Dumping surplus heat from solar panels

[Steve Firth]

Not a good solution with metered water.

[Steve Firth]

I was thinking of dumping surplus heat into four tonnes of water in a buried tank. The tank would be used as a low-grade thermal store to pre-heat incoming water.

[Tim W]

Steve Firth wibbled on Thursday 26 November 2009 10:23

That's why I asked if the OP had any calculations regarding the expected overheating.

If it's more or less an emergency dump and maybe it will shove a few dozen gallons down the drain in August, then it's economics of the cost of that water vs other solutions.

One could always dump the water into a rainwater butt instead (subject to careful engineering considerations regarding not having a butt full of very hot water!). Chances are the times any overheating may occur will coincide with times of the garden needing some water. Still paying for it, but at least it's not wasted...

OTOH if there are so many panels that the system overheats for 4 hours every day in mid summer, then I agree - don't want to be dumping that much water down th drain...

[Tim W]

Steve Firth wibbled on Thursday 26 November 2009 10:23

Your rainwater tanks presumably?

[Jim]

It's a badly designed system then: either increase the store size or have fewer panels (or shade some of them in summer when they aren't needed).

[Steve Firth]

Yes. I thought that getting them very hot from time to time might help to stop the water becoming septic. Not so sure of what to do if I need to irrigate with hot water.

[Vortex4]

I've been as thorough as I can with the arithmetic. Incremental cost (in money and hassle) of a larger array versus smaller is so minimal it's silly not to do it. Of course an oversized array will extend the season in spring and autumn as well

The store will be big: 475 litres, but I calculate that from a room temperature start it will be "full" in maximum 1.5 sunny summer days....that is without energy usage.

• About 2 Megajoules per C of temperature rise. That's nearly 2 degrees for every kWh of energy in. Arrays alleged output of 30kWh/day = 60 degrees rise in the same period.

I do expect to need to lose a lot of energy and being metered, I do not want to waste water doing it.

Shading plus heat dump radiator is the solution I think!

[Tim W]

Jim wibbled on Thursday 26 November 2009 11:07

I don't think you could call it badly designed if it's overspecc'd to do more useful work in times other than bright sun.

Yes - but you still need a dump system - for when someone forgets to adjust the blinds.

[AlanD]

I have a Navitron 30 tube panel, a small (due to space limitations) 130L vented cylinder (with 2 coils) and a Resol controller with datalogger, which has been installed for 2.5 years now. There is a large expansion vessel on the system and it has never dumped it's fluid or gone into thermal shitdown. I have a heat-dump system which uses the CH system....

The CH system has 2-port valves for each zone (4 of them). I have a bathroom towel radiatior, plumbed downstream of the DHW primary coil, so when heating the cylinder with the boiler the flow goes Boiler>pump>valve>cylinder>towel rad>boiler.

The heat dump output of the Resol controls a DPDT relay to open the zone valve for the HW circuit, which in turn powers on the CH circulating pump. The other side of the relay's NC contacts are used to *prevent* the boiler from firing in 'heat dump' mode, just in case. So when heat dumping water flow goes from the coil in the cylinder, to the towel rad, the via the boiler (which is off) and returns to the pump and valve and the cycle repeats.

I built the box containing the relay to control this as a replacement for a typical wiring centre, using DIN rail mounted terminals and relay. All 4 zones of my CH system are connected through it, and I've included status neons for power, each zone, boiler operation and heat dump mode. I have wiring diagrams available if interested.

We generally use most of the hot water on most days due to the small cylinder, however the solar system provides enough that I have ben able to shut down the boiler completely between late April and early October in the last 2 years.

According the Resol controller, it has 'heat dumped' for a total of 7 hours in that time, so not much. Most of it was when we were away in the summer - I left the bathroom window open a little and no towels on the rad during this time to help it.

I have the datalogger output if you are interested in the system's performance history. It has stored 6MW of energy so far IIRC.

Alan.

[Andrew May]

Would it be feasible to use some sort of reverse ground source device? No heat-pump but just a long pipe running underground to dissipate the heat.

Andrew

[Vortex4]

Dead useful input Alan.

You know the more I think about it I am being a little neurotic in worrying right now.

Very easy to implement a heat dump through the CH system as you say. I'm thinking I should do that on initial setup and worry more in the summer if it's clear there is a problem.

I was envisaging a wiring and "control centre" made from IMO DIN bits and bobs.

Thanks!

[Steve Walker]

Or offer the surplus to your next door neighbour in exchange for them building a pool and letting you use it!

SteveW

[Doctor Drivel]

Go to the Navitron web site forum. They are into this. Lots of people who have done it.

[john]

at ...

the ... Feasibility Study into the use of Solar Power in Eastside [.pdf doc - 2.2MB]

(Re your QTH, the PDF suggests a miserly insolation of just 800Watts per mtr^2 peak .)

[john]

. SPF are well-

Yes. 75% is reasonable for the tubes themselves but it's the overall 'aperture' area that calls the shots. Vortex4 quotes a reasonable 1.3kWH per day per mtr^2. Which if you backtrack the numbers gives about a 30% conversion efficiency.

[john]

You're working with a total of 6mtr^2 of panelling. I'd assumed just the single quoted panel. Would therefore imply an payback amortisation of say 50 years (just on the panelling), which is completely non-viable. Despite this, it's a really interesting project if the costs can be ignored.

[Vortex5]

You're being far, far too cynical and not thinking holistically about the proposition.

Remember the panels will be used alongside a large thermal store.

Gas boilers are horribly inefficient, especially if all you are asking a

25kw boiler to do is heat just 150 litres of water to 60C every day, as happens every day for 5 or 6 months of the year. By example in the quarter to 23 July (86 days) I purchased 3091 kWh of gas. 36kWh per day. To take 150l from 20C to 60C needs only 7kWh....and in reality there's always energy left from the previous day so even that number is too high. You're paying more like 15-20p/unit for the "useful" energy derived. Not 3 or 4p

A/ I do not expect my boiler fire up at all for 6 months of the year. That's a saving of over £150 per year straight out of the traps.

B/ Thermal store will ensure heating will benefit from solar (to a greater or lesser extent) whenever the heating is on. ALSO thermal store will ensure boiler will be doing a small number of long high temperature burns - with no cycling - which will also be much more efficient. I expect (but cannot quantify) significant savings here. I hope they will be another £150.00.

C/ Thermal store give me the option of adding other heat sources too. Backboiler (for biomass, junk mail, garden waste); Ground source heat pump....but that's too anorakky even for me.

Will report back in 12 months....but I believe no more than 15 years payback for the whole project.....plus money saved keeping me out of the pub.

[dent]

Yes, I see that with mine. In May to October 2008 I used my boiler 5-6 times. This year hasn't been so good, but still no more than about 15 times.

Yesterday my panel was making a contribution still, albeit only a little. =A0ALSO thermal store

Yes. Noticeably less cycling.

I fitted a 14kw wood burner with back-boiler as well. Free wood from the garden, old pallets, etc...

dan.

[David Hansen]

On Thu, 26 Nov 2009 15:40:07 -0800 (PST) someone who may be [e-mail] wrote this:-

The murky midlands.

That refers to Met Office data, the best of which I could find being where there is a figure of around 200 hours in summer months. Divide by 31 days and that is around 6.5 hours per day.

That is also a measure of direct sunlight on an instrument, do they still use the ones with paper in them? However, that does not mean that the solar panel will only operate when there is direct sunlight on it.