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EXCLUSIVE – details on Minnesota greenhouse-algal biodiesel-aquaculture project

August 9, 2012  By Treena Hein

A new greenhouse in Minnesota in the Silver Bay Eco-Industrial Business Park uses nutrient waste from aquaculture.

Energy Edge has exclusive details on a recently-opened energy-innovative greenhouse in Minnesota that integrates algae-produced bio-diesel and aquaculture. It’s part of the Silver Bay Eco-Industrial Business Park, and the University of Minnesota is a project partner.

The answers below were provided by Silver Bay Eco-Park Project Coordinator Bruce Carman.

How much bio-diesel is being produced with the algae on a monthly or annual basis? How big are the tanks?


Carman: We are currently estimating that we will be able to harvest 8000 gallons of ‘algal oil’ annually from this facility.

The ‘algal oil’ is actually a byproduct of our Aquaculture-Aquaponic-Algal Growth water flow system. The primary purpose of the algae is to clean the water of the fish waste so that the water can be reused within the system.

There are nine tanks containing fish within the system.  Each tank is 8 ft in diameter and 5 ft high. There are also four troughs 16 ft wide x 40 ft long x 3 ft tall. Three of the troughs grow produce and the fourth grows algae.

Originally, the ‘algal oil’ was to be used to heat the greenhouse, but this has changed. 

Carman: The ‘algal oil’ that is harvested can be used for biodiesel processing, but can also be used for the production of cosmetics as well. We [have currently] decided to use a natural gas back-up generator [for our project] due to the sensitivity of the water-flow system. Should we have a problem power, we only have 10 minutes to react and get the water flowing again or we start to suffer fish loss.

Because biodiesel generators are relatively new, we did not think we could take the chance [right now]. If we did decide to use a pure biodiesel generator, we were leaning to a CAT 25kw. Because the water flow system within the structure is almost entirely a ‘gravity flow’ our power requirements are minimal and therefore only one generator is required.

How much greenhouse space do you have? What crops are you growing?

Carman: The overall size of the structure is approx. 8600 ft2.  Of that, approx. 5600 ft2 is greenhouse and that is where the four troughs are located. The troughs will used to grow produce and herbs such as lettuce, parsley and cilantro.

Is the power from the generators being used for cooling this summer (or will they just be used for greenhouse heating in the winter)?

Carman: The specific purpose of the generator is to supply back-up power in case of a power outage. The water flow system is heated through the use of wood pellets and 2 200,000 btu wood pellet boilers. Because of the quantity of water within the system (55,000 gals), the water is the actual heat source medium for the facility. The facility is located on the shores of Lake Superior and therefore air conditioning is not a requirement currently.

How many fish are involved?

Carman: Each tank holds about 2000 lbs of fish at harvest time. The facility was designed so that there would be a monthly harvest of fish and that is why there are nine tanks. In our enlarged weekly harvest facility, there will be a total of 36 tanks.

Please describe how you are using fish waste as greenhouse fertilizer.

Our system is all based on the flow of water. The water in the fish tanks becomes loaded with organic nutrients from the fish waste. The water flows from the tanks through some mechanical filtering for solids and pH level controls and then into the produce troughs where the plants absorb the nutrients, essentially cleaning the water, and then into the algal growth trough where the algae further clean the water and re-oxygenate it. From the algal trough, the water flows to a sump pit, where it is pumped back to the fish tanks.

For the original recent Energy Edge story on this project, visit here.

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