## 1 Introduction  
Waste heat recovery (WHR) has huge potential to increase the energy efficiency of industrial plants thus reducing their fossil fuel consumption and the related CO2 emissions. Indeed, many industrial processes reject heat at different temperatures, determining the quality of the waste heat as high (>400 \({}^{\circ}\)C), medium (100-400 \({}^{\circ}\)C), or low (<100 \({}^{\circ}\)C) [1]. In general, a WHR system may use the technically and economically usable part of this waste heat in different ways such as (i) direct use without upgrading, (ii) use after upgrading through heat pumps, and (iii) electricity production. In the case of low and medium grade waste heat, organic Rankine cycle (ORC) systems have attracted particular attention in recent years due to their reliability, flexibility, safety, and applicability for low-temperature power generation [2]. ORC systems have already been deployed at medium (50-300 kW) and large scales (>300 kW) by industries as a mature and reliable technology as reported in [3], yet several challenges must be faced at the micro (<10 kW) to small (10-50 kW) scales. At these scales, the systems' primary costs and low conversion efficiencies are still limiting their wider adoption. Moreover, the latter becomes significantly more visible in cases where the system operates in off-design conditions. The instability of the heat source is usually the main cause of the off-design conditions, which result in system efficiency degradation at best or pump cavitation or turbine damage at worst. In addition, the variations of the heat sink condition, the electric load instability, and the working fluid loss due to leakage are the other prominent causes of the system being in off-design conditions. Therefore, understanding the ORC system off-design characteristics is necessary to realize a system with reliable on-site performance that can be integrated with the other energy systems. In the literature, an extensive number of papers can be found dealing with experimental studies of ORC systems. For instance, 68 references were used in an analysis published in 2018 on the experimental database of ORC systems [4]. Hence, only a non-exhaustive number of references dealing with subcritical ORC systems using R134a as the working fluid are reported hereunder. [PERSON] et al. [5] developed a test bench to investigate the performance of a scroll expander with a displacement of 6.8 cm\({}^{3}\) using R134a. The expander maximum power and efficiency were about 1.17 kW and 70%, respectively. A car scroll compressor was converted into an expander in an ORC system in [6] using R134a. The best performances of the system were reported as an efficiency of 3.3% and 279.58 W in the range of the heat source temperature of 75 to 95 \({}^{\circ}\)C. An air screw compressor was converted for use as an expander in a micro-scale ORC unit with R134a in [7]. Their test bench was a very-low-temperature WHR system with a heat source temperature range of 55 to 65 \({}^{\circ}\)C. The efficiency of their system was reported as 1.2% to 4.56%. [PERSON] et al. [8] devised a heat pump/ORC system for building applications. In the ORC mode, their system could produce a maximum thermal efficiency of 4.2% and a gross power output of 3.7 kW using the scroll expander and R134a as the working fluid. In this study, a non-regenerative ORC unit was tested using R134a in off-design conditions. The system consists of an open-drive, lubricated scroll expander; a plunger pump; and two plate heat exchangers as the evaporator and the condenser. In addition to the experimental performance of the ORC system and its main performance, experimental data were used to simulate the components of the system. The main novelties of the numerical investigation rely on:  
* Experimental study of a micro-scale ORC system with the scroll expander shaft speed considerably lower than typical values due to the electric generator requirements.