Comparison ALTEK SC-LH1-30 vs ALTEK SD-T2L-24

— Vacuum. All solar collectors that use vacuum-based thermal insulation are called vacuum — including flat models (see the relevant paragraph). However, in our catalogue, only tubular devices that are not related to thermosiphons (see the relevant paragraph) and are capable of operating all year round are included in this category.

In all tubular models, the role of absorbing elements is played by specially designed vacuum tubes that transfer solar energy to the water inside and, at the same time, almost do not release heat to the outside. It ensures high efficiency and minimum heat loss. Another important advantage of such devices over flat-plate collectors is their increased efficiency in terms of receiving energy: the tubes work well at almost any angle of sun rays and even in cloudy weather. At the same time, tubular vacuum collectors are also noticeably easier to install. The structure is installed in parts: first the frame, then the heat exchanger housing, then the tubes themselves. And most models allow you to change only individual tubes in case of breakdowns.

If we compare vacuum collectors with thermosiphon ones, vacuum ones are more efficient and can be used for heating (including in the cold season, at temperatures below zero), but it is more complicated and more expensive.

— Flat. A relatively inexpensive type of solar collector and the simplest type of s...uch device, massively presented on the market. On the front of such a device, there is a transparent coating (made of special glass or transparent plastic), under it there is an absorbing layer (absorber) with a heat-conducting system, and a thermal insulating layer is provided on the backside.

Theoretically, such systems are capable of heating the water inside to a temperature of about 200 °C. At a low cost, they have good efficiency in the warm season. On the other hand, flat-plate collectors have a low degree of thermal insulation, which significantly reduces their efficiency in the autumn-winter period. There are improved varieties of such devices — in particular, devices that use a deep vacuum instead of a heat-insulating layer (do not confuse them with vacuum collectors — see the relevant paragraph). They can work at low temperatures; however, they are more expensive, and the actual efficiency is still highly dependent on the angle of the sun rays.

Also, note that flat-plate collectors can be quite difficult to install: the collector has to be lifted and installed as a whole, which in some conditions causes inconvenience. Yes, and in the event of a breakdown, you have to change such a device entirely.

— Thermosiphon. Thermosiphon is a specific type of vacuum collector (see the relevant paragraph). They are designed for use in the warm season, from spring to autumn. In winter, when the temperature is below zero, the water in such collectors freezes and they become useless.

On the one hand, thermosiphons are less versatile than other vacuum models: they are limited in time of the year and cannot be used for heating (in cold weather, when heating is most relevant, the collector becomes useless). On the other hand, such devices have certain advantages: they are simpler, cheaper, more compact and easier to install. Among the best options for using thermosiphon systems are summer cottages, hotels and other places where people stay mainly in summer.

— Hybrid. A specific type of equipment that combines the capabilities of a solar collector and a photovoltaic cell. The solar cell, usually, is located on the outside, and under it is the collector itself. An interesting feature of such models is that at high air temperatures and intense sunlight, they are more efficient in generating electricity than traditional solar panels. The fact is that photovoltaic cells do not tolerate heating up to temperatures of 50 °C and above — their efficiency drops sharply. And in a hybrid cell, the solar collector also plays the role of a cooling system, removing excess heat from the solar cell and reducing its temperature. On the other hand, the thermal efficiency of such models is lower than that of specialized collectors of a similar size — a significant part of the solar energy is absorbed and dissipated by a solar cell. Another disadvantage of such devices is their high cost. In addition, solar energy requires not only batteries but also complex control systems, storage batteries, etc.; and although the energy itself is free, the equipment for its production is also expensive. Thus, this option is much less common than other types of solar collectors.

— Open. Open collectors are called collectors that work without additional pressure in the water circulation system. Usually, such a device is equipped with a tank in the upper part, into which a supply of water is poured; after that, the water by gravity flows to the tap. At first glance, open systems are not very convenient: they need to be placed higher (and the pressure will depend on the height difference between the collector and the water tap), while it is necessary to think over the way to fill the tank (bring a hose with a pump to it), and the purpose of such devices is limited domestic hot water supply and heating pools. On the other hand, such collectors are extremely simple, inexpensive, do not require a connection to the mains and can work even in the absence of electricity (as long as there is water in the tank).

Another design option is devices without a tank powered by a circulation pump. However, they are less common, mainly among models for heating pools (see "Suitable for").

— Closed. Closed collectors require high-pressure operation (about 5–6 bar) and are designed for direct integration into the DHW supply system. In this case, the indirect principle of operation is usually used for heating — the transfer of heat from the water in the collector to the water in the DHW system through a special heat exchanger.

Such devices are noticeably more compli...cated and expensive than open ones. At the same time, they are more versatile and efficient; they can be used for domestic hot water and heating. In addition, you can install the heater at any height. It does not affect the pressure in the system, unlike open design.

The main application the solar collector is designed for. It is highly undesirable to deviate from these recommendations: the specific features of the design and operation depend on the purpose, and in the “non-native” mode, the device will at best be inefficient, and at worst it may fail and even lead to accidents.

— DHW. Application in domestic hot water supply systems is a classic option, the vast majority of modern solar collectors are made for. The specific method of embedding in the DHW system may be different: in particular, open models use direct water heating, and closed models use indirect heating (for more details, see "Loop system"). Anyway, solar heating can be very handy for providing hot water. It can play both an auxiliary role (to save energy during the main heating or as a backup in case the hot water is turned off), and the main one (for example, in a country cottage or other similar place where there is no hot water initially).

— Heating and DHW. Devices designed for use both for hot water supply and heating. For DHW, see the relevant paragraph for more details; however, not everything described there is true for this category. For the collector to be effectively integrated into the heating system, it must meet certain additional requirements. First of all, such an application is allowed only for closed devices (see "Loop system") — the heating circuit operate...s under fairly high pressure and forced circulation, an open operation scheme is not applicable here. Secondly, the “heating” collector must allow year-round use (see “More features”) — after all, the heating issue is most acute in the cold season, and not all models can work at low outdoor temperatures.

— Swimming pool heating. This category includes high-performance solar collectors that can be used for heating water in the pool, as well as for other purposes that require a constant supply of large amounts of hot water — for example, the operation of underfloor heating systems or a set of bathtubs. Of course, they can also be used in a more traditional format — for example, for DHW systems; however, the described tasks associated with large consumption of heated water remain the main specialization.

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.

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.

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.

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.

The total number of tubes provided in the design of the solar collector (vacuum or thermosyphon, see "Type").

This parameter largely depends on the area of the device: for a large collector, more tubes are required. However, there is no hard dependence here. Devices of similar size may differ in the number of tubes. In general, this parameter is quite specific, it is used in some formulas for calculating the required collector power.

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.