# Parametric Analysis of a Polygeneration System with CO\({}_{2}\) Working Fluid
## 1 Introduction  
Renewable energy utilization is an important weapon for tackling critical energy problems such as fossil fuel depletion, global warming, increasing energy demand and increasing electricity prices [1; 2; 3]. Furthermore, trigeneration and polygeneration systems are highly efficient units that can produce numerous useful outputs simultaneously [4; 5]. Thus, the concept of using renewable energy sources to feed polygeneration systems is a viable and environmentally friendly solution for future sustainable systems. Another important aspect of these systems is their use of environmentally friendly working fluids, which are usually natural fluids such as CO\({}_{2}\), NH\({}_{3}\), propane and butane [6]. However, these fluids present some limitations, which are related to performance, toxicity and flammability. CO\({}_{2}\) seems to be the most promising fluid as it can operate both in transcritical and supercritical configurations and it is not toxic, not flammable and it is a cheap working fluid [7]. Moreover, there are alternative working fluids (not natural) that have zero ODP and a not so high GWP (<1000) that can be used, like R32, R245 ca, R245 fa, R365 mfc and R1336 mzz [8]. In the literature, there are many studies that examine polygeneration systems driven by renewable energy sources (solar, geothermal, biomass and wind) [9]. [PERSON] [10] studied a system with an organic Rankine cycle (ORC), absorption heat pump and parabolic trough solar collectors, which presented a 20% exergy efficiency. A similar configuration was examined and optimized by [PERSON] and [PERSON] [11], who found a global maximum exergy efficiency of 29.4% with an energy efficiency of about 150%. A trigeneration system driven by solar energy was studied by [PERSON] et al. [12]. This system incorporated an ORCand a double-stage absorption heat pump and had 13% exergy efficiency and 96% energy efficiency. [PERSON] et al. [13] investigated a unit for fresh-water, cooling and electricity production that was driven by a parabolic trough collector, which had an exergy efficiency of 42%. In another interesting work, [PERSON] et al. [14] examined a cogeneration unit for cooling and electricity production based on absorption technology. They conducted an experimental investigation and found the maximum electricity production was up to 0.7 kW, while the maximum cooling production was up to 8 kW. The combination of solar and geothermal energy has been studied by [PERSON] et al. [15]. They studied a configuration with an ORC and absorption chiller, which had 76% energy efficiency and 7.3% exergy efficiency. [PERSON] et al. [16] studied a polygeneration system driven by solar energy and biomass. This system produced heating at two temperature levels, cooling and electricity. The energy efficiency was 51.3% and the exergy efficiency was 21.8%. [PERSON] et al. [17] studied the use of a trigeneration system with a Stirling engine as the primary mover, which was driven by biomass heat input. They found that the recovery of the engine waste heat was a promising idea for improving the system's sustainability. The next part of our literature review focused on trigeneration systems that use CO2 as the working fluid. [PERSON] et al. [18] studied a Brayton power cycle with an ejector device for power and refrigeration production, which also produced heating from the turbine outlet heat. The system was driven by compound parabolic solar collectors and it showed 28.8% exergy efficiency and 53% energy efficiency. [PERSON] et al. [19] studied the modification of the previous system with extraction, which resulted in a 22.5% exergy efficiency enhancement. [PERSON] et al. [20] studied a complex cycle with CO2 as the main working fluid. This system was driven by methanol and it includes a typical air-Brayton cycle, which feeds heat to a CO2 cycle for power and refrigeration. The heating produced by this system is practically steam, which is created by the waste heat of the air-Brayton cycle.