Intro: Paddling Pool Solar Water Heater
Simple & CHEAP backyard solar water heater, based around a kids clam shell paddling pool! The approach uses a wide & shallow (100-150mm) water tight container,& in high summer (or with high angle tropical sun) this readily provides 60 litres of shower temperature water. It can be left at ground level, OR perhaps elevated enough for a good head to supply a simple garden hose connected backyard shower.
Sturdy plastic bag camping showers abound of course, but these hold only 20 litres, and can be heavy to lift up into position. Further more they may become dangerously hot, and sunny positions are not always the most private for later ablutions either! Having a hose fitting allows a length of garden hose to be run into a discrete location (perhaps even to an indoor bathroom ?)
More ambitious plumbed in solar water heating installations can easily cost $$$$,& require pumps or well placed header tanks. Such systems annoyingly work best at just the sunny time of year when you may be on holiday elsewhere-or when you are short of gardening water that could be doing double duty solar showering! This simple one can just be rolled out as needed.
Mark a position low down on the plastic shell wall, suitable for fitting an access pipe. (An old stainless 900mm x 900mm shower base could instead be used, but these bases are now quite costly even as scrap,with inconvenient bottom outlets, & can be difficult to drill anyway).
If threaded stock is not available,an old tap can provide both a useful threaded 15mm pipe and the hex screw rings. Hacksaw free this off cut, then drill and file the plastic shell base to suit. Remove any metallic burrs & sharp edges.
Fit the pipe off cut and tighten the securing rings, using plumbers hemp or PTFE tape to make a water tight seal. If done neatly the paddling pool can still be safely used normally by kids & toddlers.
A hex cap screwed onto this pipe end retains normal use of the shell as a paddling pool,and also aids it's draining. Your lawn will no doubt appreciate the watering!
A 15mm-20mm threaded adapter, fitted to the outlet,then allows normal cheap garden hose fittings to be simply used, with long hose runs if need be.
Of course a bit of elevation will allow more convenience! Ensure the support is up to the weight however ...
Clear agricultural grade plastic approximately 1m x 1m (~US$3) was found suitable as a cover, but this naturally will need securing or weighting against wind. Corrugated baby PVC also performed well, as did glass from an old window. Use whatever is convenient.
Ensure any plastic top cover is well secured against wind and animal damage! Although screwing down may tempt, this makes cleaning harder,so consider a lift off wooden framed support.
A bottom layer of gravel-or even black paint-may assist with solar pickup (and stability in winds),but water itself is a strong infra red absorber.
Case study 1: Sat. Dec 6th 2008-Wellington,NZ (early summer "down under" of course). Latitude 41 South. Note research assistant "Molly" testing progress!
Under clear skies,with max air temp. ~25 Celsius,no wind & VERY bright sun,60 litres of tap water (initially at 20 Celsius) warmed from 11 am at ~4 degrees C an hour, reaching a very cozy 42 C by 4pm. By mid night,under a ~15 degree evening, this had cooled back to 22 Celsius.
Using Q = m x c x deltaT, then 60 x 4185 x 22 = 5.52 Mega Joules of energy gained. At ~US 20c a 3.6MJ "unit" this is worth just ~US 30 cents,(& over 100 days of summer capable of saving US$30 = ~paying for itself), but the "solar shower" convenience may be much more valuable, especially off grid.
The benefit of dirty feet and salty body washing remaining outdoors of course are near priceless for harassed summer housekeepers!
If the sun remains too strong at "splashing time", then a ground level setup can perhaps even be dragged by an adult across grass into nearby shade.
An old stainless steel shower base will be sturdier and hold more water, plus (being uniform) it'll be easier to cover too. However plumbing into the usual bottom drain may be a hassles without assorted fittings or brazing gear.
This one has been in the kids tree hut since 2000, and is covered with perspex for safety. rather than glass. It still works a treat - on Jan 27th 2013, with afternoon air temps ~ 25 Celsius, the ~100 litresof water within had warmed to an almost too hot 48 Celsius by 5pm as the sun left it. ( The location was Wellington, New Zealand lattitude 41 South - Dec-Feb is NZ high summer of course)
Case Study 2: Wellington (NZ)-assorted occasions NZ summer 2007/2008 This steaming setup was installed on a sturdy shed roof, giving a 2.8m head above ground level,and back filled from a garden hose when empty. Measured clear sky high summer heating rates of ~4-5 degrees C per hour were typically observed, with cooling rates only ~1-2 degrees/hour. By mid afternoon,under clear sky conditions ,water temperatures could reach ~50 Celsius & a "solar shower" sometimes was almost too hot !
Case Study 3: Wellington NZ -December 1st 2008 ( a very warm but overcast day): Outside peak conditions, early summer water heated from 20 Celsius to 36 degrees Celsius water by 6:30pm,and remained still pleasantly warm until well after dark.
Conclusion: The approach offers a versatile, educational & cost effective approach to DIY seasonal solar water heating, with temperatures to blood heat (37 Celsius/98 Fahrenheit) readily achieved- even under non optimum solar conditions. The "backyard benefits" of solar heating significant volumes of water to modest temperatures outwaeigh heating a small volume to a higher temperature. Water rarely will get to an unsafe temperature, making the setup safer for children too.
EXTENSION: Such enhancements as side insulation, double glazing top cover,heat exchanges, water depth & perhaps even "solar salt ponds" (or solar PV powered pumping) could readily be explored for greater performance.