Ashrae Solar Technologies

The following article was published in ASHRAE Journal, April 2007. ©Copyright 2007 American Society of Heating, Refrigerating and AirConditioning Engineers, Inc. It is presented for educational purposes only. This article may not be copied and/or distributed electronically or in paper form without permission of ASHRAE.

Solar Technologies & The Building Envelope By Paul A. Torcellini, Ph.D., P.E., Member ASHRAE; Shanti D. Pless, Associate Member ASHRAE; Ron Judkoff, Member ASHRAE; and Drury Crawley, AIA, Member ASHRAE

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dvances in on-site renewable energy technology have brought the concept of zero-energy buildings within reach.1 Many

single-story residential and commercial buildings have enough favorably oriented roof area to make achieving zero energy technically feasible, assuming no major solar obstructions exist and that energy efficiency has been aggressively implemented in the building design. As the number of stories increases, the potential to have a zero-energy building within the building’s footprint decreases. As efficiencies of photovoltaic (PV) cells increase, the potential to have zero-energy buildings increases. Energy efficiency strategies have traditionally been the most cost-effective way to reduce overall building energy use. However, efficiency can reduce building energy consumption only so 14

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far. Because of diminishing returns, at some point the next increment of eff iciency becomes more expensive than an equivalent increment of PV. Aggressively deploying energy efficiency

strategies can achieve 50% to 70% energy savings. To reach a net zero-energy goal, we need to be very efficient and collect and use renewable energy sources. This article focuses on available methods and technologies to integrate on-site solar energy technologies with building energy efficiency. We will discuss daylighting, PV, transpired solar collectors, Trombe—thermal storage—walls (which were named for the French physicist who developed them in the 1960s), and solar hot water systems. For each topic area we include a descripAbout the Authors Paul A. Torcellini, Ph.D., P.E., is the team leader for Commercial Buildings Research at National Renewable Energy Laboratory, Golden, Colo., under the Center for Buildings and Thermal Systems directed by Ron Judkoff. Shanti D. Pless is an engineer on the same team. Drury Crawley, AIA, is a technology development manager in the Office of Building Technology, U.S. Department of Energy, Washington, D.C.

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NREL/PIX04117 Warren Gretz

Daylighting/Clerestories

Photograph 1: This building uses two clerestories to provide 75% of the lighting in the space. Overhangs minimize solar gains in the summer.

tion of the technology, some examples of the integration into the building envelope, and some cautions in their use.

devices (TDDs) (Photograph 2). Well-designed daylighting systems offer dramatic electrical energy savings and cooling load reductions. However, the design of good Daylighting daylighting systems is non-trivial, and we Lighting is the largest single end use in have observed many daylighting systems commercial buildings, at 24% of the total in the field that do not achieve their design primary energy used.2 From a national intent. In previous work,3 we defined six perspective, the potential for daylighting elements of good savings is signifidaylighting design cant. Based on the to achieve energy Tubular Daylighting 1999 Commercial savings. If even one Buildings Energy of these elements is Consumption Surneglected, savings vey, nearly 80% of from daylighting the total floor area in are seriously comcommercial buildpromised. A good ings has an exterior daylighting design ceiling or is within benefits from com15 ft (4.6 m) of an puter simulation, so exterior wall and that the trade-offs therefore has good between lighting, potential to be at heating, cooling, least partially dayand ventilation can lit.3 To reap the benbe understood. Too efits of daylighting, much glazing, and the electric lighting lighting electrical Photograph 2: A large flat roof is ideal must be controlled savings can be nefor tubular daylighting, as shown in in response to the gated by increased this retail building. available daylightHVAC loads. Too ing. This strategy is appropriate in all little, and electricity loads will be greater climates and almost any building type. than necessary. Also, the daylighting design Typical daylighting components include must be well integrated with the external windows, clerestories (Photograph 1), roof and internal geometry of the building. The monitors, skylights, and tubular daylighting optical properties of the glazing, placement, April 2007

The Elements of Good Daylighting The daylighting system has to be integrated with the envelope and trade-offs with heating and cooling understood to maximize wholebuilding energy savings. Even with good integrated design, the following six elements of a daylighting system are needed to maximize daylighting savings. • Design buildings to provide daylighting to all possible zones. • Provide for glare mitigation techniques in the design. • Provide automatic dimming daylighting controls for all daylit zones. • Interior designs should complement the daylighting. • Integrate the electrical lights with the daylighting system. • Commission and verify postoccupancy energy savings. presence or absence of overhangs and light shelves, and many other factors can all have a profound impact on energy savings. Daylighting must be carefully detailed to ensure its success. Glare, high contrast ratios, or computer screen reflections in the field of view can impair occupant comfort and productivity, or even lead to disabling of the system by the occupants. Lights must automatically dim or turn off in response to natural lighting, in a way that is not disturbing to occupants. Interior designs and electric light designs need to complement the daylighting system. Postoccupancy commissioning and calibration of the entire lighting system (daylighting and electric lighting) are usually necessary for the system to operate to design expectations (For more information, see http://windows.lbl. gov/pub/designguide/browse.htm). Photovoltaic Systems

A range of PV products are available to integrate with the building envelope. PV panels are available in a variety of configurations, including roofing products, glazing products, overhangs, and stand-alone products. According to a ASHRAE Journal

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Photovoltaic Shingles NREL/PIX13572 United Solar Ovonic

Roof-Mounted Photovoltaic System

NREL/PIX13789 Spire Solar Chicago

Photograph 4: PV shingles integrated in a traditional tri-cut asphalt roofing system.

Photograph 3: Flat roof-mounted system at the Art Institute of Chicago, 130.6-kW PV system.

recent analysis of the PV market on residential and commercial buildings, the building integrated and roof-top mounted PV market segment has grown at approximately 30% per year, and now accounts for roughly 60% of all new PV installations globally.4 PV systems are durable, have a long life, and show little degradation over time. All these systems convert solar energy into electricity. (For more information on the operation of these systems, see www.eere.energy.gov/solar/pv_basics. html.) The current installed cost of PV in buildings with no credits or incentives is about $7.50 to $10/W. This works out to a cost of about $0.25 to more than $0.40/ kWh. U.S. federal tax cred-

its are available to help offset the cost of PV systems (www. energy.gov/taxbreaks.htm) as well as energy efficiency strategies. The commercial building deduction, which was enacted in the Energy Policy Act of 2005, allows taxpayers to deduct the cost of energy-efficient equipment and strategies installed in commercial buildings. The amount deductible may be as much as $1.80/ft2 ($19.40 m2) of building floor area for buildings that achieve a 50% energy savings target. In addition, homeowners and businesses will receive a credit of up to 30% of the cost of installing a solar PV system or a solar hot water (not for pools or hot tubs) system. The solar energy tax credit is capped at $2,000 for residential systems. Commercial tax credits for PV and solar hot water systems have no cap. In some states, rebate programs further offset the costs (www.dsireusa.org). The maximum benefit occurs when solar systems are integrated with other building components. Examples include roofing material that incorporates PV systems (Photographs 3 and 4), overhangs that are designed to shade the building from the high summer sun, and shade structures over walkways or PV panels integrated into the curtain wall of the building (Photograph 5).5 Certainly PV systems are premium items and often difficult to cost justify, even with rebates and tax incentives, but integrating these into the architecture helps to minimize the cost implications. PV panels can be used as overhangs, be used to make a roofing system more durable, or be used as part of an

Solar Technologies: Supply- or Demand-Side Traditionally, there has been a distinction between supplyand demand-side technologies. Supply-side technologies, which are often called energy producers, collect energy and transform it to a usable commodity. The most common examples in the context of this article are technologies that gather solar energy and make electricity. Electricity can flow back and forth between the grid and the building, be easily measured, and be bought or sold. In a gray area are solar hot water systems. This technology gathers solar energy and puts the energy into hot water. It could be bought and sold on a community or district hot 16

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water loop, but typically it is used only to reduce the energy loads of the building. Sometimes it is thought of as supply side and sometimes as demand side. Other solar technologies, such as passive solar heating and daylighting, are often thought of as demand-side technologies—their primary purpose is to reduce the energy loads of the building. But passive solar heating, unlike efficiency measures that address the quality of a building enclosure or the effectiveness of mechanical/electrical/plumbing systems, does add newly gathered energy into the building’s conditioned spaces.

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Transpired Solar Collector

NREL/PIX13783 Spire Solar Chicago

NREL/PIX05802 Warren Gretz

Curtain Wall with Photovoltaic Glazing

Photograph 6: A transpired collector on a warehouse.

Transpired Solar Collectors

Photograph 5: Curtain wall with integrated PV glazing.

uninterruptible power supply. Many buildings already have UPS systems or backup generation systems; in some cases the PV system can meet this role without having generators. Some cautions to consider: When sizing PV systems, do not overestimate their energy production. A good starting rule is a system can produce 1 kWh of energy annually for every 1 W of dc installed capacity. Also, when looking at buildings that are charged a maximum demand, do not assume that the PV system will reduce a building’s peak demand. Often, the building peak demand does not coincide with the PV system peak output. To fully realize a demand saving, the demand of the entire facility must be managed. Using an average, or “virtual” rate for energy may overpredict the cost savings. Design grid-connected PV systems to avoid parasitic standby loads. Previous work has shown that, during nighttime hours when some PV systems are in standby mode, the inverters and transformers consume electricity. Nighttime parasitic PV system losses accounted for 7% to 37% of the total PV output depending on how often the system was in standby mode.3 An automatic circuit or inverter control that disconnects the PV system from the grid at night should be implemented when isolation transformers are used. Finally, carefully analyze PV systems that are in glazing systems to make sure that they do not create a cooling load on the building. The choice of glazing with the PV system may cause a cooling load that is more than the power output of the PV system. A PV system that adds energy costs is a sad story! 18

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Transpired solar collectors (Photograph 6) are ideal for buildings that require preheat for ventilation air. In these systems, air is pulled from the outside through small holes in the skin of the building. Heat is transferred as the air moves through the holes. The building’s intake air fan moves this energy to the building with the fresh air. More than 60% of the incident radiation hitting the surface is transferred to the building as heat. The system is simple and can be incorporated into a traditional outside air HVAC system. For a schematic, see www.eere.energy.gov/buildings/info/documents/pdfs/28545.pdf. Life-cycle analysis of this system often costs less than $0.01/kWh for heat added to the building. In new construction, installation costs are typically in the range of $6 to $7/ft2 ($65 to $75/m2), though if the sheet metal facade replaces a more expensive facing, such as brick, there may actually be a net reduction in cost. With retrofit applications, costs are usually higher than with new construction. Color does have a small effect on the absorptance and emittance of the collector—black is best—but many architecturally pleasing colors such as darker blues, browns, and grays have been used with only a small penalty in efficiency. A caution to consider—these systems are designed to preheat ventilation air, not to provide all the heating for a space. Trombe Walls A Trombe wall is a high-mass wall, typically made of solid concrete or grout-filled concrete masonry unit, with a glass cover. Solar energy absorbs into the wall and the heat moves through it to the interior of the space. The glazing minimizes the radiant heat transfer back to the environment. Properly integrated, a Trombe wall can enable a building envelope to go from a net-loss feature to a net-gain feature. The Trombe wall also has the advantage that the energy is delayed and radiated into the space at a later time—when the building needs more heat. Although many factors

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Trombe Wall

NREL/PIX01694 Warren Gretz

NREL/PIX10022 Thomas Wood

Trombe Wall

Photograph 7: Trombe wall at the visitor center at the National Renewable Energy Laboratory in Golden, Colo. This Trombe wall has alternating daylighting and direct gain panels intermixed with Trombe wall panels. The clear panels are oriented to the southeast for quick morning warm-up. The wall is shaded in the summer by the horizontal beams and allows direct gain in the winter.

influence the performance of the wall including the solar gain as a function of time, the emittance of the interior surface, the thermal mass wall material, and the glazing system, the delay is typically about one hour per inch (25 mm) of wall thickness. Therefore, an 8 in. (203 mm) Trombe wall will radiate to the space about eight hours after the sun hits the exterior surface. Also, the warm surface on the interior of the Trombe wall makes a good radiant surface that improves thermal comfort for the occupants. A Trombe wall is a passive solar technique that does not bring direct beam radiation into the building. This is a plus because it can help avoid direct sunlight that often causes glare problems for occupants. Some designers integrate windows into the thermal storage walls to permit some daylighting to enter the rooms located behind these walls, as shown in Photograph 7.

Photograph 8: South elevation of the Zion National Park Visitor Center in Springdale, Utah. Note the daylighting glass above the Trombe wall. Both elements are part of this curtain wall system. The PV system is tied to the building’s uninterruptible power supply to maintain operations when the utility grid is not available.

As a caution, the Trombe wall will add some heat in the summer due to diffuse solar radiation, even with the overhangs. Additionally, the insulation values of these walls are low. To optimize the performance, the annual net effect has to be considered when designing a Trombe wall, including any additional cooling loads. The performance of Trombe walls can also be diminished if the wall interior is not open to the interior zones. Based on previous experiences with Trombe walls,6 the heat delivered by a Trombe wall in a residence was reduced by more than 40% because kitchen cabinets were placed on the interior of the wall. The wall design at the Visitor Center in Zion National Park includes cast-in-place concrete projections attached to the interior of the wall. These projections were included to ensure bookshelves were not placed against the Trombe wall.

Use the Envelope to Create Low-Energy Buildings Use the envelope as the first method of creating lowenergy buildings. Using whole-building simulations, engineer the envelope to reduce purchased energy loads with features such as increased insulation, window tuning, and selfshading, and, at the same time, integrate architecturally innovative technologies such as passive solar heating, Trombe walls, and daylighting. The best way to reduce the cost of low-energy features and ensure successful integration is to incorporate them into the architecture early. If solar features such as daylighting and Trombe walls can be integrated into the 20

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architectural statement, they can be included as part of the architecture costs. Then consider integrating active solar systems, such as PV, solar hot water, and transpired solar collectors, into the building envelope. Once the building envelope has been designed to reduce loads and generate on-site thermal and electrical energy, size the HVAC and lighting equipment to meet the remaining loads. Use the mechanical and electrical lighting systems to compensate for what cannot be accomplished by architectural form and envelope, not to correct for an architectural design that is climatically ill conceived.

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As a final Trombe wall caution, placement of the footing insulation is critical, as Trombe wall performance can be diminished by three-dimensional heat transfer to the ground. By thermally decoupling the Trombe wall footings from the ground with insulation, unnecessary heat loss is avoided and more heat from the Trombe wall is supplied to the building. The Trombe walls shown in Photographs 7 and 8 all use overhangs to minimize the heat entering through the wall in the summer. In many cases, distinguishing between the Trombe wall and the glazing is difficult. The heat transfer for the Trombe wall in Photograph 8 has been measured and provides 30% to 50% of the total heating load for this building7 with little or no incremental cost. Solar Hot Water Systems

Solar hot water systems collect solar energy and put that energy into a heat-transfer fluid, usually water or a water/glycol mixture. The hot water can be used for domestic hot water, space heating, or cooling through absorption or desiccant systems. Traditionally, these systems have been roof- or ground-mounted with minimal integration, although sometimes a solar hot water system is the roofing product. More common is using solar hot water systems for heating pool water. Typically, water drains out of the panel when heat is not available, minimizing the chance of freezing the panels. Where freezing is a problem, an active protection system or climate-appropriate heat transfer fluid must be used to prevent

freezing liquid in the panels and lines, which may complicate the system. Another application is using a rooftop solar hot water system that provides heat for restrooms and washing facilities. This system circulates glycol to the panels. A heat exchanger and a tank are used to store the heat for later use. Solar hot water systems will work best when there is a consistent annual demand for hot water, and when the hot water draws tend to occur in the afternoon and early evening. Using these systems for spacing heating usually means the systems will be idle for a substantial fraction of the year, resulting in partload operation. Typically, these systems are designed only for domestic hot water. If a solar hot water system is to be used for space heating, consider thermally driven cooling technologies such as desiccant dehumidification or absorption chillers for space cooling during the summer months. This will maximize the usefulness of solar hot water systems. What to Look for in the Future

During the past few years, there has been a proliferation of new solar products that better integrate with building systems. This article highlighted a few of these. In the future look for many more products. Also, look for integration of multiple solar technologies, such as PV integrated with solar hot water systems and PV integrated with daylighting systems. These synergies can reduce net investment costs that result from multiple uses and increase the overall capture of solar energy for use in the building. What This Means for ASHRAE Members

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The advances in solar technologies can be used along with aggressive efficiency strategies to vastly reduce the energy needs of buildings. Lower lighting and cooling loads from properly sized daylighting systems can enable down-sized cooling systems. Trombe walls and transpired collectors reduce heating loads. Solar water systems reduce hot water loads. Inclusion of these technologies in buildings requires engineering expertise. ASHRAE members are ideally suited to support design teams with optimized integrated solutions that provide comfort while substantially reducing the energy footprint of buildings. This role extends far beyond that of selection and design of mechanical equipment sufficient to make the building work. References 1. Torcellini, P. and D. Crawley. 2006. “Understanding zero-energy buildings.” ASHRAE Journal 48(9):62−69. 2. U.S. Department of Energy. 2006. “2006 Buildings Energy Data Book.” http://buildingsdatabook.eren.doe.gov. 3. Torcellini, P., et al. 2006. Lessons Learned from Case Studies of Six High-Performance Buildings, National Renewable Energy Laboratory Report No. TP-550-37542. www.nrel.gov/docs/fy06osti/37542.pdf. 4. Incorporating PV in Buildings: A Gathering of Eagles, Sustainable Buildings Industry Council. www.sbicouncil.org/PDFs/Eagles0105.pdf. 5. Jalayerian, M. and S. Eich. 2006. “Blending architecture and renewable energy.” ASHRAE Journal 48(9):70−76. 6. Balcomb, J.D., G. Barker, C.E. Hancock. 1998. “An Exemplary Building Case Study of the Grand Canyon South Rim Residence.” NREL/TP-55024767, Golden, Colo.: National Renewable Energy Laboratory. 7. Torcellini, P. and S. Pless. 2004. “Trombe Walls in Low-Energy Buildings: Practical Experiences.” World Renewable Energy Congress VIII and Expo.

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