The idea of a solar collector is very simple — the sun’s rays heat the liquid flowing through the tubes, which are made of a material that absorbs heat well. In this way, it is possible to effectively obtain hot water for domestic summer needs. Such solar systems work especially successfully in summer. At this time, an ordinary black plastic tank can also cope with the role of a solar system for obtaining hot water, which many summer residents use, building, for example, an impromptu shower from such tanks. But solar energy can be used more efficiently.
How is a solar collector system arranged?
Modern collector solar system is arranged as follows. The sun’s rays heat the coolant in the collector. This liquid circulates from the collector to the heat exchanger in the heating device — a water boiler. Circulation is carried out using a pump. The solar system also includes a control unit (controller) that regulates the operation of the pump, includes protection against overheating and performs some other tasks.
Types of collectors-solar panels
The most important node is solar collectors. They account for most of the cost of the system. There are several options for the device of solar panels, which can be reduced to two types. Each of these types has its own advantages and disadvantages that determine their scope.
In them, tubes with a heat-carrier liquid are placed under a copper absorber plate and enclosed in a common housing. From above, the absorber plate is closed with protective glass, from below, under the tubes, there is a layer of thermal insulation.
Flat-plate collectors are 20–30% cheaper and less efficient, since tubes with coolant have worse thermal insulation (part of the heat goes into the atmosphere). Paradoxically, this shortcoming turns into a virtue, for example, during snowfall.
In them, each tube with a coolant is mounted in a separate cylindrical glass case with a reflective coating. The air from the case is pumped out, hence the name — “vacuum collectors”.
Tubular collectors practically do not heat up due to vacuum, and the snow does not melt on them. Worse tubes tolerate hail, they are more fragile. In general, tubular collectors are good in ideal climates, where hail is rare and snow does not occur. Tubular collectors provide an almost constant supply of heat throughout the day, this is due to the fact that the sun’s rays illuminate the surface of the glass cylinder in the same way, regardless of the position of the sun in the sky.
Comparison of properties of flat and tubular collectors
|Work during the day
|Efficiency varies greatly during the day depending on the angle of incidence of the sun’s rays.
|Efficiency is almost constant throughout the day
|Snow on the panel melts by itself due to partial heat leakage
|The snow does not melt, it needs to be cleaned
|Durability, impact resistance
|High (needs a stronger base)
Collector selection criteria
When choosing a collector, pay attention to the quality of solar panels, system components and the life of the absorber (surface exposed to sunlight).
The cost of a solar system depends on its performance, which is determined by the collector area, geographic latitude, time of year, and a number of other characteristics. The cheapest are Chinese, German panels are more expensive, but their service life is usually higher, and they can be recommended for important tasks, for example, year-round hot water supply.
An accurate calculation of the system should be done by an experienced specialist. Simplistically, we can assume, for example, that a system with a collector with a usable area of 3 m² in the middle zone and in winter can provide the preparation of approximately 150 liters of hot water (with a temperature of about 50 ° C) in 2–3 hours in 2–3 hours. Practice shows that for a small family (two or three people) a solar system with a collector area of 2–4 m² and a boiler with a capacity of 200–300 liters is enough. Such a system will cost about 100–300 thousand rubles. The cost of one module (with an area of approximately 2 m²) of the collector ranges from 200–250$. (Chinese manufacturers) up to 50–600 $. (Ariston, Buderus, Viessmann and other European manufacturers); another 40–600 $. you will have to pay for the boiler and 10–200 $. for the controller, pump and materials needed for installation.
In a small house, solar energy makes it possible to provide up to 60% of the energy needed to produce hot water.
How to install the collector
Heliopolis should be located on the unshaded side: facing the south side or with the location west — east. If such an arrangement is not possible, a correction factor is introduced into the calculations. It is also desirable that there are no tall trees nearby. The angle of inclination of the collectors relative to the horizontal surface is selected according to the geographic latitude and climatic conditions, the optimal angle depending on the season of peak use. If, for example, we design a solar system for preparing hot water in a cottage on the Black Sea coast with a May-September seasonality, this angle will be 20–25 °. But if there is a need to support heating with this solar system, the angle should be 40–45 °.
Solar collector panels are placed in an inclined plane so that during the day they are provided with an angle of incidence of the sun’s rays as close to direct as possible. The optimal slope of the collector plane corresponds to the geographic latitude of the area and is, for example, 57° for world. The direction the panel is facing in the Northern Hemisphere should be south. And of course, the solar collector should not be blocked from the sun by other objects. It is not always possible to withstand all conditions, therefore, when mounting collectors, prefabricated or welded metal structures are widely used, which can be mounted both on the roof and on stand-alone stands.
Choosing a boiler for a solar system
For solar systems, special boilers with a built-in heat exchanger are used, do not confuse them with the usual storage water heaters. Since the production of hot water by the solar system is uneven depending on the time of day, these boilers are chosen with a margin of capacity in order to accumulate heat with their help. Therefore, containers with a volume of 300 liters or more are often used. Similar bivalent water heaters are available from Ariston, Buderus, Viessmann and other manufacturers. In addition to the solar thermal preparation of hot water, these boilers usually provide for the possibility of additional heating from the heating boiler.
Do not forget that if the heat is not removed to heat the water in the boiler, then it is necessary to provide a way to somehow dump the excess heat.
How to avoid overheating
A serious problem with the irregular use of solar systems is overheating of the collector. Overheating can be dealt with in several ways. The easiest option is to use water as a coolant. When water boils, steam is discharged through a special valve, and then the lack of water is taken from the water supply system. This method is bad because the water freezes, so it can only be used where frost is rare (in this case, the system is supplemented with frost protection with forced heating of the coolant).
Another option — draining the coolant and filling the collector with air when the pump is turned off (Drain Back system) — requires a competent calculation of the system, slopes and pipe volumes. There is also the possibility of adjusting the temperature of the coolant by changing the operating modes of the system by the controller. For example, the controller starts a pump at night, which pumps the heated coolant through a flat collector; the system works in reverse. This method is good in the presence of a flat collector (tubular ones are not suitable) and a capacious boiler. Another interesting development was proposed by Viessmann. Its collectors use material with variable reflectivity. When the temperature rises (say, above 90 °C), the absorption capacity of the material decreases several times, and the coolant stops heating up.
In some cases, a malfunction of the solar system can be guessed from the appearance of the collector. So, the appearance of frost on the tubular collector indicates a violation of the tightness of the glass cylinder. It has no thermal insulation, and this section of the collector produces very little heat.