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|   Reports & papers |
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|                   | Solar Energy Systems for the Million Solar Roofs Initiative
**June 1998 **|**  document 98-19**

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.

**Links to related sites:**
- Million Solar          Roofs Initiative
- Oregon Office          of Energy
- WSU Cooperative          Extension Energy Program (Renewables)
- Eugene Water & Electric Board
- Sacramento          Municipal Utility District (Solar Program)

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