Building Efficient Solar Panels For A Solar Water Heating System

One way of making a solar water heating system is to use solar panels: these are small solar collectors, each in a waterproof, well insulated box which can be mounted anywhere convenient and connected to a hot water cylinder. Commercial solar panels have cases made of aluminium or glass fibre which will not rot or weaken when continuously exposed to the effects of sunshine and rain. Any homemade collector must be as weatherproof as possible otherwise it will fall apart before it has a chance to provide you with any appreciable quantities of hot water. However, the only possible material for a homemade collector casing is wood because other materials are not easy to handle without a proper workshop.

The materials

The size of your solar panel will be governed by various factors: the total weight—it is no good building a beautiful solar panel and then finding that you cannot lift it into place; the size of glass that can safely be used; and the size of the heat absorbing plate.

The radiator

The best choice for the plate on a homemade installation is a steel central heating radiator. You can also use a car radiator. Be sure to use the modern steel rather than old fashioned cast iron radiators. Modern radiators contain only a small amount of water so when they are used as solar collectors the sun does not have to heat up a large volume of cold water  before the collector can begin working. This means that it will respond quickly to short periods of sunshine and will collect more heat.

The glass

The glass should be 4mm thick horticultural glass if you can obtain it: it is cheaper than window glass because it tends to give a slightly distorted view like the glass in Victorian windows. Horticultural glass comes in standard sized panes made for conventional and Dutch light greenhouses, the Dutch light being the sort which uses large panes of glass in wooden frames. Dutch light glass 4mm thick is available in the following standard sizes:

  • 1410mm x 730mm
  • 1651mm x 730mm
  • 1422mm x 730mm
  • 1676mm x 730mm
  • 1613mm x 730mm
  • 1778mm x 730mm

Try to make your collector the right size to fit one of these standard panes. You can buy the glass from a major outlet (ask the merchant for horticultural glass) or perhaps from a plant nursery which uses Dutch light greenhouses. If all else fails you will have to buy ordinary 4mm glass and have it cut to size by your local glass merchant. Do not use glass larger than 1410mm x730mm: you will not be able to manage a panel bigger than this as it will be too heavy and unwieldy to lift and fit into place.

The dimensions

The size of the glass determines the overall size of your solar panel, and this will then give you the dimensions of the radiator for the absorber plate. The most important thing to look for when buying a radiator is that it should have inch iron tappings (to accept pipe connections) at all four corners. To fit the size of the glass described above you will need a radiator no bigger than 650mm x 1200mm, otherwise you will not have enough room in the casing for the connections to the radiator.

Durability

If your solar collector is to be cost effective in its saving of the cost of heating water by conventional means it must last for a long time, say fifteen to twenty years. If it falls apart or corrodes after only a couple of years it will have been nothing but an expensive toy. For this reason it is worth using the best materials and taking care to make a very good job of the construction. We would also recommend that you do not mount the collectors on a roof, because once they are up there you will not be able to check them and maintain them easily. To climb a roof you need ladders, roof ladders and in some cases scaffolding, all of which tempts one to leave the collectors to look after themselves and not bother to climb up to look at them. If you do not check them and repair any leaks of rainwater or damage to the casing they will deteriorate very quickly and will become useless. We suggest that you mount the solar panels near the ground so that you can see them easily and check whether they need any repairs.

Assembly – The box

Start building your solar panel by making the box. The sides should be made with 32mm x 175mm wood planed all round, and this will bring the finished dimensions to 26mm x 169mm. You can order the wood planed all round instead of doing it yourself; the planing machine at the timber yard will do it quickly and efficiently. The collector will be exposed to the worst possible weather, stuck outside in frost, rain and bright sunshine, so buy the wood Tanalised; and, when you cut out the parts for the solar panel from the bought timber, give all cut edges three coats of wood preservative before assembly. If you cannot obtain Tanalised wood you will have to give the wood three coats of preservative. Do this when the box is assembled, having treated all the joints before assembly as described below. Wear rubber gloves when putting on the preservative, use an old brush and take a bit of time over the job. This may sound like a lot of work, but if you omit all this preservative treatment the panel will start to deteriorate quite quickly. The box should be 740mm wide and 1390mm long. Make the top and bottom first, cutting them down to the correct width or slightly over so that they can be sanded down. Mark out the rebates, making sure that the inner faces are 688mm apart, and that the projecting piece of the joint is 13mm thick. Use a marking gauge for accuracy, and to ensure that all the joints are the same size. Cut out the waste with a tenon saw. The sides can now be cut out, and should be 1364mm long. Take some time with the marking out and sawing to length to make sure that the edges are at right angles. Now give the cut ends of the sides and top and bottom three generous coats of wood preservative. Cut out a base for the panel from 6mm WBP plywood (WBP is the external grade). It should measure 688mm x 1338mm : when you mark it out check that the diagonal measurements are equal as this will ensure that the piece of plywood is truly square. Paint it thoroughly with preservative. Now make the supports for the base from Tanalised 25mm x 25mm wood. Cut two of these 1338mm long for the sides and two 638mm long for the top and bottom. Again give the cut edges three coats of preservative. When all the preservative is dry, ideally a week after the last coat was applied, you can start to assemble the collector. Begin with the sides by marking a line 13mm in from each end, then nail and glue a piece of 25mm x 25mm wood to the bottom inside edge of each side, lining it up with the mark you have just made at each end. The nails should be galvanised or sherardised lost head, 38mm long, and the glue must be Cascamite or a similar waterproof wood glue. Next, in each end, mark the positions for three 63mm no 12 screws. The screws should be galvanised or otherwise treated against corrosion. Drill the holes, countersink them, and place one end in position on one side. Mark the location of the holes in the end of the side pieces by pushing a bradawl through the hole, and drill pilot holes at the marked spots with a 2mm drill. When all the ends have been marked out and drilled and the pilot holes are drilled (mark the ends and sides so that you know which side goes with which end), apply waterproof glue liberally to the joint and screw in the screws. When the sides and ends are joined together apply more glue to the top of the strips that are nailed to the bottom of the sides, and nail on the plywood base with galvanised or sherardised 19mm lost head nails. This will hold the box square. When the glue is quite dry nail on the other strips of 25mm x25mm wood using plenty of glue, so as to support the plywood. When the glue has dried drill three 10mm holes in the bottom edge of the box positioned so that the lower edge of each hole is in line with the top surface of the plywood. Use a small triangular file to shape the lower edge of each hole. These holes allow any condensation that occurs inside the collector to drain away. Finally, give the box four coats of water-repellent preservative wood stain inside and out, making sure that you cover every surface and the inside of the holes. Black would be an appropriate colour, but the choice is yours. The stain is preferable to paint because paint forms a film on the surface of the wood and will eventually crack and peel, so you will have to scrape it off and rub down before repainting. The stain soaks into the wood without forming a film and you can add new coats as required without any further preparation. Cut two pieces of 50mm x 50mm Tanalised softwood each 688mm long and nail two spacers of 25mm x 50mm to the bottom of each. The spacers allow any condensation in the solar collector to run out under the 50mm x 50mm members, which are the supports for the absorber panel. Give the supports and spacers four coats of the stain you used for the rest of the box. When dry the supports should be screwed into the box through the plywood base, using galvanised or sherardised screws 63mm no 12 countersunk into the plywood. Finally cut four pieces of 25mm x 25mm aluminium angle 169mm long. These are to protect the exposed grain of the timber case. Mark positions for holes on the angles, making sure that the upper holes are at least 35mm from the end of the angle. The holes have a diameter of 5mm to take 38mm no 8 round head screws. To make it easier to drill the holes, because the drill point will slide on the aluminium, first mark the positions of each hole, then take a centre punch, put it on the mark and hit it with a hammer. This will leave a small dent in the metal which will hold the point of the drill and prevent it from slipping about. When the angles are drilled hold them in place on the collector box, mark the positions of the holes and make pilot holes for the screws in the wood with a bradawl. Before you screw the angles in place coat the back of each with a layer of mastic, a sticky sealing material. This can be bought in two forms, either as a cardboard cartridge which fits into a mastic gun, or as a ‘self-propelling’ cartridge. The cartridges for the gun are cheaper but you also have to buy the gun. You can use a mastic gun one-handed which is sometimes an advantage, and it tends to make a neater job because it is easier to control the flow of mastic. At the end of the self-propelling cartridges there is a plastic wheel which you turn to force the mastic out. The best and most expensive mastic is silicone, which should last over twenty years. Before using any mastic check with the manufacturers that it can be used with aluminium. Some types may require that you degrease the metal or use a special primer before applying the mastic itself. When you have put mastic on the angles screw them into place, and be sure to use stainless steel, aluminium alloy, galvanised, sherardised or cadmium-plated screws to prevent rust. Some mastic will squeeze out round the edges of the angles and this should be cleaned off with a razorblade or Stanley knife after it has had a few hours to set. When you have put on the corner angles the box is finished and you can put it aside and turn to the solar collector itself.

The radiator

The collector is a single panel steel radiator no larger than 650mm x 1200mm. The exact dimensions will depend on the manufacturer, but the important thing is to get a radiator with I inch iron tappings at all four corners. When you buy the radiator make sure you buy two plugs to block up two of the holes. Buy two fittings, each with a inch male iron at one end and a 22mm copper compression fitting at the other. (This strange mixture of metric and imperial measurements is unavoidable since radiator connections have not yet gone metric.) Wrap PTFE tape round the thread of the two plugs and screw them into diagonally opposite holes one at each end of the radiator. Next screw the male iron/compression fittings into the remaining two holes in the radiator again using PTFE tape to prevent leaks. Tighten all the fittings and plugs with a spanner. Block off one of the compression fittings with a blanking-off disc, using PTFE tape to seal the screw thread. Fill the radiator with water through the other fitting and stand it over a sheet of dry newspaper. If you find no spots of water on the newspaper after an hour you can assume that you have made all the connections tight enough. Remove the blanking-off disc and fit it to the compression fitting through which you filled the radiator, turn the radiator the other way up and repeat the test. It is better to do this than to assemble the collector panel and then find a leak. When you have finished the leak test, empty the radiator, remove the blanking-off disc, and give the upper surface of the radiator a coat of matt black blackboard paint. Cut some glass fibre or mineral wool insulation 75mm thick and lay it in the bottom of the box that you made earlier. Do not be tempted to use expanded polystyrene insulation because, although it looks neater than glass fibre, if the collector stands in the sun with no water flowing through it will reach a high enough temperature to melt the polystyrene. If you want to you can paint the surface of the insulation with matt black paint to make it look neater, but painting glass fibre is a thankless task and best avoided.

Assembling box, radiator and pipes

You can now lay the radiator in the case and mark on the top and bottom edges the positions for the holes through which the pipes to the radiator will pass. Drill the holes carefully with a brace and a 25mm bit, and paint the inside of the holes thoroughly with more preservative stain. Put the radiator in the box and connect two short lengths of 22mm copper pipe to the compression fittings. Give the nut on the fitting one full turn with a spanner after hand tightening it, to give a watertight connection. The pipe should protrude about 150mm from the ends of the case; where it passes through the holes, it should be thoroughly sealed with silicone mastic. It is important to make sure that the sealing of the pipe to the casing is done well so that rainwater will not seep inside the solar panel and cause damage. Complete this part of the job by hammering a 50mm galvanised clout nail into the collector support at each side of the radiator to keep it in position.

Glazing

The final step is the glazing. Lay the solar panel on a flat surface and measure the width of the bottom edge, which should be 740mm. Cut a piece of 25mm x 25mm aluminium angle to this dimension, drill it with four holes 5mm in diameter to take 19mm no 8 round head screws. You will find that if you try to fit this angle in place it will bend inwards slightly because it overlaps the two corner angles. To overcome this slight problem make some washers by drilling holes in a piece of 25mm x 25mm angle and then cutting it up with a hacksaw to give you a number of small square pieces of metal with holes in. If you put one of these washers under the angle at each screw hole they will pack it out level. This angle should be bedded on mastic like those at the corners. Now lay the glass, which should be clean and dry on both sides, on top of the casing: leave an equal gap on both sides, and put the glass flush with the edge of the angle you have just fixed at the bottom of the collector. Cut two more pieces of angle, each 1415mm long, and drill them for no 8 screws. Run a bead of mastic along the edge of each angle where it will meet the glass, and coat the side of the angle that will touch the wooden side of the collector box with mastic (try to ensure that the mastic does not stick the glass to the wood). Using spacing washers cut from bits of angle as described above, screw the angle to the side of the box making sure that it is well pressed down on to the glass. Do the other side and then cut a further piece for the top of the collector, long enough to cover the two side pieces. This should be drilled and fixed just as the side pieces were, but you may need more mastic where it meets the glass because it will be raised by the thickness of the side angles above the surface of the glass. Your solar panel is now finished but the work has only just begun. The panel described has an absorbing area of about 0.7m2, which is the area of the radiator. To achieve the 4m2 recommended by the Building Research Establishment you will need six of these panels. If you estimate that each square metre of solar collector will heat fifty litres of water you could say that a household would need two of these 0.7m2 panels for each person. If a lot of people live in your house you will have to spend a long time building panels. Leave one of your panels unglazed, with the radiator not fixed in, if you are building a pumped circulation system. This will allow you to fit in a temperature sensor to control the pump. When the sensor is fixed in place, as described below, the panel can be completed like the others.

Mounting the completed collector

Before turning to the problem of plumbing you must somehow mount the collector. The angle between the panels and the horizontal is not too critical and can be anything from 300 to 60° without having much effect on performance, but an angle of 45° to 60° will allow the rain to run off the panels better than if they are at a shallow angle. Ideally, your solar collector should face south, but it can face 30° either side of south without suffering a noticeable reduction in efficiency. What is more important is to see that there are no obstructions that will cast shadows on your solar panels which should if possible have a clear field of view 45° either side. It is quite a good idea to see where any shadows will fall, particularly in spring and autumn, before finally siting and installing your panels.

The supporting framework

Most solar panels are mounted on roofs but we would not recommend this for homemade ones because it is so difficult to maintain them when they are up on a roof. A better solution is to build a framework to support the collectors and fix it to the side of the building. The supporting frame should be made of Tanalised timber, using plenty of waterproof glue on the joints. If you make the legs long enough the collectors will form a south-facing verandah or pergola, or could be used to shade a window. The part that is fixed to the building should be bolted to the wall with 10mm Fischerbolts about 100mm long; alternatively, it can be screwed with zinc-plated no 12 screws 88mm long, and plastic plugs. It is easiest to fix this piece first and then build the frame on to it, taking care to have all the frames in line with one another. The supports should be at 600mm centres so that you can leave a sufficient gap between adjacent solar panels to reach the screws if you need to reglaze them. To fasten the collectors to the supporting structure use 50mm lengths of 50mm x 50mm aluminium angle, fastened to the solar panel with 25mm no 8 round head screws, and to the wooden supports with 50mm no 10 round head screws. All screws should be zinc-plated or sherardised. Finally, connect the collector panels — one method is for pumped systems and the other for gravity circulation or thermosyphoning systems. The connecting pipes must be lagged with waterproof insulation. To run the pipes into the building you will have either to use a large diameter masonry drill at least 300mm long to go through a normal brick cavity wall, or else to remove a brick and then mortar the hole up. The drill will make a neater job and if you hire a percussion drill for a day it won’t take long to make the necessary holes. Pack round the pipes with silicone mastic once they are in place to keep out the rainwater.

The storage tank

The pipes are now inside the building but there is still a whole lot of plumbing to sort out. You will now have to buy a hot water storage tank for your solar heated hot water. The system which makes the best use of the collected solar energy is the one in which the solar heated water forms the cold feed to your existing hot water cylinder. This system means that when it is not very sunny the collector will be able to supply warm water to the solar tank, and when this warmed water enters the existing cylinder it will need less energy to heat it to a usable temperature than if the feed to the cylinder were cold mains water. The solar hot water tank should have a volume of 50-60 litres for every square metre of collector area. The-table below lists the sizes of indirect copper cylinders made to BS 1566. This type of cylinder with a coiled pipe heat exchanger in it is the correct one to use for a solar water tank. Look for the code BS 1566 and the British Standards ‘kitemark’ stamped on the side.

Diameter height capacity

in mm in mm in litres

  • 300 1600 96
  • 350 900 72
  • 400 900 96
  • 400 1050 114
  • 450 675 84
  • 450 750 95
  • 450 825 106
  • 450 900 117
  • 450 1050 140
  • 450 1200 162
  • 500 1200 190
  • 500 1500 245
  • 600 1200 280
  • 600 1500 360
  • 600 1800 440

When you have selected a size it is worth checking with the plumbers’ merchant to see if a slightly smaller tank would be a lot cheaper: the thickness of metal used changes from one tank size to another, making one size up to twice the price of the size below it. Ask the merchant if you will need a cylinder with a protective anode in it. This may be needed if you live in an area which has what the water people call ‘aggressive’ water, to prevent corrosion of the cylinder. If in doubt, buy a cylinder with an anode and play safe. The cylinder will have four connections on it, two for the indirect circuit to the heat exchanger in the cylinder, one for the cold feed and one for the hot water outlet. When you buy the cylinder you will need two connections for 22mm copper pipe to fit the cold feed and hot water outlet, and two connections for the solar circuit, measuring 28mm in diameter if it is a thermosyphoning system and 22mm if it is a pumped system. The plumbers’ merchant will supply the necessary bits to enable you to fit these pipe sizes to the tappings on the cylinder if you explain what pipe diameters you want to use. You will not need a boss on the cylinder for an immersion heater, and if there is one you will have to buy a plate to blank it off.

Plumbing – Thermosyphoning

The final problem is how to connect the solar collectors to the solar hot water cylinder. You can connect the whole thing up so that the heated water flows round the circuit under its own power, relying on the fact that heated water is less dense than cold water. If you use this thermosyphoning system the pipes between the collector and the heat exchanger in the cylinder should be at least 28mm in diameter (which is very expensive in copper pipe) and there should be, if possible, a continuous slight upward slope in the pipes from collector to tank; if this cannot be arranged it is essential that the pipes are at least horizontal. The base of the hot water cylinder must be at least 600mm above the top of the collectors, and can be put considerably higher than this. All the pipes must be well insulated so that you do not lose all the heat collected before it passes into your hot water cylinder. Where the pipes are outside the house the insulation must be water resistant so you should probably use the pre-formed cylindrical insulation (strips of glass fibre will not be waterproof), but check with the suppliers that it is really waterproof. Inside the building you can use anything as long as it will insulate. Do not forget to insulate the solar hot water cylinder, preferably with about 150mm or 200mm of glass fibre or mineral wool. It is possible to save money on pipework for a thermosyphoning system by using polythene (Alkathene) pipe, the black heavy duty type used for water supply on farms and the like, not garden hosepipe. This pipe has a larger outside diameter than the same internal diameter of copper pipe because the polythene pipe has thicker walls, and to avoid spending large sums on fittings you can buy plastic Hozelock fittings similar to those sold for garden hoses but more robust for the thick polythene pipe. If you are able to find a supplier of these fittings they will be very useful, but not many places seem to keep them. It is worth asking around. You should use polythene pipe with an internal diameter of 25mm or 32mm for a thermosyphoning system. The pipe is safe for water as hot as 60°C, and your solar panels are unlikely to produce water as hot as that; but as the pipe becomes hot it will become quite flexible and you must support it at intervals of about 600mm with pipe clips otherwise it will sag and your carefully arranged upward slope will be lost. If you can afford it copper pipe is much easier to manage because it is rigid and can withstand hot water, but plastic is good for temporary and experimental systems. If you decide on a thermosyphoning solar system you should spend some time planning the location of collectors and tanks so as to keep the lengths of pipe between collector and solar cylinder as short as possible to reduce heat loss and, more importantly, cost.

Using a pump

If you make a pumped system you will find that you can use smaller diameter pipe, 22mm if it is copper or stainless steel. The layout of the pipes will also become less critical: they can slope up or down as required to thread them through to the cylinder. You will of course also need a pump. Use the kind made for central heating installations, the important factor being the height to which the water must be pumped, which is called the head. Some pumps have adjustable heads so they can be adapted to different uses, so buy one of these if you are not sure what the head will be in your system. If your system is to be efficient the pump must operate only when there is enough solar radiation to make it worthwhile, otherwise you could end up cooling the water rather than heating it. You will need a temperature differential controller which measures the temperature of the cylinder and the temperature of the collectors, and switches on the pump when the collectors are hot enough. If fou know about electronics there are circuit diagrams available for building a controller, but f you cannot manage this you can buy one ready made. The controller will have long leads with temperature sensors on the ends. One of these sensors must be fixed securely to he absorber plate of one of your solar panels in good contact with the metal surface, and the other must be fixed to the hot water cylinder. Ask the manufacturers for their recommended fixing methods. The wire from the sensor must De led out from the back or side of the collector box, and the hole through which it passes must De sealed with silicone mastic to keep out dirt.

Protecting the collector

Whatever sort of system you build it will need protection from frost which could burst the colector panels, and from corrosion which could shorten the life of the solar installation. Corrosion can occur between copper fittings and iron adiators and you should use a corrosion proofer such as Fernox MB-1, available from plumbers’ merchants, which is added at the rate of one part MB-1 to every twenty parts water in the solar circuit. You can see now why the indirect system must be used: you do not want corrosion proofer coming out of the hot water tap. Fernox also make an anti-freeze called FP-1 which includes a foaming agent and a nasty taste so that if there is a leak into the hot water system you will get a warning. You should use a 25 per cent solution of FP-1 which will give protection down to —14°C which is probably sufficient for most areas. Check local weather records to see how cold it may become in a really bad winter and ask the manufacturers for their recommendations if it is colder than —14°C. Calculating the collector’s capacity To work out the water content of your system you will need to know the water content of the radiators used in the solar panels (ask the suppliers or the manufacturers), the water content of the coil of pipe in the solar hot water cylinder (again ask the manufacturers) and the length of pipe, including fittings, in the circuit. The water content of copper pipes is as follows:

external diameter water content in litres

in mm per metre

  • 15 0.15
  • 22 0.32
  • 28 0.54

Finally do not forget the volume of any header or overflow tanks. One quarter of the total calculated volume must be anti-freeze and one twentieth must be corrosion proofer, so work out the amount of proofer and anti-freeze needed, put them into the header tank, and turn on the water to fill the system up. All you have to do now is to wait for a sunny day. You can clearly see from all this that a solar panel system will involve you in a great amount of work and expense, and it will take many years to collect enough useful energy to pay back the cost of its construction. If you decide to build solar panels take great care to do it properly so that they will last a long time, and remember to check them regularly.

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