In recent years, the expansion of photovoltaic power stations has led to an increase in comprehensive utilization methods such as photovoltaic-plus, with agricultural photovoltaics emerging as one of the primary models. This integration of photovoltaic technology with agriculture significantly advances modern and large-scale agricultural practices. Additionally, land constraints are a pressing issue for photovoltaic development in central and eastern regions, but combining photovoltaics with agriculture introduces a novel model and format. Cultivating mushrooms like shiitake under photovoltaic solar sheds organically merges two industries, enabling resource integration, complementary advantages, non-interference, and co-development, thereby greatly enhancing land resource utilization efficiency. From my perspective, this approach leverages the dual benefits of the solar system for energy generation and agricultural production, creating a sustainable synergy.
The growth of shiitake mycelium is sensitive to temperature. According to studies, the suitable temperature range for mycelial growth is 5–32°C, with an optimum at 24–27°C. At this optimum, the mycelium grows vigorously, appearing white and robust. Overall, shiitake mycelium tolerates low temperatures but not high ones. It can survive at -10 to -8°C for 30–40 days, but at 34°C, growth ceases; at 36°C, it suffers severe damage, turning yellow; and above 40°C, it dies quickly. In central and eastern China, summer temperatures are high, often reaching 40°C or more except in some cool mountainous areas, making natural summering difficult for shiitake. To address this, we conducted experiments on shiitake summering in photovoltaic greenhouses in Leping, Jiangxi: Mode 1 used spray cooling and fan-wet curtains in photovoltaic multi-span greenhouses, and Mode 2 involved building an inner shed within the photovoltaic greenhouse for heat exchange cooling via spraying. The results are summarized below, emphasizing the role of the solar system in temperature regulation.
In this article, I will delve into the materials, methods, and outcomes of these experiments, incorporating tables and formulas to elucidate key points. The integration of the solar system into agricultural practices is not just about energy efficiency; it’s about creating microenvironments that support crop growth under adverse conditions. Throughout, I will highlight how the solar system contributes to sustainable agriculture, and I will use mathematical models to analyze temperature dynamics and growth rates. The goal is to provide a comprehensive overview that can guide future implementations of photovoltaic agriculture, particularly for temperature-sensitive crops like mushrooms.
Materials and Methods
The experiments utilized specific materials and setups to test the summering modes. Here, I describe the components in detail, focusing on how the solar system infrastructure supports these endeavors.
Materials and Key Facilities
The shiitake mushroom sticks were sourced from a local photovoltaic agricultural company, with specifications of 18 cm × 58 cm. The photovoltaic multi-span greenhouse was equipped with spray facilities, fan-wet curtains on both sides, and automatic temperature recorders. Construction materials included wooden strips (3 m × 4 cm, error within 2 mm), bamboo poles (diameter 3–5 cm, smooth to avoid damaging bags), new plastic film (thickness >0.001 cm), non-woven fabric (new, absorbent, ≥250 g/m²), absorbent sponge (thickness >5 mm with good water absorption and conductivity), and plastic steel wire (diameter >4 mm). These materials were selected to ensure durability and effectiveness within the solar system environment.
| Material | Specification | Purpose |
|---|---|---|
| Mushroom Sticks | 18 cm × 58 cm | Shiitake cultivation substrate |
| Wooden Strips | 3 m × 4 cm, error ≤2 mm | Framework for inner shed |
| Bamboo Poles | Diameter 3–5 cm | Support structure |
| Plastic Film | Thickness >0.001 cm | Covering for insulation |
| Non-woven Fabric | ≥250 g/m² | Absorption and moisture retention |
| Absorbent Sponge | Thickness >5 mm | Heat exchange medium |
| Plastic Steel Wire | Diameter >4 mm | Structural reinforcement |
Specifications of Photovoltaic Multi-Span Greenhouse
A single photovoltaic multi-span greenhouse measured 100 m in length and 50 m in width, covering an area of 0.5 hectares. The lower chord bar was 3.2 m above ground, and the roof height was 4.5 m. Except for the glass photovoltaic panels, the sides and north slope of the roof were covered with black-white film. This design is integral to the solar system, allowing light transmission for energy generation while providing shade for agriculture. The greenhouse’s structure facilitates the integration of cooling mechanisms, which are crucial for summering experiments.
Experimental Methods
The experiments compared two cooling modes within the solar system framework. Both aimed to maintain temperatures suitable for shiitake mycelium growth, using the photovoltaic infrastructure as a base.
Mode 1: Spray Cooling with Fan-Wet Curtains
To reduce summering risks, mushroom sticks were placed close to the ground in a single layer. Temperature control involved direct spraying on the sticks using the greenhouse’s spray facilities, with fan-wet curtains activated based on external temperatures. This mode relies on the solar system’s ability to support water and energy inputs for cooling. The temperature dynamics can be modeled using heat transfer equations. For instance, the rate of temperature change in the greenhouse can be expressed as:
$$ \frac{dT}{dt} = \alpha (T_{ext} – T) – \beta S + \gamma W $$
where \( T \) is the internal temperature, \( T_{ext} \) is external temperature, \( \alpha \) is heat transfer coefficient, \( \beta \) is cooling rate from spraying, \( S \) is spray intensity, \( \gamma \) is effect of fan-wet curtains, and \( W \) is airflow rate. This formula helps quantify how the solar system’s components interact to modulate temperature.
Mode 2: Inner Shed with Spray Cooling
An inner shed was constructed within the photovoltaic greenhouse, measuring 48 m in length, 6.1 m in width, and 2.7 m in height. The outer layer consisted of plastic film, sponge, and absorbent non-woven fabric from inside out. Ten daylight lamps were installed for lighting. A drainage trench was dug outside the shed base, and a door allowed access for personnel and forklifts. Inside, wooden racks with five layers were set up, with 25 cm spacing between layers and the bottom layer 30 cm above ground to prevent waterlogging. The racks varied in width (40 cm, 60 cm, and 90 cm), supported by wooden strips and bamboo poles. The layout included aisles of 80 cm between racks. Sealing was crucial to prevent leaks during spraying or film fluttering during fan operation. Wooden parts below soil were treated for anti-corrosion, and drainage systems ensured smooth water flow. This setup exemplifies how a nested structure within a solar system can enhance microenvironment control.
Temperature management followed a schedule: spraying facilities were turned on around 7:30 AM, with fan-wet curtains off and the inner shed closed for ventilation; at 8:00 PM, fan-wet curtains were activated for ventilation, and spraying was stopped. Automatic temperature recorders were placed on the fifth layer and bottom layer. The solar system’s energy output powers these devices, ensuring continuous monitoring. To analyze temperature effects on mycelial growth, we can use a growth rate model based on temperature:
$$ G(T) = G_{max} \cdot e^{-\left( \frac{T – T_{opt}}{k} \right)^2} $$
where \( G(T) \) is growth rate at temperature \( T \), \( G_{max} \) is maximum growth rate, \( T_{opt} \) is optimum temperature (e.g., 25°C for shiitake), and \( k \) is a constant representing temperature sensitivity. This formula underscores the importance of maintaining optimal temperatures within the solar system.
Results and Analysis
The experimental period in July-August 2016 experienced more sustained and extreme high-temperature days than usual, as shown in the table below. This data highlights the challenges faced by the solar system in regulating temperatures.
| Maximum Daily Temperature (°C) | Number of Days |
|---|---|
| 40 | 4 |
| 39 | 13 |
| 38 | 8 |
Mode 1: Direct Spray and Fan-Wet Curtain Cooling
During high temperatures, spraying could temporarily reduce space, stick surface, and ground temperatures below 32°C. However, within half an hour after spraying, space temperature rose to around 38°C, stick surface to 35°C, and ground temperature fluctuated less. The photovoltaic greenhouse frequently exceeded 32°C, and direct spraying created high humidity, unsuitable for mycelial growth. Consequently, infection rates were high. Prolonged exposure to high temperatures and humidity weakened mycelial vitality and consumed nutrients, adversely affecting fruiting. This outcome indicates that Mode 1 within the solar system is insufficient for reliable summering, as temperature control is inconsistent. We can model this instability with a differential equation:
$$ \frac{dH}{dt} = \delta (T – T_{threshold}) – \epsilon C $$
where \( H \) is humidity, \( \delta \) is humidity increase rate from temperature, \( T_{threshold} \) is critical temperature (e.g., 32°C), \( \epsilon \) is humidity reduction from cooling, and \( C \) is cooling capacity. This shows how the solar system’s cooling mechanisms must balance temperature and humidity.
Mode 2: Inner Shed with Spray Cooling
In the inner shed, temperatures at 1.5 m height (fifth layer of racks) were generally controlled within 32°C, reaching 33°C only during extreme external temperatures above 40°C. The lower layers were 1–2°C cooler than the fifth layer. Automatic temperature data revealed that high-temperature intervals occurred daily from 14:00 to 16:00, necessitating adequate spray water volume, sometimes increased during these periods. The inner shed effectively buffered against external heat, demonstrating the solar system’s potential for creating stable microenvironments. The temperature profile can be described by a steady-state heat equation:
$$ \nabla^2 T = -\frac{Q}{k} $$
where \( \nabla^2 \) is the Laplacian operator for heat distribution, \( Q \) is heat generation rate from external sources, and \( k \) is thermal conductivity of shed materials. This formula illustrates how the inner shed’s insulation properties, supported by the solar system’s structure, mitigate heat ingress.
To quantify the improvement, we can compute the temperature reduction efficiency \( \eta \) for Mode 2:
$$ \eta = \frac{T_{ext} – T_{in}}{T_{ext} – T_{target}} \times 100\% $$
where \( T_{ext} \) is external temperature, \( T_{in} \) is inner shed temperature, and \( T_{target} \) is desired temperature (e.g., 32°C). For instance, with \( T_{ext} = 40°C \) and \( T_{in} = 33°C \), \( \eta = \frac{40-33}{40-32} \times 100\% = 87.5\% \), indicating high efficiency within the solar system setup.
Discussion and Implications
The integration of photovoltaic agriculture with mushroom cultivation aligns with the principle that photovoltaic power generation requires sunlight, while edible fungi need shading. Combining photovoltaic power stations with cultivation greenhouses enhances land use efficiency without altering soil quality, yielding combined benefits of electricity and mushroom production. However, summer high temperatures in central and eastern China pose challenges for year-round shiitake production. Traditional methods include summer soil-cover cultivation, which often yields poor quality, or transporting sticks to cooler high-altitude areas for summering, which risks heating during transit and increases costs, making large-scale production difficult.
Our experiments show that Mode 1 struggles to maintain temperatures at or below 32°C, posing high summering risks with low yield and poor quality. In contrast, Mode 2, using an inner shed within the photovoltaic multi-span greenhouse, meets temperature requirements for shiitake summering, effectively reducing risks. The wooden racks and bamboo poles can last over eight years, while the outer film and sponge need replacement every two years. Cost analysis reveals an additional production cost of 0.2–0.3 yuan per stick, but summered sticks can hit the market earlier than conventional autumn-cultivated ones, commanding higher prices and generating considerable returns. This approach enables near-year-round production, boosting capacity and competitiveness for enterprises leveraging the solar system.
From a broader perspective, the solar system’s role in agriculture extends beyond energy generation. It facilitates controlled environment agriculture (CEA), where parameters like temperature, humidity, and light are optimized. For shiitake, the growth rate as a function of temperature and humidity can be modeled with a multivariate equation:
$$ G(T, H) = G_{max} \cdot e^{-\left( \frac{T – T_{opt}}{k_T} \right)^2} \cdot e^{-\left( \frac{H – H_{opt}}{k_H} \right)^2} $$
where \( H \) is humidity, \( H_{opt} \) is optimum humidity, and \( k_T \), \( k_H \) are constants. This emphasizes the need for precise control within solar system infrastructures.
Economically, the solar system integration can be evaluated using a cost-benefit analysis formula:
$$ NPV = \sum_{t=1}^{n} \frac{R_t – C_t}{(1 + i)^t} $$
where \( NPV \) is net present value, \( R_t \) is revenue in year \( t \), \( C_t \) is cost in year \( t \), \( i \) is discount rate, and \( n \) is project lifespan. Revenue includes electricity sales from the photovoltaic system and mushroom yields, while costs cover construction, maintenance, and operational expenses. For instance, if a solar system greenhouse produces 10,000 kWh annually at $0.15/kWh and 5,000 kg of mushrooms at $5/kg, with annual costs of $20,000, the NPV over 10 years at a 5% discount rate would be positive, demonstrating viability.
Furthermore, the solar system contributes to sustainability by reducing carbon footprints. The energy generated offsets conventional power use, and the agricultural output supports local food systems. We can quantify this with a carbon savings metric:
$$ \Delta C = E_{pv} \cdot EF_{grid} – E_{ag} \cdot EF_{ag} $$
where \( \Delta C \) is net carbon reduction, \( E_{pv} \) is energy from photovoltaics, \( EF_{grid} \) is emission factor of grid electricity, \( E_{ag} \) is energy input for agriculture, and \( EF_{ag} \) is emission factor for agricultural inputs. This formula highlights the environmental benefits of integrating solar systems into agriculture.
Future Directions and Conclusions
Looking ahead, advancements in solar system technology, such as bifacial panels or integrated energy storage, could enhance temperature control and energy efficiency. For example, smart systems using IoT sensors could automate cooling based on real-time data, optimizing resource use. Mathematical models like fuzzy logic or machine learning algorithms could predict temperature fluctuations:
$$ \hat{T}(t+1) = f(T(t), H(t), S(t), W(t)) $$
where \( \hat{T} \) is predicted temperature, and \( f \) is a function learned from historical data. This would make solar system agriculture more resilient to climate variations.
In conclusion, the solar system offers a robust framework for integrating photovoltaics with mushroom cultivation, particularly for summering challenges. Mode 2, with an inner shed, proves effective in maintaining suitable temperatures, enabling year-round production. By leveraging formulas and models, we can optimize design and operations, ensuring economic and environmental sustainability. The key is to view the solar system not just as an energy source but as a holistic solution for agricultural innovation, supporting food security and renewable energy goals simultaneously. As I reflect on these experiments, it’s clear that the synergy between solar systems and agriculture holds immense potential, and continued research will unlock further opportunities for scalable, efficient production systems.
To encapsulate, the solar system’s integration into agriculture transforms land use paradigms. Through experiments like these, we validate practical approaches that balance energy and food production. The use of tables and formulas in this article underscores the scientific rigor behind such integrations, providing a roadmap for future endeavors. As the world seeks sustainable solutions, the solar system stands out as a versatile tool, adaptable to diverse agricultural needs while contributing to global energy transitions. In my view, this synergy is not merely additive but multiplicative, creating value that exceeds the sum of its parts.