Solar electrical water heating

I'm interested in reducing my fuel/carbon foot print and estimate that most goes on heating water.I have a farly normal fully pumped gas HW/ CH system with a HW tank, with two insulation jackets. Tank stat set at 55
A number of neighbours have gone over to Solar for hot water using the vacuum collector system and it seems a very good idea, however the cost of significant (circa £3K) and requires a fair bit of specialised plumbing, for example a new tank with 2 coils.
I wondered about the cost/benefit of using solar electricity to heat water.Sounds crazy but my tank already has a top-up immersion heater and I wondred if this could be run from batteries. Looking on ebay I noticed that there are 12/24v heaters than can produce 500 watts and can replace an existing immersion heater .
I know that the specific heat capacity of water is very high so heating a water tank would require a lot of fully charges batteries and hence panels to charge them. However every degree you can heat the tank for free is one that does'nt need gas right ?
Electric also has some advantages. It might be possbile to do this in an incremental way thus spreading the cost i.e adding panels/batteries as you can afford them.It could also incoporate a wind charger to get some power in all light conditions i.e at night. Finally the plumbing might not need changing at all to incorporate this meaing this falls within the realm of a purely electrical installation + some mechanics to fit the panel/ wind charger
I understand that electro voltaics are pretty inefficient, but I wondered if this approach might be cheaper or even more cost effective than the full solar hot water panel system ?
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Forget it. You will never recouple the cost. It is quite easy to do a solar hot water system. A two coil tank is a waste most of the time the water from the sun will not be hot enough. A better solution is to use a second tank. The first tank just a normal tank ideally with a 28mm coil is used to preheat the water and the output from this goes into the feed of the tank connect to the boiler. This system is cheap to implement you can do it with an old radiator painted black and insulated in a box with a glass front. Provided the radiator is below the tank you won't even need a pump because the action of heating the water will do the work.
Better still leave this god forsaken country behind and go somewhere warmer where fuel costs aren't a problem.
If you are interested in doing your own eco projects there are some nice books that some eco centre does in wales. I forget the name. Do I few searches on the internet and I think you'll fine it.
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Plenty of bargains in Spain?
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On Fri, 29 Aug 2008 09:05:33 -0700 (PDT), snipped-for-privacy@tesco.net wrote:

I think you mean the Centre for Alternative Technology (CAT).
http://www.cat.org.uk /
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wrote:

I found the Centre for Alternative Techology disappointing. It hasn't changed much since I went the first time in 1982. They do have a super-insulated house, but not everyone can go and build one. There are plenty of sales leaflets for companies, but nothing they make at the centre. It would be far better to be able to buy a product from them that you can see demonstrated rather than have to deal with sales people at unknown companies. They didn't even sell a signle solar cell ! If that's not bad enough, my TomTom directed me to drive up a fenced off footpath which I thought was funny.
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They have a strong bias against photovoltaic cells. On my first (and last ever) visit, I asked about them and was directed to their resident "expert". He told me that no photovoltaic cells had ever been made that delivered more energy during their working life than had been consumed in their manufacture. So CET didn't like them - in other words HE didn't.
When I researched photovoltaic cells, I found that his claim was not true. Taking into account the less than ideal latitude in the UK, plus allowing for them not rotating to follow the sun, and therefore losing some more efficiency, it seems that they will generate three or four times the energy it cost to make them.
So they may not be the greenest method of electricity generation, but they are nowhere near as bad as he suggested.
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On Fri, 29 Aug 2008 09:05:33 -0700, snipped-for-privacy@tesco.net wrote:

This is fine *except* that almost all boilers won't accept water preheated to any great degree (i.e. more than 25C or so) without damage to the cold water input side. And even then, almost all of them don't modulate well enough to cope with the higher temps you could potentially get from even a UK preheater.
BTDTGTTS
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PCPaul wrote:

What's that got to do with the price of eggs?
The boiler is still feeding the secondary in the cylinder it always did in exactly the same way. It's just that when it gets topped up as you use hot water the input water is a lot warmer than it used to be!
Now if you had a combi boiler (puts on hard hat) this might be an issue - but this is plainly not the case here.
Andy
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On Sun, 31 Aug 2008 14:29:43 +0100, Andy Champ wrote:

Whoops. My mistake. I've got a combi and a stupidly designed evacuated tube system (done by the previous owners). You're right, feeding the cold inlet to a cylinder should work fine. Presumably having the smallest single coil cylinder to be preheated by solar would work best in that case?
Dual coil cylinders are made from unobtainium and welded with rocking horse farts, judging by the price.
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wrote:

So you don't know how to do it. It doesn't mean it can't be done using a preheated tank.
If your boiler can't take hot water input then fit a thermostatic mixing valve on the output, one side connected to the boiler and the other to the preheat tank. Set the temp to about 20C below the boiler and it will take as much preheated water as it can and mix it with the minimum of hot to get the temp selected. All these "problems" go away if you think for a few seconds.
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On Sun, 31 Aug 2008 17:05:09 +0100, dennis@home wrote:

No. No they don't.
Combi boilers, even ones that can modulate the heat well, are rarely able to give a 'minimum of hot' - mine for instance can only modulate down to 8.5kW output (from 25kW). If the scheme you suggest above is used, then on a hot day the solar water is likely to be >40C and the amount of water needed to top that up will either not be enough to trigger the boiler or it will rapidly overheat and short-cycle. Either way I don't get a nice stable hot supply out.
It's a nice idea, but in practice I think there are issues with it. If you can prove me wrong I'd be happy to try it.
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I'm glad you posted that, too much power being a problem wouldn't have occurred to me.
So I take it you have a evacuated tube panel warming a hot water tank that then feeds a combi. If the water is cold there's no problem, the combi does the work, if its warm the combi still does the work and modulates down to 8kW(t). The problem arises when the water is too warm to absorb 8kW and causes the combi to short cycle.
At what temperature does this happen? For example knowing the hot water tank is at 40C I can predict short cycling will happen, but I'm happy to shower at 40C, so a thermostatic bypass on the feed to the combi set at 40C would sort that problem.
At 10 litre/minute a rise of 11.5C consumes 8kW if my calculations are at all right. In fact just checking my cold tap I only get about 4 litres/minute but it is an old galvanised pipe.
AJH
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On Mon, 01 Sep 2008 16:44:11 +0100, AJH wrote:

Having too much of a good thing is an everyday occurrence for me ;-)

Sadly, no. There is no tank. I mentioned this system before, then failed to provide a diagram. Oops. I'll get onto it Real Soon Now(TM).
The evacuated tube collectors feed directly into a mixer which mixes cold with it to keep the temp down to what the boiler can cope with, i.e. 25C. Or at least if the idiot plumbers (solar installers?) had done it tirght at their *second* attempt it would - actually they have fed the hot output from the boiler to the output of the mixer then taken the hot input from the mixer to the cold input of the boiler, so effectively it does NOTHING. It's currently a rented house, though, so I'm waiting until we (possibly) buy it before sorting it out.

That's correct.

I suppose it might just work out to cap it at a sensible temperature before we get issues. I'm more likely to fit a dual coil tank or this idea might develop into a workable one when I work it out more thoroughly. Adding a single preheat tank and a mixer would certainly be easier than changing to a dual-coil tanked system instead of combi-fed direct heating..

I hadn't run the figures but that sounds about right. And like it could actually work out as a sensible solution. Hmm.
As for showers, although there is a nice combi installed, the previous owners have a 9.6kW electric shower fitted. Go figure. I suppose I could feed the shower directly from the panels for now with some temporary push fit connections, and just get a decent flow through it for a change. Lots to ponder on there...
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Don't forget this is a diy group. £3k sounds a but steep.
Look at http://www.navitron.org.uk /.
I reckon you could buy the parts for a rudimentary solar HW setup for under £1500 including collectors, tank, pumps and controls.
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I'm interested in reducing my fuel/carbon foot print and estimate that most goes on heating water.I have a farly normal fully pumped gas HW/ CH system with a HW tank, with two insulation jackets. Tank stat set at 55
A number of neighbours have gone over to Solar for hot water using the vacuum collector system and it seems a very good idea, however the cost of significant (circa £3K) and requires a fair bit of specialised plumbing, for example a new tank with 2 coils.
I wondered about the cost/benefit of using solar electricity to heat water.Sounds crazy but my tank already has a top-up immersion heater and I wondred if this could be run from batteries. Looking on ebay I noticed that there are 12/24v heaters than can produce 500 watts and can replace an existing immersion heater .
I know that the specific heat capacity of water is very high so heating a water tank would require a lot of fully charges batteries and hence panels to charge them. However every degree you can heat the tank for free is one that does'nt need gas right ?
Electric also has some advantages. It might be possbile to do this in an incremental way thus spreading the cost i.e adding panels/batteries as you can afford them.It could also incoporate a wind charger to get some power in all light conditions i.e at night. Finally the plumbing might not need changing at all to incorporate this meaing this falls within the realm of a purely electrical installation + some mechanics to fit the panel/ wind charger
I understand that electro voltaics are pretty inefficient, but I wondered if this approach might be cheaper or even more cost effective than the full solar hot water panel system ? ------------------------------------------------------------------------------- Don't do the solar PV method, it will take a life time to pay for itself. I'd go for a 'solartwin' system, its a compromise system for £2K diy install and no hot water tank changes. Plenty of info online and on YouTube etc etc.
Steve
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If you already have a tank. We didn't so had to install one - but not with two heating coils.
We're extremely happy with ours, our last gas bill was £8.95 for the quarter.
Mary
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The summer quarter, though!
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Well, thanks for pointing that out to me :-)
I know from experience that it varies mostly according to our use of gas fires in the coldest of weathers.
My point is that we hardly use the boiler at all.
Mary

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On Sat, 30 Aug 2008 10:27:33 +0100, "Mary Fisher"

Two old boilers in one house would seem a bit OTT.
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Understood. I'm impressed.
By the way, how did the Arctic cruise go?
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