Biomass is an agricultural residue of biological origin, as well as a renewable energy source, which often remains unexploited. Biomass can be exploited as a solid, liquid or gaseous fuel, reducing the energy costs of agricultural investments. The aim of the approved research project is the development of a cogeneration system (production of electricity and thermal energy for exploitation), based on the thermodynamic Organic Rankine Cycle (ORC) and used fuel the residual greenhouse biomass. The results of the project are expected to improve the economic depreciation of an agricultural investment (e.g., income increase from agricultural crops, due to reduced electricity and heating consumption), and also to significantly contribute to the evaluation of biomass management and its efficient exploitation.
Figure 1. Biomass combustion system illustration with conventional ORC integration for the exploitation of exhaust gases
Figure 2. ORC cycle with recuperator (a) and presentation of the corresponding thermodynamic states on a temperature-entropy diagram (b)
The cogeneration system that will be developed In the frame of the research project will have a thermal and an electrical power of 200 kW and 20 kW respectively. The properly controlled combustion of biomass will take place at 130oC, while the condensing temperature of the ORC cycle will be about 50 οC, given as thermal power to the greenhouse. The studied system will cover the thermal needs of a greenhouse of 300 m2.
The applied ORC cycle is described in the diagram below (Figure 2).
The specific objectives expected to be achieved are:
Figure 3. Simple ORC layout thermodynamic cycle with isentropic expansion of isothermal and quasi-isothermal expansion (IE-E)
An important research element has been the selection of the appropriate working fluid. According to the European Refrigerant Regulation, also known as the F-gas Regulation, the use of organic fluids with high Global Warming Potential (GWP) has begun to be limited in order to reinforce the use of environmentally friendly fluids. In addition, all fluids should have zero Ozone Depletion Potential (ODP). In this context, and in order to ensure the future use of the Greenco unit without any restriction, the R1224yd (Z) has been chosen, which is one of the most efficient for heat transfer in the range below 120 oC and also has a low GWP.