**Carlo Carcasci \*, Lapo Cheli, Pietro Lubello and Lorenzo Winchler**

DIEF—Department of Industrial Engineering, University of Florence Via Santa Marta, 3, 50139 Florence, Italy; lapo.cheli@unifi.it (L.C.); pietro.lubello@unifi.it (P.L.); lorenzo.winchler@htc.de.unifi.it (L.W.)

**\*** Correspondence: carlo.carcasci@unifi.it

Received: 24 January 2020; Accepted: 26 February 2020; Published: 2 March 2020

**Abstract:** This paper presents an off-design analysis of a gas turbine Organic Rankine Cycle (ORC) combined cycle. Combustion turbine performances are significantly affected by fluctuations in ambient conditions, leading to relevant variations in the exhaust gases' mass flow rate and temperature. The effects of the variation of ambient air temperature have been considered in the simulation of the topper cycle and of the condenser in the bottomer one. Analyses have been performed for different working fluids (toluene, benzene and cyclopentane) and control systems have been introduced on critical parameters, such as oil temperature and air mass flow rate at the condenser fan. Results have highlighted similar power outputs for cycles based on benzene and toluene, while differences as high as 34% have been found for cyclopentane. The power output trend with ambient temperature has been found to be influenced by slope discontinuities in gas turbine exhaust mass flow rate and temperature and by the upper limit imposed on the air mass flow rate at the condenser as well, suggesting the importance of a correct sizing of the component in the design phase. Overall, benzene-based cycle power output has been found to vary between 4518 kW and 3346 kW in the ambient air temperature range considered.

**Keywords:** ORC integration technologies; advanced thermodynamic cycles; decentralised energy systems; benzene; toluene; cyclopentane
