Absorber area
The total area of the absorbing surface of the collector. For kits with multiple collectors (see "Number of collectors"), the area for one device is indicated.
Note that the meaning of this parameter depends on the type of collector (see the relevant paragraph). In flat devices, we are talking about the working area — the size of the surface that is exposed to sunlight. In tubular models (vacuum, thermosiphon), where tubes play the role of an absorber, the total surface area of the tubes is taken into account — including that which is “in the shade” during operation and is not heated by the sun. Special reflectors can be used to overcome this problem.
All of the above means that only collectors of the same type and similar design can be compared with each other in terms of absorber area. If we talk about such a comparison, then a large area, on the one hand, provides greater efficiency and heating speed, and, on the other hand, it accordingly affects the dimensions of the device and the amount of space required for its installation. Thus, the total area of a flat collector approximately corresponds to the area of the working surface; it is slightly larger, but this difference is small. But in tubular models, there is a paradox when the total area is less than the absorber area.
Aperture area
Collector aperture area; in sets of several devices (see "Number of collectors") is indicated for one collector.
The aperture area is, in fact, the working area of the device: the size of the space directly illuminated by the sun. In flat models (see "Type") this size corresponds to the size of the glass surface on the front side of the collector; in this case, the aperture area is usually either equal to the area of the absorber (see the relevant paragraph) or slightly less (because the edges of the collector can cover the edges of the absorbing surface. But in tubular collectors (vacuum, thermosiphon), the aperture area can be measured in different ways, depending on the presence of a reflector. If it is present, the working area is equal to the absorber area, since the tubes are irradiated from all sides. If a reflector is not provided, then the aperture area is taken as the sum of the projection areas of all tubes; projection length at this corresponds to the length of the tube, the width to the inner diameter of the glass bulb or the outer diameter of the inner tube, depending on the design.
The aperture area is one of the most important parameters for modern solar collectors; many performance specs depend on it. At the same time, by recalculating these specs per 1 m2 of the aperture area, one can compare different models (including those belonging to different types) with each other.
Tube type
The type of tubes used in the design of the solar collector — vacuum or thermosiphon (see "Type").
— Coaxial vacuum direct heating. The simplest type of vacuum tube: is a hollow absorber tube enclosed in a glass vacuum tube. Such a tube has double walls, between which there is a vacuum, which provides the necessary degree of thermal insulation. And the term "direct heating" means that the heat carrier circulates directly in the inner tube, receiving heat through contact with the walls of the absorber.
The main advantages of direct heating tubes are simplicity and low cost. It is considered that they are poorly suited for year-round collectors. However, modern technologies allow for a very high degree of thermal insulation, due to which year-round systems of this type are also available on the market today. The situation is similar with the use in closed systems (see "Loop system"): direct heating elements are somewhat less suitable for such an application than more advanced vacuum tubes (like a heat pipe). Nevertheless, in addition to open ones, there are also closed systems with direct heating. However, the disadvantage of this option anyway is the relatively low efficiency.
— Coaxial vacuum heat pipes. The outer shell in such an element is glass, with double walls and a vacuum between them (like a thermos), but the inner part is just a heat pipe — a sealed flask (usually copper) filled with a special medium liquid with a low evaporation t...emperature. The upper part of this tube is led into the manifold (heat exchanger case), it has an enlarged size and plays the role of a radiator. The whole system works as follows: sunlight heats the heat pipe, and heating medium vapours rise to its upper part, where they condense and transfer heat through the radiator walls to the water moving along the manifold. The condensate flows back to the bottom of the heat pipe, after which the process is repeated.
Coaxial tubes with a heat pipe are more complex in design than direct heating systems, and, naturally, are more expensive. On the other hand, they are more efficient and can be used without restrictions in high-pressure closed collectors, as well as all-weather systems. In addition, devices with this principle of operation are easy to repair: if one of the tubes breaks, you do not need to change the entire manifold — just replace the tube itself. This does not cause any particular difficulties and can be carried out directly at the installation site, without dismantling the entire structure.
— Coaxial vacuum U-type. Vacuum tubes equipped with U-shaped heat exchangers. Such a heat exchanger has the form of a thin pipeline passing from the manifold body along the entire length of the tube and back; the pipeline is usually shaped like the letter U, hence the name. The manifold itself, usually, is made two-pipe: cold water enters the collector through one pipe (the inputs of U-shaped heat exchangers are connected to it), and the heated water is discharged through the other (the outlets of the heat exchangers are connected to it).
Such a design allows to achieve high efficiency rates in combination with excellent thermal insulation: water does not come into direct contact with the walls of the absorber, which is especially important when used in cold weather. And with the use of U-type tubes in closed systems (see "Loop system"), there are no problems either. Among the drawbacks, in addition to the rather high cost, one can name high hydrodynamic resistance and sensitivity to the quality of the heating medium. In addition, such collectors are difficult to repair: the pipes and manifold are a single unit, and to fix problems it is often necessary to remove the entire structure from the roof, and it is impossible to replace a single pipe.
— Feather vacuum tubes. Feather vacuum tubes are a kind of modification of heat pipe systems (see the relevant paragraph). In them, the heat pipe is placed not in the inner tube, but on a flat absorber, and the whole structure is installed inside a glass flask from which the air is evacuated. Feather systems are highly efficient because the absorber does not heat the air inside the flask, but transfers almost all the energy to the heating medium; however, they are not cheap. In addition, such systems are quite difficult to install, and if the tube fails, it will inevitably have to be changed entirely (although there are usually no problems with the replacement itself). It is also worth noting that feather tubes are more dependent on the angle of incidence of light than devices with a traditional round absorber.
Efficiency
Collector efficiency.
Initially, the term "efficiency" refers to a characteristic that describes the overall efficiency of the device — in other words, this coefficient indicates how much of the energy supplied to the device (in this case, solar) goes to useful work (in this case, heating the medium). However, in the case of solar collectors, the actual efficiency depends not only on the properties of the device itself but also on environmental conditions and some features of operation. Therefore, the specs usually indicate the maximum value of this parameter — the so-called optical efficiency, or "efficiency at zero heat loss." It is denoted by the symbol η₀ and depends solely on the properties of the device itself — namely, the absorption coefficient α, the glass transparency coefficient t and the efficiency of heat transfer from the absorber to the coolant Fr. In turn, the real efficiency (η) is calculated for each specific situation using a special formula that takes into account the temperature difference inside and outside the collector, the density of solar radiation entering the device, as well as special heat loss coefficients k1 and k2. Anyway, this indicator will be lower than the maximum — at least because the temperatures inside and outside the device will inevitably be different (and the higher this difference, the higher the heat loss).
Nevertheless, it is most convenient to evaluate the specs of a solar collector and compare it with oth...er models precisely by the maximum efficiency: under the same practical conditions (and with the same values of the coefficients k1 and k2), a device with a higher efficiency will be more efficient than a device with a lower one. .
In general, higher efficiency values allow to achieve the corresponding efficiency, while the collector area can be relatively small (which, accordingly, also has a positive effect on dimensions and price). This parameter is especially important if the device is planned to be used in the cold season, in an area with a relatively small amount of sunlight, or if there is not much space for the collector and it is impossible to use a large-area device. On the other hand, to increase efficiency, specific design solutions are required — and they just complicate and increase the cost of the design. Therefore, when choosing according to this indicator, it is worth considering the features of the use of the collector. For example, if the device is bought for a summer residence in the southern region, where it is planned to visit only in summer, relatively little water is required and there are no problems with sunny weather — you can not pay much attention to efficiency.
Expansion vessel
The volume of the expansion vessel supplied with the solar collector.
The expansion vessel is designed to compensate for the thermal expansion of the liquid in the heating system. With an increase in temperature and the volume of liquid, its excess enters the expansion vessel, which avoids a critical increase in pressure and damage to system components. The volume of the tank must be large enough to handle thermal expansion at the maximum intended operating temperature. One can calculate the minimum capacity using special formulas; they take into account the total volume and characteristics of the heating medium, temperature differences, design features of the heating system, etc.
Note that the expansion vessel is possible to buy separately; however, in some situations, it is more convenient (or even cheaper) to purchase it with a solar collector.
Electric heater
The presence of a heating element — a tubular electric
heater — in the design of a solar collector.
The heating element consists of a heating filament, a metal tube in which it is enclosed, and an insulating material with good thermal conductivity (sand, oil, etc.) that fills the space between the filament and the tube. Such a device plays the role of an additional heater that increases the overall efficiency of the collector: if the heating from the sun is not enough, you can additionally connect the heating element and achieve the required temperature. However, electricity is required, and for powerful heating elements, a specific connection format may also be needed (directly to the distribution board, or even to three phases of 400 V). Also, do not forget that this function is associated with additional costs: if solar energy comes “for free”, then you have to pay for electricity. Nevertheless, a solar collector with a heating element is still much cheaper than a purely electric boiler or a boiler of the same power.
