## 2 Heat-to-Power Applications
### Supercritical Steam Rankine Cycles  
There are several methods to improve the thermal efficiency of a Rankine cycle. One way frequently used to increase the cycle's efficiency is to increase the temperature of the steam entering the turbine. However, this temperature is restricted by metallurgical limitations imposed by the materials and design of the components and primary piping. The equipment and piping must withstand high pressures and great stresses at elevated temperatures. With new material development it is possible to go to higher temperatures during superheat and reheat. To balance that with the wetness at the last turbine stages, higher boiler pressure is required. Most thermal power plants are currently designed to operate on the supercritical Rankine cycle (i.e., with steam pressures exceeding the critical pressure of water 22.1 MPa, and turbine inlet temperatures exceeding 374 \({}^{\circ}\)C). Supercritical fossil fuel power plants present efficiencies of around 43%. Moreover, efficiencies of 48% are reached for very complex coal-fired power plants that operate at \"ultra-critical\" pressures (i.e., around 30 MPa) and use multiple stage reheat (see Figure 2) [21].  
\begin{table}
\begin{tabular}{c c c c c c} \hline
**Fluids** & **Mol. Wt.** & \begin{tabular}{c} **NBP** \\ **(\({}^{\circ}\)C)** \\ \end{tabular} & \
\begin{tabular}{c} **p\({}_{\text{c}}\)** \\ **(bar)** \\ \end{tabular} &
\begin{tabular}{c} **t\({}_{\text{c}}\)** \\ **(\({}^{\circ}\)C)** \\ \end{tabular} & **GWP** \\ \hline R744 & 44.01 & \(-\)56.6 & 73.8 & 31.1 & 1 \\ R170 & 30.07 & \(-\)88.6 & 48.7 & 32.2 & 3 \\ R744a & 44.01 & \(-\)88.5 & 72.4 & 36.4 & 298 \\ R41 & 34.03 & \(-\)78.1 & 59.0 & 44.1 & 97 \\ R32 & 52.02 & \(-\)51.6 & 57.8 & 78.1 & 550 \\ R1270 & 42.08 & \(-\)47.7 & 46.6 & 92.4 & 2 \\ R290 & 44.09 & \(-\)42.1 & 42.5 & 96.7 & 3 \\ R152a & 66.05 & \(-\)24.0 & 54.20 & 113.3 & 120 \\ C3H4 & 40.06 & \(-\)23.2 & 56.3 & 129.2 & 150 \\ R717 & 17.07 & \(-\)33.3 & 113.3 & 132.2 & 0 \\ R600a & 58.12 & \(-\)11.8 & 36.30 & 134.6 & 3 \\ \hline \end{tabular}
\end{table}
Table 1: Working fluids suitable for supercritical operation.  
Various Layouts  
Even though most supercritical units are coal-fired, in 2014, the Commonwealth Scientific and Industrial Research Organization (CSIRO) of Australia set a world record in generating \"supercritical\" steam using solar power with a field of more than 600 directional mirrors (heliostats) directed at two towers housing solar receivers and turbines, generating steam at a pressure of 23.5 MPa and 570 \({}^{\circ}\)C [22]. This step change proved that solar has the potential to compete with the peak performance capabilities of fossil fuel sources.  
One very efficient way to ameliorate the performance of classic thermal power plants is the combined cycle gas turbine (CCGT) system configurations. Attaining higher overall thermal efficiency of the CCGT necessitates optimizing the entire plant and the three major components: the gas turbine (GT), the heat recovery steam generator (HRSG) and the steam turbine (ST). However, among the three components of the CCGT plants, the performance of the GT comes into first as the predominant influencer on the performance; with which plant efficiency can approach the 55% target and even more [23].  
Another concept, proposed by [PERSON] et al. [24], investigates an improved 1000 MW supercritical coal-fired power generation system incorporating a supplementary regenerative supercritical CO\({}_{2}\) cycle. The overall system efficiency of the proposed system reached as high as 46.0%, 0.4%higher than that of the reference power plant and could soar to 46.9% when the exhaust flue gas temperature dropped to 100.0 \({}^{\circ}\)C.