Practical solar thermal energy power stations

Designers of solar thermal power stations may not realise that a few minor changes are all it takes to:

• Reduce the cost of solar thermal power and
• Provide reliable 24 hour, 7 days a week operations.

A substantial capital cost is the heliostat field that focuses solar energy on the tower where elecricity is generated. Two design options are available to lower this significant cost component.

• Improve the efficiency of power generation in the solar tower, and
• Simplify the two-axis tracking technology that repositions each heliostat as the sun moves across the sky.

Another substantial cost is the choice of components to provide power generation during cloudy periods and at night. The simplest design option is also by far the lowest cost solution:

• Use an alternate energy source to maintain energy production at all times.

---

Improving efficiency

The size and cost of the heliostat field is directly related to the efficiency of power generation.

•  A 10 megawatt power station that achieves only 25 percent thermal efficiency requires a heliostat field able to focus 40 megawatts of solar thermal energy on its receiving tower.
• The same power station with generating technology that achieves 50 percent thermal efficiency requires a heliostat field able to focus just 20 megawatts of solar thermal energy on its receiving tower.

The cost of the heliostat field is cut in half by this increase in efficiency.

Simplify the two-axis tracking technology

The cost of actuators to reposition heliostats is another significant component in the power station cost. Usually two actuators are needed for each heliostat.

Every heliostat in each row of heliostats needs to rotate by the same angle and at the same rate each day as the sun tracks from east to west. Just one actuator connected via a fixed beam is all that is required to provide the east-west movement of each row of heliostats. Another actuator on each heliostat is still required to provide tracking for the vertical movement each day.

Improve the efficiency of power generation

Concentrated solar thermal power stations typically use steam turbines for power generation. These are able to achieve thermal efficiencies around 25 - 35 percent at best.

A far more efficient technology is combined-cycle generation using a solar-air turbine in the first cycle with a steam turbine in the second cycle.

• A conventional internally heated combined-cycle gas turbine power station achieves around 60 percent thermal efficiency. Using an externally heated solar-air turbine turbine reduces the efficiency to around 50 percent. 

Use an alternate energy source until energy storage is much cheaper

Concentrated solar thermal energy varies steadily through the course of each day and can fluctuate in very short intervals if clouds cast moving shadows over the heliostats.

• A simple and effective solution is to incorporate a temperature sensor on the output from the solar-air turbine to control a gas burner  fitted to the turbine.