Solar thermal systems collect and store heat energy. Four main methods are used to concentrate the collected solar energy: parabolic troughs, parabolic dishes, power towers and solar ponds.

Parabolic Through Collectors

A solar parabolic trough generates electricity by heating fluid by concentrating the suns rays. A system of parabolic trough shaped mirrors is used to focus sunlight onto a thermally efficient receiver tube containing a heat transfer fluid. The fluid, which can reach temperatures around 400C is then used to produce superheated steam which can then be used to drive conventional turbine generators.

In order to operate efficiently the parabolic troughs must be able to adjust to ensure that the sunlight is focused on the receiver tubes. This is done using a tracking mechanism to rotate the troughs as the sun moves across the sky. Some parabolic trough systems can also rotate around a second axis to better track the sun for further improved efficiency.

Parabolic Dish Systems

These systems use a satellite dish-shaped array of mirrors to focus solar energy onto a receiver at the focal point of the dish. The receiver contains a fluid that is heated up to 1000C and is used to directly generate electricity in a small engine attached to the receiver. They track the sun on two axes, following it across the sky during the day and adjusting for the angle above the horizon which changes with season. This makes them the most effective concentrating collector.

Power Towers

A solar power tower system uses a circular field array of large individually-tracking mirrors (heliostats) to focus sunlight onto a central receiver located on the top of a tower. The receiver contains a fluid that absorbs the heat energy which is then used to drive a turbine generator or to provide heat directly for industrial applications. The temperature achieved by the receiver can range from 538C to 1482C. Like most solar thermal generators the heliostats are aligned using a dual-axis, computer controlled tracking system.

Solar Ponds

One factor that focusing mirrors and solar cells are restricted by is that they cannot generate electricity at night unless a thermal storage system is used. Solar ponds overcome this problem by using a salt-gradient pond for thermal storage. These ponds have a high concentration of salt near the bottom, an insulating (non convecting) middle layer and a convecting top layer with low salt concentration. Sunlight hits the pond and its solar energy is trapped in the bottom layer due to its high salinity. This heated water can not rise due to its greater density and therefore sits on the bottom heating up until it nearly boils. The middle layer acts as an insulator and prevents convection and therefore the top layers stay relatively cool. Electricity can be generated from the heated water day or night by piping a heat transfer fluid through the bottom layer for use in power generation.

| 01.01.2009 | Read more | Print |