## 2 Heat-to-Power Applications
### Supercritical CO\({}_{2}\) Power Cycles
#### 2.3.3 Experimental Results  
Experimental work carried out by [PERSON] et al. [43; 44; 45; 53] was first of its kind and provided significant results. A strong correlation between variation of solar irradiation and thermophysical properties of the cycle working fluid was demonstrated. Collector outlet temperature and pressure increased with the amount of radiation recovered. Collector efficiency performed well, and displayed a thermal efficiency above 50% and comparison with water proved supercritical carbon dioxide had better heat transfer characteristics [44]. The estimated power output was 820 W. The maximum temperature and pressure of around 187 \({}^{\circ}\)C, and 9.26 MPa were reached and the system yield an overall efficiency of around 9%, well above that of a solar cell, 8.2% [43]. [PERSON] et al. [47] used light bulbs as electrical load and the system investigated produced around 1 kW electrical power, and yielded an efficiency of 5%. They found that the piston expander performed poorly and displayed an isentropic efficiency as low as 22%, and concluded it wasn't suitable. They furthermore indicated that an increase in the mass flow rate causes the pressure in the evaporator to increase. [PERSON] et al. [48] evaluated experimental performances of a low-grade heat driven CO\({}_{2}\) transcritical cycle and few results were obtained. Tests results showed a correlation between CO\({}_{2}\) cycle mass flow rate and turbine's inlet and outlet pressures and subsequently with the generated power. Power output varied with working fluid and thermal oil mass flow rates, and reached a maximum around 500 W. System overall efficiency exceeded 10% and increased with the mass flow rate. The maximum turbine isentropic efficiency was 45% and showed poor performance when CO\({}_{2}\) mass flow rate was increased. From the experimental results obtained these authors were able to develop and implement a model in TRNsys to predict the impact of ambient air temperature on the system. [PERSON] et al. [49] tested CO\({}_{2}\) transcritical and R245 fa subcritical cycles driven by the same heat source in view of performance comparison. Input parameters were kept in closed ranges, 50-70 kW thermal heat, 0.2-0.3 kg/s working fluid flow rate, oil temperature of 139 \({}^{\circ}\)C and 130 \({}^{\circ}\)C for CO\({}_{2}\) and R245 fa, and ambient air temperature of 24 \({}^{\circ}\)C and 17 \({}^{\circ}\)C, respectively. Test results on both configurations showed that pressure at different parts of the system, power generated, and turbine efficiency increased with working mass flow rate while cycle temperatures decreased with an increase in mass flow rates and poor performances of the turbines were observed. Maximum cycle pressure recorded was around 90 bar for the transcritical cycle, i.e., more than seven-fold that of the subcritical one, 12 bar. Heat transfer analysis in the evaporator/gas heater showed that the transcritical cycle has the potential to perform better than the subcritical configuration. Positive variation of heat input, affect both cycle temperature and pressure. Measured isentropic efficiencies of expanders were below 40%. However, the authors failed to bring out important results such as cycle power output and system efficiency of both systems in similar conditions for meaningful comparison. [PERSON] et al. [50] aimed at improving trans-critical operation by cycle architecture modification and compared four configurations: basic cycle (B), cycle with regenerator (R), cycle with preheater (P), and cycle with both regenerator and preheater (PR). They were able to show that integrating a regenerator and a preheater is the best option as it provided best performances: 34 kW power output, a thermal efficiency of 7.8% and an exergy efficiency of 17.1%. It should be noted that of all configurations, the basic cycle showed lowest performances. However, results obtained were not realistic as the system did not produce any work, and performances were overestimated. [PERSON] et al.