Implementation of flexible operational schemes for buildings in a district with smart energy systems

Abstract

In the face of the growing share of fluctuating renewable energy sources and changes in the energy system as a whole, flexibility on the demand side is becoming more and more important. As focusing solely on electricity-only solutions does not pro-vide sufficient system flexibility, the next focus area are both electrical and thermal solutions. The research project summarized in this thesis aimed at investigating the energy flexibility potential of a small district. Most of the research included in the thesis used the district of Nordhavn in Copenhagen, Denmark as a case study. The study consisted of three main parts. The first part was based on a literature review and focused on earlier findings on energy flexibility in context of heat demand in buildings and district heating systems. The second part focused on the flexibility already available in the district heating systems and investigated in detail the poten-tial of using a heating grid as short-term heat storage. The third part investigated the flexible operation potential of a small district with buildings connected to a district heating network. Moreover, the challenges related to implementation of Demand-Side Management in the district heating systems were also briefly discussed. It was concluded, based on the findings available in scientific literature, that ther-mal energy systems are characterized by inherent inertia significantly greater than the electrical systems and electrical energy is more difficult to store than thermal energy. As such, treating electrical and thermal systems as an integrated whole can increase energy system’s flexibility. However, when investigating the possibility of using the flexibility of thermal energy systems, the physical characteristics of the heat demand side and thermal energy storage have to be accounted for. It is because the flexibility available depends on factors such as weather conditions, occupant behaviour, and storage losses. Additionally, a generalised schematic of flexibility sources in the built environment was proposed and their different types were pre-sented based on the proposed framework. Subsequently, the flexibility already available for the district heating network operators in the current systems was analysed. The possibilities discussed include large-scale heat storage, substation control and heat storage in the district heating network pipes. The investigation of the potential for using district heating grid as heat storage to reduce peak heat demand was based both on the results of a pilot test and a simulation study. Different scenarios regarding supply temperature increase level and its duration, as well as different heat demand in the area were analysed. It was shown, that the strategy can be used as a supplementary method of peak reduction. However, the potential of this solution when used solely on its own is limited – the peak decrease based on simulations was 4.8% and the peak decrease estimated based on measurements 7.2%. Based on the study of existing literature, the heat storage in buildings has been identified as a possible source of flexibility for the thermal energy systems. To inves-tigate the potential for flexible operation of buildings connected to the district heat-ing network and for the building thermal mass to be utilized as short-term thermal energy storage, a simulation-based study was conducted. The model in the study consisted of a grid model with connected buildings. It has been shown that there is significant potential for flexible operation of buildings supplied from the district heating systems. The achieved load shifting, defined as the change in energy use in the morning, in all scenarios was between 35% and 49%. Two different control signals for the demand-side management strategies, heat load in the district heating system and heat production costs, were also discussed. However, this solution has not been introduced in the district heating systems outside of pilot tests. As such, also the challenges related to its implementation in the existing district heating sys-tems that have to be addressed were also discussed. The overall conclusion of the study is that thermal energy system have signifi-cant potential for flexible operation and as such can play an important role in the transition to more sustainable energy supply based on renewable energy sources.