Solar Water Heating and savings???

Bob-Dubery · 2011-01-03T07:23:05+00:00

Let me nail my colours to the mast before I go any further. I think SWH is a very good idea and I want it to work - for reduction of CO2 emissions and for re...

Solar Water Heating and savings???

singemonkey

Aja, I'm confused. If, in 1/B, heat loss is irrelevant, how is 1/A actually better? 1/A takes advantage of the insulation, but 1/B doesn't need it. 1/A must surely use as much energy over the day as 1/B uses to heat from ambient twice a day? After all, the same amount of heat loss is being compensated for?

Of course the other factor is the availability of hot water all day - which is useful if, like me, you keep wildly erratic ablution times (I've just run my bath right now, at 13:00 or thereabouts, as it happens ?)

@Deeftes: UV couldn't be converted to heat by, say, using a material that absorbs UV? In the same way that microwaves are converted into heat in your microgolf? Or am I way off base here?

Seems to me though that the bottom line is that whichever method you use, the solar panels are adding heat into the system. The more added, the less electric city is required to pop up to a lovely simmer = lighter load on your wallet. Of course it all depends on how effective the solar system is. Bob's doesn't sound too hot ☹

Gearhead

A system that looses heat but is not kept at the same temperature will gradually go to a lower temperature. This lowers the average temperature and therefore the average heat loss. Thus it is more efficient in terms of energy. Less so if you want to run yer bath long after it has been reheated.
Singe is right about the UV. The sun emits lotsa wavelengths and the trick of designing a proper solar collector involves using coatings to transfer a broad band of these wavelengths.

vic

singemonkey wrote: Aja, I'm confused. If, in 1/B, heat loss is irrelevant, how is 1/A actually better? 1/A takes advantage of the insulation, but 1/B doesn't need it. 1/A must surely use as much energy over the day as 1/B uses to heat from ambient twice a day? After all, the same amount of heat loss is being compensated for?

Of course the other factor is the availability of hot water all day - which is useful if, like me, you keep wildly erratic ablution times (I've just run my bath right now, at 13:00 or thereabouts, as it happens ?)

@Deeftes: UV couldn't be converted to heat by, say, using a material that absorbs UV? In the same way that microwaves are converted into heat in your microgolf? Or am I way off base here?Seems to me though that the bottom line is that whichever method you use, the solar panels are adding heat into the system. The more added, the less electric city is required to pop up to a lovely simmer = lighter load on your wallet. Of course it all depends on how effective the solar system is. Bob's doesn't sound too hot ☹

the IR (infrared) portion of the sun's spectrum heats things up (with a bit of UV-Vis contribution)....in the case of microwaves...what happens here is that the frequency of the MV's excite water mols (in the foodstuffs) to vibrate and rotate which is then converted to heat energy....I think... :?

StephenG

singemonkey wrote:
Of course the other factor is the availability of hot water all day - which is useful if, like me, you keep wildly erratic ablution times (I've just run my bath right now, at 13:00 or thereabouts, as it happens ?)

if u at home during the day and require hot water then the solar system will benefityou as the majorityof the hot water u will use during the day will be solar heated.

if the house is empty during the day, then the expense of a solar system is a complete waste, as you will achieve the same or better savings with a R120 geyser blanket and a R600 timer.

Bob-Dubery

singemonkey wrote: Seems to me though that the bottom line is that whichever method you use, the solar panels are adding heat into the system. The more added, the less electric city is required to pop up to a lovely simmer = lighter load on your wallet. Of course it all depends on how effective the solar system is. Bob's doesn't sound too hot ☹

Well I don't know what's good and what isn't. It gets plenty hot enough on clear summer days, but it doesn't STAY hot. Loses quite a bit of heat in a couple of hours, so I suspect insulation is a problem.

Here's a sort of history.

1: About 30 months ago I got the solar geyser installed. At this time it was part of a serial system such as Gearhead describes. This was also about the time that our billing hassles started so I don't know how much it was saving us. The water was noticably hotter in the summer months. I ran the 2nd, electric geyser on a timer.

2: About 4 months ago we notice that the water is not that hot anymore. I call the solar heating guys. They say there has been a loss of glycol from the panels and they top it up. Things improve a bit.

3: November. We get an electricity meter. Hoorah!

4: I start thinking about this and my thoughts are along the lines of "That solar geyser is doing all that work and then that hot water sits up on the roof all day whilst the water in the bottom geyser gets cold and has to get heated up again". So I conclude that the way to go is ONE geyser, the solar geyser, with the electrical backup.

5: The conversion is done. Things get worse. Remember we had strings of overcast, wet days? We aren't getting much joy. The salesman tells me to turn the element on but to pas op because the solar geyser doesn't have a thermostat and things could get ugly. I ask him if a thermostat can be fitted and he says that's illegal.

6: I think on point 5.

7: I phone the manufacturers of the geyser. They tell me that not only does the geyser have a thermostat but by law it must.

8: I climb up on the roof and have a good look. There's something up there that looks a lot like a thermostat.

9: I call the sparkie. He says the thermostat was a goner and because of that there was no current to the element. So he replaces the thermostat and things start getting hot again.

10: So this brings us to the current setup. Solar Geyser, glycol in the panels, electical backup heating (run on a timer now we know the geyser won't explode), split system with geyser higher than the panels. The panels are not the evacuated whatever thingies, but the black flat panels.

11: Water gets hot (on clear days) but it doesn't stay hot for more than a couple of hours. Even if we heat electrically. EG I heat until 6 in the am so I can get my early morning shower. By 9 the water is luke warm at best until the solar gets time to do it's work. By 3 pm it's pretty good. By 6 it will be getting cool again unless we switch on the element (which we do).

So it seems there is a big loss of heat somewhere.

NB: All external hot water pipes are clad.

deefstes

singemonkey wrote:@Deeftes: UV couldn't be converted to heat by, say, using a material that absorbs UV? In the same way that microwaves are converted into heat in your microgolf? Or am I way off base here?

Oh absolutely, one such material would be melanin. In fact that is the very mechanism by which melanin protects our skin from UV radiation, by absorbing the UV and dissipating the energy as heat. Another suitable material would be a solar cell which absorbs radiation across the spectrum spectrum although the energy is converted to electricity as opposed to heat so yet another conversion is necessary.

Either way, the amount of energy carried in the UV wavelengths is really very low. Of the irradiance provided by sunlight about 53% is in the IR wavelengths while only 3% in UV. The rest, of course, is in the visible light spectrum. Which means, basically, that "harvesting" IR radiation is about 18 times as efficient as UV radiation.

guidothepimmp

jirr.. all this talk of geezers and timers has my.head spinning
?

Gearhead

@Vic: take it from me or look it up: fotons of all wavelengths give off heat and impulse upon impact upon a body.
Saying you don't think anything but infrared light gives off heat is like saying you should like to be exposed to the emitted light from an atomic bomb: not infrared so you won't get fried.

vic

Gearhead wrote: @Vic: take it from me or look it up: fotons of all wavelengths give off heat and impulse upon impact upon a body.
Saying you don't think anything but infrared light gives off heat is like saying you should like to be exposed to the emitted light from an atomic bomb: not infrared so you won't get fried.

The issue is more complex.....You're right about photons and so on...also about the nuclear explosion...of course you will get fried by thermal radiation (IR and UV-Vis) if you're close enough. However most distant casualties are from the shockwave and high energy short wavelenght ionizing gamma and X-ray radiation.... It all depends on the 1."concentration" of the radiant energy and 2.its wavelength. For example, UV light is absorbed by many materials (including water) which re-irradiates the absorbed UV energy as longer wavelenght IR radiation (this gives you the sensation of heat,albeit minimal).... The UV-Vis portion of the spectrum does indeed contribute to heating things up, but in an indirect way.
In my line we regularly work with short wavelength UV (180-280nm) and this does not heat up any irradiated material such as silica,aluminium, and solvents.
The burning/warm sensation of a (burnt) skin exposed to sunlight is not from heat of the UV-Vis per se, but rather from inflammatory reactions of cells as a consequence of exposure to sunlight.

Gearhead

As I pointed out earlier, UV is absorbed by materials used for coating proper solar collectors - hence the violet color most of them have. They do produce about as much heat from UV as they do from IR to be useful in normal circumstances. In fact, they have a built-in mechanism that stops them from producing too much heat while conditions are good. The medium is supposed to flow down into the collection tube which sits inside the vacuum tube. If the collector tube is very hot, the medium will evaporate before touching most of the tube. The lower the collector tube temperature, the more efficient the heat transfer to the medium. The output therefore is less dependent on cloudiness than one would expect, whether you think that IR collection actually is 18 times more efficient (show me that calc and I'll show you the error) or not.

Heath

I had a system which i installed for my geyser , simplie put 10 2amp panels , into marine batterys into an inverter with fail over should the batterys get to flat (mine was set 60%) worked a charm and seriously brought down my account as well as led lights which also ran on the batterys . tube heaters are useless good idea and will work great with no rain. but useless in jhb imo , the whole system cost a small fortune but worked .

vic

deefstes wrote:
singemonkey wrote:@Deeftes: UV couldn't be converted to heat by, say, using a material that absorbs UV? In the same way that microwaves are converted into heat in your microgolf? Or am I way off base here?
Oh absolutely, one such material would be melanin. In fact that is the very mechanism by which melanin protects our skin from UV radiation, by absorbing the UV and dissipating the energy as heat. Another suitable material would be a solar cell which absorbs radiation across the spectrum spectrum although the energy is converted to electricity as opposed to heat so yet another conversion is necessary.

Either way, the amount of energy carried in the UV wavelengths is really very low. Of the irradiance provided by sunlight about 53% is in the IR wavelengths while only 3% in UV. The rest, of course, is in the visible light spectrum. Which means, basically, that "harvesting" IR radiation is about 18 times as efficient as UV radiation.

Remember most UV radiation (unlike IR rad) is absorbed by the atmosphere....I thus cannot see how it can have a major direct effect in heating things up...?

Gearhead

sorry, IR is lower intensity radiation and gets absorbed by the atmosphere whereas the high-intensity UV gets through more. Which btw is also why this factor of 18 is wrong when taken at the earth's surface.
Imagine designing solar panels for a living and not figuring this one out! You would have a product that would not produce much warm water on cloudy days but on a sonny day fry itself!?
One more thing @Vic: where do you figure the shockwave comes from when nothing ever gets heated other than by IR?

vic

Gearhead wrote: sorry, IR is lower intensity radiation and gets absorbed by the atmosphere whereas the high-intensity UV gets through more. Which btw is also why this factor of 18 is wrong when taken at the earth's surface.
Imagine designing solar panels for a living and not figuring this one out! You would have a product that would not produce much warm water on cloudy days but on a sonny day fry itself!?
One more thing @Vic: where do you figure the shockwave comes from when nothing ever gets heated other than by IR?

I dunno nothing about the factor 18 mentioned by Deefstes....the shockwave comes from thermal radiation produced by the nuclear fission (IR, UV-Vis,) causing the air to expand rapidly.... AS WELL as the resultant fast moving air mols causing friction, producing much heat...eg. why does a tyre (or nozzle) gets hot when you inflate it rapidly ?

quote "IR is lower intensity radiation and gets absorbed by the atmosphere whereas the high-intensity UV gets through more".
sorry..simply not true; fact is that 98+% of high freq UV is absorbed by the atmosphere....longer wavelength UV is absorbed less...some IR is also absorbed.
Remember I indicated earlier that some materials absorb UV radiation but re-emit the energy as longer wavelength radiation...much of which is in the IR region (which we feel/detect as heat)
PS: I'm not talking about photovoltaic panels (which use UV-Vis to operate...right ?)

PeteM

This is becoming quite a heated discussion (pardon the pun) for a non-related guitar subject. ?

vic

PeteM wrote: This is becoming quite a heated discussion (pardon the pun) for a non-related guitar subject. ?

Agreed ! I withdraw... ?

Come to think of it...I think we're talking past one another...and Bob's problem is not solved :-[

Cheers.

Ray

But anyway, ol' Bob can go and gooi in a nuclear reactor now and laugh the whole solar heating nonsense off.

PeteM

Ray wrote: But anyway, ol' Bob can go and gooi in a nuclear reactor now and laugh the whole solar heating nonsense off.

:roflmao:

Bob-Dubery

Solar heating IS making use of a nuclear reaction, and a rather large one at that.

Bob-Dubery

I feel the need to post an update here because I have some new information.

There's a leak somewhere in the system.

Thursday last week I noticed that my water meter runs all the time. I ran around looking for dripping taps etc. I found that I turned off a certain stopcock/valve thingy then the meter stops spinning. This kills the water supply to the bathrooms and the geyser. Further poking around has revealed a stopcock up in the roof that controls the supply to the geyser. Turn that off and the meter stops spinning. So the leak is either
1) In the geyser
2) In the feed to the geyser
3) A hot water tap

One of these plumbing sniffer type crews came along. They couldn't find a leak, but they did say that they can go to a higher pressure but that that may cause damage elsewhere. Hmmm.....

It's not a BIG leak, but by rough calculation it's costing me about 1/2 kilolitre a day, which I'm not thrilled about. Also if there's a constant flow out of the geyser then that will slow down heating (electrical or solar).

So investigations continue. In the meantime I turn off the water and the electrical supply to the geyser when we're at work or when we go to bed.