The European Union and its Member States have committed themselves to achieving a 20% share of renewable energy by 2020. If the focus remains solely on solar thermal systems for domestic hot water (DHW) preparation, as in Spain, then the solar contribution will be very limited. Central Solar Heating Plants combined with Seasonal Storage (CSHPSS) systems enable high solar fractions of 50% and more. Most CSHPSS demonstration plants in Europe have been built in Central and North Europe, mainly in Denmark, Germany and Sweden. South Europe has little experience. This article presents a thermoeconomic cost analysis of CSHPSS systems. The objective of thermoeconomics is to explain the cost formation process of internal flows and products of energy systems. The costs obtained with thermoeconomics can be used to optimize the design of new plants and to control the production of existing plants. A simulation study on solar assisted district heating systems with high solar fractions and seasonal thermal energy storage was carried out with TRNSYS taking into consideration the meteorological conditions in Zaragoza (Spain). A CSHPSS plant was designed for a district of 500 dwellings with an annual thermal energy demand of 2,905 MWh/year. The process of cost formation has been analyzed considering the very specific features of the CSHPSS designed system: free solar energy, seasonal and DHW thermal energy storage, continuous variation of the operation due to highly variations of solar radiation and energy demands (hourly and seasonal). These features impose important difficulties in the calculation of the costs of internal flows and products in this type of systems.

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