Wednesday, 30 June 2010

Solar Electric Dreams......

Our friend Andy Rankin came to lunch on Sunday.

He is proprietor of Midsummer Energy, a firm specialising in solar and wind-powered renewable energy installations for houses and boats.

(I'm not saying this because he is a mate, but if you are interested in getting or increasing your renewable energy systems, look no further. Andy is totally honest and a great bloke: really helpful. In business to make a living, not a killing. We've recommended him to lots of fellow boaters and they all say the same...)

Enough of the ringing endorsements: Google Midsummer Energy for the website.

Anyway, Jackie cooked a lovely lunch of chicken breasts in herbs with hot whole tomatoes and asparagus, boiled baby spuds and hot flatbread.

It was utterly Yummy!

(If you are new to Pippin ramblings, by the way, then apologies for being a bit 'foodie'. However, Jackie does work in television production as an edit producer. She's just started a new job on a Famous Celebrity Chef's latest programme, so it seemed sensible to try out one of his recipes.....!)

So, over lunch and more than two (after two, we aboard Pippin consider it rude to continue counting.....) bottles of very agreeable white, Andy and I had a chat about increasing Pippin's solar bank. It's already pretty big, but I personally think you can never have too much electricity, especially if it's produced without involving oil companies, the tax man, or destroying the planet etc etc.

What I had in mind would have greatly improved our renewable output in winter, when times is hard for solar, but would also result in a considerable over-production in summer. Even Pippin's silly big 1340 amps of battery bank gets fully charged on a good sunny day by the current set-up.

What we need then is some means of what electrical engineers call call 'Load Dumping'.

The meaning of this somewhat inelegant phrase is fairly simple: It is the getting rid of the excess power in such a way as to get some use out of it.

The quickest and easiest way I know of disposing of electricity is through a heating element, so I suggested putting an extra element in our hot water tank. This would then heat our hot water and help to further reduce (if not almost eliminate) our gas consumption during the summer months.

A highly desirable outcome!.

However, Andy reckoned that a 12 volt element would be too weedy to do much heating, and running a meatier one through the inverter would result in a net loss of power overall, even in strong sunshine.

'Hmmm.' I thought. 'Pity'....

But the story doesn't, quite, end there.

Now I'm well known as a Drama School graduate, not as an engineer, but I didn't spend all the time in science classes looking out of the window and waiting for the bell......

After Andy had left for Cambridge, I had the following thought:

If you reduce the air pressure in or around a vessel containing liquid, it can be made to boil at a lower would it be possible to construct a solar-electric vacuum-pump? This would be attached to a pressure vessel/water tank. Phase one of load dumping lowers the pressure to a level where, in phase two, a twelve volt heating element is used to heat the water to a useful temperature.

If you are clever and possessed of lots of ologies, please leave a comment, even if it's to say it won't work.

I'm just intrigued, but don't know the science or have the maths to take the idea further.

Of course, one could also use the extra power to do some welcome cooling......

Twelve-volt air-conditioning unit , anyone?.....


  1. John,

    If it boils at a lower temperature under pressure what would happen when you release that pressure?

    Mark C.

  2. John: in the words of Montgomery Scott, you cannae break the laws of physics, Captain!

    Reducing the air pressure won't help you - the water will just boil at a lower temperature, which is why cups of tea made on Mt Everest are disgusting!

    What you need is a 12v immersion heater with lots of grunt. A typical domestic 240V one outputs 3kW, the same as a kettle. The biggest 12V one I can find is 500W, but I reckon that would do you fine - as that's more than the total output of your solar panels and wind generator combined.

    Then you need a voltage-sensitive relay circuit (which I could make for you fairly easily) to engage the immersion heater only when the battery bank is nearly fully charged, and to disconnect it when the battery level drops below a given threshold. Bob's your uncle. Give me a ring if you want more details.

    Mike PJ

  3. In fact, having done the maths, a 500W element should take about 3 hours to bring 30 litres of water (guessing your calorifier size) to 60C from 20C, so probably quite feasible...

    Immersion heater here:

  4. Look at Bones' blog - she got your tiller pin!