## 1 Introduction
However, transcritical configurations present higher investment costs than subcritical arrangements owing to their higher operating pressures and larger size of some components (turbines, pumps and evaporators) [60; 61]. Nevertheless, transcritical ORCs appear a promising technology to also exploit geothermal sources from an economic perspective [60; 61; 62]. [PERSON] et al. compared different ORC configurations (subcritical and transcritical, saturated and superheated, simple and regenerative cycles) with different organic fluids (isobutane, isopentane, R134a, R245 fa, R1233 zd, and R1234 yf) for the exploitation of a 40 kg/s geothermal source at 180 \({}^{\circ}\)C [60]. A bi-variable optimisation was adopted based on the exergy efficiency maximisation and payback period minimisation. The comparison between optimised configurations demonstrated that transcritical systems present a lower payback period compared to subcritical arrangements for all the investigated working fluids, with decrease up to 90%. Furthermore, the authors demonstrated that when the payback period is fixed, the transcritical ORCs provide higher net power, larger long-term economic profit, and lower specific investment cost compared to subcritical systems. The advantages of transcritical configurations were also confirmed for geothermal source temperatures between 140 and 200 \({}^{\circ}\)C. Similar results were found by [PERSON] et al., who observed a significant increase in the gross power and a noticeable decrease in the payback period comparing transcritical and subcritical power plants with geothermal source temperatures between 100 and 190 \({}^{\circ}\)C and different working fluids [61]. Furthermore, [PERSON] et al. highlighted that transcritical systems guarantee lower specific costs with respect to subcritical ORCs when the ratio between the critical temperature of the organic fluid and the geothermal source thermal level ranges between 0.8 and 0.9 due to the higher energy production and efficiency that overcomes the larger investment costs. Furthermore, the investigation demonstrated that supercritical ORCs offer better energy and economic performance for high geothermal sources [62]. Nevertheless, to the best of the authors' knowledge, few works are present in the literature concerning geothermal transcritical configurations. The proposed unit is able to work at partial loads to fulfil the energy demand of the final users, to increase the system flexibility and to respond efficiently to load variations [30]. It is worthy to note that most of the papers present in the literature do not focus on off-design performance [63]. The internal regeneration has been considered to maximise the electric power and achieve higher efficiency with respect to the mean value of typical geothermal plants (close to 12%) and isobutane has been selected as working fluid owing to its good performance and low specific costs, according to the literature [64; 65; 66; 67]. In particular, isobutane is considered one of the organic fluids with zero ODP (ozone depletion potential), suitable for supercritical ORC applications [33]. Even though the relatively high pressures lead to higher operating costs, the integrated system reaches higher global efficiency for high thermal level geothermal sources. [PERSON] et al. found that high pressure values are more appropriate for high expander inlet temperatures to maximise the system efficiency [68]. The authors show that only a few investigations are focused on working fluid temperatures larger than 180 \({}^{\circ}\)C. Despite this, in the range of temperatures 180-240 \({}^{\circ}\)C isobutane revealed good energy performance in transcritical ORCs. In particular, the authors showed that the best isobutane cycles are supercritical for the highest values of temperature of the geofluid investigated (180 \({}^{\circ}\)C and higher). [PERSON] et al.