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Impact of solar share on optimal supply technologies and seasonal thermal storage size in solar district heating systems

District heating systems are on a way to the fourth generation of the district heating (4DH) systems which can merge different sectors such as: heating, cooling, power and transport. In addition to this, they can integrate various low-temperature renewable energy sources such as geothermal and solar. Due to the high interconnectivity of energy streams and various possible technologies, optimization of such systems presents a challenge which needs to be tackled. Solar district heating systems are well-known and proven technology in EU with solar shares reaching more than 50%, despite high related investment costs. In order to integrate solar energy and district heating systems, detailed analysis should be carried out in order to select the most suited combination of classical district heating supply technologies, thermal storage size and solar share. In addition to this, power-to-heat technologies are also present in these systems, therefore electricity market should also be taken into account. In this paper, relation of solar share on optimal supply capacities and seasonal thermal storage size in the solar district heating systems has been studied. Optimization was carried by using mixed integer linear programing, with the code written in Julia and solved by using Cbc solver which is part of JuMP Julia package. The hourly based model is capable of optimizing capacities and one-year operation of supply technologies such as: electrical heater, heat pump, heat-only boiler, cogeneration and seasonal thermal storage, including solar thermal collectors’ capacity. Objective function is defined as minimization of total cost of the system, which includes discounted investment and the overall running costs, and total environmental impact of the system. This paper shows which technologies and capacities should be utilized in order to minimize total cost and environmental impact. The impact of increased solar share on the total greenhouse gas emissions, total discounted cost, seasonal thermal storage and utilized technologies size has been presented. In order to choose the most suited solution, decision making criteria has been modelled which takes into account economic feasibility and total environmental impact of the system.

solar district heating, optimization, mixed-integer linear programming, seasonal thermal storage

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