High humidity levels inside greenhouses are a by-product of growing crops indoors. 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.
Liquid desiccant dehumidifier installed inside a greenhouse. (Image courtesy of Flowers Canada).
Flowers Canada (Ontario) set about to find a solution to this challenge, accessing funding through the Greenhouse Renewable Energy Technologies (GRET) 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 the moisture and then heats the brine to regenerate it, and a heat recovery ventilation (HRV) system that is located outside the greenhouse and warms up the 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,” explains Dr. Jingjing Han, Research Engineer with Flowers Canada (Ontario).
Building on findings from that initial project, Flowers Canada was successful in receiving funding from the Greenhouse Competitiveness and Innovation Initiative (GCII) to conduct more in-depth research into the four technologies and get a better understanding of how they can integrate into existing greenhouse control systems.
“Finding ways to reduce energy costs and increase efficiencies is a top priority for Ontario’s greenhouse growers,” says Ontario Minister of Agriculture, Food & Rural Affairs Lisa Thompson. “By continuing the implementation of programs such as the Greenhouse Competitiveness and Innovation Initiative, we are furthering our investments with the greenhouse industry, finding ways to reduce their carbon footprint, and supporting farmers as stewards of our environment.”
An Ontario flower greenhouse had three systems – MRD, LDD, and HRV – installed in the GRET project in 2018, an herb greenhouse had four LDD units installed, and a tomato greenhouse in Leamington had an ERV system installed.
Despite some 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 some useful assessments about the technologies being tested.
“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.
The LDD and MRD systems were found to be most cost-effective during the late fall, winter, and early spring, but their efficiency drops off if the outdoor air temperature is above 10C or is high in humidity. HRV provides growers with economic benefits during cold and dry months, and the ERV system is effective at helping to reduce condensation on the glass greenhouse cover.
According to Han, there are two things greenhouse growers should take away 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 of the systems can result in energy cost savings of more than 10% during months like 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.
This project was supported through 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 Ministry of Agriculture, Food and Rural Affairs (OMAFRA). Project partners included the Ontario Greenhouse Vegetable Growers, Enbridge, Nortek Air Solutions, and participating farms.
“It is not feasible for an individual grower to do this type of research with this range of technologies on their own, and we couldn’t have done the project without the funding we were able to access from GCII,” concludes Han.
For more information:
Agricultural Adaptation Council