Waterbag Solar Water Heating Collector Using Inflatable Film Cover and Soap Bubble Insulation

This is an innovative new wrinkle on the Integral Collector Storage (ICS) design from Nick Pine.

With ICS solar water heaters, the collector and the storage are combined into a single unit. Basically they consist of a water container that typically sits inside of of a glazed enclosure. Solar radiation heats the water container during the day. This is an example of a commercial ICS design.

One of the downsides of this nice simple design is that it tends to lose quite a bit of heat at night through the glazing. Nick has addressed this problem by filling the area between the water container and the glazing with soap bubbles at night. The soap bubbles are good insulators and greatly reduce the night heat loss.

The design has several other innovations:

  • A film bag is used to hold the water, which reduces the cost of the water container.

  • A greenhouse polyethylene film cylinder is used for the glazing and is inflated to hold its shape -- a significant cost saving over conventional glazing. Similar to a hoop style greenhouse.

  • The north half of the glazing cylinder is reflectorized to reflect more solar onto the water bag.

One upshot of the inexpensive materials used in Nick's design is that it is not expensive to build a solar water heater with a large collection area and a large storage capacity -- this increases the solar fraction.

To transfer the heat stored in the bag to the domestic water, a large coil of PEX pipe is immersed in the bag, and the cold water from the house takes ones pass through this large coil of PEX and is heated by the water in the bag. This heat exchanger scheme has been used quite successfully on the $1K solar water heater.

So, here is your chance to be a solar pioneer and be the first to build one of these!

Description From Nick

A 12'x6’x10” greenhouse polyethylene film 450 gallon water bag inside a 16’x8’x8’-tall greenhouse poly film half-cylinder inflated with air during the day and soap bubble foam at night. Bubbles are formed with a 500:1 expansion ratio by forcing air through a pipe with holes in a rectangular shallow trench between the 2 bags containing a 10% detergent solution.

Daytime Configuration

DIY solar waterbag collector


Nighttime Configuration

water bag ics soap bubble insulation night


Thanks to Jay Burch for these diagrams.


This might sit on a warm flat roof of a city row house or a commercial building. Roof weight is a potential issue at 52 lb/ft^2, which is enough to resist a 90 mph wind with a sliding force of 8’x0.00256x90^2 = 166 pounds per linear foot.

The outer poly film might have 3 tension straps over the outside to avoid wind fatigue.

Here's a nice $8.86 12-volt inflator...

If a 16’x8’x8’-tall greenhouse poly film half-cylinder is inflated with air during the day and soap bubble foam at night. The outer cylindrical enclosure could be inflated to about 0.25 inches of water (a typical inflation pressure for a double wall greenhouse) -- this would stiffen the poly film, lessen the need for structural support, and keep it from flapping in winds. Greenhouse poly film can be inflated to 25 MPa (3.6 psi) inside an 8 ft diameter cylinder with 157% elongation of 0.006 film before failure. Alternatively, the outer cylinder could be built as hoop house greenhouses are with double curved 1 by 3's or painted PVC pipes bent into a half circle hoop (plain PVC rots polyethylene).

Zelon’s Swedish patent (US No. 3672184, June, 1972) described insulating shop windows at night with soap bubble foam. Professor John Groh at U. Arizona measured US R3 per inch for soap bubble greenhouse insulation in 1968. Professor Otho Wells at U. New Hampshire did later greenhouse experiments.  In 1995, Bill Sturm built a 12,000 ft^2 tomato greenhouse with a soap bubble foam roof in Calgary, Alberta and measured an 84% propane energy savings with and without foam on alternate nights at 20 F below zero.

Performance Cost and Labor

Solar fraction: near 100% in an average (TMY2) year

Technical risk: medium, mainly because a) tiny cold soap bubbles (1/16” with a 50 F mean temperature) are as good as fiberglass insulation, but warmer bubbles are poorer, and b) warm water under cold soap bubbles will tend to create convection currents…
Longevity: Replace greenhouse film ($32) every 4 years

Hardware Item Cost
Poly Film $32
Foam Board $80
PEX Heat Exchanger (300') $165
Foam generator $150
Total Hardware $427

DIY labor: 3 days?


Soap Bubble Generators

Soap bubble insulation has been used successfully on greenhouses in cold climates.

Bill Sturm showed me a shop vac blower attached to a horizontal 2" PVC pipe with holes in a trough with a 10% detergent solution. I'd like to use that 12V $8.86 inflation pump with a small PV and battery and an arduino controller.

Bubble Insulated Greenhouse in Canada.

Solar Bubble Build -- a greenhouse using soap bubble insulation.

I think the point-source bubble generators shown at the links above are overly-complicated and expensive, about $1K each, IIRC, vs a shop vac and a 2"x100' PVC pipe, with no need to make foam flow long distances through ducts and around corners.

Any other ideas for providing effective night time insulation?

Comments, Questions?


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