Power Tower Design

Power towers (also known as 'central tower' power plants or 'heliostat' power plants) capture and focus the sun's thermal energy with thousands of tracking mirrors (called heliostats) in roughly a two square mile field.  A tower resides in the centre of the heliostat field. The heliostats focus concentrated sunlight on a receiver which sits on top of the tower. Within the receiver the concentrated sunlight heats molten salt to over 1000 degrees Fahrenheit. The heated molten salt then flows into a thermal storage tank where it is stored, maintaining 98% thermal efficiency, and eventually pumped to a steam generator. The steam drives a standard turbine to generate electricity. This process, also known as the "Rankine cycle" is similar to a standard coal-fired power plant, except it is fuelled by clean and free solar energy.

Advantages and Disadvantages of Power Tower Plant

Advantages:

• Although the heliostat solar tower approach to solar power production isn't as commercially developed as the solar parabolic trough system, it is more commercially developed than either the parabolic dish - stirling engine or linear fresnel systems.
• Since the heliostat solar tower system produces steam to generate electricity with a conventional Rankine steam cycle, this system can be hybridized. In other words, it can be designed to use a fossil fuel (typically natural gas) as a supplementary fuel, allowing electricity to be generated when the sun isn't shining.

• The advantage of this design above the parabolic trough design is the higher temperature. Thermal energy at higher temperatures can be converted to electricity more efficiently and can be more cheaply stored for later use. Furthermore, there is less need to flatten the ground area. In principle a power tower can be built on a hillside. Mirrors can be flat and plumbing is concentrated in the tower.

Disadvantages:

• The heliostat solar tower system produces a fluid temperature greater than that of the single axis tracking, parabolic trough and linear Fresnel system, but less than that of the two axis tracking, parabolic dish - Stirling engine system. Thus it cannot achieve an efficiency for conversion of electricity from thermal energy as high as that of the parabolic dish - Stirling engine system.

• The disadvantage is that each mirror must have its own dual-axis control, while in the parabolic trough design one axis can be shared for a large array of mirrors.

Status of Commercialization of Power Tower Plant

The heliostat tower solar plant isn't as commercially developed as the parabolic trough system, but the 10 MW Solar One and Solar Two demonstration plants have already produced solar power over a 17 year period. The first commercial heliostat solar tower power plant, started operation at 20 MW in April 2009, in Seville, Spain. Several others heliostat/tower solar power plants are in the planning or construction phase worldwide.