Solar Photovoltaic and Solar Thermal Technologies

Solar Photovoltaic and Solar Thermal Technologies

There are two basic categories of technologies that convert sunlight into useful forms of energy,
aside from biomass-based systems that do this in a broader sense by using photosynthesis from
plants as an intermediate step. First, solar photovoltaic (PV) modules convert sunlight directly
into electricity. Second, solar thermal power systems use focused solar radiation to produce
steam, which is then used to turn a turbine producing electricity. The following provides a brief
overview of these technologies, along with their current commercial status.

1. Solar Photovoltaics

Solar PV modules are solid-state semiconductor devices with no moving parts that convert
sunlight into direct-current electricity. The basic principle underlying the operation of PV
modules dates back more than 150 years, but significant development really began following
Bell Labs’ invention of the silicon solar cell in 1954. The first major application of PV
technology was to power satellites in the late 1950s, and this was an application where simplicity
and reliability were paramount and cost was a secondary concern. Since that time, enormous
progress has been made in PV performance and cost reduction, driven at first by the U.S. space

program’s needs and more recently through private/public sector collaborative efforts in the
U.S., Europe, and Japan.
At present, annual global PV module production is over 150 MW, which translates into a more
than $1 billion/year business. In addition to the ongoing use of PV technologies in space, their
present-day cost and performance also make them suitable for many grid-isolated and even gridconnected
applications in both developed and developing parts of the world. PV technologies
are potentially so useful that as their comparatively high initial cost is brought down another
order of magnitude, it is very easy to imagine them becoming nearly ubiquitous late in the 21st
century. PV systems would then likely be employed on many scales in vastly differing
environments, from microscopic cells to 100 MW or larger ‘central station’ generating plants
covering square kilometers on the earth’s surface and in space. The technical and economic
driving forces that favor the use of PV technologies in these widely diverse applications will be
equally diverse. However, common among them will be the durability, high efficiency, quiet
operation, and lack of moving parts that PV systems offer, and the fact that these attributes
combine to provide a power source with minimum maintenance and unmatched reliability.
PV system cost and performance have been steadily improving in recent years. PV
manufacturing costs have fallen from about $30 per watt in 1976 to well under $10 per watt by
the mid-1990s as can be seen in Figure 5. Installed PV system costs today are about $8.00 to
$12.00 per watt, depending on the level of solar insolation at the site and other factors. These
installed system costs are expected by some analysts to reach a range of from $3.00 to $6.00 per
watt by 2010, and if this is achieved PV systems could achieve a sales level of over 1,600 MW
per year by that time.

2. Solar Thermal Systems

Solar thermal power systems use various techniques to focus sunlight to heat an intermediary
fluid, known as heat transfer fluid that then is used to generate steam. The steam is then used in a
conventional steam turbine to generate electricity. At present, there are three solar thermal power
systems currently being developed: parabolic troughs, power towers, and dish/engine systems.
Because these technologies involve a thermal intermediary, they can be readily hybridized with
fossil fuels and in some cases adapted to utilize thermal storage. The primary advantage of
hybridization and thermal storage is that the technologies can provide dispatchable power and
operate during periods when solar energy is not available. Hybridization and thermal storage can
enhance the economic value of the electricity produced, and reduce its average cost.

Parabolic trough solar thermal systems are commercially available. These systems use parabolic
trough-shaped mirrors to focus sunlight on thermally efficient receiver tubes that contain a heat
transfer fluid. This fluid is heated to about 390° C. (734° F) and pumped through a series of heat
exchangers to produce superheated steam that powers a conventional turbine generator to
produce electricity. Nine of these parabolic trough systems, built in 1980s, are currently
generating 354 MW in Southern California. These systems, sized between 14 and 80 MW, are
hybridized with up to 25 percent natural gas in order to provide dispatchable power when solar
energy is not available.
Power tower solar thermal systems are in the demonstration and scale-up phase. They use a
circular array of heliostats (large individually-tracking mirrors) to focus sunlight onto a central
receiver mounted on top of a tower. The first power tower, Solar One, was built in Southern
California and operated in the mid-1980s. This initial plant used a water/steam system to
generate 10 MW of power. In 1992, a consortium of U.S. utilities joined together to retrofit Solar
One to demonstrate a molten-salt receiver and thermal storage system. The addition of this
thermal storage capability makes power towers unique among solar technologies by allowing
dispatchable power to be provided at load factors of up to 65 percent. In this system, molten-salt
is pumped from a “cold” tank at 288° C. (550° F) and then cycled through the receiver where it
is heated to 565° C. (1,049° F) and finally returned to a “hot” tank. The hot salt can then be used
to generate electricity when needed. Current designs allow storage ranging from 3 to 13 hours.
Dish/engine solar thermal systems, currently in the prototype phase, use an array of parabolic
dish-shaped mirrors to focus solar energy onto a receiver located at the focal point of the dish.
Fluid in the receiver is heated to 750° C (1,382° F) and used to generate electricity in a small
engine attached to the receiver. Engines currently under consideration include Stirling and
Brayton-cycle engines. Several prototype dish/engine systems, ranging in size from 7 to 25 kW,
have been deployed in various locations in the U.S. and elsewhere. High optical efficiency and
low startup losses make dish/engine systems the most efficient of all solar technologies, with
electrical conversion efficiencies of up to 29.4 percent. In addition, the modular design of
dish/engine systems make them a good match for both remote power needs, in the kilowatt
range, as well as grid-connected utility applications in the megawatt range.
System capital costs for these systems are presently about $4-5 per watt for parabolic trough and
power tower systems, and about $12-13 per watt for dish/engine systems. However, future cost
projections for trough technology are higher than those for power towers and dish/engine
systems due in large part to their lower solar concentration and hence lower operating
temperature and efficiency. By 2030, the U.S. Department of Energy forecasts costs of $2.70 per
watt, $2.50 per watt, and $1.30 per watt, respectively, for parabolic trough, power tower, and
dish engine systems.