Solar thermal collectors
The sensor is a component of a solar thermal system, responsible for capturing the sun's energy and converts it into heat.
The use condition the type of sensor used; for example, if we heat a pool to a temperature of 25-28 ° C, in spring, we need a simple sensor, as the ambient temperature easily be of this order or even higher. Moreover, if we heat a fluid up to temperatures of 200 sensors we need to concentrate radiation and transferred to a small volume of fluid.
Currently, we can differentiate between the following main types of sensors on the market
- The sensors planes or flat plate.
- Scavengers radiation concentration: parabolic trough CPC concentrators incorporating reflectors.
- The vacuum tube.
In solar applications at low temperature, ie at temperatures below 80C, such as pool heating, producing hot water or even to supply heat, is mostly sensors flat plate used with or glazed uncovered, depending on the application.
Specifications of the solar collectors
The solar panels core components of solar thermal systems, must meet minimum quality requirements. These services are certified by an accredited and must follow a defined laboratory test protocol.
The technical information you need to know about a solar collector is as follows:
- Instant performance curves made by an accredited laboratory.
- Catchment Area.
- Transport and handling instructions of the sensor.
- Capacity and type of heat transfer fluid recommended by the manufacturer.
- Recommended flow rates and pressure drops.
- Maximum operating pressure and test pressure.
- Constitution of the absorber materials and fluid circuit.
- Materials and constitution of the cover of the box.
- Sealing System.
- Types and thicknesses of insulation.
- Temperature stagnation.
- Requirements for maintenance.
Physical principles of operation of the solar collector plane
The solar collector operates from the application of the following physical principles:
The black body (absorber)
The solar radiation is partially absorbed by the body. The rest is reflected or passes through them.
The relationship between these effects depends
- The nature of the body.
- The state of the surface.
- The bulk of the body.
- The type of radiation. The wavelength.
- The angle of the sun.
The greenhouse effect
This effect is generated in some transparent bodies, which are normally only traversed â € <â €
This radiation is reflected can not go into again. A portion of this energy heats the glass and the glass forwards in and out.
Some plastics (such as polycarbonate) have a glass-like behavior (let through shortwave radiation from the sun and stop long-wave emissions coming from the absorber plate).
The third of the physical principles involved in the operation of the sensors is the insulation of the assembly from the outside, usually consists of an inner coating of the container box.
Components of a flat solar collector
The flat solar collector with cover glaze is the facilities used to produce hot water. This team consists of the following elements:
The absorber is formed usually by a metal foil, typically copper (good thermal conductor) which darkens basically
1. A thin film of heat black paint, which can withstand temperatures above 100 ° C work.
2. A selective treatment based on electrochemical deposition or paints with metallic oxides having a high absorption of solar radiation (shortwave) and a low emission of heat (long wave).
The absorber includes a grid of pipes through which circulate the heat transfer fluid.
Its function is to isolate the sensor for external environmental conditions, though let sunlight - that cause the greenhouse effect. It usually consists of a single sheet of tempered glass (resistant) with low iron content (very transparent) 4 mm thick approximately.
This element, as in other applications, serves to prevent heat loss from inside the sensor -specifically the absorbidor- outward and usually consists of sheets of synthetic foam (polyurethane, cyanide, fiber glass, etc.) located on the sides and in the back of the pickup.
The housing serves to accommodate the other components. This closure is typically formed by an anodised aluminum that will ensure joint resistance even in extreme conditions. Also, the housing will have a condensate drain holes in the bottom.
flat solar collectors
The flat plate solar collector has a good cost / effectiveness in moderate climates and a large number of applications (domestic hot water, pool heating, support for heating, preheating industrial fluids, etc adapted correctly .).
We can distinguish two basic types of flat plate collectors, depending on the configuration of the absorber: the parallel of "grid type" in the vertical and horizontal versions and the series of "coil type". Basically, the difference between the two is:
- The parallel configuration favors that can stratify the temperature sensor with the largest circulation of water through the branches inside the sensor and obtain a temperature differential of about 10 in good yield.
- The series configuration consists of a single continuous circuit with low volume of circulating water and a temperature differential higher, in good yield.
Operation of flat solar collector
The operation of a sensor is very basic. In fact, any Solrecibe body exposed to an energy flow which heats and therefore increases the temperature.
A body at a given temperature emits energy around it, in the form of radiation, and this depends directly on the temperature difference between the body temperature and room temperature.
If refrigerate the sensor by passing a fluid through the interior, this heat is used, whereby it is achieved that a part of the collected energy is transmitted to the fluid as useful energy. The rest is still losing energy as radiation from the sensor to the external environment. In this case, the temperature is always less than equilibrium.
If we get good performance, the sensors have to work at the lowest possible temperature, as long as it is sufficient for the intended use given.
The harnessed energy is removed from the sensor by the heat transfer fluid, generally consisting of a mixture of water with antifreeze and corrosion inhibitors.
Other types of solar thermal collectors
planes Sensors without cover
As its name suggests, the collectors without cover element consisting essentially absorber generally comprises a set of plastic tubes, EPDM, or polypropylene.
This type of solar collector are very economical and easy to install as they usually have flexible configurations to place any superficie.Además, are resistant to corrosion and allow direct heating configurations as in the case of pool heating.
In contrast, the collectors without cover with plastic absorber tend to yield curves highly inclined because, having no glass have very good optical performance but, instead, quickly lose their performance when the ambient temperature is below the temperature work or high wind speed.
Therefore, such sensors are only desirable in low temperature applications where the operating temperature is near room temperature, for example, to extend the swimming season in outdoor pools. A variable of this type of sensors without cover sensors are embedded metal plate and offset, specifically designed for closed circuit applications.
This variant has a better thermal performance of the sensor and allows use to produce hot water or other low temperature applications. Another example of this type of sensor available in our market, is a multifunctional cover that combines the qualities of a stainless steel metal cover maintenance with the efficiency of a solar collector selective surface.
parabolic trough solar collectors
The solar concentrator systems are specially designed for high temperatures. Its principle of operation is based on the projection of the whole incident a reflective surface on a particular point radiation.
Until recently, the use of solar thermal systems CCP concentration was restricted to the field of research, industry and electricity generation, as this technology requiring complex tracking systems to locate and supermarkets.
Domestically, it was not justifiable to install sensors with these requirements to reach high temperatures of over 200 ° C.
But currently available in the market, concentration sensors "compact parabolic trough" (CPC) and having a format similar to the dimensions of the flat plate collectors.
The sensors consist of a pipeline with a selective coating which longitudinally traverse the sensor and acting absorber.
These pipes are solar radiation reflected by the curved walls that surround them, thus achieving a higher amount of energy per unit area of the absorber (W / m2).
The result of all this is a sensor capable of generating high temperatures up to 200 ° C with standard radiation levels.
Therefore, such sensors are used both to produce and support sanitary hot water heating or cooling equipment to feed by adsorption or absorption.
Solar collectors of vacuum tubes
This sensor consists of a set of cylindrical tubes, consisting of a selective absorber, a reflector located on settlement and surrounded by a transparent glass cylinder.
Between the transparent outer tube and the absorber interior it is evacuated. With this, the losses by conduction and convection is prevented from absorbing surface and this fact can achieve temperatures of over 100 ° C and use much more solar radiation.
Currently, there are two types of these sensors, quite different in terms of heat transmission from the vacuum tube to the primary circuit techniques.
On the other hand, vacuum gauges with direct fluid, have a major gauge of the primary circuit on top of the sensor; each tube is connected to the sensor through a circuit outward and return. By this circuit circulates the primary circuit fluid to be heated during the tour.
The internal circuits of the tubes have two basic configurations. Concentric separate circuit or circuit
The other trend of heat transfer in the vacuum tubes is called heat pipe.
In this system, as in the previous case, the sensors have a main sensor in the primary circuit at the top but in this case, the vacuum tube has a central copper tube sealed and filled with an alcoholic mixture.
This tube is connected to the solar collector absorber by a condenser pipette, which will feature heat-transmitting element to the primary circuit.
Solar radiation absorber and makes heat evaporates the alcohol mixture inside, which rises to the condenser at the sensor.
At this point, assign the heat in the primary circuit and the alcohol mixture are returned to the bottom of the sensor by its own weight is condensed.
One of the specific characteristics of this type of sensors is the possibility, depending on the model, bend each tube individually. This feature is very useful in locations for building requirements, necessary to mount the sensors disoriented south.
Connection of solar thermal collectors
A defining and assembling the solar field, we must bear in mind that you should make a distribution of the sensors in groups.
These groups of collectors should always consist of units of the same model and a distribution as uniform as possible.
There are two basic types options or to group two or more sensors
From series or parallel type. In addition, you can configure a catchment area by combining the two groups, which is what we call clusters or mixed circuits.
In the series circuit, the output of the first sensor is connected directly to the input of the next, and so on. The fluid temperature sensor input exceeds each of the preceding sensor, so that the output of a group of sensors can obtain higher temperatures than if we work with the temperature swing of a single sensor.
This connection type has the disadvantage that the performance of the sensors decreases proportionally with the increase of the working temperature; therefore, this type of connection is only used in very specific applications, and with a maximum of 6-10 m2 of collectors connected in series, according to the climatic zone.
As for the hydraulic behavior of this configuration, the total flow of the group of sensors will be equivalent to a single flow sensor and, on the contrary, the loss caused by the group will be equal to the sum of loss all load sensors.
In the parallel connection, both the input and output sensors are connected to common points of entry and exit to other collectors.
With this configuration, the inlet fluid temperature is the same for all sensors and so with outlet temperatures, so that the output of the battery or group of sensors collect temperature and if we work with temperature swing of a single sensor.
Therefore, all the sensors work on the same point on the yield curve. This connection is the most common solar thermal systems in low temperature.
Hydraulically, this configuration presents a total flow rate of the group which is the sum of the partial flows each sensor, while the pressure drop of the group, will be equivalent to that of a single sensor.
From the diagrams reproduced in figures you can see the basic differences in connecting sensors comprising a battery.
Mixed Connecting sensors
In some cases, the volume of the sensors and / or the need for higher temperatures result in facilities that combine the series arrangement and we call this type paralelo.A mixed.
Balancing the field of view
There are basically two methods for balancing the flow of heat transfer fluid in the collector field
Elements of connecting sensors
The field of solar thermal collectors are, by definition, a hydraulic circuit which transmits the heat received from the sun in the heat transfer fluid. An important aspect on site systems are connection between the various sensors, the solar field and the pipelines that transport and distribute the heat transfer fluid to the point of consumption or heat exchange.
The connections between sensors rely heavily on the manufacturer, because the market there are a variety of finishes and terminal connection between sensors.
Last review: September 28, 2015