## 4 Discussion  
The data presented in this study illustrate that energy use in greenhouses in the EU varies considerably depending on the type of greenhouse, geographical area, and crop grown. In advanced greenhouse systems, heating is the dominant energy requirement and in some studies accounts for up to 99% of all energy inputs. In our view this needs more detailed research with more data points from accepted methodologies as 99% appears high. In any case, our review indicates that energy requirements for heating and cooling in energy intensive systems are so considerable that other energy inputs, such as fertilizers, are extremely minor. Indeed, it has been estimated that the heating and cooling of greenhouses represents 1.5% of Europe's total energy consumption [41]. Methods of heating greenhouses vary throughout the EU; gas boilers are generally popular, as are air-unit heaters for small  
Figure 10: Energy inputs in low energy intensity tomato production systems (GJ/ha) (based on [15; 22; 34]). Figure 9: Energy inputs in high energy intensity tomato production systems (GJ/ha) (based on [15; 28; 40]). scale installations [42], as well as cogeneration (Combined Heat and Power) in certain countries (mainly The Netherlands) [9]. In most cases, heating and cooling systems are powered by energy from fossil fuels but in recent years sustainable sources of heat, mainly geothermal, have been growing rapidly. In less energy intensive systems, overall energy requirements per hectare are significantly less (50-70 times less energy per hectare) as compared to high energy systems, but generally still multiple times higher than the energy requirements of open-field agriculture. The mixture of energy inputs in these systems are split more evenly, depending on the crop and production system, between direct (lighting, heating/cooling, irrigation, machinery use) and indirect (fertilizers and pesticides). It is important to note that indirect energy sources, mainly fertilizers, constitute a considerable amount of energy inputs in low energy intensity greenhouses (6-27%). Studies also indicate that there is significant geographical variation in energy intensity between greenhouses in the EU as, in general, more advanced greenhouses are located in northern Europe and basic greenhouse structures in southern Europe. An FAO study which focuses on greenhouse production in South-Eastern Europe (including non-EU countries) highlights that around 18% of greenhouses are glasshouses and 82% plastic greenhouses (a higher proportion than the EU average), and that of all area under greenhouses in the region 97% are not heated. By contrast, in Northern Europe, especially in the Netherlands, where most greenhouses are heavily managed, it is normal for greenhouse cultivation to be heated, consuming around 11,000 GJ/ha [9]. This difference can to a large extent be attributed to climatic conditions as crops grown in Northern European greenhouses for large parts of the year are dependent on external heating sources. However, it is likely that other factors, including market conditions, accessibility, knowledge, also play a role. Identifying these areas where energy use is concentrated can help support the development and implementation of energy efficiency measures (EEM) and renewable energy sources (RES) interventions that support a transition of the entire greenhouse sector. In recent years, various studies have investigated and proposed EEM in different forms: [PERSON] et al. (2021) discuss a range of practical EEM for greenhouses located in Mediterranean countries [10], while [PERSON] (2012) finds that energy demand in greenhouses in northern Italy can be reduced by 30% by the usage of better insulated transparent materials. [17]. [PERSON] et al. (2020) conduct a review of different control strategies for improving the energy efficiency of greenhouses finding that traditional control strategies combined with intelligent algorithms have become a popular way to reduce the energy demand of greenhouses and can lead to substantial energy savings [43].