Solar Heating of High Tunnels
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Transcript of Solar Heating of High Tunnels
Ground is slow to warm
Cold soils inhibit plant growth and can cause nutrient deficiencies
Require supplemental heat during cold spells
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Traditional Outside
The high tunnel warmed the air to 80F by 11:00 AM, but the air temperature dropped quickly at night. Nighttime lows in
tunnel equaled outside (empty tunnel).
Heat the soil beneath the high tunnel
Air is heated in solar panels, pumped through tile two lines three feet below the soil surface.
Fans push air from the soil through the solar panels back to tile lines
Fans are controlled by thermostats:
Fan starts when air in solar panel reaches 125F, stops when temperature in soil panel drops to 85F
Two lines of T-tape per bed
“Dick’s Super Soil”◦ Decomposed peat,◦ pH 7.4, high CEC,
low potassium
Cold soil can cause nutrient deficiencies, root diseases, reduced germination, poor growth
Heating the soil could cause the air temperature to be warmer at night
Sun warms soil,
Soil emits infrared radiation, which heats air
Soil continues to emit IR
Infrared transparent Infrared trapping
plastic
Warm soil would emit a large amount of infrared radiation, altering the nighttime temperature of the high tunnel
Unheated Tunnel – April 10-11
Heated tunnel – April 10-11
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Traditional Tunnel Outside
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Heated Tunnel Outside
Solar panels could not keep tunnel above freezing during coldest part of the winter◦ Cold winter -40 two times in January
Soil-heated tunnel was warmer than unheated tunnel◦ 10 degrees warmer (mean temperature)◦ 20 degrees warmer (overnight lows)
Soil temperature: 31.4 Overwintered spinach, kale in heated tunnel
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Temperature in heated tunnelBlue line = airBrown line = soil
Soil rose above freezing on March 4 in heated tunnel Soil rose above 45 on March 15 Nighttime lows stayed above 45 on March 15 10-15 degrees warmer than unheated tunnel between
March 7 and April 15
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Heated Tunnel Traditional Tunnel
Soil temperature in raised bed: March-April
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Heated
Outside
Planted tomatoes and cucumbers on March 15
Tomatoes grew well, but cukeswere stunted
Started cutting spinach and kale first week of March
Radishes in early May
• Started picking tomatoes on June 8
• Started selling tomatoes and cucumbers at local farmer’s markets on June 16
High yields both in tomatoes and cucumbers
Quality was excellent at first
Roll sides manually Put shade cloth on
first week of July Thermostatically
controlled exhaust fan at the peak of tunnel
Core of tomatoes was hard Yellow shoulders Okay for processing, but not for fresh market Gave extra tomatoes at the market Assumed problem was weather related
Gray color right inside the skin
Difficult to see without cutting tomato
Hard center core Not in heirloom
varieties Most likely due to
potassium deficiency
Began in late July Applied weekly foliar
potassium (KDL) and continued to early fall
Problem disappeared three weeks after starting the KDL sprays
Amended soil with greensand
Green beans Radishes Kale Carrots Lettuce
Everything was good except onions
Aphids in peppers, eggplants in August
Controlled with ladybugs
Poor fruit set in eggplants and peppers in spite of enormous plants
Will buy bumblebees early next year
Overwintered KaleOverwintered Spinach
Sales increased 35% in 09
Tomatoes and cucumbers were 8 weeks earlier in heated tunnel than in unheated tunnel
Good year to have a heated high tunnel
All crops were better in the heated tunnel than in unheated tunnel or outside garden
Good quality Few diseases Picked tomatoes
from June 8 to November 15
Special thanks toMinnesota Department of AgricultureSustainable Agriculture Grants
Questions