Dumping surplus heat from solar panels

[Vortex4]

My green project for 2010 is to install a large thermal store in my home, along with a solar array for hot water. This kind of panel:

of the big issues with solar collectors (especially larger ones) is the dumping of surplus energy once the store is "full". An issue for the summer.

To manage the solar array I will be getting one of these:

which has a bunch of nifty features, amongst which is the ability to generate a relay output on "over-temperature".

My thinking is that in an overtemperature situation a 3 port valve could then divert the solar fluid (water/glycol) to some kind of heatsink. But what kind of heatsink? (remember this will be running at 90-95C).

The obvious solution is to install some kind of radiator outdoors [caged because of the potential temperatures!]. But that seems boring.

Other suggestions I have had:

• Heat a hot tub (don't have one. don't want one)

• Heat a swimming pool (great idea but somewhat over budget)

• Heat a greenhouse

• Lose the heat through some kind of buried pipe (Hassle and MDPE certainly won't like the temperatures)

Any other suggestions?...bearing in mind this situation will likely only occur on hot summer afternoons.

D

[dennis]

Sell it to next door. Install a blind on the panel.

[Vortex4]

Thanks,

Actually the blind idea has been suggested.

It's also been suggested I heat the pond and farm alligators.

[dent]

It depends upon whether you want to use the spare heat, or simply avoid overheating the tank. I've got a solar system (bought in kit form from the extremely helpful

) coupled to a HeatWeb thermal store. When the heat store has reached its maximum temperature, the solar controller (a Resol BS4) simply stops pumping any more fluid around the solar circuit. The fluid that is in the panel boils off, and the gasified solar fluid is absorbed into an expansion vessel connected in the circuit. When the heat store loses temperature again, or when the solar panel stops collecting heat, the expansion vessel lets the gas condense back into the panel, and things start off again. I think this is called `stagnation'. I've not had any problems with this working over the last few years.

It would be great to use the spare heat in some way though, and any of your other suggestions sound good (except heating the greenhouse - that will be quite hot on a sunny day without extra help!). In my case though I decided that it wasn't practical or worthwhile to do.

dan.

[Tim W]

Vortex4 wibbled on Wednesday 25 November 2009 11:20

One of the stock solutions is to have a 2 port valve between the DHW (assuming youy heat the hot water from the store) and dump some down the drain. Not optimal on wasting water mind, but it is a very simple solution that can be implemented mechanically with a Danfoss RAVK remoted sensor valve on a 2 port body. Sensor is strapped on the return from the solar panel and the 2 port valve is between DHW and drain. It's probably the method I would use.

The other standard method is to run the heating circuit up, ensuring there is at least one radiator or UFH circuit that is always on and dump the heat there.

I think dumping the HW is the simplest - the amount of water wasted is dependent on getting enough sun to actually overheat the store - any idea how likely that is in your case?

If you have a bypass radiator (no TRV or user valves) then the even more simplest arrangment is use your relay output to switch on the rad pump.

If you wanted a custom heatsink - then how about a car radiator from a scrappy mounted on a north wall? Again, how much power do you need to lose?

On an aside - all of these solutions depend on the electricity not failing.

What happens if it does and your panels cook? I've always wondered about that.

[Vortex4]

+++++

Snap. Will be going for the new Xcel 2009 from

The biggest one is 475 litres.

Amazing piece of kit. Not cheap but I'm comfortable with that taking a 10 year view.

My summer gas usage (no cooking) is about 30kWh/day. That's the input energy

The boiler is a 15 year old Potterton non condensing and non modulating so I guess anly about 66% efficient...so I guess about 20kWh/day gets into the current tank.

Will be deliberately oversizing the solar to maximise the length of the season using a 30kWh/day array like:

my calculation from a room temperature start the store will be up at 90C in a couple of days if no energy is drawn off.

Q. I assume your system is pressurised. Does your system not have a safety over pressure valve? In which case do you not lose fluid every time there is an overheat?

[Vortex4]

Thought about this. Got a water meter and quickly discounted it!

Still open to reconsidering.

I thought about the bathroom towel rail, but I guess there could be 3kW to lose!!

Agreed. I think I may need to get some kind of roller blind (you can get them for greenhouses) to cover the tubes when away for any period of time.

I would imagine the safety pressure valve would open and you lose expensive antifreeze!

[dent]

They are very good, and very well made. I've only got a 210l version, as that was all I could fit in the space available, but on a sunny day it will heat up enough to supply two day's worth of hot water.

The HeatWeb stores have overheat protection built-in as well, so they dump to the C/H if it gets too hot. The Resol controller is set to a lower temperature than the overheat thermostat so it should stop heating the store before it reaches this point.

Mine easily reaches 80c on good days. At 90c the store's own overheat would kick in, but I've only ever seen that happen when my woodburner with back-boiler is going full bore.

It is pressurised, and there is an overpressure valve, but the expansion vessel seems to cope with the overheat mostly. Over the course of a year there is a bit of a pressure drop, which is probably through the over-pressure value, but I only need to top the system up about once a year.

The other advantage of this system is that it will fail-safe if there is a power failure because the panel will simply stagnate.

dan.

[Andy Dingley]

Catch tank for that. Saves contamination, saves antifreeze, notifies you that you've over temperatured.

SMS or email messages from the controller are another option. Easy to do these days.

The _big_ problem is in overheating the tubes themselves. Thermomax tubes (at Thermomax prices!) have an internal widget that shuts down the heatpipe when overheating, Navitron's don't. So you _must_ either put a blind over the tubes (ideally weighted to fail safe) or you must keep coolish water circulating through the tube header tank. Blinds are good, because otherwise a pump / power fail in the Summer could be expensively damaging.

As a heat dump, it seems popular to either overheat the heat store (safely vented etc., and maybe with cold mains makeup water auto- available) or else to divert to a coldwater heat dump, such as a crude coil in a water butt or pond. This should ideally be a separate circuit, in case of leakage.

[Vortex4]

Water butt: Excellent idea.

Actually the location concerned would be ideal for a water butt....or better still the (huge) galvanized loft tank which will be redundant after this engineering is complete.

[That's assuming I can get it out of the loft]

[Vortex4]

Out of interest, the Resol solar controllers seem to have some quite sophisticated anergy logging/monitoring capability.

I'm curious to know what is the difference in solar "yield" between a midwinter sunny day and midsummer. Is this something you've ever looked at?

david

[dent]

I installed my Barilla panel mid-summer, and didn't plumb it in until the winter. The tubes lasted several months in direct south-facing sunlight, with no circulating water and no blinds, and suffered no ill effects whatsoever.

dan.

[Tim W]

Vortex4 wibbled on Wednesday 25 November 2009 14:14

Angle grinder...

[Tim W]

dent wibbled on Wednesday 25 November 2009 14:26

I presume that is the principle behind gravity drain down systems? Pump stops and water drains out of the panel into a tank... If I understand it, that's a pretty failsafe system.

[dent]

They do. I've got the extra sensor pack which measures additional temperatures (e.g., flow and return temp) to figure out energy gains more accurately, but after the initial burst of enthusiasm for checking it daily, I've forgotten about it now. I think you can get networked versions now which tie in to your computer to make it easier to log, but mine doesn't do that.

'fraid not. The panel still makes a bit of a contribution to the thermal store even in the middle of winter, but I've not measured how much.

dan.

[Andy Dingley]

My first solar system was an old heating oil tank in a greenhouse (antifreeze and a heat exchange coil inside), used as a pre-heater.

[Andy Dingley]

Yes, but it only works for flat panel systems, not heat pipe tubes. Simplest of the lot are the solar-powered controllerless pump systems, like Mary's.

[Tim W]

Andy Dingley wibbled on Wednesday 25 November 2009 15:00

Ah - thank you. Always wondered about that...

[Onetap]

Evaporative cooler? Power stations mostly still use cooling towers to dump heat because they're most efficient. If there's any sort of recirculating pond involved, there's legionella concerns. If the water evaporated is hard, it will leave limescale deposits.

Or a dry cooler (finned coil, like a car radiator with a fan) which is what most AC systems acquired when the owners got scared of legionella. You could shift some heat by natural convection if you installed a coil in a stack with cool air entering at low level. Any redundant chimneys from ground floor fires?

[Gordon Henderson]

It will (probably) only be a problem if you don't have a few showers/bath each day in the summer, or use any hot water - so basically when you're on holiday.

My solution is to dump the store into the central heating system - It's only going to happen when I'm away, so ...

However I'm in the process of building my own controller to let me do that via the rather creative process of running the CH pump with the boiler off and alternating the three way valve between CH and tank every 5 miuntes...

(Even just running it through the boiler dumps heat initially without then shunting it through the radiators)

I have the TDC3 controller right now and it has a mode and a 2nd relay output that can do that (I think)

Big radiator on the north side of the house.

The power issue that someone else mentioned is a real concern, but I have a big UPS for that, and I have already run the CH off it during short power cuts. (fortunately uncommon here)

Gordon