Performance analysis of radiation and electricity yield in a photovoltaic panel integrated greenhouse using the radiation and thermal models

Highlights

• PVIG performance of inside radiation and electricity yield is evaluated.

• The thermal model is developed to predict accurate hourly electricity yield of PVIG.

• PVIG shows the advantage in uniform distribution of inside radiation.

• PVIG with checkerboard layouts increase the annual electricity yield by 2.2–3.2 kWh m⁻².

Abstract

The agricultural photovoltaic panel integrated greenhouse (PVIG) are attempted to improve the utilization of solar energy for sustainable agriculture. This study proposes a numerical method to predict the radiation distribution and the electricity yield of a PVIG throughout the year using 3D radiation model and thermal model. The radiation model is validated in a PVIG in the southeast of China with 36 rooftop PV panels of straight-line layout and the PV array covering 25% of the greenhouse roof area. According to simulation of conventional greenhouse and PVIG with straight-line (La), crisscross (Lb) and checkerboard (Lc) PV panels’ layouts, the performance of different PVIG is evaluated in terms of the non-uniformity of the radiation distribution, the radiation intensity and the electricity yield of PV panels. The non-uniformity of the hourly radiation under the La layout is not much different from the layout Lb and Lc, and the cumulative weekly radiation under the layout of La is increased by 8.3% and 12.7% in the experimental PVIG, respectively. The thermal model of PV panels mounted on the greenhouse roof is developed to accurately predict the PV electricity yield, considering the impact of greenhouses and external environments on PV panels. In contrast to the layouts of La and Lb, the annual electricity yield of PV panels with the layout of Lc raises by 3.2 and 2.2 kWh m⁻² respectively. Compared with conventional greenhouse, the distribution and intensity of cumulative weekly radiation are not significantly affected by the optimal PV array covering 25% of the greenhouse roof area. These results indicate that the 3D radiation model and thermal model can be used as an efficient tool to optimize the performance of PVIG.

Introduction

Solar radiation plays a dominant role in the greenhouse environment, which directly affects plant physiological processes. However, solar radiation is often too intense in some high-radiation regions, or too high for some plant species in summer, the excess radiation can be applied to electricity generation by using PV modules.

Some researchers have developed the greenhouses with PV modules mounted on the roof, which can generate electricity to drive the greenhouse control systems (Yano et al., 2009, Faisal Mohammed et al., 2007, Yano et al., 2007). Sonneveld et al. (2010) investigated the feasibility of the electricity-producing greenhouse with hybrid PV cell thermal collector modules, and found that the electricity-producing greenhouse can be operated independently of fossil fuels. Nurdan and Levent (2017) developed a nearly zero energy greenhouse, which uses PV modules to assist the ground source heat pump. Ganguly et al. (2010) presented a numerical model of hybrid power system in a floriculture greenhouse covered by solar PV cells for the energy supply analysis, and the research indicated that the integrated power system was viable for powering stand-alone greenhouses. Cossu et al., 2016, Allardyce et al., 2017 developed a semi-transparent PV module integrated in a greenhouse system to increase the solar radiation conversion efficiency and crop yield respectively. Hassanien et al. (2016) discussed numerous PV technologies for solar energy applications in the greenhouses, and found solar energy can reduce energy consumption of agricultural greenhouses. However, the PV panels mounted on greenhouses inevitably affect the solar radiation intensity and the radiation distribution inside the greenhouse, which will directly affect crop growth and productivity in the greenhouse. It is necessary to analyze the radiation distribution, intensity and electricity yield of a PV panels integrated greenhouse (PVIG).

The radiation distribution in the greenhouses has already been extensively investigated using a field test method (Critten and Bailey, 2002, Lamnatou and Chemisana, 2013a, Lamnatou and Chemisana, 2013b, Cossu et al., 2014, Cossu et al., 2018, Sudan et al., 2015) that measure the hourly solar radiation intensity of each observation point to experimentally verify the daylight factor. Farkas et al. (2001) measured the inside and outside radiation using photosynthetically active radiation (PAR) sensors to analyze the light distribution in greenhouses. Meir et al. (2012) tested the PAR distribution in three multi-span greenhouses in summer and winter days to evaluate the effects of gutters and roof openings. Abdel-Ghany and Kozai (2006) measured the outside and inside radiation of a greenhouse to analyze the net solar radiation on the greenhouse floor. Soriano et al. (2004) analyzed the radiation uniformity by testing the spatial uniformity of incident radiation in the multi-span greenhouses with three different roof geometries using scale models. Nevertheless, the incident radiation distribution is particularly affected by the layout of PV panels mounted on the greenhouse roof. Marrou et al. (2013) investigated the spatial distribution of the PAR under the PV panels by field measurement, and the research verified that PV panels are not necessarily detrimental to crop production when mounted on an optimal position. The radiation distribution was measured in a greenhouse with straight-line and checkerboard layout PV panel, and found that the PV panel of the checkerboard layout provided a more uniform spatial distribution (Yano et al., 2010, Kadowaki et al., 2012). Trypanagnostopoulos et al. (2017) evaluated the effect of the PV roof covering on the crop production by measuring the solar radiation. Cossu et al. (2014) investigated the effect of solar radiation caused by PV panels covering 50% area of the greenhouse roof by measuring the solar radiation of the observation points. The PVIG system reduced the availability of solar radiation inside the greenhouse by 64%. However, field tests are more costly in some complex terrain, and inevitably take a long time to complete all possible meteorological conditions.

Some numerical methods have been proposed to analyze the solar radiation in a greenhouse. Boulard and Wang (2002) presented a simple greenhouse model to predict the solar radiation distribution inside a tunnel greenhouse based on the path of the sun, considering the greenhouse geometry, covers transmittance and sky conditions. Cossu et al. (2017) proposed an algorithm to estimate light distribution of different canopy heights in PV greenhouse. The incoming total solar radiation in a greenhouse can be calculated by using 3D-shadow in Auto-CAD (Gupta et al., 2012) and collecting the amounts of total solar radiation on each surface of greenhouse (Cakir and Sahin, 2015). The computational fluid dynamics (CFD) is widely used in the analysis of climate simulation in the greenhouse (Nebbali et al., 2012, Baxevanou et al., 2018, Lamnatou and Chemisana, 2013a, Lamnatou and Chemisana, 2013b, Fatnassi et al., 2015, Chen et al., 2015). However, these models are hard to predict the hourly solar radiation and radiation distribution in the greenhouse for a total month or a year because of the accuracy and computation burden.

The electricity yield of the PV panels mounted on the greenhouse is also an important part for the utilization of solar energy, which is affected by the temperature of PV cell. Some thermal models have been developed to predict electricity yield of PV panels (Hoang et al., 2014, Barroso et al., 2016). However, without consideration of the interaction between the PV panel and the greenhouse, these thermal models cannot be used for accurate electricity yield forecast of PVIG.

The software such as Ecotect and Daysim can be used to simulate daylight and the solar radiation incident on building envelope throughout the year (Berardi and Wang, 2014, Mandalaki et al., 2014, Ahadi et al., 2017). Therefore, a 3D model of the PVIG based on Ecotect and Daysim is developed to precisely predict the radiation in a greenhouse throughout the year. Taking the greenhouse effects into account, the thermal model of the PV panels is investigated to predict the electricity yield of PVIG accurately. In order to increase the utilization of solar energy and satisfy radiation requirement for the plant’s growth in the PVIG, the performance of a greenhouse with various PV layouts is evaluated in terms of the non-uniformity of the radiation, radiation intensity, and electricity yield using the radiation and thermal models.