Solar water heating

[The Natural Philosopher]

Indeed. We ave a crude system. When the sun shines we open the curtains. When the sky goes dark we close them.

I estimate this nets about 1000 a year solar gain.

[Peter Parry]

I'm not sure DIY construction of solar photovoltaic panels is really going to catch on :-).

I've helped build such a system for warming a swimming pool and it works quite adequately - it's also very large and pretty hideous. Where we built it it was possible to hide it behind a large hedge but not many people have a farm to play with.

[Tony Bryer]

No because if you leave the money in the bank you still have the £800. If you spend it on solar heating then you have a solar heating system, but I have seen no evidence that you are likely to get more than a fraction of this back when you come to sell your property. So what you've really done is spend the best part of £800 on an annuity that will pay you £20 per year (increasing by the rate of fuel inflation) for the rest of the period you stay in that house.

[AJH]

Your figures suggest to me that an insulated preheating water cylinder will consume all of the target cost alone if we restrict the investment to domestic hot water heating.

Yet a simple solar air heating system with low thermal inertia seems to look good if the incident solar energy is available when it's cold.

I've seen it posted that a well insulated house, to modern UK standards, will have DHW costs about 1/3 total heating costs. Solar DHW needs a large thermal store to bridge cloudy periods and contributes marginally in the winter months, so savings of ~GBP60 would seem available to me. Just installing the thermal store is over GBP400 and I already possess a number of secondhand panels, it's not worth my while installing them yet.

So where do I find typical figures for solar insolation for London UK in the "heating" season October to May.

Then how cheaply can I make a solar heated roof collector for air with a couple of sensors operating a fan?

I don't know what poly carbonate two or three wall roofing sheets cost or how they transmit in the infra red region but with the inner surface painted matt black and air circulated through the inner air gap wouldn't that make a cheap collector?

AJH

[Tony Bryer]

If you really want to wade through some numbers (UK average, not London specific) check out SAP-2005 appendix H

[Mary Fisher]

People 'waste' their money in all sorts of ways.

How would you spend what others spend on pv?

[meow2222]

The money position of each person and what interest they can get varies from one to another. Some will invest in mutual funds, some in a bank account paying under 1 or 2%, some in accounts paying 4 or 5%, some are borrowing at various rates and so on. So in reality the cost of tying up money does vary. 5% is a fairly good ballpark figure though.

Since the majority of people in Britain are in debt, mostly with mortgages, this also affects what will be a typical cost.

And we havent even raised the question of what having =A3800 in the bank is worth. Imho having money available for life's surprises is worth its value several times over, and its an unwise person that doesn't. So as ever, mileage varies.

NT

[Grimly Curmudgeon]

We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember Peter Parry saying something like:

True. Otoh, as time goes by older generation PV panels become surplus fairly cheaply - being in the right place at the right time helps a lot, though. Best place seems to be the US for much cheap surplus.

True also. I'm fortunate in having a large building with a South-facing gable wall which can accomodate several panels at ground level.

[samc]

people are looking at this the wrong way givern the above would any one ever buy a car,computer,tv e.t.c .

[meow2222]

If theres one thing I agree with you on, its that solar heating _can_ be made to pay. But most solar heat systems are designed with inadequate skill and will never pay back their cost.

NT

[meow2222]

Well it depends, you can use a header tank as a preheat. You can even use a plastic one if youre willing to limit the temp rise in a proper safe manner. There are so very many options with solar.

OK, though youre looking at only one option there. For example its quite possible to install solar heating without adding any water tank.

3 ways off the top of my head this can be done: 1=2E black radiators between incoming supply and header tank 2=2E flat panels or hosepipe panels heating the header tank 3=2E vac tubes heating the main HW tank 4=2E black radiators feeding the shower cold line. 5=2E flat panel and reflectors heating the main hw tank

Also bear in mind a solar system will not save all your HW spend. At least not unless its highly specified, which would cost more.

Is there a reason you specifically want it on the roof? Its a lot less work at ground level: no air ducts, no fan, no need to use ultra-reliable construction, and no need to haul it onto the roof.

Lets see what a low/minimum cost ground panel might cost:

For an 8'x8' panel, 4 pcs 1.5x2.5 timber @ =A31.60 each =3D =A36.40

4pcs 2 x 0.5 timber, =A31.60 2 layers 8'x8' greenhouse poly =A3? white shading for summer: =A31 poundland shower curtains: =A32 black shadecloth: =A3cant remember, it was pence per sqft though, so say ITRO 5px64sqft =3D =A32

Total materials cost: =A312 + polythene, so under =A320 per 8'x8' panel. So for =A3100 you could have 320 sqft of hot air panels.

I'd at least consider adding some reflectors too to increase output. The more days it makes a useful contribution the better, and low concentration helps with that as well as just power output.

Its certainly an option.

You can get better effficiency with black mesh cloth absorbers, and 2 layers of greenhouse polythene is a lot cheaper than polycarb. Probably so are used dg units. ISTR seeing polycarb twinwall for somewhere very vaguely in the region of =A320-=A360 for smaller sheets than 8x8.

The trouble with solar heating is the options are almost endless. Anyone new to the subject is quickly lost in a sea of choices. There are also numerous other approaches we haven't discussed, such as mirrors, troughs, heliostats, hydronic UFH systems, trickle panels, etc etc etc.

NT

[meow2222]

cost is a first approximation of energy. I use it here because the traditional approach of looking at the energy embodied in the physical parts of the product is far too inaccurate, since it takes no account of the real energy costs of real world businesses. The whole point of these figures is to demolish 2 false claims:

1=2E that solar dhw will pay back its money investment 2=2E that solar dhw will pay back its energy investment

Cars are usually a good investment. Computers are usually a good investment. Both have financial payback, and both improve quality of life. A =A32000 solar dhw system does neither.

TVs are a waste of upto a quarter of people's lifetime. All those hopes and dreasm people have, if only they had the time.. they do, they just choose to waste it away on junk tv.

NT

[Mary Fisher]

I've posed that question lots of times. Nobody has ever given an answer.

Mary

[David Hansen]

On 6 Mar 2006 15:53:46 -0800 someone who may be [e-mail] wrote this:-

Only if the building being heated is a single room.

If the building has several rooms then one needs to get the hot air into these rooms.

[David Hansen]

On 7 Mar 2006 06:58:47 -0800 someone who may be [e-mail] wrote this:-

The only problem is that your "figures" did not demolish anything. For a start you assumed a very low level of savings on the cost of heating domestic hot water.

[Mel]

[e-mail] a =E9crit :

I'm curious to how you figure =A320 a year in savings... (not having all =

the information, the inital posts are no longer....)

If I look at dshw system here in France, I have;

min cover : 50% (lets be conservative) =3D 1000kWh electric avoided average cover : 65% (for the most proactive region) =3D 1500kWh electric =

avoided electricity costs : 0.11=80/kWh

so yearly savings =3D 110=80 to 165=80

(the rest of the equation here is, for flat plate hot water collectors cost (installed, all taxes included) : 5500=80 subsidies : tax credit 50% materiel costs (say 50% of 4000=80) local subsidies - bring total subsidy up to the 80% EC limit in many=20 regions) current loan rates : approx 4% insurance included current savings account interest rates : approx 4% (best case) approx=20

1.5% (average case))

I'

[Mary Fisher]

Indeed!

Mary

>

[Mark]

Hmm How much is the average family's hot water expenditure over the two summer quarters, a suggested saving of £20 seems awfully low even for a DIY jobie.

Im interested to know because my own consumption is not typical as we have three B+B rooms that are normally occupied throughout this period. When I fitted it 20 years ago it made a useful reduction in the amount of oil used during the summer. But in honesty I cant now remember by how much.

-

[nicksanspam]

Would that be 20 UK pounds per year? Sounds like a very poor water heater. It seems to me that American families pay about $20 per month for hot water.

Would you believe 154.43 pounds?

Or both. Solar water heaters are typically undersized and overpriced.

Would you believe 249.24 pounds?

There's no reason a draindown system can't work in wintertime.

How about a solar-pond type water heater instead?

It's more rational to say

I need hot water. What's the most economical system?

Nick

[Derek Broughton]

I don't know how you get your figures, but the average North American uses some 40 THOUSAND US gallons of hot water yearly. The water heater usually runs in excess of 120F, from a coldwater supply well below 70F. Raising

40,000gal * 50F = 320,000lb * 50F = 16,000,000 BTU, which if I am not mistaken is going to cost around $25/million BTU to heat. So that comes out to $400/year. A tad more than £20, and well worth replacing even with a system that only generates half your hot water.

Ack! I'm beginning to sound like Nick. GOTO 100.