Structures & Equipment
Bringing down the curtain on winter energy costs
By Duane Van Alstine
Used correctly, energy curtains can lead to cost savings during the colder winter months.
By Duane Van Alstine
The frigid temperatures that accompany a Canadian winter are hardly a surprise, but for a grower, the high energy costs associated with operating a greenhouse through the cold weather can be shocking. As the temperature drops, energy use climbs, and with temperatures falling well below freezing, heating a greenhouse to maintain an ideal crop environment can be an incredibly costly endeavor.
It’s likely that energy costs will continue to increase, so as a year-round grower, finding cost-effective solutions for protecting your greenhouse environment while conserving energy is necessary. It’s for these reasons that more growers are realizing the benefits of energy curtains, an effective and inexpensive method of maintaining the proper climate in your greenhouse during the cold Canadian winters.
Energy curtains are retractable components made from composite materials, and they are designed specifically for the horticultural industry. Also referred to as energy screens or thermal blankets, energy curtains can provide multiple benefits to your growing environment, but a major benefit are the energy savings realized throughout the winter as your curtains work to maintain a consistent environment. The curtains are typically installed just above or below the trusses in your greenhouse, but most often above to accommodate your supplemental lighting. They can be opened or closed across the greenhouse, from gutter to gutter, or they can cover and retract across the length of the greenhouse. How your curtains are installed depends on the needs of your environment, but typically a flat curtain system will be the most inexpensive to install while offering the greatest reduction in space that requires heating.
During the colder winter months, your energy curtains act as a barrier by keeping heat below the curtain in the growing zone. The types of curtains you install will directly determine your energy savings.
The three most common categories of curtain materials are nonporous, porous and semi-porous. Nonporous materials, such as poly film, are generally not ideal for greenhouse curtains as they don’t allow for condensation from the roof to penetrate. The water will pool up on the curtains and the weight of the excess water can cause damage to your curtains or cause the supports to fail. On the other hand, while porous materials allow for condensation to drip through, they do not insulate the growing area well enough. A semi-porous material is generally the most practical choice for both shading and heat retention. They’re usually made of strips of alternating pieces of aluminized and clear polyester or acrylic fabric that’s woven together tightly. Condensation is allowed to get through the material, but there is little exchange of air from either side of the curtains when closed. The aluminized curtains are engineered to reflect the infrared radiation given off by the structure, the benches and the plants back into your greenhouse which is especially important during the winter. Be sure to work closely with experienced industry professionals to decide what material is best for your greenhouse environment.
Using your curtains in the winter
When used properly, your energy curtains can help modify the climate of the growing environment to improve the conditions inside. The curtains can be closed when they’re able to improve the environment, but can be opened if outside conditions are favourable. Ideally, curtains are used for shading if the natural light outside is too great, if energy consumption is too high, or if the humidity is too low. Orchids, for example, are highly sensitive plants that require shading as they can burn if the sunlight allowed into the growing area is too intense, but since light levels are usually lower in the winter, curtains are used more often when light levels cannot keep up with the required temperature. Most greenhouses are made of either glass or poly, neither of which have a high R-value, so the curtains provide the insulation necessary in the cold weather to maintain the desired climate.
Typically, your curtains will be closed completely at nighttime when there is no useable natural light. Since a majority of the heating in a greenhouse takes place at night, having your curtains closed entirely will help retain the heat inside the greenhouse while keeping out the cold, winter air. However, while curtains can certainly provide benefits to your greenhouse during the winter, it’s important not to fall into the trap of overusing them. In order for your curtains to provide the greatest benefit in the fluctuating temperatures of winter, your curtains should begin closing before the sun sets and be opened just after sunrise. These opening times will vary depending on solar radiation levels, but ultimately you need to strike a proper balance and ensure that the curtains are not closed too often. Many growers will install a double layer of curtains which provides even more insulation, and while this may sound costly, for growers with greenhouses in cold climates, the return on investment is quickly earned back in energy savings. When used properly, growers with curtain systems tailored for winter use are seeing energy savings of anywhere from 20 to 75 per cent with nighttime energy savings specifically on the higher end.
Automate your energy curtain system for maximum efficiency
Similar to your supplemental lighting, irrigation and ventilation, you will see the most benefit from your energy curtains if they’re controlled by a central environmental computer. Not only do your energy curtains affect the amount of natural sunlight that’s allowed into the greenhouse, but it affects lighting, temperature, airflow and humidity, which can be a lot to consider if you’re only using a manual or semi-automatic operation. You need to set your parameters to control exactly what temperature you want the inside of your greenhouse to stay at, while also considering radiant heat and heat from supplemental lighting. It’s recommended that you look back at your controls over as many seasons as necessary, and look at the tendencies and averages of the winter season to determine how you should set your parameters for the current winter season. Some growers set their curtains to open or close based on light levels in the greenhouse so they don’t have to change their parameters seasonally. However, it’s equally important to set your parameters based on temperatures to ensure your curtains will help maintain a consistent climate in your greenhouse, despite fluctuating temperatures outside. The most important thing is to not become complacent with the parameters you have set. The biggest mistake a grower can make is not making the necessary changes to their controls seasonally, which can have negative effects on their crops and can result in a lot of wasted energy.
Ultimately, the curtain system and material that work best in your greenhouse will depend on various factors including the crop environment, the style of greenhouse and the regional climate of the area your greenhouse is in. There are numerous options available, so be sure to work with knowledgeable professionals to ensure you’re getting the best curtains at the best value for your needs.
Duane Van Alstine has been in the horticulture industry for 20 years, previously having been the operations manager of a 380,000 square foot facility. In 2011, he left the greenhouse to take on Special Projects for GGS Structures Inc. For more on energy curtains, visit ggsstructures.com