Small scale wind turbine and underfloor heating

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Small scale wind turbine and underfloor heating
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dougal

Joined: 15 Jan 2005
Posts: 7184
Location: South Kent

Posted: Wed Aug 17, 05 5:01 pm Post subject: Re: Small scale wind turbine and underfloor heating

Yes back to the original post...

skyeten wrote:

Has anyone got any experience or ideas on linking a small wind turbine into an underfloor heating system ?
I live on the Isle of Skye and hence have access to a lot of wind !, my house has underfloor heating which is pretty good but I want to minimise the cost of burning heating oil which has gone up by nearly 100 % over the last year or so.

I thought of installing a small water tank with an immersion type heating element in it powered by the output from the turbine, this would link in to the underfloor heaing system.

How many kw does your current boiler produce?
Unless you dramatically improve the insulation, you are still going to need the same amount of heating power, which is measured in kilowatts (kw).

Jerome's article (previously linked, but its moving around today!) shows that a 5kw (max) wind turbine is about 5m in diameter and would be expected to sit on a 12 to 25 metre high tower.
Is that what you meant by a "small" wind turbine?
Now 5kw is a pretty puny central heating boiler - perhaps suitable for a smallish flat in the south of England.
My guess is that you are going to need more heat than that.

I'd propose two ways in which you could make good use of that 5kw.
1/ Install a Ground Source Heat Pump instead of your Oil-fired boiler.
There are lots of ins and outs but that 5kw could be multiplied to provide you with something like 20kw of heat - probably plenty. And when the wind isn't blowing it'll be topped up from the electricity grid.
A ground source system is likely to cost (after grants) perhaps as much as �10k.
2/ Install a thermal store. Here we are into plumbing!
The idea is that your *boiler* heats a *big*, insulated hot water tank. And then the tank heats your house.
The advantage of this system is that you *can* *add* immersion heaters into that tank, powered from your turbine, and then you will only use the boiler to "top up" your heat requirement.
But the wind turbine is only going to give you 5kw of heat - max. So you are still going to use quite a lot of oil.
You could install the store and a *really* small turbine - and it'll do you noticeable good. And then increase the generating capacity later...
But the advantage is that, apart from the turbine and its tower, it should be a fairly cheap solution, perhaps less than �1000 (turbine NOT included!).( Jerome's article shows a 5kw turbine (with mains tie) costing �17k, but potentially being reduced to ��8,500 by grant aid.)
AND, should you later wish to install a heat pump, running it through the thermal store will maximise its efficiency, by reducing the frequency of on/off switching to allow longer runs (and best use of Economy7).

BTW, note that on the basis of these numbers it would be *much* more cost effective to use a heat pump than to "triple-up" the generating capacity...

tahir

Joined: 28 Oct 2004
Posts: 45674
Location: Essex

Posted: Wed Aug 17, 05 5:07 pm Post subject:
What about using the heat store to supply hot water too?

Jonnyboy

Joined: 29 Oct 2004
Posts: 23956
Location: under some rain.

Posted: Wed Aug 17, 05 5:59 pm Post subject:
Out of interest, my wood burning stove does central heating and DWH. It's maximum output is 12.5kw, IIRC oil fired boilers can pump out around 25kw, but you will certainly need a lot more than 5kw.

dougal

Joined: 15 Jan 2005
Posts: 7184
Location: South Kent

Posted: Wed Aug 17, 05 6:15 pm Post subject:

tahir wrote:

What about using the heat store to supply hot water too?

Well, the thing is, you want 40/50C or thereabout for the underfloor and 55/60C for tap hot water.
So you either boost to the taps or blend down for the CH. Or have something separate!
I think most plumbers would blend down for the CH. And for the HW as well. The hotter the store, the more heat you can store in any given volume - but the more important the store insulation is.
And thats all just fine - until you add Ground Source. Or even solar. (Skye? Hmmm)
IMHO as previously beaten to death, for a Ground Source system, you want the store at as LOW a temp as you can get away with.
And you really need fancy evacuated tube solar collectors to generate a high enough *temperature* to make a useful contribution to a high temp store...

tahir

Joined: 28 Oct 2004
Posts: 45674
Location: Essex

Posted: Wed Aug 17, 05 6:20 pm Post subject:
On our exproject one of the architects we spoke to suggested converting an outbuilding into a heat store/solar collector, it had a good amount of south facing wall and he said we could use that given the low temperature required and the potentially large collector area. He said that he always preferred to keep the heatstore away from the dwelling if possible as any leaks etc could prove costly.

dougal

Joined: 15 Jan 2005
Posts: 7184
Location: South Kent

Posted: Wed Aug 17, 05 6:43 pm Post subject:

Not entirely sure I follow that one...
You may be confusing/introducing other considerations!

"Leaks' from the thermal store. Its usually water. And a store looks like, hey it *is* a (big) hot water tank. Costly? No more than any other plumbing problem. Big? 1000 litres or so. The cost of the water is insignificant.
But heat leakage happens all the time, despite the insulation - and will leak from transfer piping between a remote store and the dwelling.
I'd rather have the heat leakage *into* the building.
BUT if your heat source is "free" then efficiency considerations, like heat leakage, are less significant considerations.

There is an alternative to a 1000 litres of very hot water, and thats a swimming pool full of warmish water, which loses heat more slowly because of the lower temperature, but being really big is fearful to super-insulate, so is often incorporated into the house basement - so heat leaks *into* the building...

However when you started mentioning south facing walls as collectors, I think you are getting into passive solar Trombe walls - a different matter entirely - and where it is good efficiency practice to depart from the classic Trombe design, and separate (with insulation) the collector and store (which might be water, but much cooler than the 80/90C common for a "thermal store" tank as discussed for Skye).
And because most folk want to have *some* windows on the south side of their house, having your 'modified-Trombe' collector on an out-building perhaps makes some sense... (although its normally to provide air heating after dark.)

footprints

Joined: 26 May 2005
Posts: 234
Location: North Wales

Posted: Wed Aug 17, 05 8:06 pm Post subject:

Quote:

I live on the Isle of Skye and hence have access to a lot of wind !

I live on Anglesey and the wind wrecks the garden/fruit trees etc. I have always thought that I might at least draw some comfort from having the garden savaged by the wind if it was spinning a small turbine. My first thought was also using water as a store for the energy. In awe of the brain power debating the subject, would like the outcome translated to .........

tahir

Joined: 28 Oct 2004
Posts: 45674
Location: Essex

Posted: Wed Aug 17, 05 8:38 pm Post subject:

dougal wrote:

"Leaks' from the thermal store. Its usually water. And a store looks like, hey it *is* a (big) hot water tank. Costly? No more than any other plumbing problem. Big? 1000 litres or so.

Yup he was talking about a large volume of water being released for whatever reason from the heat store, he was talking of much more than 1000 litres though (can't remember how much exactly)

tahir

Joined: 28 Oct 2004
Posts: 45674
Location: Essex

Posted: Wed Aug 17, 05 8:41 pm Post subject:

dougal wrote:

There is an alternative to a 1000 litres of very hot water, and thats a swimming pool full of warmish water, which loses heat more slowly because of the lower temperature, but being really big is fearful to super-insulate, so is often incorporated into the house basement - so heat leaks *into* the building...

He's working on a house in Essex right now that uses this kind of technique but with the loft space acting as a solar collector/heat exchanger, it relies on a huge amount of tepid water as the heat store, I think he wanted to create something similar but use an existing outbuilding for the store rather than spend extra on foundations.

dougal

Joined: 15 Jan 2005
Posts: 7184
Location: South Kent

Posted: Wed Aug 17, 05 9:19 pm Post subject:

Here's a description of what a plumber or heating engineer might call a "thermal store" or a "heat bank" - there's a difference but at this point I'm jiggered if I can think of it...

https://www.plumbingpages.com/featurepages/HWdpsThermstore.cfm

And here's a PDF about using a thermal store in conjunction with underfloor, wind and solar...
https://www.cibse.org/pdfs/3ahill.pdf

Edited to add the link to the PFD

Last edited by dougal on Wed Aug 17, 05 9:42 pm; edited 1 time in total

dougal

Joined: 15 Jan 2005
Posts: 7184
Location: South Kent

Posted: Wed Aug 17, 05 9:27 pm Post subject:

footprints wrote:

...I live on Anglesey and...I have always thought that I might at least draw some comfort from having the garden savaged by the wind if it was spinning a small turbine.

Size is the problem.
The much-discussed Windsave is said to generate 1kw maximum in a Force 6 gale. Its blades are something like 6ft across.
Jerome's article talks about blades 15 ft in diameter generating 5kw maximum.

How many kw is your current boiler?
And how big is a *small* turbine?

dougal

Joined: 15 Jan 2005
Posts: 7184
Location: South Kent

Posted: Wed Aug 17, 05 9:37 pm Post subject:

tahir wrote:

dougal wrote:

And there you have one reason why its conventional to store *huge* quantities of tepid water in the basement!

And, since heat tends to *rise*, the majority of the heat leakage is *into* the house!

skyeten

Joined: 17 Aug 2005
Posts: 4

Posted: Thu Aug 18, 05 1:47 pm Post subject:

Thanks for all the info......

The wind turbine I had in mind was about 1.5Kw and the idea was to supplement the boiler not replace it.

At the moment in winter the underfloor heating runs from 8-11am and from 6-9pm and on windy days the boiler is running continuously for the 6 hours, this may not be the most efficient way of running the system but its the only way we have tried.

My thinking was that using a wind turbine to heat the floor overnight would create a heat store and not require the boiler to work so hard to heat the place up during the day. The floor would not have the time to cool down so would require less heating to bring the house up to temp.

Someone must have done it....

There is a very small stream which runs nearby but it frequently dries up to no more than a trickle. I dont have any large area of land available to install a geothermal system.

Thanks again.

alan

sean
Downsizer Moderator

Joined: 28 Oct 2004
Posts: 42219
Location: North Devon

Posted: Thu Aug 18, 05 1:51 pm Post subject:
I would have thought that underfloor heating would be more efficient if it ran continuously. What's your house built from?

dougal

Joined: 15 Jan 2005
Posts: 7184
Location: South Kent

Posted: Thu Aug 18, 05 3:55 pm Post subject:

skyeten wrote:

The wind turbine I had in mind was about 1.5Kw and the idea was to supplement the boiler not replace it.
At the moment in winter the underfloor heating runs from 8-11am and from 6-9pm and on windy days the boiler is running continuously for the 6 hours, this may not be the most efficient way of running the system but its the only way we have tried.
My thinking was that using a wind turbine to heat the floor overnight would create a heat store and not require the boiler to work so hard to heat the place up during the day. The floor would not have the time to cool down so would require less heating to bring the house up to temp.
... I dont have any large area of land available to install a geothermal system.

I'm concerned that your heating needs increase with the wind.
That suggests that draughtproofing and other insulation could bring great benefits. Never mind how little Building Regs suggest you can get away with - the question is how *much* can you physically fit! The more severe the *local* climate, the more important this is.
I gather (or presume from what you say) that your underfloor system is set into a concrete slab, which will indeed act as a big storage radiator. But it itself should be well insulated...

The normal thing with underfloor would be to essentially leave it running 24 hours.
It is going to be very *unresponsive* to turning the boiler on and off.
With a conventional radiator system you can turn the heating on only when anyone is around, but the point of underfloor is to provide steady, 24 hour warmth.

Is the boiler a *condensing* one?
Condensing boilers deliver more heat from every litre of oil, AND work most efficiently at the low temperatures required for underfloor.
An old boiler might use 30% more oil than a condenser to provide the same heat. (Quite a lot is wasted straight up the chimney/flue.)
Condensing *gas* boilers "modulate", that is to say adjust, the flame size to match demand, thereby hopefully allowing it to burn in a steady state, rather than be forever switching itself on and off.
My understanding is that domestic oil-fired condensers don't modulate.

What controls are fitted? A room thermostat?
You imply that the boiler is *firing* continuously for the two 3 hour periods you have programmed.
Which suggests that the room is not reaching the temperature set on the room stat...

Part of the reason that a Thermal Store is recommended with underfloor is that it prevents the boiler firing only for short, inefficient bursts, when the Room Stat is frequently changing its mind as to whether or not the room is hot enough.
You, seemingly, aren't hitting that...

The basic idea of boiler sizing is to choose a boiler which can deliver enough heat to the radiators to maintain the design temperature by matching the heat loss in the worst weather.
Simplifying things, if you provide heat at a fixed rate - say 10kw - the house will stabilise at some temperature above the outside temperature - say 15C above. You can raise the temperature uplift by either improving insulation or chucking in more energy.
It takes *much* more energy to raise the temperature difference just a little - which is why insulation becomes massively more important with colder (and windier) outdoor temperatures.
If your house is losing heat at a rate of 10kw, you have to supply heat at 10kw to maintain temperature - for example by firing a 20kw boiler for half the time, or a 40kw one for a quarter of the time (six hours a day - see where I'm going?)

Do look at your Boiler and see its output. It'll be on the serial number plate, I expect. Might be in kw or (for an old one) in BTU per hour.
I'd guess 28kw, and that your average consumption therefore is 7kw.
So a 1.5kw turbine going flat out in a gale - could provide 20% of your heating... But I doubt it'll be flat out much of the time, although probably the generation would be "in step" with demand...

So I doubt that a 1.5kw turbine would give *more* than a 20% saving.

And remember that is also assuming that the turbine is in "good" air, not sheltered or even affected by turbulence from builings, for example.

I'd repeat my suggestion that the *way* to integrate it is via a "thermal store" - just as was described in the PDF I linked previously (albeit that was integrating wind and ground source - BTW only speak of "geothermal" if you've got hot springs, etc!)

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