Absorber material
The material from which the absorber is made. It is a layer that absorbs solar energy. It is the main part of the collector; the general specs of the device largely depend on its design.
In most modern models, regardless of type, the absorber is made of copper with a special coating. This metal has a high thermal conductivity and effectively transfers heat to the heating medium. And the coating is used to improve the absorption of sunlight, reduce its reflection and, accordingly, achieve good efficiency indicators.
Another option found in solar collectors is aluminium. It is somewhat cheaper than copper and weighs less, but it is inferior to copper in terms of thermal conductivity and performance.
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.
Max. pressure
The maximum pressure of the heating medium for which the collector is designed. This parameter is indicated only for closed models (see "Loop system") — by definition, open models operate at atmospheric pressure.
The maximum pressure allowed for the collector must not be lower than the operating pressure in the heating system (DHW, heating, etc.) to which it is planned to be connected. And ideally, you should choose a device with a pressure margin of at least 15 – 20% — this will give an additional guarantee in case of various failures and malfunctions.
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.
Glass transparency coef. t
The transparency coefficient of the glass used in the design of the collector.
This parameter describes how much solar radiation falling on the glass passes through it without obstacles. The higher the value of t, the less heat is lost when passing through the glass, the more energy is transferred to the absorber and the higher the collector efficiency. The transparency coefficient is one of the most important factors affecting the efficiency of the device (along with the absorption and reflection coefficients of the absorber — see the relevant paragraphs).
Absorber absorption coef. α
The absorption coefficient of the absorber used in the collector design.
This parameter directly affects the overall efficiency of the absorbing coating and the efficiency of the device as a whole. The absorption coefficient describes how much of the solar energy reaching the absorber is absorbed by it and transferred to the heat carrier. Ideally, this parameter should reach 100%. However, it is extremely difficult and unreasonably expensive to achieve this. Therefore, the absorption coefficient is usually somewhat lower — about 95%; this is more than enough for the efficient operation of the collector. The rest of the energy is reflected as radiation; for more details, see “Absorber emissivity coef ε". Also note here that in the design of tubular collectors, tubes with a special inner coating are often used, which returns the reflected rays to the absorber and increases the actual absorption coefficient.
Absorber emissivity coef. ε
The absorption coefficient of the absorber used in the collector design.
This parameter directly affects the overall efficiency of the absorbing coating and the efficiency of the device as a whole. The absorption coefficient describes how much of the solar energy reaching the absorber is absorbed by it and transferred to the heat carrier. Ideally, this parameter should reach 100%. However, it is extremely difficult and unreasonably expensive to achieve this. Therefore, the absorption coefficient is usually somewhat lower — about 95%; this is more than enough for the efficient operation of the collector. The rest of the energy is reflected as radiation; for more details, see “Absorber emissivity coef ε". Also note here that in the design of tubular collectors, tubes with a special inner coating are often used, which returns the reflected rays to the absorber and increases the actual absorption coefficient.
Stagnation temperature
The stagnation temperature is the maximum heating medium temperature reached in the stagnation mode.
The term "stagnation" in this case means stagnation of the heating medium in the collector, due to which the thermal solar energy supplied to the device is not removed from it. Such a situation can arise when the heat or hot water supply is stopped when the circulation pump is turned off when the circuit is aired or clogged, etc. In this case, the heat carrier temperature can rise significantly up to 200 °C or more. Note that this mode, although unfavourable, is not an emergency — serious problems in the system can occur only with multiple stagnations for a short time.
The stagnation temperature is generally a reference parameter, it does not affect the basic performance and is not the main selection criterion. However, it is generally believed that higher readings are indicative of a higher level and advanced collector design. This is partly justified: a high-temperature model must be efficient enough to absorb a large amount of energy and reliable enough to withstand contact with a heated heat carrier.
