Impact of Zero Energy Buildings on the Power System: A study of load profiles, flexibility and system investments

Abstract

This thesis investigates the impacts of introducing energy efficient and electricity generating buildings, called ZEBs, into a power system with a high share of renewables. Detailed knowledge of electricity demand is essential for power system planning and operation. EUs 20-20-20 targets will increase the development of more energy efficient buildings as all new buildings shall be “nearly net zero energy buildings” (ZEBs) by 2020. The result from this ambition is that ZEBs, with lower energy demand and onsite power generation, will significantly change the way buildings are integrated in the power system. System operators must consequently prepare for changes in load profiles. This PhD entails extensive research on the hourly load profiles of ZEBs, by investigating the differences between the existing building stock and new energy efficient buildings. Whether heat is supplied by a thermal source or electricity is essential for the total electric profile of a building. Hence, the heat load and electric specific load are evaluated separately. The different topology and utilisation of the buildings called for evaluation of eleven different building categories, of which nine non-residential and two residential building types. To assess the impact on the power system of a large implementation of ZEBs, a methodology for aggregating the total load profile of the Norwegian building stock is developed. The methodology allows for evaluation of the effect of introducing any percentage of highly energy efficient buildings in the energy system. The building’s net electric load profile, which reflects how the building interacts with the electricity grid, is influenced by two main factors. First, which energy technologies are available within the building, and second, how the technologies are operated/controlled, i.e. if a smart control is applied and how it is designed. A ‘smart’ energy management system can utilise the available energy technologies within the building in a least-cost way, but still such that the energy demand is met. To evaluate the cost-optimal net electric load-profile, an optimisation model is developed which minimises investment and operational costs over the building’s total lifetime. The model can choose between ten different energy technologies and, identifies the main factors that affect the energy system design within ZEB buildings. With the hourly or sub-hourly (15 min) operational time resolution, the net electric load profile can be evaluated for different technology designs and tax schemes. The following are some of the main results obtained in this PhD work: Load modelling • The annual heat demand of passive energy efficient buildings is about 50-60 % lower when compared to existing buildings, but the shape of the hourly heat load profile is similar for the two. Hence, the heat profile is determined by the operational pattern of the buildings, whereas the total heat consumption is dependent on the standard of the building. • The electric specific demand of passive energy efficient buildings does not differ substantially from existing buildings, about -6 % for offices, and the shape of the hourly load profile is similar. Consequently, the electricity load of buildings seems to be less dependent on the technical standard of the building than the thermal load. ZEB optimisation • Results show that PV is always as part of the ZEB’s energy system design. Hence, on a building level, ZEBs have large exports of electricity to the local grid in summer, and import in winter. • The net electric load profile of ZEBs are determined by the choice and size of energy technologies, i.e. the design within the building, and how they are operated. • Each member state in the EU is free to decide their own definition of ZEBs. The results in this work show that the energy technology design of the building is influenced by the following elements of the ZEB definition: • the metric of the weighting factor (primary energy (PE) or CO2) - Using PE or CO2 is of lesser importance than the value of the weighting factors. • the value of the weighting factors - A high PE for electricity will lead to smaller PV capacity in ZEBs - For all-electric ZEBs (heated by heat pumps), the value of the PE for electricity does not matter as long as it is > 0. • the level of ZEB (‘strictly’ or ‘nearly’ ZEB) - the ZEB level is the most important factor for the size of the PV. • what energy consumption is included (partly operational, all operational, or all operational & embodied) - including less energy consumption in the ZEB balance will lower the PV size. • Therefore, when policy makers determine the elements of their national definition of ZEB, it is of vital importance to be aware of how these elements influence the ZEB building’s interaction with the surrounding power grid. Aggregated system analysis • A large implementation of ZEBs has two effects on the Norwegian energy and power system; 1) decreased demand in winter due to the passive building standard, and 2) increased electricity production due to the on-site PV generation. • Initial analyses on the operation of the Nordic electric power system in 2030 with the EMPS model, show that a large implementation of ZEBs in Norway lowers the electricity prices, which reduces the thermal power production (coal, oil and bio) and stimulates to increased consumption (dependent on the demand elasticity). Furthermore, the reduced electricity prices and increased power availability leads to increased export out of the Nordic region. About 70- 80 % of the increased power availability is exported out of the region due to the characteristics of the hydropower production. The peak prices are also reduced. • Initial analysis on the investments in the Scandinavian energy system towards 2050 with the TIMES-model, of a large implementation of ZEBs in Denmark, Sweden and Norway is performed. The findings show that the energy system surrounding the ZEB buildings will adapt to this forced change. As in the power system analysis, electricity prices are found to decrease, which lowers the incentives for investment in new wind capacity and CHP capacity. In the building sector, the lower electricity prices increases the role of electric heating, and the consumption of district heat and biomass is reduced. In this analysis, the export from Scandinavia in 2050 is also increasing.