Concentrated Solar Power: Solar Power Tower

One of the more commonly implemented Concentrated Solar Power (CSP) systems is the Solar Power Tower system. This system is a central tower that receives focused sunlight because they use a central receiver system. Computer-controlled mirrors, known as heliostats, track the sun along two axes and can move according to the sun’s position, focusing the rays onto the central receiver system at the top of the tower. The heliostats can either be curved or flat, but generally flat mirrors are used as they are less expensive than that of their curved alternatives. Additionally, the heliostats use a significant amount of space of the surrounding land, which can firstly require a substantial initial investment, as well as impacting the local landscape and the environment. In the receiver a fluid is heated and used to generate electricity in a Rankine cycle. Older towers used water as the working fluid in the Rankine cycle, so it vaporises into steam directly because of the focused sunlight. However newer systems utilise molten salts, such as sodium and potassium nitrate, as the heat transfer fluid, transferring the heat to water, boiling it in a Rankine cycle. This change is due to the fact that molten salts have a high heat capacity, which can be used to store the energy before using it to boil water. Storing the heat energy for late recovery allows electricity to be generated continuously over all 24 hours of the day, nullifying the effect of solar being an intermittent energy source. The temperature of steam in the Rankine cycle can reach highs of up to 550ºC.

Figure 1. Schematic Diagram of Solar Power Tower

Link to image: https://www.sciencedirect.com/topics/engineering/solar-tower

However, the Power Tower system demands significant volumes of water for cooling, which has resulted in investigations into the use of air cooling. However, they have high efficiencies due to the high temperatures reached at the central receiver system, while also having better energy storage than some of the other systems because of the use of molten salts and the fact that they are heated up to higher temperatures initially. Solar power tower is the second most implemented CSP system in the world, but I believe that once the necessary investigations take place to reduce costs, Power towers can be implemented globally, more frequently.