Techno-economic performance of community sized solar heating systems in Nordic conditions

Abstract

Regional size solar heating systems allow an efficient use of clean energy technologies and passive buildings. The objective of the thesis is on the design, techno-economic performance, optimization and comparisons of various solar heating system architectures in Nordic conditions for regional demand. These systems provide space heating, domestic hot water and electricity to the community. Detailed simulation is crucial to study the combined effect of renewable energy sources, mechanical components, storages and buildings. In the study, TRNSYS and TRNBuild are used for energy systems and building dynamic simulation respectively. MOBO is used for multi-objective optimization. The thesis includes four publications. The publications contain following topics: developing of control strategies for a basic system, long term parametric study of three systems alternatives, influence of technical failures on the performance of an optimized system, and comparison between centralized and semi-decentralized systems designs. The results showed that parallel short-term storage tanks charging, with temperature difference control at collector outlet, reduced the energy consumption by 20 %, compared to serial charging of the tanks. It is found that by placing a heat pump connected between two short term tanks, using solar charged borehole storage to charge the tank, improved the system performance. Additionally, seasonal storage charging and heat demand played an important role on the performance and investments. However, these systems are sensitive to certain technical failures, inversely they may not be sensitive to certain issues. Results showed that de-stratification, solar circulation pump control and heat pump COP can cause large deviations from the ideal conditions. Semi-decentralized system, where low temperature grid is centralized and high temperature grid is decentralized can clearly outperform centralized system, in terms of the life cycle cost by 35 %. Shape of the seasonal storage varies depending on the stored heat and distribution network typology. The renewable energy fraction of up to 92% can be attained. The thesis shows that it is technically possible to build high performing solar heating networks in Nordic climate. A simulation based optimization problem of solar district and buildings together can reach an optimal or close to optimal solution set, useful for decision makers. Large size of the community allowed the effective use of the seasonal storage and solar energy. Control of each component and optimization is necessary to gain benefits of such concept. Large systems are much cheaper by unit price because of economics of scale. With decreasing trend of the collector and PV cost, economics of such system can be altered with cheap thermal storage. The technical uncertainties should be considered while realizing these systems. To validate the system a pilot project is needed. Life cycle assessment can be done to rationalize its societal impact.