## 2 Heat-to-Power Applications
### Supercritical CO\({}_{2}\) Power Cycles
#### 2.3.2 Various Layouts  
Experimental investigations on small scale transcritical CO\({}_{2}\) Rankine cycle have been carried out since the 2000s. Early attempts were carried out at the Doshisha University, Japan and regarded a solar driven medium-temperature system. And since then, few other systems have been built and tested, mainly for waste heat recovery applications. A list of various layouts is given in Table 4 and commented in following paragraphs. [PERSON] et al. [43; 44; 45; 46] conducted experimental research on a solar driven CO\({}_{2}\) transcritical cycle. In principle, their system uses a solar collector array and direct evaporation, a microturbine, a condenser, a CO\({}_{2}\) feed pump and is designed to operate in CHP mode. Due to lack of appropriate turbine/expansion machine, they installed an expansion valve and implemented an equation to estimate the power output of the system. Two heat exchangers connected in series configuration serve as condenser, cooled by a cooling tower. A piston pump was integrated to move the working fluid across the cycle. [PERSON] et al. [47] proposed a small scale transcritical system in which they integrated a 2 kW rolling piston expander coupled to a generator, shell and tube heat exchangers that can stand a pressure of 20 MPa as evaporator and regenerator and another one that can stand lower pressure as 8 MPa as condenser. A plunger pump with a rated pressure of 20 MPa was used as feed pump. A 34 kW cooling unit providing 5 \({}^{\circ}\)C cold water was connected to the condenser. An oil loop with an electric heater produced hot fluid at an approximate temperature of 210 \({}^{\circ}\)C.  
A test facility was built for investigation on small-scale transcritical CO\({}_{2}\) Rankine cycles by [PERSON] et al. [48]. It comprises an 80 kW micro-turbine CHP unit, an oil loop, a Rankine engine with integrated recuperator, and a dry cooling system. The CO\({}_{2}\) turbine was an axial, with single stage and a reaction ratio of 0.5. It was designed for 5 kWe power output, 0.281 kg/s mass flow rate and has a diameter of 144 mm. The CO\({}_{2}\) feed pump was a triplex plunger pump. Hot oil temperature and mass flow rate were varied through CHP power output and variable pump speed, respectively. As their system was designed, four two-way valves were installed so as to bypass the recuperator whenever necessary. System parameters could therefore be measured in both configurations: with and without recuperator. [PERSON] et al. [49] decided to build, test and compare a small scale transcritical CO\({}_{2}\) Rankine cycle using R245 fa. The system investigated by [PERSON] et al. [50] was then complemented with an ORC engine. The ORC engine uses copper pipes instead of stainless steel as for the CO\({}_{2}\) test rig. The ORC turbine was also an axial one with single stage. The working fluid pump was a diaphragm pump coupled to a 1.1 kW asynchronous motor, and two air condensers mounted separately and controlled by variable speed fans. As both engines were driven by the same heat source, two valves were installed for cycle's selection for comparing their performance at similar conditions. [PERSON] et al. [50] designed a small scale highly flexible CO\({}_{2}\) transcritical Rankine system in which they could study several cycle configurations: basic cycle, cycle with regenerator, cycle with preheater, and cycle with both regenerator and preheater. Heat was recovered from a heavy-duty diesel engine (exhaust flue gas and coolant water), the gas heater was a double pipe heat exchanger, and other plate heat exchangers type were used for the rest of heat transfer systems. A plunger pump was integrated to pressurize the liquid fluid in the condenser, and a needle type expansion valve was integrated instead of a turbine. [PERSON] et al.