High humidity levels inside greenhouses are a byproduct of growing crops indoors.
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Left uncontrolled, humid air can reduce crop growth and result in poor quality produce, so growers have traditionally resorted to ventilation to manage the issue. While effective, this strategy also causes heat loss, which can increase a farm’s energy costs.
The Ontario chapter of Flowers Canada aimed to find a solution to this challenge. It obtained funding through the Greenhouse Renewable Energy Technologies Research & Development Initiative several years ago to test four different energy recovery technologies for their potential to reduce grower energy needs during the peak greenhouse use period of fall through early spring.
This included a mechanical refrigeration dehumidifier (MRD), a liquid desiccant dehumidifier (LDD) that runs humid air past a brine solution to absorb moisture and then heats the brine to regenerate it, and a heat recovery ventilation (HRV) system outside the greenhouse that warms cool, dry air as it enters the facility.
The fourth technology is an energy recovery ventilator (ERV) prototype that combines the liquid desiccant approach with heat exchange into a single system.
“We are looking for alternative ways to decrease energy consumption to both reduce grower costs and reduce fossil fuel use,” says Jingjing Han, research engineer with Flowers Canada.
Building on findings from the initial project, Flowers Canada obtained funding from the Greenhouse Competitiveness and Innovation Initiative, a cost-share program funded by the Ontario government and delivered by the Agricultural Adaptation Council, on behalf of the Ontario agriculture department.
The funding was used to conduct more in-depth research into the four technologies and better understand how they can integrate into existing greenhouse control systems.
An Ontario flower greenhouse had three systems — MRD, LDD and HRV — installed in 2018, an herb greenhouse had four LDD units installed and a tomato greenhouse in Leamington had an ERV system installed.
Despite setbacks with malfunctioning units that couldn’t be fixed due to supply chain shortages and changes in crops and production strategies, the researchers were able to gather and analyze enough data to make useful assessments.
“All systems are able to control humidity much better than conventional ventilation, but each has their own advantages and disadvantages,” notes Han, adding that none of the systems were effective throughout the entire year.
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The LDD and MRD systems were found to be most cost-effective during the late fall, winter and early spring, but their efficiency drops if the outdoor air temperature is above 10 C or high in humidity. HRV provides growers with economic benefits during cold and dry months, and the ERV system is effective at reducing condensation on the glass greenhouse cover.
According to Han, there are two things greenhouse growers should take from the research:
- Any of the systems can be useful tools for humidity control with reduced heat loss, but they must be properly integrated into the greenhouse’s internal control system to operate cost effectively.
- Energy and cost savings are most impactful from October to March. Some systems can result in energy cost savings of more than 10 per cent during January, February or March, but the relative price of energy (e.g., natural gas vs. electricity) plays a significant role relative to cost savings among the dehumidification systems.
Project partners included the Ontario Greenhouse Vegetable Growers, Enbridge, Nortek Air Solutions and participating farms.