Solar collectors - what type to choose?

Choosing the right solar water heater system

We love modern homes not only for the architecture, even more, not so much for the architecture, but for the care of the environment. The environment does not end inside our home, it is wherever we go, so it is important that our home is safe not only when we are inside but also outside. This safety is also achieved by seeking to reduce harmful emissions to the atmosphere so that we can breathe clean air. This is facilitated by the use of clean energy sources, one of which is solar energy. In this article, we will talk about the heating of water by the sun, which will be used in the bathroom, in the kitchen, or for heating.

What is a solar thermal collector?

We have already talked about photovoltaic (PV) solar collectors that convert solar radiation into electricity. Solar thermal collector, or solar collector (from the word helios —Greek., the sun), much easier. It is a device that collects heat directly from solar radiation. It can be used as the main source of heat, and together with other traditional sources operating on biofuel or electricity.  On the Internet, there are many models of solar panels made by home craftsmen, but there are also available factory solar installations that can be used to heat water and space heating.
In a warm climate in a thermal solar collector, it is possible to use water passing through its panels, but in a cold climate, glycol is used for the prevention of freezing of panels.

The efficiency of solar thermal collectors:

The panel's efficiency is partly determined by the rate of absorption of solar radiation, and partly by its emissivity; in other words, not only how much heat it can absorb, but also how much it will emit (or lose) before warm water is delivered to its destination.
Earlier models had absorption coefficients in the range of 90-95% (solar radiation absorption efficiency), but they also had a high loss coefficient — in the range of 55-95%, so most of the accumulated heat was lost, already in the panel.
These models also used standard black paint, while modern panels are made with a specially designed coating to absorb and retain heat.

Despite the fact that the modern generation of solar collectors is very efficient, their competitors-solar photovoltaic panels-are more efficient than thermal solar panels, so they are considered a more profitable investment. This does not mean that the quality and efficiency of solar thermal collectors is low, it is simply believed that it is better to spend money on solar photovoltaic cells, and then use the generated electricity in a conventional electric water heater. This is due to the ever-evolving technologies and lower costs in the photovoltaic industry, while the technology of production of solar collectors and the cost remained quite stable over the same period. They are still good, the problem is that the competition continues to increase.
The premise for this philosophy is that at times when hot water is not needed, the heat collector brings no benefit. If the solar installation were photovoltaic rather than thermal, solar radiation would always be absorbed to generate electricity, to power other household appliances, to store in batteries, or to return to the power supply systems that would soon appear in Russia. It's hard to deny this logic; however, there are situations where thermal solar energy is useful, so we'll describe the options.

Tubular solar collectors ©Viessmann

This is the most common type of solar heat collector you've probably seen on rooftops. The panel of the vacuum collector most often consists of glass pipes, inside which there are copper tubes, with a darkened plate covering the tube to absorb heat. Glass tubes are hermetically sealed and only part of the copper fittings remains open. Each tube is installed in the collector separately.
This makes it easy to replace the pipe if the vacuum seal breaks. They are also easier to mount-instead of lifting one heavy panel onto the roof, it can be disassembled into separate pipes and then assembled into panels.
Hermetically sealed air is an excellent heat insulator, so in winter the panel is almost unaffected by the outside temperature. And in hot summer you can touch the pipes with your bare hand, despite the fact that the inner tube could burn.

The designs are different, of course, but the usual flat plate collector is something more than a shallow box with a copper tube that is stretched through it, covered with a metal plate absorber and a transparent cover. Cool liquid passes through the copper tubes under the surface of the collector and in the process of passing is heated.
Both flat and tubular collectors use a mixture of glycol and water in the tubes, so a special tank is needed to heat the water, in which the coil of the heat exchanger passes.

Which is better flat or evacuated tube collectors?

Each type of solar collector has advantages and disadvantages. The air inside the hermetically sealed glass tubes in the vacuum collectors provides much better insulation than flat plate collectors, but some of the solar energy is lost when passing through the gaps between the tubes. 
In a flat plate collector, more heat is lost than in a tubular one, but it is able to collect more energy since its entire surface absorbs sunlight.

Thus, in the absence of other factors, the summer flat solar collector will produce more energy than a vacuum, because it has a large surface area, and the ambient temperature is not a problem. 
Conversely, in winter the air temperature causes much greater energy losses in a flat solar collector than in a tubular solar panel, so the design of a tubular vacuum solar panel is more efficient.

The choice of design that is best suited for your home depends on the intended use. If you want to reduce the cost of heating water in the summer in the country, you will probably benefit from a flat solar collector. If you intend to use it together with the boiler for heating rooms and heating hot water in the winter in the house for permanent residence, you will benefit more from the vacuum heat collector, because it works better in winter compared to flat geo collectors.

In systems with glycol, you need a heat exchanger for hot water, space heating, or both. The heated liquid from the solar collectors heats the water when it passes through the coil in the tank. An additional coil in the tank heated by gas or electricity will heat the water if the solar collector is unable to maintain the desired temperature or will not keep up with the consumption of hot water.

Maintenance and durability

Solar collectors can last 20-25 years, depending on the operating conditions, and require almost no maintenance. A significant disadvantage of the use of solar collectors in cold climates is the use of a mixture of water and ethylene glycol in equal parts. In winter, ethylene glycol is necessary to keep the liquid in the tubes from freezing. 50% mixture of ethylene glycol does not freeze to -35°C. in Summer the temperature of flat-plate collectors can reach 200°C, and tubular solar collector 295°C. When heated above 150 °C, the thermal decomposition of ethylene glycol begins, which contributes to the formation of corrosion and scale. Therefore, it is important that the panels use some cooling components included in the design, whether manual or automatic. In addition, ethylene glycol is a toxic substance.

How reasonable is it to use the sun to directly heat water?

The need to use ethylene glycol as a heat transfer fluid and the problems that arise with it lead to a decrease in the popularity of solar collectors in cold climates. Reducing the cost of photovoltaic solar panels and the flexibility of their use makes solar collectors less competitive as long-term investments.

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