1. Introduction
Solar energy has received worldwide attention due to its potential benefits for the environment and the sustainability of humankind. As a major renewable energy source, solar energy can be generated without causing a harmful impact on the environment [
1]. Although it has a high production cost of
$126/MWh compared to existing energy sources such as coal (
$88/MWh), natural gas (
$71/MWh), and nuclear (
$69/MWh) [
2], solar energy can be competitive if the government imposes an additional tax on greenhouse gas (GHG) emissions (e.g., carbon dioxide equivalent (CO2e)). According to Pehl et al. [
3], coal, natural gas, and solar energy generate 109 kg CO2e/MWh, 78 kg CO2e/MWh, and 6 kg CO2e/MWh, respectively. To make solar energy competitive among energy sources, many countries are imposing an additional tax on GHG emissions [
4]. For example, the U.S. is planning to charge
$0.025 per kg CO2e as GHG emissions [
5], and South Korea supports
$0.052 per kg CO2e (
$0.31/MWh) [
6].
To resolve the investment issue of solar energy, the construction of photovoltaic (PV) power stations has become popular not only in the U.S., but also in countries of Northeast Asia such as Korea, China, and Japan. China is aiming to reach 700 GW of solar power capacity by 2030 under the plan of China’s new Intended Nationally Determined Contributions (INDC) [
7]. Japan will have solar PV capacity of 53 GW by 2030 [
8]. Similarly, the Korea government has enacted the 3020 renewable energy policy to increase PV power capacity to 36.5 GW by 2030 [
9]. Thus, solar energy capacity in South Korea has gradually increased every year. It was 2367 MW in 2018 and 3789 MW in 2019. In particular, electricity generation by solar modules accounted for approximately 51% of the total renewable energy capacity (7429 MW) of South Korea in 2019.
In a temperate climate region (e.g., Northeast Asia), it is critical to design an agrophotovoltaic (APV) system with an appropriate structure (i.e., distance and tilt angle between the solar panels) for the maximum amount of electricity generation because, unlike in desert areas, strong solar radiation is only available for a few hours a day. Countries in Northeast Asia (e.g., South Korea and Japan) have daily average global solar radiation of 11–13 MJ/m
2, whereas countries in the Middle East (e.g., Qatar) have daily average global solar radiation of 18–20 MJ/m
2 [
10]. The average duration of bright sunshine in South Korea is 6.83 hours per day. This means that electricity generation is only available during that time [
11]. To meet the goal of the 3020 renewable energy policy to have a PV power capacity of 36.5 GW by 2030, an additional land area of 623 km
2 will be needed to construct PV power plants.
Nevertheless, it is challenging to secure enough space for the construction of PV power plants to provide a capacity of 36.5 GW. This is because the land available for construction is currently used for crop production (i.e., farmland). Compared with the mountainous area in the eastern region of South Korea, the western region with a wide plain area and high daily solar irradiation is preferred for constructing solar plants [
12,
13,
14]. However, due to the limited land area and high costs of land in the northwestern region, only the southwestern region is available for construction. The major issue is that most land in the southwestern region (i.e., Jeollabuk-do and Jeollanam-do provinces) is farmland. According to statistics from 2019, 30.58% (483,457 ha) of the total farmland in South Korea (i.e., 1,580,957 ha) is in the southwestern region. Given that 47.97% (974,525 ha) of the southwestern region (i.e., 2,031,400 ha) is mountainous, only the remaining area (52.03%) is available as a habitable area [
15]. Because habitable areas include lands used for other purposes (e.g., houses and commercial buildings), farmland (45.74% of habitable areas) is the only option for the construction. Thus, it is necessary to prevent any harmful impact of the construction of solar plants on farmlands so that the food supply of South Korea remains secure. In fact, it is contradictory to generate renewable energy for human survival at the expense of farmland.
To overcome the potential problems associated with impaired food production, an APV system must be considered. It should be devised to generate electricity from solar radiation without causing any adverse impact on crop growth [
16]. In this study, the most appropriate APV system structure among two types of solar modules (i.e., monofacial and bifacial solar modules) was determined based on their impacts on crop growth. Five crops were used in this study: sesame (
Sesamum indicum), mung bean (
Vigna radiata), red bean (
Vigna angularis), corn (
Zea mays), and soybean (
Glycine max). For the experiment, an APV system at the Jeollanamdo Agricultural Research and Extension Services in South Korea was constructed (see
Section 2 for more detail). Based on the two different solar modules, three different structures causing shading ratios of 32%, 25.6%, and 21.3% were considered in this study. We not only investigated the impacts of different structures on crop growth and yield but also analyzed the monetary benefits for farmers.
The rest of the paper is organized as follows.
Section 2 describes the structure of the APV system constructed at the Jeollanamdo Agricultural Research and Extension Services in South Korea.
Section 3 introduces the management of the five subject crops (i.e., sesame (
Sesamum indicum), mung bean (
Vigna radiata), red bean (
Vigna angularis), corn (
Zea mays), and soybean (
Glycine max)) under the APV system and the estimation models of electricity and profit from the APV system. The measured data are analyzed and the most efficient structure of an APV system in a temperate climate region is identified in
Section 4. Finally,
Section 5 concludes this study and suggests additional studies needed in the future.
2. Agrophotovoltaic Systems
The subject agrophotovoltaic system (APV) is located at the Jeollanamdo Agricultural Research and Extension Services in Naju-si (35.0161° N, 126.7108° E), Jeollanam-do, South Korea.
Figure 1 shows an overview of the subject APV system with an area of 4410 m
2 (63 m × 70 m). In
Figure 1a, three different shading ratios are shown: 32%, 25.6%, and 21.3%. According to a previous study [
17], the shading ratio for an APV should be at most 33% to generate solar power in South Korea. Thus, in this study we considered these three ratios, widely adopted in the nation. The solar modules with shading ratios of 32%, 25.6%, and 21.3% had areas of 1575 m
2 (63 m × 25 m), 1701 m
2 (63 m × 27 m), and 1134 m
2 (63 m × 18 m), respectively. The shading ratio (
Rshade) was computed according to Equation (1):
where
is the area (m
2) shaded by the solar modules and
is the area (m
2) used to install the APV system. Thus, the shading areas of the solar modules with shading ratios of 32%, 25.6%, and 21.3% were 504 m
2, 435 m
2, and 242 m
2, respectively.
As shown in
Figure 1b, each solar module was installed on a frame with a height of 5.42 m. The height of the pillar cover (or a support) was 0.81 m. By having sufficient height between solar modules and the ground, enough solar radiation could reach the crops under the solar modules. The installation costs were
$0.35 per watt (W) for monofacial solar modules and
$0.40 per watt (W) for bifacial solar modules. The subject facility had 161 units of 405 W monofacial solar modules and 161 units of 390 W bifacial solar modules. The prices of the monofacial (i.e., LG405N2W-V5) and bifacial (i.e., LG390N2T-A5) modules were
$0.35/W and
$0.40/W, respectively. The advantages of bifacial solar modules in terms of electricity generation were determined in [
18] through numerical experiments, so we considered both monofacial and bifacial solar modules in this study. The unit prices of the monofacial and bifacial modules were
$141.75 and
$156.00, respectively.
Table 1 shows the construction costs of the subject APV system. The total construction cost of the subject facility was
$167,843.39. This included the solar module cost (28.56%), structure cost (47.27%), electric distribution system cost (22.52%), and other costs (1.65%). Due to additional costs (i.e., structure, electric distribution system, and other costs), the total cost was 3.5 times higher than the cost of the solar modules. The bifacial solar modules were more expensive (10.05%) than the monofacial solar modules in terms of unit construction cost. The lifespan of the subject APV system is expected to be 25 years, which is widely adopted or assumed in the solar power industry and in studies [
19,
20,
21,
22].
5. Conclusions
In this study, we proposed an efficient structure for an APV system in a temperate climate region. Two types of solar modules (a monofacial solar module and a bifacial solar module) with different shading ratios (i.e., 21.3%, 25.6%, 32%) were considered as alternatives. The proposed structure was established at the Jeollanamdo Agricultural Research and Extension Services in Naju-si (35.0161° N, 126.7108° E), Jeollanam-do, South Korea. The southeastern region selected is the most preferable location due to its potential capacity for electricity generation. The daily electricity generation quantities of the monofacial solar module and bifacial solar module were 1.57 kWh/unit and 1.74 kWh/unit, respectively. Because of the different efficiency in terms of electricity generation, a bifacial solar module is more beneficial than a monofacial solar module to farmers in terms of profit. Under the SMP policy, the annual profit of the APV system based on corn farming is $16,420.57 per hectare. The bifacial module with a shading ratio of 21.3% was selected as the most profitable APV type. On the other hand, if both REC and SMP policies are applied to an APV system, soybean farming with a 32% bifacial module can make $40,781.04 per hectare annually. Nevertheless, considering that the price of the REC radically changes over time, a farmer needs to consider the APV system as a way to make an extra income in addition to crop production. The government also needs to pursue a policy for stable REC prices so that farmers can have stable and sustainable incomes from APV systems. As a result, the proposed APV system can produce not only monetary benefit from electricity generation but also profits from crop sales. This can be a new revenue model for farmers. Since most farmers cannot run their business during the winter season in a temperate climate region, this result is critical to their survival. In addition, by generating cleaner energy, the subject system can contribute to the survival of humankind in the future.