Explorations in the Food-Energy Nexus: Organic Photovoltaics Applications to Greenhouse Crop Production Systems

Abstract

The integration of photovoltaic technologies with agricultural systems - known as ‘Agrivoltaics’, is seen as a path forward for sustainable development, increasing the productivity of land and reducing competition for space between food and energy production. Relative to open-field farming, greenhouse crop production systems are more resource-use efficient, achieving higher annual yields per unit area using less water and pesticides. Because of these advantages, greenhouse agriculture is increasingly seen a solution to two global challenges: the first is the need to intensify agricultural production to meet the needs of a growing human population; the second is the need to reduce the environmental impacts, namely ecosystem degradation and high resource demands, of the agricultural sector overall. Even so, greenhouses can be highly energy-intensive to operate due to the climate control systems and other electrical components needed to provide and maintain proper conditions for plant growth. Thus, the effective integration of PV technologies with greenhouses can strengthen the role of greenhouse agriculture in sustainable intensification strategies, reducing industry reliance on fossil-fuel based energy sources.At the same time, the emerging technology of organic photovoltaics (OPV), based on polymer and small molecule semiconductor materials, is gaining traction in research and industry for its niche applications in the PV market, particularly in building-integrated PV (BIPV) schemes. Within the BIPV domain, greenhouses are seen as a uniquely suitable candidate for OPV integration, emphasizing key technological advantages of OPV devices, namely: facile chemical tailorability, which enables customization of material transparency and light absorption and transmission properties to complement plant light requirements; compatibility with a variety of substrates, including conventional greenhouse glazing/covering materials; and overall design versatility (lightweight, mechanical flexibility) that facilitates easy installation onto greenhouse structures. Recognizing the growing interest in OPV applications to greenhouses, a pilot greenhouse study was conducted to demonstrate opportunities and challenges that exist in this design concept, specifically for greenhouses in arid regions characterized by high solar insolation. The outdoor electrical performance of large-area, semi-transparent, roll-to-roll printed OPV arrays deployed on a gothic-arch, polyethylene-covered greenhouse was evaluated for a five-month period (October 2019–February 2020); integrating 3-D surface modeling and solar modeling tools, methods to estimate incident irradiance components for flexible PV on a curved greenhouse roof surface were developed. These methods were then applied to assess the performance, and more specifically, analyze the effects of varying irradiance conditions on the electrical behavior of the OPV devices deployed on the greenhouse for a five-month period. The OPV arrays showed better power conversion efficiencies (PCE) at low incident irradiance levels, and in the morning and midday periods compared to the afternoon period. The average PCE for the six OPV arrays measured was 1.82%. Maximum power point (Pmax) and short-circuit current (Isc) showed strong dependence on incident irradiance, and direct irradiance in particular; although, at the highest irradiance intensities there was a clear dampening effect seen in both parameters for three of the OPV arrays. Open-circuit voltage (Voc) reached limiting values around 200 W m-2, occurring in the initial daylight hours, and then showed slight decline in afternoon periods. Fill factor (FF) values also peaked around 200 W m-2; in three of the OPV arrays, FF showed slight decline at higher irradiance levels, while it was relatively unaffected in the other three arrays. Average daily FF values were low, ranging between 33.1–43.3%. On average, the OPV arrays had a 38.6% loss in overall PCE over the measurement period, ranging between 31.6–50.1%. In the second phase of the pilot study, the OPV arrays were installed as a shading element for a greenhouse tomato crop production system from March–July 2020. This demonstrated a readily available application for greenhouse-integrated semi-transparent OPV in high-light regions, wherein shading methods are required to continue production in the spring and summer seasons. During the hottest months of the measurement period (May–July), the OPV shade provided a suitable climate for tomato crop production, stabilizing canopy temperature during times of day with the highest solar radiation intensities, performing the function of a conventional shading method. Constrained solar radiation levels in the OPV section for three weeks following transplant in early March, in which the daily light integral (DLI) values were estimated to be lower than the recommended minimum of 15 molPAR m-2 day-1, resulted in more vegetative growth and delayed fruit development and ripening compared to the Control section, indicating shade avoidance behavior, and leading to lower average yields in the first 3 (of 10 total) weekly harvests. Beginning with the fourth harvest, however, yield, fruit number, and fruit mass in the OPV and Control sections were similar. Trends in yield productivity for the OPV-shaded plants showed increased performance during high light intensity periods in May/June. The light utilization efficiency (LUE), which is the relationship between cumulative yield and cumulative PAR radiation, measured in g molPAR-1, was approximately twice as high in the OPV section (21.4 g molPAR-1) compared to the Control section when it was unshaded (10.1 g molPAR-1); during the period in which the Control section was shaded, the LUE increased to 18.2 g molPAR-1 in that section. These pilot studies brought to light the wide range of design factors involved in determining the scalability and commercialization of OPV-integrated greenhouse systems. Thus, a comprehensive analytical framework involving four scales of analysis – material, module, system, and regional – was utilized to evaluate the current research status and future directions for OPV applications in greenhouses; opportunities are identified in OPV greenhouse system design that would contribute significantly to its overall desirability, feasibility, and viability as an effective design solution. This analysis also makes clear the increasing need for a cross-disciplinary and inter-scalar approaches to the conceptualization and implementation of integrated food and energy systems going forward.