The idea of using the heat of the sun to heat water is not new. In many summer cottages there is a special barrel or other similar container exposed to the direct rays of the sun. In the morning, ice-cold water from the well is poured into the barrel, and by the evening it warms up to a comfortable temperature and is quite suitable for washing, washing, or, say, watering heat-loving plants.
Modern solar systems work on a similar principle. The sun’s rays heat the heat-carrier fluid in a special collector, which enters the tank with a heat exchanger and through the latter prepares water for the needs of the consumer. As a coolant, as a rule, a mixture of water and antifreeze is used, which does not freeze at sub-zero temperatures. In addition to the above elements, the system usually includes a circulation pump that pumps liquid (there are, however, systems with natural circulation), as well as electronic monitoring and control devices that regulate the operation of the system.
What are the most common mistakes buyers make when choosing a solar system?
As a rule, they want to get 100% of the energy from the solar collector, not considering that the sun cannot be “turned off”, that is, stop heating at will. As a result, the system overheats, especially due to the lack of water intake on hot summer days. The installation must be selected specifically for the hottest period, and on colder days, use an additional heating source or solve the problem of heat dissipation, for example, provide curtains that cover the collector.
Why are solar collectors not used to heat water in radiator heating systems in winter?
There are several reasons for this. In particular, in winter a lot of heat is consumed at night, therefore, during the day it is necessary to create a significant supply of hot water in the buffer tank for heating at night, which leads to additional costs (not counting the large number of collectors). In addition, a mixture of polypropylene glycol with water circulates in the solar collector system, and water circulates in the radiators. Heat transfer from radiators with polypropylene glycol will be lower, so you will have to increase the number of heating devices. As a result, the system will be expensive, with a long payback period. Ariston has several types of solar systems, both with natural and forced circulation. Natural circulation systems — mainly for seasonal use. Their maximum effectiveness should coincide with the period of application (summer). Electricity is not required for operation, but a heating element can be connected if necessary for reheating at night. The efficiency of such systems is lower than forced ones, but at the same time the level of protection against overheating is higher.
Product Specialist, Marketing Department “Ariston Thermo Rus”
Let’s take turns
On average, a solar system covers approximately 60% of the energy demand for hot water production per year. In summer, the solar system is able to fully provide the cottage with it. In winter or on rainy, cloudy days, it is used in conjunction with other energy sources. As auxiliary equipment, for example, a gas, diesel or solid fuel boiler can act. In this case, the collector and the boiler are connected to the so-called bivalent tank — that is, a tank with two built-in heat exchangers. There are also boilers with three heat exchangers for connecting three different sources, but they are less common.
The “solar system — boiler” bundle is so popular that most of the major manufacturers of heating equipment (Ariston, Baxi, Bosch, Buderus, De Dietrich, Viessmann) offer models of both types. Thus, it is possible to assemble a system with a single control device. For example, for Ariston, the operation of the solar system and the boiler is controlled using the Ariston Sensys control device, for Viessmann, the Vitosolic controller.
Collector design diagram
Flat-plate collectors work more efficiently in summer and with direct solar radiation, while vacuum collectors, on the contrary, perform better in scattered radiation conditions in winter and during partly cloudy periods.
Pipe or flat?
The most widespread are flat-panel and tubular models of solar collectors. They have their own advantages and disadvantages.
Flat-plate collectors look like solar panels. And no wonder: their outer plane is a rectangular panel of impact-resistant glass. Under it is an absorber — an element that absorbs solar radiation. The surface of the absorber, turned towards the sun, is blackened with a special coating, and heat exchanger tubes with a heat-transfer fluid are laid under it (see figure). Due to the selective light transmission of the glass inside the panel, a greenhouse effect is created: the sun’s rays freely penetrating inside heat the absorber, which begins to reflect long-wave rays, the latter do not pass through the glass and cannot leave the collector.
In vacuum tube collectors, instead of a flat absorber, vacuum double-walled glass tubes with a reflective coating applied to their inner surface are used. The design works like a “thermos in reverse”: the sun’s rays pass through the glass and heat the tubes of the heat exchanger with the heat carrier located inside.
It is generally accepted that solar collectors are effective only in the southern regions and countries, however, a number of projects of private houses with solar systems have already been implemented throughout the European part of Europe, in the Urals and Siberia
How many “squares” do we need?
The area of the solar collector depends on the estimated performance of the solar system. For example, you want to receive hot water for washing and household needs. First of all, you need to choose a heating tank of the right size, the algorithm for calculating its capacity is the same as for storage boilers with electric or gas heating (for choosing a boiler, see the article “Hot Heart of the Tank”, No. 3/2014). The volume depends on the number of users and the type of bathroom equipment (for example, a tub or a shower cabin).
Having determined the volume of the boiler, you will know the performance of the solar collector and, accordingly, calculate their area and calculate how much space is needed for installation. For a more accurate calculation of the cost, additional parameters may be required, such as the slope of the roof and the angle between the horizontal projection of the perpendicular to it and the southern direction. For the calculation, ready-made algorithms are used, so that a professional can easily do this job. Many manufacturers and installers do the calculation for free.
How much do squares cost? The price largely depends on the manufacturer. Inexpensive Chinese can be purchased for 10–200 $. for a collector module of 2 m². A similar European-made product costs 3–4 times more. Another 30–600 $. the controller and boiler will cost.
Usually solar collectors are installed in such a way that they are located higher than the DHW cylinder. The pipes connecting the collector and the water heater are laid with a constant slope and should be as short as possible. It is necessary that they are resistant to temperatures up to 150 ° C and a pressure of 6 bar, so it is better to use copper pipes. As for thermal insulation, there are also certain requirements for it: thickness, resistance to high temperatures, ultraviolet radiation, etc. The best option is to use special Duo Tube pipelines: the direct, return lines, together with the cable for the collector sensor, are combined into a common casing made of thermal insulation (with UV protection), and on top covered with a material that will withstand the blows of bird beaks.
Head of Renewable Energy Sources De Dietrich at Rusclimat Thermo
Optimum slope of the collector plane
Solar collector panels are placed on an inclined plane so that during the day the sun’s rays fall on them at an angle as close as possible to a straight line. The optimal slope of the collector plane corresponds to the geographic latitude of the area and is, for example, 57° for world. For the Northern Hemisphere, the southern direction of the “view” of the panel is suitable (say, the southern slope of the roof). Of course, other objects should not block the collector from the sun. It is far from always possible to comply with all conditions, therefore, when mounting collectors, prefabricated or welded metal structures are widely used.
The easiest way is when the required slope of the panel corresponds to the slope of the roof. In this case, no additional inclination is needed, and the collector will be mounted on a profiled metal mounting rail. The tire is laid perpendicular to the rafters and rests on them with the help of special rafter hooks. The distance between the hooks is calculated according to the reference tables and depends on the distance between the rafters and on the snow load. The rafter hooks rest only on the rafters or on the intermediate battens (in this case, an additional supporting angle is used) and should not rest directly on the roof. The connection of pipelines is carried out by means of press fittings or brazing.
Installing a collector is a responsible operation, because you will most likely find out about the mistakes of inexperienced or careless installers when it will be almost impossible to fix them. Therefore, it is better to contact installers with experience in such work. A representative of the company will inspect potential installation sites and make a conclusion about the possibility of placing the collector, as well as calculate the engineering elements of the mounting structure, taking into account wind and snow loads.
In other countries, solar collectors are used not only for heating, but also for heating water in swimming pools: in this case, solar systems with a special external heat exchanger are usually used.
Most often, solar systems are used for domestic hot water with two heat exchangers in the tank (one for the collectors, the other for the boiler). The efficiency of solar systems is much higher than is commonly believed. So, in Bosch systems, a collector with an area of only 1.9–2.4 m² is quite enough to generate the required amount of water. However, the efficiency depends entirely on how well the system is designed. In EU, collectors of both types have become widespread. The Bosch range includes flat plate solar collectors made in Germany. They are the most reliable and able to work effectively all year round. The collectors are equipped with weather resistant glass and an absorber with a special structure that increases the efficiency of the devices.
Product manager at Bosch Thermotechnika
Installation of a solar collector on the roof
Comparison of flat panel and tube collectors
|20–30% lower than the vacuum of the same class
|Work during the day
|Efficiency varies greatly during the day, from a minimum at sunrise to a maximum when the sun is at its zenith, then the efficiency decreases again to a minimum
|Due to the tubular shape of the collector and the mirror effect, the sun’s rays are used more efficiently and the efficiency is almost
does not change during the day
|Work in the cold
|Efficiency is 30–40% lower than that of vacuum
|Higher (by 30–40%) efficiency due to lower heat losses
|High (damaged section can be replaced)
|High (needs a stronger base)