There are a few high price solutions to this problem, but nothing really useful or portable. This device I present here can be carried around (if empty!) and used anywhere where you want to have warm water. It is quite stable and cheap.
Step 1: Parts
- 2m x 2m of 0.5mm black PVC pond liner. 0.5mm is enough and of course it should be black! That should cost you something around 8-12Euro.
- 150-200ml of PVC-Glue. Most people think that PVC is not glueable, but that is not really true. You can actually buy this kind of glue I think it consists of soluted PVC in butanol. In Germany it is sold in cans of 200ml and 1000ml by 'Heissner' for example. But ebay should give you a lot of alternative sources. This stuff is either for pond construction or repair of inflatable boats. This might cost you something around 10.-Euro for the small can up to 30.-Euro for the liter.
- 30cm of PVC-tubing 12-20mm diameter with suitable adapters to your favorite hose-coupling
- a cheap paint-brush, because it will be destroyed after use. Some PVC-glues come with brushes in the cap of the can. Watch out for these!
- a few old rags
- 250ml of benzine, for cleaning purposes
- a flat space of 2x1m somewhere outdoor in the shade
Step 2: Preparation
You might want to work on the floor if it is rather clean, but make sure that no dust or dirt gets on the places you want to glue. It is especially important to have a flat stable surface to really press the adhesive area.
Keep in mind that you cannot correct the two parts with glue once they are brought in contact!
It is best to stick the upper part of the mattress to a rugged tubing by the mean of tape and then roll it until the middle is reached. This way you can unroll the upper part centimeter by centimeter as you glue your way to the outer part. As the PVC-glue is extremely fast drying, you should only work on about a 5cm-strip from the inside. Then unroll the upper part, press the two parts together and continue with the next 5cm-strip. Before you applicant the glue, wipe down the area with a rag of benzine to clean it from dust and oil. Then you cover both sides with a small amount of glue, wait till it is nearly dried on the surface and stick them together. Press them really tight to make a good contact. It is not important to press them for a long time as the glue will immediately work. But try not to move it too much for the next few hours after gluing. And of course don't fill it with water for at least 24 hours!
Step 3: The Shapes
The S-Shape: This shape makes one long tube inside the mattress where the water can flow from the inlet to the outlet. The advantage of this design is the long way the water has to flow to go through the mattress. So adjusting a small pressure to the inlet can give you a big temperature-difference between output and input. On the other hand, the long way provides quite a lot of resistance against the water flow and a small solar-driven waterpump might not be enough to really get water through. But you can work with it on an interval base, that means: fill it with cold water from the hose and let it rest for 20min, then exchange it by applying new cold water from the hose. This way you could fill your pool in a while with warm water. Another disadvantage is the relatively high need for glue. You would need up to 200ml for one mattress.
The Point-Shape: This shape only sticks points of the upper layer to the lower layer. You know this type of shape by a lot of air-mattresses. The advantage is the low amount of glue you need and the low resistance for the water flow. To a mattress with this shape you can install a solar-driven water-pump for a continuously heating device. There are cheap water pumps that work with 12V at only a few hundred mA. These can be operated with a cheap 12V-solar module letting you heat a bath tub of water during a sunny autumn day.
Step 4: The Adapting Hose
It is up to you where you want to attach the inlet. I decided to make both connections on one side, so I could get easy access to both. But you might also, especially in connection with the S-shape, connect the tubes on opposite sides of the mattress. It's up to you!
Step 5: The Testing
The minor leaks like those shown on the pictures are not a real problem. You don't really loose much water through them.
Another word on the UV-stability of pond liner: Without the protective water in the pond the liner is really not very stable to UV. So this means, this heating device is not for a continous installation on a roof or somewhere else. For this purpose it is also not stable enought. Think of your waterfilled mattress flying around in the next thunderstorm, that is something you would not like to see.
The advantage of this device are the low costs and the low weight of it. So take it out when you need it and store it somewhere indoor when not needed!
Step 6: The Usage
Use it with a solar driven pump and heat up a large amount of water over a few days.
Use it for a warm shower, put it on a flat roof and wait till the desired temperature is reached.
Use it while you're camping or after a bath in a cold lake.
Build a few of these devices to increase the heat gain.
Any more ideas...?