Low-temperaturesolar thermal power plants are solar systems conceived for thermal storage. They provide useful heat at temperatures below 65 degrees Celsius through solar energy. They are commonly used as a domestic source of heat energy in small buildings.
These are the components of a low-temperature solar power plant:
Solar collectors: there are several types of collectors, but the most common to be used in a house are flat plates due to their low price. The energy conversion between solar and thermal energy is produced here.
Two water circuits: the primary circuit is closed; the fluid goes to the collector, which increases its temperature. Then, the heated fluid goes to an exchanger to transfer this energy to the secondary circuit.
Heat transfer fluid: it is a fluid treated chemically to improve its thermal efficiencies and to decrease its freezing point.
Heat exchanger: it is the device where heat is transferred from the primary circuit to the second one. Therefore, it works as a diathermal wall because the fluids are not mixed.
Storage tank: it is a regular water storage tank well-isolated where we accumulate heated water waiting to be used.
The circulation of the water inside the circuits can be obtained through a thermosyphon or a pump. A thermosyphon system takes advantage of the difference in density of the water at different temperatures making hot water go up and cold water down. On the other hand, if we use a circulation pump, we will need an external supply of electrical energy.
Uses of Low-temperature Solar Power Plants
There are several uses for this technology. The following applications are the most important:
1. Production of Sanitary Hot Water (DHW)
The main application of solar thermal energy is the production of sanitary hot water (DHW) for the domestic and service sectors. Domestic hot water is used at a temperature of 45 degrees Celsius. This temperature can be easily reached with flat solar collectors that can reach an average temperature of 80 degrees Celsius.
2. Solar Space Heating
Solar space heating is a process of using solar energy to heat up a living or working space. For example, solar thermal panels are installed on the roof of a building and connected to a water heating system. The sun's energy heats up the water in the solar thermal panel, and the hot water is then circulated through the building to heat it up.
This system can be used to supplement or replace traditional fossil fuel-based heating systems, and can provide significant cost savings. Solar space heating can be used in both residential and commercial buildings, and is particularly well suited for use in passive solar buildings where sunlight is utilized to help heat the building.
Solar heating systems are a complement to the traditional heating system, especially for systems that use make-up water below 60 ° C.
For heating with solar input, the system that works best is the underfloor heating system (circuit formed by a network of pipes through the floor), since the temperature of the fluid that circulates through this circuit is about 45 ° C, easily achievable by solar collectors.
3. Pool Water Heating
The use of solar collectors allows energy support in outdoor pools, lengthening the bathing period.
It must be considered that the legislation of some countries does not allow the heating of outdoor swimming pools using conventional energy sources, which are usually non-renewable energy sources.
We can use solar collectors without glass in this application. These solar panels are simply made up of a large number of tiny metal or plastic tubes arranged in a serpentine way through which the water circulates.
Strategies for Conversion from Solar to Thermal Energy
The solar radiation collection system comprises solar collectors connected to each other. Its mission is to capture solar energy to transform it into thermal energy, increasing the temperature of the fluid that circulates through the installation.
The most widespread type of thermal collector is the flat plate solar collector that achieves a temperature of around 60°C at a low cost.
What Is a Flat Plate Solar Collector?
The flat plate solar collector is formed by a metal plate that is heated by solar radiation with its exposure to the Sun (absorber). This plate is black in color so that it does not reflect incident solar radiation. Normally the solar panel is placed in a box with a glass cover. Water is circulated inside the box through a coil or a circuit of tubes so that the heat is transmitted to the fluid through a thermodynamic process.
The effect that occurs is similar to that of a greenhouse, sunlight passes through the glass plate and heats the blackened plate. Glass is a "solar trap", as it lets the radiation from the Sun pass (short wave) but does not let out the thermal radiation emitted by the blackened plate (long wave) and as a consequence, this plate heats up and transmits the heat to the liquid circulating through the tubes.
There are also solar vacuum collectors on the market. They consist of metal tubes covering the metal tube containing the working fluid, leaving a chamber that acts as an insulator between them. They have a very high yield, but their cost is also high.
Parts of a Flat Plate Solar Collector
Solar thermal collectors are made up of the following elements:
Cover: The cover must be made of transparent material. The function of the cover is to minimize thermodynamic heat losses.
Air channel: The air channel is a space that separates the cover from the absorbent plate and may or may not be empty.
Absorbent plate: The absorbent plate is the element that absorbs solar energy and transmits its heat to the liquid that circulates through the pipes. The main characteristic of the absorbent plate is that it has to have a high absorption of solar radiation and low thermal emission.
Tubes or pipes: The pipes touch the absorbent plate to maximize heat energy exchange by thermal conduction. The liquid circulates through the lines that will be heated and will transport the heat to the accumulation tank.
Insulating layer: The purpose of the insulating layer is to cover the system to avoid and avoid thermal losses. The insulating material should have a low thermal conductivity for the best possible insulation.
How Could It Impact Climate Change?
Solar thermal power plants could help to reduce greenhouse gas emissions and climate change. If widely adopted, they could make a significant dent in the amount of carbon dioxide released annually.
In addition, solar thermal power plants can operate with a smaller footprint than traditional power plants, further reducing their environmental impact.
What Are High-temperature Solar Power Plants?
High-temperature solar thermal power plants use solar energy to generate electricity at temperatures above 1000°C using the Rankine cycle.
Two types of high-temperature solar thermal power plants are parabolic troughs and power towers.
Parabolic Trough Power Plants
Parabolic trough power plants use long, curved mirrors to reflect and concentrate sunlight onto a receiver tube that runs the length of the mirror.
The receiver tube is filled with heat-transfer fluid and heated to very high temperatures (up to 400°C) as it flows through the receiver. The hot fluid then flows through a steam generator, transferring its heat to water, creating steam that drives a turbine connected to an electrical generator.
Solar Power Towers
Solar power towers - also called central tower plants or heliostat plants - use an array of flat mirrors (heliostats) to reflect and concentrate sunlight onto a receiver located on top of a tall tower.
The receivers can reach temperatures as high as 1500°C. Water is pumped to the top of the building and flows down through the receiver, where the concentrated sunlight heats it. The hot water then flows through a steam generator, creating steam that drives a turbine connected to an electrical generator.
