Energy performance assessment of photovoltaic greenhouses in summer based on coupled optical-electrical-thermal models and plant growth requirements

Abstract

Photovoltaic (PV) greenhouses are widely used to regulate internal irradiance and air temperature to create optimal climatic conditions for plant growth. Therefore, comprehensive multidomain energy models of PV greenhouses are more likely needed to improve the system performance. However, few studies investigate optical-electrical-thermal characteristics and PV greenhouse evaluation indicators based on crop growth requirements. The aim of this work is to develop coupled optical-electrical-thermal models to assess the energy performance of the PV greenhouse with different PV roof coverage ratios during the summer months. In addition, the models of photosynthetically active radiation (PAR) and net photosynthetic rate (PN) are presented based on the irradiance and temperature of the PV greenhouse to evaluate the dynamic response of crops to interior climate conditions. The models are validated with experiments on the PV greenhouse in Kunming, with a total area of 26.25 m2. The electricity supply and demand systems include PV modules (3.285 kWp), a battery bank (24 kWh), an air–water heat pump (AWHP), and spray combined with fan cooling. Two cooling scenarios are analysed and compared by varying the PV module coverage ratio. Scenario 1 (S.1) is AWHP cooling, and scenario 2 (S.2) is spray combined with fan cooling. The optimum interior temperature and PAR of the PV greenhouse are 22 °C and 300 μmol·m−2·s−1, respectively. The simulations indicate that if the proportion of the PV layout on the greenhouse roof is greater than 20%, the year-round PAR inside the greenhouse will be below the threshold value of 550 μmol·m−2·s−1 at which the strawberry PN is saturated. For every 10% increase in PV roof coverage, the interior air temperature decreases by 0.02–0.56 °C corresponding to a daily cooling load reduction of 0.45–1.02 kWh/d, while the PV generation increases by 1.7–3.19 kWh/d and the maximum electricity consumption of S.1 and S.2 drops by 0.66 kWh/d and 0.52 kWh/d, respectively. Moreover, the high self-consumption-sufficiency balance (SCSB) values show that the PV and battery sizes are 2.22–2.78 kWp (40–50% coverage) and 15.72–15.75 kWh for S.1, and those in S.2 are 1.11–1.67 kWp (10–30% coverage) and 0.66–3.22 kWh. The coefficient of performance (COP) of the system reaches a maximum of 0.45–1.12 at 40% coverage in S.1 and 1.88–3.01 at 10–30% coverage in S.2. When the PV module coverage is 40% in S.1, the levelized costs of electricity (LCOE) and levelized costs of cooling (LCOC) are the lowest at 0.0760USD/kWhel and 0.0693USD/kWhc. Similarly, the lowest LCOE and LCOC of S.2 are 0.0306USD/kWhel and 0.0210USD/kWhc, as the PV module coverage is 30%. The study will be beneficial for applying photovoltaic modules, battery banks, and cooling technologies in the greenhouse, especially considering the dynamic crop response to internal climates and the energy performance assessment.