## 2 Heat-to-Power Applications
### Supercritical CO\({}_{2}\) Brayton Cycles
#### Experimental Results  
[PERSON] et al. [73] developed models of closed recuperated Brayton cycle driven by heat from a nuclear reactor and these models were subsequently validated. For the model validation, they used nitrogen as working fluid and a capstone C-30 microturbine power generator to provide a small-scale closed Brayton cycle, the SBL-30. The 30 kWe microturbine operated at 1144 K turbine inlet temperature with a shaft speed of 96 kRPM and was modified to integrate an electrical heater and a water-cooled chiller. [PERSON] et al. [74] later on investigated a split flow recompression Brayton cycle based on previous research carried by [PERSON] et al. [75; 76] during which they designed and tested the cycle components. In the proposed architecture two separate turbines receive separated flows. The system design integrates two TACs of 123 kWe rated power each (hermetically sealed pressure vessels), two recuperators (610 and 2232 kW), electrical heaters (780 kW), and a pre-cooler (531 kW). Efficiencies of turbines and compressors were 87% and 68%, respectively. The experiment aimed at simulating the operation of a solar thermal power plant. The split flow recompression Brayton cycle configuration tested by [PERSON] et al. [75] showed poor performance. For a turbine inlet temperature of 404 \({}^{\circ}\)C (9.89 MPa), a power output of 16.6 kW, and a cycle efficiency of 4.9% were obtained. At design conditions, the system was expected to produce 135 kW power output and a cycle efficiency of 15.2%. During the experiment, thermal losses at various parts of the system were detected and quantified, about 40% of total heat input. Rotating losses also contributed to system inefficiency. In fact, the high density of the working fluid and the high-speed environment led to turbulent flow at the shaft. [PERSON] et al. [68] at CVR, Czech Republic, built a small-scale test facility in the framework of the SUSEN project. The facility is intended to be used for components testing at temperature up to 550 \({}^{\circ}\)C and pressure up to 30 MPa. The test rig built at CVR, aimed at testing heat exchangers and turbomachinery. Compressor operated as expected while closed loop operation of the simple Brayton cycle was not possible because of excess pressure losses. The 1 MW recuperated cycle built at SwRI [72] saw its firsts tests, and the turbine reached a temperature of 550 \({}^{\circ}\)C at 21 kRPM. Future experimental investigations are planned and will focus on taking the temperature up to the design point set at 715 \({}^{\circ}\)C/25 MPa. [PERSON] et al. [77] reported the development of a test rig at the university of Cranfield (UK) were they intend to test various components integrated in a CO\({}_{2}\) loop. In South Korea, KAERI, KAIST and POSTECH have teamed up to carry out investigation on supercritical Brayton cycles. They have developed a s-CO\({}_{2}\) integral experiment loop (SCIEL) test facility with the aim of testing cycle operation and components. Two projects were put in place for high and low compression ratio [54; 78]. The first test rig was a recuperated Brayton with two stage compression and two stage expansion. The turbines were designed for low pressure ratio (<2) and could stand a temperature as high as 500 \({}^{\circ}\)C, well suited for an SFR. The second one was a simple Brayton cycle. At the KIER, a research plan was set to investigate transcritical Rankine and Brayton cycles. Several cycle architectures with different power output were to be tested and evaluated [79]. Selected Brayton architectures were standard closed Brayton and dual Brayton. In the course of their experiment they designed and built a small-scale high speed turbo-generator (1 kWe, 200 kRPM) for a multi-purpose test loop which could operate at high temperature (500 \({}^{\circ}\)C) as simple Brayton and at low temperature (200 \({}^{\circ}\)C) as a transcritical Rankine cycle [80]. The first system designed by Echogen was a 10 MW sCO\({}_{2}\) plant based on an innovative recuperated cycle.