Siting of panels for solar water heating

On Thu, 16 Nov 2006 18:41:47 -0500 Joe Fischer wrote :

Is that average year-round, or max in January?

Sorry I gave you the wrong units: the numbers are kWh/m2/yr
-- Tony Bryer SDA UK 'Software to build on' http://www.sda.co.uk

MarkK wrote:

Solar panels for space heating! How daft is that! Who needs heating April-September? If you do, it'll be because the sun isn't shining... These things provide some heat at other times, but not enough to get your cylinder hot let alone heat the house! Super-insulated eco-houses excepted, of course.
Mark

http://www.builditsolar.com/ & http://www.builditsolar.com/Projects/SpaceHeating/Space_Heating.htm
NT

Richard Bates wrote:

3. Do not believe ignoramuses who suggest that you will not get much heat - that did apply to the old flat panel stuff, still being sold by B&Q, but the vacuum tubes now being used on the continent are vastly superior. I recently went to a demo and noted a cylinder water temperature of 48 c being obtained on a relatively cloudy and cool day. I am reliably advised that on a warm summers day, the tube manifold can reach well over 100 c and hence so much heat is being soaked up that one can have 3 to 4 free baths and still have to dump excess heat into a loft radiator - that's how good they are. 4. If you have already signed up and paid a deposit for a flat panel system do your best to get out of it.

You'll notice the thread is posted to uk.d-i-y as well as other ngs. This tells me the OP is in Britain.
First, flat panels give much better ROI than vac tubes Second, British winters are mostly overcast, and flat panels work ok on indirect sun, whereas silvered evacuated tubes work a lot less efficiently under these conditions.
3rd, it is all more complex than that, but suffice it to say that flat panels are very much a going concern in Britain.
An optimally designed system would have a mix of both flat panel and vac tube, with each heating a separate part of the system. Flat panel is best for mid-temp water, as it gives much more output per �/$. Vac tube is best for the final max temp water, as it gives high output temps that flat panels cant consistently deliver. However, the ROI on the flat tubes will be much poorer, so spending some of the money on flat panels will much improve total annual output.
NT

On Thu, 16 Nov 2006 18:09:22 -0500, Joe Fischer wrote:
I agree with that. If you can. Sounds didgy. > I would cancel the order until the details are worked out.

On 16 Nov 2006 12:09:52 -0800 someone who may be "Will" wrote this:-

However, when their surveyor came to measure up he said that the west-facing roof was inadequate and the panels would have to be sited on the gable-end wall (facing south).

Generally that is the case.

I can't see how this would be an improvement. Even though the panels would be angled away from the wall (mimicing the roof's pitch) they would certainly be obscured by the angle of the roof for at least part of the morning and part of the evening.

I find that difficult to visualise without looking at the house. However, the sun is usually weak in those positions.
Someone mentioned planning permission in the thread. You didn't say which country you are in, but if you are in Scotland then an installation mounted on the house wall doesn't need planning permission, provided a few conditions are met. An installation mounted on a roof only needs planning permission if it sticks up by more than a certain amount. The certain amount is no problem for flat panel systems, but is a problem for evacuated tubes, where the header may stick above the limit. Beware that the natural reaction of planning officials is to say no, you may need to quote chapter and verse to them before they agree that an installation doesn't need planning permission.
-- David Hansen, Edinburgh I will *always* explain revoked encryption keys, unless RIP prevents me http://www.opsi.gov.uk/acts/acts2000/00023--e.htm#54

Will,
I too have been experimenting and trying to get all my energy from renewables. First was to have a structure that needed a minimum of energy. I have a simple structure with 12" thick walls filled with fiberglass insulation.
I use no power for heating or A/C.As long as there is light outside it is enough to serve my needs inside ... the roof has clear sections insulated with bubble wrap.
But my experience with solar panels show them to be an abject failure.
1. Cost is high. My 165W high voltage panel cost over $700 2. My 165W panel has never delivered more than 90W 3. The panel can only supply current about 5 hrs per day 4. The panel only supply useful power on sunny days 5. The panel loses lots of efficiency on hot days 6. Any shadow on the panel is the same as complete panel in shade 7. Can lose 30% or more if panel is not perpendicular to sun's rays 8. Charge controller must be very efficient or more is lost 9. For anything practical a huge number of panels and batteries is necessary
The solution I have arrived at is hybrid. I use my one panel to charge batteries when I'm away. When away, my refrigerator is my only load (about 120W with 30% duty cycle). My battery bank is 10 70AH car batteries. All together they cost about as much as my 1 PV panel. I don't use deep discharge because they are much more expensive and I haven't seen an inverter that works below 10V. Most shut down at 11.5V.
My real power source is a 3hp diesel engine driving a car alternator capable of delivering 40A. I run it on waste vegatable oil (WVO) at low RPM (~1,000). I route it through the same high voltage (up to 53VDC) charge controller that I use with the PV panel. It does a splendid job of keeping the batteries charged and only runs 4 to 5 hours per day. It uses hopper cooling so I get my hot water from it as well.
The batteries have enough charge to get me through the night with no other power source. I use small fluorescent lights and all that is usually running is my refrigerator. I use less than 50AH over night and the diesel recovers that in the morning in less than 2 hours.
My average daily consumption is about 1.2KWH.
Regards,
Todd Marshall Plantersville, TX
Will wrote:

We moved into our two storey, reconstituted stone-built, pitched-roof house in April.
We decided early on to get as much of our energy as possible from renewables.
A couple of weeks ago we signed up for a solar heating system. I've looked at a few and this seems like a very good system.
The marketing wonk who visited us (who had obviously been in the pub first) declared that our directly west-facing roof would be perfectly adequate to site the panels. It is a very clear prospect as it faces a flat field, and there are no trees in the line of sight.
However, when their surveyor came to measure up he said that the west-facing roof was inadequate and the panels would have to be sited on the gable-end wall (facing south).
I can't see how this would be an improvement. Even though the panels would be angled away from the wall (mimicing the roof's pitch) they would certainly be obscured by the angle of the roof for at least part of the morning and part of the evening.
Am I wrong? The alternative, according to the surveyor, would be double the number of panels with half on the east pitch and the other on the west.
Many thanks
Will.

"Joe Fischer" wrote in message

The roof pitch should be ignored, and the panels mounted so they get the most direct sun, but sometimes that makes an unsightly installation.

I disagree that solar water heating panels are unsightly, they look like a large Velux window, flat and dark.

Mary

"Todd" wrote in message

Will,
I too have been experimenting and trying to get all my energy from renewables. First was to have a structure that needed a minimum of energy. I have a simple structure with 12" thick walls filled with fiberglass insulation.
I use no power for heating or A/C.As long as there is light outside it is enough to serve my needs inside ... the roof has clear sections insulated with bubble wrap.
But my experience with solar panels show them to be an abject failure.

The OP was asking about solar water heating, not electricity production :-)
My experience of solar water heating is very good.
Mary

On Fri, 17 Nov 2006 12:58:52 -0000, Mary Fisher wrote:

"Joe Fischer" wrote in message
The roof pitch should be ignored, and the panels mounted so they get the most direct sun, but sometimes that makes an unsightly installation.
I disagree that solar water heating panels are unsightly, they look like a large Velux window, flat and dark.

You appear to have misunderstood the observation about ignoring the roof pitch. For best efficiency the panels need to be set at an appropriate angle which can be steeper than the roof pitch. This means that the panel must stand proud of the roof. Which *is* unsightly but it is also more more efficient.
Your observations about the appearance of the panels also refers really to the style of low-efficiency flat panel that you favour. An efficient vacuum tube array does not look like a Velux. OTOH Velux windows are also unsightly.

Steve Firth wrote:

On Fri, 17 Nov 2006 12:58:52 -0000, Mary Fisher wrote:
"Joe Fischer" wrote in message The roof pitch should be ignored, and the panels mounted so they get the most direct sun, but sometimes that makes an unsightly installation. I disagree that solar water heating panels are unsightly, they look like a large Velux window, flat and dark.
You appear to have misunderstood the observation about ignoring the roof pitch. For best efficiency the panels need to be set at an appropriate angle which can be steeper than the roof pitch. This means that the panel must stand proud of the roof. Which *is* unsightly but it is also more more efficient.

Is that "much more" efficient? I wonder.
For someone in England the altitude of the sun is about 90 - 52 +-23.5 = 14.5 to 61.5 degrees with the lowest value in midwinter.
So about 38-11.75 = 26.25 degrees from vertical is a good panel angle for winter heating. The efficiency varies roughly as cos(angle from optimum). For a roof making an angle of 50 degree with the vertical, the efficiency is
cos(50-26.25 deg) = cos(23.75 deg) = 91.5% (a vertical mounting has cos(26.25) = 89.7% efficiency)
so you only lose about 8.5% efficiency with a flush roof mounting. Not too bad.
Toby

Solar thermal panels for space heating is one of the ways we can save lots of fossil fuels in the future.
"MarkK" wrote in message

"Eric Sears" wrote in message
Technically your surveyor is right - in the northern hemisphere they should face south at approximately the angle of your latitude.
Correct, and a little shade in the early morning and late evening is neither here nor there as there's little heat in the sun's rays at those times. Look at graphs for solar energy vs time of day, and vs orientation, angle, etc.
I don't understand, from your description, how they can "mimic" the roof pitch, unless there is a small angled roof facing south - in which case that is where the panel should be placed.
He just means that they'll be mounted at an angle rather than vertically.
Of course, if the panels were for space heating, you might want the greatest input in the morning, in which case the east facing roof might be better. There are a number of factors to consider.
Solar panels for space heating! How daft is that! Who needs heating April-September? If you do, it'll be because the sun isn't shining... These things provide some heat at other times, but not enough to get your cylinder hot let alone heat the house! Super-insulated eco-houses excepted, of course.
Mark

On Fri, Toby Kelsey wrote:

Steve Firth wrote: On Fri, 17 Nov 2006 12:58:52 -0000, Mary Fisher wrote: "Joe Fischer" wrote in message The roof pitch should be ignored, and the panels mounted so they get the most direct sun, but sometimes that makes an unsightly installation.
I disagree that solar water heating panels are unsightly, they look like a large Velux window, flat and dark.
You appear to have misunderstood the observation about ignoring the roof pitch. For best efficiency the panels need to be set at an appropriate angle which can be steeper than the roof pitch. This means that the panel must stand proud of the roof. Which *is* unsightly but it is also more more efficient.
Is that "much more" efficient? I wonder.

In winter, maybe, and that is for a south slope roof, the original poster has an east-west sloped roof, which is a problem.

For someone in England the altitude of the sun is about 90 - 52 +-23.5 = 14.5 to 61.5 degrees with the lowest value in midwinter.
So about 38-11.75 = 26.25 degrees from vertical is a good panel angle for winter heating.

Actually, vertical or almost vertical is better in winter for space heating, and does not overheat the panels as much in summer. Also, mounted on a south facing vertical wall, snow cover can give 50 percent more thermal energy in winter, or even polished aluminum laying flat can almost double the thermal energy. And in a region where hail is likely, the vertical mount protects any plastic or glass cover.

The efficiency varies roughly as cos(angle from optimum). For a roof making an angle of 50 degree with the vertical, the efficiency is
cos(50-26.25 deg) = cos(23.75 deg) = 91.5% (a vertical mounting has cos(26.25) = 89.7% efficiency)

Is that year round, or only in winter?

so you only lose about 8.5% efficiency with a flush roof mounting. Not too bad. Toby

On the west slope roof there would be zero efficiency until nearly noon, even if the panels have the north edge of each higher than the south edge with the panels spaced wide apart so they don't shade each other.
The big advantage of the south facing wall mount is the near optimum efficiency in winter, with the possibility of extra reflections to exceed 100 percent rating of the panels, and the reduction of thermal energy in summer. (This was discussed at length with graphs in a 1959 ASHVAC proceedings publication).
But that is for space heating (and it has a short pay back time for low cost flat panel collectors), and for domestic hot water, it may also be suitable, with maybe one or two panels mounted for summer sun.
Joe Fischer

Joe Fischer wrote:

Is that "much more" efficient? I wonder.
In winter, maybe, and that is for a south slope roof, the original poster has an east-west sloped roof, which is a problem.

With an east-west slope the heating would be much less, and may even be better with a cloudier sky.

For someone in England the altitude of the sun is about 90 - 52 +-23.5 = 14.5 to 61.5 degrees with the lowest value in midwinter.
So about 38-11.75 = 26.25 degrees from vertical is a good panel angle for winter heating.
Actually, vertical or almost vertical is better in winter for space heating, and does not overheat the panels as much in summer.

In fact my angles were based on the midday sun position, so a more vertical position is probably better for capturing the sun during most of the day, and a vertical position is better than I calculated.
Is overheating a big issue? Could you automatically cover the panels?

Also, mounted on a south facing vertical wall, snow cover can give 50 percent more thermal energy in winter, or even polished aluminum laying flat can almost double the thermal energy. And in a region where hail is likely, the vertical mount protects any plastic or glass cover.

I can see those issues could outweigh any marginal differences in efficiency.

The efficiency varies roughly as cos(angle from optimum). For a roof making an angle of 50 degree with the vertical, the efficiency is
cos(50-26.25 deg) = cos(23.75 deg) = 91.5% (a vertical mounting has cos(26.25) = 89.7% efficiency)
Is that year round, or only in winter?

That's only at midday for the winter months, so a bit misleading. In fact the all-day efficiency of a vertical position is better than that, but I haven't done the calculation. Other factors also affect the efficiency, for example if the sun is not visible until it has risen say 5 degrees above the horizontal that changes the calculation. Or the circulation and heat transfer may be faster in a vertical orientation. It's better to do a direct comparison in the field if possible.

On the west slope roof there would be zero efficiency until nearly noon, even if the panels have the north edge of each higher than the south edge with the panels spaced wide apart so they don't shade each other.

And the same for the east side after noon, and with the poor angle you would probably get less than 1/4th the normal heating effect.
Toby

Mary Fisher wrote:

"Joe Fischer" wrote in message The roof pitch should be ignored, and the panels mounted so they get the most direct sun, but sometimes that makes an unsightly installation.
I disagree that solar water heating panels are unsightly, they look like a large Velux window, flat and dark.

....And unsightly..like a large Velux window. ;-) ..

Mary

On Fri, 17 Nov Toby Kelsey wrote:

Joe Fischer wrote: In winter, maybe, and that is for a south slope roof, the original poster has an east-west sloped roof, which is a problem.
With an east-west slope the heating would be much less, and may even be better with a cloudier sky.

Popcorn clouds maybe, but full clouds provide very little heat, especially for any kind of focusing collector like a trough, dish, or vapor deposited vacuum tube. PV does get the same voltage with diffuse daylight, but much less amps.

For someone in England the altitude of the sun is about 90 - 52 +-23.5 = 14.5 to 61.5 degrees with the lowest value in midwinter.
So about 38-11.75 = 26.25 degrees from vertical is a good panel angle for winter heating.
Actually, vertical or almost vertical is better in winter for space heating, and does not overheat the panels as much in summer.
In fact my angles were based on the midday sun position, so a more vertical position is probably better for capturing the sun during most of the day, and a vertical position is better than I calculated.
Is overheating a big issue? Could you automatically cover the panels?

Sure, but the sun doesn't rise due east in winter, and doesn't set due west, so south vertical or near vertical is better for winter.

Also, mounted on a south facing vertical wall, snow cover can give 50 percent more thermal energy in winter, or even polished aluminum laying flat can almost double the thermal energy. And in a region where hail is likely, the vertical mount protects any plastic or glass cover.
I can see those issues could outweigh any marginal differences in efficiency.

It is more true for high latitudes, I think most of Europe and the UK is higher than I am at 38 N.

The efficiency varies roughly as cos(angle from optimum). For a roof making an angle of 50 degree with the vertical, the efficiency is
cos(50-26.25 deg) = cos(23.75 deg) = 91.5% (a vertical mounting has cos(26.25) = 89.7% efficiency)
Is that year round, or only in winter?
That's only at midday for the winter months, so a bit misleading. In fact the all-day efficiency of a vertical position is better than that, but I haven't done the calculation. Other factors also affect the efficiency, for example if the sun is not visible until it has risen say 5 degrees above the horizontal that changes the calculation. Or the circulation and heat transfer may be faster in a vertical orientation. It's better to do a direct comparison in the field if possible. Toby

There may be charts according to latitude, this has been studied a lot, but really under implemented, and it is a shame now with fuel prices getting higher.
There is something really wrong if my car is warm when the sun shines regardless of temp, while the house can be cooler than our side with no heat.
Joe Fischer