solar collector 5 : absorbers 2

time to finish the absorbers. i had a sheet of coated plywood - the kind used for concrete-pouring - cut in to 15cm wide strips. i glued and screwed these to the sides of the absorbers plywood base. the sides stick out 10 cm at the back and about three and a halve at the front.


the back gets filled with insulation and covered with plastic. everything is glued together with tec7. the insulation is what's left over after insulating the water tank and is there to prevent the heat collected at the front to be radiated out at the back. the material used at the back has to be very heat resistant since the collectors can get very hot (temperatures in the absorber can get up to well over 100°c!)
the holes where the copper tube comes out of the collector are also sealed with tec7. i used a PU based black paint to seal all the edges of the board and all the screw holes.
it all looks a bit scruffy done ;-) but let function prevail over form and i'm pretty confident it can take at least 25 belgian winters!


the front needs some glazing. just like in a greenhouse you want to trap the heat inside. the material normally used for this kind of application is either (hardened) glass or double layer polycarbonate.
however i stumbled across this PVC material that is very thin - only one mm - very easy to work with - it can be cut with a pair of scissors - and, unlike traditional PVC, can take high temperatures and is UV resistant. it also came cheap.
the use of this material is experimental though. when it lasts 10 years i'll be happy. when it fails - when ever that is - i'll replace it with something more traditional.


i decided it's not necessary to make the window removable so i just glued it on to the top. when it has to be removed it can simply be cut off. this is the easiest and cheapest way of mounting the glazing.



the absorber for the first time out in the sun (it's on it's side here). before installing it on the wall i leave the absorber in the garden for a while to check how it holds out in sun and rain.

solar collector 4 : absorbers

time to start building the actual absorbers or collectors. at this moment i'm building two collectors each about one square meter large that coupled together will function as one. in a later stage i'll build a third and a fourth collector.

these are the parts that'll make up the collector : ten meters of soft copper tube, one m² of aluminum sheet and plywood and a couple of hundred stainless steel staples.




the collector is build on a sheet of plywood. this material is a good insulator, it can withstand relatively high temperatures (usable to about 100 °c) and tough. the copper tube is laid out in a serpentine pattern. on top of the copper comes the aluminium sheets that are bend around the copper.



in the example i'm working from silly putty is used to bond the aluminium and the copper but i decided not to use it : it's meant to fill all the gaps between the copper and aluminium to optimize heat transfer. however silly putty is an excellent insulator and a thin layer of silly putty seperating all of the copper and aluminium would actualy prevent heat transfer. i estimate that about half the tube is in contact with the aluminium which should be just fine.


the aluminium is 0.3 mm thick and i bought it in pieces 20 by 40 cm large. it's easy to work with and bending it exactly to the copper tubes diameter was easy. the other end of the sheets fits underneath the tube so the copper is completely surrounded by aluminium. the aluminium is stapled to the plywood.

last step are a couple of coats of heat resistant paint - the kind that is used to paint engine blocks - to enhance heat absorption.


next step : the frame.

solar collector 3 : the water tank 2

time to put in the tank insulation and lining.
it's essential the tank is as well insulated as possible. the more insulation the better the system works.
there are many kinds and types of insulation but there are only a few with the right properties for this application. the material has to be resistant to relative high temperatures, mechanical stress (since the tank is insulated on the inside the material will have to resist the water pressure and weight of the water) and have an as low as possible thermal conductivity.
the material of choice is polyisocynurate or PIR ; very tough with great thermal properties (it's actually twice a good an insulator as rock wool with an R-value of 8 per inch). i'm using 10 cm thick boards that should insulate the tank pretty well. PIR is not only a super material but also super rare : it was only available in ghent - a city 50 K from where i live - and it took me three hauls with our small car to get it all home. ;-)




next up is the pond lining. EPDM liner is the best quality - lasts forever and resists 135 °C - but exceedingly expensive. pvc liner doesn't last as long and is less heat resistant but it can still withstand temperatures up to 80°c (tank stays below 65°c) so it'll do the job just fine.

just cut to size and fold :-)


and of course one of the center pieces of this installation : the heat exchanger between the solar heated water in the tank and the tap water; here in the shape of a hundred meter long coil of PE tube.

solar collector 2 : the water tank 1

the first thing to construct was the water tank. the tank has to be large enough to hold about 500-1000 liters of water and insulated well enough to keep the water hot for at least a couple of days. of course the more water the tank contains the more heat you can store but also the longer it takes to heat that body of water.
this tank will hold 650-700 liters and i think that is a good size for the 4 m² collector surface i'll be using.
of-the-shelf insulated tanks can be used for this but they are very expensive. buying an ordinary stainless steel or plastic water tank and adding insulation is another option but also not very cheap or particularly easy.
i decided to go with the wooden design where you build a large wooden box or crate covered on the inside with insulation board and pond liner.


this is the finished box. its basically a cube with a size of one meter. it's build in osb with some 5*5 timber for support on the bottom and top. it's glued and screwed together and should be able to withstand the water pressure (which is at its deepest point less then 10 % above atmospheric pressure).


the other thing i did was bend the copper tubes that will go in the actual collector. you see them on display on the water tanks lid.
i decided to use continues stretches of soft copper so i would have no soldered connections or couplers in the collector decreasing the risk for leaks dramatically. it took a lot of care to bend the long pieces of copper pipe into a serpentine shape but i managed to do three without screwing up.
two of these serpentines will go in the two identical collectors i'll be building first. the third - somewhat smaller - serpentine will be used for another collector i'll be building later.

solar collector 1

a while ago we decided to start using the power of the sun to at least partially power/heat our house. next to installing a 2 kV PV solar array on the roof i also planned on building a solar collector to provide hot water.

after doing some research i went for a non pressurized flow back system using only water. i found an excellent example to work from right here http://www.builditsolar.com/Experimental/PEXColDHW/Overview.htm. in this system the water in a large insulated tank is pumped through a collector sitting on the roof and thus heated by the sun. inside the tank there is a large (100 meter) coil of tubing acting as a heat exchanger. the tap water absorbs the heat of the water in the tank through the coils side walls. the heavily insulated tank is capable to hold its heat for days-weeks depending on water use and outside temperatures.

there's an additional set temperature tankless gas boiler in the water circuit to provide hot water when the sun hasn't been out for a while.

the reason i decided to go for a DIY project is cost and return. compared to electricity producing PV arrays solar collector hot water systems are very expensive and give only a modest return on investment. the goal is to spend 1500 euros - not including the gas boiler - on the installation as opposed to the 5000-7000 that would be spend on an of-the-shelf system (also not including the boiler).