Solar water heating

[Stanley]

-- I have been looking at some archived items and web items about this.

Basically it seems fairly straight forward. I need a South facing solar heating panel with a grid of tubing filled with a non freezing fluid. A small pump then transfers this heated fluid to an indirectly heated cylinder (let's call it a solar tank) in the loft and plumbed in between the existing header tank and the existing hot water cylinder.

This is all very fine, but when the sun goes in cool water will be circulated which will then start to cool the solar tank. What I need is a pair of sensors that will detect when the water in the solar tank is hotter than the temperature in the solar panel and will then switch off the pump till the solar panel warms up again.

Has anybody tried this sort of heating supplement and addressed the particular problem of the switching control on the pump.

Any other tips.

Delete SPOILER if you want to email me.

[Set Square]

You could probably get away with a single thermostat which senses the temperature of the panel - on the basis that it isn't worth circulating the water unless the panel is above a specified temperature. This would be simpler than something measuring temperature difference.

[John Rumm]

That "specified temperature" however is going to vary depending on the temperature of the stored water though. No point in saying run the pump if temp over 30 degrees when the tank is sitting at 50!

[gribblechips]

The easiest way to do it is to buy a complete control board with a pump. A few compression fittings and plug it in.

Try:

[John Rumm]

whatever the author was on please ;-)

[Ed Sirett]

We have something of a mutual lock on why we don't do solar heating. The professional kit is very expensive (costs 2-3 boilers) the price would fall dramatically if there was a market for them. There is no market because the price is very high...

If you make you own then everything changes (I have not done so mainly because I don't have anywhere to put the pre-heat tank.)

The peice of kit you require is a differential thermostat which will switch the solar circuit on when there it is worth while. Is a couple of thermistors, an op-amp and a relay.

CAT and other scources of 'green' info have the plans.

[Mark]

mine works on the HR cheapskate principle , the pump is run by a elec solar panel when it voltage drops below a set limit the pump switches off. Designed on the hope that if theirs not enough sun to run the pump then its not hot enough to heat the water, I cant claim its efficiency is anything other than poor but it does help save money (no gas were I live)

[N. Thornton]

As soon as I read that I knew it was going to go wrong.

A system like that gives you excessive install costs and poor working efficiency. Only if you use loads of HW throughout the day is that the best way to go.

The opposite type of system is a drain down panel: the HW is pumped into the panel when its hot, and when the pump stops the panel drains down. Far less to install, and the captured heat goes directly to the HW, and can heat the tank up, which the other system cant.

yep, alt.solar.thermal. You may waste your money otherwise. While we're here, what is your aim behind installing solar thermal?

Regards, NT

[G&M]

If you use a dc pump connected to a solar cell in the corner of the solar panel, the speed of the pump is moreorless proportional to the heat being collected by the panel and so the system self-adjusts to the amount of sunlight. And of course after dark it stops dead.

[N. Thornton]

A neat trick, but the control efffected does not match that required for best heat harvest. Despite that, it works quite well. But if you can make a differential stat you can expect more return.

NT

[Mike]

I am thinking of doing this next year but am wondering about the pros and cons of using gravity circulation vs a small pump.

We have the (luxury) of being able to put the panels on the pitched roof of a single storey extension and the pre-heat tank higher up.

Gravity, Con is the pipework size and possibly increased heat up times.

Pumped, Con is cost of differential thermostat (Cat kit looks like £5! of bits) and the pump (£???). Has anyone found a source for a suitable pump.

I suspect I will try gravity and if performance is poor retrofit a pump.

Also, it occurs to me that by adding an additional pipe from the bottom of the main tank to the bottom of the pre heat tank, and ensuring the pre-haet is fully below the main tank you can allow gravity circulation when the pre-heat is hotter than the main. This makes it more like a combined tank. Has anyone got any thoughts on thhis? Have I got something wrong?

[Michael Mcneil]

Never mind about all that, just try buying a South facing solar heating panel

[Brian Sharrock]

snip

FWIW; - while serving the Queen, out in Cyprus, we were quartered in houses (flat roofed) which had solar heated hot water systems. The installation was quite simple, and _passive_;-

A 'green house' glass fronted collector box which consisted of a black-painted domestic radiator -acting as a 'sucker' rather than emitter of radiation, with glass fibre insulation backing it. The 'radiator' collector was cooled - (its heat was transferred) to a bog-standard indirect hot water cylinder which stood on a framework _above_ the collector plate. The 'hot' water was fed from a small header tank which itself was mounted above the domestic hot water cylinder. Take-off was via the normal top of the h-w cylinder, which had an electrical immersion heater as the only other energy source for heating water. There was nothing that hadn't come out of a plumber's bag. No sensors, no pumps, no control valves; just gravity and thermal expansion of the "as it becomes hot- it rises" variety.

These installations are very common throughout the eastern Med.

If I try to replicate this in UK, I'd be inclined to have a primary loop, radiator- coil with anti-freeze as we do freeze. However in Cyprus freezing is not uncommon away from the coast in winter time and they seem to manage.

[If anyone reading this thinks;- Ah but Cyprus is closer to the sun - I invite them to measure the distance from Sol-Cyprus (35N) and compare it to Sol-UK (50N) I think you'll find its Ninety-odd million miles and not a lot of difference!]

[tony sayer]

In article , Brian Sharrock writes

Mum... its the "angle" that the Sun hits the gubbins at and IIRC the amount of atmosphere it has to go thru that makes the difference otherwise the UK would be as hot as the equator...discuss;)

[Ed Sirett]

The gravity stuff might work, but givern the difficulty is getting gravity HW systems to work when the boiler is not directly below the coil I'd be wary.

If you dose the circulation water with inhibitor then a simple £30-£40 CH pump on minimum should be fine.

[Ed Sirett]

Yep the saolar collectors are alledged to gather a lot of heat on cloudy days.

[G&M]

But so will the solar cell - the dc pump will run slowly to match. But I agree a fully controlled system would be better if one is able to build one. However the solar cell to dc pump is nothing if not easy to wire.

[Ed Sirett]

When people ask me and I say the total kit is around £2000+installation they tend to drop the subject. I wonder if it would be possible to contruct the kit from one of the CAT plans for a lot less? Wooden box, rock wool, polycarbonate sheet, radiator, black paint.

IIRC Laying the panels flat on a roof does not make much difference annually but it kills the winter efficiency which is when it is needed most. In fact it might even be better to hang them vertically.

[Brian Sharrock]

IIRC. the solar heat flux normal - at ninety degrees -to the sun is 300W per metre squared. This doesn't sensibly vary from equator to pole: however at the equator ,on a particular day, that's 300W directly onto each square metre while at the pole it's glancing past each square-metre. Hold your metre-square at ninety degrees to the sun at you'll intercept all the flux at every latitude. For solar-panels purposes the angle of dangle should be Lat-Decl; but this'll vary -23 degrees to -23 degrees between seasons. Remember the good-old "Sine" rules from trig' classes and you'll realise that the sun is only normal to a fixed panel at noon - at noon-six hours it's only glancing off the panel, at noon +six hours it's 'gone-past' the panel the other way. Sensible 'levels' greater than Sine 45 (300W * .7) will be obtained from 09:00~15:00. [>200W/H for six hours ~= 1.2 KwH]

[G&M]

I'm using wooden box (few quid), old slightly misted double glazing panel (free), Kingspan (offcut), coil of 10mm pipe (£25), black paint, solar cell and dc pump (£40) into an old hot water cylinder (free). Certainly a lot less than £2k even if it isn't quite as efficient.