Tuesday, May 18, 2010

The Story So Far... (part 2)

Originally, I just wanted to run my work laptop on solar power.  I thought, "How hard can it be to power a 30W load for 8 hours a day, every day?".  Turns out the answer is, "pretty hard".

The problem is that solar power is very variable and unreliable.  Some days you get loads other days you get nearly nothing.  The batteries help but they are fragile beasties and if not kept charged up fully will soon die (like in a matter of days if left completely flat).  So you are always balancing using the stored solar power in the batteries with not using so much that you kill them.

One thing this leads to is any big constant loads like the water heater are very difficult to run.  Even if you've got enough solar panels installed to run it, you will almost never know if you've got enough solar power coming in to run it.  If not, then the batteries start to drain and you risk not having enough power for the night.

The first problem is that water heaters use an enormous rate of power consumption (typically 3kW).  I've only got 1.8kW of solar panels.  The answer was to reduce the power used by the heater.  The easiest way to do this is by reducing the voltage fed to it.  At 230V, the heater will use 3kW of power.  At 110V, it will only use about 650W.  It will of course take much longer to heat a tank of water at this rate but in the summer, you can count on up to 6 hours of solar power on a good day... More than enough to heat a tank of water to 55'C.

I found an old tool transformer at a car boot sale (I love car boot sales - why buy new when you can buy randomly old?).  Conveniently, this does just what I need; converting 230V solar power to 110V.  So now I only need 650W of reliable solar power to run the water heater.

It wasn't as bad as it looks... Honest.  Just flaky paint and some external rust.  The guy let me have it for £15, not bad considering it was a custom wound 4kVA unit - more than man enough to run a water heater continuously without getting hot or catching fire or 'owt :)


With a bit of Hammerite, things were looking a lot more ship-shape (or at least transformer-shape).

And then off it went to its new home in the airing cupboard.

That leaves you with problem number two...  You could turn the water heater on and off by hand while watching the power levels and the window for clouds to appear and spoil your fun.  I did this at first but quickly grew tired of running up the stairs to the airing cupboard.  So what was needed was an automatic way to measure the solar power, the condition of the batteries and then turn the water heater on and off so that you keep the batteries charged up and only use incoming spare solar power to heat the water.  In off-grid living, this is called a dump load controller.  Something that takes all the spare energy and dumps it somewhere useful (water heaters are the usual dump load of choice).

Coming up in Part 3... A whole lot of bodgery involving a kit from Maplins, a doorstop laptop, some retro 90's software development, seashells and polyfilla...

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