Reducing the cost of heating greenhouses is a major challenge for greenhouse growers, especially those located in cold regions. Various techniques have been applied to reduce greenhouse heating costs during the winter season. This study provides a comprehensive review of different energy saving techniques that can be applied to reduce heating costs, including energy efficient greenhouse designs, use of energy efficient covers, use of thermal curtains, energy efficient management of indoor microclimates, selection of energy efficient greenhouses. efficient heating system andthe potential for the use of alternative energy sources. The energy saving potential of different design parameters (shape, orientation) is highly location dependent and the energy saving potential of most passive heating systems (water tanks, rock bed) may not be suitable for commercial greenhouse production in cold regions. Alternative energy sources such as industrial waste heat, geothermal energy and wood biomass can be a viable option to reduce the cost of greenhouse heating for large-scale production. However, it is important to consider the balance between the agricultural needs of plants and the energy saving potential of various energy saving techniques, as well as the economic feasibility of energy saving systems in greenhouses. It can be concluded that this study can be useful for greenhouse growers, researchers and producers to create sustainable energy efficient greenhouse production in cold regions.
Introduction
Environmentally controlled agriculture, such as crop production in greenhouses, is becoming very popular due to its high yields, which are 10-20 times higher per unit area than outdoor production (Nederhoff & Houter, 2007). But high energy costs in greenhouse production are a major challenge. The second largest operating cost after labor in greenhouse production in cold regions is energy cost. Approximately 65-85% of the total energy consumed in greenhouses is used for heating, while the rest is used for electricity and transportation (Runkle and Both, 2012). Heating in greenhouses (mainly) and represents about 70-85% of total operating costs, excluding labor-related costs (Anifantis et al., 2017, Rorabaugh et al., 2002). However, depending on the type of greenhouse, environmental control practices and, most importantly, the location of the greenhouses, heating energy can account for up to 90% of total energy needs (Kristinsson, 2006). Therefore, reducing greenhouse heating costs can make greenhouse production more economical and sustainable. To reduce heating costs, greenhouses need to be energy efficient and able to use renewable energy sources such as solar, biomass and geothermal heat. Many studies have been conducted to reduce the heating costs of greenhouses; depending on the location of the greenhouses, strategies such as energy efficient structural design, use of energy efficient covers, improved heating and ventilation systems, energy efficient management of indoor microclimates and use of renewable energy sources can be implemented. The essence of these techniques is mostly to increase solar heat gain and reduce heat loss from the greenhouse. Figure 1 shows the typical heat loss pattern of a conventional greenhouse located in Saskatoon (52.1°K), Canada, through various heat transfer phenomena (Ahamed, Guo, & Tanino, 2018a). It is also important to consider the balance between the agronomic needs of crops and the energy saving potential of different techniques (Sanford, 2011). Therefore, information on energy efficiency strategies and their impacts on plants, as well as the economic feasibility of available heating energy saving technologies for conventional greenhouses would be useful for greenhouse growers, researchers and producers. Sethi and Sharma (2008) reviewed and evaluated various passive heating technologies available for agricultural greenhouses worldwide. Cuce, Harjunowibowo, and Cuce (2016) reviewed energy saving strategies for greenhouse systems, which are mostly oriented towards renewable and sustainably 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 the best of our knowledge, there are no recent reviews examining available energy saving techniques to reduce heating costs in conventional greenhouses. Therefore, the aim of this study is to present a comprehensive review of potential techniques to reduce the heating cost of traditional style winter greenhouses.
Energy efficient design of greenhouses
In order to reduce the heating cost of conventional greenhouses in cold regions, an energy efficient shell design is very important. The aim of energy efficient design is to increase solar energy gain and energy retention inside the greenhouse. Design parameters that greatly influence the heating demand of the greenhouse shell include its shape, orientation and, in the northern hemisphere, the characteristics of the north wall.
The use of covers to reduce the cost of heating greenhouses
The choice of covering materials for greenhouses depends on several factors such as capital cost and maintenance cost, its impact on plant growth and yield, local climate and technical support (Papadopoulos and Hao, 1997). Greenhouse cover or glass is the main factor that greatly influences energy consumption in greenhouses (Papadakis et al., 2000). Good coating materials should have high transparency to global solar radiation, especially in photosynthetically active media.
Energy saving potential of the energy curtain
Thermal curtains or night curtains are often used to reduce the loss of thermal radiation to the sky on winter nights. The use of a thermal curtain can reduce nighttime longwave radiation loss in the greenhouse by about 40-70% (Andersson, 2010, Bakker, 2006, Chandra and Albright, 1981). Sethi and Sharma (2008) reported that the use of thermal curtains in greenhouses can save about 23-60% of heating energy, depending on the location and type of thermal curtain and the heating energy.
Energy saving potential of insulation
Greenhouse covers have very little insulation as maximum light transmission is beneficial for plants. This chapter examines the energy savings from insulating conventional greenhouses, including air gap insulation in the double-layer cover, insulation in the side wall and basement wall.
Energy saving potential of indoor microclimate management
Optimal control of indoor microclimates is crucial to reduce the cost of heating greenhouses. Significant heating energy can be saved through effective management of indoor greenhouse microclimates, including indoor setpoint temperature and relative humidity.
Heating contribution of additional lighting
For a modern greenhouse located in northern latitudes (above 40° North in the United States and above 50° North in Europe), supplementary lighting is essential because shorter day lengths and reduced solar radiation affect plant growth in greenhouses (Nelson, 1985). The supplementary lighting used for greenhouses depends on the daily light integral, which varies depending on location, time of year and crop types; the choice of lighting is also influenced by lamp cost, electricity cost and heating requirement.
Energy saving potential of heating systems
The heating system in greenhouses can be in active or passive mode or a combination of both. The active mode of heating systems means that heat is supplied from other sources to increase the internal temperature. In passive mode, 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 temperate climates and the active mode of the heating system is usually used for commercial purposes.
Use alternative energy to reduce the cost of heating greenhouses
The rising prices of fossil fuels and the environmental impact of fossil fuel use are encouraging greenhouse growers to consider alternative fuels for greenhouse heating. Possible alternative sources for greenhouse heating could be industrial waste heat, geothermal heat and wood biomass.
References (212)
- JA Adaro et al.Geothermal contribution to greenhouse heatingAppliedEnergy (1999)
- MS Ahamed et al.Quasi-steady state model for predicting heating requirements of conventional greenhouses in cold regionsInformationProcessingin Agriculture(2018)
- H. Al-Hussaini et al.Use of shallow solar pools as a heating source in greenhouses in cold climatesEnergyConversion and Management (1998)
- R. Andrews et al.Environmental and economic assessment of greenhouse waste heat exchangeJournalofCleanProduction (2011)
- AS Anifantis et al.Thermal energy assessment of small-scale photovoltaic, hydrogen and geothermal independent system for greenhouse heatingRenewableEnergy (2017)
- BJBaileyReducing thermal radiation in greenhouseswith thermal curtains greenhouses-energii-kullanimi/ Journal of Agricultural Engineering Research (1981)
- GC Bakos et al.Greenhouse heating using geothermal energy Geothermics(1999)
- M. Balghouthi et al.Experimental investigation of thermal and moisture behavior of wet and dry soils with buried capillary heating systemSolarenergy(2005)
- T. Bartzanas et al.Effect of heating method on greenhouse microclimate and energy consumptionBiosystemsEngineering(2005)
- H.BenliEnergy performance analysis of ground source heat pump system with latent heat storage forgreenhouseheatingEnergyConversion and Management(2011)
