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High-tech greenhouses are major consumers of energy. This energy consumption is responsible for considerable carbon emissions and contributes to climate change and global warming. Light-emitting diodes (LEDs) have been promoted as a solution to the energy problem of illuminated greenhouses, due to their high efficacy in converting electricity to light. Current LEDs consume around 40% less electricity than the commonly used high-pressure sodium (HPS) lamps, while emitting equivalent light intensities.Despite their high efficacy, adoption of LEDs in greenhouses has been limited. High investment costs, lack of knowledge regarding the influence of LEDs on the greenhouse climate and crop, and low trust towards their claimed benefits, have all been cited as obstacles for adoption. Greenhouses with LEDs also require more heating than greenhouses with HPS lamps: due to their higher efficacy, LEDs contribute less heat to the greenhouse, which must be compensated by the heating system. Therefore, it has so far been unclear precisely how much energy can be saved by a transition to LEDs in greenhouses, and what factors contribute to this potential saving.This thesis explores how LEDs influence the energy consumption of greenhouses. It investigates the consequence of replacing HPS lamps by LEDs in terms of lighting demand, heating demand, and total energy use; analyzes how LEDs influence the greenhouse climate and energy balance; and examines further possibilities for energy saving by LEDs in future scenarios.Chapter 1 provides an overview of the greenhouse energy problem, using the Netherlands as an example. The chapter shows that the advent of LEDs in greenhouses has been accompanied by great expectations which have so far failed to materialize. The chapter suggests that a transparent and quantitative assessment of the potential benefits of LEDs will help adjust expectations towards this new lighting technology and promote the trust of growers. Process-based mathematical modelling is proposed as a method towards achieving these goals.Chapter 2 investigates the discipline of process-based greenhouse modelling. The chapter shows that a considerable number of greenhouse models are published, and sets out to understand the reasons for the existence of this multitude of models. In Section 2 of the chapter, substantial background on the concept of process-based greenhouse modelling is provided. Modelling studies published between 2018 and 2020 are categorized according to their objectives, types of greenhouse, and equipment they consider, and a model inheritance chart is presented, showing how current models are based on earlier works. Moreover, a comparison of modelling validation studies is performed.Based on this analysis, possible reasons for the abundance of greenhouse models are suggested, including a lack of model transparency and code availability, and a belief that model development is in itself a valuable research goal. The chapter ends with recommendations for the future advancement of the discipline. These include promoting model transparency and availability of source code, and establishing shared datasets and evaluation benchmarks.Chapter 3 presents GreenLight, a process-based model for a greenhouse with a tomato crop, which describes the influence of HPS and LED lamps on the climate, crop, and energy use. GreenLight’s performance is evaluated against data from a greenhouse with HPS and LED lamps. The model is found to perform reasonably well, with a relative error in the range of 1-12%. The model is offered in an open source format at https://github.com/davkat1/GreenLight, making it available for further inspection and extension by others.Chapter 4 uses GreenLight to predict the influence of replacing HPS lamps by LEDs in a greenhouse. A wide range scenarios is considered, including varying climates and multiple settings for indoor temperature, lamp intensity, and greenhouse insulation. In all scenarios, LEDs are found to reduce the energy demand for lighting by 40%, but to increase the demand for heating. This results in a total energy saving by transition to LEDs in the range of 10-25% for most scenarios. An important factor influencing how much energy can be saved by a transition to LEDs is be the ratio between the lighting and heating demand before the transition.Chapter 5 presents a novel concept for greenhouse climate control: heating by light. Since lamps provide heat as well as light, the chapter suggests that illuminating at high intensities could eliminate the need for heating. This approach could be very efficient, as it uses lighting both to enhance crop growth and to maintain the indoor temperature. LEDs offer new possibilities in this direction since they can be installed at very high intensities. The scenarios explored in this chapter show that heating a greenhouse exclusively by lamps is possible if sufficient lamp intensities are installed and a heat harvesting system is used to maintain indoor temperatures when the lamps are off. The chapter also shows that if no changes are made to the lamp control strategy, increasing the lamp intensity in the greenhouse typically results in a higher energy use, and a lower energy efficiency.Chapter 6 provides a general discussion, with further outlook described in Section 6: Section 6.1 points out that several reports predict that the energy use of greenhouses will greatly increase in the coming years, mainly due to lighting. It is suggested that the higher efficacy of LEDs might actually incentivize growers to illuminate. In this way, LEDs could contribute to an increase rather than a decrease of greenhouse energy use. Therefore, the rest of the section highlights possible avenues to reduce the energy use and carbon emissions of greenhouses, aside from LEDs. Finally, using some of the lessons learned in the current work, Section 7 provides a short discussion on a new type of growing system: sunless cultivation, also known as plant factories or vertical farms.Chapter 7 is an appendix, providing a user’s guide and a description of the GreenLight model, including a detailed description of the lamps, blackout screen, and heat harvesting sub-models. This description could serve as an aid for future researchers who wish to further use or extend the GreenLight model. |
