Solar Panels Inside a Greenhouse for Transplant and Crop Production Funding Purpose System Overview Results Uses Benefits Cost/Payback Lessons Learned Calculation Tool Funding • VT REAP Grant Renewal Energy for Ag. Grant Program Purpose • Use sun, water, and wet soil to grow plants out of season • Increase sustainability of farm by decreased fossil fuel use • Create tool to replicate system System Overview Results Agribon Cover Germination Chamber Greenhouse High 108.7 109.9 Low 7.6 2.8 Average High 83 83.4 Average Low 29.4 26.4 Reflectix Cover Germination Chamber Greenhouse High 116.4 114.1 Low 41.2 24.9 Average High 90 92.8 Average Low 50.5 41.3 Uses Transplant Examples • Spinach 2/15 • GH Eggplant 3/5 • Field Tomato 4/5 • Winter Squash 5/3 Uses Crop Examples Ginger 3/11 Benefits Heating small volume is more efficient Decreased fossil fuel use/ CO2 output Cold weather growing capacity Bottom heat for plants Using stored energy provides a buffer Low maintenance/ operation cost Cost/Payback Initial investment Materials $6850 Labor (100hrs@ 30/hr ) $3000 Total $9850 Operating costs Electricity (3KWh/day @ .12/KWh) $.38/day $.38 X 365 Days $138.70/yr. Energy Collected Average of 3.3 kWh/day x .$.12/kWh $.40/day $.40 x 365 days $146/yr. Cost/Payback Payback from Energy saved… With labor $9850/$146 67.5 yrs. Without Labor $6850/$146 47 yrs. Payback with transplants… 90 trays @ 10 6-packs/tray 900/6-packs Cost of production $500 Gross Sales @ 3.50/6-pack $3150 Net profit $2650 System with labor $9850/$2650 3.7 years * Payback time decreases with added crop or successions of seedlings. Tool Energy Use Calculation Tool for Solar Heated Soil Table User inputs highlighted in yellow Calculated Values in Blue Important notes highlighted in green Soil Table Calculations Oct. - April Units Notes Soil Table Length 40 ft User Input Soil Table Width 4 ft User Input Soil Table Depth 1 ft User Input Soil Table Temperature to be Maintained 45 °F User Input Total 24 Hour Heat Loss 9670 Btu Q day + Q night Thermal Storage Calculations Water Storage Tank Length 3.10 ft User Input Water Storage Tank Width 3.10 ft User Input Water Storage Tank Depth 3.75 ft User Input Water Storage Tank Insulation Thickness (Blue Board) 4 in User Input Water Storage Tank Volume 270 gal Weight of Water in Tank 2248 lbs Water Storage Thermal Mass Available for Heating 86842 Btu Q=(MCpΔT)-Heat loss Soil Table Weight 9920 lbs Based on Wet Soil 62lb/CF Soil Table Thermal Mass Available for Heating 121520 Btu Q=MCpΔT Total Thermal Mass 208362 Btu Q of soil + Q of water tank Note: Soil Table Heat loss is accounted for in 1st section, water storage tank heat loss is accounted for in second section. Energy Input Calculations Heat Loss from Storage Tank (BTU per 24 hours) 3084 Btu/day Heat Loss from Soil Table (BTU per 24 hours) 9670 Btu/day Assumed Heat Loss from Piping (10% of total) (BTU per 24 hours) 1275 Btu/day Total System Heat Loss (BTU per 24 hours) 14029 Btu/day Solar Heating Inputs Rated Thousand BTU/panel a day 9.4 KBtu Use SRCC data for Water Heating (Cool Climate) Mildly cloudy Solar Collector Efficiency 81% Manufacturer's data Total Number of Solar Panels for daily loss 2 Total Number of Solar Panels for 3 day loss (assuming some cloudy days) 6 Total Number of Solar Panels to maximize hot water tank mass 13 Total Number of Solar Panels to maximize soil table mass 18 Total Number of Solar Panels to maximze tank and soil table mass 31 Backup Heat Source Energy Requirement Heat Loss from Seed Table (BTU per 24 hours) 9670 Btu/24 hours calculated above Heat Loss from Seed Table (Watts per 24 hours) 2826 Watts/day (Btu*0.012178)*24 Heat Loss from Seed Table (BTU per hour) 569 Btu/hr (Btu/day)/17 based on heat loss for 17 hrs Heat Loss from Seed Table (Watts per hour) 166 Watts/hr (Watts/day)/17 based on heat loss for 17 hrs Lessons Learned Greater capacity for thermal storage in the soil of the germination chamber than in the water of the thermal storage tank. It would be worthwhile to research redesigning the system to exclude the hot water storage tank and solely rely on the thermal storage capacity of soil. Based on the data collected, it appears that to maintain the soil temperature up to 43°F passive solar gain alone is adequate. This implies that a well-constructed insulated germination bed could suffice. Efficiency of design related to maximizing solar energy gain. The observed data shows many days where it was hot enough to collect solar energy but there was no energy gain in the system. The reason for this is because the water in the storage tank was hotter than the temperature in the solar panels.
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