  The synthesis of energy systems is a two-stage optimization problem where
design decisions have to be implemented here-and-now (first stage), while for
the operation of installed components, we can wait-and-see (second stage). To
identify a sustainable design, we need to account for both economical and
environmental criteria leading to multi-objective optimization problems.
However, multi-objective optimization leads not to one optimal design but to
multiple Pareto-efficient design options in general. Thus, the decision maker
usually has to decide manually which design should finally be implemented.
  In this paper, we propose the flexible here-and-now decision (flex-hand)
approach for automatic identification of one single design for multi-objective
optimization. The approach minimizes the distance of the Pareto front based on
one fixed design to the Pareto front allowing multiple designs. Uncertainty
regarding parameters of future operations can be easily included through a
robust extension of the flex-hand approach.
  Results of a real-world case study show that the obtained design is highly
flexible to adapt operation to the considered objective functions. Thus, the
design provides an energy system with the ability to adapt to a changing focus
in decision criteria, e. g., due to changing political aims.
