The function of a solar thermal installation is to take advantage of solar energy to generate heat. The solar panels of these facilities capture the heat of the solar radiation that falls on them to heat a fluid. The different ways to take advantage of this hot fluid allows us to use this type of renewable energy in multiple applications.
A solar thermal installation consists of:
- Solar collectors
- Primary and secondary circuits
- Heat exchanger
- Accumulator, pumps
- Glass of expansion
- Main control panel.
Solar collectors are the elements that capture solar radiation and convert it into thermal energy, into heat. Solar collectors are known for flat plate, vacuum tube and absorber collectors without protection or isolation. The flat (or flat plate) collection systems with glass cover are the common ones in the production of sanitary hot water. The glass lets the rays of the Sun pass, they heat metal tubes that transmit heat to the liquid inside. The tubes are dark in color, as dark surfaces heat up more.
The glass that covers the collector not only protects the installation but also allows to conserve the heat producing a greenhouse effect that improves the performance of the collector.
They consist of a closed aluminum shell resistant to marine environments, an eloxat aluminum frame, a silicone-free perimeter gasket, thermal insulation that respects the rock wool environment, high transparency solar glass cover, and finally by ultrasonic welded tubes.
The solar collectors are composed of the following elements:
- Cover: It's transparent, it can be present or not. It is usually made of glass although they are also used in plastic, as it is less expensive and manageable, but it must be a special plastic. Its function is to minimize the losses by convection and radiation and therefore it must have a solar transmittance as high as possible.
- Air channel: It is a space (empty or not) that separates the cover of the absorbent plate. Its thickness will be calculated taking into account the purpose of balancing the losses by convection and the high temperatures that can occur if it is too narrow.
- Absorbing plate: The absorbing plate is the element that absorbs solar energy and transmits it to the liquid that circulates in the pipes. The main characteristic of the plate is that it must have a great solar absorption and a reduced thermal emission. As common materials do not meet this requirement, combined materials are used to obtain the best absorption / emission ratio.
- Tubes or conduits: The tubes are touching (sometimes welded) the absorber plate so that the exchange of energy is as large as possible. For the tubes circulates the liquid that will heat up and go to the accumulation tank.
- Insulating layer: The purpose of the insulating layer is to cover the system to avoid and minimize losses. Why the insulation is the best possible, the insulating material should have a low thermal conductivity.
Types of solar collectors
The three main types of solar collectors are:
- Flat plate thermal solar collectors
- Thermocouple solar vacuum tube collectors
- Parabolic solar thermal collectors
In flat plate solar collectors, the core of the system is a vertical gate of metal tubes, to simplify, which conduct the cold water in parallel, connected by a horizontal tube down to the cold water intake and above by another similar to the return.
In the system of thermal solar collectors of vacuum tubes, the metal tubes of the preceding system are replaced by glass tubes, encapsulated, one by one, in another glass tube between which the vacuum is made as insulation.
In parabolic solar collectors, the collectors have a parabolic shape to concentrate all the solar radiation received on a surface at a point through which the thermal fluid passes. These systems are generally used to generate steam and with the steam to drive turbines with which to obtain electricity. It is a way to obtain electrical energy through solar energy if you use photovoltaic panels.
The primary circuit of a solar thermal installation, is a closed circuit, transports the heat from the collector to the accumulator (system that stores heat). The heated liquid (water or a mixture of substances that can transport the heat) carries the heat to the accumulator. Once cooled, it returns to the collector to reheat, and so on.
The heat exchanger heats the drinking water through the heat captured from the solar radiation. It is located in the primary circuit, at its end. It has a serpentine form, since this way, it is possible to increase the contact surface and, therefore, the efficiency.
The water that enters the accumulator, as long as it is colder than the coil, will heat up. This water, heated during sun hours, will be available for later consumption.
The accumulator is a tank where the heated water useful for consumption accumulates. It has an entrance for cold water and an outlet for hot water. The cold enters underneath the accumulator where it is with the exchanger, as it heats it moves upwards, which is where the hot water will come from for consumption.
Internally, it has a system to avoid the corrosive effect of the hot water stored on the materials. On the outside it has a layer of insulating material that prevents heat loss and is covered by a material that protects the insulation from possible dampness and shock.
The secondary or consumption circuit, (open circuit), enters cold supply water and at the other end the heated water is consumed (shower, sink, ...). The cold water passes through the accumulator first, where it heats the hot water until it reaches a certain temperature. The hot water pipes from the outside must be covered by insulators.
The hydraulic pumps, if the installation is forced circulation, are recirculation type (there are usually two per circuit), working one half of the day, and the couple, half the time remaining. The installation consists of the clocks that run the system, they exchange the pumps, so that one works the first 12 hours and the other the remaining 12 hours. If there are two pumps in operation, there is the advantage that in the event that one stops working, there is a substitute, so that the process can not be stopped when one of them fails. The other reason to consider, is that thanks to this exchange the pump does not suffer so much, but it is left to rest, cool, and when it returns to be in good condition (after 12 hours) it is put back on track. This causes the pumps to be able to lengthen the operating time without having to do any kind of previous maintenance.
In total and as defined above, there are usually 4 pumps, two in each circuit. Two in the primary circuit that pump the water from the collectors and the other two in the secondary circuit that pump water from the accumulators, in the case of a forced circulation type installation.
Nothing to do with the hydraulic pumps used in a hybrid system of solar energy and hydraulic power.
The expansion vessel absorbs volume variations of the heat transfer fluid, which circulates through the collector ducts, maintaining the adequate pressure and avoiding losses of the mass of the fluid. It is a container with a gas chamber separated from the one of liquids and with an initial pressure which depends on the height of the installation.
What is most used is with closed expansion vessel with membrane, without mass transfer on the outside of the circuit.
The pipes of the installation are covered with a thermal insulation to minimize the thermodynamic loss with the surroundings.
There is also a main control panel in the solar thermal installation, where temperatures are shown at every moment (a thermal regulator), so that the operation of the system can be controlled at any time. The clocks responsible for the exchange of bombs also appear.