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SUMMARYOngoing changes in energy systems offer the opportunity to reconsider the design of our current energy infrastructures. As many new grid participants like combined heat and power plants involve other forms of energy in addition to electricity, the consideration of multi-energy networks represents a useful planning approach. This approach consists in an integrated analysis of combined infrastructures for conversion, transmission and storage of electricity, heat and chemical energy carriers. The integrated operation of systems with different energy carriers offers a new degree of freedom to system planners: energy can be generated, stored, and transmitted in various forms before being distributed to consumers in the adequate form. Different possible solutions for the design of multi-energy systems will exhibit differences in costs and risks. In order to adequately assess these two quantities, mean-variance portfolio theory is applied. Portfolio theory has been used in several cases of electricity generation planning. This paper shows the required modeling adaptations for the analysis of portfolios with multiple energy outputs and illustrates the application of portfolio theory in the context of multi-energy infrastructures. The presented method can be used for the integrated planning of energy systems including the generation, the transmission and the possibility of additional conversion of multiple energy carriers.

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In this paper a model for maximizing exergy efficiency in multi-carrier energy systems is introduced. Based on modeling concepts developed in the project "Vision of Future Energy Network", e.g. the Energy Hub concept, exergy is modeled in the context of energy systems that involve multiple energy carriers such as electricity, natural gas and heat. In the context of this integrated consideration of multiple energy carriers, the exergy approach allows to take into account the quality of different energy carriers. Hence this modeling approach provides the possibility to identify from a system perspective how the available energy content of different energy carriers can be exploited as efficiently as possible to satisfy a given demand for final energy carriers. In order to illustrate the proposed exergy analysis method, we compare it with a previously developed cost optimization and apply both methods to an example system consisting of an electricity and natural gas system interconnected by Energy Hubs.Index Terms-Exergy efficiency, multiple energy carriers, Energy Hub, cogeneration. I. INTRODUCTIONMaking more efficient use of energy is considered one of the crucial levers to increase sustainability of energy systems. Efficiency measures can make important contributions to cope with the growing demand for energy on a worldwide scale and to mitigate climate change. Several studies have identified energy efficiency opportunities for a number of countries [1]. The energy strategy of ETH Zurich [2], which formulates a vision of a transformation path for the energy system in the 21 st century, also emphasizes the importance of an efficient conversion of primary energy carriers to final energy carriers. These conversion processes involve a number of different energy carriers of diverse quality.In order to characterize the quality of different forms of energy, exergy is a meaningful measure. Exergy is derived from the Second Law of Thermodynamics and describes the available useful energy. In contrast to the First Law of Thermodynamics stating the conservation of energy, exergy accounts for the irreversibility of conversion processes. Therefore, the exergy concept can be used to identify how the available energy content of different energy carriers can be exploited as efficiently as possible. Considering exergy is a powerful approach, which has already been introduced in legal frameworks and directives related to energy planning [3].