Research on 100% renewable energy systems is a relatively recent phenomenon. It was initiated in the mid-1970s, catalyzed by skyrocketing oil prices. Since the mid-2000s, it has quickly evolved into a prominent research field encompassing an expansive and growing number of research groups and organizations across the world. The main conclusion of most of these studies is that 100% renewables is feasible worldwide at low cost. Advanced concepts and methods now enable the field to chart realistic as well as cost-or resource-optimized and efficient transition pathways to a future without the use of fossil fuels. Such proposed pathways in turn, have helped spur 100% renewable energy policy targets and actions, leading to more research. In most transition pathways, solar energy and wind power increasingly emerge as the central pillars of a sustainable energy system combined with energy efficiency measures. Costoptimization modeling and greater resource availability tend to lead to higher solar photovoltaic shares, while emphasis on energy supply diversification tends to point to higher wind power contributions. Recent research has focused on the challenges and opportunities regarding grid congestion, energy storage, sector coupling, electrification of transport and industry implying power-to-X and hydrogen-to-X, and the inclusion of natural and technical carbon dioxide removal (CDR) approaches. The result is a holistic vision of the transition towards a net-negative greenhouse gas emissions economy that can limit global warming to 1.5˚C with a clearly defined carbon budget in a sustainable and cost-effective manner based on 100% renewable energy-industry-CDR systems. Initially, the field encountered very strong skepticism. Therefore, this paper also includes a response to major critiques against 100% renewable energy systems, and also discusses the institutional inertia that hampers adoption by the International Energy Agency and the Intergovernmental Panel on Climate Change, as well as possible negative connections to community acceptance and energy justice. We conclude by discussing how this emergent research field can further progress to the benefit of society.

Globally, PV prosumers account for a significant share of the total installed solar PV capacity, which is a growing trend with ever-increasing retail electricity prices. Further propelled by performance improvements of solar PV and innovations that allow for greater consumer choice, with additional benefits such as cost reductions and availability of incentives. PV prosumers may be one of the most important enablers of the energy transition. PV prosumers are set to gain the most by maximising self-consumption, while avoiding large amounts of excess electricity being fed into the grid. Additionally, electricity and heat storage technologies, heat pumps and battery electric vehicles are complementary to achieve the highest possible self-consumption shares for residential PV prosumer systems, which can reach grid-parity within this decade in most regions of the world. This research finds the cost optimal mix of the various complementary technologies such as batteries, electric vehicles, heat pumps and thermal heat storage for PV prosumers across the world by exploring 4 different scenarios. Furthermore, the research presents the threshold for economical maximum battery capacity per installed PV capacity, along with self-consumption ratios, demand cover ratios and heat cover ratios for 145 different regions across the world. This is a first of its kind study to conduct a global analysis of PV prosumers with a range of options to meet their complete energy demand from a future perspective, up to 2050. Maximising self-consumption from solar PV generation to meet all energy needs will be the most economical option in the future, for households across most regions of the world.

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Globally, the heat sector has a major share in energy consumption and carbon emission footprint. To provide reliable mitigation options for space heating, domestic hot water, industrial process heat and biomass for cooking for the energy transition time frame up to the year 2050, energy system modeling relies on a comprehensive and detailed heat demand database in high spatial resolution, which is not available. This study overcomes this hurdle and provides a global heat demand database for the mentioned heat demand types and in a resolution of 145 mesoscale regions up to the year 2050 based on the current heat demand and detailed elaboration of parameters influencing the future heat demand. Additionally, heat demand profiles for 145 mesoscale regions are provided. This research finds the total global heat demand will increase from about 45,400 TWhth in 2012 up to about 56,600 TWhth in 2050. The efficiency measures in buildings lead to a peak of space heating demand in around 2035, strong growth in standards of living leads to a steady rise of domestic hot water consumption, and a positive trend for the worldwide economic development induces a growing demand for industrial process heat, counterbalanced by the efficiency gain in already industrialised countries. For the case of biomass for cooking, a phase-out path until 2050 is presented. Literature research revealed a lack of consensus on future heat demand. This research intends to facilitate a more differentiated discussion on heat demand projections.