Economic and environmental aspects of the component sizing for a stand-alone building energy system: A case study Samuel F. Fux * , Michael J. Benz, Lino Guzzella ETH Zurich, Department of Mechanical and Process Engineering, Sonneggstrasse 3, 8092 Zurich, Switzerland article info Article history: Received 8 June 2012 Accepted 19 December 2012 Available online 6 February 2013 Keywords: Stand-alone energy system Renewable energy sources Optimal component sizing Model predictive control Mixed-integer linear programming Global warming potential abstract When designing a building energy system based on renewable energy sources, a major challenge is the suitable sizing of its components. In this paper, a simulation tool is presented for determining the optimal sizes of the main components of a stand-alone building energy system which integrates both thermal and electric renewable energy sources. Since the control of this multisource energy system is a non-trivial, multivariable control problem, particular emphasis is placed on the energy management system. A control structure based on model predictive control is proposed, whereas the underlying optimal control problem is formulated as a mixed-integer linear programming problem. The simulation tool developed is successfully applied on the specific case of an alpine lodge. A set of potential configurations, each being optimal with respect to both the net present costs and the global warming potential, is generated by analyzing the system for various component sizes. Out of this set, the decision makers can choose the most cost efficient configuration fulfilling their specifications. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Due to sustainability concerns, the contribution of renewable energy sources (RES) to the energy supply of buildings is con- tinually increasing. This substitution of conventional fossil energy sources is assisted by governmental regulation [1]. However, in order to fully exploit the potential of RES based energy systems, both economically and ecologically, the suitable sizing of the components of an energy system is of crucial importance. To tackle this challenging task, simulation tools have been widely used for the optimal sizing of both electric and thermal RES based energy systems. Simulation studies have been performed, for instance, to determine the optimal component sizes of electric systems com- bining photovoltaics (PV) and energy storage technologies such as batteries [2e5] and/or hydrogen tanks [6,7]. Based on simulations, numerous publications have analyzed the optimal sizing of electric systems integrating both PV and wind generators, such as in [8e 11]. Examples concerned with the application of simulation tools for optimizing the sizing of the thermal solar collector (TSC) and the heat storage (HS) tank of solar hot water systems are reported in [12e16]. In contrast to those sizing studies, which focussed either on an electric-only or a thermal-only RES based energy system, this study addresses the optimal component sizing of a stand-alone building energy system integrating both electric and thermal RES. In addi- tion, it proposes the inclusion of a sophisticated control strategy into the design process. For this purpose, a simulation tool com- posed of mathematical models of all subsystems is developed. Since the performance of energy systems consisting of multiple energy sources and storage systems strongly depends on the control strategy used, an energy management system (EMS) based on model predictive control (MPC) is implemented. This simulation tool is then used to study the economic viability and the environ- mental impact of different component configurations of the energy system of an alpine lodge. This paper is structured as follows: The next section describes the configuration of the energy system considered. The simulation models used to predict the system performance and the proposed EMS are introduced in Section 3 and Section 4, respectively. The criteria used to assess the sizing of the system components are discussed in Section 5. Finally, Section 6 summarizes the simulation results, and Section 7 lists some conclusions. 2. System description The energy system under investigation is the one of an alpine lodge located in the Swiss Alps at about 2900 m above sea level [17]. From March to September the lodge can accommodate up to 120 visitors per day, while the rest of the year it is closed. The lodge is only accessible on foot or by helicopter. Because of its location far * Corresponding author. Tel.: þ41 44 632 58 64. E-mail address: samuel.fux@alumni.ethz.ch (S.F. Fux). Contents lists available at SciVerse ScienceDirect Renewable Energy journal homepage: www.elsevier.com/locate/renene 0960-1481/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.renene.2012.12.034 Renewable Energy 55 (2013) 438e447