Performance assessment of the integration of semitransparent solar cells with different geometry of greenhouses under different climate regions

A lack of resources and suitable farming lands, climate change, and a rapidly growing population are some of the main concerns of the societies that pose security challenges to the governments. Creating controlled environments for cultivation, growing plants, and farming, such as greenhouses, may assist in overcoming these challenges. Greenhouses can significantly increase land use efficiency in agriculture by increasing crop yield and harvesting throughout the year, which has long been proven effective. The history of greenhouses for farming dates back to Roman times, and there are different barriers to their applications. An example is the provision of energy to control the cultivation conditions of plants in greenhouses, particularly for heating and cooling hot and cold climate areas. On the other hand, based on the global energy trend, decentralized energy production based on solar energy is highly regarded. In the same way, that households can harvest solar energy, greenhouses can also benefit from solar energy. However, because greenhouses need sunlight to cultivate plants, reducing sunlight using common photovoltaic panels is not logical. By incorporating semitransparent solar cells into these greenhouses, the issue of reduced sunlight could be addressed, and further efficiency gains could be achieved by reducing energy demand in these greenhouses. This research investigates the energy supply system’s integration with greenhouses consumption. First, we assess different conventional types of greenhouses in terms of energy demand. Then, we investigate the energy demand with organic photovoltaic (OPV) integration for each type. Finally, the best design of the greenhouse for OPV integration is recommended. Results show that flat arch geometry is the best choice for dry and cold climates, while sawtooth geometry showed better improvements in tropical climates. In both temperate/mesothermal and continental/microthermal climates, A-frame geometry showed superiority in energy saving. Simulations revealed an annual electricity generation for a unit floor area of the greenhouses to be 173.7 kWh/m2 to 247.9 MWh/m2 for the optimum structural geometries that decrease the energy consumption of greenhouses. Additionally, the results show that the installation of the OPV can decrease energy consumption from 15 to 58% based on the greenhouse’s location and structural geometry.


Introduction
The growing rate of the population results in an increasing demand for food and energy and also a shortage of fresh water. This demand might be one of the most critical challenges in the future for humankind, specifically in developing countries, which have food and energy challenges. Greenhouses are one of the solutions for these challenges, but since most of these greenhouses are in rural areas, they might not access the energy grid. One of the approaches to address this issue is solar panel integrated greenhouses, which supply part of their energy demand. Based on the report of the World Health Organization, the Food and Agriculture Organization of the United Nations estimates a Responsible Editor: Philippe Garrigues 70% increase in food production compared to 2009 (World Health Organization 2009), and another research in 2019 has predicted that food demand by 2050 will increase by 47% (Gouel and Guimbard 2019). This demand increase necessitates the use of technology and the updating of traditional agricultural methods. Among the various challenges of agriculture, such as soil, water, climate, and air, supplying the required energy is also a significant challenge. With the development of renewable energy technologies, many energy supply challenges have decreased, especially in areas with low access to the national grid. Among the various renewable sources, solar energy has many advantages; on the one hand, it is used for photosynthesis and plant growth, and on the other hand, it can provide electricity or heat energy.
In recent years, the decreasing cost of solar cell production has turned photovoltaic (PV) into one of the promising technologies to produce low-cost and carbon-free electricity for the rising energy demand. A large portion of the costs for solar electricity generation is related to land. So, it is more cost-effective to mount the PV panels on top of buildings or other structures to reduce the electricity cost. PVs have different generations; in agriculture, in addition to buildings, it also has applications such as supplying the energy needed to pump water. The first generation of PV is a polycrystal or monocrystal of silicon. The second generation of PV, known as the thin film, has reduced the panels' thickness, granting them flexibility. Recently the third generation of solar panels, i.e., organic photovoltaic (OPV) developed specifically to be used in the construction industry. These semitransparent solar panels can be mounted on the building's wall or windows to produce electric energy. Due to the semitransparent nature of these OPVs, there seems to be a vast capacity for applying this technology in agriculture and farms.
The use of greenhouses can be considered a viable solution for reducing water consumption and increasing crop yield. By providing a controlled environment for crop cultivation, greenhouses allow for the optimization of temperature, light, and other essential growing conditions. As a result, crops can grow more efficiently, leading to greater yields and reduced water usage (Cook and Calvin 2005). However, this improvement comes at a cost; energy is required for heating and cooling to maintain the desired climate inside these greenhouses, which decreases the revenue of the greenhouse by integrating PV panels in the greenhouses, if not all, at least a portion of their energy demand could be addressed. This results in offsetting the costs of final products. Nevertheless, these PV panels will block sunlight through the ceiling, harming crop yield.
There are multiple solutions to overcome this problem. The first approach is partial shading; in this method, the PV panels will cover a portion of the roof. Studies show that covering 20-30% of the roof will not significantly impact crop growth rate (Kadowaki et al. 2012). Another approach is to implement semitransparent solar cells (SSC). The SSC or organic photovoltaics (OPVs) are the best options for integrating greenhouses since the light can pass through them, offering more advantages than conventional PVs. OPVs can potentially lower the cost of electricity generation due to their production methods (Lucera et al. 2017).
Furthermore, these OPVs are lightweight and flexible and can be manufactured in different shapes, which offer possibilities beyond conventional PVs. Usage of this technology can allow photosynthetically relevant light, which is required for crop growth to pass through the solar cells while harnessing the energy in the remaining spectrum and converting it to electricity. This selectivity can be achieved by manipulating the molecular structure of the OPVs.
In recent studies, Waller et al. have reported the cooling effects of organic photovoltaics (OPVs) implemented on greenhouse roofs (Waller et al. 2021). They used flexible, semitransparent OPV arrays as a roofing shade for a greenhouse tomato production system in the arid southwestern USA. The OPV shade stabilized the canopy temperature during high solar radiation intensities, demonstrating that using OPVs as a seasonal shade element in hot and highlight-intensity regions is feasible. However, it is worth noting that in some regions, during specific times, excessive solar radiation can surpass the requirement of crops in the greenhouse, requiring the deployment of shades to reduce the temperature inside. In such cases, implementing OPVs can improve cooling by reducing solar radiation and supplying a portion of cooling equipment's electricity demand (López-Marín et al. 2012).
In recent years many researchers studied the performance of SSC-integrated greenhouses. Yano et al. studied the performance of PV panels mounted inside the greenhouse at different tilt angles. Their results show that the low tilt-angle PV modules mounted inside the roof in a north-south orientation generate the most electrical energy (Yano et al. 2009). Kläring et al. have modeled a greenhouse with PV panels providing partial shading to model the crop yield based on the different solar radiation intensities. They stated that the effect of high irradiances would be minimal in crop yield (Kläring and Krumbein 2013). Quaschning et al. studied the PV thermal system performance. They studied 61 sites to determine the best locations for different types of PV technology (Quaschning 2004). Juang et al. analyzed the system dynamics of an off-grid greenhouse in an arid environment and reported their findings on the input and output of the system (Juang and Kacira 2013). Marucci et al. studied the different shading using PV in the tunnel-shaped greenhouse by installing transparent PV panels in a checkerboard arrangement. They stated that at some times of the year, the shading was outside the greenhouse, while sometimes, it was partially inside. However, the shading never exceeded 40% of the greenhouse . Colantoni et al. studied the climate changes inside the greenhouse and their effects on the crop, with 20% of the roof covered by PV panels. They have reported the distribution of solar radiation, temperature and humidity variability, lighting, and crop production for the greenhouse .
Integrating solar cells as a greenhouse cover material has significantly impacted greenhouses' performance. In order to fully understand the effects of such integration, it is essential to consider the greenhouse shape features, as these can play a significant role in the effectiveness of the installation of OPVs on the greenhouse structure. Cossu et al. reported the shading degrees of various types of PV-integrated greenhouses and showed that the yearly global radiation decreased by an average of 0.8% for each additional 1.0% PV cover ratio while it increased by 3.8% for each additional meter of gutter height (Cossu et al. 2018). The study's results highlight the importance of considering the greenhouse's orientation, pattern, and height when installing PV/OPV panels. Furthermore, the authors suggest that a checkerboard pattern and N-S orientation can help improve the uniformity of light distribution and support sustainable mixed systems for both energy and crop production. The lack of studies on the best greenhouse structure geometry for these solar panels highlights the need for further research in this area.
Organic photovoltaic (OPV) modules have been studied as a potential shading solution for greenhouses (Peretz et al. 2019). In a study, Friman-Peretz et al. evaluated the feasibility of using semitransparent, flexible OPV modules as greenhouse shading material. The radiometric and thermal properties of an OPV module were measured, revealing about 20% transmissivity and 65% absorptance in the photosynthetically active radiation range and a mean daily power conversion efficiency of about 0.8%. Despite these promising results, the modules' high cost and short life duration are currently limiting factors.
Many other pieces of literature have studied the usage of the OPV in greenhouses. Emmott et al. conducted a techno-economic analysis of photovoltaic greenhouses using organic photovoltaics, evaluating the potential for spectral selectivity and determining the efficiency and spectrally resolved transparency of various commercially available and non-commercial polymer materials (Emmott et al. 2015). This study suggested that OPV greenhouses could have great potential if cost targets are met but also emphasized the importance of developing new, highly transparent electrode and interlayer materials, along with high-efficiency active layers. Friman-Peretz et al. studied the impact of semitransparent OPV modules on a greenhouse tunnel housing a tomato crop, focusing on energy partitioning, radiation, vapor pressure deficit (VPD), and air temperature (Friman- Peretz et al. 2021). The results showed that the radiation variability in the OPV tunnel was lower on cloudy days, with most of the net radiation converted into latent heat. However, additional experiments are needed to determine the best arrangement of OPV modules on the roof. La Notte et al. reviewed hybrid and organic photovoltaics in greenhouses to reduce energy demand and dependency on fossil fuels (La Notte et al. 2020). The effects of PV technologies and module arrangements on energy production and plant growth were analyzed, focusing on new PV technologies, such as organic, dye-sensitized, and perovskite solar cells, due to their semi-transparency and flexibility. Ravishankar et al. evaluated the benefits of integrating semitransparent organic solar cells in greenhouses in the USA through an energy balance model, finding that these systems can have an annual surplus of energy in warm and moderate climates and that sunlight reduction can be minimized with appropriate design (Ravishankar et al. 2020). Friman-Peretz et al. compared the microclimate, crop performance, and physiological parameters of a greenhouse tunnel shaded by OPV modules with those of a conventional shaded and unshaded tunnel (Friman-Peretz et al. 2020). The results showed that there was no significant difference in mean seasonal air temperature and humidity between the tunnels, but the OPVshaded tunnel had a higher mean daily light integral and a higher yield compared to the control tunnel. The authors suggest that the use of OPV modules as shading elements in greenhouses has the potential to improve crop yields and energy efficiency.
OPV modules can reduce greenhouse heat load and generate electricity by using renewable energy while working as a semitransparent shading material. Friman-Peretz et al.'s study found OPV modules to have 20% transmissivity, 15% reflectivity, and 65% absorption in the photosynthetically active radiation range, with a mean daily power conversion efficiency of 0.8% and a heat transfer coefficient of 6.0 W/ m 2 -K (Peretz et al. 2019). According to Emmott et al., OPV greenhouses could have tremendous potential, and semitransparent OPV devices using PMDPP3T with a low band gap and PCDTBT with a mid-band gap can improve performance. There are, however, some limitations, including high costs and a short lifespan (Emmott et al. 2015). In a study by Magadley et al., the lifetimes and electrical performance of solar PV modules integrated into a greenhouse were investigated . Power conversion efficiencies reported by the authors ranged from 1 to 3%, with typically combined outputs of 105 Wh on a sunny day and 81 Wh on a cloudy day. Its initial efficiency was reduced after 15 days of burn-in.
Module degradation was accelerated by tunnel integration. In this study by Magadley et al., OPV modules' electrical behavior and lifetime were compared outside and inside polytunnel greenhouse roofs. A study was undertaken to find a greenhouse installation location that could reduce degradation and prolong the OPV module lifetime. As a result of exposure to harsh weather conditions, mechanical stresses from the movement of greenhouse plastic sheeting, and dust accumulation, the modules degraded at different rates. During the measurement period, efficiency values declined to 47%, and fill factor values decreased by 21% inside the polytunnel. They suggested that the lifespan of OPV modules could be improved by installing them within a tunnel and avoiding mechanical stress by not attaching them directly to the polyethylene cover. OPV arrays that were integrated into a greenhouse were evaluated for electrical performance by Waller et al. (2022). Solar irradiance incident on the greenhouse's curved roof resulted in a non-homogeneous distribution of solar radiation across the module surfaces. The OPV arrays had an overall power conversion efficiency of 1.82%. This reduced efficiency in the afternoon compared with morning and midday and a loss in normalized efficiency of 32.6%. OPV greenhouses need more robust OPV devices and more accurate performance monitoring strategies.
Some articles focus on integrating organic solar cells (OSCs) in greenhouses and their impact on energy consumption, plant growth, and indoor climate. In the study by Friman-Peretz et al., the impact of non-homogeneous shading by semitransparent OPV modules on the energy partitioning and spatial variability of air temperature, vapor pressure deficit (VPD), and radiation in a greenhouse tunnel was considered (Friman- Peretz et al. 2021). The results showed that on cloudy days, the radiation variability in the OPV tunnel was smaller than on sunny days, while the variability of air temperature and VPD did not change much with diffuse radiation. The authors found that most of the net radiation in the tunnels was converted into latent heat, with higher spatial variability of radiation within the tunnel when the solar elevation angle was high. The agronomic aspects of plant growth under the OPV modules were briefly presented.
Ravishankar et al. studied the impact of semitransparent organic solar cells (SOSCs) on a greenhouse's growth and indoor climate (Ravishankar 2021). Fresh weight and chlorophyll content of lettuce did not differ clearly when SOSCs were added to a greenhouse structure. Additionally, they observed that SOSCs provide an opportunity for light and thermal management of greenhouses through device design and optical coatings, which should be explored further.
In two similar greenhouse tunnels, one shaded by organic photovoltaic (OPV) modules, Friman-Peretz et al. examined tomato microclimate, yield, and physiological parameters. The two tunnels' seasonal air temperature and humidity did not differ significantly at noon (Friman- Peretz et al. 2020). On the other hand, in 2018, OPV tunnel radiation levels were much lower, and radiation distribution was less homogeneous. As a result of the OPV, the leaf temperature varied from higher to lower in 2018 and 2019. In 2020, La Notte et al. reviewed hybrid and organic photovoltaic systems in greenhouses (La Notte et al. 2020). Considering the semi-transparency and flexibility of organic, dye-sensitized, and perovskite solar cells, they examined the effects of PV technologies on energy production and plant growth. The authors concluded that reducing the energy demand and dependency on fossil fuels is crucial for improving the sustainability of greenhouses and that renewable technologies represent a key option to meet greenhouse energy demands.
Furthermore, researchers have researched modeling and design potential for organic solar cells in greenhouses. With a detailed energy balance model, Ravishankar et al. determine the benefits of OSC integration on greenhouse energy use (Ravishankar 2021). Wang et al. developed highperformance, eco-friendly semitransparent organic solar cells (SOSCs) for use in greenhouses (Wang et al. 2021). A newly designed quaternary blend achieved 17.71% power conversion efficiency. As well as exhibiting suitable photon transmission windows for plant absorption, the quaternary blend exhibited excellent photovoltaic properties. In SOSCs, plants grew favorably under the filtered light, with growth comparable to glass. As a result of their work, the authors concluded that their approach could provide an eco-friendly greenhouse photovoltaic solution.
Dipta et al. evaluated semitransparent organic solar cells in agro-photovoltaic greenhouses using a 3D greenhouse model developed to simulate and compare light interaction and crop growth with traditional and semitransparent technologies (Dipta et al. 2022). Semitransparent organic solar cells increased tomato crop dry ground weight by 46% compared to conventional silicon cell agro-photovoltaic greenhouses. The authors conducted a comprehensive model analysis to evaluate how semitransparent solar cells might improve crop growth in greenhouses. A greenhouse tunnel experiment with OPV modules (OSCs) and a control tunnel during two summers are presented by Friman-Peretz et al. Microclimate, yield, and physiological parameters are examined.
Although there have been many studies on implementing PV/OPV panels in buildings to produce electricity, only a tiny portion of these studies are about installing OPVs in greenhouses. Even in these studies, only the effect of installing PV/OPV panels on one type of greenhouse geometry has been studied, while it should be noted that there are many different geometries for the greenhouse, and different geometries can play a significant role in the effectiveness of the installation of the OPV on the greenhouse structure.
Although checking the effect of the geometry is an essential factor in energy harvesting and the percentage of the shadow inside the greenhouse, however, there has been no attempt to find the best greenhouse structure geometry for these solar panels to be mounted on.
The other important factor that has been neglected in the few papers that have studied the effect of the OPV on greenhouses is that they have reported energy consumption 1 3 annually. This is very good from the energy consumption view, but it is essential to consider that a greenhouse may not operate throughout the year and only be operational in specific months.
In this study, we try to address these research gaps to identify which greenhouse geometry is best for OPV integration. Also, pinpoint the geometries with the highest potential in producing electricity and those with the lowest net load. To achieve this, we study a variety of greenhouse structure geometries to compare energy consumption for maintaining microclimate conditions inside the greenhouse and the potential of electricity generation for each of them to determine the best geometry. Moreover, report a detailed review of the energy consumption and electricity generation of different structural geometries in different climates to cover most of the planet's climate conditions.

Modeling
There is a vast range of approaches for modeling a microclimate in a greenhouse, from simple static models to more complex dynamic models. A list of these models has been reviewed by Sethi et al. (2013). Simple models consist of only sizing the heating and ventilation of the greenhouse, while complex dynamic models might consider energy and mass balance over the various component of the system, from soil and air to structural parts. Typically, dynamic models are much more accurate since they utilize more precision in predicting the energy model of the system, which results in a closer comparison to the actual demands of the system and also a better simulation of climate change inside of the greenhouse, which may improve the crop yield by leading decision-making variables to their optimum solution.
Greenhouses need to preserve an acceptable change in their climate throughout the day, which may include both cooling and heating demand. Cooling demand is required mainly during the noontide, where sun irradiance meets its maximum peak. To avoid this heat load, greenhouses commonly deploy shades to prevent overheating and exceeding the allowable temperature range. With advancements in semitransparent organic solar cells and the commercialization of this technology, these solar cells could be used as semitransparent ceiling panels, which not only reduce solar radiation during noon but also reduce the total energy demand of the system by producing electricity.
This study focuses on modeling the microclimate and energy demand of a greenhouse with different structural shapes and compares the total energy demand for two cases where semitransparent organic solar cells have been used as ceiling versus commonly used materials. Figure 1 shows the conceptual model of the proposed system. Many commercial software packages exist capable of dynamically modeling a greenhouse environment (such as Energy Plus, ESP-r, etc.). The Design-Builder software has been used for this study to model the proposed system. The Design-Builder software is capable of integrating many types of solar panels, including the semitransparent type, which is used in this study, into the greenhouse structure to model the energy demand (for cooling, heating, and ventilation) and electricity production. This software provides the possibility of defining custom OPV specifications to match the requirements.
The system's energy balance was simulated in five different locations to consider different climates and solar radiation types. The mean monthly outdoor temperature and humidity over the years for each location were used to visualize an accurate climate over the year. Figure 2 shows the model structure and procedure flow diagram for the greenhouse structure selection.
The present model aims to solely assess the overall balance of energy and investigate the percentage of energy saving and whether or not this semitransparent organic solar cell integrated greenhouse could achieve a zero-energy demand. However, using any type of solar cell will inevitably reduce crop yield due to diminishing solar radiation delivered to the plants. This modified solar radiation may affect the plant's growth rate, but the impact could only be assessed indirectly due to the inability to model these changes. Since the photosynthetically active radiation (PAR) spectrum in 400-700 nm is only partly absorbed by semitransparent organic solar cells, effects on crop yield should be minimal; however, it should be noted that plants are also able to absorb some UV and far red spectrum in 320-400 nm and 650-730 nm, respectively (Ravishankar et al. 2020;Runkle and Fisher 2004).

Simulated model
In order to compare the energy consumption and system loads of an OPV-integrated greenhouse with a typical greenhouse, multiple models with different geometrical structures have been developed with the aid of the Design-Builder software, which is used to simulate the energy usage, the cooling/heating loads of the system, and the electricity generation of the OPV panels.
The primary energy flows in the proposed model, as shown in Fig. 3, include energy transfer to/from (1) air by ventilation, (2) ambient by radiation from greenhouse structure, (3) ambient by radiation from plants, (4) ambient by convection from greenhouse structure, (5) soil by conduction, and (6) internal heater or cooler. There are other ways for energy to transfer between the greenhouse and the environment, but they can be omitted due to their small values compared to the ones mentioned above.
The heat transfer to the greenhouse can be based on two sensible and latent sections. For heavy plant rooms with 24-h low-medium internal gains from equipment and transient occupancy, the latent load is considered to be based on four sections: (1) gains, the lumped gains into space from people, equipment, lights, etc. The coefficient of 50 W/m 2 is considered for the greenhouses. This coefficient is referred to as power density.
(2) For occupancy, this section includes occupancy levels, times, and metabolic activities and sets the metabolic rate according to the activity level within the space. The metabolic factor accounts for people of various sizes. It is considered 1 for men, 0.85 for women, and 0.75 for children. In the greenhouses, this coefficient is considered to be 0.9. The density of the population is considered to be 0.025 people/m 2 for greenhouses. The greenhouse working profile is 5 days a week from 9 am to 5 pm. (3) For other gains, there are also some other loads, including computers, office equipment, catering, process, and miscellaneous equipment. The main load in this section includes greenhouse equipment with a 24-h workday profile and general lighting with a workday profile from 9 am to 5 pm, and considering the latent load of agricultural products, this coefficient is considered to be 50 W/m 2 . (4) For environmental control, the data related to the environmental and comfort requirements of greenhouses is as follows: the heating set-point temperature is considered to be 22 °C; the cooling set-point temperature is considered to be 26 °C; the fresh air levels required per person are defined by mechanical ventilation air change rates that are considered to be 10 l/s-person; the humidity in the zone at the time of maximum sensible load is considered to be 60%.
Sensible heating is the heating effect of the HVAC system action on the heat balance, in particular, the heating effect of introducing air that is warmer than the zone air. Likewise, sensible cooling is the cooling effect of the HVAC system on the zone. Note that these are not always directly related to heating and cooling coil energy delivery, mainly because of the effect of free cooling from the outside air. So, for example, even if there is no cooling coil operational at a particular time, the sensible cooling output on the heat balance can be high due to the introduction of relatively cooler outside air into space through mechanical ventilation. These sensible heating/cooling outputs will also include a component due to fans (if operational) which will tend to warm air that moves through it.
The material and other parameters used for the model simulation (e.g., OPV panel type, building structure material) are shown in Table 1.

Climate condition
Assuming tomatoes as the target crop of the greenhouse (which is one of the largest greenhouse crops globally (Heuvelink 2018)), indoor conditions are set to 22 °C < T < 26 °C and 17 °C < T < 28 °C for occupancy and unoccupancy, respectively; while relative humidity is kept within the range of 70-90% (Ahamed et al. 2018;Schwarz et al. 2014).
Two fans on the north and south end walls provide ventilation, providing 1 air change per hour (ACH) (ASHRAE 2015, Ravishankar et al. 2020. Air infiltration is considered to be 1 ACH (ASHRAE 2015). Cooling and heating systems are both electrical, with COP of 0.83 and 1.67, respectively.
To obtain a comprehensive model, energy demand for both cooling and heating loads, also greenhouse microclimate conditions in five different locations based on the five different climates, has been simulated. These are the most known climates on the earth that covers almost all of the globe. We have selected a representative city for each of these climates to compare the results in different climates and to be able to select the best geometrical structure for each climate. Table 2 lists the different climate conditions and their representative cities.
It should be noted that there is also a polar climate; however, due to impracticality and not being suitable for establishing a greenhouse (due to not having proper sunlight), we omitted this type of climate.

System geometry
As illustrated in Fig. 4, eight different geometry for the greenhouse structure have been compared to compare energy balance. These shapes consist of the gable, flat arch, tunnel, ridge and furrow, sawtooth, skillion, uneven, and A-frame roof shapes. While solely comparing the energy balance of these geometries may not be a wise comparison due to the different internal volumes of these geometries, since the ultimate goal of the greenhouse is to grow plants, it is meaningful to fix plant area as the main variable and presume different internal volume as an inevitable choice, because wall height cannot be changed since it will result in different radiation absorption from the sun and environment. Except  for flat arch and tunnel shape geometries, the height for all shapes is equal. In order To reduce structural shadow, the greenhouse is orientated north-south. Walls are made up of 2 layers of polycarbonate with a thickness of 3.5 mm with 3 mm of air between them. The angle for a slope-shaped roof is typically 27 to 30° (ASHRAE 2015) applied to gable shape roof; for other roof shapes, an equal height as same as gable shape roof is considered for the purpose of equaling the internal volume of the greenhouse.

Solar radiation and lighting
Obtaining detailed information on solar radiation is important for calculating the greenhouse's thermal load and the power generation of solar cells. Sun radiation on the roof area should be calculated for the solar cell's power generation. The Design-Builder program can report the electricity generation by defining the OPV specification in the software. Spectrally engineered semitransparent organic solar cells (SOSCs) have been developed for greenhouse applications. A quaternary SOSC with 17.71% power conversion efficiency is achieved using newly designed multi-component blends. It is noteworthy that SOSCs with 13.08% PCE and a plant growth factor of 24.7% were developed through non-halogenated solvent fabrication, demonstrating promise as an eco-friendly greenhouse photovoltaic (Wang et al. 2021). In this research, we considered an efficiency of 8% for simulation. This PCE is less than the values mentioned so far and brings our results closer to commercialization. As a result, we are not only close to advances in organic solar cell efficiency but also close to industry efficiency.
For all the greenhouse structural geometries shown in Fig. 4, the floor areas are identical; except for the tunnel geometry, the heights are identical too. However, due to the shape of the greenhouse, the roof area where the OPVs are installed is different. Table 3 shows the OPV area corresponding to each of these structure types.
As we see in Table 3, the highest OPV area is related to the A-frame geometry, while ridge and furrow and sawtooth geometry are second and third. Uneven, gable, and flat arch geometries have a similar OPV area, and tunnel geometry has the lowest area.

Result and discussion
In this study, we try to identify the best structural geometry for the OPV-integrated greenhouse by comparing the energy consumption, cooling/heating loads, and electricity generation in each geometry. This comparison has been conducted in five cities representing the most dominant climates on the earth. In the following, we discuss the findings of this study.

Net energy usage
The greenhouse's net load/energy usage is the summation of the heating and cooling loads minus the generated electricity of the OPV. The heating or cooling load is the amount of heat energy needed to be added to or removed from a space to maintain the temperature in an acceptable range. To compare the energy usage of different geometries, we have simulated the cooling and heating loads for different geometries for five cities with different climates. The summation of the cooling and heating load as the net load is shown for these five cities in Fig. 5.

Tropical climate
In cities like Miami, with a tropical climate, from May to October, the sawtooth geometry has a minimal load compared to others; however, this geometry requires more load in other months. From November to April, the flat arch geometry shows the lowest load. Nevertheless, the sawtooth geometry has the minimum annual load. So, if the greenhouse is not used in only some seasons, the sawtooth geometry is the most promising structure for this climate.

Dry climate
In cities with a dry climate like Tehran, we see some similarities with the tropical climate. The sawtooth geometry is the best choice for lowering energy consumption in hot months like July and August. The ridge and furrow geometry also shows similar behavior as the sawtooth geometry with slightly more energy consumption. Nevertheless, interestingly, the f lat arch geometry, contrary to what we saw in the tropical climate, only has the lowest load in temperate seasons, i.e., from April to May and from September to October. In winter, A-frame geometry shows the most promising energy consumption. Though if we consider the annual energy consumption, the flat arch geometry is the best choice.

Cold semi-arid climate
In a cold climate such as Phoenix, we also see similar patterns to the previously discussed climates, as sawtooth geometry is the best choice for hot seasons from May to September. Moreover, the flat arch geometry is the best from October to April. As for the annual energy consumption, the flat arch geometry is the best choice.

Temperate/mesothermal climate
For temperate/mesothermal climates like what we see in Barcelona, although the sawtooth geometry shows minimal energy load like previous climates in the hot season from June to August, the load is similar to the other geometries, especially the flat arch geometry. In this climate, we see very different behavior in the net load of the greenhouse in other months from October to May, as A-frame geometry shows the minimum load in these months. Unlike the previous climates, A-frame geometry also is the best choice for annual energy consumption.

Continental/microthermal climate
In temperate/microthermal climates such as Toronto, the energy consumption in different geometries is somewhat similar. Like the temperate/mesothermal climate, the A-frame geometry shows superiority in energy saving throughout the year, especially from September to May, while with a little difference, the flat arch geometry is slightly better from June to August.

Solar panels power generation
Electricity generated by semitransparent OPV panels mounted on the greenhouse rooftop was simulated by defining the OPV's specification inside the Design-Builder software.
As illustrated in Fig. 6, the result shows that the maximum solar gain (electricity generated) took place from May to July, which is logical due to the sun positioning in the sky. Results show that regardless of the climate, the maximum electricity generated is related to A-frame geometry, which is logical as in this geometry, two side walls are covered by solar panels, and because of its shape, the coverage area of solar panels is higher than other geometries. After A-frame, skillion, sawtooth, flat arch, and ridge and furrow geometries show maximum potential for electricity generation, while uneven, gable, and tunnel geometries show the least potential.
Tropical climate (Miami), temperate/mesothermal climate (Barcelona), and temperate/microthermal climate (Toronto) show a similar monthly electricity generation that peaks at approximately 22.5 kWh/m 2 , while Phoenix and Tehran show a higher value that peaks at approximately 30 kWh/m 2 . Table 4 shows the annual electricity generation by the OPV for different structural geometries in different climates.
As we previously discussed in the "Net energy usage" section, each climate has an optimum structural geometry to minimize the energy consumption of the greenhouse. Table 5 shows the summation of the cooling and heating loads, electricity generation, and net energy consumption of the optimum structural geometry in each climate for OPV-integrated greenhouses and typical greenhouses. The maximum self-sufficiency in OPV-integrated greenhouses happens in tropical climates, as about 50% of the required energy of the OPV-integrated greenhouse can be covered by the OPV's generated electricity. In Temperate/mesothermal climates, this self-sufficiency goes down to 34% and 30% for dry and cold climates. The continental/microthermal climate shows minimum self-sufficiency of 20%.
As we see in Table 5, the best structural geometry for the OPV-integrated greenhouse and the typical greenhouse is the same except for the tropical climate. So, it seems a fair inference that if the structural geometry of an existing greenhouse has been optimized for local use, installing OPV on them should also produce the best results in reducing energy consumption. Even if OPV is not installed on the best structural geometry, it still offers a generous reduction in energy consumption. To determine the energy-saving percentage after installing the OPV, we should compare the net load of the OPV-integrated greenhouse to the load of a typical greenhouse to find the improvement; since installing the OPV on the greenhouse not only decreases the energy consumption by generating electricity but also by providing a selective shade on some wavelengths that reduce thermal load inside the greenhouse with little side effects on crops growth rate. We can see the effect of reducing thermal load by comparing the load column of Table 5 for both OPVintegrated greenhouses and typical greenhouses. Table 6 shows the energy-saving percentage achieved by installing OPV on typical greenhouses. The results show that installing OPV can decrease energy consumption somewhere between 15 and 58% based on the location and the structural geometry of the greenhouse, which is quite a significant value. We can see that the highest energy saving is in tropical climates, while the lowest happens in continental/ microthermal climates. We should expect an approximately 30% improvement worldwide by installing the OPV on the greenhouses.
If we consider the best structural geometry in Table 5 and compare the best OPV-integrated greenhouse to the Fig. 6 Monthly generated electricity of the OPV installed on different greenhouse geometries in different climates best typical greenhouse (cross geometry compare), we can determine the maximum energy saving that can be achieved. Table 7 shows the maximum energy-saving percentage for different climates.

Conclusion
Greenhouses can significantly improve the efficiency of agriculture by increasing crop yield; however, this increase in effectiveness is accompanied by deficiencies. One of these deficiencies is the energy consumption of the greenhouse to the point that the costs of the final products may be comparable to those of regular farms. One of the methods to decrease the costs related to greenhouses is to decrease the dependency on grid energy usage, which has been possible due to recent developments in semitransparent solar cells. In this study, different structure geometries for the greenhouse have been compared to determine the best geometry for mounting OPV panels in terms of the greenhouse's electricity generation and energy consumption.
Results demonstrated that from the dry to the cold and tropical climates, we see a similar pattern that shows the sawtooth geometry is the best choice for hot seasons, while the flat arch geometry is the best for cold seasons.  However, the number of months that the sawtooth geometry can be used in the hot seasons is different in these three climates, from 6 months in the tropical climate to 4 months in the cold climate and 2 months in the dry climate. The flat arch geometry is the best choice for the rest of the year. If we consider the annual energy consumption, except for the tropical climate, where the sawtooth geometry is the best choice, the flat arch geometry shows more promising results for the other two climates. As for the two other climates, temperate/mesothermal and continental/microthermal, the A-frame geometry shows significant energy savings compared to other types of geometries. Although the flat arch geometry also shows better results compared to the other geometries, it is not comparable to the A-frame in terms of energy consumption.
Simulation results revealed that the annual electricity generation of the OPV could vary from 107.7 to 274 kWh/ m 2 depending on the structural geometry and climate. For the optimum structure, we find that for tropical (Miami), temperate/mesothermal (Barcelona), and continental/ microthermal (Toronto) climates, it will be 197.9 kWh/ m 2 , 180.3 kWh/m 2 , and 173.7 kWh/m 2 , respectively, while for the dry (Tehran) and cold (Phoenix) climates, it will be 247.9 kWh/m 2 and 235.1 kWh/m 2 .
Also, we find that in four of the five climates, the best structural geometry for the OPV-integrated greenhouses and typical greenhouses is the same, so installing an OPV on the currently existing greenhouses that their structural geometry has been optimized for that area is probably going to produce the best energy saving; so we recommend installing the OPV without a need to re-optimizing the existing greenhouses.
The comparison between the energy consumption of OPV-integrated greenhouses and typical greenhouses has shown that the best energy saving will happen in tropical climates with 58.2% saving, while the lowest will happen in continental/microthermal climates with 23.1%. The dry, cold, and temperate/mesothermal climates show an improvement of 31.2%, 32.2%, and 35.5%, respectively.

Data availability
The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.

Declarations
Ethical approval Not applicable.

Consent to participate Not applicable.
Consent for publication All authors are agreed to publish this work.

Competing Interests
The authors declare no competing interests.