A theoretical comparison of costs between greenhouses and indoor farms: a case analysis in Ohio

Leafy green production under controlled environment is a fast growing industry sector in North America. Various production systems are used but information on the productivity and costs is limited. Therefore, a site-specific theoretical analysis compared operational costs to produce leaf lettuces in two selected types of systems designed to achieve a similar production capacity: 1) a modern hydroponic greenhouse with supplemental lighting and 2) an indoor hydroponic farm with sole-source lighting. Both have the same growing surface area (1028 m(2)) located in Columbus, Ohio. A series of assumptions were made for simpler comparisons applicable to other cases. Preliminary analyses suggested that the distinct difference between the two systems would be for heating costs and electricity costs for lighting and cooling. Costs for supplies, materials, and labor are assumed the same regardless of the system and that other cost differences for items such as water exist but are negligible. Both facilities can operate year-round maintaining daily light integral at a constant level (12 mol m(-2) d(-1)) by shading (greenhouse only) and lighting. Air temperatures are maintained at 21/16 degrees C day/night except summer months in greenhouse. Because of the possible variation of harvested head weight of lettuce grown in greenhouse, all the costs are expressed as per-head values. The results showed that production costs were always higher in the indoor farm than in the greenhouse examined in our analysis, with an annual average difference of $ 0.12 per head. The difference becomes lower in winter ($0.01-0.07), due to intensive heating and supplemental lighting in greenhouse. Although the difference in capital expense can be large, the potential difference in operational costs seems to be relatively small especially during cold months. Strategies to further reduce costs such as improving lighting efficiency or CO2 enrichment are discussed based on sensitivity analyses.