Thermal solar - would this work?

[amilford]

Don't know if anyone here has dabbled with diy solar thermal?

We are extending and remodelling our house and as part of it will be re-doing hot water system.
We will have a hot water cylinder in loft space (bungalow) heated by immersion heater(s).
I have also acquired a second pressurized hot water cylinder which has a coil as would be used by a boiler.

So I had in mind putting the second cylinder in series with the main cylinder, so the cold water supply to the main cylinder passes through the second cylinder first. Then I would heat this second cylinder using thermal solar. So it would effectively be pre-heating the water before it enters the main cylinder. Thus reducing the work the immersion heater has to do.

As part of the works there will be a 30 sqm flat roof extension with EPDM covering over 22mm OSB over insulation.

I had an idea of routing a 10mm groove zig-zag pattern in the OSB and laying 10mm continuous copper pipe in the groove so this would lie just under the EPDM. Maybe filling the groove with the pipe in it with resin to remove any air gaps.
On another property with EPDM flat roof I have noted that it gets really hot in the top layers of the roof, so the thinking in this design is that the solar heat on the EPDM would go into the 10mm copper, and then circulated into the coil of the pre-heat cylinder.
A pump and a controller to only run the pump when the temperature of the copper pipe is above that of the tank completes the system. (and all the usual expansion vessels, pressure relief valve - all the usual stuff on closed systems)

Clearly I'd fill the 10mm copper and its pipe with an anti-freeze mixture similar to used with heat pumps.

I can't mount normal thermal panels on the existing sloping roof as this is already full of solar PV.
I am reticent to mount off-the-shelf solar thermal panels on top of the EPDM since I don't want to pierce the EPDM and risk leakage.

Anyone any experience of solar thermal and any (constructive) criticism?
Thanks for reading...

 

[Sideways]

As you have solar PV already, why not add more and use a solar energy diverter ?
That's how we went.
PV is less efficient for heating but if you can put 3kW into an immersion heater it will put 65C in your tank no problem. It is also waay more versatile, and more "intense" energy.
We have 24 panels and those meet our electricity needs for 9+ months of the year plus our hot water consumption over the 6 brighter months.
We use about 4kW of immersion per bathtub.

All this said, we're planning to switch to octopus flux, they pay a decent export rate so we will be better off exporting as much surplus electricity as possible and heating the water with gas.

With PV you just don't need to worry about plumbing.

 

[amilford]

As you have solar PV already, why not add more and use a solar energy diverter ?
That's how we went.
PV is less efficient for heating but if you can put 3kW into an immersion heater it will put 65C in your tank no problem. It is also waay more versatile, and more "intense" energy.
We have 24 panels and those meet our electricity needs for 9+ months of the year plus our hot water consumption over the 6 brighter months.
We use about 4kW of immersion per bathtub.

All this said, we're planning to switch to octopus flux, they pay a decent export rate so we will be better off exporting as much surplus electricity as possible and heating the water with gas.

With PV you just don't need to worry about plumbing.

We could - but would have to be a separate inverter etc as we already have full amount of PV allowed for FIT payments. Ours was installed 13 years ago and we benefit from the very generous FIT payments at that time.
Also to put on flat roof would raise issue of penetrating EPDM.

 

[Spectric]

Remember that with a pressurised system you need to have full control of the heat source, easily done with a gas boiler or emersion heater.

 

[Sideways]

When your pipes leak inside the osb under the edpm, you'll be in for a costly reroof.
You need the water above the edpm not under it

 

[Fitzroy]

I like the idea, and with a continuous plastic pipe (like an underfloor heating pipe) I think the leakage risk is manageable. The bit I would be worried about is the thermal efficiency of the unit.

In most solar water panels the tubes that contain the water are painted black and directly exposed to the sunlight. The heat transfer is thus pretty effective, intimate contact between fluid and tube material and intimate contact between black surface and tube material. As a result you are only dealing with conduction in a good conductor(metal tubes), convection in a good convector (water/liquid), and radiation from a hot body (sun).

In your model you are introducing an additional element, either at worst an air gap between black surface and pipe or at best a resin filled gap. Both air and resin are pretty poor conductors of heat. The other thing your design has is a much lower surface area than commercial options, you are effectively limiting your surface area to the diameter of the tube.

I think these changes would result in pretty poor heat transfer to your fluid. You could model it to your heart's content and I expect you'd be incorrect. The best thing to do before committing a big spend would be to make up a good sized panel 0.5m² ish and test it. Flow a hose pipe though the panel, measure inlet and outlet temperature, and flowrate (how long to fill a bucket) and you can calculate the heat pick up.

A fun exercise/project but my gut feel, as a process engineer who has worked with heat transfer for 20+yrs, is that the performance will be poor.

Fitz.

 

[Sachakins]

Why not lay some insulation panels, glued on top of epdm, router grooves in that, lay copper in and spray it all black. Overlay with safety glass, glued to insulation all around edge.

 

[Spectric]

I like the idea, and with a continuous plastic pipe (like an underfloor heating pipe) I think the leakage risk is manageable.

How do you prevent the pipes freezing ? Small bore pipes in that enviroment would be at risk from freezing.

 

[John Brown]

How do you prevent the pipes freezing ? Small bore pipes in that enviroment would be at risk from freezing.

"Clearly I'd fill the 10mm copper and its pipe with an anti-freeze mixture similar to used with heat pumps."

 

[Just4Fun]

We have a good amount of solar thermal. It works well and I plan to add to it with some DIY solutions. When I installed our existing system it was more cost-effective than PV. I think that has changed though, and I think an electrical solution would be better today.

To consider your idea, I am sure it would work. Cost would be fairly low so it should be worthwhile financially. You could improve heat collection by using aluminium collector plates to go in the groove under the pipe with a flat area either side of the pipe below the EPDM. You can buy them or make them from empty beer cans. I have done both and both work.

I would be uneasy about running the pipes under the EPDM where they could not be accessed. If you do that make sure you run the kind if pipe used for under floor heating. Do not use copper and do not bury any joints where you can't get at them.

You could consider a drain back system to avoid freeze risk but probably not easy with a flat roof.

 

[Terry - Somerset]

Re-use and recycle. Old radiator, paint it matt black, mount on pads to sit on roof.

Consider/trial whether efficiency would be improved with frame and glass cover.

Fill with antifreeze at a level which eliminates freezing risk.

Cheap - if it works great, if not remove.

 

[simonridout]

Terry. You remind me of my diy swimming pool heating 30 yrs ago, water circulated through old radiators, painted black, in a poly tunnel. It was very effective.

 

[Jones]

I think it would work but not that well . A proper evacuated tube system would be more efficient. There are brackets and systems available to ensure the roof stays waterproof .

 

[Stuart Moffat]

+1 for jones evacuated tubes (copper rods are heated not black water filled pipes. Heat then transferred to water in a manifold and pumped through 10 mm pipes to your tank. mine very efficient 10+ years on. Surprisingly little fluid in the system.

 

[HamsterJam]

Some years ago, I was involved in fitting equipment on top of buildings we didn’t own and most had flat roofs which we were not allowed to puncture.
We used a system of weighted sleds and rubber pads (made from recycled car tyres).
Maybe you could devise something similar for direct solar hot water or more PV.
Of course you will need to consider the loading on the structures underneath.

 

[amilford]

Thanks all for your comments - some things to mull over....

 

[Ozi]

In this country we are obsessed with putting panels on the roof, do you have any suitable ground or wall space for black painted radiators if not the proper vacuum panels

 

[Rocky Allen]

No idea about the solar thermal idea, but don't worry about more PV affecting your FIT. You just have to follow the rules and inform your FIT supplier. If your roof is strong enough the panels can be fitted in weighted plastic trays so as not to penetrate the roof.
Could be a good opportunity to replace your existing (ageing) inverter with a new larger hybrid and a battery?

 

[Dabop]

In this country we are obsessed with putting panels on the roof, do you have any suitable ground or wall space for black painted radiators if not the proper vacuum panels

There is a good reason to go roof mounts- at least with PV panels- most dust is carried within a metre of the ground, so roof mounts stay clean far longer than ground mounts
For thermal, it doesn't matter that much obviously, but at ground level, you often have shading issues, where on the roof shading tends to be less of an issue...
Less shading means more heating...

 

[Ozi]

There is a good reason to go roof mounts- at least with PV panels- most dust is carried within a metre of the ground, so roof mounts stay clean far longer than ground mounts
For thermal, it doesn't matter that much obviously, but at ground level, you often have shading issues, where on the roof shading tends to be less of an issue...
Less shading means more heating...

Which is why solar farms employ a man with a squeegee