Solar Controller

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The open-source hardware design for the Tirna Solar Controller can be downloaded by clicking the blue coloured box below:

Solar Controller Schematic


Water can be heated by pumping it through a panel designed to trap solar radiation and convert it into heat in a collector in close thermal contact with the water. Thus, the heat is transferred to the water. The water is normally pumped through the panel from the bottom of a water tank and returned to the top of the same tank. Stratification keeps the hot water returned from the panel from mixing fully with the cold, heavier water below it, so we don’t need to be able to heat the whole tank to the desired temperature before we can obtain hot water.

This technique for obtaining free hot water is used routinely in Mediterranean latitudes where the sun is so strong, and the skies are so clear, that “thermo- syphoning” can be used. No pump is needed, and this is the simplest possible way to collect free hot water. However, in more northern latitudes and especially where the sun can suddenly appear and then disappear as clouds pass over, we need to be able to start and stop the flow of water through the panel as the trapped solar energy follows the fluctuation of the incident solar energy. We need a pump and a controller.

Our Controller and Panel

Tirna Electronics Ltd is currently developing a sophisticated, microprocessor based, controller which controls up to 3 tanks automatically or under user control. However, we believe there is a need for a simple controller which can be installed and maintained by the user with nothing more than a cheap, widely available voltmeter.

If the user is DIY minded, they should also be able to make and install their own panel, with the help of a plumber, if necessary. If you would prefer to buy the controller or the panel, then we will be producing both panel and controller. Both will be available in limited quantities by the end of the first quarter of this year.

We are publishing the design of the controller now as a reassurance of its simplicity. There is no microcontroller, and no software, and the design is fully described as we believe that you cannot maintain and fix a circuit you do not understand. If you want to build your own controller you are free to do so but it would be easier to register your interest with us, tell us what you want to do with it (we may be able to advise on some points), and wait for our PCB and kit of parts.

When more than 20 people have registered an interest in the PCB, we will have it made. It will be double sided, plated-through and all components will be through- hole. The quad op-amp will be socketed for serviceability though we would be surprised if you ever had to replace it. None of the components will be static sensitive. CMOS is banned! The schematic is extensively commented, and we will try to answer any further questions you may have regarding this design.

It could make a good school electronics club or 1st/2nd year university undergraduate project and again, we will try to help anybody proposing to use it in that way. It’s hard to beat “learning by doing” and students are motivated by projects which fulfil a social need.

Whether you make your own controller and panel or buy them from us, we recommend the book by Lee Rose, “Solar hot water – choosing, fitting and using a system”. ISBN: 978-0-9549171-9-7, Published in 2012 by Low-Impact Living Initiative, Redfield Community, Winslow, Bucks. This excellent book contains 265 pages of informative, and essential information.