This is an update on the simple solar water heating system that Nick Pine and I have been working on.
This is the third prototype. It is showing some promise, but still has at least one unresolved problem with high nighttime losses (see performance section). Any ideas for resolving the performance problem, or improving the design would be most welcome.
The Goals for this project are:
An efficient solar heater with
performance comparable to commercial units.
A long life with little
Four season operation in cold
Easy to build -- a weekend project
(well, maybe a long weekend)
Low cost -- less than $500 with all
new, high quality materials (much less if you have a good scrap pile, or
Use only readily available materials.
* This does not mean that it produces 100% of your hot water in the winter -- it would take a much larger collector to do this. Nick would argue that the collector should be made large so that it does meet nearly all your year round water needs, and that is certainly possible.
This solar water heater consists of:
An insulated pond that stores solar
heat (about 4ft by 8 ft and 11 inches deep on the prototype)
A pipe coil immersed in the pond
that your water flow through on its way to your heater. The water in the
pipe coil is preheated by the pond water.
A combination absorber and
insulating lid that absorbs solar energy. The insulated lid also reduces
heat loss from the pond.
A small solar electric pump that
pumps water from the bottom of the pond to the top of the absorber, where it
picks up heat and then drains back into the pond.
A glazed cover that is high on the north side, and slopes down to the pond level on the south side. The north, east, and west walls of the cover are reflectors to direct additional sun onto the absorber.
Schematic cross section of the solar pond water heater.
The pond is well insulated, and lined with an EPDM membrane for a long, leak free life. The prototype is sized to meet a good fraction of the hot water demand for two people -- it can be made larger to satisfy larger demands.
In operation, when the PV panel is exposed to sun, it starts the pump up. The pump pumps water from the bottom of the pond up onto the EPDM absorber. The water forms a shallow pool on top of the EPDM that is about a half inch deep. The water heats up as it flows around on top the absorber, and eventually this heated water finds the drain that drains it back down into the pond.
When there is no sun, the PV panel does not generate enough power to run the pump, and it stops. The water in the pool on top the EPDM drains back into the pond. This leaves you with a pond storing hot water that is well insulated, and has low heat loss.
When the house calls for hot water, the initial water comes from the water that is already in the pipe coil, and is up to the same temperature as the tank water. The coil itself holds enough water that just the water in the coil will satisfy most single hot water demands. For the prototype, the 200 ft of 1 inch diameter poly pipe holds about 8 gallons. The coil can be made longer or larger in diameter to hold more water if desired -- a 300 ft coil of 1 inch poly pipe might be a good choice at 12 gallons. If more hot water is needed than can be supplied by the water in the pipe coil itself, then the water is still heated as it flows through the pipe coil, and the water coming out of the pipe coil outlet will be preheated, but not up to the full pond temperature. The pipe coil is acting as a heat exchanger to pick up heat from the pond. The draw tests shows how this works for the prototype.
The final design will have a thermal switch that will shut the pump off when the pond temperature reaches 140F. This is to protect the black poly pipe pipe and the EPDM liner. This can be done with a simple $10 thermal snap switch.
This is still and experimental concept. Here are some known potential problems -- there may be other ones unknown ones as well -- as always.
The High Density Polyethylene pipe
may not stand up to a combination of high pressure and high temperature.
It will definitely be at the limit of its intended use.
This is why the tank temperature must be limited to 140F (or so) with the thermal switch.
I used the lowest pressure NSF approved PE pipe I could find (100 psi) just to get an idea if this is a problem or not. Using higher pressure PE pipe (160, or 200 psi) would provide more margin.
PEX or PEX-AL-PEX might provide somewhat more temperature capability than the HDPE pipe, and would be a good candidate to try.
If all else fails, a shorter, smaller diameter coil of copper pipe could be used -- copper is such a good conductor that the pipe coil would not have to store a substantial amount of water.
The life of the EPDM absorber may
not be up to expectations, although it has been used successfully in this type
of application before. The EPDM pond lining should have a very good
The small pump used in the
prototype will almost certainly not have a long life. One of the
remaining challenges is finding a small 12 VDC pump that pumps about 2 gpm and
will have a long life at 140F, and does not cost an arm and a leg.
One caution is that the pond is
filled with water that is hot enough to scald. The water is protected by
the glazing layer, and by the absorber/lid, but this is still a hazard worth
considering. If this is a concern, then using something like twinwall
polycarbonate glazing with good glazing supports might be one solution.
The construction is intended to be simple, use readily available materials, and require no special tools.
The construction of the water heater is covered here in detail.
The performance testing is covered here.
There is currently an unresolved problem with high losses during the night (see the Day/Night test under link above).
Any comments on this would be welcome.
We may be a bit biased on this, since we came up with this design in an effort to answer some of the short comings we saw in existing solar water heaters, but here is how we see the tradeoffs.
Batch Water Heater:
Batch solar water heaters are a nice simple design that can provide a high fraction of your hot water in warm climates. They can also be used in cold climates, but must be drained for the winter. Another shortcoming of this design is that the tank temperature can drop quite a bit at night, because of the relatively high heat loss out the glazing. There are some schemes that provide for night time tank insulation, but these generally require that someone deploy the insulation at night and remove it in the morning.
It can also be difficult to find a good tank for the batch heater. If you can't find a used one in good shape, then you pretty well have to buy a new water heater and strip off the insulation and fittings to make a batch tank.
Our proposed design eliminates these short comings at the cost of a bit more complexity, and few (not many) more dollars.
Commercial Drainback and Closed Loop Systems:
These systems work fine, have relatively low maintenance, and a long life. But, they are expensive -- several thousand dollars. People into building things can tackle one of these systems, but it is a fairly involved task, and the components are expensive.
The goal of the new design is to provide equivalent performance and life to these commercial systems at a much lower cost.
Work on the earlier prototypes of the solar pond water heater can be found here:
Horizontal Pond Domestic Hot Water Heater
experimental concept for solar heating water that uses a pipe coil in a
glazed pond to collect energy.
A 2nd prototype with many changes is in work. Current status of Proto 2.