Published on July 17th, 2009 | by Fred Etcheverry4
Solar Thermal Hybrids Are Hot
Aora-Solar, an Israeli company, has constructed, licensed and launched the first solar thermal energy (STE) natural gas hybrid electric generator (June 2009). At the time of this post, it is being tested in Kibbutz Samar in southern Israel. Such hybrids can lower the carbon footprint of existing natural gas power plants. The Israeli design is modular and permits small plants that can serve communities without long distance line losses. Florida Power and Light is scheduled to launch their first hybrid in 2010.
As I discussed in a previous post, electricity can be generated by high temperature or concentrated solar power (Solar Thermal: The Other Solar Energy). Basically, solar rays are concentrated and directed to a heat collector that transfers their heat to a heat engine (usually a closed circuit steam engine) that drives an electric generator.
Several schemes have been used to concentrate the sun’s rays. Curved mirrors or lenses or an array of plane (flat) mirrors direct the rays toward a collector. Each scheme has particular advantages and disadvantages.
Aora-Solar design uses an array of plane mirrors on motor driven platforms. These “heliostats” must steer their reflected beams onto the collector located on a thirty-meter high “power tower.” Once this would have been a formidable task, but modern microcomputers have simplified it. The power tower then provides heat to supplement natural gas used by an existing power plant.
One of the most popular STE schemes is to use mirrors having a parabolic curvature in one dimension and are long “troughs” in the other. These mirrors focus the sun’s rays on a tube that runs the length of the trough. A fluid flowing through this tube carries the heat to the heat engine.
In this scheme the troughs are aligned parallel to the sun’s daily path across the sky so that the rays are always focused on the tube. The mirrors do need to be adjusted to accommodate seasonal changes, but these are slow and easily programmable.
Florida Power and Light uses troughs in its hybrid. The advantage of troughs is that they can be combined to build almost unlimited “solar thermal farms.” Their main disadvantages are that they produce lower temperature outputs and must be installed on flat terrain. Typically, troughs produce about 600 C compared to 1000 C + for power towers. Heat engines are more efficient when provided with higher input temperatures.
The economics of the Israeli design it exciting. Natural gas plants are run as “peaker plants.” That is, they only go online to provide surges of power when there is a sudden high demand for electricity.
Peaker plants are necessary to maintain the grid, but their infrequent use makes for expensive electricity. Not only must the resulting electric power pay for fuel in such plants, but also it must pay for capital investment and the plant’s standby workforce. Hybridization of existing peakers can make them into base-load plants with peaking capability by taking advantage of unused capacity. Natural gas would only be used for peaking. Eventually, biofuels could replace natural gas.
The Aora-Solar power tower design allows existing plants to be retrofitted with 100KW modules. Power towers can be installed on rough terrain. These power towers’ high temperature output permits driving natural gas plants directly without need for any fuel assist.
The flexibility of the Israeli design permits small community to tap into solar power while avoiding power wire loss from large distant generators. These modules can be installed quickly and economically on almost any terrain. Aora-Solar is negotiating with power companies in Jordan and Australia.