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Small scale wind turbine and underfloor heating
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skyeten



Joined: 17 Aug 2005
Posts: 4

PostPosted: Wed Aug 17, 05 10:55 am    Post subject: Small scale wind turbine and underfloor heating  Reply with quote    

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.

Any input appreciated....
Thanks
Alan

dougal



Joined: 15 Jan 2005
Posts: 7184
Location: South Kent
PostPosted: Wed Aug 17, 05 12:12 pm    Post subject: Reply with quote    

Hi Alan and welcome!

Briefly, think in terms of the energy usage.
6kw is a small central heating boiler, but a fairly large (domestic) wind turbine.
Have you seen:
http://www.downsizer.net/Projects/Sustainable_Building%10Energy_Efficiency/Wind_power_to_the_people!/

If you have lots of wind, its a shame not to use it! But substituting wind for electricity is likely to offer a better payback than substituting wind for oil...

That said, since you have underfloor, I'd wonder whether you should be thinking of ground source heating. Skye? Do you have access to land that could be dug up to lay the pipes? Is it good and wet land? (or even better, have you access to water?). A ground source heat pump can deliver 4kw for every 1kw of electricity used. (Maybe more.) Think of that as dividing the cost of electricity by at least 4... Run it (largely) on Economy 7...
AND put up your own wind turbine to further reduce the electricity bills...
But of course, as usual, insulation should be the first priority!

tahir



Joined: 28 Oct 2004
Posts: 44268
Location: Essex
PostPosted: Wed Aug 17, 05 12:31 pm    Post subject: Reply with quote    

The only problem with attaching a Ground Source Heat Pump to an existing system is that they need a low temerature system (i.e. loads of slightly cooler pipework) for optimum efficiency, I haven't got technical details but you need to speak to a couple of different suppliers before making any decision.

dougal



Joined: 15 Jan 2005
Posts: 7184
Location: South Kent
PostPosted: Wed Aug 17, 05 1:05 pm    Post subject: Reply with quote    

tahir wrote:
The only problem with attaching a Ground Source Heat Pump to an existing system is that they need a low temerature system (i.e. loads of slightly cooler pipework) for optimum efficiency, I haven't got technical details but you need to speak to a couple of different suppliers before making any decision.


skyeten wrote:
... my house has underfloor heating ...


Underfloor heating *needs* a low temperature heat supply.
A heat pump's efficiency falls off as the output temperature rises.
That's why the two are a match made in heaven!
Heat pump + Radiators = low efficiency, high expense.
Heat pump is not such a great idea for hot water (but better than straight electric!) - which is a minor consideration compared to space heating.

Add in underfloor pipes set into a concrete slab and you have a useful thermal mass to act like a giant storage radiator... allowing you to run mainly on Economy 7. (Though I'm not suggesting Economy10 should be ruled out - its a detail to be explored.)

And yes, speak to lots of suppliers, and get them to put you in touch with their existing customers...

Last edited by dougal on Wed Aug 17, 05 1:09 pm; edited 1 time in total

tahir



Joined: 28 Oct 2004
Posts: 44268
Location: Essex
PostPosted: Wed Aug 17, 05 1:07 pm    Post subject: Reply with quote    

I spoke to "an authority" on GSHPs in the UK when I was consuiderting the technology and he told me that MOST systems he's seen have been poorly designed and in his opinion had a negligible benefit.

dougal



Joined: 15 Jan 2005
Posts: 7184
Location: South Kent
PostPosted: Wed Aug 17, 05 1:45 pm    Post subject: Reply with quote    

There is no question that they do work.
An efficiency factor of about 4 is a reasonable overall expectation for space heating via underfloor. (Most suppliers claim rather more - 4 is realistically conservative.) Perhaps only 2 for raising to 60C for hot water. Even less for higher temperatures for radiators.
To get the best out of them, you really do *need* underfloor, ideally in a slab for thermal mass. AND good insulation.
Such systems are quite common in Scandinavia, where insulation standards are much higher - and I think oil is more expensive in comparison to electricity.

There seems to be an element of "black art" to determining an appropriate size for the ground collector.
The wetter the ground, the smaller collector should be needed.
Access to reliable running water (not dried up, not frozen!) permits the smallest collector.
The smaller the collector, the cheaper the installation.
But if its too small, bang goes the efficiency, and possibly chills anything growing above the collector. (I'd specify some means of conveniently adding more collector, should it prove necessary...)

If Ground Source is going to make sense anywhere in the UK, Skye sounds like THE place. Being remote, I'd expect oil (and gas from a tank) prices to be among the highest in the UK. I'd strongly doubt that mains gas would be an option... Population density being low, there's a great chance that there's land space for the collector. And it does rain rather a lot on Skye, so there's the groundwater benefit too.
And I'd expect the annual heat *demand* to be much higher than in, say, Essex... and so running cost assumes more importance compared to initial capital cost.
The essence of a heat pump is to multiply the heating benefit you get from each unit of electricity. (By "stealing" it from that big solar collector outdoors.)
Some "greenies" are against heat pumps on principle because they depend on centrally generated mains electricity, with its 50% generation and transmission efficiency.
However, it could provide a great synergy with home electrical generation - and if nothing else, reduce by a factor of 4 the wind turbine generating capacity required.

As a technology, it really can work.
The economic practicalities depend on the alternatives available - and essentially *demand* underfloor (ideally in a slab) - which is why it is so rarely suitable for a retro-fit...
But I've no idea what foul-ups Tahir's man may have seen...

tahir



Joined: 28 Oct 2004
Posts: 44268
Location: Essex
PostPosted: Wed Aug 17, 05 1:49 pm    Post subject: Reply with quote    

He was talking mainly about systems being designed to run at higher than optimum temperatures, thereby to a large degree any benefits.

I'll try and dig out the exact stuff later.

dougal



Joined: 15 Jan 2005
Posts: 7184
Location: South Kent
PostPosted: Wed Aug 17, 05 2:03 pm    Post subject: Reply with quote    

dougal wrote:
Underfloor heating *needs* a low temperature heat supply.
A heat pump's efficiency falls off as the output temperature rises.
That's why the two are a match made in heaven!
Heat pump + Radiators = low efficiency, high expense.
Heat pump is not such a great idea for hot water (but better than straight electric!) - which is a minor consideration compared to space heating.


tahir



Joined: 28 Oct 2004
Posts: 44268
Location: Essex
PostPosted: Wed Aug 17, 05 2:09 pm    Post subject: Reply with quote    

dougal wrote:
dougal wrote:
Underfloor heating *needs* a low temperature heat supply.
A heat pump's efficiency falls off as the output temperature rises.
That's why the two are a match made in heaven!
Heat pump + Radiators = low efficiency, high expense.
Heat pump is not such a great idea for hot water (but better than straight electric!) - which is a minor consideration compared to space heating.



I know you know that, but in the guy's experience a LOT of jobs had not been desiggned with that in mind (at least not to a low enough temperature)

dougal



Joined: 15 Jan 2005
Posts: 7184
Location: South Kent
PostPosted: Wed Aug 17, 05 2:29 pm    Post subject: Reply with quote    

I'm just amazed that someone would say *"MOST"* ground source systems were designed ignoring those fundamentals...
Or that the buyers would stand for it, and cough up regardless...

Afraid that I had taken it that your man had a 'down' on the technology, which definitely does work, but certainly doesn't make economic sense for everyone.

tahir



Joined: 28 Oct 2004
Posts: 44268
Location: Essex
PostPosted: Wed Aug 17, 05 2:41 pm    Post subject: Reply with quote    

He told me that he has been called into some quite high profile jobs that have been incorrectly designed, the (professional) guys at the AECB seem to have a lot of respect for him so I assume he knows what he's talking about.

tahir



Joined: 28 Oct 2004
Posts: 44268
Location: Essex
PostPosted: Wed Aug 17, 05 2:49 pm    Post subject: Reply with quote    

This was part of it, i can't find the bit where he talks about the poorly designed systems he's been asked to look at

john cantor wrote:
I have been installing heat pumps (in small numbers) for many years now. I have always had some doubts about their integrity, and have never considered them a 'renewable'. In 1983 I visited Sweden where heat pumps have a high status because 60% of their electricity comes from Hydro, then they make sense. In Denmark it was a different story, environmentalists were worried that the use of heat pumps could tip the pro-nuclear argument. (I wonder what they think now).

Heat pumps are complicated since there are many variables that affect the systemís efficiency. The COP (coefficient of performance, ratio of output over input) can generally vary from 2 to 4 (or possibly wider). I am concerned that many Ground Source Heat Pumps currently being installed will only give an average yearly COP of around 3 (some give less), for Air Source the actual figures will be worse. If you look at manufacturer's data it often looks quite impressive, but when you most need that heat, an Air Source will struggle. (this is another debate)

The real question that I have wanted to answer for over 20 years is this; what is an environmentally acceptable COP? Clearly a COP of 2 would only be acceptable in Norway where 100% electricity is renewable. In the UK I consider that a COP of 4 would be better than oil or LPG, but this is more of a gut feeling than based on facts. This is such a difficult discussion since there are again, so many factors, Nuclear etc.

To attain a COP of 4, you need a well-insulated house, an underfloor system designed to work at about 35įC (you may struggle to find a UK installer to cope with that). And the ground source collector must be big and deep. Weather-compensated control is a must.
This type of installation is expensive. Heating domestic hot water is a common requirement, but remember, this is less efficient since temperatures required are higher.

Unfortunately, the vast majority of buildings in the UK use large amounts of energy. Many potential customers end up with oil or even storage heaters because they are cheaper. Heat pumps must be better than these options. It is a matter of practicality. In remote areas, as Chris says, it is better to transport electricity than other fuels.

The environmental impact (+ or -) of heat pumps could be very significant. On the one hand, very high efficiency types displacing oil/LPG or gas, possibly using a wood fired back up in mid winter. On the other, low efficiency installations. In this case the money would be far better spent increasing insulation levels. We seem to be heading somewhere in the middle at present. But as for growth of reversible air-conditioner heat pumps, these are an abomination.

tahir



Joined: 28 Oct 2004
Posts: 44268
Location: Essex
PostPosted: Wed Aug 17, 05 2:53 pm    Post subject: Reply with quote    

Here's something more relevant to the original query:

Nick Grant wrote:
I might consider a wind turbine dumping to a heat store in a rural situation - with super insulation as Chris says. Could do sums to see if its better to use a heat pump to triple output or just add more wind turbine and energy efficiency for the money. However I agree with Pete that designing an urban house with electric heating is almost certainly wrong.

dougal



Joined: 15 Jan 2005
Posts: 7184
Location: South Kent
PostPosted: Wed Aug 17, 05 4:54 pm    Post subject: Reply with quote    

Tahir, I think John Cantor is saying pretty much what I said.
He is bothered about where the electricity comes from, hydro, nuclear or whatever.
He emphasises (just like I did) the importance of insulation and underfloor, and the relative inefficiency for domestic hot water heating.
And he's a bit more pessimistic than I am on Coeffiecient of Performance or multiplication factor. Which varies greatly with the output temperature, (lower the better), and collector efficiency, (wetter the better) - as well as the design of particular heat pumps. Just for space heating, using underfloor, with wet ground and a modern pump, I don't think 4 is too unreasonable.

But mainly he's worried about where the electricity comes from...
And in this case...


(Air source *is* a different matter, and in general I'd agree wholeheartedly with Mr Cantor that these are an "abomination".)

tahir



Joined: 28 Oct 2004
Posts: 44268
Location: Essex
PostPosted: Wed Aug 17, 05 4:59 pm    Post subject: Reply with quote    

I couldn't dig out the bit I was talking about but he was definitely scathing on the (lack of) design of a lot of GSHP systems

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