Solar Thermal Energy - What is it and How does it Work?
Posted on February 01 2019
Solar thermal energy, also known as solar thermal power, is used to describe a group of technologies that capture heat energy from the sun and utilize it for our daily needs. The system has two basic components: the reflecting mirrors that collects and concentrates the sun rays and the receiver or collector that absorbs heat from the sunlight. These systems harvest solar thermal energy and utilize it for cooling and heating purposes, and also to generate electricity on a large scale.
Parabolic trough. via helioscp
A renewable energy resource
Every year humans use about 35 billion barrels of crude oil to run the entire universe. That is a lot of oil! It is even hard to imagine how much it pollutes our mother earth. Furthermore, with the speed humans are using crude oil, it is estimated that by 2070, the Earth will run out of oil and out of coal in a century. So, sooner or later, turning to renewable energy resources is the ultimate fate of the creatures of the Earth. Besides hydro-power plants, solar PV cells, and biomass, solar thermal energy is another renewable resource of energy that we can harvest to generate power for our daily use.
Active solar thermal system. via home.uni-leigzip
Active and Passive solar thermal energy
Primarily, the solar thermal energy can be divided into two sub-categories: active and passive solar thermal power systems. Passive systems are designed such that they can absorb the maximum amount of heat energy from the incoming sun rays, such as in it does in a greenhouse or solar ovens. In an active system, engineers design structures that have moving parts including fans, pumps, solar trackers etc. to absorb heat from the sun at optimal angles, redirect it towards the receiver, and circulate the thermal fluid or water in the system. Active solar thermal power systems can be used for commercial or residential properties to heat water or drive solar driven chillers for air-conditioning.
Solar oven (passive system). via moneycrashers
Electricity is generated from solar thermal energy via the concentrated solar power (CSP) system. This is much like an active system used for residential or commercial heating, but it operates at a much higher temperature ranges from 400°C to 600°C. The two commonly used electricity generating systems of CSP are the parabolic trough system and central tower system. Both the systems work on the same principle of harvesting heat from the sun, but they differ slightly in the method. We have explained them briefly below.
1. Parabolic Trough System
In the parabolic trough solar thermal energy system, bent and angled mirrors are used to concentrate the sun rays onto a tube that is carrying thermal fluid. The concentrated sunlight heats-up the thermal fluid up to 400°C, which is then pumped towards the heat exchanger. In the heat exchange, stored water is evaporated into pressurized steam, and it then turns the turbine which is connected to a generator to generate electricity. The steam is cooled down into its liquid form (water) to repeat the process. Energy generated from here is then transferred to the grid for distribution and usage.
Parabolic trough system. via mtholyke
2. Central Tower System
The central tower systems use a slightly different method for harvesting heat from the sun. It uses a large group of mirrors, also known as heliostats, that reflects the sunlight onto the tower. The picture below will give you a good idea of how it looks like. The heliostats are mounted on a solar tracker so it can always get the right angle and project maximum sunlight onto the tower. The concentrated sunlight can produce temperatures of up to 600°C which is used to boil water and produce pressurized steam. The steam then drives a turbine which runs a generator to generate electricity. Electricity is then inserted into the grid for customers to use.
Central tower solar thermal system. via cosmomagzine
Pros and Cons of the solar thermal energy
As this system generates electricity from the forever free source of energy - the Sun, it is a clean and renewable source of electrical power. Another advantage of generating electricity from solar thermal energy systems is the low cost of maintenance. As you know that these systems have a few moving parts, maintaining such solar farms is inexpensive as compared to solar PV farms.
Household solar thermal water heater. via yougen
In both the thermal power systems, massive amounts of mirrors are used to concentrate the widespread sunlight onto a targeted receiver. The harvested heat is then used to make pressurized steam which in turns rev the generator and produces electricity. The two systems, although working on the same principle, has a few differences. The parabolic trough system can be installed in urban areas on a small-scale for power generation or use it for commercial water heating purposes. On the other hand, the tower can only work efficiently if installed in large spaces that get a lot of sunshine year-round. That is why most of the time you will only find the central tower CSP system installed in deserts.
With concentrated solar thermal energy systems, electrical energy production is only possible in wide-open areas like deserts that get a lot of sunlight throughout the year. However, this is also a factor that reduces the efficiency of the system. The energy conversion process involves evaporation and the hot desert conditions result in a loss of the stored water. Water is already difficult to transport to the desert from rivers and reservoirs, and barely ever available under the ground in such areas, thus the efficiency of the system is debatable. Furthermore, although the CSP systems are good for large scale power generation their upfront cost is being challenged by the recent rapid growth of technology in the PV market.
Crude oil vs green energy. via postgrowth
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