## 1 Introduction
[27; 28] provided a performance comparison of low-grade ORC using R245 fa, pentane and their mixtures based on the thermo-economic multi-objective optimisation and decision makings, and tested the operation characteristic of a R123-based ORC subject on WF mass flow rates and heat source temperatures. Applying the same objective function of maximum net power output on geothermal ORC system, [PERSON] et al. [29] concluded that R600a/R601 mixtures generated between 11% and 4% extra power than R600a for geothermal water inlet temperatures between 110 \({}^{\circ}\)C and 150 \({}^{\circ}\)C, respectively. [PERSON] et al. [30] observed that using zeotropic mixtures as the WF in lieu of a pure fluid such as R245 fa, results to more than 24% increase in power production for ORC with 100 \({}^{\circ}\)C geothermal water. [PERSON] et al. [31] identified that WF mixture is more cost-effective than pure WF and R245 fa/isopentane and R245 fa/pentane were suggested for 100-180 \({}^{\circ}\)C heat sources amid four mixtures and five pure WFs. [PERSON] et al. [32] analysed the performance of ORC using zeotropic mixtures of R227 ea/R245 fa, butane/R245 fa and RC318/R245 fa for a 120 \({}^{\circ}\)C hot air heat source. Outcomes revealed, however, that the mixtures can deliver high thermal efficiency but result in a poor economic performance. [PERSON] and [PERSON] [33] reviewed the zeotropic mixtures used in ORC, described both the advantages and problems based on previous studies for the specific heat sources. [PERSON] et al. [34] investigated experimentally R245 fa, R123 and their mixtures to produce maximum net power on simulated low-temperature industrial waste heat. Four mass fractions of R245 fa:R123 were used to test the system performance at the heat source temperatures of 110 \({}^{\circ}\)C and 120 \({}^{\circ}\)C with a specified mass flow rate of a heat source of 0.15 kg/h. They found that the case of pure R245 fa generated a maximum net power of 1.56 kW with an electrical efficiency of about 3.9% when heat source temperature is fixed at 110 \({}^{\circ}\)C, while, the case of mixture R245 fa:R123 = 2:1 generated a maximum net power of 1.66 kW with an electrical efficiency of about 4.4% when heat source temperature input was fixed at 120 \({}^{\circ}\)C. [PERSON] et al. [35] provided provisional design procedures of zeotropic mixture WFs for ORC. They showed that the estimated mixtures have superior performance than the ideal pure WF. They recommended that the proposed zeotropic mixtures would have the identical basic characteristics like the best pure WFs and as well possess temperature glide matching with the cooling source. The numerous studies described above indicate the increasing attention to the subject of ORC applications for low-temperature applications and use of WF mixtures. However, in spite of this proliferation of studies trying to solve an ORC system's performance using mixtures, and their contribution to the ORC application's use particularly in low-grade heat source, to the authors' knowledge, there is still insufficient research into or investigation of the micro-scale solar ORC system's using mixtures for use in buildings detailed and complete thermo-economic and sustainability performance investigations. Hence, the present study attempts to demonstrate the thermo-economic feasibility, sustainability, and potential advantages of using zeotropic mixtures as WFs in an ORC system to generate heat and power driven by low-temperature solar heat sources for building applications. A thermodynamic model has been developed with an ORC using mixtures with and without heat regeneration.