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4 Kwh/m2A Short Of A Picnic: Should I Care?


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#21 SteamyTea

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Posted 16 January 2016 - 09:22 AM

Should not be that hard to work out how much extra PV you need for every extra m2 of window area

#22 recoveringacademic

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Posted 16 January 2016 - 09:40 AM

View PostSteamyTea, on 16 January 2016 - 09:22 AM, said:

Should not be that hard to work out how much extra PV you need for every extra m2 of window area

Steamy, I have a couple of degrees under my belt, a few other qualifications of which I'm quite pleased and I am not stupid. But I am more proud of a grade 6 (a scraped pass) at 'O' Level Maths than anything I've ever done before or since (late 1960s). I can even do the mental maths necessary to navigate a light aircraft successfully (mostly fuel calculations) because I've memorised some simple rules of thumb - all you can think of when a good dose of turbulence hits you.

But from first principles, work out how much PV we need to compensate one extra square metre of window? Even thinking about doing it increases my tinnitus.

Edited by joiner, 16 January 2016 - 09:43 AM.


#23 jsharris

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Posted 16 January 2016 - 09:59 AM

Ian, it doesn't really need much math. Go to PVGIS ( http://re.jrc.ec.eur...g=en&map=europe ) and enter your location (lat and long) and the angle of the panels in both planes. If you enter 1m² into PVGIS it will tell you how many kWh 1m² of PV will generate for you in a year. You only need to compare that figure with the floor area x 4 kWh per year of additional losses and you can see how much PV you would need.

#24 SteamyTea

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Posted 16 January 2016 - 10:14 AM

Just the difference in power really.
So say a window allows in an extra 200W or thermal energy, at the same time, a PV module next to it would collect about 15% of that, so 30W. But as one generally puts PV at a different angle (around 35 to 40° on the roof), at the important times it may be getting more, say 60W.
Now you can take that 60W and put it via an ASHP in cooling mode and get 180W of cooling out of it. Which is pretty close. So you may find that you only need an extra 20% by area of PV to offset any extra.
By the very nature of PV working when it is sunny, and only getting solar gain when it is sunny, you you have less of a problem timing wise.
You could also just run a bit of extra ventilation when the sun comes out.

Heating losses are similar, but probably at a lower level as most of the time the temperature differences are relatively small.

The key point is that it is 'offsetting' energy usage and not instantaneous correction, something you can't do in a plane low on fuel, unless you pray a lot.

The trick when modelling this is to not treat the internal temperature as static, allow it to vary by a degree or so around the optimal mean. Modelling is only getting the most accurate picture you can, it is not reality itself.
Take Jeremy's situation with his East facing window. When he moves in, he may find that is is less of a problem as he can open the back door (which is fairly close to the window) for a few minutes.
He may also be in that area earlier in the day than he is now, not not working on the house, but loafing around in a dressing gown and slippers (actually he will be in the garage making a 5 kW electric bike).

So basically, add in the extra glass, put some more PV on of you can, don't worry about gain to much as it is a short term, infrequent problem in the UK, get a comfy chair and enjoy the view of the newts multiplying.

#25 SteamyTea

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Posted 16 January 2016 - 10:17 AM

View Postjsharris, on 16 January 2016 - 09:59 AM, said:

Ian, it doesn't really need much math. Go to PVGIS
I assumed Ian had done that. ASSUME will make an arse out of you and me.

Is there a ST calculator and that may give a good approximation of solar gain.

#26 recoveringacademic

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Posted 16 January 2016 - 10:32 AM

Ahhhh!OK, got it. Happy bunny now. :)

#27 SteamyTea

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Posted 16 January 2016 - 10:38 AM

So what sort of comfy chair you got.
I am sure we all have opinions on them.
Having worked in High Wycombe, we got involved in a lot of furniture design (usually the tooling for cushions).

Two of my favourite chairs are cheap collapsible ones. They are over 20 years old now and still good.
Can put them just about anywhere, easy to store and suit my body shape a treat.

#28 joe90

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Posted 16 January 2016 - 10:43 AM

Ian, I came to this thread a bit late but I agree with everyone, your home should make you smile every time you look at it, or sit in it. It's got to be what you both want, not something you can prove with maths is good for some bods with calculators.

I am having more glass than I think I should ( probably not going to do Passive hous calcs) because I , like you , want to see the great outdoors and enjoy the sky and the view. I have a theory that the overheating issue will only be for a very short time in the year , but the shoulder months and even the winter will be enhanced by warm sunshine even on a frosty day. ( when I will have my small wood burner going:). )

Like you I am having thoughts about blinds etc for the south and that is very achievable. Go with your gut feeling and build what you want.

Life is too short!!!



#29 reddal

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Posted 16 January 2016 - 01:47 PM

One way to reduce the impact of lots of glazing (on overheating) is to choose glass that doesn't let as much of the solar gain through.

I think we have some variation of this kind of glass - http://ememento.sain...405692202_1.pdf

Which claims to block 30% of the solar gain compared to standard thermally insulating glass. The u-values are nothing special - but in a trade off between heat losses in winter vs less overheating in summer I would choose less overheating every time. Its back to the same point - heating is easy - cooling is hard.

Maybe you can get glazing where it isn't a trade off? ie has excellent u-values and blocks lots of solar gain? I think you can also get glass that blocks more solar gain when the angle of sunlight is high in the summer - but lets it in in winter when the angle is lower.

If you are having lots of glass I would worry more about these issues than heat loss.

- reddal

#30 recoveringacademic

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Posted 16 January 2016 - 02:44 PM

View PostSteamyTea, on 16 January 2016 - 10:14 AM, said:

don't worry about gain to much as it is a short term, infrequent problem in the UK, get a comfy chair and enjoy the view of the newts multiplying.

I'm considering making the Temporary Amphibian Fencing (TAF) permanent. (PAF)

#31 SteamyTea

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Posted 16 January 2016 - 02:53 PM

Rent out deck chairs at a pound an hour to other self builders with a similar problem.

Edited by SteamyTea, 16 January 2016 - 02:53 PM.


#32 stones

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Posted 16 January 2016 - 03:25 PM

I'm very late to this one as well, but wholeheartedly agree with what has been said, and build a house that has the features you want, the view, the feel, rather than being obsessed by an arbitary figure. So what if it costs a few pounds more per year to run than a passive house, you are still going to end up with a house that is far superior to the vast majority of houses in the UK.

We went through exactly the same process with our last house, with a passive type house builder quoting for us. They were obsessed with achieving passive house performance at all costs, which for them meant ditching north and east facing windows. What they couldn't understand or accept was why we wanted to maximise the views we had and have a feeling of light and spaciousness as opposed to the 'high performance house' that they would build.

#33 NeilW

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Posted 16 January 2016 - 03:25 PM

Let's not forget that the Denby Dale Passivhaus has a huge south facing atrium that lets loads of light into the building.

#34 recoveringacademic

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Posted 16 January 2016 - 03:31 PM

View PostNeilW, on 16 January 2016 - 03:25 PM, said:

Let's not forget that the Denby Dale Passivhaus has a huge south facing atrium that lets loads of light into the building.

And massive external blinds on that face, to say nothing of the delightful owner who freely admits to leaving a window slightly open some nights to help him sleep. Lovely generous bloke.

Edited by recoveringacademic, 16 January 2016 - 03:32 PM.


#35 VIPMan

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Posted 16 January 2016 - 06:31 PM

Ian - Have you filled in the summer ventilation sheet in your PHPP? I suspect many people open windows at night (night purging) in hot weather and the night ventilation section allows you to see how effective this might be. Certainly a cheap solution if this can be achieved without compromising security.

#36 recoveringacademic

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Posted 16 January 2016 - 07:16 PM

View PostVIPMan, on 16 January 2016 - 06:31 PM, said:

Ian - Have you filled in the summer ventilation sheet in your PHPP?

:huh: