Recycling Initiative: Solar thermal power plants - Solar Thermal Tower Power Plants

In solar thermal tower power plants, hundreds or even thousands of large two-axis tracked mirrors are installed around a tower. These slightly curved mirrors are also called heliostats; a computer calculates the ideal position for each of these, and a motor drive moves them into the sun. The system must be very precise in order to ensure that sunlight is really focused on the top of the tower. It is here that the absorber is located, and this is heated up to temperatures of 1000°C or more. Hot air or molten salt then transports the heat from the absorber to a steam generator; superheated water steam is produced there, which drives a turbine and electrical generator, as described above for the parabolic trough power plants. Only two types of solar tower concepts will be described here in greater detail.

Open Volumetric Air Receiver Concept

The first type of solar tower is the open volumetric receiver concept (see Figure 4a). A blower transports ambient air through the receiver, which is heated up by the reflected sunlight. The receiver consists of wire mesh or ceramic or metallic materials in a honeycomb structure, and air is drawn through this and heated up to temperatures between 650°C and 850°C. On the front side, cold, incoming air cools down the receiver surface. Therefore, the volumetric structure produces the highest temperatures inside the receiver material, reducing the heat radiation losses on the receiver surface. Next, the air reaches the heat boiler, where steam is produced. A duct burner and thermal storage can also guarantee capacity with this type of solar thermal power plant.

Pressurized Air Receiver Concept

The volumetric pressurized receiver concept (see Figure 4b) offers totally new opportunities for solar thermal tower plants. A compressor pressurizes air to about 15 bar; a transparent glass dome covers the receiver and separates the absorber from the environment. Inside the pressurized receiver, the air is heated to temperatures of up to 1100°C, and the hot air drives a gas turbine. This turbine is connected to the compressor and a generator that produces electricity. The waste heat of the gas turbine goes to a heat boiler and in addition to this drives a steam-cycle process. The combined gas and steam turbine process can reach efficiencies of over 50%, whereas the efficiency of a simple steam turbine cycle is only 35%. Therefore, solar system efficiencies of over 20% are possible.



FIGURE 4. Schematic of two types of solar thermal tower power plant, showing (a) an open volumetric receiver with steam turbine cycle and (b) a pressurized receiver with combined gas and steam turbine cycle

Comparing Trough and Tower

In contrast to the parabolic trough power plants, no commercial tower power plant exists at present. However, prototype systems – in Almería, Spain, in Barstow, California, US, and in Rehovot, Israel – have proven the functionality of various tower power plant concepts.

The minimum size of parabolic trough and solar tower power plants is in the range of 10 MWe. Below this capacity, installation and O&M costs increase and the system efficiency decreases so much that smaller systems cannot usually operate economically. In terms of costs, the optimal system size is in the range of 50–200 MWe.