Capturing solar energy for domestic hot water or high-efficiency heating
Solar panels are very efficient at taking advantage of solar radiation. In our latitudes the sun generates up to 1000 Watts per m². With photovoltaic panels we can capture 200 W / m², with thermal energy it rises to 800W / m², four times more! Solar panels are much more profitable than photovoltaic panels and much less expensive. The "home made" solution Eric Lafond offers us easily reaches 500W / m² for a cost of 15 € per m².
This  shows the solar power you can expect to receive, depending on your geographical position and the season.
Solar thermal panels are particularly useful for domestic hot water production. In this case they are called solar water heaters.
3 - 4m² (32 - 43 sq feet) of solar thermal panels will cover 90% of the hot water needs for a two person household throughout the year. The hot water tank will take over during cloudy days. If there are more inhabitants, and therefore more water being consumed, you need to increase the size of the solar panels. For example, 6m² (64.5 sq feet) for 6 people.
Eric's complete system - which includes home-made panels, supply pipes, coolant, solar balloon, circulator, and a regulator - will be profitable in two to three years. The panels installed at his house are in their eighteenth year.
These thermal panels are designed in the same way as those on the market: a solar collector containing a heat transfer fluid is sandwiched between an insulator and a sheet of glass. In this case, we will use the grill you find at the back of a fridge for the solar collector.. And we will use the door of the fridge as the insulator. The glass is from old double glazed windows. You will find many fridges in landfills or recycling areas, and double-glazing windows at many glaziers.
A big thank to Riké, who shared his know-how with us, from his 20 years of experience in the world of energy, and to the members of the Grand Moulin collective who welcomed us to the training they organized, particularly to Karine, Sylvain and Pascal. Thanks also to Jean-Loup for the explanation of glass cutting and soldering, and to all the other volunteers of the participative building site for their help.
The thermal solar panels that we are building will have to be installed facing directly south, ideally forming an angle of 60° with the horizon, (or vertically on the outside wall, if the former is not possible). Rooftop usage of these panels is much less efficient in the winter and causes overheating in the summer (more details in the installation phase).
To maximize the effectiveness of the thermic solar energy, it is necessary to minimize the inertia in the light uptake and to maximize it in the storage or the diffusion. The panels work as soon as there is a sunbeam thanks to their low inertia. The heat is retained for a long time with its great volume and its good insulation.
The pipes of the refrigerator grill have a small diameter (4mm), so there is a low volume of heat transfer fluid to heat in the panel. This low inertia quickly increases the temperature as soon as the sun appers from behind the clouds and then heats the hot water balloon. The greater the diameter of the pipes, the more time it needs to heat the larger volume of fluid. and the less efficient the system becomes.
To ensure the temperature of the panels increases quickly, the space between the insulation and the glass need to be as small as possible. So the panel has to be as thin as possible, whilst ensuring that the grid doesn't touch either the insulator nor the glass. Otherwise those elements will conduct heat away from the panels.
The temperatures can exceed 150°C (302°F) inside the panel, so it is essential to use materials resistant to heat and to UV radiations. Do not use sticks or paints with solvents which are not resistant to UV radiations. Here, we use PU putty and acrylic paint. For a good longevity, also make sure to use rot-proof local wood.
On winter nights, it can get very cold inside the panel. The different materials, insulation, metal, wood and glass will expand differently between winter night and summer sun. The joints that unite them must be thick to absorb the deformation, if they are not thick enough they will tear off.
The specific feature and ingenuity of these solar thermic panels is the use of refrigerator grills. But beware, not all grills are good and they must be used in the right direction! The grills must be provided with cooling fins and painted black (see photo). They must not be in galvanized steel, as the paint will not adhere. Similarly, some refrigerators are equipped with pipes connected by wire, but the surface of these grids is insufficient for this use.
The right grilles have a direction of assembly, the fins, comparable to louvers, must capture the sun. In a sense the sun will pass through, it's bad, in the other direction it will capture the rays, it's the good sense! They must be perpendicular to the sun's rays.
Tip: take the grid and lift it between the sun and yourself, in one sense the rays will pass and not in the other.
Refrigerator grills are covered by a refrigerant, harmful to the environment, the greenhouse effect and the ozone layer. In France the refrigerating gases must be recovered. However, there are exemptions for equipment containing less than 3 kg of fluid until 2025. The domestic installations (refrigerator and air conditioner) are degassed before continuing their upgrading process. We can open the circuit without too much conscience.
Be careful, you will do something that may involve risks.
The information and advice given in this tutorial come from joint workshops and are neither perfect nor exhaustive. If you don't have certain tools or if you don't feel competent, don't hesitate to ask around for help. Remember to wear your safety equipment, to work in ventilated areas and do not put yourself in danger. Be careful, calm, and of course critical of any false good ideas you may have ("that's how it will go...").
Refrigerators are numerous in waste collection centres, it is necessary to identify those which have the adequate grids (see Preface - Grid) and of the largest possible dimension. Pinching the pipes at the outlet of the compressor will limit the exhaust of refrigerant gases. Cut the pipes as close as possible to the compressor to ensure maximum length with the grid. Unscrew the grid. Wash the grid with soapy water. Blow a blower into the pipes to remove impurities. Seal the pipes with tape to prevent impurities from getting into them, as they may have a small diameter and become obstructed.
Refrigerator doors are filled with insulating foam. By recovering them and assembling them they form the back of the panel. They must be flat (and not curved), it does not matter if there is a chamfer on the edges, it will be filled with mastic.
Ideally, the panels should be between 1.5 and 2m², if they are larger they will be heavy and therefore more complex to install. The glass may break if the panels deform. If they are smaller it will be necessary to increase the number of panels, therefore more work to do.
Grilles of similar size, slightly smaller than the doors, should be assembled. For a panel of about 2m² you generally need 3 to 4 grilles for 2 or 3 doors. In our case we have 3 grids and 3 doors.
The refrigerator doors form the structure of the panel and are its insulator. They will be glued side by side, in the height. The difference in thickness of the doors does not matter, they are aligned on the front face, on the interior side of the panel.
This step consists of connecting the solar collectors (refrigerator grid) to the heat transfer fluid circuit via two feeders (copper tubes). The feeders must have a diameter equal to the sum of the diameters of the pipes they feed, in addition, in our case, 3 pipes of 3mm inside diameter, it is necessary a feeder of at least 9mm inside diameter.
The feeders may be on the top or bottom of the frame, this does not change the proper operation of the panel. To be adapted according to the installation of each one.
To concentrate heat on the drids, it must not be in direct contact with the door metal sheet. They are spaced by cork spacers. Cork is rot-proof and resistant to high temperatures.
To create a greenhouse effect and limit convection between the grille and the outside, the panels will be closed with glass. Glassmakers get rid of old windows, especially double-glazing, by asking them nicely they can be recovered free of charge. Ideally, a glass thickness of 4mm is required for vertical panels and 5mm for inclined panels, more subject to hail and bad weather. It is not necessary to have thicker glasses, it reduces their performance.
Depending on the hot water requirements and the amount of sunlight, several solar thermal panels may be required. To make additional panels, repeat the previous steps. However, unlike the blind panel, the feeders must be through, i.e. the two copper pipes, hot and cold water, must protrude at the bottom of the frame on each side. The diameter must increase by 2mm in each additional panel: 12mm for the blind panel, 14mm in the second, 16mm in the third, etc. Care must be taken to properly connect the hot water feeders between them and also for cold water feeders.
To capture maximum solar energy, calories, the solar panel(s) must be perpendicular to the sun's rays for two reasons :
Solar panels are rarely mobile and their angle is therefore fixed to the installation. Solar power is much stronger in summer than in winter, not to mention the length of days. Solar energy is at least three times more important in summer than in winter (see tutorial energy in the home), so the panels are sized and oriented for the most critical period: winter.
In the middle of winter, in France, the sun has a zenith at 30° with the horizon, this is its maximum height. In summer, at the same latitudes, it rises to 60°. Ideally, the panels will be installed at a 60° angle to the horizon, south-facing. Otherwise, to limit the losses, one can put them vertically against a wall, it is more interesting and less dangerous than on roofs.
In summer, the solar power being much more important, the angle does not matter, the panels will quickly be very hot, even too much. A shading is interesting to limit overheating, a cap on a vertical panel does very well.
The solar thermal panels must be positioned as close as possible to the hot water tank to minimize heat loss.
The panels must be connected to an exchanger tank. In addition to the standard electrical resistance, an exchanger passes the heat transfer fluid through the storage tank to transfer heat from the panels to the domestic water. You can find these exchange tanks in DIY stores. They cost 15 to 30% more than 100% electric balloons but will pay for themselves very quickly. Otherwise, some good tutorials allow you to make them yourself from a classic tank.
The system must be equipped with a regulator and a circulator. In our case the circulator is switched on when the temperature of the panels is 10°C higher than that of the storage tank, it is switched off when this difference is less than 5°C. This prevents the balloon from cooling at night or when the sun is shy. The tank is also equipped with an expansion vessel to absorb the expansion of the heat transfer fluid on sunny days. These elements are generally supplied with the solar heat exchangers. You can also make them yourself.
It is advisable to use a heat transfer fluid in the system. If it is used in winter it must be antifreeze, otherwise the system must be drained.
It is possible to use the same solar thermal panel system in low temperature heating. The heat transfer fluid from the panels is fed directly into the heating pipe network in the floor or walls. However, in solar energy, heating is much less obvious than domestic hot water. Indeed, for heating, a maximum of energy is required when it is least available: in winter, when the need for domestic hot water is spread over the whole year. Moreover, the energy required for heating the house is 6 times greater than that of hot water (see tutorial energy in the home), so it will require 6 times more panels for heating than for hot water.
For example, depending on the year, Riké covers 10 to 30% of its heating needs with 14m² of panels, while it covers over 90% with 6m² of panels dedicated to heating its 350 litre hot water tank.
A simpler solar heating solution to implement was documented in February, it is a [Solar heating slate version