## 4 Development and Market Status of Supercritical Energy Conversion
### Pathways in Future Development  
The use of water as a working fluid in supercritical power plants is the largest application of any fluid at supercritical pressures in an industry. However, there are a number of areas where other supercritical fluids are used currently, or can show performance benefits in the near future [120; 121; 122] as given below:  
* Supercritical carbon dioxide as a refrigerant in air conditioning, refrigerating systems and power systems. * Supercritical refrigerants in thermodynamic cycles for heat and power conversion. * Supercritical carbon dioxide, near-critical helium and liquid hydrocarbon used as cooling media. However, at the moment there is no general adoption of supercritical fluids in thermodynamic cycles. This is primarily due to the unknown behavior of the supercritical fluid in the heat exchangers at design and during operation. It is known that in the supercritical region the thermophysical properties change drastically with temperature and pressure. The near-critical point is actually a region around the critical point where all thermophysical properties of a pure fluid exhibit rapid variations. In contrast to the refrigerants used in ORCs or heat pumps, supercritical heat transfer has only been investigated in some detail already for CO\({}_{2}\) and water [123; 124]. An example of data found for water is given in Figure 6. These unknowns now pose a high risk for potential manufacturers which are typically small to medium sized companies. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Plant/Company Name** & **Country** & **Type** & **Working Fluid** & **Output (kW)** & **Year of Commissioning** \\ \hline \hline
\begin{tabular}{c} Granite \\ Power \\ \end{tabular} & Australia & Solar & GRANEX R & 30 & 2014 \\ \hline TAS & USA & Geothermal & R134a & 22,000 & 2012 \\ \hline Atlas Copco & Canada & Waste Heat & Butane & 7500 & 2012 \\ \hline Turboden & Italy & Geothermal & - & 500 & 2012 \\ \hline San Emidio & USA & Geothermal & R134a & 8000 & 2013 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Selected supercritical ORC plants in operation. Figure 6: Variations of selected thermophysical properties of water near pseudo-critical point: pseudo-critical region at 25 MPa is about \(\pm 25\,^{\circ}\)C around pseudo-critical point. In addition, there are regions of heat transfer enhancement (HTE) and heat transfer deterioration (HTD) [125]. However, even for CO\({}_{2}\) and water, there is no consensus on the physical explanation. Yet, it is clear that these effects lead to flow instabilities during operation and they complicate the design of heat exchangers. For ORCs this becomes even more of an issue as the heat source frequently shows intermittent behavior as waste heat availability and solar irradiation changes. As such, understanding the flow behavior during heat transfer to supercritical refrigerants is a crucial step to design supercritical ORCs and to avoid unstable operation. Unfortunately, this specific research is mostly lacking in scientific literature. Another technology that influences the economy, safety and reliability of a high-pressure plant and sits at its core are the high-pressure pumps and compressors [126]. The challenge in designing pumps operating at high pressure include proper sealing, fatigue, wear, installation and vibration which come up in such applications. In these conditions, high stresses and strains are generated in the liquid containing parts leading to fatigue and hence, non-conventional materials and techniques of design and manufacturing are required. From the point of view of increasing efficiency of the power plant by increasing steam inlet pressure and temperature, there is a reduction in the power demand from almost all auxiliary equipment except the boiler feed pump. The reason behind this is the linear dependence of the power needed to drive the feed pump on pressure. This results in a substantial increase in the feed water pumping power, even though there is increase in the pump efficiency [126].