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Old 09-08-09, 09:14 AM   #1
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Default DIY thermal differential controller (solar hot water pump controller)

In anticipation of using a solar hot water system, I am working to design a DIY thermal differential controller. The main reason behind this is that it is a simple device that costs $130+ for a commercial controller. I know I can make one for less than half that price. Plus, if I put together good documentation others should be able to easily construct them as well.

So, what is a thermal differential controller anyway? A thermal differential controller is what is used to control the pump between the solar panel and hot water tank. It senses when the solar panel is hotter than the tank and starts up the pump so that the water can go get warmed up. When the panel cools off, the controller then turns the pump back off so it doesn't cool down the tank. Sounds pretty simple doesn't it? Well, it is and thats why I don't want to pay $130 for one!

The setup is pretty simple. You have a micro controller and two sensors. One sensor goes in the panel and ones goes in the tank. You also need a relay of some sort to switch your pump on and off.


For the micro controller, I am going to use an arduino. It is very popular micro controller and is more than capable of handling this small task. I'm also some what familiar with using it.

The sensors will be 10k thermistors. These are what are commonly used on the commercial controllers. This makes for quick and easy replacement should the user choose to go that way. The other option is making your own sensors which will save a fair bit of money. This is the route I will pursue.

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Old 09-08-09, 07:31 PM   #2
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I'm lazier than you. Wouldn't a solar powered pump be easier? If there is enough solar energy to run the pump then should also be enough to solar energy to heat your water. You would still have monitor the temperature of the tank to ensure it doesn't get too hot.
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Old 09-08-09, 10:39 PM   #3
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A PV powered pump will work okay. However, it will start up before the panel has warmed up and it will shut off after the panel has already cooled down. So, you will loose some efficiency.

With a normal thermal differential pump, you set it so the pump only comes on if the solar panel is 8 or more degrees hotter than the water tank. You also shut it off when the solar panel is only 4 degrees (sometimes more) hotter. These temperature differentials are set to compensate for thermal losses through the lines. The longer the line between the tank and collector the greater the temperature differential. It of course matters how much insulation is used around the lines as well. In the commercial controllers, these temperature differentials can be set to as much as 24 degrees.
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Last edited by Daox; 09-09-09 at 09:48 AM..
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Old 09-09-09, 07:51 AM   #4
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I've been in contact with Gary over on BuildItSolar to get an idea of what features he would like to see and what is important in such a controller. Does anyone here have any input?
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Old 09-09-09, 09:46 PM   #5
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The easy way is two temperature sensors and a comparator with adjustable offset and hysteresis.
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Old 09-10-09, 06:13 AM   #6
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Quite right, NiHaoMike. However, that wouldn't leave much room for additional features. For instance, Gary commented that it would be nice to have an LCD display that shows a couple things. He also mentioned a max tank temperature is a nice feature or displaying the number of hours the pump has been in use. I'm not sure if I'll end up going that far with it, but I'd like to know that I have the option.
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Old 09-11-09, 10:58 PM   #7
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Then add all the fancy stuff like control outputs to start use of hot water on the spot (such as washing dishes), PWM pump control, thermostat for home heating that uses excess hot water production, maybe even Ethernet connectivity.
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Old 09-12-09, 12:01 AM   #8
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Default Differential Controller...

Daox,

Great idea on the controller.

There is a variant on the differential controller idea... it's a controller which is specifically for hydronically heated homes. I'm not so sure what the general name of the controller is, but the feature is that it monitors the temperature outside the house as well as inside the house. Hydronically heated houses have a very large thermal mass (AKA: concrete radiant floor). This thermostat can sense the temperature drop (or rise) before the inside sensor can and helps prevent undershoot and overshoot conditions.

I'm not so much of a circuit hacker, but I suspect that the 'adjustable offset and hysteresis' features could apply tho your project and with some tuning could apply to this type of thermostat as well.

Bst Regards,

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Old 09-12-09, 08:29 AM   #9
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Mike, that is definitly doable. Perhaps, once the base model is done I/we can start developing add on modules for it for doing additional things. I was also thinking about integrating data logging (perhaps thats what you meant by ethernet though). I'm really no electronics expert though. Even adding a simple LCD screen will be a learning experience for me.

How would PWM control of the pump help though? As far as I can tell, the larger the temperature differential between the fluid flowing through the panel and the panel itself, the more efficient then panel will be. I'd imagine slowing down the flow would decrease efficiency.


That is a very interesing idea AC Hacker. I like the idea of a 'smart' hydronic controller. You'd definitely be able to get some extra energy savings that way. I'll have to think about using that as a possible add on.
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Old 09-12-09, 11:46 AM   #10
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Default Differential Controller Curcuit...

Daox,

Just to prime the thought-pump...

...have you Googled: app note op amp differential

...such as here:

app note op amp differential - Google Search

...or Googled: differential controller circuit

...such as here:

differential controller circuit - Google Search

Also, as far as the ethernet monitoring part of your project, Ward Cunningham, the guy who invented Wiki (the structure upon which Wikipedia is built) has offered to help me get the Homemade Heat Pump Manifesto project on line, with monitoring data publicly available in real time. Obviously, I will be sharing these development details with you as they come available to me.

Best Regards,

-AC_Hacker

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