Thermodynamic analysis of a novel multigeneration energy system based on heat recovery from a biomass CHP cycle

• Novel heat recovery based energy system is proposed.
• There is one energy input while system has 5 useful outputs.
• Combustion excess air increases district heating capacity but declines efficiency.
• Steam generation has the most effect on system efficiency.

A multigeneration energy system with one fuel intake (sawdust biomass fuel) and five useful outputs is proposed and energy and exergy analyses are carried out to assess its performance. Instead of using a simple heat exchanger to satisfy district heating needs, applying a deaerator is found to result in 10% more hot water mass flow rate for the same conditions. The energy and exergy efficiencies of the multigeneration system are found to be around 60% and 25%, respectively, while the corresponding energy and exergy efficiencies of a biomass system with only electricity generation are 11% and 13%, respectively. When investigating the effect of adding various product outputs to biomass power generation, steam generation and then domestic hot water production are found to have the greatest enhancing effects on the system efficiencies. Heat recovery from exhaust gases for district heating and wood drying is found to enhance the energy efficiency more than the exergy efficiency. Also, due to the size of the heat recovery system, which is smaller than the biomass CHP cycle, district heating and drying cannot increase the energy and exergy efficiencies of the primary system like steam generation. A parametric study shows that the biomass fuel input rate affects significantly the district heating heat load and the electricity generation rate, in a linear manner. However, increasing the biomass input rate has no effect on the CHP system energy and exergy efficiencies, while increasing the exergy efficiency of the entire system and decreasing its corresponding energy efficiency slightly. Of the several heat recovery options from exhaust gases, electricity generation and wood drying result in the highest exergy efficiency while district heating and drying lead to highest energy efficiency.