Hi all,

My name is Karl, this is my first post but I’ve been reading and researching using this superb forum for quite a while now.
Thank you all for sharing your obviously hard earned knowledge, it really is appreciated and has help develop my knowledge and interst in renewables and heat engineering.

I have a question which I can’t quite find a definitive answer to, having searched for a while.

Is it ok to you use one coil for multiple solar thermal inputs?

Some background and sorry if this is too much info but I have seen from general responses here that the more detail, the better able people are to provide input.
I have an existing 3 coil 1000ltr thermal store supplying my DHW via a full height stainless steel 7.5m2 coil, works brilliantly.
Direct connection (store is open vented) of a 24kw wood burner via laddomatt and digital stove control. Rarely used except for depths of winter when the family really enjoy the live flame experience and I get that smug satisfaction knowing that we are loading the thermal store with heat too.
Upper coil, 2m2 supplied by 15kw pellet boiler, tops up my DHW on less dull days and via 2 port valve inputs heat to the UFH when needed (mainly early hours of morning (2-6 am) all very efficient.
Lower coil 3m2 supplied by 6.6m2 flat plate solar thermal, provides 100% DHW production for c.8 months of the year. Remaining 4 months it contributes c.50% DHW and some (c.40%) space heating as UFH take off is 60% up the store so once the temp sensor at 60% hits 40c the heat is pumped to the UFH until the temp at 60% drops to 35c. So the UFH gets 40c input from solar most days during heating season, average is about 1 hour recovery from 35c to 40c for the store for 15 mins supply to the UFH all whilst keeping the upper 40% at DHW temp, very happy with the stratification.
I now have an additional 50 EV tubes to add to the system but no more coils available. I’d like to T into the lower solar coil and use it for both solar arrays, (existing flat plate and new EV tubes) obviously running through separate pump stations with non return valves and separate controllers. I have some direct connection ports left so running through a PHE could be another option but would involve more pumps and more plumbing. Not my preferred option.

Could I ask what people think about sharing a coil for two heat inputs?
I have seen it done before for oil CH when a boiler stove has been added so I know it’s technically possible and accepted practice in some places. I have never seen or heard of it done with solar though.

Many thanks in advance for all replies,
Karl

I think the 'Du--ley Neutraliser (not to advertise! ...but you may want to think solid fuel stoves) was designed for just this. Haven't come across one in over 10 years, though, I think.

So I have taken a look at the type C and type R neutralisers and I actually have something similar to the type R in the heat distribution side of my system.
It’s made by a different company in Ireland for “system linking” and works well, though it looses a lot of heat as it’s uninsulated. I must get that sorted.
Using something similar to twin the solar inputs would definitely work but seems overkill to be honest and I can’t see, for my purpose anyway, how it would offer any benefit over simply T’ing the two solar inputs close to the coil at the thermal store. Both solar circuits are pressurised, pumped and will run through a pressurised coil. I’m not mixing open and pressurised systems. Plus both are protected against thermo syphion too so all the key benefits offered by a neutralizer are already addressed.
Maybe I’m missing something...?

If there is a danger It may be that you've got two pumps on different circuits pushing, or pulling, heated glycol at quite possibly differing temperatures. This I may create problems if one circuit is slower/cooler than the other and you could end up with thermal back pressure on the slower pump or indeed the array.

Posted By: gyrogearPer link, & depending where you live, winter insolation should be around 40 % more than summer...

Would you care to explain that claim a little more, especially what 'per link' means?

That link helps a bit. But I don't see where your 14.43 kWh/day vs 10.12 kWh/day/ per square meter numbers come from. I see

Measured in kWh/m2/day onto a vertical surface

Jan 1.11
Feb 1.83
Mar 2.14
Apr 2.50
May 2.70
Jun 2.54

Jul 2.62
Aug 2.61
Sep 2.35
Oct 1.96
Nov 1.31
Dec 0.98

No numbers anywhere near as big as yours, and the peak in the summer as expected, though it is showing the double-peak as expected for a vertical surface.

Thanks Nick and Owlman, some good open questions there.
I see combining systems as one large array really but totally accept there could be an averaging of temperatures across both and/or back pressure issues or worse.
I guess I’ll just have to connect it and see how it goes. It won’t cost much as 90% of the work is already in place.
I’ll do it and circle back, though it could be a couple of weeks.

Thanks for the comments Gyrogear and Djh.
I should say that I am in the north of Ireland so have quite low kWh per m2.
For my latitude, I always understood that vertical solar instalation performed better in winter as the sun was lower on the horizon so the array was closer to 90degrees to the sun. Less that optimal angle in summer but that was generally compensated by the overall higher insolation.
I’ve never seen figures as high as you mentioned gyrogear, were you quoting the total kWh for my particular 6.6m2 array?
I checked the link and it’s quoting figures closer to what djh mentioned and that I’ve seen from other sources when researching previously but admit that it’s not an area that I’ve spent a massive amount of time on. Where are based gyrogear, are you closer to the equator and are those figures typical for you?
I’m happy to be educated more on the subject and greatful for everyone’s input!

gg, I am self-confessedly rubbish at mathematical manipulation, but while I see the point re winter being twice as long as summer, I cannot then see how you can come up with a potential yield figure *per day*. Or are you saying that the insolation per *2 days* of winter is greater than that for *one* average day of summer?

Confusedly yours,

Nick

Posted By: gyrogearFrom the table, aggregating the daily insolation for the 8 months of winter (Oct; Nov; Dec; Jan; Feb; Mar, Apr; May = 8 months), gives 14 kWh per square meter per day.

No! When you add numbers like that the units don't somehow stay the same. They would become, in this case kWh per square meter per 8 days. Similarly the summer figures would be in units of kWh per square meter per 4 days. We then need to divide by 8 and 4 respectively to get them back to the same units (per day) so we can compare them. That gives:

winter = 14/8 = 1.75 kWh per metre² per day
summer = 10/4 = 2.5 kWh per metre² per day

and then it's quite clear that as you would expect, it's sunnier in the summer than the winter.

edit: you can of course compare the total generation over the whole season but then you'd do better to move to Sweden where it is said they have only two seasons; winter, and July. So they get even more energy in winter, since it's 11 months long.

Well I’ll be, you really do learn something every day. Well I do at least, especially here!

So in summary, my understanding is that, if you look at a single days insolation in one of the 4 summer months the kWh per m2 per day is higher than a single day in one of the 8 winter months.
However 😀
If you consider that winter is 8 months long, then..... the aggregate insolation (for want of a better term) over the 8 months is higher than the 4 months of summer.

Did I get that right?

So my off the cuff statement re winter gains not being as good as summer was clearly wrong.
I did mean the gain per individual day but I hadn’t factored the “shoulder months” making winter 8 months long.
I wasn’t specific in that so I’m glad to have been educated, thanks everyone.

I will still circle back once I have something to share on the sharing of the coil in the thermal store.

Now, off to research more on the vertical mounting of the 50 EV tubes, I’ve heard mixed advice saying that they “need” to be at a slight angle for the small amount of liquid inside to condense at the top and rollback down to be heated again.

But by angling the collectors with a winter bias you can get better collection in the winter (when you would need to make best use of what there is) at the cost of less than max collection in the summer (when you probably don't need all you can collect)

Posted By: Peter_in_HungaryBut by angling the collectors with a winter bias

The numbers we're discussing ARE with a winter bias. Have been all along.

<blockquote><cite>Posted By: owlman</cite>For Karl;
If you proceed with your "Tee in" experiment It'd be wise to look at pipe sizes e.g. first the coil bore then secondly the respective sizes of the two ST pipes presumably 22mm. If you end up "throttling down" the flow in the coil when both arrays are pumping it could cause one array ( the weaker), to go into stagnation and therefore be ineffective, defeating the whole object.
Less of a problem if the coil is oversized.</blockquote>

Thanks Owlman, I have the same concern but thought that temperature rather than pressure might cause that effect so I intended on setting/playing with the delta T on the FP to only trigger the pump at a similar point to the EV array. Thus “pumping” the system at similar temperatures, reducing the risk of the EV array sending higher temps to the FP array and wasting energy.

Here are the details of the systems, I’d be very interested in opinions;

The coil is 3m2 with inputs sized at 3/4 inch copper.
The FP solar is plumbed in 3/4 inch copper completely.
The EV tubes will be plumbed in 3/4 copper externally, will join into 1 inch copper once they come through the south facing wall (reusing spare wood burner flow and return pipe work as already runs 95% of the same route) and then reduces down to 3/4 inch copper at the “Tee” joint with FP solar entering the solar coil.

It’s worth mentioning that the FP pipe run is c. 12 meters of 3/4 inch copper total and the planned EV pipe run is c. 20 meters copper total (with c.6m being 3/4 inch copper & 16m being 1 inch copper).
I have A rated willo pico modulating pumps that I can use if that would balance pressure?

Perhaps it’s just going to be a case of plumb it and see....

Sharing the 1 inch pipe is not an option with my particular set up as the FP pipe work comes from the roof and enters the plant room directly from above whereas the pipe work intended for the EV runs along the ceiling of the basement and enters horizontally. Im not explaining very well but trust me, it’s not practical.
Good idea though Owlman and I’m sure it would help.

I’ll keep the 450ltr option as a plan b, that could be quite effective with both solar arrays heatings a smaller volume of water, then when it hits the desired temp, the pump transfers it to the thermal store. The idea is growing on me.
I’ll see how the coil share goes first as it’s an easy connection and almost ready.