May 21, 2019


Analysis of all the data that will lead to the right choice of solar water heater, offering multiple benefits for a home and its owner.

Man has always been aware of the benefits of using solar radiation to serve his needs. Over the last decades technological advances in the field of materials have allowed the development of efficient systems to exploit this free energy, both in the form of photovoltaic and in the form of solar thermal.

The difference between the two is that they first convert the solar radiation directly into electricity, while the latter in thermal. Thermal energy is produced by heating a medium (typically water or other fluid) through the greenhouse on special surfaces (slabs) made of special absorbent and reflective materials and forming the solar panels. The water is then pumped into a storage tank and can be used either for daily use or for assisting the existing central heating system.

Historically, the solar water heater began to be used since the 1970s when the oil crisis triggered the exploitation of alternative energy sources. It was widely disseminated in the 1980s to reach Greece in 2004 alone, installed over 3 million square meters of solar collectors.

In other Mediterranean countries, their use is now a mandatory legal basis for all new constructions (Israel). Especially for the production of hot water, Greece with about 300 days of sunshine every year is definitely the ideal country for the exploitation of this free energy. The Greek solar thermal system is one of the most advanced in the world, with exports reaching up to 50% of its production.

The operation of the solar water heater

However, in order to understand the operation of the solar water heater, it would be better to first look at the different species that exist. Thus a first separation can be made in an open or a closed circuit.

Solar Open Circuit: It is the simplest and cheapest in its construction and operation, as the water itself is the heated medium and is heated directly by the collectors. Easy to install and maintain, has the unique disadvantage that it can not be used in severe cold conditions.

Solar Closed Circuit: The second and most sophisticated type uses a separate heated medium consisting of another fluid (eg oil), which circulates in an independent circuit without coming into direct contact with the water. So, with the help of a heat exchanger, the water heats the water without mixing them. The advantage of the second type is that it provides increased protection against frost because in the first case antifreeze agents can not be used. In addition, the heating medium may also have anti-corrosion properties thereby increasing the overall strength of the system over time.

A second categorization can be done based on the capabilities of the storage container. So the solar water heaters can be distinguished in double or triple energy:

 Dual Energy: Their operation is carried out with the simultaneous exploitation of either solar or electricity. In cases where solar radiation is not sufficient (eg overcast) it uses the electricity to achieve the desired water temperature. For this purpose an electrical resistance with a thermostat inside it is installed.

 Triple Energy: It is the same as double energy, but it also has another entrance to take advantage of the hot water of the radiator produced during the operation of the central heating boiler.

The solar collectors and the storage tank

However, regardless of their type, all solar water heaters share some common features in their operation, which are none other than their components, as they all use the same principle. Two of these are: The solar collectors where the solar radiation is absorbed and converted to thermal and the storage tank that is basically the tank that keeps the hot water used.

Solar collectors: they are sometimes called “mirrors” and are the heart of the system. They consist of four (4) parts, the radiation collector plate usually made of titanium-coated aluminum or copper, the heating medium flow tubes, the cover plate (crystals) and the insulated frame on which all are fastened the other parts. In recent years, so-called “vacuum tubes” have also emerged. While sharing the same operating principle with conventional ones, they have much higher performance and can achieve much higher temperatures inside.

Their operation, as mentioned above, is based on the greenhouse effect developed between the absorbent plate (usually made of black) and the glass coating (mirror). Solar radiation is absorbed by the plate by forcing it to emit heat. The heat is then trapped by the glass, forcing it to return back to the plate, reinforced by a very large percentage, further increasing the temperature and therefore the performance of the system. This heat is then absorbed by the heating means through an exchanger and by means of the pipelines located at the back of the absorption plate or between it is transferred to the water of use.

Storage tank: Nowadays, the term “boiler” is basically what the English name means, that is, a large kettle. It is essentially a large cylinder made of steel (stainless steel to more expensive models) or copper (coated with some glass) or some other antioxidant coating to avoid electrolysis phenomena at welding points. It is insulated from 5 to 7 centimeters on the outside (usually polyurethane), so that warm water can be stored at the desired temperature with the least possible loss. Its capacity can range from 100 to 300 liters usually for home use, but its size is indissolubly connected to the surface of the collectors to be installed. Typically, a solar water heater with a capacity of 100 liters can from 2 square meters of collectors heat about 150 liters of water at 35 to 40 degrees. Storage of water in the tank can be done in two ways, whether natural or forced.

Physical circulation: As the phrase says, the solar thermal water heater exploits the fluid flow phenomenon due to temperature difference (and therefore density difference). Because it is known from physics that the energy transfer is always done from the hot (light and light) point to the cold (dense and heavier) until both come to temperature equilibrium, it is easy to understand that there is physical flow without the help mechanical means e.g. a circulator. The only prerequisite for the cold medium to be at a higher point than the hot, so for the simple solar water heaters we usually see the tank always above the solar panels.

Forced circulation: in this case, circulation of the heating medium into the system is carried out by means of circulators, valves and other control systems. The latter periodically measure the water temperature in the storage tank and accordingly instruct the heating medium to circulate to the collectors. It is more expensive than a solution, but it offers great advantages to the overall performance of the system and the immediacy of on-demand water.

Key factors for final performance

The final performance of a solar water heater in a building is dependent on many parameters that must always be considered seriously. Properly dimensioning the system based on the real needs of the people staying in the building is usually a one-way street and if implemented, it leads to very good savings results. For this purpose, we take into account a whole lot of data such as daily hot water needs, location, orientation, the location of the storage tank, and the climate of the area. Proper and careful installation is also a very important factor that plays a major role in system adequacy. Incorrect placements even on the slopes of the collectors have a great effect on performance and should always be done by specialized garages.

Cost of installation … from 250 euros

Installation costs depend on the size and needs of the occupants. Today, there are systems ranging from 250 euros for 80 liters capacity, without transport and installation costs. A typical 200-liter facility, however, that comfortably covers the needs of a four-member family will cost approximately EUR 1,000 with the installation and VAT. If we think that using other sources of energy (oil, electricity or gas), to warm the same amount of water daily, we need 200 to 250 euros per year for the same result, one can easily calculate the cost of the initial expense. The pleasure of the solar thermal systems is that after depreciation they can continue to feed us free of charge with hot water for many decades yet, basically making their choice one way nowadays