Solar collectors. Sizing a solar hot water system. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Alternative energy sources

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A solar hot water system can serve as the sole source of hot water or include a backup system using conventional fuels to meet increased or unexpected hot water demand. The size of the system is usually determined by the number of rooms, people and the amount of hot water required. There are several basic configurations of solar water heaters. In the most general terms, they are divided into two types: active systems, equipped with pumps and controls that allow the solar heat to be directed to a thermal storage tank, and passive systems such as a thermosyphon, which use the natural circulation of hot water.

When creating a solar water heating system, it is important to immediately determine how much hot water will be used on average during the day. Based on this figure, the dimensions of the system (collectors, storage tank) are calculated.

The main component of a solar installation is the solar collector. Most often, flat-plate collectors are used, consisting of an absorber plate (absorber), on which solar radiation is converted into heat and transferred to the coolant fluid, thermal insulation at the edges and under the absorber, a box that contains all this and provides the necessary ventilation of the glass or plastic cover.

If glass is used for the coating, it is important that the iron content of the glass is low or no, so that at least 95% of solar radiation passes through the glass. Most often, a single layer of glass is used. If plastic is used, it must be UV resistant. Polycarbonate plates have shown excellent results in practice.

The absorber is a plate with tubes attached to it, through which the coolant flows. It is made from copper, aluminum or stainless steel. Copper absorber tubes have been proven to be the best, as steel tubes are highly susceptible to corrosion. It is important that the absorber is resistant to ultraviolet radiation from the sun and exposure to high temperatures, which can reach 100-140°C for collectors with conventional and 150-200°C - with selective coating.

The construction of a flat collector requires the soldering of pipes and their connection to the plate. The closer the tubes are in contact with the plate, the greater the heat transfer of the liquid flowing in them. The absorber is often coated with a special selective black paint that absorbs the sun's rays and traps heat radiation inside. Ordinary black paint under the influence of high temperatures evaporates from the surface of the metal. Under normal conditions, black paint radiates more heat instead of transferring it to the heat transfer fluid.

The body of a solar collector is made from a variety of materials: wood, plastic, steel and aluminum are used with varying degrees of success, but aluminum is by far the best of the listed materials. It is weather tolerant, low maintenance and comes in black, eliminating the need to paint the outside of the solar panel. Many years of practice have shown that plastic is of little use for the manufacture of various solar panel components. It is not suitable for external parts, as it degrades under ultraviolet rays: it fades, loses its hardness and cracks. Plastic has a high coefficient of expansion, meaning it expands and contracts so much that it is difficult to seal the joints. The use of steel cases is also associated with difficulties. Firstly, the panels need to be tinted regularly, and secondly, they react chemically with copper components.

Solar collectors are usually installed directly on the roof of a building or on a frame mounted on a flat roof or on the ground. You can also make manifolds part of the roof. Sometimes there are difficulties in sealing the space between the collector and the rest of the roof space.

The size of the solar collector depends on the daily need for hot water. On average, one person consumes up to 50 liters of hot water per day with a temperature of 55 - 60°C (washing and showering, excluding laundry). It has been proven that to heat 50 liters of water per day, the average area of ​​solar collectors should be 1-1,5 m2. The price of the collector depends on its size and the cost of its installation. The latter is most easily carried out when the solar system was taken into account when developing a project for building a new house. The architect can then include collectors in his project in advance, both from an aesthetic point of view and from an economic point of view.

For typical solar collectors with a selective absorber, heating water by 8-45 degrees, there are standard rules:

• Hot water consumption averages 50 liters per person per day.
• 1-1,5 m2 of solar collectors are needed to heat 50 liters of water per day.
• The storage tank should hold 40-70 liters of water per 1 m2 of the solar collector, or 80 liters per person.
• The heat exchanger in the storage tank must transfer at least 40-60 W / ° C per m2 of the solar collector at a temperature of 50°C.

If these rules are followed, a typical solar collector in Central Europe will be able to provide 60-70% of annual hot water consumption and produce 350-500 kWh/m2 per year. In large buildings (hotels, hospitals, apartment buildings), the collector area and tank volume per inhabitant are smaller, but a detailed analysis of demand and local climatic conditions is needed to accurately determine the optimal size of the system. Experience shows that solar systems for hot water heating should be as simple as possible and not too large.

Example

A family of 4, which consumes 200 liters of hot water per day, needs a 6 m2 collector. Such a system generates up to 3000 kWh of environmentally friendly energy per year. If the oil boiler is replaced with a header, the fuel oil savings are at least 300 liters per year.

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