Solar energy can be converted to electrical energy in two ways: First, with photovoltaic (PV) cells and second, with solar energy collectors that use the sun’s heat to run turbines coupled to electric generators. If you are contemplating either system for powering your facility, you want to weigh both their advantages and disadvantages. PV panels, for instance, is the most widely used type of a solar generator today and its application continues to grow with the demand for clean energy. However, it has one drawback; high ambient temperatures reduce the semiconductor solar panel output efficiency. In other words, a clear, cool day is better for PV panel efficiency and output power than a hot day.
Special SolFocus CPV arrays are made to track the elevation and azimuth of the sun’s path. They are designed for optimum stiffness and provide tracking accuracy within 0.1 °.
Three CSP Approaches
In contrast, the second system uses passive solar energy collectors, commonly
called Concentrated Solar Power or CSP systems. It deserves consideration
because it does not contain the temperature-sensitive semiconductor devices
that PV panels do. Another benefit is that CSP systems are less expensive than
PV systems of the same capacity.
CSP systems contain parabolic mirrors or lenses that direct the sunlight onto a
receiver located at the focal point. The receiver is filled with a heat transfer fluid
such as oil that absorbs the heat energy. The heated oil is pumped through a
heat exchanger or steam generator, which converts water on the secondary side
to steam. The steam turns a turbine generator (and the pumps) to generate the
electricity. When the steam exits the turbine, it returns to the liquid phase in the
condenser – and the cycle repeats.
CSP systems come in three configurations; a parabolic dish, a solar trough,
and a solar power tower. The parabolic dish provides the highest efficiency
of the three types. It usually contains a servo system that positions the dish
in two axes to track the position of the sun while maintaining the receiver at
its optimal focal point.
Concentrated Photovoltaics (CPV) arrays from SolFocus, Inc. are a step up from the more common flat PV arrays. CPV arrays, claims the manufacturer, are able to achieve 25% efficiency and concentrate the solar energy up to 500 times more than the flat PV panels, which typically reach about 17% efficiency.
Concentrated Photovoltaics (CPV) arrays from SolFocus, Inc. are a step up from the more common flat PV arrays. CPV arrays, claims the manufacturer, are able to achieve 25% efficiency and concentrate the solar energy up to 500 times more than the flat PV panels, which typically reach about 17% efficiency.
The trough system is similar, but in this design, the parabolic reflectors are long and linear as opposed to being circular dishes. The receivers comprise long tubes located at the focal line of the reflectors. These reflectors also track the sun, but only about one axis. And, similar to the parabolic system, the fluid is either oil or a molten salt, which reaches extremely high temperatures and converts water to steam in a heat exchanger. The steam runs a turbine generator. An added benefit is that some of the hot fluid is stored in tanks to be used during low light levels or during the night. The trough system efficiency is not as high as the parabolic dish system, but it is the least expensive and currently the most widely installed type.
Finally, the solar power tower has its receiver located at the top of a tower surrounded by an array of reflectors called heliostats. The heliostats are at ground level and track the sun during the day to keep the concentrated light beam on the receiver. Solar power towers have high efficiency and storage capability.
Hybrid Systems
CSP plants can generate electrical power on their own and store backup power during periods of low light levels. Their heat storage capacity is a sufficient buffer to provide power during nights, cloudy days, and seasonal changes that affect the amount of solar energy available. During these times, the hot fluid is released from storage tanks to drive the turbine generators. CSP plants can also be combined with other types of generating systems, such as those built for fossil fuel consumption. This is because they use the same kinds of turbine generators. The US Department of Energy predicts that more utility companies will consider CSP as an addition to their existing fossil fuel power plants.
New power generating plants are being built around the hybrid concept. For example, NextEra Energy Inc., Juno Beach, Fla., (previously known as Florida Power and Light Company), recently began constructing a new facility called the Martin Next Generation Solar Energy Center, the first hybrid solar energy plant and first utility scale solar facility in Florida. The plant will not require as much fossil fuel during the day to produce the heat needed to run turbines. The added CSP capacity will reduce natural gas consumption by about 41 billion cubic feet and reduce the need for more than 600,000 barrels of oil over the next 30 years.
CSP systems do not use photovoltaic cells and can withstand high ambient temperatures in addition to the heat generated by concentrated solar energy, which is collected by the parabolic troughs and transmitted to the receivers. The receivers contain the working fluid, which converts water to steam to drive the turbine generator. The system is designed to be connected to a common utility power grid.
