Paul Westaway wanted to make his solar water heater more efficient, so he created a cool gadget. He put together a differential controller to direct the circulating pump that sends the water through the solar panel or woodstove to the storage tanks. The controller turns on the pump when the temperature of the solar panel or woodstove coils is higher than the temperature in the storage tanks. When the temperature in the panels or stove is lower than the temperature in storage, the pump turns off.
I'm interested in doing something similar to my woodstove and am curious about design / construction details on the heat exchanger, especially how to support and seal the pipes where they pass through the woodstove. How much of what kind and sizes of pipe(s) did you use?
One safety concern when putting a heat exchanger on a woodstove: if closing valves (including solenoid valves that close when power is off) can close off a section of pipe that passes through the woodstove, a pressure-relief valve is required to prevent a possible steam explosion. I've seen pictures of a woodstove with the top blown off. While you may know better than to close both valves with a fire going, your kids, spouse or the next homeowner are likely to forget. I'd guess a standard hot-water-heater pressure relief valve would work.
I have a woodburning stove (installed for many years). It is gravity fed so no question of explosion if pump fails. However if you use a pump you must have a circuit that is gravity fed outside the pump circuit that can dump heat, if not you will be sitting on a "time bomb" the clock for the bomb being a random time when the pump fails.
Now, I’m puzzled: This electronic enhancement to move cold to hot seems so obvious, I thought all Solar H20 heaters would have naturally already had that functionality. While I do not yet have a Solar Water Heater for my house, I do have a Solar Water Heater for my pool. Simple plumbing circuit pushes water from the pool to the filter to the roof panels and dumps hot water back into the pool. Simple.But Living in South Florida, the pool even gets TOO hot in July, so I run the pump at night; effectively taking the hot pool water thru the panels and cooling it in the night air. Seems obvious - even necessary, and I’ve been doing it for 10 years. So, commercial Solar Water Heater don’t do this-? I think you’ve got a nice market on your hands, go for it!
JimT, you are wasting a lot of electricity, first to run pool water through the panels all day until they are well above the desired temp and then again at night to cool them down.
What if you have a simple vacating tube design for the solar panels? As soon as the desired pool temp is reached, the pump is stopped and the panels vacated.
Of course the solar panels will get hot when not filled (cooled) with pool water, so you need to verify that they can stand being in the sun and not full of water, but that operational condition must be possible anyway, for example in the case of a power outage. As far as I know, all panels must allow for this.
So, simply lower the max setpoint of your pool water controller to your desired water temp and enjoy a nice swim with lower power bill!
Diff controller is a fine choice for the application. May suggest putting in some hysteresis so it does not switch on/off a lot. I have a question for you. What did you choose as a pump for the heated water? I am planning a similar system and was curious what would be a good choice for a hot water circulating pump.
I agree that the guts is the controller, and hysteresis is probably programmable if it's a reasonable product.
But I don't understand why the part numbers for the standoffs, terminal strips and lights are listed, but the controller is not. "DTC Deluxe Dual Temperature Controller" is just a generic description. The project seems like it's just about the enclosure, and choosing the right controller was skipped over.
I hope we can find out which is the ordering code for the dual temp controller.
NOTE that the two relay outputs of the controller are wired in parallel and the 4 differential input sensors include *two* sensors for the water heater. This makes it a safe bet that all you need to control separate flows through the solar panel and stove independently, is to remove the wire bridges and wire each relay to one pump, so a pump for the solar panel loop and an additional pump for the stove loop.
The general working principle of this hot water heater is not new, about 15 years ago I installed a "Luigjes" Hottop solar boiler (it appears it was a design only available in The Netherlands) where Mr Luigjes had produced a series of differential temp controllers for boiler vessels with two heat exchanger loops, one being mounted near the top of the vessel and having the temp sensor also placed high up, this one was to keep the upper level of the boiler hot with a backup energy source such as a gas heater or (in my case) city-supplied hot water from a nearby power plant. Note that this rather early version of Hottop temp controller was produced in limited quantities and that was evident from the controller board execution, it looked like someone soldered it together in their garage. Today the Hottop boilers are still produced, the original Luigjes appears to have found its place with Heliomax BV in Breda, the Netherlands.
The lower heat exchanger was connected to the solar panel and as soon as the temp controller sensed the temp in the panel to be higher than the boiler temp, the pump was started and the energy from the solar panel caught in the fluid and transported to the boiler. Note that the diff temp controller had two temp sensors to sense the difference between (lower) vessel temp and solar panel temp. In addition it also had a safety trip point: if the solar panel temp got close enough to the boiling temp of water, then the pump was stopped to avoid steam production and overpressure. Part of the solar water loop was empty (air) so that the solar panel could drain completely and neither freeze nor boil when the pump was not in action. (this put requirements on the way this loop was plumbed!)
The main vessel of the Hottop solar boiler contained the city water that was used for showering, washing and so on. I added a thermostatic mixer on the hot water output at the top of the boiler so that even if the outflow would be close to boiling hot (95 deg C) then it would be mixed down to a safer 55 deg C, which allowed me to run it directly into my washers and seriously save on electricity for water-heating (my cheap washers had a cold water inlet only, but could take up to 60 deg water directly and skip the electric heating program)
Note that the design of the Hottop allows you to have only one vessel (typically 50 gallon = 200 liter or more) and not need any other hot water production, except to keep the "top" of the boiler at temp when no solar heat is available, either with an electric heater or a gas fired heater or dedicated furnace loop.
I found that a far simpler means of solar water heating was to submerge a black plastic sheet in the pool when it was not in use. The sheet did ned to be well below the surface, so as to heat a larger volume of water, it could even be on the bottom, out of the way. It was not hard to remove for serious pool use, and it consumed no energy, and it was very cheap.
Most gadget freak write-ups, it appears to me, are incomplete in one way or another. The result is that the missing order number for the dual temperature controller is to be expected. Just attempt to find a circuit, or some other critical build information. Usually some vital information is missing. That is what to expect.
I grew up on Popular Electronics. Every project was very well documented with schematics and wiring diagrams and parts lists. I seem to see too many of these GF articles not even up to the standards of PE in the good old days - 50's and 60's.
This column needs to be presented with better editing. I agree that many of these GF cases are incomplete and have indecipherable or incorrect schematics and incomplete parts lists like this one and dead end links. Even Design News has had too many errors lately.
I know the audience is full of engineers but who needs to have work so hard to read the article? When these projects are presented with a half baked presentation it is disrespectful to the author and to the readers. And a pain in the @ss to read. It takes the fun out of it.
So PLEASE put a little time into editing and put everything, photos, schematics, BOM's etc into the article on the same page or have links that work.
Nowadays, I would update the power supply with a voltage regulator IC and update the op amp, but I think you'll like the simplicity. Hint: two precision thermistors, a 25-turn pot and a voltage comparator op amp.
If you revamp and update this device, you could submit it to the Design Gadget Freak column. We pay $500 for complete Gadget Freak entries. If you're interested, send me a note at firstname.lastname@example.org and I'll send along the details.
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