Winter optimised solar to drive underfloor heating - many urgent questions

[duncanlaw]

I start installing 4 KWp solar array on my Victorian house in S London on Tuesday.

THE PROJECT

But since space heating is a bigger carbon challenge I am planning to put winter optimised solar thermal along the ridge/party wall of the rear addition facing ESE. This will the pass through 2 or 3 collectors, again winter optimised, on the S facade on the way down to the heat store in the basement which will drive at least the ground floor underfloor heating. I have already installed double/triple glazing in 95% of windows and am fitting 180mm cork on the back of the house and have 180mm of woodfibre under most of the ground floor. So the heat load should be vastly reduced.

I looked at Hybrid PV but this would have yielded huge amounts of hot water in the summer and low grade heat in the winter. I looked at having a heat-pump to upgrade that. But in the interests of simplicity I've gone with PV (with dreams of using airborne heat from the backs of the panels in Spring and Autumn to add warm ventilation to the house.

THE PROBLEM

I want to harvest as high grade heat as I can in the winter so that it is useful to the UFHeating and perhaps even a DHW use. What is the best kit to use.

The Panels
I was planning to use  - spam link removed - on the ridge to get the biggest array possible (4.5m2 , £1190) to prewarm the water which would then be boosted by 2 or 3 evac tube panels on the S facade, - spam link removed - concentrating sydney tube panel 2.06m2 aperture area 0.78m absorber area, direct flow, £1472 the pair) plus a 1.5m 20 tube - spam link removed -

Is this a good idea? The ridge panels will get morning sun but won't get much if anything beyond midday in winter. The winter optimised facade mounted ones are building shaded in early morning but will get full sun from say 10.30 on a winters morning. Will the large area of flat plate cool the water in the afternoon. - spam link removed -
The - spam link removed - has a small collector area but with parabolic reflectors so in grey would receive less but in sun I guess yields a significantly higher temperature due to the concentration. It stagnates at 350 degrees.

Does anyone have experience of the - spam link removed - Does it work in our climate? Is it worth the extra for the peak heat it will deliver on sunny winter days or should I go for volume from the flat plate and attempt to boost it through a couple of evac tubes panels...

I want to do something cost effective (which must be the flat plate) but also carbon optimal and effective in my attempt to make a Victorian house liveable with no fossil inputs)

The heat store
I was planning to use a - spam link removed - a £3K bit of kit with super-stratifying cone technology inside which I think is brilliant. But it is probably oversized (and priced) for my needs. So in the first instance we are going with an unused but 12 year old - spam link removed - 210l twin coil tank used as a heat store, which I will superinsulate against cool temperatures in my basement. I really want to get return temps as low as possible and facilitate stratification without mixing. Should I go for an external heat exchanger for the solar input? If I want to get DHW should I do the same out of the highest point of the tank with the return from the heat exchanger coming back into the tank at a low level? How effective are they? What are the typical return temp to get DHW at say 45 degrees on mostly a flow rate no more than 10l min?

Oh I wish I'd posted this sooner. Or hired an engineer. I need to make decisions by Monday 26 Sept!

I can send anyone pics, roof plans...

Thanks in advance for all your useful inputs.

Duncan

-multiple spam links removed  -

 " but please note that attempts to raise business or to promote a competing product on the Navitron forum is not acceptable and such posts will be deleted. Repeat offenders will be excluded from the forum.

The decision of the Admins, Moderators and Navitron Ltd is FINAL in all aspects of application of the Forum rules"

[desperate]

Links to competitors gear all over your first post might look a little cheeky to some.................

Frankly a few square meters of anybodys panels aren't going to supply much usefull heat during the winter, and leaving important decisions this late in the game is just asking to waste a stack of money.

If you really want to do something usefull, get the pv installed and spend some time planning the heating for later.

Desp

[ecogeorge]

Have you thought about covering the  tins in glass with insulation behind? 

[SteveH]

 Mirrors are what you need...

[martin]

 

[duncanlaw]

 
Sorry if my links to competitors products were deemed spam. I was considering using a balcony evac tube panel from Navitron but they discontinued it. I was not trying to promote them. I was hoping for some open discussion of the different technologies out there which I know is an exercise in comparing apples and kumquats but still worth doing especially if people have actual experience of the products in question. Do you all only use Navitron?

I have been thinking about this for years. But every time I re-engage in serious research or look again at the roof new possibilities occur to me. At some point the scaffolding up and the decisions must be made. Up to that point I continue to seek the best solution. This is a demonstration project as much as to heat my house. I want to avail myself of the best thinking (much of it available here) and the best products and technologies even if as an early adopter I have to pay 'uneconomic' prices for them. I know someone who for most of the year heats a house insulated to similar standard to mine with a 30 tube panel at 30degrees.

I'd still like some considered input.

So are we really not allowed to mention other products on this forum? If not I guess it's back the AECB for me.

[djh]

...

I can see why all your links were removed since you made so many on your first posting, but it does make it very difficult to have any sensible discussion about what you're proposing.

My first thought is that it all sounds very complicated, so I'd examine it carefully with a view to simplifying it if possible.

Will the large area of flat plate cool the water in the afternoon.

Yes, if I've understood your design correctly. Selective absorbers will reduce the loss but running the primary loop through a lot of extra piping that is not contributing to the heating is a good way of increasing losses. There's less of an effect with heat-pipe evacuated tubes because there's less water in the manifolds and the heat pipes act as 'thermal diodes'.

I want to do something cost effective (which must be the flat plate)

That sounds like you're saying that flate plate collectors are more cost effective than evacuated tube collectors. That isn't generally the opinion here, I think, especially with regard to winter performance.

BTW, what is a 'ridge/party wall'? A picture would help.

[dhaslam]

Of course extra panels will help and they are particularly useful in the spring because air temperatures lag behind  the increasing solar gain at that time of year.    For off season heating  you do need to use evacuated tubes and you still need to be able to use a low temperature output.   Underfloor heating that will work at 25-30C  is fine.  There would be days that the tubes will not heat a store to 25C so you need a few days storage at least.    In summer you need to be able to use or store the surplus heat.    

[martin]

You CAN have a reasonable discussion without using brandnames - "a flat plate solar panel", or an "evacuated tube panel" is quite adequate to let us know what you're talking about - anyone who makes a first post choc-full of what can be construed as spam links is regarded with the deepest suspicion - we get a colossal amount of spam of all sorts, and waste an inordinate amount of time dealing with it, so it gets very short shrift....... here's a recent post illustrating the problems  - http://www.navitron.org.uk/forum/index.php/topic,15032.msg168328.html#msg168328

[duncanlaw]

Thanks - useful opinions. Point taken about spam. But it does make it difficult to discuss. 'Evacuated tube panels' does not express the technology I am interested in which has a parabolic reflector panel behind 6 tubes giving an aperture are of .98m2 per panel from an absorber area of only .39m2. Now my limited physics tells me that that is great when the sun shines as it concentrates the radiation on a small surface resulting in a high temperature but in diffuse light it is one expensive tiny panel. Should I just pray that the IPCC prediction of warmer greyer winters doesn't materialise. These panels also have the benefit that they are narrow allowing me to position at the highest point along ridge where my house backs onto the next one in the terrace. See drawing:



This company gives me comparison figures for their flat plate and their concentrated tube in KW generated p m2 per year. Flat plate 272 - Evac tube 324. But this gives no indication of seasonal production ie what they produce in the winter when you really need hot water at a useable temperature. Also for the price of the 3.1m of evac tube I could get 9m of flat plate so the yield in KWs per £ is better for the flat plate. BUT WILL IT BE AT TEMPERATURES I CAN USE? The data just isn't out there in figures that allow even most installers to compare like for like.

Thinking I should put a flat plate up alongside the evac tubes and run sensors from all of them. But I'd rather listen to someone who can work it out in advance and install a system that works optimally.

Thanks again.

[KLD]

Duncan

The biggest problem I can see with solar heating support in the UK is the lack of sunshine in winter   JohnS has just posted a link to historical data. I've extracted the monthly sunshine hours from the weather station at Heathrow for the past 10 years, averaged the monthly figures, and plotted them versus the calendar month. See below. There are only a quarter as many hours of sunshine in winter as compared to summer. Combine that with the much reduced intensity, and you'll end up with ... not very much.

A cloudy day more or less per month will probably introduce more variation than a few percentage points extra efficiency.
There are loads of posts on these forums every year in spring where people express their renewed surprise at how sudden solar thermal springs to life in March (-ish).

Klaus

[Drawmer]

To get any worthwhile heating out of solar in the winter, you will need to invest in and have the roof space for a vast amount of panels. You will also need to store the heat so that in December when the sun goes down at 16:00 you can still be warm indoors at 23:00.

In the summer, you would have so much heat, you would need to be dumping it (lots of it)

Looking at Klaus' diagram the difference in energy collection between June and December is huge. My PV system produced nearly 5 times as much power in the best month than the worst.

In my opinion, trying to do this in the UK is a financial non-starter.
When we need the heat, the sun isn't shining.

[duncanlaw]

Thanks. Useful graph and thoughts.

I am 'winter optimising' the panels, putting them at a steep angle so that they are as near right angles to the low aspect sun and therefore capture more of it than your panels at the recommended 30degrees, and very much less than them in the summer. If nec I can reach out of back windows and put a cover over the S facing ones. I have underfloor heating on the ground floor which itself is quite a few litres of water and external insulation (walls down to 0.2 U value) so a lot of insulated thermal mass vis the whole of the back of the house that is my heat battery. I used to use 11,000 KWh gas to keep the house not very warm in the winter. There are now no draughts and the windows are down to 1.1 u value.

So has anyone used concentrating solar evac tubes?

Duncan

[KLD]

So, what sort of energy input do you now need after all the insulation? Ideally you'd know daily requirements?

Klaus

[martin]

"So has anyone used concentrating solar evac tubes?" - I can't recollect it being mentioned on the forum, I think there's been the odd lash-up "experiments", but nothing commercial. My understanding is that they offer no practical advantage over conventional tubed systems (which to my mind are superior in every way to flatties)
To be frank, I think you're attempting something similar to roof-mounting a wind turbine - it is possible, but effectively pointless - you can't take energy from where there isn't much - the idea of using solar hot water systems for domestic heating surfaces every once in a while, but there is so little power available from the sun in midwinter (when you most need it), that economically, it really is a non-starter -  it would cost an inordinate amount of money to ever generate anything worthwhile IF the sun shines - if it doesn't you'll get diddly squat....