A bioshelter is a new, more complex form of greenhouse that can meet important basic human needs. Bioshelters imagine a new synthesis between people and their local ecology: an early exploration in weaving together the sun, wind, biology and architecture on behalf of humanity. The architecture protects the ecosystem inside from extremes of wind and cold, and absorbs the energy from sunlight that causes plants to grow and heats the air.

The ecosystem includes soil life, diverse plants, fish, insects, frogs, and people. Food is produced as nutrients cycle through plants and soil. Water is captured, stored, warmed and directed to crops and ponds.

Water Tubes

The Holy Grail of Thermal Mass for Solar Greenhouses

Click here for a downloadable PDF of this report by Earle Barnhart

“Alchemical processes depend on the maintenance of steady temperatures, and much experience is needed in the design of furnaces to precisely regulate the heat and draft …”

-quote from Old Alchemy

Passive solar heat storage in water tubes

These Greenway Passive Solar Heat Storage Tubes absorb and store excess heat from greenhouse air during the day and release the stored heat into the greenhouse at night. Water is the best material for heat storage, storing the most heat for its volume. Each tube, made from Teflon, is 100% transparent, letting sunlight pass on through to growing plants in the greenhouse. Heat is absorbed automatically from warm air, and is released automatically back into colder air. Water tubes are most efficient if hung high in a greenhouse where the air is warmest and above the soil and crops.

Benefits of water tubes

  • Automatic: the air and heat move naturally
  • Silent: no fans, no power, no fuel, no burners
  • Simple: no mechanical or digital controls, so you can see them working
  • More attractive than ducts

The air in a greenhouse on a sunny day will get hotter and hotter, and usually it is vented out of the greenhouse and wasted.

A transparent tube of water hanging in the greenhouse will absorb the heat from the hot air and store it as warm water. Later at night when the greenhouse cools, heat is released from the warm water and heats the greenhouse air. Water is the best material to store heat; it holds the most heat for its volume.

When warm moist air cools on a cold water tube, drops of condensation will form on the surface. Condensation will be visible while absorbing heat. At night, it evaporates off and the tube is clear.

During the daytime, the process works as follows:

  • warm air touches tube
  • heat moves into water inside
  • warmest water rises to top
  • cooled air sinks downward

During the night, the now-warm tube radiates heat and the air touching tube becomes warm, rising upward.

Notes from personal experience

When warm moist air cools on a cold water tube, drops of condensation will form on the surface. When absorbing heat, the condensation will stay visible. When heat is released at night, it evaporates off and the tube is clear. The water at the top of the below tube is very warm and is not cooling any air.

The water at the top of this tube is very warm; too warm to absorb heat and form condensation.
The temperature in this tube is 100F.

Trial and Error

Using 2 polyethylene tubes 2″ and 4″ diameter, the water tubes will fail in a year or two.

Using 2 Teflon tubes, they remained inert and clear and stable shape.

Using 2 nylon tubes, we had a catastrophic failure every 3 months with cloudy water and bulges in the bottom.

Plastic rods bend and distort; so does wood; aluminum rods are better.

We found that 4 inch diameter tubes are very heavy and hard to handle.

Sealed bottoms are necessary, simple tying and clamps will leak

Hanging in freezing air, eventually the water in a tube will freeze.  Water expands about 9% in volume when frozen.

Water tubes are more effective if they are placed high in a greenhouse and distributed in many places.

Other thermal mass

Why Not Black?

Plants in a greenhouse need the maximum amount of light possible to photosynthesize and grow.

A clear tube in front of a plant will let light go through and hit the plant. A clear tube will only only absorb heat from nearby air.

A black tube in front of a plant will absorb light, leaving less light will hit the plant. A black tube will absorb both heat from nearby air and light that strikes it, which turns into heat. A black tube will get hotter than a clear tube.

Black containers of water are useful to absorb light that will not strike a plant, such as at the north wall. On sunny days black metal drums of water can store large amounts of heat.

For people, black walls can be slightly depressing to look at and spend time near.

6 responses to “Bioshelters”

  1. Can anyone tell me where you can buy the hanging water tubes? Thanks, Mary


    1. At this time, as far as we know, no one makes or sells hanging water tubes. We hope that someone sees this information and begins to make and sell them. We’re looking forward to the day when you can get them by Amazon. When that day comes, remember to order the ones made with clear fluorocarbon film.


  2. Teflon? Seriously?


  3. Hello, I understand that one can not buy these water tubes, but can you please give more details on how they were produced? What kind of raw materials did you get? How did you seal the bottom? How was the top secured to the hanger? How think was the material you used? What materials did you use for the hanger? In the end, what diameter tube did you use?
    Thank you


    1. Hi Igal!

      The material is Tefzel, an ETFE Film that is similar to Teflon (inert, transparent, strong). I had them made to order at the Ingeneven Co. It is probable that other companies that make things from Tefzel can also make them. American Durafilm is another company. I tried using polyethylene and nylon, but they fail/split/break in about 6 months. The Tefzel has lasted 12 years so far.

      The tubes are made of ‘fold-flat tubing’ .004″ thick. A flat tube 4 -5 inch wide will make a tube about 3 1⁄2 inches diameter. I tried several sizes, and finally ended up with a tube 3 1⁄2 inches diameter and 2 feet long. If they are bigger in diameter or longer, they are too heavy to easily handle and hang.

      The bottom is heat-sealed by the company. The top seal is made of a commercial plastic clamping product. The original plastic rod and clamp was not strong enough and bend and came apart. I now use aluminum rods and drill a hole in each end to hang them from. It is strong enough to hold
      a tube up to 4 foot long by 4″ diameter.

      In practice, you fill the tube and then attach the clamp and then hang it up.

      Hope that helps!
      -Earle Barnhart


  4. Hello, it is possible to use a glass bottle, instead of teflon (or tefzel, ecc)? Thank you


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