KIEAE Journal

In 2021, the smart farm supply area in Korea was 6485ha, an increase of approximately 16 times from that over eight years ago (405ha) in 2014 when statistics monitoring began¹⁾. Following the development of the "Agricultural Food ICT Convergence Expansion Plan"in 2013, the government has been supporting the expansion of smart farm sites since 2014, with an emphasis on facility horticulture. In 2017, smart farms were chosen as one of the major creative growth projects in the direction of the economic strategy of the government. Since then, efforts have been made to develop them as future growth sectors by improving technological innovation and competitiveness. With focus on productivity improvement by scale-up, facility integration, and convergence with information and communications technology devices, smart farms are becoming energy-consuming businesses and electric energy-oriented enterprises. The increasing energy usage and electrification in the agroforestry industry contradict the national energy transition and carbon-neutrality goals.

Although the 2050 carbon-neutrality vision was established in October 2020, practical negotiations including sectoral objectives are lacking[1]. In the Netherlands, where greenhouse agriculture has grown, attempts to preserve greenhouse energy and reduce carbon dioxide emissions have been underway since the 1990s. In 1997, the Dutch greenhouse business sector signed the Glasshouse Horticulture and Environmental Agreement with the government²⁾. The agreement called for a 65% improvement in energy efficiency in 2010 as well as a 4% decrease in renewable energy consumption compared to those in 1980[2]. Following this agreement, greenhouse crop productivity was increased based on the concept of a greenhouse system, which improves energy efficiency, beginning with "closed greenhouses." Closed greenhouses are sustainable greenhouses because they eliminate windows for ventilation and improve the energy efficiency[3], or proof of the future[4] has been established. Semi-closed greenhouses have been established in Korea since 2017; however, research on them is lacking³⁾. The aims of this study were to examine the concepts of closed and semi-closed greenhouses, assess the necessity and present the energy efficiency improvement element technology, and recommend future directions.

From 2005 to 2020, 25 overseas publications and professional journal articles on closed and semi-closed greenhouses were examined. Semi-closed greenhouse research has mostly focused on improving the energy efficiency, agricultural output, and irrigation water conservation. In this study, we focused on energy efficiency and the technologies employed to achieve it. A semi-closed greenhouse is a large-scale smart greenhouse, also known as a commercial or corporate greenhouse. It is equipped with air-conditioning and heating, ventilation, dehumidification, and environmental control technology. As a first step in ensuring energy efficiency in greenhouses, greenhouse covers and shade films that combine insulation and light transmittance are typically examined. These passive features were excluded in this study because they are frequently considered in existing greenhouses. In this study, active aspects that occur in large-scale open greenhouses supplied with facilities were primarily covered.

A total of 17 (65%) of the 26 research on the Dutch climate involved testbed experiments, monitoring, simulations, and literature reviews. Furthermore, 7 examples (27%) were for Europe, including France, Germany, and Spain, and the remaining two cases were from North America, indicating a lack of studies on suitability to the four unique seasons of Asia. In particular, six cases (23%) specifically investigated cooling and dehumidification, among facility component technology elements, and much research was devoted to surplus heat treatment in summer. It is critical to investigate system configuration and capacity planning suited for the conditions in Korea, where the operation of cooling and dehumidifying facilities is expensive owing to high temperature and humidity, leading to the loss of the summer cropping season. Furthermore, studies were undertaken to demonstrate the impact of introducing semi-closed greenhouses by measuring the energy saving based on the technical features and system design of greenhouse facilities compared to those of current greenhouses. Empirical and monitoring approaches were utilized for quantification verification in 11 cases (42%), simulations in 7 cases (27%), and theory and literature research in 9 cases (35%). This study extracted technical aspects and improvement concepts from a literature survey, and further research on energy savings in semi-closed greenhouses applicable to Korea's environment should be continued.

A closed greenhouse is defined as a greenhouse in which the roof ventilation windows are closed[6]. Its development started with the concept of generating electricity from the heat generated in a summer greenhouse[9]. When the windows that enable surplus heat and steam to escape are closed, more heat is created than a greenhouse requires. An early plan was to provide the heat of a greenhouse to surrounding homes[8]. Thus, closed greenhouses have been referred to as solar greenhouses[5], heat production[8], zero-fossil energy use greenhouses[11], large solar collectors[15], and Sunergy[18].

Closed greenhouses provide a yearly excess of solar energy. The majority of solar energy is produced in summer; however, because heat energy is required in winter and spring, the timing of energy demand and supply is difficult[10]. Summer heat is stored in aquifers and used to meet the winter heat demand, resulting in reduced yearly energy usage[5]. The use of aquifers as heat storage is a common feature of Dutch closed greenhouses (Fig. 1.).

Fig. 1. 
Closed greenhouse concept (Source: [6][10][15])

Owing to the high cost of removing the extra latent heat in summer, a closed greenhouse with all windows removed is not economically feasible[6][10][11]. In the process of examining the economical sealing rate, the concept of a partially closed greenhouse that transfers the heat generated in a closed greenhouse to an open greenhouse by combining closed and existing open greenhouses appeared (Fig. 2.).

Fig. 2. 
Forms of conventional, closed, and semi-closed greenhouses (Source: [27])

A totally closed greenhouse may save 19% energy consumption, whereas combining closed and open greenhouses can save 33% main energy use. 1 The reduction in the primary energy consumption is proportional to the area of the closed and open greenhouses[6], and the appropriate closure rate is estimated based on climatic conditions[9]. A semi-closed greenhouse is one that combines may be entirely closed and partially opened greenhouses by installing a very small ventilation window and an air conditioning system. Semi-closed greenhouses manage some ventilation heat through strategically positioned vents. Basically, an air treatment unit (ATU) that regulates the exchange of air between the outside and the inside is used. In addition, an air conditioning system for a closed greenhouse that provides the air processed therein to the greenhouse via a distribution system is applied. (Fig. 3.)

Fig. 3. 
Semi-closed greenhouse composition

First, the energy requirements of a greenhouse may be decreased by recycling and storing the heat released by the existing ventilation. In a semi-closed greenhouse, a heat pump, combined heat and power generation, and a quarterly energy storage system using an aquifer are set to generate 35% primary energy saving[6] and 60% secondary energy saving[5]. Second, the positive pressure in a closed greenhouse improves production by managing the carbon dioxide (CO₂) content and reducing the entry of insect repellants and fungal spores. In a closed greenhouse, the concentration of CO₂ can continuously be in the range of 1000–2000ppm, which increases the photosynthesis of plants and increases the production by 22. The amount of water for irrigation is decreased by 50% owing to the lowered evaporation loss by ventilation[26]. Moreover, the usage of herbicides and insecticides is reduced by 80% because the positive pressure greenhouse environment prevents pest invasion[6][7][8][9][14][26].

Despite these advantages, the problems of semi-closed greenhouses necessitate complex equipment to control the internal climate, such as efficient energy storage systems, air distribution ducts, air conditioning systems, dehumidifiers, heat exchangers, and forced ventilation systems. In addition, a passive insulating structure and a screen system are used to reduce the heat loss from the greenhouse itself. The initial investment cost is increased by these facility systems and greenhouse constructions[4][21]. This suggests that farmers must acquire professional training to operate the equipment incorporating these new technologies[15].

Because the technical level of a facility impacts performance, a semi-closed greenhouse is beneficial for application to large-scale greenhouse projects of at least 1ha[28], and it can balance the high investment costs by ensuring a high production[26].

If the sealing rate in a semi-closed greenhouse is high and the cooling capacity is raised, the microclimate of the greenhouse becomes lesser dependent on the external environment, and the regional features disappear[26]. However, as the cooling capacity increases, the energy consumption also increases. This reduces the effect of the energy savings obtained using a semi-closed greenhouse. To meet the objective of introducing a semi-closed greenhouse, which is to enhance the energy efficiency and economic feasibility, an appropriate cooling technique and capacity must be chosen based on the external climate.

As summarized in Table 2., Dutch and Spanish greenhouses plan heat storage in the forms of aquifers or rock formations, whereas French greenhouses do not provide heat storage separately. Heat pump systems are utilized at all three locations for cooling and heating. A top cooler, fog system, and fan pad system are used for cooling along with each heat pump. Air conditioning facilities focused on ATU are established in greenhouses in the Netherlands and France. A Spanish greenhouse lowers CO₂ emissions by combining biomass and wind power as the electrical energy sources for the facility. Although the greenhouses summarized in Table 3. cannot be considered to reflect the complete climate of the Netherlands, Spain, and France, common technical aspects of semi-closed greenhouses may be determined. The results suggest that, in particular, the seasonal heat storage system and cooling technique are set in various methods to match the features of the location and the greenhouse environment.