100 per cent renewably-powered Alaskan greenhouse
By Treena Hein
The Chena Fresh hydroponic greenhouse, which uses LED lights and other energy-efficient technologies, is powered completely by renewable energy. So is the rest of the site where it is located – Chena Hot Springs Resort, about an hour’s drive north of Fairbanks, Alaska.
Chena Hot Springs Resort has the only geothermal district heating system operating in Alaska. The current system serves all 46 buildings onsite using 165°F water at 250gpm. The first geothermal well at Chena was drilled in 1998.
The greenhouse, built several years ago, was again showcased at the 7th Annual Chena Hot Springs ‘Renewable Energy Fair’ on August 26, 2012. Almost 800 people attended, including congressional delegates.
Chena installed a small test greenhouse in 2004. From Chena’s website:
“The hoop house is divided into two areas; an 800ft2 grow area and a 200ft2 air-mixing area. A 2″ radiant fin tube is used around the perimeter of the grow area. 165°F water is pumped from one of the geothermal production wells through the fin tube. The same water is also pumped through a radiant air exchanger in the air-mixing area. The water from both heat sources is then returned into the outdoor Rock Lake. The exterior of the hoop house is 6 mil greenhouse poly-film. A second layer is added on during the winter season. Warm air from inside is then blown in between the layers to inflate the roof and end walls creating an insulation of warm air. Cold air in the winter season is drawn into the air-mixing area and mixed with the warm air from the radiant air exchanger. The warm air is then transferred into the grow area via a ventilation tube. This method prevents any cold air from coming into contact with the plants. A heated arctic entry is also used to prevent large amounts of heat loss or too much cold air from entering at once.”
(Chena’s new greenhouse – described below – in winter.)
Chena then constructed a 6000 ft2 greenhouse to provide the resort’s restaurant with a greater variety of fresh produce on a year-round basis. They also sell the produce to other local restaurants and one market. Chena produces hydroponic lettuce, tomatoes, herbs, fodder (animal feed), and small fruits for the restaurant, employee meals and livestock. More than 500 heads of lettuce a week, and annually 25,000 lbs of tomatoes, 275 tons of fodder, and 400 lbs of zucchini are produced.
“In our new greenhouses, the concept of the hoop house was taken one step further once we had demonstrated successful results in the original house. Two Poly-Tex, XA-300, gutter-connected greenhouses, with a total area of 4,320ft2, were erected on a radiant heated concrete slab. The same 165°F water is pumped through the concrete slab. Each 30′ x 72′ greenhouse has its own environment controller and is maintained at different temperatures. One greenhouse is used for growing hydroponic tomatoes.
There are approximately 450 tomatoes consisting of six Dutch varieties; one cherry tomato variety, one grape tomato variety, one beefsteak tomato variety, and three intermediate cluster tomato varieties. These tomatoes are grown using a Dutch technique in Dutch buckets with perlite and drip irrigation. The other greenhouse is used for growing hydroponic lettuce using a NFT (Nutrient Film Technique) bench system. This bench system is capable of growing approximately 2,000 head of lettuce on a rotating schedule, producing approximately 100 head of lettuce per week. The nutrient solution is monitored and adjusted by its own controller also. We are currently growing four different varieties of lettuce and plan to add a few more varieties and some herbs.
We recently completed Phase II of the new greenhouse project. This phase includes two aspects; additional heating for the winter season and a 28′ extension to both greenhouses. Radiant fin tubes will be added along the perimeter of each greenhouse. The 28′ extension will work in the same way as the air-mixing area of the hoop house. This extension will also be utilized as a work area, display the current projects in the greenhouse, and offer public viewing into the grow area.
In Alaska, short summers offer long days (16-22 hours) of natural light with little or no night period. Meanwhile, winter requires 100% crop lighting October till March with little to no natural light. The result of ongoing research activities, plant varieties best suited for Chena conditions have been selected, and continually evaluated for peak production.
Chena’s commercial designed, double-poly greenhouse is suited to maintain a growing temperature of 70-75°F for tomatoes, and 60-65°F for lettuce when the outside temperatures drop below -60°F a di_erence of 130°F protected by a merely a thin plastic wall. Outdoor summer temperatures often reach 80°F, and if uncontrolled, the greenhouse temperatures will quickly exceed a detrimental 120°F. Controlled environment management and production techniques are utilized to maintain a proper environment for the crop throughout the year.”
“Chena Hot Springs is also working in partnership with the University of Alaska Forestry and Agriculture Experiment Station on a controlled environment research project for investigating the opportunities of establishing similar projects throughout the State of Alaska, using geothermal or other waste heat streams.”
Other site features
A water ram provides gravity-fed water for the greenhouse. Water rams have been around since 1797, when they helped gold miners to supply water pressure to sluice boxes or dredges. The 38°F water comes from a small dam on a nearby creek and fills a supply tank, which is not necessary for the ram pump, but it leaves the option to use pressurized water for other projects. From the supply tank, water goes through a drive pipe to the ram pump. The water ram pumps 20 per cent of the water into a 4200 gallon water tower, and the remaining 80 per cent goes back into the creek. The water ram pumps 1200 gallons per day. The water is used for drip irrigation in the greenhouse and gardens.
Solar water heaters are also used at Chena. These are close-coupled solar domestic water systems made of all-glass evacuated tubular collectors with heat pipes, which are ideal for use in small units, houses and other applications where a cheap, reliable hot water supply is required. They can also be used together with an auxiliary electric boost, thus providing hot water even on a low-sunshine day.
Chena also has a plastic-to-oil machine is able to process polyethylene, polystyrene and polypropylene (all plastics numbered 2, 4, 5, and 6) into gasoline, kerosene, diesel, and a heavy oil called mixed oil.
Keep visiting Energy Edge for more on northern greenhouse development.