In the previous two articles I gave an introduction to solar thermal technology. In this article, I will talk about concentrated solar thermal (CST) technology.
I wrote in one of my previous articles that the total energy incident on the Earth’s surface in one hour is enough to power the global energy needs for one full year! While that is true, the flux (energy per unit area) at any point is quite modest. To use it as a heat source it has to be concentrated. We all know that the Sun’s rays by themselves don’t burn paper, but when they are concentrated on a paper using a magnifying lens, it can be burnt! That’s the basic principle of CST technology.
There are three main types of CST technologies in use today: linear concentrating systems, a solar tower system, and a dish/engine system.
Linear Concentrating Systems
Linear concentrating systems collect the sun’s energy using long rectangular, curved (U-shaped) mirrors. The mirrors are tilted toward the sun, focusing sunlight on tubes (popularly called “receivers”) that run along the length of the mirrors. The reflected sunlight heats a fluid (popularly called Heat Transfer Fluid or HTF in short) flowing through the tubes. The hot HTF is then used to generate steam which drives a conventional steam-turbine generator and produces electricity.
There are two major types of linear concentrating systems: parabolic trough and linear Fresnel reflector systems.
In parabolic trough (PT) systems, receiver tubes are positioned along the focal line of each parabolic mirror.
Notice the people in the image. That will give you an idea about the actual size of the collectors.
A typical block diagram of a PT system is as can be seen in figure 15.2 below.
As can be seen in the block diagram, there are four main parts of the system:
This consists of rows of highly reflective parabolic mirrors mounted on support structures which can be tilted about an axis to track the diurnal movement of the sun from East to West. The receiver is at the focal line of the mirrors.
The receiver is encapsulated in a glass tube and the annular space between the tube and receiver is evacuated to prevent thermal loss. The receiver tube is given a special coating, which along with the glass cover leads to better absorption and transfer of heat to the HTF flowing inside the receiver. The HTF flows in and out of the receiver through header pipes. However, since the focal line moves considerably while the trough is tracking the sun, the header pipes have to be connected to the receiver tubes with flexible couplings. These couplings also have to be leak-proof because the HTF can catch fire if exposed to the atmosphere.
The most commonly used HTF is synthetic oil whose maximum operating temperature is around 400°C. So the maximum outlet temperature of the HTF is restricted to 390°C. On the other hand, since the HTF freezes at 13°C, care has to be taken that the temperature doesn’t fall below this level.
This is where the heat is transferred from HTF to the feed water to produce superheated steam. This steam drives the steam turbine which is coupled to a generator. The heat exchanger is equivalent to the boiler in a conventional power plant.
This is where the steam turbine is which produces electricity. The steam exiting the turbine is condensed using wet or dry cooling and condensed water goes to the feed water pump which pumps is back to the heat exchangers to again convert it steam. Wet cooling option requires considerable amount of water (3-4 m3/MWh). This is an issue especially if the power plant is located in dry and arid areas. If air cooling is employed, it decreases the cycle efficiency which increases the cost of generation.
This is where excess heat is stored to keep the plant when there is a cloud cover or after sunset. A power plant with thermal storage needs to generate more power than the rated capacity of the plant so that something extra can be saved away for later use. While storing, the thermal storage medium (TSM) from the cold tank passes through the heat exchangers, absorbs the heat, and gets stored in the hot tank. While releasing, the exact opposite happens.
A linear Fresnel reflector (LFR) system, which a relatively new development, also has all the components as a PT system (except the heat exchanger) but with some notable differences.
It uses an array of flat or slightly curved mirrors to reflect sun’s rays on to a linear receive which is stationary. Slight curvature in the mirrors is needed to reduce the aperture area. These mirrors are relatively small and mounted close to the ground where the wind loads are much less. This reduces the overall cost of the system which is obviously beneficial.
Although these mirrors are tilted differently with respect to each other, they need the same amount of rotation to track the sun. Therefore a single control system can be employed for all of them.
A modified version of the LFR is the compact LFR (CLFR) system.
In a CLFR system, the sun’s rays can be reflected either one of the two receivers depending on the sun’s position. This feature decreases the shadowing and blockage and therefore requires less land area than an LFR system.
Unlike PT reflector, LFR does not have a sharp focus line. Therefore, the small diameter tube similar to one in a PT system cannot be used. Also, the width of the mirrors used varies which leads to a variation in aperture width of the receiver. So a standard receiver configuration hasn’t yet evolved for LFR systems. Enclosing the receiver in a transparent tube and evacuating the annular space in between is complex. Hence, all receiver tubes have a transparent cover (without evacuation) at the bottom to reduce convection losses.
Water is used as the HTF in LFR systems and direct steam generation is the common mode of operation. This is why heat exchangers are not required. However, using water as the HTF has a disadvantage as well, and it is that both the liquid and vapour phases co-exist in the horizontal tubes which has problems of instability due to phase change. It also causes non-homogeneous temperature distribution due to difference in heat transfer coefficients of the liquid and vapour phases, which may lead to thermal stress. If superheated steam is generated, these problems become more severe.
Since direct steam generation is employed in LFR systems, only buffer thermal storage in the form of steam accumulator is possible. Steam accumulators occupy large volumes, and so extensive thermal storage is not a viable option.
I will talk about solar tower and dish/engine systems in the next article.
Writer, Publisher, Entrepreneur