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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.
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