## 3 A Novel Model of the Reversible Rankine-Based Thermodynamic Cycle (RRTC)  
### A Proposed Design of the Reversible Rankine-Based Thermodynamic Cycle (RRTC)  
The Organic Rankine cycle (ORC) is a Rankine cycle utilizing an organic working fluid instead of water, whereas the heat pump cycle (refrigerating cycle) is a reversed Rankine cycle. The main operating purpose of the ORC is to generate electricity, whereas the main operating purpose of the heat pump is to heat (or cool) the enclosed space. The thermodynamic processes of these cycles are different. However, it was found that, taking into account the design, these cycles have very similar components, such as heat exchangers (one heat absorber and one cooler), a machine to transfer and pressurize the working fluid (a pump in the ORC and a compressor in the heat pump), and an expander. The literature review showed that the ORC system [8; 10] or heat pump [25; 26] is usually designed as a stand-alone unit for a single application (generating power or heating/cooling). Continental climate and seasonal temperature fluctuations are characteristic of many areas globally. In these areas, the application of the ORC and heat pumps is especially interesting. However, the applied designs of these systems, as described above, may lead to significant investment costs due to the necessity of installing two separate units. As an alternative, a promising and novel reversible Rankine-based thermodynamic cycle (RRTC), comprising a heat pump and the ORC, is proposed. The proposed design of this system and its thermodynamic processes are illustrated in the \(T\)-s diagram in Figure 1. It should be noted that, in Figure 1, the maximal and minimal cycle temperatures for both cycles are not the same because the heat pump utilizes waste heat during the operation. In this case, if there are no waste heat recovery/sufficient heat sources, the maximal and minimal cycle temperatures for both cycles may be in the same range. HE1 and HE2 represent heat exchanger 1 (as the heat absorber for the ORC and the heat sink for the heat pump) and heat exchanger 2 (as the heat absorber for the heat pump and the heat sink for the ORC). EXR, CPR, and LP are expander, compressor, and liquid pump, respectively. Moreover, CV and GM are control valves and the reversible generator-motor. In terms of practical aspects, installing a reversible generator-motor may entail several considerations, such as shaft layout and regulation of the shaft when the pump or compressor is not in use. This configuration will also have a significant impact on the efficiency of the generator (which generates the electricity) or the motor (which drives the pump/compressor). To improve the efficiency of a reversible motor-generator, several different factors must be considered. As a result, future experimental study of reversible motor-generators will be interesting. Counter-flow heat exchangers, as illustrated in Figure 1 for both HE1 and HE2, and indicated by the direction of the input/output and the working fluid inside the cycle, are used to create a more efficient system. Since the heat exchangers in this system are designed in a fixed geometry, and these devices behave as both condenser and evaporator for ORC and heat pump modes (and vice versa), regulating the mass flow rate to match the energy balance occurring in this heat transfer is a possible practical aspect of this design. Figure 1 shows a system that may be utilized for two different tasks (i.e., a switch-like system). This system may be appropriate for continental climate areas and those with limited heat sources that may require cooling at a specific period (i.e., not continuously). The implementation of this RRTC may be beneficial since it is more cost effective than installing two separate devices. In the RRTC, some investments, such as pipelines, heat exchangers, and other components may be reduced. This is the main advantage and innovation of the proposed system.