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CfSH Solar Thermal ENE7 Credits


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#1 Fubar

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Posted 04 February 2015 - 06:30 PM

I hope this is the correct bit of the forum...

I'm just going through pre-assessment, and am having some minor conflicts with the assessor (good natured ones). According to ENE7 I have to have 10 or 15% energy supplied by low/zero carbon technologies to gain the credits.

The house is a 100m2 detached, designed to a good standard ( not state of the art, but ok). I'm using the limited south facing roof for 4m2 of solar thermal, (not PV as I don't like them). backed up by woodburning stove with back boiler warming a 300l thermal store. i will have a modern combi attached (that I already own, so I'm using it) to top up the hot water temperature to the taps and rads if neccesary.

The question is why doesn't the solar thermal cover 10-15% of the total carbon footprint? assuming on average a 1kw return, i can't see me using 8 to 10kw on average consumption. A, I missing something here?

The assessor reckons that 4kw of PV would gain me the credits..... madness I say, madness,

#2 ProDave

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Posted 04 February 2015 - 06:35 PM

What do you have against solar PV?

For a start, it pays you money AND you get to use the electricity it generates.

A solar dump controller will put any surplus energy into an immersion heater rather than exporting it. And a solar PV system has the potential to contribute something to water heating on a dull day that solar thermal will be doing nothing.

If you have the roof space, have both. All boxes ticked.

#3 jsharris

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Posted 04 February 2015 - 06:39 PM

The problem is that solar thermal isn't very efficient. Sure it seems as if it should be, as peak efficiency is a lot better than PV at around 60 to 65%, versus 16 to 20% for PV, but solar thermal only delivers heat when the panels/EV tubes are hotter than the tank (so if the tank is warm it could be mid-morning before the solar thermal does anything at all) and overall you're probably only going to get a fraction of the apparent heat output from it. Also, as soon as your tank is hot the solar thermal stops delivering any energy, so is just sat there taking up space and doing nothing useful.

PV has the advantage that it delivers energy whenever there is light around, no matter what your tank temperature. It also exports unused energy to others, so makes a much bigger overall difference in terms of overall carbon footprint.

Edited by jsharris, 04 February 2015 - 06:39 PM.


#4 Fubar

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Posted 04 February 2015 - 07:09 PM

thanks for the quick replies! I have good access to logs, so a thermal store makes sense, solar thermal would be good for summer months to go with it. I have only got approx 4m2 of SSE facing roof space (dormer), so going photovoltaic would be a large initial outlay and mean ditching the solar thermal. i'm more bothered about making a house that works well for us than this CFSH stuff. Dual PV/thermal is big money I believe.

As an aside, re photo-voltaics, i work in specialist electrical current measurement and am well awarte of the issues that substations and the grid in general is going to be having with the proliferation of domestic/small commercial PV arrays. i have alimited budget and am going for a fairly conservative build by grand designs standards.

#5 jsharris

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Posted 04 February 2015 - 07:25 PM

If you're budget limited, them PV makes MUCH more sense than solar thermal, as it's a LOT cheaper per effective delivered kWh. Solar thermal is pretty expensive, plus needs maintenance and relies on having pumps, thermostats, controllers, antifreeze replacement every couple of years etc. The best orientation for PV is arguably east/west, rather than south, as that gives more useful output for most people (how often does anyone need maximum power around midday?).

I understand the issues the infrastructure is having with moving from a centralised, to a distributed, power generation model, but that's going to happen with the proliferation of wind farms of solar farms anyway, so the grid will just have to get rid of their extremely crude transformer tapping control mechanism and done something more sensible to maintain grid voltage and frequency.

#6 Fubar

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Posted 04 February 2015 - 10:44 PM

I appreciate the replies.
Food for thought, certainly, if we gave up the thermal/solar + storage then going photovoltaic would mean ditching the wood burning stove for starters. (the backboiler would be essential as a method of controlling the heat). That would be a shame, as we like the stove idea ( we have 2 in our current single skin brick house). My mate is an Arborist and I have a van :-) We certainly wouldn't want to be using the stove during summer though. Would it then just make sense to use PV to power an Immersion heater element in a thermal store? or just export at the feed in tarrif and use the gas combi to provide hot water?
cheers

Edited by Fubar, 04 February 2015 - 11:03 PM.


#7 jsharris

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Posted 05 February 2015 - 07:50 AM

First off, have you checked the house heating requirement with the wood stove output, and taken into account the limited modulation that a wood stove has? A house built to a fairly reasonable insulation standard, with good airtightness and effective heat recovery almost certainly won't need anywhere near as much heat as a typical small wood stove will put out. For example,, when it's -10 deg C outside and we want the inside of our 130m² house to be around 20 to 21 deg C, we only need an actual whole house heat input (for every room) of around 1.5kW. The living room only needs around 200 to 300 W, far, far less than a wood stove would put out.

We're using an immersion powered by excess PV and it is very effective in summer, but only moderately so in winter. We aren't on the gas grid, so had to resort to using a small air source heat pump, but had we had gas then I'd have fitted a small condensing gas boiler, as the cheapest, cleanest and least CO2 producing option.

Worth noting that even the very best wood burning stove produces around 100 times the particulate and toxic emissions of a good modern diesel car, and is very far from carbon neutral, because there is a 15 to 20 year lag between burning wood now and it regrowing and re-sequestering the CO2. My personal view, given the known health issues with wood burning, is that it isn't something I'd want near my house. Some countries are already banning or restricting their use because of the toxic emissions problems, but sadly there are some who refuse to accept the hard evidence of harm. It reminds me a bit of the smoking and health arguments from 40 or 50 years ago, in some ways.

#8 Fubar

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Posted 16 February 2015 - 06:15 PM

Thanks for the info JSH, have spoken to a local renewables bloke (who came recommended), am going down the route of PV+immersion heater, small gas combi (already owned) and ditching the S-Thermal.
cheers

#9 warby

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Posted 17 February 2015 - 05:18 AM

What a wonderful result for the forum.
An open minded person sensibly changing his mind when he is shown an alternative strategy from JSH, nice one.
I never fail to be impressed by the help provided.

Martin

#10 Fubar

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Posted 17 February 2015 - 07:34 PM

hi, I have found this forum full of interesting info, whilst I prefer to be as "eco" as possible without spending uneccesary cash, am more interested in making our new home something that is "us" that we will be living in until I croak......We certainly aren't property developers.

My background is in AC electrical current measurement, so who knows, I maybe able to help in return at some point.

Cheers