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Author Topic: Combining coils in a thermal store  (Read 4261 times)
woodi
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« on: June 15, 2019, 08:05:48 PM »

I'm installing a gledhill thermal store, which has the usual solar coil, direct solid fuel connections, but also has a sealed primary coil, which has exactly the same area as the solar coil. I already have slightly more solar thermal panel area than is suggested for the tank, and have access to additional panels if I need them.
What I don't have is a boiler - I'm only using solid fuel, the solar coil, and some immersion dump from solar and wind. My question is this - can anyone see an issue with combining the solar and primary coils and adding the extra solar thermal? I could add them in series or parallel, again, any thoughts?
cheers
Steve
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8kw PV - 40x80w; 8x100w;13x300w Victron 150/70, 100/50, 100/30 MPPT; 3x Victron Multiplus 24/3000/70; Color Control GX Monitoring; 1000ah@24v traction batteries - 50 tube solar thermal - 3kw Bornay Inclin grid tie/ac coupled wind turbine - wood fuel cooking & heating. 28kw Ponast wood pellet boiler
oliver90owner
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« Reply #1 on: June 16, 2019, 06:48:51 AM »

Gravity or pumped? I could see the possibility of air locks with a gravity system, dependent on how the pipework runs....  Might also have similar with parallel coils even with a pumped system...

Needs some thought before getting it wrong!
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brackwell
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« Reply #2 on: June 16, 2019, 08:17:10 AM »

The suggested size of tank re panels is to do with the tank getting overheated in peak output/little draw and therefore the necassity to having a seperate cooling coil/rad.  ie nothing to do with transfer rates of heat. As the heat output of the panels increases and temp rises so will the transfer rate.  So i guess i am struggling to see what benefits another coil will bring against the issues stated and trying to control coils operating in two different temp regimes.
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woodi
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« Reply #3 on: June 16, 2019, 09:05:38 AM »

It's pumped. Yes the worry about overheat is always there (we've had some excitement over the last 6 years), so I'm not rushing into this. Rather than add an additional panel, would combining the two coils simply make better use of the panel area I already have.
The cylinder is 210 litre; I have a 20 tube (2m) evac tube panel, and two elderly 2m 5 tube (100mm tubes that carry 15mm pipe) panels that are perhaps 50% as efficient.
Max panel area for this tank is specified at 2.9m2 on the solar coil. On the setup I have, nothing gets too out of hand.
Other option I have is to add a secondary dump tank that accepts flow once my TS gets up near temperature.
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8kw PV - 40x80w; 8x100w;13x300w Victron 150/70, 100/50, 100/30 MPPT; 3x Victron Multiplus 24/3000/70; Color Control GX Monitoring; 1000ah@24v traction batteries - 50 tube solar thermal - 3kw Bornay Inclin grid tie/ac coupled wind turbine - wood fuel cooking & heating. 28kw Ponast wood pellet boiler
sbchapman
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« Reply #4 on: June 18, 2019, 10:32:55 AM »

If the Gledhill store is similar to mine (Boilermate 2000 with solarpod, total volume ~210 litres) you could use a solar controller with a secondary relay output that activates the CH pump when store exceeds the target temperature. I have 3.2m2 of evacuated tubes which is great for spring / autumn, but things can get spicy in summer without additional measures.
The only problem with this is inherent hysteresis that results from solar stratification - by the time the main (upper) tank temperature has dropped below target, you've dropped the whole store down by 20C+, with solar pump running at 100% to try and bring the solarpod back up to temp again and continuing to heat the upper store!
The way I got around this was by installing a timed relay between the secondary output from the solar controller and CH pump, allowing fine tuning of when and how long the CH is allowed to run when dumping heat via radiators. So, say temperature is reached at 1500, the solar controller turns on relay #2 but the circuit is only closed when the timed relay is also on. The timed relay is on two minutes every 15 minutes or so, which allows the store temperatures to settle.

PS I agree with comments above about secondary coil - it doesn't buy you much - you'd end up running the solar less frequently but with a higher deltaT?
« Last Edit: June 18, 2019, 10:35:36 AM by sbchapman » Logged

Thermal: 4x Schott ETC16
PV: 3.9 kWp (20xSchott Mono Perform 195Wp)
Boilermate Thermal Store
SMA SB400TL-20
Clearview Pioneer 5kW
Woodwarm 9kW
Windsmithy Arthur 6kW
1 anemometer (sadly no turbine)
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