4.18 * 27.7 C *0.8 L/min * 1,000 mL/L * 1/60 min/sec = 1,544 W per 1.4 m^2. This is 110% which shows his estimated flow rate is too high. It appears he was just watching the temp change to estimate the turn-over in tank volume which would give too high a flow rate because the water is mixing. The inlet temp would get hotter before tank volume had actually turned over. By my estimate of efficiency, his flow rate at the beginning was actually about 0.44 L/min.

I saw in a different forum that there is some concern about the plastic aging way too fast when it is touching water. This idea could use a solar pool heater plastic piping ($160 for 80 ft^2 on amazon) and this type of insulation backing, frame, and polystyrene front cover.

A polystyrene or polycarbonate cover reflects 5% from it’s front surface and 5% from the back for a 10% loss. Glass, polypropylene, acrylic, and plexiglass reflects about 8% total from front and back. Thick plastics like plexiglass are also going to absorb a bit of the infrared. A 2-pane window is only needed for fairly cold places because it also causes a penalty of 8% or 10% from reflection. The front surface of the corrugated plastic would lose another 4% from reflection if it is clear and the liquid is black.

A decently insulated system like yours will lose about 10% from heat escaping. So 75% is easy, and it’s hard to do much better no matter how professional the design. The objective is to transfer heat to the fluid without raising the air temp between the corrugated plastic and the front window (your polystyrene). Higher temps in the air gap mean higher energy loss out the front. Your plastic method doesn’t lose the additional 4% from reflection that occurs in a black water design, but the front side of the black-painted plastic is heating up the air gap a bit more than ideal which is why you’re getting 60% instead of 75%. You could probably get up to 70% with your method.

If you make your collector 4×8 and lay it long along the ground tilted back 30 degrees from the vertical and then lay one 4×8 insulation board on the ground in front of it with the reflective side up, you’ll get 30% more light for 3 or 4 hours/day. Add another insulation board at the top (tilted 30 degrees forward from the vertical) and you’ll get another 30%, bringing your “efficiency” to 1.3*1.3*0.60 = 101%. It’s basically a trough design. These 30 degree angles are sufficient for all of the U.S. (the reflection boards reduce losses from not being at an ideal angle). The longer the better to get the early morning and late afternoon. If you add a car radiator on the inside of the house with one 4×8 collector and two reflectors, you should easily transfer 6 m^2 of sunlight to the house (4 kW average for 5 hours in the winter, equal to nearly three 1,500 electrical heaters, enough for 1,000 sq ft).

You do not want an air gap between the surface of the water in a black water design and the corrugated plastic (or whatever you use to contain the water) because then you’ll lose 8% more reflection: 4% from the inside of the plastic and 4% from the water surface, black or not. When the water touches the plastic it’s like a “coupling”. It’s the transition from air to plastic/water/glass AND vice versa that causes the reflection loss, based on a difference in the index of refraction (1.0 for air and ~1.5 for plastic, water, and glass).

Has anybody tried corex? – its plastic corigated cardboard. Its only £7 for an 8′x4′ sheet. Its light and easy to cut and handle. weight per mm surface area in contact with the water and surface area per m2 would be good as well.

Might break down with UV but could be coated and its available in black.

The top and bottom tubes might need some work.

It is a great idea and construction! I run a non-profit “green issues” website in Hungary. Would it be possible to republish this content by translated on my site? of course I would display your site as source.

thanks.
attila

Good luck.

We did a 4 hour test on a clear day, readjusting the panel angle a few times during the test. The full spreadsheet is available but I was not sure if it could be posted here. It is a 1.814 square meter panel with 37.85 liters (10 gal) of water in the system. We used a 55 gallon plastic drum for the tank. The tank and hoses were not insulated.

Starting temp was 53.8 F. At the 1 hour mark the temp was 92.7 F, average power was 952 watts, 52% efficient. At 2 hours, 117 F, 768 watts, 42%. At 3 hours, 127 F, 593 watts, 33%. At 4 hours, 130 F, 464 watts, 26%. We also did a stagnation test with no water in it, and it got up to 152 degrees F on a 45 degree day. We are looking forward to mounting it permanently and testing reliability/longevity. One thing we still need to do is get UV clear paint to help protect the panels from UV breakdown, and see if that affects the efficiency much.

Now the question is, did anyone read my above post about the hot box with a simple car heater core, or car oil cooler in it and a fan? Using the air in the box to transfer the heat?
Just a black box with fiberglass insulation and glass front. Simple, cheap.
I’m going to build it, I did some small scale tests and things look good heat wise, but if someone already did this and has bad data I don’t want to waste my time.
As for the heat box I’m talking about, there would be no problem with it getting hot when turned “off”, the wood, fiberglass insulation and glass front would handle it.
You can “reverse” it in summer (nights) when it’s cooler outside than in the house, open a “summer” vent hole (with screen) and let cool outside air circulate inside.
And, don’t worry, you can’t patent a black solar box.

So please, comment, or did I not explain it properly? (feel free to ask questions)

I am however concerned about freezing problems, more due to glued joints at the ABS pipe at the top and bottom of an assembled panel bursting. I plan to have a large holding tank of water used directly with the panels without a heat exchanger to the holding tank so I’m not too interested in using a bunch of anti-freeze. I am planning to build a drainback system using these panels, and just painting them black.

I also saw that polypropylene does not tolerate UV light well and will become brittle and break after long exposure. Coroplast can however be made special order with a UV absorber mixed in with the polypropylene. I am getting some regular Coroplast from a local sign shop that does not have the UV protection, so I’m looking into paints that absorb the UV. If the prototype works well I’ll look into getting UV protected Coroplast for additional panels.

I’d like to use this design to heat a hot tub (in addition to a bio-gas powered water heater: read more about biogas here: http://solarcities.blogspot.com/)

I’m a bit worried about chemicals leaching into the water. I’m not planning on drinking the hot tub water, but are there any issues with exposure to the plastic chemicals?