Models for flexible building operation in the Nordhavn district energy system

As part of the transition from a traditional carbon-intensive energy system to a sustainable energy system, a high proportion of electricity production from renewable energy sources has already been achieved in many countries and it is anticipated to increase further. This means that the controllability of the supply side is limited, which reduces the stability of the power grid and increases the need for balancing strategies. Energy flexibility is proposed as a way to facilitate the management of the energy system while integrating a large proportion of renewable energy sources. In this way, the traditional energy management approach, in which demand dictates the production, can be extended to decentralized energy storage and flexible load shifting, so that both demand and production are controlled to some extent to stabilize the energy network. The residential building sector has a great potential for flexibility, as it is responsible for a very large share of all energy consumption and part of this energy demand can, with appropriate control, be shifted in time, in order to increase the flexibility of the demand side. There are both thermal and electrical loads in buildings that can become flexible. The potential energy flexibility of a building depends on its physical characteristics, the local energy grid and on occupants energy-related activities. The present research forms part of the “EnergyLab Nordhavn – New urban energy infrastructure” project, which is a smart city research and demonstration project based on the newly developed Nordhavn district in Copenhagen, Denmark. The present thesis analyses the potential flexibility of low-energy residential buildings as a contribution to design and operational decisions, considering the relation of the building with the energy infrastructure and the occupants. The main objectives of the thesis were: i) to investigate the physical potential of low-energy buildings to facilitate flexible heating operation by using the thermal mass of the buildings as heat storage, while maintaining thermal comfort; ii) to investigate the operational flexibility potential of low-energy buildings by proposing methodologies that make it possible for the heating system to be operated in such a way as to meet the flexibility requirements of a given district energy supply system; iii) to address the occupants’ energy-related activities in relation to flexible electricity loads, using Danish time-use survey data to create a realistic daily electricity demand profile for Danish households, which can be used when modelling energy flexibility of dwellings. The most important findings of the research performed in this thesis may be summarized as follows: In the first part of the work, it was shown that by using only the inherent structural thermal mass of the buildings as storage, there is physical potential for flexible heating operation of low-energy buildings. In cases of abundant production of renewable energy in the system, the buildings can act as storage absorbing thermal energy in the structural thermal mass. In addition, low-energy buildings can remain without any heat supply for many hours without jeopardizing thermal comfort, if there is need to withhold the heat supply for a certain period of time. The results depend on the duration of the change in the heating system and the time of the day that it was performed. It was observed that the energy flexibility potential is strongly affected by the boundary conditions, namely the ambient temperature, solar radiation and internal heat gains. Focusing on the thermal mass of different concrete elements, measurements from a custom-made set of temperature sensors cast inside concrete walls and ceilings at different depths, confirmed that all the internal concrete layers examined contributed to the physically available heat storage potential of a building. In the next step of the work, the operational flexibility potential of low-energy buildings using their thermal mass as heat storage was evaluated in relation to the local district heating. The demand of the district heating system was used for rule base scheduling and the marginal heat production cost was used for cost based scheduling of the building’s heating system. It was shown that highly effective heat load shifting can be achieved to reduce consumption during the district heating system’s peak load hours, and that costs decrease can be achieved. Some possibly adverse effects were identified, as increasing the flexibility of heating system operation may result in increased total energy use and new peaks in the heating load of the building. Nevertheless, higher energy use may be considered acceptable, if it costs less to be produced and can be beneficial for the environment because the additional energy comes from renewable sources. The magnitude of the benefits that could be achieved was found to be associated with acceptable changes in energy use and in thermal comfort of the occupants of the building. The role of building occupants was addressed in relation to household electricity loads, as there is some potential for achieving flexible electricity loads in buildings by rescheduling the use of domestic appliances and possibly the heating system, i.e. heat pumps or electric heating. Creating realistic daily household electricity demand profiles was undertaken as the basis for flexibility modelling of electricity household loads. Using information collected from a large group of Danes, i.e. Danish time-use survey (DTUS) data, about the timing of household activities, two modelling approaches were implemented to model the occupant energy-related activities. The resulting occupant energy-related activities profile was linked to electricity demand by using data on appliance ownership and power ratings. A realistic daily electricity load profile for Danish households was obtained, one that can be used in future modelling of energy flexibility of dwellings.