Hot water from the sun

Submitted: Wednesday, Jan 12, 2005 at 11:40
ThreadID: 19293 Views:2620 Replies:11 FollowUps:10
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Having used the plastic solar showers for a number of years, I have been wondering if there might be a better way to heat the water quicker. I have experimented with black plastic containers which work OK but no quicker than the solar shower bags - although the containers are very much easier to fill.
I would love to try a container that is maybe only an inch wide with a larger surface area, but can't find one. I have approached plastics companies, who aren't interested unless thousands are produced at a time.
I am wondering if the water would heat quicker if a good convection type current could be set up. I notice the water in plastic containers gets extremely hot very quickly near the upper surface but this takes a long time to spread the heat to the lower surface.
For the technically minded:- If a length of black plastic tube entered the container at both the upper and lower sides, would this be likely to set up a convection current whereby the hotter water moves to the bottom and the cold to the top. If so, what length of pipe should be used and would the diameter be critical? Would it help if the centre point of the pipe could be curled up in some way, thereby forming a type of 'snail shell' pattern so the sun also heats the water in the pipe. The water in the pipe would heat up very quickly I would imagine. Would this assist the 'convection'.
Has anyone done any successful experimenting regarding simple water heating from the sun?
I only need to heat between 5 and 10 litres at a time.
Cheers to all,
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Reply By: Rob! - Wednesday, Jan 12, 2005 at 11:57

Wednesday, Jan 12, 2005 at 11:57

An interesting concept. I don't think that simply adding a hose to the top and bottom of a tank will move any water around, unless the hose is the heating element.

Hot water will always flow higher then cold water so perehaps you can try a two part system with a storage tank and a large surface area "heating element". If you have the storage tank placed higher then the heating element then the coldest water will flow down to the heating element - warm up and flow up. Something like residential solar hot water systems.

Are you hoping for this to be permanantly attached to the car or something that is set up when camping?

Good luck and keep experimenting.


AnswerID: 92614

Follow Up By: Dave - Wednesday, Jan 12, 2005 at 13:21

Wednesday, Jan 12, 2005 at 13:21
Thanks R,
Re your first sentence:- My thinking is that the tube would make a fairly good heating element. With such a small amount of water in it, I am sure it would get very hot very quickly. This being the case, the trick would be to make the water in the tube to constantly move into the container and be replaced by the cooler water still in it.
My plan would be to carry a couple of containers which could be easily filled by dunking in a river and then just place them in the sun.
FollowupID: 351509

Reply By: Member - Camper (SA) - Wednesday, Jan 12, 2005 at 12:03

Wednesday, Jan 12, 2005 at 12:03
Hello Dave,
I had two black plastic water heaters years ago which had deeply corrugated surfaces. these would heat water efficiently provided the quantity was small. trying to maximise the amount of hot water by overfilling was not on because it took too long to heat so much water.
So to try and get technical without, many qualifications to support me, it seems obvious to me that the ratio of water quantity to surface area exposed to the sun is critical here. Obviously the more water in close contact with the heating effect of the sun (thro the black plastic) the better off we are going to be. I guess then that if we put the water into a long plastic tube of say 35mm diameter we are going to heat that water faster than if it is in a black plastic tank. So if it is practicable to use a pipe as a tank and get enough volume this way it could do the job.
If you have a container of water with piping from top to bottom as you describe you will get the water at the top heating up but that lower down will not until conduction within the container heats it all, even then the water at the top will always be hotter. The plastic pipe in your scheme will not allow convection to happen.
If you have a much longer pipe fixed as you describe and have a 'snail coil' which is situated below the whole tank then you will get a convection current thus: the heated(therefore lighter) water in thepipe will flow up to the top of the tank and the cooler(heavier) water from the tank will flow down to replace it. Bingo circulation.
If the tank is black too you will be collecting heat there as well.
I'm tempted to say that this will work better if the tank is tall rather than squat, but I"m getting a migrane thinking about it (which is camper-speak for I am not sure and have to give this some more thought).
Anyway thanks for the entertainment.
If you get water to boil and we meat some time a cuppa from the system would be great!
AnswerID: 92617

Follow Up By: Member - Camper (SA) - Wednesday, Jan 12, 2005 at 13:21

Wednesday, Jan 12, 2005 at 13:21
Me again without migrane.
In the interests of theory a tall tank or at least greater height difference between the heated water and cool reserviour will give you a greater heat-syphon effect.
I'm amazed at how many replies you have gotten and how much good problem solving advice there is in them.
Go forumites!!!
FollowupID: 351508

Follow Up By: Dave - Wednesday, Jan 12, 2005 at 13:26

Wednesday, Jan 12, 2005 at 13:26
Thanks Camper, interesting reply.
Feel like making the migraine worse - What length of 35mm pipe would be required to heat 5 litres?
FollowupID: 351510

Reply By: hoyks - Wednesday, Jan 12, 2005 at 12:04

Wednesday, Jan 12, 2005 at 12:04
A lay flat type tube would be the best bet to provide the largest surface area. The other option would be a coil of 12mm micro irrigation tube with a small 12V bilge tube pump to circulate the water. They use a similar system to heat swiming pools and you know how hot the water in a hose gets lying in the sun.

Trying to set up a convection current wouldnt be too sucessful as the water at the top would get hot and try to stay at the top of the container and the cool would settle to the bottom.

If you used the hose thing mount the pump so it _Affordable_Storage_Drawers.aspx water from the bottom of the container and emepties into the top which would encourage uniform heating of the water column. You could also substitute the plastic hose coil for a copper pipe that you could throw it in the fire when the sun wasn't out
AnswerID: 92618

Follow Up By: hoyks - Wednesday, Jan 12, 2005 at 12:16

Wednesday, Jan 12, 2005 at 12:16
Irrigation tube is real cheap too if you wanted to have a play.

The idea of having the heating coil below the tank and letting convection do it's thing is a good one provided you get all the air out of the line.

Get a bucket, a $5 roll of 12 or 19mm tube and fill them both with water put one wnd of the hose at the bottom of the bucket and the other end just below the water level at the top. Throw the coil in the sun preferably in a way that would alow the water to naturaly move upward as it ir heated (a vertical coil would be good)You could also drop in some food color on the cold side to see how quickly the water moves . Or you could just stick your hand there and see if it is hot.
FollowupID: 351499

Follow Up By: hoyks - Wednesday, Jan 12, 2005 at 12:21

Wednesday, Jan 12, 2005 at 12:21
Instead of a bilge pump you could also use a windscrean washer pump to boost the circulation, Wreckers almost give them away. Not the largest in water volume moved but cheap
FollowupID: 351501

Follow Up By: flappan - Wednesday, Jan 12, 2005 at 12:44

Wednesday, Jan 12, 2005 at 12:44
I have thought of doing this a few times (including for heating spa water).

A black drum, with a series of black irrigation pipes. May a set of say 12mm pipes with some smaller (6mm ?) pipes running across between them.

I think the trick would be to keep the water in contact with the black pipe for long enough to heat it up (and why use smaller pipe).

A small 12v pump (heck maybe the DSE ones would work) , run it off a small 12v battery with a solar charger.
FollowupID: 351502

Follow Up By: flappan - Wednesday, Jan 12, 2005 at 12:46

Wednesday, Jan 12, 2005 at 12:46
quote: Instead of a bilge pump you could also use a windscrean washer pump to boost the circulation, Wreckers almost give them away. Not the largest in water volume moved but cheap

I think the slower you move it . . . the better. Longer contact time.

I dont think we are talking about a pump to actually shower from here , just to heat the water.
FollowupID: 351504

Reply By: Top Cat - Wednesday, Jan 12, 2005 at 12:15

Wednesday, Jan 12, 2005 at 12:15
A few years ago I had to do a quick copper repair job to my sisters hot water has a solar system attached as well which consisted of a heap of black tubed matting that lay on the roof.

I reckon you could get hold of some of that and probably adapt it to whatever size u wanted pretty easily and of course be able to use the natural movement of hot water rises principle to circulate it into a holding tank.........maybe a mat that could be used on your roof rack??

AnswerID: 92620

Follow Up By: hoyks - Wednesday, Jan 12, 2005 at 12:17

Wednesday, Jan 12, 2005 at 12:17
For that stuff try a pool/spa shop.
FollowupID: 351500

Reply By: Member - Sand Man (SA) - Wednesday, Jan 12, 2005 at 12:57

Wednesday, Jan 12, 2005 at 12:57

If there was a more efficient way to heat water, I believe it would be commercially available by now.

As good as anything mate, is the old gas burner. Heat a couple of kettles of water and add to cold water in a container until you get the desired temperature.

Of course, there is a more expensive alternatives such as the Coleman, or Primus gas heated water systems that also come with a shower attachment, or units that attach (permanently) to the vehicles engine cooling (heating) system.

Depends on how sophisticated you want to get and how much you are prepared to spend.

Me, I use the kettle method and a cheap battery operated shower unit immersed in a 20 litre rectangular container that you stand in. (SWMBO gets first shot usually and I use recirculated water which is still warm enough)

Your existing black plastic "solar" shower is also a practical alternative.
You could also add (gas) heated water to this container, to speed things up.

I'm diagonally parked in a parallel Universe!

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AnswerID: 92626

Follow Up By: Dave - Wednesday, Jan 12, 2005 at 13:43

Wednesday, Jan 12, 2005 at 13:43
Thanks Sand Man, but I reckon by now everyone would know how well gas heats water.
The thing is, I just love the thought of using the sun as a free effective 'power' source. I think it is a practical & fun thing to think about. Sure, the end result is a hot shower, but the starting thought is the real issue.
As regards your first sentence, I feel that if you were correct, the human race would never invent anything new. Bear in mind we have only had digital cameras a short time. Also I am sure there are many vested commercial interests that stop excellent ideas from hitting the shop shelves from time to time. I am sure that re-inventing the wheel is a fair pursuit and I am just as sure our world of leisure in 10 years time will include many items not available today.
FollowupID: 351512

Follow Up By: firestang - Wednesday, Jan 12, 2005 at 16:42

Wednesday, Jan 12, 2005 at 16:42
Just a quick thought , how about use the black tube as suggested and have a decent sized very shiny stainless bowl and have the tube coiled in it , maybe this would use the satekite dish type colection of the suns rans and somehow concentrate it at the tube?
FollowupID: 351536

Reply By: Member - Blue (VIC) - Wednesday, Jan 12, 2005 at 15:46

Wednesday, Jan 12, 2005 at 15:46
I think the biggest problem lies in the amount of energy required to heat water. If I remember my physics correctly, if you take 1kg of steel and 1kg of water it takes 14 times more energy to elevate the water temp by 1degree than it does to get the same result with the steel.
Back on the subject, I have seen a troopy with a mass of black hoses attached to the side just like a home solar system, nobody around to ask how well it worked though...
AnswerID: 92660

Reply By: Flash - Wednesday, Jan 12, 2005 at 16:08

Wednesday, Jan 12, 2005 at 16:08
Good luck Dave- I like your thinking.
The only thing I would add at the risk of repetion is that any tube must basically rise throughout it's length or the water will probably not flow- assuming no pump.
Good luck.
AnswerID: 92664

Reply By: uncle - Wednesday, Jan 12, 2005 at 16:54

Wednesday, Jan 12, 2005 at 16:54
I have a similar system to the sandman,we use a 10 litre galvanized bucket,fill it with water ,and then just drop it on the fire!Keep stirring until your water suits your temp preference,and bingo!Then we just drop the 12 volt shower system that we got from bbq's galore in the bucket.Ive rigged up a 3 meter peice of conduit into a circle and it fixes to the side of the caravan.The wife has put a shower curtain around it and it works great..If I can sneak in a photo while she's using it you'll see how it works.But at the end of the day it's a simple idea and does us fine.
AnswerID: 92671

Reply By: Member - Mozza (NSW) - Wednesday, Jan 12, 2005 at 17:05

Wednesday, Jan 12, 2005 at 17:05
an uncle of mine uses the 'hot/warm' water out of 3-4 solar showers.. and empties into one big black (sealable) drum (used as a storage drum when not n use hence it has 2x uses!) - that is presurrised via an air compressor which uses a standard outlet threaded into the top of the drum lid... pushes the hot water up a standard peice of hose line fitted with on/off tap and shabang.. hot shower.. works on very similar principles to a keg and magic box setup with the gas pressurising the keg.. except the beer is taken over the ice chiller to get cold...
AnswerID: 92673

Reply By: Patrolman Pat - Wednesday, Jan 12, 2005 at 21:41

Wednesday, Jan 12, 2005 at 21:41
I'm a plumber by trade and I've seen black poly pipes left in the sun under pressure explode due to the water inside superheating. I'd make sure any system tried was vented to atmosphere somewhere.
AnswerID: 92716

Reply By: Member - Camper (SA) - Thursday, Jan 13, 2005 at 17:58

Thursday, Jan 13, 2005 at 17:58
Hi again,
My calculations and those of three others, one a professor of engineering,( because I couldn't figure out where the decimal point went in the answer) is 5.194m for 5litres.
Which is about 1m per litre.
"Oow me 'ead"

PS you could heat water well by wrapping suitable tube around your lower radiator hose. With the tank plumbed as we have discussed. You would need to vent it as Pat says in case it boiled, which would be unlikely but still important to guard against.
You don't get heat unless you are driving, or a total environmental vandal.
AnswerID: 92852

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