Heating systems
Hay varios sistemas de conseguir calentar el agua por el sol, aunque la mayoría se basan en la utilización de colectores solares planos.
Installation with collectors and independent accumulation
Flat solar collectors are used and sometimes other types, such as vacuum tubes, although flat ones are usually much cheaper because they are manufactured in large series. The collectors are connected to an accumulator tank[20] that, on average, would have the necessary water capacity for one day of use (between about 22...30 L/day per possible inhabitant of the building and at a temperature of 60 °C).
The connection of the collectors is normally made with a specific circuit (primary circuit), which runs through a heat transfer unit, and goes to an exchanger in the storage tank. This independent circuit has two important advantages:
The regulation of the operation of the primary circuit is done through a device called differential thermostat. It consists of an electronic control unit and two thermal probes; The probes are located, one at the outlet of the collectors and the other in the accumulator tank, and they send the temperature measurement of each of these points to the control unit that compares them: when the temperature of the collectors is higher than that of the tank, it starts the recirculation pump, otherwise it stops it. In cold places, where frost is rare, the differential thermostat can start the pump when the collector probe registers temperatures equal to or lower than 4 °C, heating the collectors with the accumulated water and avoiding the use of antifreeze, which is expensive; It is not convenient to use this possibility in places with frequent frosts as the accumulated water may become too cold and the collectors may freeze.
As has been said, it is neither economical nor practical to make an installation capable of providing the necessary energy to heat the water at any time, so auxiliary energy must be added to this system. For reasons of energy saving, two independent accumulators must be used; In the first the water will be heated exclusively by solar energy and in the second the preheated water will enter and, if necessary, it will be reheated to the distribution temperature (normally 60 °C) using auxiliary energy. Indeed, the power with which the sun heats is much lower than that of any heating system using current energy and that means that, if the auxiliary energy exchanger were in the tank, the water would be heated with the auxiliary energy much faster than with the sun and the differential thermostat would stop the pump, unless the collectors reach a high temperature, conditions in which the collectors have lower performance. If reheating occurs in another tank, the collector system takes full advantage of the sun's capacity at all times. It is a more expensive installation, but it pays for itself in a short time. And indeed, in the case of Spanish regulations, it is prohibited to add this energy to the same tank where the water heated by solar means has been accumulated.[21].
A so-called instantaneous heater can also be used, which must be of the modulating type, with which the second accumulator would not be necessary.
"Autonomous" solar heater
These are devices that allow energy from the sun to be captured and transferred to water, generally in a single device, with the necessary devices. The heated water is stored in a storage tank and can be consumed directly or connected in series with an auxiliary heater.
There are two types: In one of them, the panels have a receiving plate and tubes through which a heat transfer liquid circulates attached to it. The receiver (generally covered with a dark selective layer) captures solar radiation and heats it, while the water circulating through the tubes transports the heat to the storage tank. The tank is located at a higher height than the collector (figure on the left), so that the water circulates through a thermosyphon: when heated it increases in volume and loses density, so it rises to the accumulator, while the colder, higher density water goes down through another pipe at the back, towards the collector, where it is heated. It is a simple device with good performance, with some caveats. It does not work well in places where there is serious frost (you cannot use antifreeze, since the water that circulates is for consumption) and it does not work well in places where the water is hard, for the reasons stated above.
The second type is much simpler (figure on the right) and, if manufactured in a certain quantity, cheaper. There are several versions, more or less similar. One of them consists of a tank painted black with reflective covers that open during the day to collect a greater amount of sun, and close at night to isolate the tank from heat loss. The tank can have a transparent cover to increase the greenhouse effect.
In these two types, a reheating system with an auxiliary tank, or a modulating heater, can be added later, as in the previous case.
Another much simpler and cheaper variant consists of a black painted tray, with a flexible polyethylene cover. In the mornings it is filled with water, inflating the cover like a balloon, and at night it is emptied to use the hot water. With this system the problem of frost is avoided, but it implies being attentive to morning fillings and evening emptying.
Solar heater without cover
It is not especially used to heat water for sanitary uses, but it is a cheap heating system for seasonal uses, such as heating swimming pools. It is a surface made of rubber, plastic materials or waterproof fabrics, black in color, which has a water circuit formed by the material itself. It is spread over an exterior surface and water is circulated through the circuit. As an example of the operation of this system, we can cite the effect that the sun has on a hose placed in the sun and full of water: when the tap to which it is connected is opened, the first liters, those that were contained in it, in the sun, come out hot, the more so the darker the color of the material of the hose and the longer the hose has remained in the sun.
As mentioned, it can be used in swimming pools and, when the swimming season comes to an end, it is collected and stored until the following summer. It can also be used to heat camp showers.
It is economical and does not require too much care. In swimming pools it can be operated with the same pumps that operate the purifier, and in showers it will be necessary to install one, which may be small, with little energy consumption.