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Pointers on Greenhouse Heating and Energy Conservation

Part 1: Greenhouse Location and Design

Autumn has just arrived and with it we are noticing that day length is shortening.

This is the time of the year where our free energy source, sunlight, becomes limited. Sunlight is essential to maintain the global temperature in the ranges appropriate for plant growth. In winter, plant production is challenging because temperature and solar radiation decline to their minimum values. In addition, there are several cloudy days which further diminish photosynthesis. Nevertheless, greenhouses are used to minimize the negative effects of those environmental constraints. As winter approaches, the need for heating increases. The energy consumed to heat a greenhouse can be as high as 80% of the total energy required to run a greenhouse business.

The heat loss or gain in a greenhouse occurs as conduction, convection, radiation and infiltration. Conduction occurs when heat is transferred between two bodies in direct contact. Convection occurs when the heat is transferred via a fluid like water or air to the surface of an object. Radiation occurs when the heat is transferred without any physical contact or with the help of a fluid i.e. solar radiation. The last category is infiltration, this occurs when there is an exchange or leakage of air between the inside and the outside. In order to fully understand the different ways to save energy for heating, it is essential to study how to conserve energy. Here are some strategies:

Greenhouse location. Starting in summer, the day length decreases as latitude increases. For example, on December 21st the day length in New York (Latitude: 40˚N) is 9 hours and 15 minutes and the solar radiation for a sunny day would be 1555 J/cm2 during this day. On the other hand, in Queretaro, Mexico (Latitude: 20˚ 35’ N), for the same day, the day length is 10 hours 53 minutes, solar radiation for a sunny day would be 2783 J/cm2 during this day. There is almost twice the amount of solar radiation energy received in Queretaro, Mexico, than in New York, and as a result we can expect lower temperatures and greater heating costs in New York than in Queretaro, Mexico.

Single bay and gutter connected greenhouse orientation. In order to produce crops in the winter months in northern latitudes above 40˚N, a single bay greenhouse should be oriented east-west but the rows have to be oriented north-south to minimize shadings. For gutter connected, the greenhouses and the plant rows have to be oriented north-south.

Single layer vs. double layer covering.  It is a fact that single layer coverings lose more heat than double layer. Each covering material has different heat transfer values. In greenhouse agriculture, the units used to measure the amount of energy to heat or cool a pound of water by one degree Fahrenheit is called British Thermal Unit (BTU). To demonstrate the energy loss differences, let’s assume there are two greenhouses: one with single layer polyethylene and another with double layer polyethylene. The outside temperature is 20˚F and the inside is maintained at 60˚F during the night. The energy loss through the single layer polyethylene would be 335,000 BTUs, but for the double layer polyethylene, it would be 200,000 BTUs. By using polyethylene with infrared barrier and anti-condensation surface treatment, heat loss can be further reduced.

Greenhouse covering. The greenhouse coverings have different heat loss coefficients. For example, using the same parameters as above, the heat loss for a greenhouse covered with an 8mm twin wall polycarbonate would be 183,000 BTU. The savings would be 45% and 9% compared with a single layer polyethylene and a double layer polyethylene, respectively.

Single bay vs. gutter connected greenhouse heat loss. A single bay greenhouse will lose more heat than a gutter connected greenhouse because the single bay greenhouse will have more surface area exposed to the outdoors. For example, if there are three greenhouses similar as the above greenhouse, the heat loss for a single layer polyethylene would be 335,000 BTUs times three single bay greenhouses equals 1,005,000 BTUs of energy loss. On the other hand, if those three greenhouses are gutter connected, the heat loss would be 785,000 BTU’s, or 22% less than the three single bay greenhouses.

Insulation. Insulation materials can be placed at the base of the walls of the greenhouse (even below the surface of the soil) up to the plant height. The insulation material can be as simple as extruded polystyrene.

Infiltration or leakage. The infiltration rate for a greenhouse is measured as air exchanges per hour. A new greenhouse with glass covering will have 1.5 times the amount of air exchanges per hour when compared to a double layer polyethylene film. An “old construction”, glass greenhouse in good condition typically has two times the amount of air exchanges per hour compared to a new glass covered greenhouse and four times more if the “old construction”, glass greenhouse is in poor condition. It is impossible to avoid infiltration, but it is possible to minimize it by sealing holes to the outside and by tightly closing vents, louvers and doors.

Vent Louvers Pointers on Greenhouse Heating and Energy Conservation.JPG

“Poorly sealed vent louvers can cause significant air infiltration and heat loss.
Source: Premier Tech Horticulture.”

Windbreaks. It is recommended to install windbreaks around the greenhouse facility. The windbreaks can be trees, although with anything that is used, they must be placed at a certain distance to avoid shadows especially during winter on the south side. The windbreaks serve as a physical barrier for the wind to decrease air velocity and subsequent air infiltration.

Energy blankets or curtains. On a clear night, heat energy is lost rapidly from the earth into space via long wave radiation. On the other hand, energy loss diminishes during a cloudy night because the clouds trap long wave radiation, serving as a natural blanket. Likewise, energy blankets or curtains installed in greenhouses serve to reduce heat loss at night. They are installed just below the gutter and are retractable so they can be closed during the night and retracted during the day. When closed, there is less volume air to heat and the long wave radiation bounces back to the leaves of the plants and other surfaces. During summer, the energy blanket can serve as a shade, thus decreasing the heat load in the greenhouse. Energy curtains are often made from aluminized polyester and depending on how they are woven, they can be porous, semi-porous or impervious to moisture.

Shade curtain Pointers on Greenhouse Heating and Energy Conservation.jpg

“Partially retracted shade curtain will help keep heat in the greenhouse at night.
Source: Premier Tech Horticulture.”

Environmental controls. It is important to do routine maintenance and calibration of temperature sensors and/or thermostats. When the temperature sensor does not read the appropriate air temperature, the heaters can run too long or too frequently or they do not heat when needed. When the heater is running too infrequently and the temperature is too cold, plants grow more slowly and it is possible to have problems with insects and diseases. On the other hand, when the heater is running too often, the extra heat generated is often not cost-effective and is therefore wasted. The temperature sensors have to cover the entire greenhouse area in order to have a better heating efficiency.

Although it is easier to save energy if the greenhouse is properly designed from the beginning,  the strategies described in this article provide some of the techniques used to save energy. It is very important to maximize heat energy savings because the price of fossil fuel is volatile, even though the price of oil is currently at its lowest level since March 2009.

In part 2 of this series, we will describe the strategies for heating a greenhouse with the use of fossil fuel, electric energy and alternate fuel sources.

PRO-MIX® is a registered trademark of Premier Horticulture Ltd.

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