SWH prototype project part 6 - prototype conclusions

Here's a bit of a belated update to where we're up to with the prototype panel. For the last few weeks it's been sitting out in the orchard soaking up the rays, and using the energy from said rays to warm up its little tank of water. We set it up on some legs, cunningly constructed from a pair of old doorframes nailed together. It's now sitting at an angle of about 50° to the horizontal*.



















(*For the best all-year-round collection, we would set the panels at an angle equal to our latitude - here it's 34.26°. Setting it up a bit higher favours winter collection (by facing the lower winter sun better), and slightly reduces the chances of the panels overheating in summer. I did lots of complicated calculations to try and figure out the best angle, but these remained somewhat inconclusive - so I reverted to a rule of thumb suggested by some people who seemed to know what they were doing...)

At present there's no pipework in place to enable one to actually make use of this warm water, but that doesn't mean this setup isn't useful. Running a closed system like this allows us to calculate, to a ball-park figure at least, how well it's performing. By measuring the tank temperatures at the start and end of the day we can calculate fairly accurately how much useful energy we're collecting (obviously we have no way of measuring the energy losses, but we can estimate efficiency by comparing our daily energy collection figures with solar insolation data collected over a couple of decades at a weather station not far away).

























Here are some numbers. If you have some objection to numbers, feel free to skip to the pictures and chatty bits lower down!

Some simple thermodynamics also tells us that the hotter the system is running, the higher the heat losses, and therefore the lower its efficiency. Which just means that a record temperature in our tank doesn't necessarily equate to the greatest amount of energy being harvested.

The greatest daily (useful) energy collection we've witnessed so far was 10.6MJ, with our 59l tank rising from 17°C in the morning to 61°C in the late afternoon. Comparing to our insolation figures for January, (9.4MJ/m² per day x 1.79m² = 16.8MJ per day), this suggests an efficiency value in the region of 63%**.

The highest tank temperature we've recorded, after it had been sitting out for a few successive clear days, was a mighty 75°C (with the panel sitting at 81.5°C, and water entering the tank at 78°C). But since the water was already sitting at nearly 50°C from the previous day, this equates to a much less impressive 5.3MJ, and 32% efficiency.


















(**All these calculations contain hefty doses of approximation and a little guesswork. We take several measurements in the tank, as the water tends to stratify in layers of different temperature. You can stick your hand in and feel the temperature lower significantly as you move it downwards in the tank. Also, the weather station's figures are averages over the whole month, for many years. If we happened to get an above average clear sunny day, then this would tend to push our efficiency figure upwards. So we might surmise that we're operating somewhere in the range of 50% - 65%.

Also, the system in it's current state could be improved significantly by the simple expedient of insulating the connecting tubes, by insulating the tank better, and by not opening it up every hour to stick a thermometer in!)

Bear in mind that these measurements are all for the middle of winter - in midsummer we would expect to gather at least twice as much energy.

In case all that maths really wasn't your thing, here's a few of the conclusions I've been coming to about this, and therefore what I would keep or change for the design of the large system:

• The basic design seems to be working well, and we have suitable solar resource and a suitable location and use for it in the hospital.
• The 1.2mm aluminium is seriously hard to work with. Find some thinner stuff.

• Get someone who knows what they're doing to do the copper soldering. Q'bad's finest looks pretty, but I wouldn't want to guarantee its quality.

• Two layers of glass is really heavy, not to mention prone to breakage. Try to get hold of some of that polycarbonate stuff they use for greenhouses.
• Don't let the panels sit outside in the sun with no water in them - it hit 103°C the other day - I suspect the polystyrene insulation in the back of the panel melted a bit. Find some insulation that has a higher melting point. Work out how to shade the panels in the event that they're sitting out empty.
• Don't use whatever this material is for lining a tank with!

It appears to degrade over a few days in the hot water, leaving this interesting greenish flaky stuff:

• Don't drop the thermometer into the tank!

What You See Is What You Get...

...but isn't necessarily what you wanted.

This week I and my 'boss', Mr P, at the workshop went down to Pindi on another shopping trip - mainly for supplies for the hospital, but also to try and reconnoitre the availabilities and prices of various materials for the solar project.

So after many deals and many cups of tea at various shops (including spending £500 on lightbulbs), we parked the van and hopped into a more nimble rickshaw to head into the narrower, busier streets of the metalworkers' bazaar.



















Riding in this thing feels very much like driving a fairground dodgem - the handling is pretty much identical - only it's more in your interest to avoid the other vehicles rather than take them on. And he can get through some minuscule gaps when he wants to. The bazaar is a bit different from your average one - the stalls/shops are much bigger and generally have piles of interesting things, and various guys making things out of other things.

Anyway, to come to the point of this blog post, here's a representative sample of one of the dual-language, triple-participant conversations in one of the shops. To give a little context, I'm trying to track down some plastic tubing for use as a heat exchanger. It'll need to stand up to nearly 100°C, and there aren't many kinds that'll do that.

(opening pleasantries)

Me: "Do you have any PEX pipe?"

(discussion back and forward between Mr P and shopkeeper in Urdu/Punjabi)

Shopkeeper: "What about this one?" (hands me a bit of tube - a yellow one, 1/2" diameter, no markings)

Me (thinks): "It's a yellow one, 1/2" diameter, no markings..."
Me (out loud): "Umm... what's it made of? It has to not melt at 100°C."

(more Urdu/Punjabi discussion)

Mr P: "Okay, how about this one?" (points out a different one - black, 1" dia, again no markings)

Me (internally): "It's black, 1" dia, again no markings..."
Me (out loud): "Do you know what it's made of? I really need to know the material it's made of before I can tell whether it's suitable or not..."

(more Urdu/Punjabi discussion)

Shopkeeper: "Also, I have this one over here..."

Me (in the privacy of my own head): "But what the @£%$!!ing $£^^@ is it made of???!!!!! Aaaargh!!"

What You See Is What You Get...
...but that doesn't necessarily mean you know what it is, and neither does the shopkeeper.











Random addendum: for ages I've been trying to get a picture of one of the painted trucks at night, because in many cases all that amazing decoration is done in reflective paint. But, not wanting to blind oncoming drivers with my camera flash (road travel here being quite dangerous enough already!), I had to take a short movie and grab a frame from it - not the full glory, but you get the idea: