In recent years, due to the circular economy, the use of green energy forms, such as biofuels and biogas from anaerobic digestion of fermentable materials (e.g., agricultural and livestock residues) has entered our lives. According to the International Energy Agency it is estimated that the needs in 2040 will be 48% higher than in 2012 so all political decisions have converged on an urgent need for the use of more and more renewable and green energy. Considering the overall economic activity of these sectors in the region of Thessaly, the aim of this study is to highlight the residues from agricultural and livestock activities in the primary sector and calculate the annual biomass production, the methane and biogas potential, the electrical and thermal energy that can be produced from these wastes, as well as the solid residue that can be used to improve the soil of the region. The study was based on data referring to the years 2015 to 2020. The production of livestock and agricultural residues, averaged over the above six-year period in the study area, was estimated at approximately 4.8 × 10⁶ t·yr.⁻¹, with livestock residues accounting for 83% and agricultural residues for 17%. Furthermore, the total residues can produce an average biogas potential of approximately 4.7 × 10⁶ m³·yr.⁻¹, while the amount of electricity that can be produced ranges from 708–1091 GWh·yr.⁻¹, and the corresponding thermal energy from 1112–1577 GWh·yr.⁻¹. As a result of the complete anaerobic digestion process, a solid residue could also be obtained for the improvement of the region’s soil, which translates into a quantity in the range of 4.01 × 10⁴ to 5.10 × 10⁴ t·yr.⁻¹.

Several years have passed since the linear economy model proved unsustainable, leading to the transition toward the circular economy (CE) model. Significant amounts of agricultural residues and waste from livestock farming units remain unutilized in fields. The anaerobic digestion (AD) method addresses this issue by generating energy in the form of thermal (TE) and electrical energy (EE). This article examines greenhouse heating using thermal energy from a biogas plant. For this purpose, a thermal load model is developed and applied in two regions, northern (Florina) and central Greece (Trikala), to assess the greenhouse’s energy requirements in areas with differing characteristics, especially during the winter months. Additionally, the economic benefits of a biogas plant from selling electricity to the grid are analyzed. Thermal energy constitutes 59.7% of the system’s total energy output. On average, the generated electrical energy amounts to 518 MW h per month, while thermal energy reaches 770 MW h per month. The biogas plant’s daily electricity consumption ranges from 1564 kW h to 2173 kW h, depending on its needs. Ambient temperatures vary between 0 °C and 37 °C, significantly influencing the greenhouse heating system’s efficiency. The biogas plant also demonstrates financial profitability, earning 504,549 € annually from the sale of surplus electricity. Furthermore, the article explores greenhouse crops in the broader Thessaly region, where tomato cultivation seems to be dominant. Greenhouse heating requirements depend on crop type, location, weather conditions, sunlight exposure, and heat loss based on covering materials. Meanwhile, the thermal energy output that can heat a given greenhouse area is directly proportional to the biogas plant’s capacity.