A simple thermosiphon system at builditsolar.

Personally, I think it would be worthwhile to separate the circuit in the collector and put a heat exchanger in the hot water tank. This would prevent contamination from bacteria in the collector and it would also enable you to run a 40/60 propylene glycol/water mixture in the collector to prevent it from freezing.

Mark, you could try a standard immersion heater. There are 300W, 500W and even a 1000W bucket heater models available from Amazon.

If you want several hundreds of Watts from a solar thermal collector, I reckon you're looking at a square yard of collector in good conditions; assuming 800W irradiation and 50% efficiency, you'd get 400W of heating from that. Since the specific heat of water is around 4.18 J·g⁻¹·K⁻¹, and 5 gallons is about 18900 g, at 400W a heater would need about two and a quarter hours to get 5 gallons from 68°F to 140°F (that is a Δt of 40 K).

Probably so, but we wouldn't put the bathroom over where we have the solar shower. Instead, we'd plan it where the water comes from the tank to the house and use the same solar collector to heat water for the kitchen sink. That's a more major undertaking that we aren't going to deal with now. Mark's solar shower is just a quick and dirty summer shower to keep us clean with minimal work in warm weather. (It also doubles as a quick, outdoor handwashing station, which has already come in handy!)

We appreciate your feedback, though! Mark was just saying that by keeping it vague, he knew you'd kick in with the calculations. So here's some more data. For the shower, we'd probably only fill it in the morning if we wanted to shower that day, so there's not a big problem about bacterial buildup at lower temperatures. If we were just looking to heat the five gallon bucket to normal shower temperatures by running water through a hose (or, more likely, black PVC pipe) on the roof, how much surface area would you recommend? Assume the water starts at 50, not 68.

I think that's what Mark plans to try next. I know that when I go out and hose off with the garden hose, it's always plenty warm!

I hadn't thought of making the water recirculate through a garden hose without a pump. I'm having trouble wrapping my head around that, but you probably know what you're talking about.

Roland,

Thanks for the data.

I really like the idea of upgrading the submersible heater, but the first two you linked me over to look like junk and the reviews say as much, but that bucket heater for 40 dollars looks handsome indeed. I could see that coming in handy on chicken killing day ,might have to compare the energy output from our propane set up to see if it might be cheaper. One guy in the review of the bucket heater said he used it as a heater for a do it yourself hot tub. That certainly has got some wheels turning in my head.

Back in the 60's and 70's I did many experiments in having a daily shower using no electricity in rugged conditions.
I found the the Guvment (Army) had done many experiments and found that the best natural collector of solar heat was green but I found that the difference in my application it was not measurably more than black. Since green hose was the cheapest and most available that is what I used. I coiled it on a frame, connected it to the top an bottom of a bucket and waited for the sun to heat the bucket. It did but only from the sun on the bucket. It had overlooked that the natural flow of cold water in at the bottom and hot out at the top would be thwarted by the coiled hose. The water in the hose and the water in the bucket was plenty warm and we both had a shower that evening. The entire group that was there for that occasion watched us with envy. They went to bed dirty. Later experiments got better. I still used a hundred feet of green hose to heat water as it was much faster than the bucket and I had it. When it was really hot I didn't bother. When it was cooler I learned to use the hose protected from the wind and covered the bucket with clear plastic. I arranged it so there was air all around the bucket and the sun could see it. That prevented wind cooling. Even on cold California days I had plenty of bath water. Invest in a hand pump pressure sprayer for winter bathing.

I think oldfool's on the money. What we're looking at here is surface area. When heating up thermal mass, we understand that only the outer surfaces get the heat, and hope to heat the whole mass by either a) recirculating so that the inner (colder) becomes the outer and can be touched by the thermal heat or b) heating the outer to a temp beyond the target and radiating that heat inward to heat the cooler inner part.

When we had a solar pool heater, it was black plastic mats with rows of much smaller tubes, so that there was more water to surface area than if we'd have had the same amount of water going through larger tubes.

I'd say any situation (green hose or otherwise) that allowed you to take the same amount of water you're planning for in the bucket system and spread it out so that you have maximum surface area for the sunlight, you should be able to increase water heat quite quickly.

How's this for simplicity? Here are some more.

The water pressuse you get depends on the height of the reservoir and the diameter of the hose, not on the type of container as such.

More pressure also means more flow, so you'll empty the bag/bucket sooner.

If you can get hold of that relatively sturdy black plastic foil, you could use that to make a watertight liner in a worm-bin line structure. bond in an outlet hose in a corner. Fill with a couple of inches of water, put a pane of clear plastic or glass on to to stop evaporation and let the sun warm it up.

My gut reaction told me that a five gallon bucket with a hose at the bottom would give more water pressure than a 1 gallon milk jug with the same hose at the bottom, but a quick search of the internet suggests that you're right --- if the top of the water in the milk jug was as high as the top of the water in the bucket, they should give equal pressure. So maybe my displeasure with our solar shower bag was due to not hanging it very high (I'm pretty short and never used a ladder) and to the smaller hose coming out of it (since it's only one gallon and would run out of water pretty fast with a garden hose coming out)?

If it wasn't so much weight it would collapse the roof, I'll bet the cheap plastic kiddie pool we used to use to soak mushrooms, filled with an inch of water then topped with some glass or plastic would make a pretty good collector.