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25 March 2007

Sorry about the time between this newsletter and the last one. It hasn't really been the sort of weather that makes me think about solar power, more like wood-cutting weather, but summer time started here today and it got me thinking again...

1) Price goes down again
2) A few thoughts about earthing
3) Heat that water

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1. Lower Price
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This really isn't anything to do with solar power, but it is to do with my Solar Power Design Manual. A couple of months ago I increased the price. The theory is this - price goes up, sales go down, total income remains the same. True, but that's not what I want. I want more people to know about solar power. So I put the price down again and that's where it will stay for the forseeable future.

So, get it now, at the new, old, lower price:

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And if you subscribe to the list on the download page, you'll get updated versions for no extra charge.

2. Earthing low voltage systems
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The other day I got an email from one of my customers asking why my wiring diagrams show only the batery neative terminal earthed, and suggesting that the solar array should be earthed as well. A good question.

The correct answer to the question is that your wiring should be in accordance with the wiring regulations of the country you're in. That may well require the earthing of the module frames, in which case you should do that. Most manufacturers provide an earth terminal; if your modules don't have one then you can use one of the mounting bolts. Remember to remove the anodising first though.

So why don't I show that in the diagrams? Two reasons really:

a) It avoids confusion. It's very important not to earth either output terminal - I've seen the negative earthed and that can short-circuit the controller, causing it to overcharge the battery. The less complex the diagram, the less likely confusion is to arise.

b) It's not necessary. Electrically I mean. It works like this: If you earth the battery negative terminal, and everything is connected to the battery, then the highest voltage that can exist in the system is the open-circuit voltage of the array. In a 12 Volt system that's 20 Volts or so, which isn't considered dangerous. Of course, if there's an inverter in the system then its output is 110 Volts or 230 Volts AC, but the input and the output are electrically isolated and the AC side will be earthed just as if it were a grid-connected circuit.

All the same, earth the module frames if you want, or if the wiring regulations tell you to. And of course, these are only my opinions and you should always gain the opinion or assistance of a qualified person if you're at all unsure.

3. Water heating
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Any of you who are familiar with solar technology will know that photovoltaics is no way to supply heat. And also that water heating panels are the most cost-effective and easiest way of displacing fossil fuel usage.

So, now our house is essentially finished, I'm going to install some solar water heating. Of course I want to do it all myself, so I looked around for a decent DIY design. And there isn't one, at least not one I'm happy with. So I've done one myself, which has got nothing to do with putting radiators in boxes and painting them black. If you're wondering, that works OK actually, but I'm looking for something a bit better.

Don't hold your breath; I've got to build it, install it and test it yet, but if it works as well as I hope it will I'll publish the design. It should all be standard parts which you can get at any hardware or DIY shop, and I've got an idea for a heat exchanger to work with mains pressure systems. I've done the calculations so I know that bit will work. And I've bought most of the bits already.

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