A solar thermal collector, also known as a solar thermal collector, is a component of a solar thermal installation. A solar collector is a type of solar panel responsible for capturing solar radiation and transforming it into thermal energy. For this reason, this renewable energy source is called solar thermal energy.
The objective of this type of solar panel is to carry out an energy transformation: the solar irradiation experienced by the solar modules is transformed into thermal energy. In some types of solar thermal installations this heat is used to generate steam, and obtain electricity, but it is not the function of the solar collector. On the other hand, photovoltaic panels have the ability to generate electricity directly in the form of direct current. Photovoltaic panels are an indispensable element of photovoltaic solar energy installations.
From a physical point of view, solar thermal collectors use thermodynamics to perform energy transformations. In contrast, photovoltaic panels do not use the laws of thermodynamics to convert solar energy, but rather it is an electrical process.
Types of solar thermal collectors
Of solar collectors there are many types. The solar collector used will depend on the use that will be given. For example, if we want to heat a pool to a temperature of 25-28 degrees Celsius, in spring, we need a simple solar collector, since easily the ambient temperature will be of this order or even higher. On the other hand, if we want to heat a fluid to temperatures of 200ºC we will need solar concentration collectors to concentrate the solar radiation and transfer it to a small volume of fluid.
Currently, in the solar energy market we can differentiate the following types of solar thermal collectors:
- The flat or flat plate solar thermal collector. This type of solar panel captures the solar radiation received on a surface to heat a fluid. The greenhouse effect is often used to capture heat.
- The solar thermal collector of concentration of solar radiation. This type of collector captures the radiation received on a relatively large surface and concentrates it through mirrors on a smaller surface.
- The solar thermal collector of vacuum tubes. This solar thermal collector consists of a set of cylindrical tubes, formed by a selective absorber, located on a reflecting settlement and surrounded by a transparent glass cylinder.
In solar applications at low temperature, mostly flat plate solar thermal collectors are used. A solar application is considered to be carried out at a low temperature when the temperature of the working fluid is below 80 ° C; such as the heating of swimming pools, the production of domestic hot water or even to supply heating. These flat plates used can be carried without glazed cover or not depending on the application.
Flat plate solar collectors
The soul of a flat plate solar collector is a vertical gate of metal pipes, to simplify, that conduct cold water in parallel, connected from below by a horizontal tube to the cold water outlet and from above by another similar to the return.
The grill is embedded in a cover, as described above, usually with double glass up and insulating behind.
In some models of flat plate solar collectors, the vertical tubes are welded to a metal plate to take advantage of the insolation between tube and tube.
Solar collectors of vacuum tubes "all glass"
In a solar collector of vacuum tubes the metal tubes of the preceding system are replaced by glass tubes. The glass tubes are encapsulated, one by one, in another glass tube between which the vacuum is made as insulation.
The great advantages of solar vacuum tube collectors are their high performance. On the other hand, in case one of the tubes breaks down, it is not necessary to change the entire panel to a new one, but only the affected tube must be replaced. On the contrary, as an inconvenience we have that, in relation to flat plate solar collectors, these are more expensive.
Solar collectors of vacuum tubes with "heat pipes" by phase change
The solar panels that use this system take advantage of the phase change from vapor to liquid inside each tube, to deliver energy to a second transport liquid circuit.
The elements are closed tubes that contain the liquid that, when heated by the sun, boils and turns into steam. These tubes are usually copper. The generated steam rises to the top where there is a wider head (condensation zone). The outer part of the condensation zone is in contact with conveying liquid. Because the transforming liquid is at a temperature below the vapor of the tube, it captures the heat thanks to a thermodynamic convection process and causes the steam to condense. The condensed liquid falls back into the lower part of the tube to start the cycle again.
The liquid in the tube can be water that will have a low boiling point to work even with insolation of infrared rays in case of cloud. The heat pipe can be wrapped with insulating materials to minimize irradiation losses.
Finally, the heat pipe is closed inside another glass tube between which the vacuum is made to insulate. Resistant glass tubes are usually used to reduce damage in case of small slush.
Concentration solar collectors
In solar energy a concentration solar collector is a solar panel that uses a technique of concentration of solar radiation to obtain high temperatures. This technique is used in high and very high temperature solar installations.
Depending on the energy application that you want to give there are different technologies to apply this technique of obtaining renewable energy. In this way, we can find, by way of example, parabolic trough solar collectors or concentrating solar furnaces to obtain higher temperatures.
Components of a solar collector
Standard solar collectors are composed of the following elements:
- Cover: The cover of a solar collector is transparent, may or may not be present. It is usually made of glass, although plastic is also used, since it is less expensive and manageable, but it must be a special plastic. Its function is to minimize losses due to convection and radiation and therefore must have the highest possible solar transmittance. The presence of the roof improves the thermodynamic performance of the solar panel.
- 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 convection losses and the high temperatures that can occur if it is too narrow.
- Absorbent plate: The absorbent plate is the element that absorbs solar energy and transmits it to the liquid that circulates through 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 absorbent plate so that the energy exchange is as large as possible. For the tubes circulates the liquid that will be heated and will go to the accumulation tank.
- Insulating layer: The purpose of the insulating layer is to cover the system to avoid and minimize losses. Because the insulation is the best possible, the insulating material should have a low thermal conductivity to reduce the thermodynamic transfer of heat to the outside.
- Accumulator: the accumulator is an optional element, sometimes it is an integral part of the solar panel and, in these cases, it is often visible immediately above or in the immediate vicinity. Very often the accumulator is not part of the solar panel but of the thermal system.
Use of solar collectors
Solar collectors are mainly used to supply hot water and heating or to generate electricity.
In the case of collectors for domestic hot water and heating, the tank stores the domestic water that contacts the fluid by means of a coil. The coil allows the fluid to transfer the stored heat energy to the water without contaminating the water. This water can be used as hot water in homes (80% integration) or can be used to complement the floor heating of the rooms (10% integration). Thermal solar panels are capable of supplying hot water in good quantities, but cannot completely replace the usual heating methods due to the lack of solar energy.
Solar collectors intended for electricity generation require that the heat exchanger be heated until it is boiling. Once the liquid has completed the thermodynamic phase change and has passed into the gas phase, it is sent to a thermoelectric turbine that will convert the movement of water vapor into electrical energy. This type of system is called solar thermodynamics and requires large spaces for the installation of solar panels and a constant presence of sun. Examples of these plants have been installed in the deserts.
Connection of solar thermal collectors
When defining and installing a solar thermal installation, it must be taken into account that a distribution of solar collectors should be made in groups.
These groups of solar thermal collectors should always be formed by units of the same model and with a distribution as uniform as possible.
There are two basic options or types to group two or more collectors: series or parallel. In addition, a collection field can be configured by combining the two groupings, which is what we call mixed groupings or circuits.
Connection of solar collectors in series
In series connection, the output of the first solar collector is connected directly with the input of the next, and so on. The temperature of the inlet fluid to each collector is higher than that of the preceding collector, so that at the exit of a group of collectors we can obtain higher temperatures than if we worked with the thermal jump of a single collector.
This type of connection has the disadvantage that the performance of the sensors decreases proportionally with the increase in the working temperature; Therefore, this type of connection is only used in very particular applications, and with a maximum of 6 to 10 m2 of solar thermal collectors connected in series, depending on the climatic zone.
Regarding the hydraulic behavior of this configuration, the total flow of the group of solar panels will be the equivalent of the flow of a single panel and, on the contrary, the loss of load caused by the group will be equivalent to the sum of the loss of load of all solar collectors.
Connection of solar collectors in parallel
In the connection of parallel solar collectors, both the output and the input of the collectors are connected to common input and output points to the rest of the collectors.
With this configuration, the temperature of the inlet fluid is the same in all the collectors and the same happens with the outlet temperatures, so that at the exit of the battery or the group of sensors we obtain the temperature as if we worked with the jump single collector thermal.
Therefore, all collectors work at the same point on the performance curve. This connection is the most common in low temperature thermal solar installations.
Hydraulically, this configuration presents a total flow of the group, which is equivalent to the sum of the partial flows of each collector, while the loss of load of the group, will be the equivalent to that of a single solar thermal collector.
Mixed connection of solar collectors
On some occasions, the volume of the collectors and / or the need for higher temperatures results in installations that combine the arrangement in series and in parallel. We call this type of connection of solar panels mixed.
Balancing the pickup field
Basically, there are two methodologies to balance the flow of heat transport fluid in a field of solar thermal collectors:
- Matching the length of the circuits. This is achieved with what is called inverted return. That is, equalize the length of connecting pipes of all solar collectors so that the fluid has the same pressure drop.
- Inserting load losses through the provision of balancing valves at the inlet of the collector batteries. Thus, at a shorter circuit length, more pressure loss is applied with the corresponding valve.