# ORC Optimal Design through Clusterization for Waste Heat Recovery in Anaerobic Digestion Plants
## 1 Introduction  
Anaerobic digestion plants have been spreading in Europe due to the favorable synergy between energy production and sanitation services [1]. Moreover, subsidies related to biogas upgrading into biomethane and its injection in the natural gas network, or its liquefaction, have made anaerobic plants profitable in several European countries [2]. All these issues make anaerobic digestion a promising waste-processing technology from environmental, energy, and economic points of view [3]. Anaerobic digestion requires electric energy for sludge piping and handling and thermal energy for maintaining the correct digestion temperature [4]. Traditionally, biogas-fueled internal combustion engines (ICEs) are used as cogeneration units [5]. However, micro-gas turbines (mGT) are preferred in some cases due to the lower maintenance cost and pollutant formation [6]. The thermal energy produced by cogeneration units is often more than that required by digesters, especially in hot climates or when a mGT is adopted. The excess thermal energy may be exploited in a waste heat recovery (WHR) system [7]. Several WHR systems have been proposed to increase the efficiency of biogas plants. [PERSON] et al. [8] proposed mGT inlet air cooling through an absorption chiller that exploits the excess thermal energy in the fluegas. Results showed that for anaerobic digestion plants equipped with a mGT, inlet air cooling is a valid technology from energy and economic points of view. A further study [9] showed that inlet air cooling effectiveness is strongly dependent on the climate conditions. In [7; 10], it was demonstrated that biogas plants could be optimized to become a polygenerative system from which cooling, freshwater, and electrical power are obtained. In a previous study by the authors [11], the biogas plant capability of powering both district heating and district cooling was analyzed. Results showed that trigeneration was a viable improvement for an anaerobic digestion system, but the small thermal load required in mid-seasons penalized the payback period. Among all the possible WHR solutions for power production, organic ranking cycle (ORC) is the most diffused and studied. The simplicity [12], flexibility [13], and technological maturity [14] of this technology have led to the proliferation of ORC modules in several countries [15]. In the last few years, several ORCs have been installed in anaerobic digestion plants to increase electricity production. Many of them have come from downsizing and customization of commercial modules [16; 17]. In a previous study [18], the authors showed that ORC is a promising technology to increase anaerobic digestion plant efficiency, even by adopting commercial modules. In [19], the authors showed that even small modifications of an anaerobic digestion plant could increase the producibility of a commercial ORC and the global efficiency of the cogeneration system. Many authors have focused their research on the optimization of ORC modules for biogas applications. In [20], [PERSON] et al. focused on analyzing three different cycle architectures: subcritical, wet-expansion, and supercritical ORC. The analysis considered fixed design conditions for the anaerobic digestion plant and the results showed that the supercritical cycle maximized the investment net present value (NPV). In [21], [PERSON] et al. proposed an ORC coupled to a biogas combustor and chose the best configuration for fifteen different operating conditions, which might occur due to changes in biogas composition. The work did not consider the ORC module part-load or off-design operations. In [22], [PERSON] et al. determined the optimum plant configuration and the optimum ORC operating fluid among 118 candidates to maximize an ORC performance coupled to a biogas engine. In [23], [PERSON] et al. presented a system for WHR from a biogas fueled mGT by using both an ORC and an absorption chiller. The system achieved an energy efficiency of 72.5% and a second law efficiency of 30.44%. In [24], [PERSON] et al.