Sizing solar panels for thermal stores

[gbranger]

I have the good fortune to be moving from comfortable gas centrally heated suburban Harrogate to wonderful dilapitated cottage on the Lizard in Cornwall. This move comes with a multitude of challenges - the 1st of which has been how to provide DHW and heating. I've looked at lots of options but have been drawn to the simplicity of 2 wood burning stoves for space heating - one of which will have a boiler for DHW and some 'wet' bathroom heat. I plan to supplement the water heating with solar panels.

I have 2 queries to help me move forward on this.

1) What area of solar panel to install - I have seen a lot of good information about the average amount  of solar energy available per sq m throughout the year but less information how much of that energy I could reasonably expect to convert into hot water. I am planning on 50% - so in summer if I get 4 kw per day on the panel I expect 2 kw 'in the tank'  would I be a million miles out ? so
 
2) Thermal store (vented)  or standard cylinder - I am drawn to the thermal store as it can provide mains pressure hot water and the potential to provide some controlled heat in the bathroom - but I understand that it would have to reach a higher temperature than a same sized cylinder to provide useful DHW. If that's the case would I need a greater area of solar panel ratio for a thermal store compared to that for a similar sized cylinder ?   
 
PS - although first post I have been an ardent 'passive' reader for weeks - thanks to all contributors for an absolute mine of useful info.

[wookey]

1) Careful with your units. '4kW per day' doesn't mean much. 4kWh per day does. You can expect to do somewhat better than 50% with a well-designed system, but using that a conservative rule of thumb would be a good start. There have been a few recent threads on the subject. Be careful with using areas and insolation level to work out incoming power - I got this wrong due to using 'gross' rather than 'aperture' area. Much better to work with known panel power outputs and avoid the whole 'which area' thing, although correct derating factors between actual insolation variation over day/angle and the quoted/tested power output at constant 1kW/m2 insolation are a bit hard to come by - and that's the number your question is really looking for.

2) In a word yes, but not by much (gut feeling, haven't actually done the sums). The extra heat required should be about 5C, although I've not seen that confirmed by anyone with a store here. How much extra panel area to get an extra 5C? good question. 10%?

Anyone got some actual numbers to hand?

[dhaslam]

In very simple terms you can think of the years sun like a tide table  that goes over six hours 123321 except with solar Jan to Dec needs twelve numbers. 123456654321.   Unlike tides the variation is not regular and the variation is slightly greater as well.   If you have a stove for water heating there isn't much point in covering water heating needs from solar in the worst four months so perhaps double or treble the normal  panel size is about it but you could go to  more without enormous expense.  I would say three or four 30 tube 47mm panels would be the about right.  Beyond that you would need a ground loop or swimming pool  dump heat in summer.   If you use a very large buffer tank it can be used for  solar in spring to autumn and solid fuel in winter. It is advisable to use a bigger back boiler on the stove that you can build up the store  in the hours that you are around.    The increase in solar energy should be very noticeable in Cornwall and winter temperatures are probably higher as well all of which should make the job easier.     

[kristen]

Slightly tangential thoughts:

I think it is worth considering a pair of thermal stores, if you have room and want to over-specify the solar collector area somewhat (to boost Autumn/Spring). Heat the main one, when that's "full" switch to heating the second.  Pre-heat the DHW through the second.

Also, if you have the option, angle your panels for winter sun (well, probably optimise for Spring/Autumn), rather than mid-summer.  if you are sticking them on a pitch roof I doubt you have much choice, OTOH on a flat roof its worth considering.

[gbranger]

Thanks Wookey - I've seen that Kw / KWh mistake corrected a few times - and I still got it wrong - there's no hope for me!

1) You've confirmed my thoughts on efficiency - if designed & installed well then a 50% factor is a fair place to start to avoid later disappointment .

2) I suppose the next question would be is "5c more than what" ? I currently have a gas combi boiler giving instant DHW - we have it set to 50c. I have a feeling that 55c may be below optimum for DHW from a thermal store. But you have confirmed that there a temp differecne has to be factored in

Comments appreciated

Cheers   

[wookey]

By '5C more', I meant that the temp drop across the DHW heat exchanger (plate or coil) in a thermal store is about 5C - i.e the water should come out about 5C less than the temp of the store. So a thermal store needs to be ~5C hotter than a plain storage tank for the water to come out of the taps at the same temp. But check this with someone who actually knows.

[David]

2) I suppose the next question would be is "5c more than what" ? I currently have a gas combi boiler giving instant DHW - we have it set to 50c. I have a feeling that 55c may be below optimum for DHW from a thermal store. But you have confirmed that there a temp differecne has to be factored in

55c at the top sensor of a Navitron thermal store should be fine for most uses. It will provide water hot enough for basins and showers. It may not be enough for a bath (depends on how big your bath is, how deep you fill it and what it is made of).

I find that anything over the low forties is fine for basins and anything over 48 or so is fine for showers. 55 would be ideal and could be boosted for a bath if necessary.

Note that when the heating is in use the demands of heating will outweigh the hot water, but provided the system is designed properly there will still be plenty of domestic hot water.

With hindsight I would like another thermometer pocket in my thermal store, about half way up, to have an idea what the temperature is there. I would also move the top sensor down a little, so the boiler heats up a bit more of the store before cutting off.

 

[kristen]

Stupid question:

Would an inline electric [or other I suppose] heater be out of the question - I mean in the mains hot water after the thermal store - to lift it to 55C if / when required.  Strikes me that may be better than using the boiler to heat the whole thermal store when there isn't quite enough solar.

I don't really want the boiler coming on at 10am and heating the whole store when its actually going to be a nice sunny day - and I have had a lie-in and want a bath!

But I dunno how to do the comparative calculation, sorry.

[dhaslam]

If the store is big enough and the  solar coil is at the bottom there should always be enough cold water to heat during the day.    Direct electric heating needs a lot of electricity and devices need to be very well made.  Having set the house on fire with one I am not so keen on them.

[gbranger]

Thanks for input folks - really helpful in getting a picture of how the thermal store delivers DHW.

I'm not attracted to over specifying the system - either in respect of additional panels or add on electrics. Simply after 'free DHW' for the bulk of the time coupled with DHW in cooler months as a by product of wood fired space heating - with the bonus of snug bathroom if possible.

This is all for personal use in a relatively small cottage. Once that's up & running I hope to provide DHW & heating over 6+ months of the year to holiday accommodation using 'over specced' solar water & thermal store. But that's for later!
I hate gas patio heaters - anyone tried patio UFH using summer surplus solar heat for those cool coastal summer evenings ?

[kristen]

"I hate gas patio heaters - anyone tried patio UFH"

Absolutely hilarious! I haven't laughed so much for years!

[Gary T]

      Overspecking the size of panel and tank looks like a good option for you provided you use the same supply for both your cottage and the holiday accommodation. That way, your total hot water demand comes much closer to tracking the seasonal variation in solar gain.
      I gather that a typical amount of tubes would vary from about 5-10 tubes per person - depending how heavily you use hot water. Include an estimated number of people for the holiday lets, and you would be about right.
      If you can afford it, get all your panels in one go - one lot of installation and scaffold etc, so costs less per tube than adding more later.

[billi]

I hate gas patio heaters - anyone tried patio UFH using summer surplus solar heat for those cool coastal summer evenings ?

I have thought of this a few times , cause me designing outdoor spaces
I am hoping they (patio heaters) will be banned soon as well as these electric outdoor heaters
Must be a huge amount of energy wasted...

I think its  worthwhile to find out how long heat can be stored in stone/concrete/sand etc outside
and use solar and wood/barbeque  to circulate hot air or water through walls , floors, benches , tables outside

 

Billi

[Ivan]

one of our exterior walls faces West. After a sunny day, the heat radiating from the wall is very obvious even at 2am - say 4hours after sundown. You need to be quite close to the wall to benefit from it though.

[dhaslam]

The difficulty is in finding a sunny day, but then nobody would be sitting outside otherwise.  The question is whether it is worth while painting a wall black specially for the purpose. Not so bad on a garden wall but it would look odd on an otherwise white house.  In this case the small piece of west facing wall at the patio doors.  The house faces exactly south.