John Jackson

Hello Benjamin,

the project is going great, you can check its progress here: slopefarming.org

John

Here's a video explaining how a thermo siphon works: https://www.youtube.com/watch?v=5IRLVCJ1olA&t=
I would like to use that to heat the pond inside the greenhouse with firewood. The method could be combined with a rocket mass heater, whereby the exhaust would be going through the pond before exiting the greenhouse. I imagine, it's a lot more efficient to heat water than to heat air, as water can store much more heat.

One of the most interesting studies I found regarding duckweed research is this one:
"Nutrient recovery from swine waste and protein biomass production using duckweed ponds (Landoltia punctata): southern Brazil" https://www.ncbi.nlm.nih.gov/pubmed/22592476
The authors grew duckweed in anaerobically digested swine manure in ponds for one year and managed to produce a yield of 64 t/ha/y dry mass with an average protein content of 35 %. They concluded that this type of duckweed farming produced 20 times more protein than soy in the same time period on the same surface area under the same climatic conditions.

So, I'm guessing, an integrated system with duckweed cultivation might be the most efficient way of farming. The highest yield obtained with duckweeds is 183 t/ha/y under lab conditions, so there is even potential for further improvement! Additionally, the protein content can be 45 %, when the nitrogen content in the pond is maintained at very high levels. http://www.fao.org/ag/aga/agap/frg/lrrd/lrrd7/1/3.htm
In contrast to using swine manure as nutrient source, with ducks that have acces to the duckweed pond, there is no need to transport their feces from A to B as they directly crap into the pond.

John Natoli wrote:I'd echo William's concern about the ducks eating all the vegetation. I have three very young ducks and they gobble up duckweed faster than I can get it in the pond which has about a 1000 sq ft surface. Looking forward to seeing your calculations on that as they may help me out!

I'm just guessing, but probably your pond has very little nutrients in it.

Here are the calculations. I can provide references for most numbers, if anyone is interested, just ask! All the masses are referring to the dry mass! So, 32 t/ha/y of duckweed yield means about 640 t/ha/y of fresh weight, because it's about 95 % water. The same goes for food waste, duck eggs, larvae, it's all specified in dry mass! Have a look at the attachment first as an orientation.

One duck needs on average: 125 g of dry feed per day with 18 % Protein, which equals 22,5 g Protein, which equals 3,6 g N. It lays 0,66 eggs per day (80 % laying ducks, each having 300 eggs per year) having 5,9 g protein which equals 0,9 g N. The duck poo contains 2,5 g N then. However, the ducks will spent 50 % of their time outside of the greenhouse, so only 50 %, 1,3 g N go into the interior pond. I would feed one duck 10 g of larvae per day. For that I need to put in 67 g of food waste (12 % protein) which would result in the production of leachate with 0,6 g N entering the pond.

Together one duck and the larvae rearing for this one duck would put 1,9 g N per day into the pond. Duckweed can take up 4,4 g N per day per m² under favorable conditions. The trick here is to keep them at the perfect density at all times: thick enough to cover the whole surface to make use of all the sunlight hitting the surface and to suppress competing algal growth underneath, but not too thick to induce crowding, which inhibits or even stops the growth of duckweed. Let’s assume, I would have a removal of 2,2 g N/m²/d with all the floating plants, then I could get 115 ducks plus the respective larvae rearing without accumulation of N in the pond.
115 ducks would produce 76 eggs having 108 g N per day, which would be the system’s output. The system’s input would be 7,7 kg of food waste for the larvae rearing with 148 g N. In this case I would have a surplus of 40 g N per day. However, I did not take into account any losses of N, like denitrification, sedimentation and volatilization of ammonia, which can be higher than 70 % of all N under high N conditions. The system buffers itself, so to say.

The area of 100 m² would produce 877 g of floating plants per day (32 t/ha/y). For one duck out of 115 that would be 7,6 g per day. That’s rather low, even when they can forage outside. However, this is based on a productivity of 32 t/ha/y of floating plants. The highest yield under lab conditions for duckweed is 183 t/ha/y, for Azolla it's 360 t/ha/y, so there is still room for improvement.

As I made many assumptions and all of this just serves as a rough estimation, I will start with a small number of ducks and fish and larvae and slowly increase their numbers while keeping dissolved oxygen, temperature and salinity (as an indirect measurement of N) of the pond in check. No pumps, no aerators, and no need to rush it.

On that basis I have to regulate the system by observing these three main parameters and readjusting them:
Dissolved oxygen too low: reduce the number of fish, put less leachate from the larvae in the pond
Dissolved oxygen too high: unlikely
Temperature too low: improve insulation, heat (I’m thinking about a thermo siphon stove with an inside copper coil tubing that lets the pond water run through it)
Temperature too high: increase ventilation, exchange water with exterior pond
Salinity too low: put more food waste into the larvae rearing, get more fish/ducks
Salinity too high: exchange water with exterior pond, put less food waste into the larvae rearing, reduce the number of fish/ducks

Mike Jay wrote:
As for the walls, have you seen the wofati construction method?  Logs and boards form the walls with bracing to keep them from tipping in.  They have plastic on the earthen side to keep them from rotting as quickly.  The pond would have to not touch the logs so a small walkway around the perimeter of the pond could ensure separation.  

Just looked into it. It says it is mainly based on Mike Oehler's concepts. I read his book about earth-sheltered solar greenhouses, it was a huge inspiration for me. Nevertheless, I am concerned about the longevity of the wood, as I assume, I will have very high humidity inside coming from the pond.

Mike Jay wrote:The concept I'm thinking for "geothermal" would be to dig the equivalent of a hand dug well about 8' deep.  Or however deep you can safely and easily do.  Or do several.  Then support the sides with stone work, block or a piece of road culvert.  The top of the well or culvert would be at the bottom of your pond depth so the pond is sitting on top and connected to the water in the deep well.

 
Are there some example of that concept or is it your own idea?

In my greenhouse design I planned on R20 for the first 4 feet underground.  By doing that, the ensuing heat loss was indeed negligible.

Sounds reasonable. But you're in zone 4, I'm in 7, maybe 8. However, if a little insulation can extend the productive season it probably pays for itself.

I'm still unsure how I would construct the walls. With the interior pond, I'm afraid any wood will rot easily. I thought about covering the earth walls with liner from the inside and just putting in some vertical steel rebars, connecting them with horizontal steel rebars to press the liner onto the earth.

William Bronson wrote:John,if the fish have free access to the plants and the ducks free access to the fish and the plants,you might find yourself without either fish or plants.
On the other hand, allowing ducks access to a pond without filters or water changes of some sort is probably asking for trouble.

That depends on how much plants, how much fish and how much ducks there are. The components need to be kept in balance. I will post some calculations on that soon.

You could segregate the plants with floating cages , but im not sure what would allow ducks and fish to coexist in a pond at this scale.

Well, ducks can only eat what fits in their beak, so with bigger fish, I don't see a problem. The same goes the other way round.

Floating your structure on the pond might be cheaper and easier than building a steel span.

Yeah, true, but I want to be able to use as much light as possible and grow floating plants even below the steel span. I will need to do some calculations there.

Mike Jay wrote:What temperature do the water plants need in order to proliferate at the needed rate?

Some duckweeds are supposed to grow even below 10°C. But I don't think there will be any significant growth below 15°C.

Will you insulate the part of the E, W and North walls that are within 4' of the surface of the surrounding soil?  I can understand the concept of not insulating the bottom of the pond so that it harnesses the semi-deep earth temps.  Just don't forget that any soil near the surface and touching your greenhouse will be frozen in the winter and will suck the heat out.

You're right about the heat loss at the upper soil layers. However, I think that amount is negligible compared to the amount I will lose through the sheet at the whole front. Therefore I plan to roll down an insulation over the whole front, when it gets really cold. Otherwise I will probably not use any insulation to keep it simple.

One idea that comes to mind is that you could dig the pond extra deep in a couple spots to tie into warmer/deeper temperatures.  Steep pond sides are a challenge but I'm imagining a 2-4' diameter pipe or culvert standing on end.  It would hold back the dirt and the water inside it could "mine" heat from below.  I'm not sure if the convection of that heat would be sufficient to actually help heat the building...

You mean geothermal heating? I just googled that, you would need to dig 33 m deep for a 1°C increase in temperature on average. Sounds like a lot of digging to me or I misunderstood your idea.

Mark Tudor wrote:Would the indoor pond be fully lined with insulation underneath to thermally separate it from the earth underneath? I'm wondering that as Tilapia require rather warm water to be productive and survive, relative to catfish. Depending how deep the pond is, the earth underneath will be a heat sink and draw heat from the water, possibly making it too cold for the tropical fish (depending on location).

You might find some other species which are more tolerant of varied conditions similar to catfish, or if the pond is deep enough a colder variety may work better.

Good point! I'm not planning to insulate the interior pond. The local average temperature is 8,5°C, which is what the deeper soil layer will have then more or less. Tilapia start dying below 10°C. I think it's quite hard to predict what the minimum water temperature will be like in the coldest months as a lot of factors are contributing. I guess I'll start with just catfish to be on the safe side and see what the first winter will be like. The emphasis is on the ducks anyway and most breeds are surprisingly tough.

This is what a mixture of Azolla and duckweed and some other floating plants looks like. These plants have an enormous capacity to take up nutrients. Azolla is the fastest growing plant on the planet followed by duckweed, both grow at an exponential rate, doubling every 2-3 days under optimal conditions.