BIG Solar Graz: Solar District Heating in Graz – 500,000 m2 for 20% Solar Fraction

District heating (DH) became a central technology for providing heat and hot water in the residential and service building sector of the city of Graz over the years. It covers with approximately 1000 GWh/a (in 2013) 39% of the overall heat demand and is planned to be extended extensively in the coming years. Current heat generation for Graz is mainly from waste heat from fossil fired combined heat and power (CHP) plants in and nearby Graz. The operator of these plants announced their closure in 2020 due to low electricity prices in the European market (gas plants) and due to maturity (coal plants). Thus vanished 80% of heat production need to be replaced. Therefore, in 2014 the Graz city senate constituted a project team to find various options for providing heat for DH in Graz and its surrounding communities for 2020/30. One promising concept is a large scale solar thermal system including seasonal storages and heat pumps. The local energy provider agreed to carry out a feasibility of such a large-scale solar thermal power plant in order to analyze its potential in detail, whereas first results of this study are presented within the present paper. The purpose of the feasibility is to determine the optimum size of the solar system to be best integrated in the future district heating system. The feasibility study includes the investigation of appropriate areas for collectors and storages, the design of a technical concept using dynamic simulation, an economical evaluation and the investigation of legal issues. The price to be compared with is the heat production cost from gas boilers. First results show the best feasibility of the system between 150,000 m2 and 650,000 m2, providing 9 to 26% solar fraction of district heat demand. While the study is ongoing first results show a high potential for the large scale solar system being a feasible concept for the urban district heating of Graz. Further in depth simulations will be done after all relevant parameters have been clarified in detail. Thereby economical optimization and the definition of the final dimensions of the whole system will be done.