Evacuated tube solar water heating and central heating system project

[SolarHeat]

Does using solar towards central heating work?
Having tried to find a definitive answer to the question and failed, I tried approaching a few installers who mainly said no and offered systems for hot tap water only, at what I felt were rather high prices for what they were supplying.
I hope my experiences may be of benefit to others, hence my posting here!
I don't want to be seen as promoting any particular company, I have no connection to any company or their product used save as a customer, hence not mentioning any manufacturers, except, I only see it as fair including Navitron as they kindly have this forum - sadly I didn't consult their expertise when specified the panel from them and am sure I could have gained from talking more with them.
I've not posted pictures as there isn't much to see - plenty of other places have pictures of panels and tanks, etc!
I designed the system but had others do the installation works, which caused many issues, but the work needed to install was beyond my time and abilities.
Cutting a long story shorter.....
After extensive research I designed a system whereby two 30 tube panels (one full size direct flow lying flat and one Navitron 850mm long tube panel mounted vertically) running on individual pumps feed into a small DHW cylinder, a wet heat store or a solid heat store.
Unfortunately, I couldn't find a controller which would do all I needed and the price of an expansion to the most appropriate looking controller was so high I decided not to add it to the system, but, with hindsight, I may have lost most of what I saved with the added complexity and time spent working around this and only using the unexpanded controller!
The solar side of the system has three options - running to 180 litre DHW store, to approx. 400 litre wet heat store and to solid heat store.
Originally, the plan was to be able to reclaim heat from the solid store but this would have been more complex and, as losses in the store were unknown, I decided against - which, with the benefit of hindsight, I believe was the correct decision.
I've found that having the DHW as primary heat receiver appears to be the most efficient usage of resources. If the DHW tank is at or above the solar panel temperature, then solar heating is switched to the wet heat store and, if this is at or above the panel temperature, then solar heating is switched to the solid store.
The solid store is the foundations of the house, a 3 bedroomed semi, and so is quite lossy but better than nothing! Also, this serves as the heat dump for the summer in the hope that some will be reclaimed on colder days - unfortunately, so far I can't give any results on this as the people doing the installation took so long we missed, to my great disappointment, the summer heat gain!
The DHW tank installed was not the one I specified and I discovered there weren't enough places to put all the sensors in that I needed. However, because of the stratification being so large (upto 50℃ so far between solar coil and top of tank) I repositioned sensors and that has worked better, on this tank, than if there were enough sensor pockets where I had originally wanted them. I was surprised how large the losses from the tank are and had to add extra tank insulation to improve efficiency - I gather this applies to most "insulated" tanks.
The wet heat store is filled with, treated, heating system water and so runs as a preheat to the old gas condensing boiler. If the wet heat store is above approximately 40℃ output temperature, the boiler is disabled until the temperature drops below. This means the wet store can be used, on its own if warm enough, to run the radiators, which were deliberately oversized for 40℃ running temperature, or to run the heating inlet to the DHW tank. The boiler runs during the night, if required, to take the DHW tank to just over 60℃ as a legionella protection.
I kept the old heating/hot water controller so that there is a nice easy heating control away from all the solar paraphernalia and, though it is normally set to off as the solar controller handles normal hot water control, the hot water control could be used as a boost. Additionally, on the DHW tank, I have a thermostat which switches the heating valve in if the heating is running and the tank falls too low at the position of the boiler input pipe on the tank.
If the wet heat store is cooler than the boiler water return flow, a valve switches it out of the circuit until such time as the flow temperature drops back below the wet store temperature.
The solar panels were located horizontally and vertically for planning and practicality reasons. Despite the vertical panel performing quite well over the winter, the main reason it was installed being for the winter gain, I feel a couple of degrees angle to the vertical would have dramatically improved results and wonder if a direct flow version would be much more efficient due to cooling slower. So far, the horizontal, direct flow, panel has produced incredible results considering the angle of the tubes, even beating the vertical panel on some of the snowy days and whilst still covered in snow. The vertical panel, it must be remembered, has tubes much shorter than the horizontal panel which undoubtedly reduces output. The controller is running frost protect mode, as I didn't dare risk the panels, etc. freezing in the very cold weather, and the non ideal angle along with poor sensor positioning by the installers means it has to run the pumps from time to time to get a proper sensor reading during the day.
I would have loved to have included a data logger in the system but didn't have a couple of hundred pounds spare for this luxury.
At the same time as adding the solar system I got super insulation put on the underside of the roof as a supplement to the already good insulation laid on the upstairs ceilings. A few weeks ago, I did a temperature test when the roof tiles were at -11℃ during the day and the roof space was at around 5℃ which again proves how important good insulation is. I added a heat recovery system to the bathroom which has worked well at reducing moisture but, so far, I'm dubious how much gain there is in relation to purchase and running costs. On the central heating I put a new super efficient pump which uses about 25% of the electricity the previous pump did, I'd have loved to put it on the solar as well but couldn't find information if it would cope with the different fluids.
We had numerous issues of bad installation including an underfilled solar system (discovered, despite my being told it was definitely OK, the first day after filling when the system temperatures hurtled too high - I'm left hoping no damage was done!) and substantial insulation missing from where it was supposed to have been installed (the lack of insulation found when I investigated, in the cold weather, why heat was leaking from all those places you can't see without close inspection!).
Results so far, even after the installation issues, moving from an electric shower and adding heating upstairs, imply around a 10% saving on energy bills compared to the previous year - these results include the November/December temperatures when it was below -12℃ at times during daytime. On very overcast misty days there is no solar gain but nearly all the other days there is some. Input to the heating system has been only a few degrees most of the winter but, a few days ago, whilst the ambient external temperature remained around freezing, the solar took the DHW from around 6℃ to over 30℃ at the solar coil position and the wet store made it to 29℃ .
Am I pleased to have the new system installed? Despite all the hassles - YES!
I am, also, now more aware than ever how many people are being ripped off by companies selling expensive systems to heat tap water and advise buyer beware - if you are unsure if it is a good deal, at very least, look at component prices and add appropriate labour charges to see what sort of price it should be!
So the answer to the question I started with appears to be yes - but do you get payback? Only time will tell!

[dhaslam]

As you are probably aware  60 tubes is about the correct size  for  a water heating system  covering 60% of the year.   For heating, in winter obviously,  you need  3-5 times as much depending on house insulation levels and it needs to be oriented towards the winter sun and have  a few days storage.

Kristen, who unfortunately is not in communication at present,  had a setup  that was in line with the above  idea  but I don't know how it performed in the  recent frosty and sunny weather, it should have worked well.
 
www.navitron.org.uk/forum/index.php/topic,10426.msg114768.html#msg114768

[Drawmer]

Does using solar towards central heating work?

It's tempting just to write:

No

But after your detailed explanation, I feel some more justification is required.

The reason why I don't believe that solar will work for heating is that when you need the heat, the solar tends to be in off mode.
And since there's no efficient way of storing the energy until we need it for the heating (summer round to winter) then to capture solar power for winter heating we need a large (expensive) system, and still need to store heat captured in the short day for release in the long night.

OTOH it does DHW fine.

[2807]

Hello SolarHeat

I live in SW France & have 100X58mm together with 100X47mm evac tubes facing south, and 100X47mm evac tubes on a west facing roof.

These provide hot water for 8 months of the year, heat a swimming pool for 5 months of the year & central heating for a 200M2 house in the winter.

You asked whether using solar towards central heating will work.

The simple answer to your question is in fact yes - solar heating will work towards central heating, however, in December & January, the daylight hours are just too short to provide sufficient heat for the hours of darkness, unless:-

1. Your property is extremely well insulated

AND

2. Your solar collectors are massively oversized.

So an alternative source of heat is normally also required.

I was away from the house for 6 weeks from the beginning of December and when I returned, the house was around 12 degrees - not warm, but not too cold either.  However, it is February next week & already, I am noticing the increased heat output of the radiators, especially as the sun is starting to climb higher in the sky & heat the 100X47mm west facing tubes, for a longer period of time each day.

Currently, the radiators are simply plumbed directly in line with the manifolds, however, from mid to late February onwards, I alter the plumbing so that the tubes heat a thermal store which provides for hot radiators later in the day after the sun has gone down.

It also helps that we are on CET so have the extra hours daylight at the end of the day.

Best Regards

2807

[billt]

I too agree that it can provide some contribution to heating, but not a meaningful one in the depths of winter, unless you live in an area that has many clear days (not the UK).

I've installed a 120 x 58mm tube system with the aim of not needing to burn any fuel between March and October. It started producing energy at the beginning of October and I fitted a calorimeter half way through October. The best day in the second half of October produced 33 kwhr; the best day in December produced 8.8 kwhr. The daily average for the 2nd half of October was 14.6 kwhr, November 6 kwhr, December 2 kwhr, January so far 3 kwhr. In December and January there have been only 12 days with clear enough skies to produce a reasonable amount of power.

The winter months have probably been a bit worse than usual and October may have been a bit sunnier than usual, bit the trend is clear; no usable quantity of heat is available from solar panels in the depths of winter in the UK.

[stuartiannaylor]

Solar heating is quite possible and doesn't have to stop in winter.
Currently systems work on the principle that they are balanced on capacity in regards to stagnation. IE there is no point installing a huge array if the majority of the time is spent at stagnation temps.
If a house is reasonably insulated or even better getting to passivhaus standards then it gets very plausible. Also the installation requires some lateral thinking.
You specify a collector array against winter conditions of low light and look at providing generation with the summer excess.

Sorry for the spam but this is the only product I know of and have mentioned it to Ivan.
http://www.rittersolar.de/english/index_e.htm are direct flow collectors with concentrating mirrors and have very high returns.
You can start looking at the heat exchange circuit in higher heats than water / glycol in thermal oil loops.
Low heat generation has a few options but this market is starting to see products arrive.
What I do like is the idea of hybrid systems where a backup burner is used in conjunction with the solar system.
These guys are trying to do field trials and I am unsure of the cost or arrangements http://www.coolenergyinc.com/
Then there is http://www.cyclonepower.com/ with there waste heat engine
Also http://freepower.co.uk/ do a series of organic rankines

The basic premise is to oversize your array and generate with the excess. 

[dhaslam]

When it is sunny in the winter the  heat generated in the panels isn't all that different than the summer.   Water heating is more efficient in winter because the incoming  water temperature  is usually much lower but of course the water never becomes very hot.   The same  situation would apply to space heating  because the heat would be used at much lower temperatures.   

In December I had two periods of very high output from the panels, one day at the beginning of December showed 28 kWh. The controller  exaggerated a bit  but there was a lot of hot water used that day  so the cylinder was constantly being fed with very cold water, near freezing,   in the middle of the day.         Of course there were some days, particularly January,  with no output.      About five times as many tubes and  enough heat storage for a week it would probably do the job.    The cost would be  a bit too high but any kind of subsidy would probably make it viable.     



     

[billi]

My conclusion is  PV on the roof , a clever Heatpump , and solar thermal panels on the house  south façade , if no space left on the roof   / and or  solar air heating panels  for heating

Simply because  the over-sizing  of PV will not just be a waste in summer for heating swimming pools or radiators as a Dump , no it supplies electricity


Billi

[Baz]

There are newish houses that achieve mostly solar heating by a combination of high mass, high insulation and large south facing windows (and shading in summer). Existing houses can to some extent be modified for the first 2 but making big windows is more drastic.
However it should be able to achieve the equivalent solar capture with contributions from lean-to greenhouse and conservatory both of which give secondry value in summer, and solar air collectors possibly replacing sections of roof rather than just being bolt on thus liberating tiles for reuse instead of using energy to fire more.
Although tube collectors are more efficient in winter than other systems I wonder if that is material and cost effective.

[Drawmer]

OK - I'm wrong; it can contribute if you have a massive system. Not sure it would be cost effective though.

Actually, I admit I forgot about passive solar collection changes to the south side of a house.
A south facing sun trap like a conservatory will take a lot of heat in, which can add appreciably to the heat in the home during the winter sun.
Often all it needs is some management of the doors into the conservatory to get some real heat into the house during the day.

[rt29781]

Hi,

We have 150 tubes on our roof in SW France.  On a clear day here at the moment we get 22kWh of heat on a normal day.  That is about half our needs so we have a woodburner and we also have an ASHP which is plumbed into the solar circuit (nominal 12kW)  See our blog for more details.  Over the past 2 weeks we have had very clear crisp days with outside temperature no more than 3C and the solar system has been working great.  As the year progresses the solar system heats the house and then the pool and in Summer we have to dump the heat.  The last 2 days we have had torrential rain so no solar at all but the ASHP and woodburner keep us toasty.  So in summary you need to have lots of clear days and lots of tubes but solar does work and it pays for itself easily.  It's a lot easier than feeding a woodburner.

[billt]

For my location in the Midlands PVGIS gives an average insolation of 22.6 W/sq.M in December and 151 W/sq.M. in July. For a random location in SW France it gives 56.6 W/sq.M in December and 182 W/sq.M in July.

You're getting 2.5 times the insolation and will have a significantly lower heating requirement, so it's reasonable that it can work there. There are probably many parts of the world where it is possible to get some useful portion of winter heat from solar. That doesn't include most of the U.K. which has low winter sun and many cloudy or foggy days when there is no usable solar energy available. It's quite possible that there are a few places in the UK where the mean temperature is high and the amount of cloud is low, but they will be few and far between. Bournemouth or Pembroke maybe, but they would be very unrepresentative.

My 120 tubes produced 14 kWhr on the 19th of January, but there have only been 4 days that have produced more than 10kWhr and 4 other days that have produced a useful amount of heat. That's because there has been no useful solar energy for most of the month due to cloud and fog. That's not enough energy to keep the water hot, let alone provide space heating.

However, I'm hoping that by the end of February, when the insolation levels have increased, and the heating demand has dropped that my system will produce some useful energy for space heating and reduce the amount of wood that I need to burn.

[rt29781]

I think a more useful measure would be to look up http://www.bdpv.fr/index_en.php for your area and see if anyone has posted PV figures.  This is a real measure of solar received in your area (so it takes into account clouds etc).  For our system we get about 3 times the kWh from the thermal system as we do for the PV system so as a rule of thumb 50 Navitron tubes are equivalent to about a kW of installed PV.

Obviously in December the figures are poor but in the UK you heat more than just in December.  As the Solar thermal system contributes in each month then you need to do sums to see what would be the total contribution.  Any contribution to the heating bill is good.  It is a no brainer for us as we can use any excess heat to heat the pool when the season for that starts.  We have a massive excess of heat in July and August which is dumped.

[SolarHeat]

Having read the responses so far, people make some interesting points.
I looked very carefully at going PV as well, to run the system, and approached a company doing PV and thermal as a single panel but they couldn't be bothered to reply to my enquiries. Currently, for price, roofspace and efficiency, I couldn't make a PV install work. One of the problems was the solar pumps as the low power ones that can cope with a PV supply wouldn't provide enough head feet.
So longer term, when the next generation of PV panels are more reasonably priced, I hope to add that in to the system but I'm also keen on using a safe and environmentally pleasant store or it is pointless if there is a power cut!
I would also have loved to do a full rainwater recycling system but as a retrofit it didn't financially add up as well as the environmental problem of replacing the UV tube every year - hopefully L.E.D.s will solve this yearly requirement soon methinks!
Updating the performance information....
Compared to the gas and electricity bills for the December to mid February period last year, it looks as if gas usage will be about the same and electricity around 10% less but of course this time it has been colder and for longer. Also, the heating has been on more due to prolonged flu! I reckon the bad workmanship by the installers has cost a few percent as well. So that looks pretty good for this time of year!
Over the last few days, with the more sunny weather, the HW tank has made it to the high 30s at the solar coil and the central heating tank has been in the high 20s several times and they may well have been higher after I'd looked. So the heating certainly isn't running off the solar but is getting a reasonable preheat of several degrees most days. Today, being milder but very overcast conditions, at 11am one panel was at 17 and not on but the other panel was over 22 and having put maximum into the HW tank had nearly got the CH tank to the same temperature.
Hope the above is of interest and maybe helpful.

[SolarHeat]

Having actually received the final energy bills for the last quarter, the electricity consumption is down around 20% and gas is the same as the same period last year.
All things considered, I'm pleased with this. I was hoping the gas would be a little less but it has been colder and the workmen left the doors open a few times!
It'll be interesting to see how things go for the whole year but this is a good start. We've already had a day when gas usage was below one unit and I reckon we should soon be down to only needing gas for cooking until next winter!