ReviewEnergy saving techniques for reducing the heating cost of conventional greenhouses
Introduction
Environmentally controlled agriculture, such as crop production in greenhouses is becoming very popular because of its high output which is 10–20 times greater per unit area than outdoor production (Nederhoff & Houter, 2007). However, the higher energy costs involved in greenhouse production are a major challenge. Energy cost is the second largest operating cost after labour for greenhouse production in cold regions. About 65–85% of the total energy consumed in greenhouses is used for heating, with the remaining portion is used for electricity and transportation (Runkle & Both, 2012). The costs for heating (mainly) and cooling in greenhouses represents about 70–85% of total operating costs excluding those costs associated with labour (Anifantis et al., 2017, Rorabaugh et al., 2002). However, heating energy could be up to 90% of the total energy requirements depending on the types of greenhouses and environmental control practises, and most importantly the location of greenhouses (Kristinsson, 2006). Therefore, reducing greenhouse heating costs could make the greenhouse production more economic and sustainable. To reduce heating costs, greenhouses must be energy efficient and able to use renewable energy resources such as solar, biomass, and geothermal heat. Many studies have been conducted to reduce heating cost of greenhouses; strategies such as energy-efficient structural design, the use of energy efficient covers, improved heating and ventilation systems, and energy-efficient management of indoor micro-climates, and use of renewable energy sources, can be applied depending on the location of greenhouses. The principles of these techniques are mostly to increase the solar heat gain of the greenhouse and reduce heat loss from the greenhouse. Figure 1 shows the typical pattern of heat loss through various heat transfer phenomena of a conventional greenhouse located in Saskatoon (52.1°N), Canada (Ahamed, Guo, & Tanino, 2018a). It is also essential to consider the balance between the agronomic needs of the plants and the energy-saving potential of different techniques (Sanford, 2011). Hence, the information related to the energy-efficient strategies and their effect on plants and the economic feasibility of the existing heating energy-saving technologies for the conventional greenhouses would be useful for greenhouse growers, researchers, and manufacturers. Sethi and Sharma (2008) reviewed and evaluated the various passive heating technologies available for worldwide agricultural greenhouses. Cuce, Harjunowibowo, and Cuce (2016) reviewed the energy saving strategies for greenhouse systems mainly renewable and sustainable based solutions such as photovoltaic (PV) modules, solar thermal (T) collectors, hybrid PV/T collectors and systems, energy-efficient heat pumps, innovative ventilation technologies, and efficient lighting systems. However, to our best knowledge, no recent reviews have examined the energy-saving techniques available to reduce heating costs in the conventional greenhouses. Therefore, the objective of this study is to present a comprehensive review of the potential techniques to reduce the heating cost of conventional-style winter greenhouses.
Section snippets
Energy-efficient design of greenhouses
An energy-efficient envelope design is very important to reduce the heating costs of conventional greenhouses located in cold regions. The goal of energy-efficient design is to increase solar energy gain and energy retention inside the greenhouse. The design parameters of the greenhouse envelope that greatly affect the heating requirement include its shape, orientation, and in the northern hemisphere, its north wall characteristics.
Use of energy-efficient greenhouse covers
The selection of covering materials for greenhouse depends on several factors such as capital cost and maintenance cost, its effect on plant growth and yield, and local climate and technical support (Papadopoulos & Hao, 1997). The greenhouse cover or glazing is the basic factor that greatly influences the energy consumption in greenhouses (Papadakis et al., 2000). Good covering materials must have high transparency to global solar radiation, especially within the photosynthetically active
Energy-saving potential of energy curtain
Thermal screens or night curtains are often used to reduce the loss of thermal radiation to the sky during the winter nights. The use of a thermal screen can reduce about 40–70% of night-time long-wave radiation loss from the greenhouse (Andersson, 2010, Bakker, 2006, Chandra and Albright, 1981). Sethi and Sharma (2008) reported that the use of the thermal screen in greenhouses could save about 23–60% of heating energy depending on the location and type of thermal screen, and the heating energy
Energy-saving potential of insulation
Greenhouse envelopes have little insulation as the maximum light transmittance become beneficial for the plants. The energy-saving from the insulation of conventional greenhouses including the air gap insulation in double-layered cover, insulation in the side wall and basement wall, is reviewed in this section.
Energy-saving potential of management of indoor microclimates
The optimum control of indoor microclimates is very important for better crop production and minimisation of energy requirement in greenhouses. A significant amount of heating energy can be saved by effective management of indoor greenhouse microclimates including indoor set-point temperature and relative humidity.
Heating contribution of supplemental lighting
Supplemental lighting is very important for a modern greenhouse located in northern latitudes (above 40ºN in America and 50ºN in Europe) because the shorter day length and reduced solar radiation affect the plant growth in greenhouses (Nelson, 1985). The supplemental lighting used for greenhouses depends on the daily light integral which varies due to location and time of the year, and crop species; the lighting selection is also affected by lamp cost, electrical cost, and heating requirement (
Energy saving potential of heating systems
The heating system in greenhouses can be in the active or passive mode, or a combination of both. Active mode of heating systems means the supply of heat from other sources for increasing indoor temperature. In passive mode, the solar energy is used to heat the greenhouse by storing the heat using different heat storage materials. Passive heating systems are suitable for small greenhouses located in moderate climates, and the active mode of the heating system is usually used for commercial
Use of alternative energy for greenhouse heating
The increased price of fossil fuels and the effect of using fossil fuel on the environment are encouraging greenhouse growers to consider alternative fuels for greenhouse heating. Possible alternate sources for greenhouse heating could be industrial waste heat, geothermal heat, and wood biomass. The following section includes the previous studies about the potential use of alternative fuel for greenhouse heating.
Conclusions and recommendations
Several types of techniques have been applied for reducing the greenhouse heating requirement thereby reducing the heating cost. Based on the review of different heating options for reducing heating cost, the following conclusions can be drawn:
- 1.
The heating energy saving potential of a greenhouse shape significantly differs depending on its location. In general, the east–west orientation of greenhouse is more energy efficient for the winter greenhouse at high northern latitudes, and the
Acknowledgment
The authors are highly thankful to the College of Graduate and Postdoctoral Studies (CGPS) at University of Saskatchewan, Canada; and Innovation Saskatchewan, Canada, for their financial support to this research (Project No. USask-ITMSR4).
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