Siting of panels for solar water heating

[Todd]

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 0
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:

[Mary Fisher]

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

Mary

[Mary Fisher]

The OP was asking about solar water heating, not electricity production :-)

My experience of solar water heating is very good.

Mary

[Steve Firth]

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.

[Toby Kelsey]

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

[SJC]

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

[Joe Fischer]

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

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.

Is that year round, or only in winter?

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

[Toby Kelsey]

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

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?

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

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.

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

[The Natural Philosopher]

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

[Joe Fischer]

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.

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.

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

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

[John Beardmore]

In message , Mary Fisher writes

Yes, but not double the price. Maybe about a third more ?

It's not without advantages. It does mean you can get more power earlier and later in the day, and more or less as much in the middle.

Cheers, J/.

[John Beardmore]

In message , Mary Fisher writes

Yes. Facing due west won't be great, but keep in mind that if you capture heat towards the evening, it won't have to be stored long before a typical family will use it.

We once inspected a system, and data logged a collector that faces 45 degrees west of south in summer.

To quote our report, "the temperature of the pipe from the solar panel shows a rise of about 5.25°C per hour on the 25th, starting at around

9:00 and continuing until around 19:00".

(The 25th was one of the few sunny days !)

Cheers, J/.

[John Beardmore]

In message , MarkK writes

I thought it was 80mm, (even in national parks ?), as long as you aren't in a conservation area or on a listed building ?

Yes - and the planners I've asked really hate it ! :)

Hmmm... I've never heard of such queries, but no doubt it's the kind of thing solicitors live for...

Cheers, J/.

[OldNick]

Good. They should look at all renewables.

Was this solar PV, or solar HWS systems, OOI ?

[John Beardmore]

In message , SJC writes

I really doubt it !

Cheers, J/.

[John Beardmore]

Optimising the orientation is similar for space heating and SDHW presumably ?

The heat gathered by a panel also depends on the area. In the UK panels are generally mounted flat to roof, partly because of aesthetics, partly because it reduces the chance of the wind ripping a panel off, and partly because adding is more area is cheaper than making a complex mounting.

As explained elsewhere in this thread it need not make things hugely more expensive.

Hmmm... Hard to get in the UK.

See above. Not sure why you restrict this comment to drain-back/down though ?

Not sure that they are so bad if you do the sums.

:)

I started out thinking that, but clients and planners have slowly ground me down.

Cheers, J/.

[John Beardmore]

:)

Ow ! Pass the tin opener and that huge can of worms !!

Cheers, J/.

[John Beardmore]

In message , Joe Fischer writes

Due west certainly isn't great, but they'll go on working into the evening longer, and our measurements of panels 45 degrees west of south show useful heat gain from 9 am on sunny summer days.

Hmmm... Little snow here, and only a few polished aluminium driveways...

Cheers, J/.

[John Beardmore]

Not in the UK.

Generally no need, and other ways to deal with overheating.

Are you assuming a convection driven primary circuit.

See the SPF web site.

Depends on the roof pitch.

Cheers, J/.

[John Beardmore]

In message , Richard Bates writes

Agreed, though I don't think their tubes are Clear Skies / LCBP approved.

Yes.

Yes, though I've seen the same with good flat plat collectors, indeed I've seen a Zen 28S collector with snow on the bottom third of it have a temperature at the top of 116 centigrade in 3pm milky spring afternoon sunlight with thin cloud.

Yes.

Not quite sure that you'd need a rad. Other ways to skin that cat.

Agreed.

What area ?

Agreed ! And don't even get me started on their '10 year safe working life' wind turbines. See for example

If anybody is curious about this I can also pass on copies of a report from CREST looking at urban wind turbines.

It seems to me that B&Q risk a backlash from their consumers if the kit doesn't recover its costs or make up for the environmental impact of the manufacturing process.

Cheers, J/.