One of the major challenges of renewable energy systems is the inherently limited dispatchability of power generators that rely on variable renewable energy (VRE) sources. To overcome this insufficient system flexibility, electrical energy storage (EES) is a promising option. The first contribution of our work is to address the role of EES in highly renewable energy system in Europe. For this purpose, we apply the energy system model REMix which endogenously determines both capacity expansion and dispatch of all electricity generation as well as storage technologies. We derive an EES capacity of 206 GW and 30 TWh for a system with a renewable share of 95%. An extensive sensitivity analysis shows that ESS requirements range from 126 GW and 16 TWh (endogenous grid expansion) to 272 GW and 54 TWh (low EES investment costs). As our second contribution, we show how the spatial distribution of EES capacity depends on the residual load, which-in turn-is influenced by regionally predominant VRE technologies and their temporal characteristics in terms of power generation. In this sense, frequent periods of high VRE excess require short-term EES, which naturally feature low power-related investment costs. In contrast, long-term EES with low energy-related costs are characteristic for regions where high amounts of surplus energy occur. This relationship furthermore underlines how EES capacity distribution is implicitly influenced technical potentials for VRE expansion.

Ram air–based thermal management systems (TMS) are investigated herein for the cooling of future hybrid-electric aircraft. The developed TMS model consists of all components required to estimate the impacts of mass, drag, and fuel burn on the aircraft, including heat exchangers, coldplates, ducts, pumps, and fans. To gain a better understanding of the TMS, one- and multi-dimensional system sensitivity analyses were conducted. The observations were used to aid with the numerical optimization of a ram air–based TMS towards the minimum fuel burn of a 180-passenger short-range partial-turboelectric aircraft with a power split of up to 30% electric power. The TMS was designed for the conditions at the top of the climb. For an aircraft with the maximum power split, the additional fuel burn caused by the TMS is 0.19%. Conditions occurring at a hot-day takeoff represent the most challenging off-design conditions for TMS. Steady-state cooling of all electric components with the designed TMS is possible during a hot-day takeoff if a small puller fan is utilized. Omitting the puller fan and instead oversizing the TMS is an alternative, but the fuel burn increase on aircraft level grows to 0.29%.

This paper presents a novel tool for the modeling of partial turboelectric propulsion systems together with a corresponding case study for a commercial single-aisle aircraft. In order to reduce the environmental impact of air traffic, radically new aircraft and propulsion concepts with a high market penetration are needed. Partial turboelectric propulsion systems seem to offer a promising option to achieve this. For the development of these propulsion systems, a preliminary design tool with a homogeneous and sufficiently high fidelity, both for turbomachinery and electric components, is needed. To address this, the authors of this publication have developed a tool based on the GasTurb software. The models developed, in particular for the electric components which together form the electric powertrain, are described here. In the case study, which demonstrates the coupling of the developed tool with an aircraft design environment, a conventional turboprop baseline aircraft is compared to a derived aircraft which features a partial turboelectric propulsion system with wingtip propellers. The latter are intended to reduce the induced drag, enabling a reduction of the aircraft's total shaft power demand compared to the conventional baseline aircraft. The comparison between the partial turboelectric aircraft and the baseline aircraft indicates that fuel reduction increases with power split. However, primarily increasing electric powertrain masses and a stagnating drag reduction result in lower additional fuel reductions for higher power splits. Despite these conclusions, the predicted induced drag reductions need further refinement as they were found to be optimistic. In summary, this publication presents a methodology and a set of physics-based component models for the preliminary design of partial turboelectric propulsion systems, so that the electric components can be investigated and optimized at the same high level of detail as the gas turbine.

As SOFC microCHP systems come closer to commercial application, Vaillant GmbH as one of Europe's largest heating appliance manufacturers and their development partner Fraunhofer IKTS focus on system development for large scale demonstration.

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