energies Article Experimental and Numerical Study of a Microcogeneration Stirling Unit under On–Off Cycling Operation Gianluca Valenti 1, * , Aldo Bischi 2 , Stefano Campanari 1 , Paolo Silva 1 , Antonino Ravidà 1 and Ennio Macchi 1   Citation: Valenti, G.; Bischi, A.; Campanari, S.; Silva, P.; Ravidà, A.; Macchi, E. Experimental and Numerical Study of a Microcogeneration Stirling Unit under On–Off Cycling Operation. Energies 2021, 14, 801. https:// doi.org/10.3390/en14040801 Academic Editor: Dimitris Katsaprakakis Received: 8 December 2020 Accepted: 25 January 2021 Published: 3 February 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional afﬁl- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Department of Energy, Politecnico di Milano University, Via Lambruschini 4/A, 20156 Milano, Italy; stefano.campanari@polimi.it (S.C.); paolo.silva@polimi.it (P.S.); antonino.ravida@polimi.it (A.R.); ennio.macchi@polimi.it (E.M.) 2 Center for Energy Science and Technology, Skolkovo Institute of Science and Technology, Bolshoy Boulevard 30, 121205 Moscow, Russia; a.bischi@skoltech.ru * Correspondence: gianluca.valenti@polimi.it; Tel.: +39-02-2399-3845 Abstract: Stirling units are a viable option for micro-cogeneration applications, but they operate often with multiple daily startups and shutdowns due to the variability of load proﬁles. This work focused on the experimental and numerical study of a small-size commercial Stirling unit when subjected to cycling operations. First, experimental data about energy ﬂows and emissions were collected during on–off operations. Second, these data were utilized to tune an in-house code for the economic optimization of cogeneration plant scheduling. Lastly, the tuned code was applied to a case study of a residential ﬂat in Northern Italy during a typical winter day to investigate the optimal scheduling of the Stirling unit equipped with a thermal storage tank of diverse sizes. Experimentally, the Stirling unit showed an integrated electric efﬁciency of 8.9% (8.0%) and thermal efﬁciency of 91.0% (82.2%), referred to as the fuel lower and, between parenthesis, higher heating value during the on–off cycling test, while emissions showed peaks in NOx and CO up to 100 ppm but shorter than a minute. Numerically, predictions indicated that considering the on–off effects, the optimized operating strategy led to a great reduction of daily startups, with a number lower than 10 per day due to an optimal thermal storage size of 4 kWh. Ultimately, the primary energy saving was 12% and the daily operational cost was 2.9 €/day. Keywords: micro-CHP; cogeneration; transient operation; mixed integer linear programming; opti- mal schedule 1. Introduction Micro-cogeneration Stirling units are a viable option for small-scale residential and commercial applications because of their high total efﬁciencies, favorable ratios of thermal- to-electric power with respect to the loads they must meet, low CO as well as NOx emis- sions, extended time between maintenances, and reduced vibrations during operation [1,2]. In contrast to the case of large industrial applications where both thermal and electric loads are relatively constant throughout the year, residential and commercial applications are characterized by large ﬂuctuations of electricity, heating, and cooling loads, which are strictly related to climatic conditions and to the end-users’ needs. On top of these load ﬂuctuations, electricity tariffs feature typically daily and weekly variations, while the option of selling the electricity to the grid can be subjected to constraints and may turn economically unattractive. Within this framework, a number of studies indicate that the optimal scheduling from the economic perspective of micro-cogeneration units, option- ally equipped with thermal storage, requires switching on and off the units several times during the day and the week [3–8]. However, some studies highlight the strong inﬂuence of transients during startups and shutdowns, demonstrating signiﬁcant effects in terms Energies 2021, 14, 801. https://doi.org/10.3390/en14040801 https://www.mdpi.com/journal/energies