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Highlights • Building internal mass (BaB) is preheated during the afternoon hours for evening heating. • Measured BaB efficiencies of 60–80% for insulated apartments. • Measured BaB efficiencies of 40–70% for low-insulation apartments. • Variable renewable energy as photovoltaic is appropriate for building as battery. • BaB thermal storage can be a viable option for consumers with small reductions in daytime electricity cost. Abstract To increase the penetration of photovoltaic renewable energy (PV RE) in the residential building sector energy mix, the mismatch between mid-day production and evening electrical energy demand must be bridged. The use of heat pumps for space heating of residential buildings creates an opportunity for energy flexible residential buildings that rely on low-cost thermal energy storage systems. This paper explores the 'building as battery' concept (BaB), i.e. the use of the structural thermal capacity as a heat storage medium in winter. In these systems the building internal mass is preheated during solar daytime hours (using a PV RE powered heat pump) and cooled by discharge into the room air after sunset, for space heating. This study presents measured and simulated performance of BaB systems used for thermal energy storage in living rooms of three different apartment buildings with variable thermal insulation levels, ranging from typical southern Europe 1980s construction to a insulated apartment (Passive House certified). To assist in interpreting the results and facilitate their integration in simplified simulation of BaB systems in smart grids the model parameters of a simplified Resistance-Capacitance model were found by fitting the experimental results. The results show that, for apartments with high thermal mass, thermal insulation is the key driver of BaB system thermal efficiency, with the highly insulated apartment reaching BaB efficiencies of 60–80%. For existing apartments with a low thermal insulation, the BaB approach is only effective when space heating is required right after sunset (just when PV RE powered heat pump charging finishes). [ABSTRACT FROM AUTHOR]

This paper investigates how the European electricity and heating system is impacted when medium-scale energy communities (ECs) are developed widely across Europe. We study the response on the capacity expansion of the cross-border transmission and national generation and storage within the European electricity and heating system with and without ECs in selected European countries. The representation of ECs has a special focus on flexibility, and we analyze the difference between flexibility responses by ECs towards local versus global cost minimization. Results show that EC development decreases total electricity and heating system costs on the transition towards a decarbonized European system in line with the 1.5 °C target, and less generation and storage capacity expansion is needed on a national scale to achieve climate targets. We also identify a conflict of interest between optimizing EC flexibility towards local cost minimization versus European cost minimization. • We study investments in the European energy market with energy community roll-out. • We compare flexibility responses with local versus European perspective. • Energy communities partly replace centralized generation and storage capacity. • Energy community roll-out shifts investments from onshore to offshore wind. • There is conflicting interest between local and European flexibility utilization. [ABSTRACT FROM AUTHOR]

This paper analyses the role of sector coupling towards 2050 in the energy system of Northern-central Europe when pursuing the green transition. Impacts of restricted onshore wind potential and transmission expansion are considered. Optimisation of the capacity development and operation of the energy system towards 2050 is performed with the energy system model Balmorel. Generation, storage, transmission expansion, district heating, carbon capture and storage, and synthetic gas units compete with each other. The results show how sector coupling leads to a change of paradigm: The electricity system moves from a system where generation adapts to inflexible demand, to a system where flexible demand adapts to variable generation. Sector coupling increases electricity demand, variable renewable energy, heat storage capacity, and electricity and district heating transmission expansion towards 2050. Non-restricted investments in onshore wind and electricity transmission reduce emissions and costs considerably (especially with high sector coupling) with savings of 78.7€ 2016 /person/year. Investments in electric power-to-heat units are key to reduce costs and emissions in the heat sector. The scenarios with the highest sector coupling achieve the highest emission reduction by 2045: 76% greenhouse gases reduction with respect to 1990 levels, which highlights the value of sector coupling to achieve the green transition. • Sector coupling increases demand flexibility and renewable energy integration. • Electricity-to-heat is a cost-effective way to decarbonise most of the heat sector. • Decarbonisation targets might not be reached without sector coupling. • Restricting onshore wind and/or grid expansion reduces the value of sector coupling. • Detailed variable renewable energy modelling is important in future energy systems. [ABSTRACT FROM AUTHOR]

• Technical, economic and environmental modeling frameworks are proposed. • Economic and environmental optima of hybrid renewable energy systems are different. • Local climatic conditions highly influence the impacts and the optimal design. • Single- and multi-objective optimization both have advantages and disadvantages. • Installing photovoltaics is the cheapest way to decrease environmental impacts. The rapid spread of renewables made it essential to design optimal hybrid renewable energy systems (HRES) with the distinctive economic and environmental impacts of each technology in mind. According to a comprehensive literature review, very few studies consider life-cycle environmental impacts in small-scale hybrid renewable energy system optimization. This paper aims to fill this gap by providing a multi-objective design framework for household-scale systems based on the technical modeling of several typical components. Solar photovoltaic, wind turbine, solar heat collector, heat pump, heat storage, battery, and as a novelty, heat insulation thickness are considered. Backup power is either drawn from the grid or produced by a diesel generator in grid-connected and off-grid scenarios, respectively. Single objective optimization using genetic algorithm resulted in the least cost and the least environmental footprint options in a case study of three different locations across Europe. Then, Pareto-optimal solutions between the two extremities were explored with a multi-objective genetic algorithm. Single objective results show substantial differences between environmental and economic optima, while multi-objective optimization proved to be an efficient tool to investigate the trade-offs between the two conflicting goals. Solar photovoltaics is proved to be the most competitive technology to reduce environmental impacts in the case of grid-connected systems. Off-grid systems, however, benefit the most from a balanced mix of different renewable energy sources. Life-cycle impacts in the design of systems involving renewables is proven to be relevant while potential applications of the framework are also revealed. Further research areas, as well as the limitations of the methodology are identified in the conclusion. [ABSTRACT FROM AUTHOR]

• Net-zero emissions are modelled for electricity, heating and cooling in Europe. • System cost decreases thanks to heat savings, but not for climate change scenarios. • Lower heat demand disfavours heat pumps as surplus electricity is used by resistors. • Cheaper heat pumps impact resistive heaters substantially, but not the opposite. To limit the global temperature increase to 1.5 °C, fossil-free energy systems will be required eventually. To understand how such systems can be designed, the current state-of-the-art is to apply techno-economical optimisation modelling with high spatial and temporal resolution. This approach relies on a number of climatic, technical and economic predictions that reach multiple decades into the future. In this paper, we investigate how the design of a fossil-free energy system for Europe is affected by changes in these assumptions. In particular, the synergy among renewable generators, power-to-heat converters, storage units, synthetic gas and transmission manage to deliver an affordable net-zero emissions system. We find that levelised cost of energy decreases due to heat savings, but not for global temperature increases. In both cases, heat pumps become less favourable as surplus electricity is more abundant for heating. Demand-side management through buildings' thermal inertia could shape the heating demand, yet has modest impact on the system configuration. Cost reductions of heat pumps impact resistive heaters substantially, but not the opposite. Cheaper power-to-gas could lower the need for thermal energy storage. [ABSTRACT FROM AUTHOR]