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  1. 1. A national laboratory of the U.S. Department of Energy Office of Energy Efficiency & Renewable Energy National Renewable Energy Laboratory Innovation for Our Energy Future Conference Paper Zero Energy Buildings: NREL/CP-550-39833 A Critical Look at the Definition June 2006 Preprint P. Torcellini, S. Pless, and M. Deru National Renewable Energy Laboratory D. Crawley U.S. Department of Energy To be presented at ACEEE Summer Study Pacific Grove, California August 14−18, 2006NREL is operated by Midwest Research Institute ● Battelle Contract No. DE-AC36-99-GO10337
  2. 2. NOTICEThe submitted manuscript has been offered by an employee of the Midwest Research Institute (MRI), acontractor of the US Government under Contract No. DE-AC36-99GO10337. Accordingly, the USGovernment and MRI retain a nonexclusive royalty-free license to publish or reproduce the published form ofthis contribution, or allow others to do so, for US Government purposes.This report was prepared as an account of work sponsored by an agency of the United States government.Neither the United States government nor any agency thereof, nor any of their employees, makes anywarranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, orusefulness of any information, apparatus, product, or process disclosed, or represents that its use would notinfringe privately owned rights. Reference herein to any specific commercial product, process, or service bytrade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement,recommendation, or favoring by the United States government or any agency thereof. The views andopinions of authors expressed herein do not necessarily state or reflect those of the United Statesgovernment or any agency thereof. Available electronically at http://www.osti.gov/bridge Available for a processing fee to U.S. Department of Energy and its contractors, in paper, from: U.S. Department of Energy Office of Scientific and Technical Information P.O. Box 62 Oak Ridge, TN 37831-0062 phone: 865.576.8401 fax: 865.576.5728 email: mailto:reports@adonis.osti.gov Available for sale to the public, in paper, from: U.S. Department of Commerce National Technical Information Service 5285 Port Royal Road Springfield, VA 22161 phone: 800.553.6847 fax: 703.605.6900 email: orders@ntis.fedworld.gov online ordering: http://www.ntis.gov/ordering.htm Printed on paper containing at least 50% wastepaper, including 20% postconsumer waste
  3. 3. Zero Energy Buildings: A Critical Look at the Definition1 Paul Torcellini, Shanti Pless, and Michael Deru, National Renewable Energy Laboratory Drury Crawley, U.S. Department of EnergyABSTRACT A net zero-energy building (ZEB) is a residential or commercial building with greatlyreduced energy needs through efficiency gains such that the balance of energy needs can besupplied with renewable technologies. Despite the excitement over the phrase “zero energy,” welack a common definition, or even a common understanding, of what it means. In this paper, weuse a sample of current generation low-energy buildings to explore the concept of zero energy:what it means, why a clear and measurable definition is needed, and how we have progressedtoward the ZEB goal. The way the zero energy goal is defined affects the choices designers make to achievethis goal and whether they can claim success. The ZEB definition can emphasize demand-sideor supply strategies and whether fuel switching and conversion accounting are appropriate tomeet a ZEB goal. Four well-documented definitions—net-zero site energy, net-zero sourceenergy, net-zero energy costs, and net-zero energy emissions—are studied; pluses and minusesof each are discussed. These definitions are applied to a set of low-energy buildings for whichextensive energy data are available. This study shows the design impacts of the definition usedfor ZEB and the large difference between definitions. It also looks at sample utility ratestructures and their impact on the zero energy scenarios.Introduction Buildings have a significant impact on energy use and the environment. Commercial andresidential buildings use almost 40% of the primary energy and approximately 70% of theelectricity in the United States (EIA 2005). The energy used by the building sector continues toincrease, primarily because new buildings are constructed faster than old ones are retired.Electricity consumption in the commercial building sector doubled between 1980 and 2000, andis expected to increase another 50% by 2025 (EIA 2005). Energy consumption in thecommercial building sector will continue to increase until buildings can be designed to produceenough energy to offset the growing energy demand of these buildings. Toward this end, theU.S. Department of Energy (DOE) has established an aggressive goal to create the technologyand knowledge base for cost-effective zero-energy commercial buildings (ZEBs) by 2025. In concept, a net ZEB is a building with greatly reduced energy needs through efficiencygains such that the balance of the energy needs can be supplied by renewable technologies.Despite our use of the phrase “zero energy,” we lack a common definition—or a commonunderstanding—of what it means. In this paper, we use a sample of current generation low-energy buildings to explore the concept of zero energy—what it means, why a clear andmeasurable definition is needed, and how we have progressed toward the ZEB goal.1 This work has been authored by an employee or employees of the Midwest Research Institute under Contract No. DE-AC36-99GO10337 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the articlefor publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, worldwide license topublish or reproduce the published form of this work, or allow others to do so, for United States Government purposes. 1
  4. 4. Using ZEB design goals takes us out of designing low-energy buildings with a percentenergy savings goal and into the realm of a sustainable energy endpoint. The goals that are setand how those goals are defined are critical to the design process. The definition of the goal willinfluence designers who strive to meet it (Deru and Torcellini 2004). Because design goals areso important to achieving high-performance buildings, the way a ZEB goal is defined is crucialto understanding the combination of applicable efficiency measures and renewable energysupply options.Zero-Energy Buildings: Boundary Definitions and Energy Flows At the heart of the ZEB concept is the idea that buildings can meet all their energyrequirements from low-cost, locally available, nonpolluting, renewable sources. At the strictestlevel, a ZEB generates enough renewable energy on site to equal or exceed its annual energy use.The following concepts and assumptions have been established to help guide definitions forZEBs.Grid Connection Is Allowed and Necessary for Energy Balances A ZEB typically uses traditional energy sources such as the electric and natural gasutilities when on-site generation does not meet the loads. When the on-site generation is greaterthan the building’s loads, excess electricity is exported to the utility grid. By using the grid toaccount for the energy balance, excess production can offset later energy use. Achieving a ZEBwithout the grid would be very difficult, as the current generation of storage technologies islimited. Despite the electric energy independence of off-grid buildings, they usually rely onoutside energy sources such as propane (and other fuels) for cooking, space heating, waterheating, and backup generators. Off-grid buildings cannot feed their excess energy productionback onto the grid to offset other energy uses. As a result, the energy production from renewableresources must be oversized. In many cases (especially during the summer), excess generatedenergy cannot be used. We assume that excess on-site generation can always be sent to the grid. However, inhigh market penetration scenarios, the grid may not always need the excess energy. In thisscenario, on-site energy storage would become necessary.Prioritize Supply-Side Technologies to Those Available On Site and within the Footprint Various supply-side renewable energy technologies are available for ZEBs. Typicalexamples of technologies available today include PV, solar hot water, wind, hydroelectric, andbiofuels. All these renewable sources are favorable over conventional energy sources such ascoal and natural gas; however, we have developed a ranking of renewable energy sources in theZEB context. Table 1 shows this ranking in order of preferred application. The principles wehave applied to develop this ranking are based on technologies that:• Minimize overall environmental impact by encouraging energy-efficient building designs and reducing transportation and conversion losses.• Will be available over the lifetime of the building.• Are widely available and have high replication potential for future ZEBs. 2
  5. 5. This hierarchy is weighted toward renewable technologies that are available within thebuilding footprint and at the site. Rooftop PV and solar water heating are the most applicablesupply-side technologies for widespread application of ZEBs. Other supply-side technologiessuch as parking lot-based wind or PV systems may be available for limited applications.Renewable energy resources from outside the boundary of the building site could arguably alsobe used to achieve a ZEB. This approach may achieve a building with net zero energyconsumption, but it is not the same as one that generates the energy on site and should beclassified as such. We will use the term “off-site ZEB” for buildings that use renewable energyfrom sources outside the boundaries of the building site. Table 1. ZEB Renewable Energy Supply Option Hierarchy Option ZEB Supply-Side Options Examples Number Reduce site energy use through low-energy Daylighting, high-efficiency HVAC equipment, 0 building technologies natural ventilation, evaporative cooling, etc. On-Site Supply Options Use renewable energy sources available within the PV, solar hot water, and wind located on the 1 building’s footprint building. PV, solar hot water, low-impact hydro, and wind 2 Use renewable energy sources available at the site located on-site, but not on the building. Off-Site Supply Options Biomass, wood pellets, ethanol, or biodiesel that Use renewable energy sources available off site to can be imported from off site, or waste streams 3 generate energy on site from on-site processes that can be used on-site to generate electricity and heat. Utility-based wind, PV, emissions credits, or other 4 Purchase off-site renewable energy sources “green” purchasing options. Hydroelectric is sometimes considered. A good ZEB definition should first encourage energy efficiency, and then use renewableenergy sources available on site. A building that buys all its energy from a wind farm or othercentral location has little incentive to reduce building loads, which is why we refer to this as anoff-site ZEB. Efficiency measures or energy conversion devices such as daylighting orcombined heat and power devices cannot be considered on-site production in the ZEB context.Fuel cells and microturbines do not generate energy; rather they typically transform purchasedfossil fuels into heat and electricity. Passive solar heating and daylighting are demand-sidetechnologies and are considered efficiency measures. Energy efficiency is usually available forthe life of the building; however, efficiency measures must have good persistence and should be“checked” to make sure they continue to save energy. It is almost always easier to save energythan to produce energy. Determining a project’s boundary, which can be substantially larger than the buildingfootprint, is an important part of defining on-site generation sources. The question arises as towhether this larger area should be considered for on-site renewable energy production.Typically, the only area available for on-site energy production that a building has guaranteed as“its own” over its lifetime is within its footprint. To ensure this area is available for on-siteproduction, many states, counties, and cities have solar access ordinances, which declare that theright to use the natural resource of solar energy is a property right. For example, the City of 3
  6. 6. Boulder, Colorado has a solar access ordinance that guarantees access to sunlight forhomeowners and renters in the city. This ordinance protects the solar access of existingbuildings by limiting the amount of shadow new development may cast on neighboringbuildings, and maintains the potential for using renewable energy systems in buildings (City ofBoulder 2006). Using a neighboring field to generate electricity is not as favorable as a roof-mounted PV system; the area outside the building’s footprint could be developed in the future;thus, it cannot be guaranteed to provide long-term generation. Wind resources for ZEBs are limited because of structural, noise, and wind patternconsiderations, and are not typically installed on buildings. Some parking lots or adjacent areasmay be used to produce energy from wind, but this resource is site specific and not widelyavailable. Similar to PV generation in an adjacent parking lot, the wind resource is notnecessarily guaranteed because it could be superseded by future development. Renewable sources imported to the site, such as wood pellets, ethanol, or biodiesel can bevaluable, but do not count as on-site renewable sources. Biofuels such as waste vegetable oilfrom waste streams and methane from human and animal wastes can also be valuable energysources, but these materials are typically imported for the on-site processes. The final option for supply-side renewable energy sources includes purchasing “greencredits” or renewable sources such as wind power or utility PV systems that are available to theelectrical grid. These central resources require infrastructure to move the energy to the buildingand are not always available. Buildings employing resources 3 and 4 in Table 1 to achieve zeroenergy are considered off-site ZEBs. For example, a building can achieve an off-site ZEB for allthese definitions by purchasing wind energy. Although becoming an off-site ZEB can have littleto do with design and a lot to do with the different sources of purchased off-site renewableenergy, an off-site ZEB is still in line with the general concept of a ZEB.Zero-Energy Buildings: Definitions A zero energy building can be defined in several ways, depending on the boundary andthe metric. Different definitions may be appropriate, depending on the project goals and thevalues of the design team and building owner. For example, building owners typically careabout energy costs. Organizations such as DOE are concerned with national energy numbers,and are typically interested in primary or source energy. A building designer may be interestedin site energy use for energy code requirements. Finally, those who are concerned aboutpollution from power plants and the burning of fossil fuels may be interested in reducingemissions. Four commonly used definitions are: net zero site energy, net zero source energy,net zero energy costs, and net zero energy emissions. Each definition uses the grid for net use accounting and has different applicablerenewable energy sources. The definitions do apply for grid independent structures. For alldefinitions, supply-side option 2 can be used if this resource will be available for the life of thebuilding. Off-site ZEBs can be achieved by purchasing renewable energy from off-site sources,or in the case of an off-site zero emissions building, purchasing emissions credits. In support ofDOE’s ZEB research needs, the following definitions refer to ZEBs that use supply-side optionsavailable on site. For ZEBs that have a portion of the renewable generation supplied by off-sitesources, these buildings are referred to as “off-site ZEBs.” 4
  7. 7. • Net Zero Site Energy: A site ZEB produces at least as much energy as it uses in a year, when accounted for at the site.• Net Zero Source Energy: A source ZEB produces at least as much energy as it uses in a year, when accounted for at the source. Source energy refers to the primary energy used to generate and deliver the energy to the site. To calculate a building’s total source energy, imported and exported energy is multiplied by the appropriate site-to-source conversion multipliers.• Net Zero Energy Costs: In a cost ZEB, the amount of money the utility pays the building owner for the energy the building exports to the grid is at least equal to the amount the owner pays the utility for the energy services and energy used over the year.• Net Zero Energy Emissions: A net-zero emissions building produces at least as much emissions-free renewable energy as it uses from emissions-producing energy sources.Low- and Zero-Energy Buildings: Examples To study the impacts of these ZEB definitions, we examined seven low-energycommercial buildings that had been monitored extensively with respect to the definitions. Eachwas designed to minimize energy and environmental impacts and used a combination of low-energy and renewable energy technologies. The buildings represent several climates and uses.They are all good energy performers; site energy savings range from 25% to 68% compared toconventional buildings that are minimally energy-code compliant (ASHRAE 2001).Understanding the energy performance of the current stock of high-performance buildings is animportant step toward reaching the ZEB goal. The lessons learned from these seven buildingsare used to guide future research to meet DOE’s goal for facilitating marketable ZEBs by 2025.The buildings studied are (Torcellini et al. 2004; Barley et al. 2005):• “Oberlin”—The Adam Joseph Lewis Center for Environmental Studies, Oberlin College.• “Zion”—The Visitor Center at Zion National Park, Springdale, Utah.• “Cambria”—The Cambria Department of Environmental Protection Office Building, Ebensburg, Pennsylvania.• “CBF”—The Philip Merrill Environmental Center, Chesapeake Bay Foundation, Annapolis, Maryland.• “TTF”—The Thermal Test Facility, National Renewable Energy Laboratory, Golden, Colorado.• “BigHorn”—The BigHorn Home Improvement Center, Silverthorne, Colorado.• “Science House” Science Museum of Minnesota, St. Paul, Minnesota. These buildings were further investigated to determine additional PV system array areaand capacity requirements to meet the ZEB goals (see Table 2). Annual electricity and naturalgas site-to-source conversion multipliers (3.2 for electricity and 1.07 for natural gas) wereapplied to each building to determine source energy use (EIA 2005). For the all-electricbuildings (Oberlin, Zion, Cambria, and the Science House), the site ZEB and source ZEB are thesame. CBF used a minimal amount of propane, and the TTF and BigHorn used natural gas forspace and water heating. Zion, TTF, BigHorn, and the Science House are single-story buildings;Oberlin, Cambria, and CBF are two stories. We used the PV system simulation tool PVSyst v3.3 5
  8. 8. (Mermoud 1996) to calculate the expected annual performance of the PV system. Single-crystalline PV modules were modeled with 0.0° tilt, as we assumed the PV system would bemounted on a flat roof of each building. These modules provide the best available output perunit area of commercially available PV modules. The Science House is the only building in thislist that is currently a site, source, and emissions ZEB; it is a net exporter with approximately30% more generation than consumption. Table 2. ZEB Example Summary Source Flat Roof Area (ft2) Building and PV Site Energy Actual Roof PV System DC Size Energy Use Needed for Source System (DC Rating Use (w/o PV) Area Needed for Source (w/o PV) ZEB and Site ZEB Size) (MWh/yr) (footprint) (ft2) ZEB and Site ZEB (MWh/yr) with PVOberlin-60 kW 118.8 380.2 8,500 10,800 120 kWZion-7.2 kW 91.6 293.1 11,726 6,100 73 kWCambria-17.2 kW 372.1 1,190.7 17,250 37,210 415 kW 25,316 Source ZEB 282 kW Source ZEBCBF-4.2 kW 365.2 1,142.0 15,500 25,640 Site ZEB 286 kW Site ZEB 4,010 Source ZEB 45 kW Source ZEBTTF-No PV 83.5 192.5 10,000 5,550 Site ZEB 62 kW Site ZEB 18,449 Source ZEB 206 KW Source ZEBBigHorn-8.9 kW 490.4 901.0 38,923 31,742 Site ZEB 354 kW Site ZEBScience House-8.7 kW 5.9 18.8 1,370 1,000 6 kWZero-Energy Buildings: How Definition Influences Design Depending on the ZEB definition, the results can vary substantially. Each definition hasadvantages and disadvantages, which are discussed below.Net Zero Site Energy Building A site ZEB produces as much energy as it uses, when accounted for at the site.Generation examples include roof-mounted PV or solar hot water collectors (Table 1, Option 1).Other site-specific on-site generation options such as small-scale wind power, parking lot-mounted PV systems, and low-impact hydro (Table 1, Option 2), may be available. As discussedearlier, having the on-site generation within the building footprint is preferable. A limitation of a site ZEB definition is that the values of various fuels at the source arenot considered. For example, one energy unit of electricity used at the site is equivalent to oneenergy unit of natural gas at the site, but electricity is more than three times as valuable at thesource. For all-electric buildings, a site ZEB is equivalent to a source ZEB. For buildings withsignificant gas use, a site ZEB will need to generate much more on-site electricity than a sourceZEB. As an example, the TTF would require a 62-kWDC PV system to be a site ZEB, but onlya 45-kWDC PV system for a source ZEB (Table 2); this is because gas heating is a major enduse. The net site definition encourages aggressive energy efficiency designs because on-sitegenerated electricity has to offset gas use on a 1 to 1 basis. A site ZEB can be easily verified through on-site measurements, whereas source energyor emissions ZEBs cannot be measured directly because site-to-source factors need to bedetermined. An easily measurable definition is important to accurately determine the progresstoward meeting a ZEB goal. 6
  9. 9. A site ZEB has the fewest external fluctuations that influence the ZEB goal, and thereforeprovides the most repeatable and consistent definition. This is not the case for the cost ZEBdefinition because fluctuations in energy costs and rate structures over the life of a buildingaffect the success in reaching net zero energy costs. For example, at BigHorn, natural gas pricesvaried 40% during the three-year monitoring period and electricity prices varied widely, mainlybecause of a partial shift from coal to natural gas for utility electricity production. Similarly,source energy conversion rates may change over the life of a building, depending on the type ofpower plant or power source mix the utility uses to provide electricity. However, for all the ZEBdefinitions, the impact of energy performance can affect the success in meeting a ZEB goal. A building could be a site ZEB but not realize comparable energy cost savings. If peakdemands and utility bills are not managed, the energy costs may or may not be similarly reduced.This was the case at Oberlin, which realized a 79% energy saving, but did not reduce peakdemand charges. Uncontrolled demand charges resulted in a disproportionate energy cost savingof only 35%. An additional design implication of a site ZEB is that this definition favors electricequipment that is more efficient at the site than its gas counterpart. For example, in a net siteZEB, electric heat pumps would be favored over natural gas furnaces for heating because theyhave a coefficient of performance from 2 to 4; natural gas furnaces are about 90% efficient. Thiswas the case at Oberlin, which had a net site ZEB goal that influenced the design decision for anall-electric ground source heat pump system.Net Zero Source Energy Building A source ZEB produces as much energy as it uses as measured at the source. Tocalculate a building’s total source energy, both imported and exported energy are multiplied bythe appropriate site-to-source energy factors. To make this calculation, power generation andtransmission factors are needed. Source Energy and Emission Factors for Energy Use inBuildings (Deru and Torcellini 2006) used a life cycle assessment approach and determinednational electricity and natural gas site-to-source energy factors of 3.37 and 1.12. Site gasenergy use will have to be offset with on-site electricity generation on a 3.37 to 1 ratio (one unitof exported electricity for 3.37 units of site gas use) for a source ZEB. This definition couldencourage the use of gas in as many end uses as possible (boilers, domestic hot water, dryers,desiccant dehumidifiers) to take advantage of this fuel switching and source accounting to reachthis ZEB goal. For example, the higher the percent of total energy used at a site that is gas, thesmaller the PV system required to be a source ZEB. At BigHorn, for a source ZEB, 18,500 ft2 ofPV are required; however, 31,750 ft2 of PV are required for a site ZEB (Table 2). This definition also depends on the method used to calculate site-to-source electricityenergy factors. National averages do not account for regional electricity generation differences.For example, in the Northwest, where hydropower is used to generate significant electricity, thesite-to-source multiplier is lower than the national number. In addition, national site-to-sourceenergy factors do not account for hourly variations in the heat rate of power plants or howutilities dispatch generation facilities for peak loading. Electricity use at night could have fewersource impacts than electricity used during the peak utility time of day. Further work is neededto determine how utilities dispatch various forms of generation and the corresponding dailyvariations of heat rates and source rates. Using regional time-dependent valuations (TDVs) fordetermining time-of-use source energy is one way to account for variations in how and when 7
  10. 10. energy is used. TDVs have been developed by the California Energy Commission to determinethe hourly value of delivered energy for 16 zones in California (CEC 2005). Similar nationalTDVs would be valuable to accurately calculate source energy use to determine a building’ssuccess in reaching a source ZEB goal. A first step in understanding regional site-to-sourcemultiplier differences is available (Deru and Torcellini 2006); multipliers are provided for thethree primary grid interconnects and for each state. There may be issues with the source ZEB definition when electricity is generated on sitewith gas from fossil fuels. The ZEB definitions state that the building must use renewableenergy sources to achieve the ZEB goal; therefore, electricity generated on site from fossil fuelscannot be exported and count toward a ZEB goal. However, this is unlikely, because buildingsare unlikely to need more heat than electricity and the inefficiencies of on-site electricitygeneration and exportation make this economically very unattractive. The best cost or energypathways will determine the optimal combination of energy efficiency, on-site cogeneration, andon-site renewable energy generation. The issue of unmanaged energy costs in a site ZEB is similar for a source ZEB. Abuilding could be a source ZEB and not realize comparable energy cost savings. If peakdemands and utility bills are not managed, the energy costs may or may not be similarly reduced.Net Zero Energy Cost Building A cost ZEB receives as much financial credit for exported energy as it is charged on theutility bills. The credit received for exported electricity (often referred to net energy generation)will have to offset energy, distribution, peak demand, taxes, and metering charges for electricityand gas use. A cost ZEB provides a relatively even comparison of fuel types used at the site aswell as a surrogate for infrastructure. Therefore, the energy availability specific to the site andthe competing fuel costs would determine the optimal solutions. However, as utility rates canvary widely, a building with consistent energy performance could meet the cost ZEB goal oneyear and not the next. In wide-scale implementation scenarios, this definition may be ineffective because utilityrates will change dramatically. As energy-efficient building technologies and renewable energyinstallations increase, the effects of large numbers of energy-efficient buildings must beconsidered in a given utility’s service area. In addition to purchasing fuel to generate electricity,electric utilities must provide dependable service, maintain capacity to meet potential loads, meetobligations for maintaining and expanding infrastructure, and provide profitability forshareholders. The fixed costs associated with these activities result in rate structures that provideonly limited incentive for consumers to create cost ZEBs. Trends in other utility sectors, such aswater districts, indicate that as buildings become more efficient, and consequently have lowerconsumptive charges, the costs associated with infrastructure are increased. If significantnumbers of buildings achieved a zero energy cost, financial resources would not be available tomaintain the infrastructure, and the utility companies would have to raise the fixed and demandcharges. For commercial buildings, a cost ZEB is typically the hardest to reach, and is verydependent on how a utility credits net electricity generation and the utility rate structure thebuilding uses. One way to reach this goal in a small commercial building might be to use autility rate that minimizes demand charges. For example, at Zion, a 73-kW PV system is neededfor a site and source ZEB at current performance levels (about 65% energy savings without PV). 8
  11. 11. To be a cost ZEB, with the utility providing credit for net electricity generation at avoidedgeneration costs, a 100-kW PV system would be needed. A cost ZEB may be technicallypossible in this case, but the following characteristics would all be required to achieve this ZEBdefinition:• High energy savings (Zion’s measured energy savings approach 65%).• Aggressive demand management to allow PV to help offset demand. Without demand- responsive controls, PV systems cannot be relied upon to reduce peak demand charges (Torcellini et al. 2004). Additionally, the low peak demands enable the building to qualify for the small commercial rate structure.• A favorable utility rate structure weighted toward energy use, not peak demand charges. Standard commercial rate structures often result in electricity charges that are typically split between peak demand and energy charges. The small building commercial rate structure for Zion, which has comparatively low peak demand rates and higher consumption rates, would not apply if the building used more than 35 kW for any 15- minute period over any time of the year. This small commercial rate includes a low demand charge of $6.30/kW for all usage that exceeds 15 kW, and an energy charge of $0.08/kWh for the first 1500 kWh and $0.045/kWh for all additional kilowatt-hours. A time-of-use rate would also be advantageous for a cost ZEB.• A net-metering agreement that credits excess electricity generation at avoided generation costs ($0.027/kWh in this case), without capacity eligibility limits to PV system sizes. Avoided generation costs refer to how the utility credits the customer for net generation and is based on the costs associated with the utility not having to generate this energy. A far more favorable net-metering agreement would credit the net generation at the full retail rate. This is considered “true” net metering, and would be the favored net metering arrangement in a cost ZEB. The net-metering agreement also must allow the excess generation credit to be used for offsetting energy-related and nonenergy charges, such as monthly meter charges, demand charges, and taxes. In the Zion net cost ZEB example, a PV system 30% larger than a site or source ZEB PVsystem would be required to reach the net cost ZEB goal. For utility rates that do not allow thenet generation credit to be applied to nonenergy charges, a net cost ZEB would not be possible,irrespective of the size of the PV system, the energy or demand savings, or how the rates weightenergy and nonenergy charges. If demand charges account for a significant portion of the utility bills, a net cost ZEBbecomes difficult. For example, Oberlin’s rate structure is not weighted toward energy ratescombined with minimal demand savings. A 430-kW PV system would be required for a costZEB at Oberlin at current levels of performance. This is 3.6 times the size of the PV systemOberlin would need to be a site or source ZEB. For this 13,600 ft2 building to be a net cost ZEB,a PV system approaching 40,000 ft2 would be required—much larger than the building footprint. If two-way or net metering is not available, on-site energy storage and advanced demand-responsive controls to manage peak demand charges should be included in the design andoperation of cost ZEBs. It may be more effective to store excess PV energy and use it at a latertime to reduce demand charges rather than export the energy to the grid. 9
  12. 12. Net Zero Energy Emissions Building An emissions-based ZEB produces at least as much emissions-free renewable energy as ituses from emissions-producing energy sources. An on-site emission ZEB offsets its emissionsby using supply-side options 1 and 2 in Table 1. If an all-electric building obtains all itselectricity from an off-site zero emissions source (such as hydro, nuclear, or large scale windfarms), it is already zero emissions and does not have to generate any on-site renewable energyto offset emissions. However, if the same building uses natural gas for heating, then it will needto generate and export enough emissions-free renewable energy to offset the emissions from thenatural gas use. Purchasing emissions offsets from other sources would be considered an off-sitezero emissions building. Success in achieving an emissions ZEB depends on the generation source of theelectricity used. Emissions vary greatly depending on the source of electricity, ranging fromnuclear, coal, hydro, and other utility generation sources. One could argue that any building thatis constructed in an area with a large hydro or nuclear contribution to the regional electricitygeneration mix would have fewer emissions than a similar building in a region with apredominantly coal-fired generation mix. Therefore, an emissions ZEB would need a smallerPV system in areas with a large hydro or nuclear contribution compared to a similar buildingsupplied by a utility with a large coal-fired generation contribution. The net zero emissions ZEB definition has similar calculation difficulties previouslydiscussed with the source ZEB definition. Many of these difficulties are related to theuncertainty in determining the generation source of electricity. Like the source definition, onewould need to understand the utility dispatch strategy and generation source ratio to determineemissions from each of these sources.ConclusionsZEB Definitions Applied to a Sample of Current Generation Low-Energy Buildings Each of these leading-edge case study buildings demonstrates the progress towardachieving ZEB goals in real-world examples. Only the Science House has achieved the site andsource ZEB goal because it is a small building with a relatively large PV system. The other one-story buildings—Zion, BigHorn, and TTF—could achieve ZEB within their roof areas for all thedefinitions except cost ZEB. ZEB is not feasible for the two-story buildings unless their loadsare further reduced. For Oberlin (currently closest to meeting a ZEB goal in a two-storybuilding), the annual PV production is still less than the best-case energy consumption scenario.Oberlin is currently installing another 100-kW PV system in the parking lot (total installed DCcapacity will be 160 kW), which will be tied into the building’s electrical system. We expectthat the building will achieve a site, source, and emissions ZEB, but that a cost ZEB will bedifficult to reach without further demand management controls. To accomplish a ZEB, the PVsystem has been extended past the building footprint. None of our sample commercial buildings could clearly be cost ZEBs with the currentrate structures. Zion could be the closest because of its aggressive demand management,favorable utility rate structure, and efficient use of energy. A cost ZEB is the most difficult ZEBgoal to reach because typical commercial rate structures do not allow for net metering such thatexported electricity can offset all other utility charges. To reach a cost ZEB goal, the credit 10
  13. 13. received for exported electricity would have to offset energy, distribution, peak demand, taxes,and metering charges for both electricity and gas use.The ZEB Definition Selected Can Have an Impact on Future ZEB Designs The zero energy definition affects how buildings are designed to achieve the goal. It canemphasize energy efficiency, supply-side strategies, purchased energy sources, utility ratestructures, or whether fuel-switching and conversion accounting can help meet the goal. Table 3highlights key characteristics of each definition. Table 3. ZEB Definitions SummaryDefini- Pluses Minuses Other Issues tionSite • Easy to implement. • Requires more PV export to offsetZEB • Verifiable through on-site measurements. natural gas. • Conservative approach to achieving ZEB. • Does not consider all utility costs (can • No externalities affect performance, can have a low load factor). track success over time. • Not able to equate fuel types. • Easy for the building community to • Does not account for nonenergy understand and communicate. differences between fuel types (supply • Encourages energy-efficient building availability, pollution). designs.Source • Able to equate energy value of fuel types • Does not account for nonenergy • Need to develop site-ZEB used at the site. differences between fuel types (supply to-source conversion • Better model for impact on national availability, pollution). factors, which energy system. • Source calculations too broad (do not require significant • Easier ZEB to reach. account for regional or daily variations amounts of in electricity generation heat rates). information to • Source energy use accounting and fuel define. switching can have a larger impact than efficiency technologies. • Does not consider all energy costs (can have a low load factor).Cost • Easy to implement and measure. • May not reflect impact to national grid • Offsetting monthlyZEB • Market forces result in a good balance for demand, as extra PV generation can service and between fuel types. be more valuable for reducing demand infrastructure charges • Allows for demand-responsive control. with on-site storage than exporting to require going beyond • Verifiable from utility bills. the grid. ZEB. • Requires net-metering agreements such • Net metering is not that exported electricity can offset well established, energy and nonenergy charges. often with capacity • Highly volatile energy rates make for limits and at buyback difficult tracking over time. rates lower than retail rates.Emis- • Better model for green power. • Need appropriatesions • Accounts for nonenergy differences emission factors.ZEB between fuel types (pollution, greenhouse gases). • Easier ZEB to reach. A source ZEB definition can emphasize gas end uses over the electric counterparts totake advantage of fuel switching and source accounting to reach a source ZEB goal. Conversely,a site ZEB can emphasize electric heat pumps for heating end uses over the gas counterpart. Fora cost ZEB, demand management and on-site energy storage are important design considerations, 11
  14. 14. combined with selecting a favorable utility rate structure with net metering. An emissions ZEBis highly dependent on the utility electric generation source. Off-site ZEBs can be reached justby purchasing off-site renewable energy—no demand or energy savings are needed. ConsistentZEB definitions are needed for those who research, fund, design, and evaluate ZEBs.ReferencesASHRAE. (2001). ANSI/ASHRAE/IESNA Standard 90.1-2001 Energy Standard for Buildings Except Low-Rise Residential. Atlanta, GA: American Society of Heating, Refrigerating and Air-Conditioning Engineers.Barley, C.D.; Deru, M.; Pless, S.; Torcellini, P. (2005). Procedure for Measuring and Reporting Commercial Building Energy Performance. Technical Report NREL/TP-550-38601. Golden, CO: National Renewable Energy Lab www.nrel.gov/docs/fy06osti/38601.pdfCEC. (2005). Time Dependent Valuation (TDV) Economics Methodology. www.energy.ca.gov/title24/ 2005standards/archive/rulemaking/documents/tdv/. Sacramento, CA: California Energy Commission.City of Boulder. (2006). Solar Access Guide, Building Services Center, Boulder, Colorado http://joomla.ci.boulder.co.us/files/PDS/codes/solrshad.pdf, last accessed May 2006.Deru, M. and P. Torcellini. (2004). Improving Sustainability of Buildings through a Performance-Based Design Approach: Preprint. NREL Report No. CP-550-36276. World Renewable Energy Congress VIII, Denver, CO: August 29−September 3, 2004. Golden, CO: National Renewable Energy Laboratory, 8 pp. www.nrel.gov/docs/fy04osti/36276.pdf.Deru, M. and P. Torcellini. (2006). Source Energy and Emission Factors for Energy Use in Buildings. Technical Report NREL/TP-550-38617. Golden, CO: National Renewable Energy Laboratory. www.nrel.gov/docs/fy06osti/38617.pdf.EIA. (2005). Annual Energy Review 2004. www.eia.doe.gov/emeu/aer/contents.html. Wash- ington, DC: U.S. Department of Energy, Energy Information Administration.Mermoud, A. (1996). PVSYST Version 3.3. Users Manual. Geneva, Switzerland: University of Geneva, University Center for the Study of Energy Problems. www.pvsyst.com/. Last accessed September 2005.Torcellini, P., M. Deru, B. Griffith, N. Long, S. Pless, R. Judkoff, and D. Crawley. (2004). Lessons Learned from Field Evaluation of Six High-Performance Buildings. Paper #358, Proceedings (CD-ROM), ACEEE Summer Study on Energy Efficiency in Buildings, August 22-27, 2004, Pacific Grove, CA. Golden, CO: National Renewable Energy Laboratory, 16 pp. www.nrel.gov/docs/fy04osti/36290.pdf. 12
  15. 15. Form Approved REPORT DOCUMENTATION PAGE OMB No. 0704-0188The public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources,gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of thiscollection of information, including suggestions for reducing the burden, to Department of Defense, Executive Services and Communications Directorate (0704-0188). Respondentsshould be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display acurrently valid OMB control number.PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ORGANIZATION.1. REPORT DATE (DD-MM-YYYY) 2. REPORT TYPE 3. DATES COVERED (From - To) June 2006 Conference Paper4. TITLE AND SUBTITLE 5a. CONTRACT NUMBER Zero Energy Buildings: A Critical Look at the Definition; Preprint DE-AC36-99-GO10337 5b. GRANT NUMBER 5c. PROGRAM ELEMENT NUMBER6. AUTHOR(S) 5d. PROJECT NUMBER P. Torcellini, S. Pless, M. Deru, and D. Crawley NREL/CP-550-39833 5e. TASK NUMBER BEC61012 5f. WORK UNIT NUMBER7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 8. PERFORMING ORGANIZATION National Renewable Energy Laboratory REPORT NUMBER 1617 Cole Blvd. NREL/CP-550-39833 Golden, CO 80401-33939. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSOR/MONITORS ACRONYM(S) NREL 11. SPONSORING/MONITORING AGENCY REPORT NUMBER12. DISTRIBUTION AVAILABILITY STATEMENT National Technical Information Service U.S. Department of Commerce 5285 Port Royal Road Springfield, VA 2216113. SUPPLEMENTARY NOTES14. ABSTRACT (Maximum 200 Words) A net zero-energy building (ZEB) is a residential or commercial building with greatly reduced energy needs through efficiency gains such that the balance of energy needs can be supplied with renewable technologies. Despite the excitement over the phrase “zero energy,” we lack a common definition, or even a common understanding, of what it means. In this paper, we use a sample of current generation low-energy buildings to explore the concept of zero energy: what it means, why a clear and measurable definition is needed, and how we have progressed toward the ZEB goal.15. SUBJECT TERMS zero energy building; zeb; commercial building; low-energy building16. SECURITY CLASSIFICATION OF: 17. LIMITATION 18. NUMBER 19a. NAME OF RESPONSIBLE PERSON OF ABSTRACT OF PAGESa. REPORT b. ABSTRACT c. THIS PAGE Unclassified Unclassified Unclassified UL 19b. TELEPHONE NUMBER (Include area code) Standard Form 298 (Rev. 8/98) Prescribed by ANSI Std. Z39.18F1147-E(12/2004)
  16. 16. Getting to Net Zero Journal Article NREL/JA-550-46382Drury Crawley September 2009U.S. Department of EnergyShanti Pless and Paul TorcelliniNational Renewable Energy LaboratoryPublished in ASHRAE JournalSeptember 2009
  17. 17. NOTICEThe submitted manuscript has been offered by an employee of the Alliance for Sustainable Energy, LLC(ASE), a contractor of the US Government under Contract No. DE-AC36-08-GO28308. Accordingly, the USGovernment and ASE retain a nonexclusive royalty-free license to publish or reproduce the published form ofthis contribution, or allow others to do so, for US Government purposes.This report was prepared as an account of work sponsored by an agency of the United States government.Neither the United States government nor any agency thereof, nor any of their employees, makes anywarranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, orusefulness of any information, apparatus, product, or process disclosed, or represents that its use would notinfringe privately owned rights. Reference herein to any specific commercial product, process, or service bytrade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement,recommendation, or favoring by the United States government or any agency thereof. The views andopinions of authors expressed herein do not necessarily state or reflect those of the United Statesgovernment or any agency thereof. Available electronically at http://www.osti.gov/bridge Available for a processing fee to U.S. Department of Energy and its contractors, in paper, from: U.S. Department of Energy Office of Scientific and Technical Information P.O. Box 62 Oak Ridge, TN 37831-0062 phone: 865.576.8401 fax: 865.576.5728 email: mailto:reports@adonis.osti.gov Available for sale to the public, in paper, from: U.S. Department of Commerce National Technical Information Service 5285 Port Royal Road Springfield, VA 22161 phone: 800.553.6847 fax: 703.605.6900 email: orders@ntis.fedworld.gov online ordering: http://www.ntis.gov/ordering.htm Printed on paper containing at least 50% wastepaper, including 20% postconsumer waste
  18. 18. This article wContentsIntroduction .............................................................................................................................. 1What Is a Net-Zero Energy Building? ..................................................................................... 2From “Real Low” to Zero ........................................................................................................ 5Calibrating Expectations .......................................................................................................... 6The Future for Net-Zero Energy Buildings ............................................................................. 7References ................................................................................................................................ 8 iii
  19. 19. IntroductionAs the futurist Stewart Brand observed, “Every building is a forecast. Every forecast is wrong.”Making forecasts progressively less wrong over time—specifically, forecasts about high-performance buildings—is the purpose of the U.S. Department of Energy’s (DOE) Zero EnergyBuildings Database. The intent of this article is to provide an overview of the DOE’s effortstoward realizing cost-effective net-zero energy buildings (NZEBs).Buildings have a significant impact on energy use and the environment. Commercial andresidential buildings use almost 40% of the primary energy and approximately 70% of theelectricity in the United States. The energy used by the building sector continues to increase,primarily because new buildings are constructed faster than old ones are retired. Electricityconsumption in the commercial building sector doubled between 1980 and 2003 and is expectedto increase another 50% by 2025 (EIA 2005). The vision of NZEBs is compelling. These highly energy-efficient buildings will use, over the course of a year, renewable technology to produce as much energy as they consume from the grid. Building owners and tenants stand to realize attractive returns on their NZEB investments while reducing carbon footprints. And, while today’s buildings are our nation’s highest energy-consuming and carbon- emitting sector, with NZEBs, our nation can gain a network of clean domestic energy assets (see Figure 1). Yet, how realistic is this vision? How close do NZEBs come to realizing their design goals? How much does it cost to design and build a net zero energy building? Thanks to data being provided voluntarily by building owners in the Zero EnergyBuildings Database, we now have some early insight into these questions and into the drivers ofnet zero energy performance.Just as important, we now have an influential community of industry leaders who are committedto pushing the boundaries of building performance and sharing the results. As part of the Net-Zero Energy Commercial Building Initiative, authorized by Congress in the EnergyIndependence and Security Act of 2007 (EISA 2007), DOE hosts industry-led CommercialBuilding Energy Alliances focused on specific interests of key sectors. To date, DOE haslaunched three alliances representing substantial proportions of the square footage in theirrespective sectors: the Retailer Energy Alliance (17%), the Commercial Real Estate EnergyAlliance (21%), and the Hospital Energy Alliance (17%) (DOE 2009a, 2009b, 2009c). EISA2007 also authorizes the Net-Zero Energy Commercial Building Initiative to support the goal ofnet zero energy for all new commercial buildings by 2030, and specifies a zero-energy target for50% of U.S. commercial buildings by 2040 and net zero for all U.S. commercial buildings by2050.Some members of the alliances take their commitment to high performance an additional step.With technical support from DOE national laboratories, each of these “National Account”companies pledges to design and construct one new building that is at least 50% more efficient 1
  20. 20. than ASHRAE Standard 90.1-2004 and to renovate a facility to achieve at least 30% savingsover standard practices. With these investments—and the detailed measurement and verificationthat will accompany them—researchers will gain a richer understanding of the best practices andtechnologies for achieving high performance, along with the operational and cost data for a solidbusiness case. By sharing this knowledge base with peers, industry leaders will spur furtherinvestment in high-performance buildings—and, over time, a critical mass of data will enableincreasingly precise modeling of building energy performance and greater certainty in net zeroenergy design.Building design professional societies also have recognized the vision of net zero energybuildings. For example: • ASHRAE Vision 2020 report (ASHRAE 2008) sets out requirements for developing the tools by 2020 to enable commercially viable net zero energy buildings by 2030. ASHRAE’s recent conference on net zero energy buildings featured more than 25 posters (ASHRAE 2009) of NZEBs, some operating close to or at net zero and others in various stages of design or construction. • The AIA 2030 Challenge (AIA 2009) calls for incrementally reducing energy use, starting with a 50% reduction over existing buildings’ energy use and increasing savings up to 2030, when new buildings will be carbon neutral. Architecture firms, large and small, are beginning to make this voluntary commitment to adopt energy-saving targets in building design and implement steps to reach the carbon-neutral goal.Policymakers also are embracing net-zero energy buildings as a key strategy for meeting energyand carbon goals. The California Public Utilities Commission, for example, has an energy actionplan to achieve net zero energy for all new residential construction by 2020 and net zero for allnew commercial construction by 2030. This action plan will provide direction for futuredevelopment of California’s Title 24 building energy codes, as well as incentives for net zerobuildings. NZEB goals also were recently announced by the European Parliament in a March2009 press release (European Parliament 2009). All European Union Member States are toensure that all newly constructed buildings produce as much energy as they consume on-site nolater than December 31, 2018.It is still early in the collection and analysis of energy-performance data. But it is already clearthat high-performance commercial buildings—some NZEBs, some “almost NZEBs”—can beconstructed cost effectively, providing productive environments for occupants, reducingoperating costs, and enhancing the competitiveness of commercial properties. Here is a synopsisof what we’ve learned so far.What Is a Net-Zero Energy Building?Calibrating expectations is a foundation for success in any net-zero energy building (NZEB)project.In the broadest sense, an NZEB is a residential or commercial building with greatly reducedenergy needs. In such a building, efficiency gains enable the balance of energy needs to besupplied with renewable energy technologies.But this broad definition leaves plenty of room for interpretation—and for misunderstandingamong the owners, architects, and other players in an NZEB project. Agreeing to a commondefinition of NZEB boundaries and metrics is essential to developing design goals and strategies. 2
  21. 21. To improve clarity, the National Renewable Energy Laboratory has created two foundationalreferences:Zero Energy Buildings: A Critical Look at the Definition (August 2006) documents four NZEBdefinitions: net zero site energy, net zero source energy, net zero energy costs, and net zeroenergy emissions (Torcellini et al. 2006a).Zero Energy Buildings: A Classification System Based on Renewable Energy Supply Options(July 2009) classifies NZEBs based on the renewable energy sources used. At the top of theclassification system is the NZEB:A—a building that offsets all of its energy use from renewableenergy resources available within the footprint. At the lowest end is the NZEB:D—a buildingthat achieves an NZEB definition through a combination of on-site renewables and off-sitepurchases of renewable energy credits. Table 1 and the sidebar “Net Zero Energy Net-Zero Energy Building Definitions Building Definitions” summarize the four • Net Zero Site Energy: A site NZEB produces NZEB definitions and four energy-use at least as much renewable energy as it uses in classifications. All NZEBs must reduce site a year, when accounted for at the site. energy through energy efficiency and • Net Zero Source Energy: A source NZEB demand-side renewable energy technologies produces (or purchases) at least as much renewable energy as it uses in a year, when such as daylighting, insulation, passive solar accounted for at the source. Source energy heating, high-efficiency HVAC equipment, refers to the primary energy used to extract, natural ventilation, evaporative cooling, process, generate, and deliver the energy to the ground-source heat pumps, and ocean water site. To calculate a building’s total source cooling. In addition, they must use supply- energy, imported and exported energy is multiplied by the appropriate site-to-source side renewable energy according to one of conversion multipliers based on the utility’s the four NZEB classifications. Column 3 of source energy type. the table discusses the ability of each • Net Zero Energy Costs: In a cost NZEB, the classification to meet the site, source, amount of money the utility pays the building emissions, and cost definitions of NZEBs. owner for the renewable energy the building There is no “best” definition or energy-use exports to the grid is at least equal to the amount the owner pays the utility for the energy accounting method; each has merits and services and energy used over the year. drawbacks, and the approach for each project should be selected to align with the owner’s • Net Zero Emissions: A net zero emissions building produces (or purchases) enough goals. But across all NZEB definitions and emissions-free renewable energy to offset classifications, one design rule remains emissions from all energy used in the building constant: tackle demand first, then supply. annually. Carbon, nitrogen oxides, and sulfur oxides are common emissions that ZEBs offset. To calculate a building’s total emissions, imported and exported energy is multiplied by the appropriate emission multipliers based on the utility’s emissions and on-site generation emissions (if there are any). 3
  22. 22. Table 1 Classifying NZEBs by Renewable Energy SupplyClassification ZEB ZEB Supply-Side Options ZEB DefinitionsOn-Site Supply Options YES: Site, Source, Emissions Use renewable energy sources available within the Difficult: Cost building’s footprint and directly connected to the If the source and emissions multipliers for a ZEB:A are building’s electrical or hot/chilled water distribution high during times of utility energy use but low during A system. times the ZEB is exporting to the grid, reaching a source (Examples: Photovoltaic, solar hot water, and wind located on the or emissions ZEB position may be difficult. Qualifying as building.) a cost ZEB may be difficult depending on the net-metering policies in the area. Use renewable energy sources as described in ZEB:A YES: Site, Source, Cost, Emissions and Difficult: Cost Use renewable energy sources available at the building If the source and emissions multipliers for a ZEB:B are B site and directly connected to the building’s electrical or high during times of utility energy use but low during hot/chilled water distribution system. times the ZEB is exporting to the grid, reaching a source or emissions ZEB position may be difficult. Qualifying as (Examples: Photovoltaic, solar hot water, low-impact a cost ZEB may be difficult depending on the net-metering hydroelectric, and wind located on parking lots, adjacent open policies in the area. space, but not physically mounted on the building.)Off-Site Supply Options YES: Site, Use renewable energy sources as described in ZEB:A; Difficult: Source, Cost, Emissions ZEB:B, and ZEB:C A ZEB:C source and emission position may be difficult and if carbon-neutral renewables such as wood chips are used or if ZEB has an unfavorable source and carbon Use renewable energy sources available off site to multipliers. This can occur if a ZEB exports energy generate energy on site and directly connected to the during times that the utility has low source and carbon C building’s electrical or hot/chilled water distribution impacts, but imports energy when the utility has high system. source and carbon impacts. ZEB:C buildings typically (Examples: Biomass, wood pellets, ethanol, or biodiesel that do not reach a cost ZEB position because renewable can be imported from off-site, or collected from waste streams materials are purchased to bring on-site—it would be from on-site processes that can be used on-site to generate very difficult to recoup these expenses by any electricity and heat.) compensation received from the utility for renewable energy generation. Use renewable energy sources as described in ZEB:A, ZEB:B, and ZEB:C and Purchase recently added off-site renewable energy sources, as certified from Green-E (2009) or other equivalent renewable-energy certification programs. YES: Source, Emissions Continue to purchase the generation from this new NO: Site, Cost D resource to maintain ZEB status. ZEB:D buildings may qualify as source and emissions (Examples: Utility-based wind, photovoltaic, emissions if they purchase enough renewable energy and have credits, or other “green” purchasing options. All off-site favorable source and emissions factors. They will not purchases must be certified as recently added renewable qualify as site and cost. energy (Green-E 2009). A building could also negotiate with its power provider to install dedicated wind turbines or PV panels at a site with good solar or wind resources off-site. In this approach, the building might own the hardware and receive credits for the power. The power company or a contractor would maintain the hardware.) 4
  23. 23. NZEB owners and designers should first use all possible cost-effective energy efficiencystrategies and then incorporate renewable energy, weighing the many possible supply optionsavailable and giving preference to sources available within the building footprint. Use of on-siterenewable options over off-site options minimizes the overall environmental impact of an NZEBby reducing energy transportation, transmission, and conversion losses.In addition to efficiency measures, demand-side strategies may include renewable energy sourcesthat cannot be commoditized, exported, or sold, such as passive solar heating, daylighting, solarventilation air preheaters, and domestic solar water heaters. Typical supply-side generationoptions include photovoltaics, solar hot water connected to a district hot water system, wind,hydroelectricity, and biofuels.From “Real Low” to ZeroOur earliest experiences were with high-performance buildings that were not designed for netzero energy. DOE monitored the performance of six of these “real low energy buildings”(Torcellini et al. 2006b).What we found is that all of these buildings delivered significant energy and cost savings underactual operating conditions—but that none of them reached the levels of savings modeled in thedesign. First, design teams were a bit too optimistic about the occupants’ behavior andacceptance of systems. Undoubtedly, some of the shortfall also rested with inherent uncertaintyof designing buildings and forecasting their performance. Energy consumption was higher andphotovoltaic (PV) energy production was lower than simulations predicted. In particular,daylighting contributed less than predicted, which meant more electrical lighting was used(Torcellini et al. 2006).Again, driving down this uncertainty—and accelerating investments in net zero energybuildings—is the motivation behind the Zero Energy Buildings Database (DOE 2009d).The database currently houses data on eight commercial buildings, voluntarily reported byowners and architects. Some data sets are modeled for yet-to-be-constructed facilities; othersinclude both modeled and end-use operating data. Each case study provides details on keyenergy efficiency and renewable energy technologies, a discussion of the NZEB definition(s)used, cost and financing information, and images of the project. In general, buildings withmeasured data in this database are fairly small (less than 14,000 ft2 [1300 m2]) one- or two-floorcommercial buildings. The next generation of net zero energy buildings, as evident in theposters mentioned previously, will include larger commercial buildings such as K–12 schoolsand medium and large office buildings.One example under construction is the new Research Support Facilities (RSF) at DOE’s NationalRenewable Energy Laboratory (NREL) in Golden, Colorado (see Figure 2). This 218,000 ft2(20,253 m2) administration and executive office space and data center will rely solely on renew-able energy available on the roof and in the parking lot (NZEB:B Classification). At the point ofbuilding turnover, the energy goals are whole building energy use intensity of 25 kBtu/ft2·yr (284MJ/m2 · yr), LEED Platinum, and 50% energy savings. By including energy performance in thedesign/build acquisition process, NREL is attempting to ensure the energy models represent whatis actually built. For example, to ensure the as-installed wall insulation represents the R-value inthe energy model, installation preconstruction meetings are held to ensure the insulation isinstalled to minimize thermal breaks. NREL also specified plug loads to be used in the energy 5
  24. 24. models, and encouraged the design team to identify efficiency strategies in the purchasing,control, and operations of the plug and process loads. NREL PIX 16277 Figure 2 Proposed orientation of the NREL Research Support FacilitiesCalibrating ExpectationsIn the quest for ever-greater precision in energy-performance measurement, we’ve uncovered theneed for greater precision in definitions. What do people mean by net zero energy performance?Are they talking the same language and making the same assumptions? Aligning goals andexpectations—as well as measurements—requires specificity.Conceptually, an NZEB is one that has no adverse energy or environmental impact associatedwith its operation. It is a building that is highly energy efficient and that is capable of fulfillingthe balance of its energy needs using renewable energy technologies, producing at least as muchenergy over the course of a year as it draws from utility grids. It is also possible to have a grid-independent NZEB, with back-up energy needs supplied from renewable resources such as woodpellets or biodiesel.To hone this concept into a measurable and verifiable definition, the National Renewable EnergyLaboratory has developed a series of papers drawing on real-world experiences, including datareported through the Zero Energy Buildings Database.To arrive at a shared definition, parties in an NZEB project must evaluate two interrelatedquestions: • How will we account for energy use? Some projects target net zero energy use at the site. Others allow for purchased renewable energy to supplement on-site renewables, with that energy use accounted for at the source. In other cases, energy cost is the predominant factor, with the goal being to offset any purchased energy with revenues 6
  25. 25. realized through the sale of on-site renewables. Still others target net zero emissions of carbon, nitrogen oxides, and sulfur dioxide. • What are our boundaries for choosing among renewable energy options? If a project targets net zero energy use at the site, that necessarily limits the renewable energy choices to sources and technologies available within the building footprint or at the site. Some projects use renewable energy sources from beyond the site (e.g., biomass) to produce power at the site, while others incorporate purchased renewable energy.Clearly, getting to an agreement on energy-use accounting and renewables options is pivotal todetermining NZEB design goals and strategies.The Future for Net-Zero Energy BuildingsOngoing measurement and verification are essential in realizing the full benefits of a net zeroenergy design. DOE recommends that NZEB status be reviewed and tracked each year throughutility bills or submetering.Realistically, a building may be designed to achieve one or more NZEB definitions but may notachieve a net zero energy position in actual operations every year. Any NZEB may fall into thenear-NZEB category in a given year, depending on weather, the condition of the building,operations, and other factors. A well-operating NZEB also may become a near NZEB duringabnormal weather years that have above-average heating and cooling loads, with below-averagesolar and wind resources.An area under study at DOE and the national laboratories is the potential for net zero energy useat a community or campus level. Clearly, buildings do not stand alone, and more data is neededto determine optimal ways to extend the boundaries of net zero energy to most effectively use therenewable energy sources available within a broader footprint.Another area of study, initiated in partnership with utilities, is the longer-term impact of NZEBson energy production. Utilities are concerned about how large-scale application of NZEBs willaffect grid stability. A primary lesson learned from some of the initial NZEBs is that peakdemand issues are even more pronounced than in typical buildings (Torcellini et al. 2006b).While NZEBs have large PV systems that offset significant energy use, their demand for gridpower peaks in the evening, during morning warm-up, or when a cloud blocks the sun. As aresult, NZEBs often do not offset peak demand.To address the potential poor load factors of NZEBs, more research is needed on energy storageand its effect on large-scale NZEB penetration scenarios. With energy-storage integration, flatterload profiles, and better load factors, utilities will be much more agreeable to the concept of largenumbers of buildings with significant on-site generation capacity.Energy consumption in our nation’s commercial building sector will continue to increase untilbuildings can be designed to use energy efficiently and produce enough energy to offset theirgrowing energy demand. DOE has set an aggressive goal to create the technology andknowledge base for cost-effective net zero energy commercial buildings by 2025, and we aregratified to have many industry leaders joining us in this quest.We invite commercial architects, engineers, and owners to use the Zero Energy BuildingsDatabase and to contribute their own data to make it an increasingly useful tool. 7
  26. 26. ReferencesAIA. (2009). AIA 2030 Commitment. www.aia.org/about/initiatives/AIAB079458. Lastaccessed September 2009.ASHRAE (2008). “ASHRAE Vision 2020.” www.ashrae.org/doclib/20080226_ashraevision2020.pdf. Last accessed September 2009.ASHRAE. (2009). Net Zero Conference. Poster Presentations.www.ashrae.org/events/page/2198. Last accessed September 2009.DOE. (2009a). Retailer Energy Alliance. www.eere.energy.gov/buildings/retailer. Lastaccessed September 2009.DOE. (2009b). Commercial Real Estate Energy Alliance. www.eere.energy.gov/buildings/real_estate/. Last accessed September 2009.DOE. (2009c). Hospital Energy Alliance. www.eere.energy.gov/buildings/hospital/. Lastaccessed September 2009.DOE. (2009d). Zero Energy Buildings Database. http://zeb.buildinggreen.com. Last accessedSeptember 2009.EIA. (2005). Annual Energy Review 2004. Washington: U.S.Department of Energy, EnergyInformation Administration. www.eia.doe.gov/emeu/aer/contents.html.European Parliament. (2009). Press Release. www.europarl.europa.eu/sides/getDoc.do?pubRef=-//EP//TEXT+IM-PRESS+20090330IPR52892+0+DOC+XML+V0//EN&language=EN. Last accessed September 2009.Green-E. (2009). www.green-e.org. (last accessed March 2009).Torcellini, P.; Pless, S.; Deru, M.; Crawley, D. (2006a). Zero Energy Buildings: A CriticalLook at the Definition. ACEEE Summer Study on Energy Efficiency in Buildings. Golden, CO:National Renewable Energy Laboratory. www.nrel.gov/docs/fy06osti/39833.pdf. Last accessedSeptember 2009.Torcellini, P.; Pless, S.; Deru, M.; Griffith, B.; Long, N.; Judkoff, R. (2006b). Lessons Learnedfrom Case Studies of Six High-Performance Buildings. NREL Report No. TP-550-37542.Golden, CO: National Renewable Energy Laboratory. www.nrel.gov/docs/fy06osti/ 37542.pdf.Last accessed September 2009.Net Zero Energy Build 8
  27. 27. Form Approved REPORT DOCUMENTATION PAGE OMB No. 0704-0188The public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources,gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of thiscollection of information, including suggestions for reducing the burden, to Department of Defense, Executive Services and Communications Directorate (0704-0188). Respondentsshould be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display acurrently valid OMB control number.PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ORGANIZATION.1. REPORT DATE (DD-MM-YYYY) 2. REPORT TYPE 3. DATES COVERED (From - To) September 2009 Journal Article4. TITLE AND SUBTITLE 5a. CONTRACT NUMBER Getting to Net Zero DE-AC36-08-GO28308 5b. GRANT NUMBER 5c. PROGRAM ELEMENT NUMBER6. AUTHOR(S) 5d. PROJECT NUMBER D. Crawley, S. Pless, and P. Torcellini NREL/JA-550-46382 5e. TASK NUMBER BEC71210 5f. WORK UNIT NUMBER7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 8. PERFORMING ORGANIZATION National Renewable Energy Laboratory REPORT NUMBER 1617 Cole Blvd. NREL/JA-550-46382 Golden, CO 80401-33939. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSOR/MONITORS ACRONYM(S) NREL 11. SPONSORING/MONITORING AGENCY REPORT NUMBER12. DISTRIBUTION AVAILABILITY STATEMENT National Technical Information Service U.S. Department of Commerce 5285 Port Royal Road Springfield, VA 2216113. SUPPLEMENTARY NOTES14. ABSTRACT (Maximum 200 Words) Buildings have a significant impact on energy use and the environment. Commercial and residential buildings use almost 40% of the primary energy and approximately 70% of the electricity in the United States. The energy used by the building sector continues to increase, primarily because new buildings are constructed faster than old ones are retired. Electricity consumption in the commercial building sector doubled between 1980 and 2003 and is expected to increase another 50% by 2025.15. SUBJECT TERMS building; energy; net-zero energy bulding; nzeb16. SECURITY CLASSIFICATION OF: 17. LIMITATION 18. NUMBER 19a. NAME OF RESPONSIBLE PERSON OF ABSTRACT OF PAGESa. REPORT b. ABSTRACT c. THIS PAGE Unclassified Unclassified Unclassified UL 19b. TELEPHONE NUMBER (Include area code) Standard Form 298 (Rev. 8/98) Prescribed by ANSI Std. Z39.18F1147-E(10/2008)

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