In the previous article, I talked about the two types of linear concentrating systems: parabolic trough (PT) systems and linear Fresnel reflector (LFR) systems. In this article I will talk about the solar tower systems.
Solar Tower Systems
Solar Tower (ST) systems use a large number of flat or slightly curved mirrors, also called heliostats, having dual-axis control (about the vertical and horizontal axes) to reflect solar radiation to a stationary receiver at the top of a tower. The heat transfer fluid (HTF) passing through the receiver converts the concentrated solar energy received by the tower to thermal energy, which is transferred to the working fluid of a conventional power block to generate electrical power.
The main advantage of a ST is that a high concentration of 200 to 1000 can be achieved. Therefore, temperatures of the order of 1000°C can be easily reached with the help of suitable HTFs. This high temperature results in higher power cycle efficiency, and an overall conversion efficiency of 25%.
Also, thermal energy storage and hybridization can be incorporated as in PT systems. Further, molten salt can be used both as HTF and thermal storage medium. When the ST is not in operation and the molten salt freezes, it simply falls down since the tower is vertical; that can’t happen if it is used in PT systems since they are horizontal. Therefore, ST systems with molten salt as HTF are becoming quite popular.
The major components involved in ST systems are explained below:
1.Heliostats: These are flat or slightly curved mirrors mounted on a backup steel structure which can be controlled around two axes (horizontal and vertical) so as to tilt the heliostats to reflect the solar rays on to a fixed receiver at the top of the tower.
The aperture areas of the heliostats used in various can vary in size considerably, from 1 m2 all the way up to 120 m2. However, all heliostats within a particular plant are always of the same size.
The main advantages of small heliostats are the ability to do mass production, ease of handling and installation, and smaller wind loads due to smaller size and proximity to ground. However, for a given power, the number of heliostats required increases quite a lot.
On the other hand, the heliostat of 120 m2 aperture area has many curved facets; in one project where such large heliostats were used, there were 28 facets arranged in 7 rows and 4 columns. With such large heliostats, each faces has to be canted properly so that the receiver size can be kept as small as possible and the concentration ratio as high as possible. That is a challenge in itself. But if it can be overcome, it becomes advantageous since the number of heliostats and controls is reduced. However, the structures of the heliostats have to withstand high wind loads and the control system has to be powerful enough to move them.
2.Receivers: This is where all the reflected and concentrated solar energy is received. Since the concentration ratio in STs is 200 to 1000, the solar heat flux impinging on the receiver is in the range of 200 to 1000 kW/m2, and therefore high temperatures of the order of 1000°C are possible. Even though the temperature is high, the thermal loss from the receiver is comparatively less compared to the receivers of PT or LFR system, since the surface area is less.
There are mainly two types of receivers: tubular and volumetric. Tubular receivers are used for liquid HTFs such as water, molten salt, thermic oil, liquid sodium, and Hitec salt. Volumetric receivers, on the other hand, use air as HTF.
a.Tubular receivers: In tubular receivers, the HTF passes through several vertical tubes and gets heated by the radiant flux reflected from the heliostats. There are two types of tubular receivers: external cylindrical receivers and cavity receivers.
In external cylindrical receivers, the vertical tubes are arranged side by side in a cylindrical fashion and the radiant flux impinges from all directions. Since the receiver is exposed to the atmosphere, it is susceptible to high convection losses.
In cavity receivers, the welded tubes are kept inside a cavity to reduce convection losses
b.Volumetric receivers: In volumetric receivers, air is used as the HTF. These receivers are made of porous wire mesh or metallic/ceramic foams. The radiation impinging on the volumetric receiver is absorbed by the whole receiver. There are two types of volumetric receivers: open volumetric and closed/pressurized volumetric.
In open volumetric receivers, ambient air is sucked through the porous receiver where it gets heated up by the concentrated solar energy. The front end of the receiver will have less temperature than inside the receiver because the incoming air cools the surface and avoids damage to the material. Julich tower plant (in Germany) uses a porous silicon carbide absorber module. The air gets heated up to 700°C and is used to generate steam at 485°C and 27 bar in the boiler to run the turbine.
Closed volumetric receivers are also called pressurized volumetric receivers, in which the HTF is mechanically charged to the receiver by a blower and the receiver aperture is sealed by a transparent window. The HTF gets heated up at the dome shaped portion of the receiver by the concentrated solar energy and is used either in a Rankine cycle via heat exchanger or in a Brayton cycle for generating electricity
3.HTF and Power Cycle: Different types of HTF can be used in STs based on the type of receiver and power cycle employed in the system. HTFs that are commonly used in STs are water, molten salt, and air. Liquid sodium, Hitec oil, and synthetic oil are the other possible candidates.
When water is used as HTF, steam is generated directly and the Rankine steam cycle is used for power generation.
If molten salt is used as HTF, a heat exchanger is used to transfer the thermal energy to water to generate steam and the Rankine steam cycle is used for power generation. Use of molten salt as HTF permits easy thermal storage. However, when the plant is not in operation, the HTF has to be drained out.
The figures below show the heliostat fields for some of the ST plants in operation around the world.
From the above figures, one can infer that the heliostats can be of pretty much any shape. What changes with the shape is the type of receiver and the controls required to concentrate the solar energy onto it. Also, as is obvious from the figures, it is not necessary that the receivers be located at the centre of the heliostat field. In PS-10, PS-20 and Julich power plants, they are located at the northern end of the heliostat field.
I hope you liked this article. In the next article I will talk about dish/engine systems. I will also compare all the different types of CST technologies with each other.
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