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Concept of the “LongPondGreenhouse”: (A) Sketch, demonstrating long rows of this greenhouse. The inner surface is almost entirely covered by the long pond. Access is also through this pond but may be only rarely needed. (B) Possible external appearance. (C) “Raceway” type pond with single harvest point for continuous 24/7 automated harvesting.


This is a concept for a simple greenhouse-like structure that has a long pond inside and is used exclusively for the production of floating plants such as duckweed and azolla. The installation has a low height (1-2 m), which helps to greatly reduce costs for the structural supports. Once the function of the greenhouse is reduced in this way, then there is only sporadic or no need for human access. There is only a small volume of CO2-enriched air above the water. The water is warm and temperature is maintained by the large amount of circulating water. The structure is mostly sealed off, in order to prevent water losses (though some way to get rid of "waste" oxygen will need to be added). A CO2 source will be required (e.g. combination with adjacent composting system, which may also be a good heat source). "Farming" this pond can be highly automated, with very simple floating robots (like toy ships that have small solar panels on them) moving the crops to the harvest points.

Other aspects

  • These "farms" could be very suitable for aquaculture in the drylands (e.g. Central Asia, Africa, Asia), as water losses are minimized.
  • This is similar to the spirulina farm concept built by Jean-Paul Jourdan in France.
  • The structural support can be something cheap like wood or bamboo (problem: this is a very humid environment, so these won't last long). Concrete supports are more durable but also more expensive.
  • Covering: similar to the Chinese solar greenhouse, a covering (straw mat, wool) could be used at night. The structure is unlikely to overheat, but may sometimes be above the optimal Azolla temperature (i.e. above 30C).
  • temperature management (heating/cooling) may be required. Heating can be from biomass or solar. Cooling can be from a diurnal (day/night) system and does not have to be electric.
  • Compost as a CO2 source. The air is highly enriched in CO2. Compost is ideal in this integrated and enclosed system. The electric motors for fans can be solar powered. We also may need air pumps to pump air into the compost. That air can be pulled from this „greenhouse“ back into the compost to retain the water (in areas of water scarcity). The compost will also serve as an extra source of heat. Azolla is very sensitive to air pollution, duckweed less so.
  • Expected losses are CO2 and water. These are likely to be minimal. Carbon can be imported for composting.
  • Downstream: cattle (+BSFs, worms), chickens, fish, pigs, later cattails; we can run the whole farm from this one source. Azolla can be lacto-fermented for storage.
  • Other ideas: ferrocement or similar technology as the liner, fully automatic (24/7) harvesting

Materials Required

  • Transparent covering/foil (polyethylene, teflon, polycarbonate, glass…). Because of the high moisture, something like fluoropolymer-based foil (ETFE/PTFE) may be preferable (see below), but is much more expensive.
  • structural supports
  • Insulating covering
  • pond liner: polyethylene foil, ferrocement, other
  • electric (battery-operated) floating "tractors" (as in: miniature boats - can be solar powered) or some other way to move plants forward. These tiny boats should also perturb the nutrients in the water to make them available to the plants.

Potential Problems

  • (most importantly:) moving water is not ideal for duckweed and azolla, as they prefer a still pond. It is better to only move the plants “as needed” prior to harvest. The “racetrack” shown in the image here is therefore probably not ideal for these plants (but good for Spirulina). Better to put the wheels underwater so that there is good water flow for nutrient dispersal but minimal disturbance at the surface
  • temperature fluctuations in the air: too high, too low (the very small amount of air makes extremes of temperature more likely; water temperature will be more stable)
  • high moisture: possible problems with microalgae, fungi, etc.

Related Wiki Pages

Possible Design Improvements

  • Due to the very high moisture, microalgae growth on the inside of the foil is likely. This may be minimized with the use of non-stick (i.e. “Teflon”, ETFE/PTFE) foil, which is far more expensive than polyethylene however.
  • earth sheltered on one side (in high latitudes especially): would provide extra insulation and structural support but block out some of the light. It would allow for a very inexpensive build with simple, straight, short beams of wood or concrete.
  • night-time cover with insulating material (e.g. straw mat, sheep wool) to minimize heat losses