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Solar co-generation, i.e., the generation of electricity and heat in a single device by concentrating the sunbeams, has the potential to significantly increase the overall system performance. The main challenge is related to the cooling of solar cells. In order to do so, it is essential to reduce the thermal resistance between the cell and heat transfer fluid. This paper features the optimization procedure of a low-cost custom concentrated photovoltaic thermal (CPV-T) receiver for a parabolic trough collector using silicon solar cells. A finite volume model for the thermal process has been developed. Hence, a fluid dynamic thermal simulation of the receiver is presented. The optimized heat sink tube geometries have been manufactured and tested in a lab environment, allowing for a comparison between modelling and experimental test results. Three possible heat sink geometries have been designed and compared regarding their overall heat transfer coefficient with respect to the non-dimensional pumping power, i.e. the ratio between the overall transferred heat and the energy required for pumping. The overall heat transfer coefficient for a finned heat sink has been increased up to 60% with respect to a baseline case without fins under similar conditions. This project has received funding from SOLAR-ERA.NET Cofund 2 joint call undertaking under the European Union’s Horizon 2020 research and innovation programme. This work has also been supported by Scientific and Technological Research Council of Türkiye (TÜBİTAK) under grant number 219M028. The authors acknowledge the Heat and Environment Laboratory, Mechanical Engineering Department, METU, and the help of Bulent G. Akinoglu and Elsen Aydin. D. Santos acknowledges FI AGAUR-Generalitat de Catalunya fellowship (2022FI_B2_00173).

Solar power generation plays an increasingly important role in the context of the energy revolution. Apart from the widely used crystalline silicon photovoltaic (c-Si PV) cells and flat plate vacuum solar thermal collectors for decentralized energy generation, concentrating solar power systems are on the rise due to their ability to reach high process temperatures and/or high photovoltaic (PV) efficiencies. In order to push technological advancements in this field, high-flux solar simulators are required for development and testing of components and modules under reproducible conditions. This paper describes the design process, manufacturing and commissioning of a versatile, low-cost, high-flux solar simulator for the investigation of concentrator photovoltaic (CPV) cells or other parts and modules used in concentrating solar systems. The design and development process of the solar simulator is described in detail, starting with the compilation of the requirement list, followed by the selection of the correct light source, all the way to the measurement technology and graphic user interface. The solar simulator is also characterized based on IEC and ASTM standards regarding spectral mismatch, concentration factor, temporal stability, as well as adjustability for a variety of measurement tasks. It can reach adjustable concentration factors of more than 200 suns in an area of 50 x 50 mm and the cost was kept below 5000 €. Finally, exemplary test results of an Azurspace 3C44 multi-junction cell under different concentration factors are presented.

A new design and retrofit approach for a concentrated photovoltaic thermal (CPV-T) system based on a parabolic trough collector is presented. The design differs from previous hybrid architectures, employing a significantly simplified topology in order to reduce costs while keeping electrical efficiency high. To achieve this ambitious goal, the absorber tube of a conventional parabolic trough collector used in thermal systems is replaced by a newly developed hybrid absorber equipped with multi-junction solar cells. This offers the advantage that existing solar thermal parabolic trough facilities can be easily retrofitted, making the system scalable and cost-effective. A scaled prototype of the system was designed and tested in Graz, Austria. During on-sun tests an average electrical system efficiency of 26.8 % with a simultaneous thermal efficiency of 48.8 % was measured using a geometric concentration ratio of 150 (DNI based). This leads to an overall average system efficiency of 75.5 %. Moreover, a solar cell peak efficiency of 30 % was achieved, one of the highest measured solar-to-DC efficiencies for a parabolic trough-based solar collector. Special attention was paid to the temperature difference between the solar cells and the heat transfer fluid in order to ensure sufficient cooling of the cells and maximize thermal efficiency. The electrical, thermal and overall efficiencies at different heat transfer fluid temperatures were measured (17 ° C - 90 ° C) and their coefficients were derived. This work serves as an experimental proof-of-concept for the application of solar cells in parabolic trough collectors, thereby opening up new possibilities for cost reduction of CPV-T systems.

Efficient energy storage is the key to modern hybrid or zero emission vehicles and low carbon mobility in general. Compared to conventional storage technologies like batteries, flywheel energy storage systems (FESSs) offer various theoretical advantages, such as high cycle life, no capacity fade over time, temperature independence, easy determination of state of charge, and complete recyclability. However, the special operating conditions of FESSs-such as vacuum, high rotational speeds, and high gyroscopic reactions, etc.-make bearing design a complex and crucial endeavor. This article describes methods of determining loads for rolling element bearings in automotive FESSs. An overview of FESS technology is given, followed by the discussion of an analytic, numeric, and empiric approach, including a detailed comparison of the different methods. Furthermore, the concept of a test bench investigating flywheel behavior in a resilient mount is described, and its results regarding the design of an FESS-tovehicle mount are discussed in depth.

Besonders seit der Etablierung genormter Testzyklen, wie z. B. des New European Driving Cycle (NEDC), herrscht landlaufig die Meinung, dass die Verantwortung einer Verbrauchsreduktion der Fahrzeuge in erster Linie bei den Fahrzeugentwicklern liegt, und daher eine rein technische Frage sei. Eine Optimierung der Verbrennungskraftmaschine (VKM), Leichtbau und ein „smartes“ Energiemanagement konnen zweifellos zur Reduktion des Kraftstoffverbrauchs beitragen, aber oftmals bergen ausere Einflusse, wie die Wahl des Fahrzeuges an sich und dessen Einsatzprofil ein erheblich groseres Potenzial. Da in manchen Fallen eine exakte Quantifizierung und Ermittlung des wichtigsten Einflussparameters nicht moglich ist, gilt es fortan das Fahrzeug als ganzheitliches, systemabhangiges Optimierungsproblem zu betrachten. Dies bedeutet gleichermasen, dass all jene Parteien, welche das Supersystem des Fahrzeuges gestalten, in einen interdisziplinaren Prozess involviert sind und Verantwortung fur dessen gemeinsame Entwicklung ubernehmen mussen. Abb. 2.1 zeigt die Wechselwirkung zwischen Hybridfahrzeug, Subsystem (bestehend aus den wesentlichen technischen Komponenten des Fahrzeuges) und Supersystem, welches jene auseren Einflusse beschreibt, die auf Fahrzeugtopologie, Betriebsstrategie und letzten Endes Energieverbrauch Einfluss haben. Auf das eigentliche Subsystem des Speichers bzw. die verschiedenen Hierarchieebenen der Systembetrachtung wird in Kap. 4 eingegangen.

Dieses Kapitel erortert Moglichkeiten der effizienteren Nutzung von Schwungradspeichern durch Anpassung der Speicherumgebung – oder des Supersystems (entsprechend der in Kap. 4, Abb. 4.2 vorgestellte Systemhierarchie) – an gegebene Speichereigenschaften.

Das Subsystem des FESS umfasst wie beschrieben alle Komponenten und Bauteile, welche fur den Aufbau und Betrieb des Speichers notwendig sind. Durch Modifikation einzelner Komponenten, wie Lager, Rotor, oder E-Maschine konnen energetische Spezifikationen des FESS beeinflusst und ggf. verbessert werden. Ziel ist die Annahrung der im Rahmen der aktuellen technischen Moglichkeiten erreichbaren Eigenschaften eines FESS an die Performance eines Referenzenergiespeichers – zumindest bis zum Erreichen der unter Abschn. 5.4 beschriebenen Threshold-Spezifikationen.

Das Supersystem des Flywheel Energy Storage Systems (FESS) umfasst all jene Aspekte und Komponenten, welche sich auserhalb des Energiespeichers befinden, jedoch in direkter oder indirekter Wechselwirkung mit dem Schwungrad stehen. Diese hierarchisch ubergeordneten Komponenten oder Einflussgrosen konnen in sich wieder ein eigenes System bilden und werden in einer holistischen Betrachtung auch oftmals sinnvoller Weise zu einer eigenen Einheit zusammengefasst. So werden in manchen Fallen alle relevanten Komponenten des Fahrzeuges zum Supersystem „Fahrzeug und Fahrzeugtopologie“ zusammengefasst, um fur die relevante Betrachtung entsprechende Systemgrenzen zu setzen.

Um die Motivation und Vorgehensweise des Designs jener Rotoren, welche unter Abschn. 7.4 und 7.5 noch genau beschrieben werden besser zu verstehen sind im folgenden Abschnitt die wesentlichen mechanischen Grundlagen zusammengefasst. Unter Abschn. 7.2 wird der Stand der Technik erhoben bzw. analysiert. Es ist anzumerken, dass das Kapitel Rotor an dieser Stelle nur vom Gesichtspunkt der Energiedichte und dem maschinendynamischen Verhalten betrachtet wird. Rotorspezifische Berst- und Versagensszenarien werden im Kapitel Gehause (vergleiche Kap. 8) betrachtet.

Schwungradenergiespeicher sind mit ihren hohen spezifischen Leistungen bei moderaten Energieinhalten bzw. ihren hohen Zyklenzahlen ideal fur Bremsenergierekuperation in dynamischen Fahrprofilen. Einige spezifische Probleme bei Einsatz dieser Speicher in Automobilen, wie die soeben erorterten gyroskopischen Lagerreaktionen und die in Kap. 8 diskutierte Crash-Sicherheit, wurden bei Stationaranwendungen komplett entfallen. Gelingt es, die bislang hohe Selbstentladung von FESS zu minimieren und gleichzeitig die Kosten zu senken, so konnte dieses Konzept eine gute Alternative zu chemischen Batteriespeichern fur die Speicherung erneuerbarer Energie darstellen. Der vielzitierte Umstieg vom konventionellen Pkw mit Verbrennungskraftmaschine auf ein voraussichtlich primar elektrobasiertes Transportsystem macht nur dann Sinn, wenn sich der Strommix zu einem Grosteil aus CO2-neutralen Energiequellen zusammensetzt. Im Jahr 2013 wurden in der EU durchschnittlich immer noch 558g CO2 pro kWh Elektrizitat produziert [1], was – wenn man die Lade- und Entladewirkungsgrade der Elektrofahrzeuge berucksichtigt – nicht wesentlich Umweltfreundlicher ist als die Energieerzeugung mittels Benzinmotor, welche in etwa 690 g CO2 pro kWh verursacht [2].

Data related to the performance of burst containments for high-speed rotating machines, such as flywheel energy storage systems (FESS), turbines or electric motors is scarce. However, development of optimized burst containment structures requires statistically significant data, which calls out for low-cost test methods as a strategic development tool. Consequently, a low-cost test rig (so called spin pit) for the investigation of burst containments was designed, with the goal to systematically investigate the performance of different containment structures and materials, in conjunction with the failure mechanisms of different rotors. The gathered data (e.g., burst speed, acceleration, temperature, ambient pressure, etc.) in combination with a post-mortem analysis was used to draw an energy balance and enabled the assessment of the effectiveness of various burst containments.

On the path to a low-carbon future, advancements in energy storage seem to be achieved on a nearly daily basis. However, for the use-case of sustainable transportation, only a handful of technologies can be considered, as these technologies must be reliable, economical, and suitable for transportation applications. This paper describes the characteristics and aging process of two well-established and commercially available technologies, namely Lithium-Ion batteries and supercaps, and one less known system, flywheel energy storage, in the context of public transit buses. Beyond the obvious use-case of onboard energy storage, stationary buffer storage inside the required fast-charging stations for the electric vehicles is also discussed. Calculations and considerations are based on actual zero-emission buses operating in Graz, Austria. The main influencing parameters and effects related to energy storage aging are analyzed in detail.Based on the discussed aging behavior, advantages, disadvantages, and a techno-economic analysis for both use-cases is presented. A final suitability assessment of each energy storage technology concludes the use-case analysis.

Hybridsysteme bieten die Moeglichkeit, praxistaugliche Fahrzyklen und einen energieeffizienten Fahrzeugbetrieb miteinander zu vereinen. Nicht jedes Sekundaerspeicherprinzip ist dafuer gleich gut geeignet. Spezifische Eigenschaften, Anforderungen und die sich daraus ergebenden Herausforderungen werden am Beispiel eines Schwungradspeichers (Flywheel) gezeigt. Zudem werden Speicherdimensionierung und Betriebsstrategien als Teil einer praxisrelevanten, holistischen Betrachtung am Beispiel des urbanen Linienbusbetriebs untersucht. Es zeigt sich, dass, wenn wesentliche Voraussetzungen wie dynamische, vorhersagbare Geschwindigkeitsprofile gegeben sind, spezifische Vorteile von Schwungradspeichern im Vergleich zu anderen Sekundaerspeichersystemen zum Tragen kommen. Dadurch ist eine anwendungsgerechte Auslegung durchfuehrbar. Dies ermoeglicht die Realisierung von Speichersystemen, die bei wirtschaftlich vertretbarem Aufwand hohe Energieeinsparungen ermoeglichen. Dabei sind hohe Leistungen beziehungsweise Leistungsdichten erreichbar, wobei das komplexe Systemverhalten mechanischer Energiespeicher eine interdisziplinaere Herausforderung darstellt. (A)

Reducing the costs of heliostats is crucial for the worldwide deployment of Concentrated Solar Power (CSP). CSP offers a number of advantages over photovoltaic at large scale power generation, such as lower cost and the option of thermal energy storage. However, the costs of a power tower plant are dominated by heliostats, which are usually 50% of the entire facility. NASA5s Jet Propulsion Laboratory was awarded funding by the DOE SunShot Initiative to develop a low-cost, rigid foam-based solar concentrator to meet installed cost goals of S75/m2 vs. current costs of — $200–250/m2. The design approach was validated by building and testing of a small-scale (6m2) heliostat prototype. Based on the findings of this prototype costs estimates for high production volume using industrial manufacturing technologies were conducted. The findings of these studies as well as lessons learned from manufacturing of the prototype are presented in this paper.

This publication demonstrates that flywheel energy storage systems (FESS) are a valid alternative to batteries for storing energy generated by decentralized rooftop photovoltaic systems. The increasing number of private PV arrays calls out for high energy storage capacities in order not to overload the grid. Despite being the current storage technology of choice, chemical batteries are still too expensive and have certain disadvantages compared to FESS, such as capacity fade over time and currently still difficult recycling. Within a research project at the Graz University of Technology a feasibility study for a low-cost, low-loss FESS was conducted. Energetic dimensioning was performed using actual PV power and electric load data recorded at a building in Austria with 6 apartment units. A low-cost flywheel system with an energy content of 5.0 kWh and 2.2 kW maximum rated power using a steel rotor and economic off-the shelf components was designed and investigated. Self-discharge of the proposed FESS design was significantly reduced using a cast silicone bearing seat, which allows supercritical rotor operation. Axial bearing loads were compensated by nearly 100% via repelling permanent magnets allowing drastic down-sizing of the bearings and further reduction of torque loss. The concept was validated by a small-scale test setup, which showed promising results. Finally, an improved design option is compared to the initially proposed FESS in terms of costs and self-discharge.

A low-cost rigid foam-based concentrator technology development program was funded by the DOE SunShot Initiative to meet installed cost goals of $75/m2 vs. current costs of ∼ $200–250/m2. Phase 1 of the project focused on design trades and cost analyses leading to a cost-optimized self-powered autonomous tracking heliostat concept with a mirror surface area in the 100m2 range. In Phase 2 30-year accelerated testing of the mirror modules based on ReflecTec film with 94% specular reflectivity bonded on composite foam substrate were initiated and completed in Phase 3. The tests with 15 coupons showed optical performance degradation of less than 5% in specular reflectance following 30-year equivalent UV testing and other abuse testing such as acid rain, bird dropping, thermal cycling, etc. A small scale prototype (3m×2m) heliostat design based on modular truss elements with removable mirror modules was developed in detail. In this phase components such as the dual-axis actuators were sized and selected based on wind load requirements and pointing accuracy demands were completed. Finite Element analyses for the mechanical structure with mirror modules were performed using three separate commercial codes — ANSYS, COMSOL and SolidWorks to validate the optical errors induced by wind loads on the structure up to 35 mph. Results indicated that the RMS deflections contributed to less than 0.4 mrad pointing error. Dynamic response of the heliostat indicated that the first 5 eigenmodes were in the 17–20 Hz range. The individual structure elements such as the trusses and c-rails were fabricated locally and assembled with the mirror facets in the lab for initial fit check and testing. The nine mirror facet surface errors were characterized using photogrammetry and verified using Reverse Hartmann techniques and showed to be in the order of 1 mrad or less. A three-level controller (main, gateway and heliostat) was architected and built. Tracking of the sun is done using NREL’s Sun Tracking Algorithm implemented in the gateway controller. Target-pointing vectors are calculated for each heliostat and conveyed wirelessly to the individual heliostat controllers for actuating the azimuth and elevation motors. The power subsystem consisting of solar panels and a battery provide 24V for the actuators and controller boards. The system was sized to provide adequate power for a period of 5hrs of operation when power is not available. Initial calibration will be performed with on-site camera tracking the sun’s image on a target located approximately 52m from the heliostat. Testing of the heliostat pointing under calm and windy conditions will be done to demonstrate overall performance that meet DOE targets of 4 mrad under 27mph winds. Commercialization efforts are underway to transition the design to the commercial sector. The project is well on its way to approaching overall cost targets and current estimates are approximately $90–110/m2 and lower costs can be achieved with alternates to the film we have identified.

Even though the r&d activities in the field of energy storage are currently being pushed all over the globe, the results are still far from satisfactory. This paper describes how the benefits of flywheel energy storage systems (FESS) can be exploited even though some properties still need to be improved significantly in the near future. Within a research project at the Graz University of Technology an FESS for application in vehicles serving urban public transportation (UPT) was investigated. The systematic design process and the problems of the proposed topology are discussed. Finally, solutions are suggested, which allow the maximum exploitation of the properties of kinetic energy storage. These recommended solutions do not only affect the flywheel design itself, but also its mode of application. This means that a holistic approach was chosen, which takes into account the whole super-system of the flywheel hybrid vehicle.

The demand for short term energy storage providing high power for electric and hybrid-electric vehicles is increasing dramatically. Stationary flywheel energy storage systems (FESS) are established as uninterruptible power supply (UPS) and represent an emerging market. In contrast, mobile FESS are currently only used in few applications such as motor sports. To enable a wider use in personal and public transportation the lifespan of the flywheel's bearings needs to be increased significantly. This paper presents an alternative approach to extend the lifespan of the flywheel's bearings by using a combination of rolling element and active magnetic bearings (CREAMB).

Photovoltaic systems have become an integral and widespread part of renewable energy generation. In combination with energy storage, PV systems offer a variety of advantages such as increased self-sufficiency of the user and improved grid stability. However, component sizing, as well as the implemented control algorithm severely influence system efficiency, total cost of ownership and payback time. In order to assess real-life performance of such systems, it is imperative to know the theoretically optimal solution. Therefore, a generic simulation framework based on a so-called ‘dynamic programming’ approach was developed, which allows benchmarking of PV systems by solving a multi-dimensional optimization problem. This paper does not only provide in-depth insight into the programming of the control algorithm but also provides detailed models of components like PV modules, battery storage, and inverters. Time-dependent performance aspects such as aging of the battery, deterioration of PV etc. are considered, allowing precise long-term predictions. The benefits of the dynamic programming approach are demonstrated by simulating characteristic use-cases including the consideration of different feed-in tariffs and energy costs. Finally, the impact of sub-optimal and over-simplified models (e.g. wrong component efficiencies) is analyzed. The developed simulation framework will be provided free of charge on request.