Monday, September 28, 2015

Earthside - Smart Floating Farms


 Here is a proposal to build floating aquaculture barges. Solar PV for power on the top level, hydroponic crops on the middle level and fish farm tanks on the bottom level.

 It's a great idea, there's just not enough detail on their website to decide if I should take them seriously. Some of my wilder ideas are better-defended with links and data, to be honest.

 The article mentions a depressing value for productivity; 2.04km² of growing area and only 7.3 tons of vegetables + 1.5 tons of fish per year. That's less than 3.6 grams per m² per year of produce and 0.74 grams per m² per year of fish. I'd be disappointed if those were the hourly production numbers, so hopefully someone mistook their per-barge yield with their barges-per-year productive area or something.

 If I were to develop an aquaculture barge like this the approach would be similar: enclosed tanks on the bottom level, a vertically-stacked hydroponics level and a light collection system above. Skip the bulk PV electrics, too expensive; use parabolic reflectors into fiber optic lines and run those directly to the plants. Plant and fish species would be native to the region so they are suited to the climate. The system is not closed; it would require nutrient imports as either commercial fertilizers or bulk waste products like manure in addition to bulk fish feed. Fresh water would be produced by evaporation and condensation using sunlight and plastic (potentially yielding sea salt as a marketable byproduct in small amounts). Fish waste water would be pumped into tanks on the roof and gravity-fed to the various hydroponic trays. Separated solids would be digested in algae tanks (probably chlorella or spirulina as a batch process); algae would be filtered out, pressed into pellets and fed to the fish. Any remaining solids would be shipped to shore for disposal. Any water needing to be pumped off the boat must be sterilized, preferably by solar heating to 90 °C or more, then sprayed through an aerating valve to cool before introduction to the local water.

 Their system is meant to include processing services, so there would still be a need for electricity. PV may work well for that, as would wind. The boat's engine could easily double as a generator to be used as a backup power source. I question how much processing is actually necessary; these are meant to be deployed very close to population centers with fresh fish and produce markets. The daily harvest could be delivered to the local fish market at minimal cost, with no disposable packaging required. The main input would be refrigeration; still power-intensive but not wasteful of materials. Vegetables typically need to be cooled, washed and packed; again refrigeration is an input, but this time we also use fresh water. Packaging can be reusable bulk bins, particularly since the product is going to a distributor rather than an end user. Instead of typical compressor technology, the cooling load might be within reach of an evaporative chiller system; that would dramatically cut the power requirements. An alternative might be to anchor close enough to shore for a power cable to be run, connecting to grid power and using solar PV or wind only to offset the bill (or perhaps as a net producer of electricity).

 I estimate that low-intensity (and low-tech) hydro techniques could yield on the order of 200 grams per m² per day of produce. This could be doubled, tripled and perhaps quadrupled using more labor, more technology or both; specifics will depend on the economics of the region. The fish section could  easily produce 1.07 kg of herbivorous fish (Nile tilapia or Channel catfish) per m³ per day using commercial feed, or could instead consume the output of approximately 3 to 9 m² of hydroponics per m³ of aquaculture tanks. Certainly any waste vegetable matter would be fed to the fish and reduce the need for feed, but we're still looking at 1.4-1.8 kg of feed per m³ per day. This would yield 320 grams of fish fillets and 747 grams of fish waste per m³ per day if processed on-site, which would reduce the need for calcium and phosphorus imports. If space-grade technology were applied yields could exceed 2 kg per m² per day of produce at great expense; fish production could be roughly doubled with staged growth tanks and growth-stage tailored feed.

 The barges would require a lot of plastic. Main mechanical needs would be water pumps, the engine and the refrigeration system. Any PV or wind system with battery backup would be one of the larger expenses. An ongoing supply of fertilizer and fish feed would be balanced against a daily yield of produce and fish; cargo transport is essential to this system. Some areas and species will require purchasing fingerling fish rather than restocking through on-site breeding. These barges should be deployed as close as possible to local markets to cut down on transportation costs. They are also meant to be in service for decades, so a modular design with field maintenance is essential. Floats should be redundant and replaceable without a drydock; these could be as simple as surplus 55-gallon drums coated in antifouling paint or as complex as composite-encased foam blocks. Entire tanks should be removable so holes can be patched while dry. People in some parts of the world might prototype this with found materials, probably using wood for most of the structure and metal roofing material for the hydro channels (and possibly the fish tanks). EDPM membranes (pond liners, rubber roofing) are an option for making the tanks watertight, especially since their service life should be in the 20-40 year range. It would be an adventure in ongoing maintenance.

Best of luck to them, both the idea people and those who will end up building the concept on their own.

channel catfish FCR of 1.5
growth schedule:
egg to fry : 8d
fry to 0.87g : 25d
0.87g to 111g : 80d
111g to 714g : 95d
 (assumes FCR 1.5, feeding 3% mass of 32% protein)
300 fish per m² 1m deep (Broussard and Simco 1976)
harvest mass 714g, 30% fillet
no consideration for growth, so water volume is calculated for the harvest size; could be optimized by growing in stages and stocking appropriately.

200 days for 300 fish to reach 714g = 1.071 kg per m³ per day
30% fillet = 321.3g per m³ per day

see for example. There is a lot of information available on aquaculture, particularly in tropical or subtropical climates. FAO has plenty of data on low-tech methods too, though some of their older stuff on combined livestock methods is no longer considered pathogen-safe.

As for vegetables, I have a spreadsheet with a large number of references on yield across a broad selection of species. It's not in publishable shape right now. As a single base example, here's a Texas A&M article on hydroponic tomatoes. The most pessimistic yield number there is 7 kg per plant and 9.37 plants per m², yielding 65.59 kg/m²*year or 180 grams per day. This is given as a range of 7-14 kg per plant, meaning 180-360 grams per day. It seems reasonable that 200 grams per m² per day using mixed species, vertically-stacked plants and highly-soluble nutrients should be a lower bound for productivity.


  1. Why put the system on a boat when you can install it at a coastal fish farm (Asia has a lot of those, built by sectioning off bits of sea with levees)?

    1. Coastal farms work fine until you run out of coast. As a coastal fish farm, the main difference vs. existing fish farms would be much tighter nutrient control. Instead of running an open or partially open system where wastes are washed out to sea with fresh water, this would be a mostly closed system where wastes are recycled into nutrients.