Pointers on Greenhouse Heating and Energy Conservation
Tuesday, September 12, 2017 | Jose Chen Lopez
Part 2. Heating Systems
In the previous article, we covered several strategies to conserve energy and how to avoid excessive heat loss. With increase concerns about global warming, it is very important to use the least amount of fossil fuel for energy to minimize the impact on our fragile environment.
In this article, fossil fuel refers to natural gas, LP gas, coal or fuel oil.
For growers, it can be very difficult in winter months to maintain the greenhouse at ideal temperatures for plants, even during the day. During winter months, the strategies to conserve energy may not be enough to keep the air temperature within an optimum range, therefore, the use of fossil fuel is necessary to heat the growing environment. The method to increase air temperature inside the greenhouse has to be affordable, efficient and safe to use. In other words, the capital invested has to be recoverable, the heat source must be close to the plant, support the entire growing area, maintain temperatures high enough to promote active growth, and finally, the fuel source has to be safe.
Central heating systems:
- Heat is transferred from the hot water pipe to an object; this type of heat transfer is considered radiant heat.
- This type of heating systems uses a boiler to heat water or to produce steam.
- Boilers can burn fuels like natural gas, wood, coal or fuel oil.
- Hot water systems are more efficient than steam systems.
- One by-product of the boiler is CO2. This gas can be trapped and delivered into the greenhouse to help the plants in the photosynthesis process.
- This system is recommended for large operations due to high installation and hardware cost.
- The warm water or steam has to be transported in pipes throughout the greenhouse.
- The efficiency can be higher than the use of forced air heater units because the greenhouse is heated uniformly; there is little need for ventilation.
- The pipes can be placed around the inside perimeter of the greenhouse in the case of a single-span greenhouse. For a gutter connected greenhouse, it is recommended to install pipes close to the gutters (to melt ice) and/or under growing benches.
- The water temperature for overhead systems can be up to 200˚F (94˚C) while the water under the benches can be up to 104˚F (40˚C).
- Floor heating is another method to heat the greenhouse with hot water. With this method, pipes are placed in the ground under a layer of concrete, gravel, sand or soil. Heating pipes form loops around the entire surface of the greenhouse. The water temperature can be up to 104˚F (40˚C).
- These systems require time to reach the desired temperature; similarly, it will take time to decrease the temperature.
- In greenhouses dedicated to vegetable production, two pipes carrying hot water are placed in the ground, parallel to the rows and between the rows. These pipes serve to heat the greenhouse and look like train rails. Trolleys can be used on these rails to transport produce, support equipment to spray plants or support equipment for people to work on plants.
- Another option for vegetable production is to place polyethylene pipes with warm water in close proximity to the plants.
“Piping for an in-floor heating system before concrete is poured over them. Source: http://blog.maripositas.org/horticulture/greenhouse-heating-systems”
Infrared radiant heater:
- Heat is transferred from the source to an object. In the case of a greenhouse, the heat is transferred from the infrared pipe to the plant or other objects. In other words, air will not transfer the heat and air temperature will not increase or impact the plant’s temperature.
- An infrared pipe or two pipes (U shape) run parallel to the gutter along the length of the greenhouse.
- The system consists of a single burner or a series of burners along the pipe.
- Fin pipes are more efficient than bare pipes; this is due to the much larger surface area of the fin pipe to radiate heat.
- This type of heating system delivers heat more efficiently when the heat is reflected in a 90˚ angle (facing the plants) since it can cover the entire surface of the greenhouse.
- The fuel source can be natural gas or propane.
- Low intensity infrared heaters are safer to use in a greenhouse than high intensity infrared heaters, because the pipes are heated up to 1100˚F (594˚C); the pipes of a high intensity infrared heater can be heated as high as 1800˚F (982˚C).
- The difference of heat irradiated to the floor between the source and the end of the pipe can be minimized by increasing the height of the pipe.
Infrared heating pipes located in the greenhouse peak radiate heat that warms solid objects below. Source: http://www.robertsgordon.com/agricultural”
Forced air heater units:
- There are two types of air heaters: vented heater and unvented heater. In the first type, the heat from the combustion is transferred to the air with the use of a heat exchanger; the exhaust gases from combustion have to be delivered outside the greenhouse through a flue pipe. The oxygen for the combustion is obtained from the outside environment. The second type utilizes oxygen from inside the greenhouse. A drawback of this type of heater is that the gases from the combustion stay inside the greenhouse. All the heat produced by the heater is used to heat the air.
- Fuel sources used for heaters are: natural gas, LP gas, kerosene and fuel oil.
- The by-products of the combustion are: water vapor, carbon dioxide, carbon monoxide, ethylene, propylene and sulfur dioxide. Plants can utilize the carbon dioxide, up to a point.
- The heaters can be mounted overhead, or placed on the floor.
- A greenhouse can have one unit or multiple units, this depends on the design.
- This type of heating system is recommended for small greenhouses.
- This type of system requires fans to move the air from the source heater to the opposite side of the greenhouse. One way to achieve this, is by the use of polyethylene tubes. A tube runs parallel to the length of the greenhouse and crop rows. These tubes have ventilation holes along their lengths, perpendicular to the tubing to distribute the heat evenly to plants.
- One heater and one overhead polyethylene pipe are enough to heat a greenhouse. However, smaller diameter polyethylene tubes can be placed on the side of the benches, under the rows or between rows.
- It is necessary to install HAF (Horizontal Air Flow) fans to move the air through the entire greenhouse. These fans help with air circulation; stagnant air can cause differences in air temperature, plants could grow uneven, and the hot and humid microenvironment may encourage the development of diseases.
- The heater can be turned on and off depending on the temperature set points on the thermostat.
- The time to heat the air is relatively low compared to central heating systems.
- The heat transferred to the plants is less efficient because the air has to be heated first in order to heat the plants (heat is transferred by convection). Therefore, the plants’ temperature cannot be higher than the air temperature.
"Overheat, forced-air heating systems. Source: Premier Tech Horticulture"
In addition to the above methods, a greenhouse can be heated with the use of wasted hot water from power plants that may be in close proximity. Another option is the use of geothermal heat in which hot water from the ground is pumped to the surface to provide heat to the greenhouse. Moreover, deep mines can provide hot air to a greenhouse.
In this article and the previous one, we covered several ways to conserve energy and heat in the greenhouse. It is imperative to consult an engineer to modify or upgrade the greenhouse, because the success of the different heating systems or the energy conservation strategies will depend on a good design.
PRO-MIX® is a registered trademark of Premier Horticulture Ltd.
The heat loss or gain in a greenhouse occurs as conduction, convection, radiation and infiltration.
As the old saying goes, “an ounce of prevention is worth a pound of cure”. The cost of greenhouse sanitation is minimal in comparison to the costs associated with fixing problems, such as weeding and chemical pest control.
In this second part of our four-part series on growing vegetables and herbs in the greenhouse, we will focus on the greenhouse structures needed to produce quality crops.
In this third part of our series on growing greenhouse vegetables and herbs we will focus on the ideal greenhouse environment.
In this fourth and last part of our series on growing greenhouse vegetables and herbs, we will address the appropriate growing medium to use.