Energy hub modeling for the design of solar thermal energy systems with short-term and long-term storage

Abstract

A mixed integer linear programming (MILP) energy hub model is developed to design a solar thermal district-heating network for a cluster of residential buildings in Rheinfelden, Switzerland. The model is employed to determine the area of roof-mounted solar thermal collectors that are required to meet the space heating and domestic hot water loads of the 11 buildings in the neighborhood. The installation and operation of electric heaters is permitted in order to supply back up heat during periods when the solar energy is not sufficient to meet thermal loads. The results of the energy hub model show that thermal energy storage (TES) is required in order to achieve solar fractions (i.e. the percentage of total energy demand that is met by solar energy) that are over 10%. Furthermore, the volume of required TES increases exponentially with the solar fraction. Due to the seasonality of space heating demand, the installation of short-term storage tanks alone is not sufficient for achieving solar fractions that are above 60%, and long-term TES units are also required. However, even when both shortterm and long-term TES units are installed, 100% solar fraction is still not possible due to the loss of heat in the TES units over time. Finally, for the size and characteristics of the neighborhood analysed in this study, the decentralised storage configuration with both shortterm and long-term storage units results in a cheaper energy system with a higher solar fraction, when compared with the centralised storage configurations.