**2. Carnot Battery Development by Technologies**

## *2.1. Rankine Cycle Systems*

Rankine cycle systems can be further divided into steam Rankine cycles, which are predetermined to work efficiently at higher heat source temperatures and high power outputs typically in the order of dozens to hundreds MW and organic Rankine cycles (ORC), which are the domain of smaller power outputs and lower heat source temperatures up to several MW and possibly down to the kW scale. CO2 cycles, typically with transcritical operation, make up a specific category. Considering the underlying principle, even systems, where the Rankine cycle thermodynamic changes are separated to different time periods, are included here. A typical example is storing heat in a liquefied gas or vapor (latent heat), while the gas itself is a working fluid in the thermodynamic conversion.

Table 1 comprehensively summarizes the CB systems utilizing the Rankine cycle for its discharge phase in the scope of commercial development (or having some commercially oriented aspects). They are sorted according to the power to heat conversion method to the resistance heated systems and fully reverse systems; then by working fluids from water to organic fluid, air and CO2 and finally to systems with a Rankine cycle heat pump as a full cycle and the systems with part of the heat pump cycle with liquefaction and storage of the working fluid. Note that eight out of 13 reported systems are conducting experimental, demonstration or even pilot operations of their systems.


**Table 1.** List of commercial development projects in CB using Rankine cycle discharging.
