Solar Energy Systems for the Million Solar Roofs Initiative

The goal of the Million Solar Roofs Initiative, announced by President Clinton in June of 1997, is to install 3,000 megawatts of solar energy systems on one million United States buildings by 2010. This initiative is intended to increase the demand for, and to lower the cost of solar photovoltaic systems, solar water heating systems and solar space heating systems, located on or near residential, commercial or industrial buildings. Doing so will help slow greenhouse gas emissions, expand available energy supply options, create high technology jobs, and improve US competitiveness in the solar energy arena. This paper describes the solar energy systems available to meet the goal of the Initiative.

Solar Photovoltaic Systems

Building-scale solar photovoltaic systems produce electric power for on-site use and for sale back to the electric grid. These systems may be connected to the utility grid , or stand-alone. Grid connection, where feasible, provides service during periods of insufficient sunlight without storage batteries or an auxiliary generator. In some areas, surplus electricity can be sold back to the utility. The photovoltaic array can be either attached to the building or incorporated into its structure. Rack-mounted systems use arrays of standard photovoltaic modules affixed to roofs or separate supporting structures. Building-integrated systems use building components such as shingles or tiles that incorporate photovoltaic surfaces.

Rack-mounted Photovoltaic Systems

A grid-connected rack-mounted photovoltaic system includes a photovoltaic array, an inverter, and protective and interconnection devices. Direct current power produced by the photovoltaic array is converted to alternating current at household voltage and frequency by the inverter. Surplus power feeds back to the grid.

The photovoltaic array may be installed on fixed racks or tracking mounts. Tracking mounts increase energy production by continuously orienting the array to the sun. However, the benefit of tracking is less significant in northerly latitudes and cloudy climates. Also, tracking systems are more costly, may need additional maintenance and are often more obtrusive than fixed mounts.

Residential photovoltaic installations generally range from 1 to 5 kilowatts in capacity . The physical size of the array depends upon the photovoltaic technology. A 3-kilowatt system using single-crystal cells would occupy about 300 square feet. A system of the same capacity using thin-film (amorphous silicon) photovoltaic materials would require about 600 square feet.

The Sacramento Municipal Utility District (SMUD) staff reports that fixed-mount photovoltaic systems installed under the SMUD PV Pioneer project operate at an annual capacity factor of about 20 percent . Based on limited solar radiation data, it appears that similar systems located in southeastern Oregon or southern Idaho (the best solar areas of the Northwest) might also operate at a capacity factor of about 20 percent. A 3-kilowatt system, for example, located in these areas would produce about 5,250 kilowatt-hours annually . Equivalently-sized systems located in southwestern and northeastern Oregon, central Idaho and western Montana would produce would operate at roughly a 16 percent capacity factor. Systems located in coastal areas of the Northwest would be less productive. Local variation can be expected.

Though currently expensive, photovoltaic systems are rapidly declining in price. In 1997, SMUD signed a five-year contract with Energy Photovoltaics of Princeton, NJ, to supply rooftop systems from 1998 through 2002. The guaranteed price for installed systems is $5,000 per kilowatt in 1998, and declines to $3,000 per kilowatt by 2002. These prices are for volume orders; individual installations are more expensive.

At 1998 SMUD prices, a 3-kilowatt system would cost about $15,000. Assuming a 25-year system life, a lifetime maintenance cost of $750 and 6.5 percent mortgage financing, the levelized cost of electricity from this system operating at a 20 percent capacity factor would be about 18 cents per kilowatt-hour. At a 16 percent capacity factor, the levelized cost of electricity would be about 23 cents per kilowatt-hour. These costs are exclusive of possible tax credits or other incentives.

The potential for continued cost reduction is excellent. If 2002 SMUD contract prices are achieved, the cost of rooftop photovoltaic systems will have declined at a real (exclusive of inflation) rate of 11 percent per year over a period of ten years. The levelized cost of power from a system installed in 2002 in southern Oregon should be about 16 cents per kilowatt-hour. Continued improvement of photovoltaic material efficiencies and production methods will support continuation of this trend. Increased production volume and system operating experience will also help resolve system integration and inverter reliability problems experienced with some earlier systems.

Building-integrated Photovoltaic Systems

Building-integrated photovoltaic systems use photovoltaic surfaces that are integrated with standard roofing, glazing and cladding products. The net cost of the photovoltaic system is reduced because photovoltaic components are substituted for the standard building materials. Other factors that are expected to reduce the cost of building-integrated photovoltaic systems include mass production of standard components, elimination of support structures, simplified engineering, reduced installation labor, and use of tradesmen and contractors normally present on site. Furthermore, building-integrated photovoltaic systems are less obtrusive than rack-mounted systems.

The basic elements of a building-integrated photovoltaic system are similar to those of a rack-mounted system. The physical size of the array is also about the same as a rack-mounted system using similar photovoltaic materials. The portion of the roof or fa?de not occupied by the photovoltaic product is finished with a compatible non-photovoltaic material.

The building-integrated photovoltaic product market is in its infancy. Manufacturers are few, and product lines are limited. Current products include shingles, tiles and standing-seam roof panels. Custom-fabricated fa?de panels, and translucent skylight, sunshade, shelter and marquee panels, are also produced. Examples of commercial products include:

  • United Solar's PV shingle mimics the appearance and function of standard three-tab composition roofing. The product is a 12- by 86-inch flexible strip, with 5- to 6-inch exposure when installed. The exposed portion is faced with thin-film photovoltaic material. The capacity of each unit is 17 watts. One distributor characterizes this as a 25-year roofing product -- equivalent to a good-quality composition shingle.
  • The Atlantis Energy "SunSlate" consists of a photovoltaic module bonded to a standard fiber-cement roofing tile. Each tile is approximately 28 inches high by 16 inches wide with 11-inch exposure when installed. Crystalline or thin-film photovoltaic materials are used, depending upon technical and appearance requirements. The peak output of a SunSlate tile using a crystalline module is 11 to 15 watts. A tile using a thin-film module will produce 4 to 9 watts, peak. The product is designed for a 40-year life and is warranted for ten years.
  • United Solar's "Architectural Standing Seam Panel" provides the appearance and function of standing-seam metal roofing. This product installs on a standard roof deck. Thin-film photovoltaic material is laminated to a metal base panel. Panels are available in standard sizes and are rated at 5 watts per square foot. The United Solar "Structural Standing Seam Panel" is a similar product, but self-supporting.

The performance of building-integrated photovoltaic systems on tilted south-facing surfaces is comparable to that of a rack-mounted system. Other orientations reduce system productivity.

Reported turnkey costs for grid-connected systems range from $5,000 to $14,000 per kilowatt. However, substantial cost reduction is expected over the next several years. Atlantis Energy, for example, will be supplying SunSlate systems to SMUD at installed prices beginning at $5,060 per kilowatt in 1998 and declining to $3,180 per kilowatt by 2002.

The total cost of a 3-kilowatt system at 1998 SMUD prices would be about $15,000. The system would replace roughly $1,500 of comparable-quality roofing, for a net system cost of $13,500. With a 25-year system life, a lifetime maintenance cost of $750 and 6.5 percent mortgage financing, the levelized cost of electricity from a system operating at a 20 percent capacity factor would be about 17 cents per kilowatt-hour. If the system operated at a capacity factor of 16 percent the cost of power would be about 21 cents per kilowatt-hour. This cost is exclusive of possible tax credits or other incentives.

Volume production of building-integrated photovoltaic components, combined with anticipated reductions in the cost of photovoltaic materials may lead to more rapid cost reduction than expected for rooftop systems. If the 2002 price provisions of the recent SMUD contract are achieved, the net energy cost of a system operating at a 16 percent capacity factor, for example, should decline to about 14 cents per kilowatt-hour.

Although building-integrated products are best suited for new construction, it is feasible to retrofit buildings having suitable roof structure, area and orientation. Because they are new products and assembled in the field, some reliability problems might be expected with building-integrated photovoltaic arrays. It is reported that some installations have experienced loss of photovoltaic cell efficiency because of overheating of the substrate.

Solar Water Heating

Solar water heating systems use solar energy to heat water for residential, commercial or industrial applications. In the Northwest, these systems normally offset about half the electrical or gas energy normally used to supply hot water. Solar energy can also be used to heat swimming pools. Because of the simpler design of pool systems and the seasonal coincidence of solar radiation and pool use, solar pool heating systems are typically more cost-effective than other solar water heating applications.

Solar Water Heating Systems

Solar water heating systems have been marketed for many years. Many different designs are commercially available. The Oregon Office of Energy, for example, currently has performance values for 77 OG-300 certified and 12 generic system configurations and sizes for the state alternative energy tax credit program. Active systems use a pump to circulate heat-transfer fluids whereas passive systems rely on natural circulation. Open-loop systems circulate potable water directly through the solar collector, whereas closed-loop systems use an intermediate heat-transfer loop between the collectors and a heat exchanger. All systems incorporate freeze protection features when used in climates subject to freezing temperatures.

Available designs include:

  • Active closed-loop antifreeze systems: These comprise about 40 percent of the 600 to 800 solar hot water heating systems installed each year in Oregon, according to the Oregon Office of Energy. A typical system consists of two collectors of 32 square feet each, a heat-transfer loop with pump and expansion tank, a heat exchanger, a potable water storage tank, controls, and an auxiliary (booster) water heater. An antifreeze solution, usually a mix of water and propylene glycol, is used as the heat-transfer fluid. When the controller detects sufficient temperature differential between the collector and the storage tank, the heat-transfer fluid is pumped through the collectors. Here it is heated by solar radiation. The hot fluid releases heat to the potable water storage tank via the heat exchanger. The auxiliary heater (generally a standard hot water tank) raises the temperature of the potable water to the desired temperature when solar radiation is inadequate. Active antifreeze systems have proven to be very reliable, but require somewhat greater maintenance than other types of systems. Because the antifreeze mixture can acidify at temperatures sometimes reached when the system is inoperative on very hot days, the pH of the fluid must be tested every four years. The fluid is replaced if it does not meet specifications.
  • Drain-back systems: Eugene Water and Electric Board (EWEB) staff reports that drain-back systems are the most common and reliable configuration installed under the EWEB solar water heating program. A typical drain-back system includes two collectors, a closed heat-transfer loop with pump and drain-back reservoir, a heat exchanger, a potable water storage tank, controls and an auxiliary water heater. Water, treated with corrosion inhibitor, is used as the heat-transfer fluid. When the controller detects sufficient temperature differential between the collector and the storage tank, the heat-transfer fluid is pumped through the collectors. Here it is heated by solar radiation. The hot fluid releases heat to the potable water storage tank via the heat exchanger. When air temperatures approach freezing, the pump shuts down. This drains the collectors and heat-transfer piping to the reservoir, which is located in a heated space.
  • Drain-down systems: A drain-down system is an active open-loop system. A typical drain-down system consists of solar collectors, a storage tank and a circulating water loop with a pump and an isolation/drain-down valve. Controls and auxiliary heater are also provided. Potable water is circulated by pump directly through the solar collectors when the controller detects sufficient temperature differential between the collector and the storage tank. When the pump is idle, an automatic drain-down valve isolates and drains the collector and loop to a sump. This feature provides freeze protection. Drain-down systems provide the advantages of fewer components and somewhat greater heat transfer efficiency than other designs. Earlier drain-down systems experienced drain-down valve reliability problems.
  • Integrated collector/storage systems: A passive design, these systems employ solar collectors having integrated heat exchangers and storage tanks. Cold potable water is supplied to a heavily insulated heat exchanger/storage tank, located at the upper end of the collector. An integrated, closed heat-transfer loop containing a freeze-resistant fluid transfers heat from the collector surface to the heat exchanger. This fluid circulates by thermosyphon action. Heated potable water is withdrawn from the storage tank as needed. Auxiliary heating is supplied by a booster heater located in the storage tank. Because of the relatively small size of the storage tank, an auxiliary hot water heater is often installed downstream. Freeze resistance is achieved by use of antifreeze heat-transfer fluid, the heated volume of the storage tank, heavy storage tank insulation, the integral booster heating unit and insulation of the potable water piping leading to and from the collectors. Compared to active systems, passive systems offer reduced electric energy consumption, greater reliability and reduced maintenance costs.

The Oregon Office of Energy (OOE) estimates that solar water heating systems located in southern or eastern Oregon will displace 3,000 kilowatt-hours annually if offsetting electric hot water heating. Similar performance can be expected of systems located in southern Idaho or western Montana. According to OOE, systems located in the Willamette Valley will displace about 2,500 kilowatt-hours annually. This value is probably representative of the coastal areas of the Northwest. Local variation can be expected. These performance estimates are based on the typical hot water load of a three-person household. Larger loads will generally result in greater offset, whereas smaller loads will reduce offsets.

The turnkey cost of solar water heating systems installed under the EWEB program ranges from $2,300 to $4,000, averaging $3,100. Costs have been stable in real terms for several years. EWEB staff estimates that the 20-year lifetime maintenance costs of the currently installed mix of systems range from $300 to $600.

A system located in southern Oregon, for example, serving a three-person household can be expected to displace electricity at a cost of about 8 cents per kilowatt-hour, exclusive of possible tax credits or other incentives. This assumes a 20-year system life, a lifetime maintenance cost of $450 and 6.5 percent (nominal) mortgage financing. In the cloudier coastal areas, a solar hot water heating would displace electricity at a cost of about 10 cents per kilowatt-hour.

Significant declines in the cost of solar water heating systems are not expected. Rather, evolutionary improvements in materials and equipment design should gradually improve system efficiency and reliability, and reduce maintenance costs.

Solar Pool Heating Systems

Solar pool heating systems are among the most cost-effective applications of solar energy. Pool heating systems are less complex and costly than solar potable hot water heating systems. Normally the full output of the system can be used. Applications may be cost-effective even with currently low energy prices.

Systems servicing pools used year-around employ glazed collectors, a closed heat-transfer loop filled with an antifreeze liquid and a heat exchanger to transfer the heat to the pool water. The heat exchanger is typically located in the pool filter system. Glazed collectors provide efficient cold-season collection of solar energy. The intermediate heat-transfer loop provides freeze protection and isolates collector materials from pool chemicals.

Solar pool heaters intended only for warm weather use employ unglazed collectors. These are fabricated of rubber or plastic materials resistant to ultraviolet radiation and pool chemicals. Pool water is circulated through the collector by the filter pump. These systems are drained during cold weather.

Older data (1989) collected for the Oregon alternative energy tax credit program indicates that solar pool heating systems installed under that program offset an annual average of 9,545 kWh of energy. A useful life of 15 to 20 years can be expected of seasonal systems using unglazed non-metallic collectors. Systems using glazed collectors and antifreeze heat-transfer systems may operate for 20 years, or more.

The installation costs of solar pool heating systems vary widely, depending upon the type, size and quality of system. Systems installed under the Oregon tax credit program cost an average of $2,100 as of 1989. This would translate to about $2,700 in 1998 dollars. The Texas Energy Conservation Office currently reports costs ranging from $2,000 to $5,000. These costs are reasonably consistent with the older Oregon data if it is assumed that most Oregon systems were of the lower-cost seasonal type. However, the Florida Energy Extension Service in 1992 reported solar pool heating system costs ranging from $3,700 to $5,000 (1998 dollars).

The 1989 Oregon cost and performance data, adjusted to 1998 dollars, yield a levelized cost of 2.6 cents per kilowatt-hour displaced. This assumes a 15-year life and a lifetime maintenance cost of $200.

No major technological breakthroughs in solar pool heating systems are anticipated. Continuing improvement in materials should lead to longer-lived and more reliable systems. Solar Space Heating

Many approaches to using solar energy for space heating were developed following the energy crises of the 1970s. Passive and active systems are the two principal approaches.

Passive systems employ direct collection and storage of heat from solar radiation. Strategically oriented glazing admits solar radiation to the building, where it warms concrete, water, or other materials having high thermal mass. These components are often integrated with floors, walls or other elements of the building structure. Heat is distributed to the building by direct radiation and natural circulation. The cost of passive systems can be low, and some simple passive measures are cost-effective even at current low energy prices. However, it is often difficult to retrofit passive systems to existing structures.

Active systems employ solar collectors, mounted on the roof, wall or ground. Air or fluid, circulated by fans or pumps, is used to transfer heat from the collectors to a thermal storage device, and from the storage device to the building spaces.

Active liquid space heating systems use many of the same components as solar water heating systems. A typical system consists of collectors, a closed primary heat-transfer loop, a heat exchanger, a storage tank, system controls and an auxiliary heat source. The primary heat-transfer fluid may be water or an antifreeze mixture. A pump circulates this fluid through the collectors where it is warmed by solar radiation. The heat is given up to a secondary working fluid contained in the storage tank. The secondary fluid is circulated from the storage tank through baseboard, radiator or radiant heating tubes when space heat is needed. Some systems use fan-coil heat exchangers in a forced-air system to deliver heat to building spaces. Liquid solar space heating systems usually provide water heating to improve the loading of the system. Liquid systems can be easier to retrofit to existing structures than passive systems or active air systems.

A typical active air system consists of collectors using air as the working fluid, ducts, fans, a rock-filled thermal storage bin, controls and an auxiliary heating device. A fan circulates air through the collectors, where it is warmed by solar radiation. The warm air can be circulated directly to building spaces, or to the storage bin where surplus heat is stored in the rock mass. Air is circulated through the storage bin to the building spaces when sufficient heat is not available from the collectors.

A concern with any solar space heating system is overheating during the summer months. Cooling costs can offset savings achieved during the heating season. A properly designed solar space heating system, however, can improve the comfort of the building and even reduce summer cooling load.

Because of variations in building design and climate, the cost of solar heating measures vary widely. The Oregon Office of Energy, for example estimates that passive solar space heating features can displace electricity at costs ranging from 2 to 10 cents per kilowatt-hour, exclusive of possible tax credits or other incentives. Some of the variation in cost is due to the difficulty in assessing which system elements are attributable to the solar space heating system and which parts are integral to the design of the building. The additional maintenance costs of a simple passive solar measures are negligible.

The prospects for more widespread application of simple passive solar heating measures are good. Simple measures such as south-facing windows and adjacent slab floors are relatively inexpensive, and additional maintenance costs are negligible. Passive designs can be cost-effective even under Northwest conditions where wintertime heating loads often coincide with heavily overcast days. Moreover, passive measures can improve building aesthetics and comfort. The most significant obstacles to increased use of passive solar space heating are intrinsic building characteristics such as orientation and room layout. Although the prospect for significant cost reductions or performance improvements is modest, greater developer and consumer awareness of the benefits of passive measures should expand their application.

Because of the expense few active solar heating systems are being installed. The cloudy heating-season climate of much of the Northwest appears to fundamentally constrain the long-term potential of active systems.