Solar Dishes
Abstract
A parabolic dish solar generation unit is a small power plant with a reflector like a large satellite antenna. Like a solar tower, it is a point focusing concentrator, but it can achieve an even higher concentration ratio, theoretically as high as 2000, and an efficiency of up to 40%. Size is limited by material constraints, and the largest practical dishes are normally around 10 m in diameter, with a generating capacity of 25 kW, although larger individual dishes have been constructed. For larger generating capacity, arrays of multiple dishes are used. Each solar dish normally has its own heat engine, often either a Stirling engine or a micro turbine. Some systems have also experimented with steam turbines with the heat cycle powered by an array of solar dishes.
Keywords
Parabolic dish; point focusing concentrator; Stirling engine; micro turbine; multiple dish arrays; dish steam generation systems
A parabolic dish solar power plant is based on a single parabolic reflector, similar in shape to a satellite antenna. This reflector focuses light to a point where a heat engine is placed. The heat engine uses the concentrated solar heat to produce electricity. In concept, the solar dish is similar to the solar tower, but on a much smaller scale. The small scale allows a full parabolic reflector to be built, but limits the maximum size of the solar dish that can be constructed. Economic and physical constraints mean that most solar dish units have a generating capacity of 25 kW or less, although some larger individual systems have been built.
A single solar dish unit can be used for distributed power applications, providing power to a single domestic dwelling or small commercial operation. For larger power plants, multiple solar dishes can be installed in arrays. In principle this can provide tens or even hundreds of megawatts of generating capacity from a single power plant, although the largest plant of this type that has been constructed had a capacity of only 4.9 MW.
The advantage of the solar dish over other types of solar thermal power plants is its efficiency. A unit of this type can use solar energy to raise the temperature of the thermodynamic fluid in a heat engine to 1000°C; solar dish power units have demonstrated an efficiency of 30% and higher. Another advantage is that the dishes can be self-contained generating units that do not require water cooling, which is often needed in other types of solar thermal power plants.
Most solar dish designs use simple, self-contained heat engines, one to each dish. However, some parabolic dish power plants have used linked dishes to create one or more heating loops to generate steam to drive a central steam turbine.
The first parabolic dish solar devices were built by a French mathematician named Augustin Mouchot. In 1978, helped by his assistant Abel Pirfre, Mouchot presented his invention at the Universal Exhibition in Paris. The solar device consisted of a truncated cone-shaped reflector with an area of 20 m2 that could produce steam pressurized to 3 bar to drive a pump capable of delivering 1500–2000 L/h of water. He is also credited with a solar refrigeration device. Mouchot’s devices were never developed commercially, and the next time solar dishes appeared was a century later during the 1970s oil crisis. Some demonstration projects based on parabolic dishes were built during the 1980s, but no successful commercial plants. Since then a number of companies have continued to develop solar dish power units, and one plant of over 1 MW has been constructed. Even so, commercialization of the technology has yet to prove successful.
Solar Dish Technology
The solar dish is defined by the parabolic reflector that is used to capture and concentrate solar energy. The parabolic shape allows all the sunlight incident upon the reflecting surface to be focused to a single point where a heat collector or receiver is placed. The actual amount of heat energy collected will depend upon the size of the dish. Most modern dishes have been less than 10 m in diameter, although one project involves a 25 m diameter dish with an aperture or capture area of 500 m2. A diagram of a solar dish power plant with multiple solar dishes is shown in Fig. 6.1.
In most solar dish designs each dish has its own heat engine that is sized to suit the amount of solar energy the dish can collect. As a general guide, a 10 m solar dish can collect sufficient energy for a 25 kW heat engine.
The construction of the parabolic reflector is the most important aspect of the solar dish design. It is not economical to construct a single glass parabolic reflector of the size required for a typical solar dish, and so most dishes are broken down into a series of smaller elements that together form the larger reflector. Silvered glass is the most effective and durable material for mirrors, and offers one method of construction, with the individual mirrors mounted onto a parabolic space frame. For the largest parabolic dish yet built, the SG4 project in Australia, the reflector was constructed from identical laminated glass on metal mirrors that were mounted onto a tubular space frame structure. The reflector was constructed from 380 mirrors, each 1.165 m2.
Other construction techniques have also been tried. Plastics are cheaper than glass, and the addition of a silver or aluminum layer to a transparent plastic support can provide a good reflective surface. However, plastics are likely to be less durable than glass. Another technique that has been tried is to stretch a flexible plastic membrane over a circular support, and then to use a partial vacuum behind the membrane to create the curved shape. This same technique has also been used to form a thin steel disk into a parabolic reflector.
The accuracy of the reflective surface is important for achieving high solar concentration levels. The dish must also be able to track the Sun across the sky. Two tracking systems have been used for solar dish projects. For larger dishes the azimuth-elevation system is preferred because it allows for better weight distribution. Under this system the disk can be rotated about two perpendicular axes, one horizontal and the other vertical. Rotation about the vertical axis changes the azimuth or compass bearing, while rotation about the horizontal axis alters the altitude or elevation. Smaller dishes often use polar tracking under which the dish moves about an axis running parallel to the Earth’s axis of rotation and at a speed of 15°/h to match the speed of the Earth’s rotation. There is also a second axis of rotation, perpendicular to this, to take into account the seasonal change in the height of the Sun in the sky.
Power Generation in Solar Dish Systems
The power generation system in a parabolic dish system normally needs to be a small, self-contained heat engine. In most cases one of two different types has been used, either a micro turbine or a Stirling engine. Steam generation, used to drive a steam turbine, is also possible, but is less common.
The key to the operation of each of these systems is efficient capture and transfer of heat energy to the thermodynamic working fluid that is used in the heat engine cycle. The heat capture and transfer is carried out using a receiver, which for a dish system may be either a tube-based receiver or a type of volumetric receiver. The heat can be transferred directly to the working fluid in the heat engine or it can be first captured by an intermediary heat-transfer fluid that then transfers the heat to the working fluid in a heat exchanger. Direct heating of the working fluid is the most efficient approach, and this is frequently used in dish systems.
Both direct and indirect heating can be applied to the Stirling engine. The latter is a closed cycle piston engine that contains either helium or hydrogen, pressurized up to 200 bar. Both gases are efficient at absorbing heat energy. The engine requires both a hot source and a cold source. The cold source is usually air from the atmosphere while the hot source is the heat from the solar collector. In the Stirling engine both heat and cold are applied to the outside of the engine. It is possible to heat a single engine directly by focusing the sunlight onto the heater plate of the engine. However, for more complex systems with multiple pistons it can be more efficient to use heat pipes to absorb the solar heat and transfer it to the places where it is required. Systems of this type have been built using liquid sodium as the heat-transfer fluid.
Stirling engines can be very efficient. With a solar input temperature of 1000°C, an efficiency of 40% is theoretically possible, although the highest efficiency that has been achieved so far is just over 31%. Even so, this is much more efficient than any other solar thermal technology. Typical Stirling engines for solar dish generation systems are between 10 kW and 30 kW in size.
Micro turbines are the other common small generating unit for solar dishes. A micro turbine is a small gas turbine, and its working fluid is air. Air is less good at absorbing heat than either hydrogen or helium, and to gain the best efficiency a type of volumetric receiver is required. This consists of a porous foam or honeycomb of a ceramic material. The receiver is exposed to sunlight through a quartz window, and the concentrated solar energy heats up the honeycomb. Air then becomes heated as it is pumped through the receiver.
The micro turbine has an open thermodynamic cycle. Air from the atmosphere is drawn into the turbine through its compressor stage, where it is compressed. This compressed air then passes through the volumetric receiver where it is heated to as much as 850°C. The hot, high-pressure air is then released through the turbine stage of the micro turbine where it generates sufficient power both to drive the compressor—mounted on the same shaft—and drive a small generator producing electricity. The still-hot air exiting the micro turbine exhaust is used to heat the air between the compressor and the volumetric receiver before being released back into the atmosphere.
The efficiency of a micro turbine is lower than that of a Stirling engine, but the units are cheaper. The micro turbine also has the advantage that supplementary heating can be added very simply by burning natural gas to boost output when the solar input falls off. As with Stirling engines, the typical size of a micro turbine solar dish plant is 10–30 kW.
In addition to the Stirling engine and the micro turbine, some solar dish systems have been designed to raise steam to drive a steam turbine. One of the earliest solar dish projects, built in southern California in 1983/1984, used a steam system. The project, called Solar Plant 1, used 700 dishes constructed from stretched-membrane reflecting elements. A system of steam loops, each one cycling through multiple dishes, was used to generate superheated steam at 490°C to drive a central, 4.9 MW steam turbine. The plant stopped operating in 1990.
More recently, solar dish development in Australia has also focused on steam generation. The SG4 large dish system is designed to generate steam directly. The 500 m2 dish should be capable of driving a 100 kW steam turbine generator. In tests it has produced steam at 45 bar and 535°C using a tube boiler.
Commercial Solar Dish Projects
There have been a number of proposals for large solar dish arrays, including some schemes involving thousands of dishes and generating capacities of hundreds of megawatts. So far, none has ever been built. However, two semi-commercial projects have been constructed, both in the United States. The first, called the Maricopa project, comprised 60 dishes, each of 25 kW, to provide an aggregate generating capacity of 1.5 MW. The project, which started generating power in 2010, was a demonstration scheme, and was intended as the precursor for a string of projects, including a 27 MW scheme with 1080 dishes and one with 34,000 dishes and a proposed generating capacity of 850 MW. Neither of these schemes, nor a third of 709 MW, ever materialized, and the Maricopa plant was decommissioned in 2011.
The second large-scale scheme was constructed at the Tooele Army Depot in the U.S. state of Utah. This project comprised 430 individual dishes, each capable of generating 3.5 kW of power for an aggregate capacity of 1.5 MW. Commissioning of the plant began in 2013, but later in the year the company that was building it filed for Chapter 11 bankruptcy. The fate of the project is unknown.
With no commercial projects under construction, solar dish technology has little chance of developing, even though it can offer high efficiency compared to other solar thermal technologies. The major difficulty has been competition from solar photovoltaic technology because the cost of the latter has fallen steeply. While other solar thermal technologies, and solar tower and solar trough plants, are suited to large-scale applications, the solar dish is by its nature a small-scale generating system. This has made it difficult for the technology to find a place in the renewable generation mix that is developing in the 21st century.