  Educate building opera-tors to ensure that they operate thebuilding as is intended10building.  Photovoltaic modules integrated into the roofingwas an additional cost; however, the marketing value of thisinvestment, coupled with the other features, created a totalcost-effective business plan.  Even before construction ofthe BigHorn Center was completed, the owner/developersaw increased sales in his existing facility, which he attrib-uted to the publicity he received for installing the PV systemand other sustainable design features.The BigHorn Center is the first commercial building inthe State of Colorado to have a standing-seam metal roof-integrated PV system, and it is the largest building-integrated PV system in Colorado.  The grid-tied PV systemis one of the first net-metered commercial buildings in Colo-rado.  The BigHorn Center is also one of the first examplesin the United States of truly integrated daylighting and natu-ral ventilation cooling systems in a retail space.3. Zion National Park Visitor CenterThe Zion National Park Visitor Center and Comfort Sta-tion, under construction at the time this paper was written,will be two of the National Park Services (NPS) most effi-cient buildings.  The design team optimized the performanceof the aggressive low-energy design strategies into the7600-ft2 (706-m2) Visitor Center and 1100-ft2 (102-m2)Comfort Station.  Design features in both buildings includedaylighting, Trombe walls for passive solar heating, down-draft cooling towers for natural ventilation cooling, energy-efficient lighting, and advanced building controls.  It is es-timated that these features result in about 10 kW of electri-cal demand savings.Zion National Park is located in a remote area of south-ern Utah, where power reliability is an issue.  For this rea-son, the NPS required an uninterrupted power system (UPS)for the buildings.  The only additional cost to convert theUPS to a PV-for-buildings system was PV array because thebattery storage and balance-of-system components werealready a part of the UPS design.A 7.5-kW roof-mounted PV system is planned to be in-stalled on the south-facing roof of the Visitor Center.  Be-cause of the daylighting and natural ventilation coolingsystems, designers anticipate that the buildings PV systemwill export power to the utility grid during the summer.During power outages, the building control system will shutdown nonessential electrical loads so that the PV/UPS sys-tem will be capable of supporting enough building opera-tions to continue business for at least one-half hour withoutany additional PV power, or all day if PV capacity is avail-able.Officials at Zion National Park are negotiating a net-metering agreement with the local utility.  When an agree-ment has been reached, this will be the first net-meteredsystem in the State of Utah.4.  Van Geet ResidenceThe Van Geet Residence is a 3000-ft2 (279-m2) homeconstructed off the utility grid at an elevation of 9200 ft(2804 m) near Idaho Springs, Colorado.  The design of thehouse was optimized to incorporate passive solar heating(including direct solar gain and Trombe walls), natural ven-tilation cooling, daylighting, and a good thermal envelope.Mass was incorporated into the building shell and someinternal walls using dry-stack concrete blocks filled withconcrete.  Insulation was attached to the external surface ofthe block.  Solar domestic hot-water (SDHW) heating andsupplementary space heating needs are met by a solar hot-water system.  Energy-efficient lighting and appliances (therefrigerator uses 75% less energy than a conventional re-frigerator) minimize electrical loads in the house.A 1000-W (rated) PV array provides electric power forthe house and powers the well pump.  Power from the PVarray is stored in a battery bank, and an inverter converts the24V DC from the PV array to 120V AC.  An automatic pro-pane generator meets the remaining loads when PV/batterypower is not available.  During the two years since thehouse was completed, the occupants have relied on the gen-erator to meet less than 10% of their total power needs.The Van Geet house is two miles from the power line.  Itwas less expensive to incorporate energy-efficient and re-newable energy technologies into the design than it wouldhave been to extend the power line to the house.5.  SummaryIn all three examples discussed in this paper, the PVsystem initial cost and physical size were minimized be-cause aggressive energy-efficiency design measures werefirst incorporated into the building designs.  It is more costeffective to reduce building loads through good design thanit is to meet building loads with renewable energy technolo-gies.  Photovoltaic and other renewable energy technologiescan be viable solutions to meeting todays building energyneeds only when combined with measures to minimizebuilding energy loads.6.  AcknowledgementsThe National Renewable Energy Laboratorys High-Performance Buildings Research Project and Photovoltaicsfor Buildings Task provided support for the energy designof the buildings described in this paper.  The Department ofEnergy provides funding for these two activities from theOffice of Building Technology, State and Community Pro-grams and from the Office of Power Technologies, respec-tively.7.  References[1]  Torcellini, P.A., S.J. Hayter, R. Judkoff:   Low-EnergyBuilding Designthe Process and a Case Study, ASHRAETransactions, V 105, Part 2, (1999) 802-810.[2] Ovonic, UniSolar Web Site, www.ovonic.com/unisolar,accessed March 27, 2000.

Photovoltaics for Buildings: Case Studies of High-Performance Buildings with PV

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Photovoltaics for Buildings: Case Studies of High-Performance Buildings with PV

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