Section 5: Calculating Production Surface Area and Space Usage Efficiency
In addition to designing the individual benches or production surfaces, the arrangement of the benches is important. Besides affecting traffic flow within the greenhouse, the benching arrangement affects the amount of space available under cover for production of crops (space usage efficiency). A greenhouse that has 10,000 ft2 under cover will not necessarily have 10,000 ft2 available for the production of crops. Some of the space will be used for walkways, work areas, and greenhouse systems. The space available for crop production should be maximized because this is the income generating area.
It is important to be able to calculate the square footage in a greenhouse, the square footage of production area under cover and the space usage efficiency. These calculations can be performed using a few simple geometric equations.
For rectangles (including squares), the length of the structure multiplied by the width of the structure provides the total number of square feet within the structure. As an example, we will determine the total square footage of space and the space usage efficiencies for two different bench arrangements in a greenhouse structure that is 38 ft wide and 60 ft long. In the first benching layout, the side benches (running the length of the greenhouse) are 2 ft wide and extend the entire length of the structure. There are 3 benches spaced across the greenhouse that are 6 ft wide. The central walkways between the benches are 4 ft wide and the end walkways are 5 ft wide.
The total area under cover is:
38 ft x 60 ft = 2280 ft2
The area in benching (production surface area) is:
2 x (2 ft x 60 ft) = 240 ft2
3 x (6 ft x 50 ft) = 900 ft2
240 ft2 + 900 ft2 = 1140 ft2
Therefore, the space usage efficiency for this benching layout is:
(1140 ft2 ÷ 2280 ft2) x 100 = 50%
If we put rolling benches into the same greenhouse structure, our production surface area and space usage efficiency changes. In this layout, there are two 2 ft wide benches along the length of the greenhouse. There is one 4 ft aisle. There are six 5 ft wide rolling benches. As before, there is a 5 ft walkway at the end of these benches on each end of the greenhouse.
The overall area under cover remains 2280 ft2
The two side benches still provide 240 ft2
The rolling benches provide 6 x (5 ft x 50 ft) = 1500 ft2
The total area in benching is 240 ft2 + 1500 ft2 = 1740 ft2
The usage efficiency is 100 x (1740 ft2 ÷ 2280 ft2) = 76%
By using rolling benches, the space usage efficiency increased from 50% to 76%, and we gain 600 ft2 of bench space that can generate revenue. We also created 600 ft2 of space over which we can spread certain production costs. In a purely hypothetical scenario, if the business is paying $5,000 per year on the cost of the structure, approximately $13.70 must be generated per day just to pay for the greenhouse. In the first benching scenario, there were 1140 ft2 of space that could generate revenue. Therefore, each ft2 of bench space must be charged approximately $0.012 per day just to pay for the greenhouse structure. A crop that occupies 1 ft2 of space (and thus each plant) for 6 weeks must be charged $0.50 to pay for that space. In the second scenario, each ft2 needs to be charged $0.008 per day. Therefore, the same crop would be charged $0.34 per plant. The business would save $0.16 in production costs per plant. These savings would go directly to the profit line of the business. All other fixed costs and some variable costs (i.e. heating) are accounted for in this way. Therefore, maximizing space usage efficiency reduces costs.
You are now at the end of Unit 2: Benching Systems. Click here to take the self-exam.
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