Energy saving technologies in greenhouses from a plant viewpoint
Part 2: Methods for reduction of energy costs.
January 31, 2023 By Dr. Mohyuddin Mirza
The primary goal for any greenhouse operator is to reduce energy costs per dollar value of crop produced, may it be vegetables, bedding plants, ornamentals, tree seedlings, strawberries or cannabis. Increasing crop yields and quality is another way of reducing energy costs. To emphasize the point of higher yields, the following table correlates gross margins sensitivity analysis of cucumber production and price of natural gas.
Table 1 shows that if the production of cucumbers is 80 fruit/plant then your dollars per sq.foot net revenue would be in negative at a gas price of $ 5.50/GJ. The figures were prepared at a certain price level and the actual numbers will change with the price of cucumbers.
In order to go to a positive revenue at $ 9.50/GJ of gas price, one has to produce 100 cucumbers.
The point is that in order to offset higher heating costs or other input costs, prepare a sensitivity table like this and know how many cucumbers need to be produced to pay for increases.
I know some growers produce 80 cucumbers per plant, get better price because of direct retailing and are able to absorb additional heating or energy costs by increasing their price. In the wholesale market, the growers have less control over the price they need to pay for the operating costs.
Methods for reducing energy costs:
There are three major ways to reduce energy costs.
- Improved maintenance of existing equipment and structures/buildings
- Capital investments to improve energy efficiency
- Switching to a lower cost fuel wherever possible.
Table 2 summarizes methods to reducing greenhouse energy costs.
Tips to reduce energy costs:
Here are a few more tips to reduce energy costs in a greenhouse operation. The possible impact on greenhouse crops productivity should always be kept in mind when considering methods to reduce energy costs. There have been many instances where growers started using much cooler temperatures to reduce energy use which resulted in plants becoming too vegetative and delay in crop maturity.
That is the first step in energy savings. The picture on page 27 shows a neglected greenhouse where the grower got side-tracked to do other things and could not plan properly. I don’t need to discuss this greenhouse more but you get the message.
The grower had also excuses to justify why the greenhouse was in this shape. The plastic did not come in time, then I got busy, then the bedding plants season started, and now, I will fix it after the season.
The grower was able to complete his bedding plant production cycle in this state of the greenhouse but a lot of energy was wasted.
Thus, proper maintenance of the greenhouse structure is essential and the first important component of energy management. Even a well-constructed double poly greenhouse will have 0.5 to 1.0 air change per hour.
Greenhouse maintenance to reduce energy costs includes repairing leaks in all areas of the outer structure, ensuring proper inflation of a double poly roof and regular replacement of roof poly to maintain anti-condensate and Infra Red (IR) properties.
Good maintenance and inflation of double poly roof systems should get the highest priority as 60 to 80% of a greenhouse structure heat loss is through the roof.
Use of proper greenhouse roofing materials is very important. It is worth mentioning that greenhouse roofing materials keep on changing based on several factors sometimes not related to heat loss properties.
In Alberta in the 1980s, glass was the most commonly used roofing material and every few years hail storms would devastate the greenhouse structures. That resulted into a switch to double poly structures from 1990 till about 2004. That is the time when growers shifted their focus to winter production and start switching to much improved glass to get maximum light transmission.
The above pictures highlight the transitions from double poly roof to glass structures to get better sunlight, high Photosynthetically Active Radiation (PAR) transmission, and low vapour condensation.
Energy screens have to be managed properly to reduce heat loss.
Thus, the point is made that a greenhouse structure should be well maintained and proper glazing or roofing materials be used to get more natural light into it and minimize heat loss.
An audit of heat loss through the structure is very useful tool to determine and plan for energy management.
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