Published on August 5th, 2011 | by Breath on the Wind12
That “Other” Solar Tower Technology
August 5th, 2011 by Breath on the Wind
Several days ago I wrote of the Giant Arizona Solar Tower (and confusing names) that is trying to find funding. Brightsource is the company that is presently building Solar Towers with an entirely different technology.
The History started perhaps with Egyptians using mirrors to illuminate the interiors of tombs. They needed nothing more than a polished metal mirror and a slave who hopefully had a hat. The mirror would have to move in two ways to follow the sun through the sky. In those times, the main energy source was human power. From that time, various mechanical mechanisms have been substituted to track the sun. The name Heliostat itself is over 250 years old. Today electricity is king. Modern heliostats are moved by motors which, in turn, allow the sun to provide electricity.
The technology of Solar Power Towers is dependent upon heliostats. Fields of them, all reflecting the sun’s light to a central tower. There, the highly concentrated sunlight produces equally high temperatures. The heat is then used in an otherwise conventional Rakine Cycle thermal power plant to generate steam that runs a turbine to generate electricity. The first of these systems were built in the early 80’s in Russia, Europe Japan and slightly later in the US. Salts with higher melting temperatures can contain more sensible heat storage and were being used for their efficiency advantage over simply heating water.
Refinements: Brightsource has announced that the theory is now ready for the next construction project and calls the technology “LPT solar thermal energy system.” They have also added two refinements. Thermal (Rankine cycle) power plants need cooling. Because the technology is often used in deserts where water is scarce they have added air cooling. Hours of sunshine would normally limit the supplied power but thermal heat storage allows plant operation into the night.
The storage system used is a two-tank molten salt storage system. In two tank systems, heat is stored in the tanks at two temperatures. A heat exchanger between them is used to heat up or cool down another medium like water or oil. By adding such a storage system, the hours of operation are increased.
The capacity factor of a power plant is the actual plant energy production compared to the energy a plant could produce running 24/7 at maximum power. This is significant to the overall energy expected from a power plant. A 100-MW geothermal plant with a 90% capacity factor would have an advantage over a 100-MW coal plant with only a 75% capacity factor if the cost of both were equal.
Photovoltaics are around 19% in Arizona. Wind and solar towers without energy storage have capacity factors around 25%. Energy storage can increase the capacity factor of Concentrated Solar Power (CSP) plants like the solar power tower to over 60%.
Advantages: CSP technologies have an advantage over photovoltaics as they are already producing heat which suggests a technologically simple method of energy storage. The other CSP technology being built around the world is the solar trough collector. The Brightsource LPT solar thermal power-generating technology can reach higher operating pressure and temperature levels. This adds to increased efficiency and economic value.
Future refinements in such technologies will certainly be heavily dependent upon new developments in thermal storage. Eventually, we may see more use of chemical heat storage for extremely long, even seasonal, storage systems.
- Brightsource solar tower projectby Flickr user pgegreenenergy
- Heloistats for solar tower by Flickr user langalex
- Other photos: Brightsource