# Parametric Analysis of a Polygeneration System with CO\({}_{2}\) Working Fluid
## Material and Methods  
### The Examined System  
In this work, a polygeneration system was investigated as shown in Figure 1. A biomass boiler was used as the energy heat input and it gives the proper energy for the system operation. The working fluid in the examined thermodynamic cycle is CO2, which is an environmentally friendly fluid. This fluid has a relatively low critical point with the critical pressure being 73.8 bar and the critical temperature being 31.1 \({}^{\circ}\)C.  
Refrigeration is produced from the evaporator and saturated vapor (quality equal to 100%) is produced (Figure 1, state point 1). This quantity is compressed from low pressure to medium pressure with the use of the compressor (state point 2). After this, a proper heat exchanger is used in order to produce low-temperature heating at 45 \({}^{\circ}\)C and state point 3 is  
Figure 1: The examined polygeneration system with CO2 working fluid. selected as 50 \({}^{\circ}\)C in all the cases. Practically, a temperature difference of 5 \({}^{\circ}\)C [28] is used in order to ensure proper heat transfer. Moreover, a gas cooler is used in order to reject heat to the ambient and so the CO\({}_{2}\) reaches state point 4. A second compressor re-compresses this quantity up to state point 5. The present configuration has intercooling in order to reduce the compressing work and to recover heating. Moreover, the recompression from state point 4 to state point 5 needs a small amount of work because state point 4 is close to the critical point of the CO\({}_{2}\) (from the supercritical side) and so the density of the CO\({}_{2}\) is relatively high, which reduces the pumping work. After the second compressor, there is a heat exchanger in order to preheat the CO\({}_{2}\) before the biomass boiler. So, the CO\({}_{2}\) is warmed up to state point 6 and goes into the boiler where its temperature is increased up to the turbine inlet temperature (TIT) at state point 7. The turbine receives fluid of high pressure at state point 7 and expands it up to state point 8 (medium pressure), and so, work is produced. The heat exchanger reduces the temperature of the supercritical CO\({}_{2}\) (state point 9) and the next step is the heat recovery in the high-temperature levels (state point 10). A pinch point of 5 \({}^{\circ}\)C was also applied in this device, and when the high heating temperature is 80 \({}^{\circ}\)C, for example, the temperature level in state point 10 is 85 \({}^{\circ}\)C in this case [28]. The gas cooler follows the heating device and heat is rejected to the ambient up to state point 11. A throttling valve is used to reduce the pressure from the medium to a low level. This device is assumed to be adiabatic, and so, the enthalpy is the same in its inlet and outlet. The state point after the valve (12) is the inlet in the evaporator and so the cycle closes. A general overview of the present system can be performed by explaining the heat exchanges of the examined system with the ambient and the external sources. The examined system takes heat from a biomass boiler (Q\({}_{\text{b}}\)) at a high temperature (e.g., 700 \({}^{\circ}\)C) and there is a heat input in the evaporator (Q\({}_{\text{ref}}\)), which is the refrigeration load in the lowest cycle temperature (e.g., 5 \({}^{\circ}\)C). The system rejects heat to the ambient in the gas coolers (Q\({}_{\text{gc,1}}\) and Q\({}_{\text{gc,2}}\)) as it has a relatively low temperature of 35 \({}^{\circ}\)C, and so these heat rates are rejected and not reused. There are also the heating heat exchangers, which perform energy recovery, and consequently, heating production. More specifically, the (Q\({}_{\text{heat,low}}\)) is produced at 45 \({}^{\circ}\)C, while the (Q\({}_{\text{heat,high}}\)) is produced at a higher temperature (e.g., 80 \({}^{\circ}\)C). The compressors consume a part of the work that the turbine produces, while there is a net-work production from the system to the grid.