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The dual-energy water heater: solar and electric

Eco-friendly but also very economical, the solar water heater allows you to heat your home all year long without contributing to global warming. Contrary to popular belief, this system works even in the most austere regions. With solar energy it's about 60% of your heating and hot water needs that will be covered each month without spending a penny once installed.


The operation of the solar water heater

The operation of a solar water heater is comparable to what is obtained by leaving a garden hose filled with water in full sun: the water in the pipe is heated by solar radiation. To take this principle on a larger scale and adapt it to domestic use, the solar water heater requires several elements.

Solar thermal collectors, usually placed on the roof of homes, capture the sun's rays and transmit heat to a heat transfer fluid usually composed of water and glycol. It is a glass box (prismatic glass that retains 95% of the sun's rays) insulating producing a greenhouse effect. It contains copper tubes, black, called absorbers. They are the ones who catch the rays and heat the coolant.

The dual-energy water heater: solar and electric: dual-energy

This liquid is then conveyed by a closed circuit insulated towards a hot water tank located inside the house.

The balloon is a perfectly insulated metal tank that contains the reserve of domestic hot water. In the storage tank, the coolant passes through a heat exchanger (a pipe coil), where it gives up its heat to the sanitary water contained by the balloon without coming into contact with it. Finally, the tank will be connected on one side to the cold water network to supply it, and on the other side to the hot water network to service the taps. After passing through the exchanger, the heat transfer fluid is routed back to the solar collector to resume the cycle.

In order to set the heat transfer liquid in its circuit, an electric pump is necessary: ​​the circulator. The pump is activated by means of a regulation device which is based on temperature thanks to a thermostat connected to a probe located in the tank and another located at the sensors. When the balloon probe is hotter than that of the sensors, the pump is deactivated by the controller. This avoids losing the heat of the balloon at night, or in winter if it is heated by an auxiliary device.

The coolant can flow in two different ways between the sensors and the balloon. With natural circulation, the liquid circulates thanks to its difference of density with the water of the balloon, warmer and less dense than it, it rises naturally towards the sensors by thermocirculation. To do this, the sensors must be placed lower than the balloon. Water heaters that operate on this principle are called thermosyphons.

The dual-energy water heater: solar and electric: water

The water heater: an extra energy

In essence, renewable energies are unstable in time: they are irregular and vary cyclically over time. For solar energy, the first case is characterized by the alternation of days of rain and good weather, and the second case by the succession of seasons.

In order to smooth out the first phenomenon, it is common to oversize the capacity of the water heater to a certain extent: more hot water is stored during sunny periods, which allows for a certain amount of time if clouds occur. The second phenomenon on the other hand can not be counterbalanced so simply, so we generally use a backup energy (fossil and continuous) to take over the sun when it is not available.

The most common case is to use an electrical resistor similar to that found in conventional storage water heaters. In case of insufficient solar collectors, the circulator will be deactivated. The thermostat coupled to the electrical resistance will detect a temperature too low in the tank and will then heat the electrical resistance to take over the solar.

Sizing well its solar water heater

The dual-energy water heater: solar and electric: electric

On average, a French family consumes fifty liters of water at fifty degrees per day per person. For a family of three, a water heater of one hundred and fifty liters is enough. For more comfort, it may be necessary to oversize this capacity somewhat: it avoids ending up in shortage of hot water in case of exceptional consumption. Be careful, however, because any over-sizing will have a certain impact on the consumption of extra energy, the total profitability of the system will be lower. In addition, the price of the material is proportional to the panel area and the storage capacity of the balloon. It will therefore be necessary to determine whether the priority is to make the installation profitable as quickly as possible, or whether comfort of use is a priority.

The overall profitability of the system will also be strongly impacted by family consumption. Indeed, the higher the hot water consumption, the faster the installation will be profitable. From a purely financial point of view, the calculation is not necessarily advantageous for a couple without children. On the contrary, a family of five will meet there rather quickly.

The dual-energy water heater: solar and electric: electric

On average, fossil energy savings achieved by installing a solar water heater are the same whatever the latitude (in France). The average sunshine is of course higher in the south, but this phenomenon is offset by the average temperature of the cold water, which is also higher in the south. The energy required to heat cold water is therefore lower, which (with a higher yield) roughly gives an equivalent financial saving.

Finally, the set temperature set on the extra energy (electric in our case), has a strong impact on savings achieved through solar energy. To maximize gains and minimize power consumption, it is important to maintain a low enough set temperature for the electrical resistance. It is also desirable to completely shut off the electric backup when weather conditions permit. Be careful, however, not to cut too fast or not to set a set too low the risk of creating a breeding ground for salmonella. There too, everything is a matter of compromise.


Video Instruction: Heating water with a solar panel