This time it's about Oil Boilers

[AMc]

I have to admit to having probs getting my head round this.

Can I rig my old bike up to a generator and claim that money too? How about the hamster wheel?

http://www.good-energy.co.uk/home_generation

There is more demand for green electricity than can be met by the current supply Most green tarriffs are (according mostly to the Green Building Bible but others too) trading Renewable Obligation Certificates (ROCs) which as far as I can determine is double counting the supply.

If you can prove the power output of your renewable power generation you can get Renewable Obligation Certificates (ROCs) which have a value to someone attempting to meet their quota of renewable generation. They buy them from you. In the case of an offgrid system - the alternative would be a fossil fueled generator - so the same rules apply as a grid connected system.

I think this seems a bit bent in that most people buying 'green' electricty think their power is coming from wind farms when it's likely coming from a fossil/nuclear dependant buying ROCs from the wind farm owner who has already sold or used the units directly.
The point is to increase renewable generation and this is obviously going to stimulate production as you can "get paid" twice for the same power output

Good Energy also retire some of their allocation of ROCs to increase the demand for more production - which is sensible business as if everyone switched over there wouldn't be enough to go around to meet their 100% claim as well as renewable production being IMHO a "good thing".

 

[Aidan]

I know nothing about oil boilers...


I think in modern buildings solar panels work acceptably from a
visual point of view but I am not convinced about them on roofs of
historic buildings like yours. I have heard of a system that is hidden
beneath slates but not seen it yet.....From the principle that hot water rises, I have always thought that the sensible place is at ground level, providing the area is not too enclosed."

Does anyone here have any knowledge of such a system?

No, but there's no reason why you couldn't have pipes on the back of tiles. It's a bit like frying eggs on paving stones; the paving stones would lose a lot of heat by conduction to the earth. if they were isolated with thermal insulation, they'd get much hotter, unless the heat could be removed/recovered/harvested. Metal 'tiles' would get hotter still.

No reason why they should be only mounted on a roof, apart from the fact possibly they can catch more sun there in most places and it's a short pipe run to the tank.

Less shading on the roof, it reduces heat gains into the building roof space and no one is likely to want to turn it into a car park or whatever.

Solar panels may look out of place on a period property, but they can be removed with minimal affect to the fabric. Similarly electric lights, flushing toilets, radiators, plumbing, televisions, cookers, etc., are also out of place. I fear that the supply of cheery peasants, who will stoke fires and remove chamber pots for a few groats a year, is much diminished in recent years.

 

[Nemesis]

I think we've already been through the removal with minimal effect etc. discussion.

I won't be spoiling my lovely roof with them, I know that.

 

[NT]

I now quote part of an email received today from the architect who is working on a small first-floor extension to our house (of which more in my second question below) - I'm sure he won't mind being anonymously quoted

"I think in modern buildings solar panels work acceptably from a
visual point of view but I am not convinced about them on roofs of
historic buildings like yours. I have heard of a system that is hidden
beneath slates but not seen it yet.....From the principle that hot water rises, I have always thought that the sensible place is at ground level, providing the area is not too enclosed."

Does anyone here have any knowledge of such a system?

There are pros and cons with roof vs ground collectors. The question of gravity driven circulation is such a trivial difference as to be immaterial for 99% of installs. It becomes important with buildings with no electricity supply, remote cabins etc.

It is possible to rely on gravity circulation with ground or wall mounted panels, but doing so is so counterproductive that its almost never done unless its the only workable option.


NT

 

[piper]

There are pros and cons with roof vs ground collectors. The question of gravity driven circulation is such a trivial difference as to be immaterial for 99% of installs. It becomes important with buildings with no electricity supply, remote cabins etc.NT

Thanks for the replies so far, and comments on the general principle of solar panels on roofs. However, by quoting our architect's extra comment about ground-mounted solar, I seem to have rather confused the issue. My real query was about this behind-the-slates system he mentions. (Thanks Aidan) - has anyone else got any knowledge of it?

 

[Nemesis]

Nope - but if I find anything out I will let you know!

I have found info about solar slates but I like my natural ones...

http://www.solexenergy.co.uk/pdfs/brochure.pdf


(I won't be fitting uPVC windows either.)

 

[Jane Smith]

I have seen a system for a new roof, where a network of pipes were installed behind the tiles in order to collect heat. It was based in New Zealand, not available in the UK, and from what I remember it couldn't be retrofitted and wouldn't provide anything like enough heat to earn its cost back here in the UK.

I'll see if I can find a link and if I can I'll post it here.

 

[NT]

There are pros and cons with roof vs ground collectors. The question of gravity driven circulation is such a trivial difference as to be immaterial for 99% of installs. It becomes important with buildings with no electricity supply, remote cabins etc.NT

Thanks for the replies so far, and comments on the general principle of solar panels on roofs. However, by quoting our architect's extra comment about ground-mounted solar, I seem to have rather confused the issue. My real query was about this behind-the-slates system he mentions. (Thanks Aidan) - has anyone else got any knowledge of it?

Such a system could prewarm water, but its hard to imagine it paying its way. Consider that its a struggle to get payback on panels with front glazing and rear insulation, and with the pipes in contact with the absorber, whereas a slate roof has no glazing, often no rear insulation, and there is the slate insulator between slate and pipe.

The flip side of the story is that there is less to install, no external roof access needed, and there's a whole acre of slate to tap warmth off. The figures / calcs would be needed to see how it performed. One thing's for sure though, this would be summer only, the winter return would be zero.


NT

 

[piper]

Such a system could prewarm water, but its hard to imagine it paying its way. Consider that its a struggle to get payback on panels with front glazing and rear insulation, and with the pipes in contact with the absorber, whereas a slate roof has no glazing, often no rear insulation, and there is the slate insulator between slate and pipe.

One thing's for sure though, this would be summer only, the winter return would be zero.NT

That's pretty much what I thought, too. Thanks for the replies though.

 

[NT]

Underslate Solar
-------------------

50m of MDPE pipe is £19, a CH pump is £30, and a cylinder thermostat £8 (screwfix.com), so a materials cost of £57. Its an easy DIY job with no access costs. The stagnation temp is not going to exceed what a header tank can handle, so you've got a simple prewarm system for under £60. It might pay its way after all.

If we take the minimum worthwhile payback as 10 years with interest at 5%, the system would have to return 15% pa to meet this criterion. 15% of £57 is £8.55 pa. Not a very demanding requirement.

£8.55 is apx 285kWh with gas at 2.5p and 83% boiler efficiency. Given the low efficiency of this system we can only expect significant heat output for 3 or 4 months a year at best.

285kWh/3.5 months = 2.7kWh / day.

50m of pipe with 16" rafter spacing (40cm) covers apx 50 x .4 = 20 m^2 of roof. So we're looking for at least 2.7/20 = .13 kWh/day/m^2 for it to pay back.


The other option is to lose the pump and route the header water to the HW cylinder via the collector pipe. Much cheaper at £19, but lower payback too.


I've missed many details out, but first glance looks like it would indeed pay its way no problem.


NT

 

[sidd]

This idea of under-tile solar is very interesting, even if it is just for pre-heating. Assuming that mains water comes in at say 11 degrees all year round, any time the air temperature is above that, then you are saving power straight away. In mid summer, that means the water coming in is 15-30 degrees just from the ambient air, and before any solar gain at all.

So if we used 10mm copper tube coil to snake the underside of a bay under the tiles, with say 3-4 of them in parallel to ensure adequate flow, backed it with multifoil and allowed for condensation runoff, then we could get some pretty good prewarming temperatures, even if that relies on drawing on it during warmer hours. (MDPE not being very conductive might not be as effective)

http://www.theyellowhouse.org.uk/themes/heatwat.html#h10

The above also leads me to the point about mains pressure water: if we remove the water tank by converting to mains water, then we lose the pre-warming that we currently take for granted.

And we'd probably want to drain the system for the winter.

 

[NT]

I had in mind backing the rafters with 2 layers of surplus wallpaper at £1 a roll. The paper layers are staggered, and the gaps give some resistance to airflow. Not much insulation there, but there's none on the slate side, so its not worth putting a lot up, and system cost is key.

There are also the questions of legionella, freezing and pump corrosion. All 3 could be fixed by running an antifreeze circuit with a heat exchanger in the header tank, or one could go in the opposite direction and lose the pump & storage option, and just drain the roof pipe for winter.


NT