Solar Thermal Power Plants.

As a future power source we always look up to the Sun, the eternal source. A source that is freely available and environment friendly, solar energy has been in use in a myriad of ways for thousands of years. However, no solar energy system has been has been able to replace the base load power plants as continuous and reliable source of power.

Photovoltaic cells or solar panels produce power for many applications but on very low scale. These have been in use at remote locations and for space applications. The poor availability of sunlight, which even in the best case is less than 50 %, non-availability due to cloud and weather conditions have made it a non-starter for base load application. High cost of the photovoltaic and solar panels has also been a hindrance in developing the solar source on a large scale. However, with carbon emissions restrictions and cost of environmental cleaning up the outlook on Solar systems as a base load system is fast changing.

The most promising and already in service in large-scale pilot power plants, is the Concentrated Solar Thermal system. This system consists of three parts.

• Multiple mirrors concentrate solar radiation on to a pipe or tube with water or oil to produce steam.
• The steam rotates a steam turbine to produce electricity in the conventional system. This part is in service in thousands of units all over the world.
• Adding a thermal storage system increases the availability beyond normal daylight hours. This part, the storage system, is the one that requires developments.

As with most renewable energy systems, sunlight is available only in daylight hours which even in equatorial regions makes the availability less than 50 %. Cloud cover and other weather related parameters makes it still worse. Added to this the proximity to load centres also have to be considered for power transmission. The deserts of North Africa, Middle East are some areas that have high solar radiation coupled with very less cloud cover and are considered potential areas for large-scale development.

The potential is truly great. With two hectares of direct sunlight capable of producing one MW , Northern African Desert areas have the potential to power the entire Europe discounting transmission issues. Anticipated targets by 2030 are in the order of 100,000 MW . This can significantly reduce Carbon emissions.

Since concentrated solar thermal systems use the normal steam cycle, this is ideal to combine with the gas turbine in the combined cycle. Concentrated solar thermal systems can make use of the steam turbine portion of the combined cycle effectively . The plant works on the normal combined cycle mode when there is low or no solar power and can switch to solar mode when the sun shines. This makes the plant suitable as a base load plant for uninterrupted power supply with less overall emissions .

Solar only may not be an answer as of now, but hybrid systems certainly are.