How many ETs c/w what size thermal store for 100% Solar DHW?

[darylp]

Hello all,

I hope someone 'technical' can help? We are trying to work out how many ETs combined with size of Thermal Store that are required for 100% solar DHW for a barn conversion. There will be 4 occupants, the ETs will be due south facing, no overshading.

I had guesstimated a 1000l Akvaterm (or similar), but the issue of thermal overload in summer has been raised, and how to dump the excess?

So, what is the recognised/recommended minimum area of collector we could expect to use for 'normal' DHW usage, combined with a Thermal Store, without having to dump large amounts of heat in the summer?

Location in 'sunny' North wales....

Thanks in advance....

[A.L.]

hello and welcome!,

since no-one else has replied, perhaps since no-one else wishes to be the bearer of bad news I will try 

-unless you are willing to store water heated in summer for winter use only an infinite number of ETs will provide 100% 

-usually you decide on a % of the total usage and aim for that

-40 x 58mm ETs will probably provide 50-75% of annual usage, depending on consumption

[Drawmer]

Not worth it. There will be days when you get two thirds of three fifths of not a lot of sun. On those days in-between November and February, then no amount of tubes will harvest what isn't there.

As posted above, it's probably best to either work with a target % days to be satisfied, or just a financial budget.

[darylp]

Thanks to the above....

Well now we have a figure to start with: 40 x 58mm ETs, for 75% isn't too bad.
Is there a correlation between ET area and TS volume, or should I stick with the Akvaterm 1000l do you think?

Cheers....

[desperate]

Hallo Daryl,

We have 60*47mm tubes feeding a 216litre hot cylinder, today it has reached the dizzying temps of 39 at the top and 25 at the bottom, it has received no other heat since last night. Over a full year we get just about 75% of our hot water supplied from the solar, there is so little energy available on dark gloomy winter days that it would be difficult for us to increase that percentage much without some kind of seasonal storage. From the beginning of march to mid October it will provide 95% of our hot water and for the rest of the year it is about 25%ish.
Are you planning to feed the store from other heat sources as well as the solar? 1000litres would be way too big for 40 tubes on their own I would think.

Seeya

Desp

[darylp]

Desp,

Thanks, that is useful information. Where in the world are you?
I should say that there will be a secondary heat source, a WBS.
Space heating will be served by a GSHP.

I suppose we could realistically aim for 95% DHW in the summer/sunny months, and around 25% in the darker times.
What area do 60 x 47mm ETs cover, please?

Cheers, and thanks again...

[A.L.]

hello again,

dimension details here - www.navitron.org.uk/product.php?proID=115

output of 60 x 47mm and 40 x 58mm about the same, ideally two volumes of about 250litres heated sequentially, but could be in same tank with switching between two solar coils

[knighty]

with a 1000 litre tank...

and a system starting from scratch (no existing system to modify)

would it not be better to have 80 x 58mm tubes min. with the 1000litre tank

and then just dump more heat on the really hot days ?

dumping heat is simple, you just need a radiator somewhere which you pump the solar water through to cool it down :-)

is it just for your hot water, or heating too ?

with a clean slate, you might as well have under floor heating fed from the tank too ?
(maybe a bigger tank.... or 2 tanks?)

[desperate]

Desp,

Thanks, that is useful information. Where in the world are you?
I should say that there will be a secondary heat source, a WBS.
Space heating will be served by a GSHP.

I suppose we could realistically aim for 95% DHW in the summer/sunny months, and around 25% in the darker times.
What area do 60 x 47mm ETs cover, please?

Cheers, and thanks again...

Daryl,
We are in south London, so a little bit south of you, but not a huge difference in insolation, we also have a WBS as a secondary heat source (16kW Woodwarm) and if all else fails a gasser on standby, so far since the snow melted in february the gasser hasn't really come on at all. You can see all the details in CRAC, show us yours board.

http://www.navitron.org.uk/forum/index.php/topic,8465.0.html


Desp

[Baz]

After a while someone will come on here and say that if your store is too big you will just get cold water etc missing the point that there is value to gathering energy not absolute heat. A better balance can be had with 2 tanks, main and feeder to hold the extra heat when available if you can sort out some switching.
Roughly 30 x 47mm tubes will give you 35 litres x 4 which is enough if your family has enough brains to turn the shower off but not if you read a recent post about the problems of power showers. 30 will cover you for 7 months of summer most of the time if you have a tank 2x daily usage heated to 80c to tide over the hot spells into the cold ones. However there are often sequences of 5+ days with poor sun.
Beyond that is not cost effective unless you are using Oh, how I wish it was better cheepo tubes (ie not navitron) but if the feeling of saving money is more important than actually saving it then get more tubes. Calculated figures roughly value the heat from 30 tubes at £80 per year as £55 from march to sept inclusive.
There is some very usefull real life excel data for 60 tubes on a post by Dhaslam on 7 Nov 2010 which shows you the output variability day to day and also how big to make your heat dump for when you are on holiday in a heatwave.

[langstroth3]

Just to give you a further example.
Weather: Clear sky all day today in N.Hampshire; but the days are of course short and sun low in the sky. Temp max 10C
40 x 47mm EV tubes on a 160L tank.
Tank at 10am: 18C top, 12C bottom
Tank at 6pm: 30C top, 28C bottom

No boiler input until after 6pm.

So, as Baz says, valuable energy collected, with the boiler having less work to do.

 

[darylp]

... many thanks to all above for the advice and information! 

I understand about the issues around sizing, just not yet how to resolve them?
Dumping excess is not a problem, we can dump into the ground around the GSHP collector 'slinky'.
What I want to avoid is the risk of having lots of lukewarm water, rather than a smaller amount of
'hot' water.   

Cheers....