Solar Panels Inside a Greenhouse for Transplant and Crop Production

Solar Panels Inside a

Greenhouse for Transplant

and Crop Production

Funding

Purpose

System Overview

Uses

Benefits

Cost/Payback

Lessons Learned

Calculation Tool

Results

• Use sun, water, and wet

soil to grow plants out of

season

• Increase sustainability

of farm by decreased

fossil fuel use

Funding • VT REAP Grant

Renewal Energy for Ag. Grant Program

• Create tool to replicate

system

Purpose

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

Crop Examples Ginger 3/11

Uses

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.

Payback from Energy saved…

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.

Cost/Payback

With labor

$9850/$146 67.5 yrs.

Without Labor

$6850/$146 47 yrs.

Payback with transplants…

Tool

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

Energy Use Calculation Tool for Solar Heated Soil Table

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 InputsRated 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 RequirementHeat 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.