ZAE Bayern Research Articles

Drifts (Diffuse Reflectance Infrared Fourier Transform Spectroscopy) - a Way to Assess the Reactivity of Solid Electrolytes with Ambient Air

All-Solid-State-Batteries (ASSBs) are envisioned to be the next-generation lithium-ion batteries (LIBs). Replacing the inflammable organic electrolyte in conventional liquid electrolyte LIBs by a non-inflammable inorganic solid electrolyte (SE) is one of the concepts to increase battery safety. Moreover, ASSBs may offer higher gravimetric and volumetric energy density compared to liquid electrolyte based technology.[1,2] However, their high reactivity with ambient air is a major obstacle which prevents ASSBs from industrial scale processing, as production under inert atmosphere is not possible on this level.[3] Thus, industrial ASSB production would have to take place in dry-rooms, leading to the question whether SE handling in dry-room atmospheres containing O2 and CO2 and ppm-levels of moisture is possible.Therefore, we will examine the reactivity of the commercially available solid electrolyte Li10SnP2S12 (LSPS) with O2 and CO2 as well as with the typical H2O concentration in ambient air, followed by reactivity studies with a mixture of CO2 and H2O vapor. For this purpose, we used Diffuse Reflectance Infrared Fourier-Transform Spectroscopy (DRIFTS), a highly surface sensitive infrared technique.The hermetically sealed DRIFTS cell containing the SE powder was set up such that it could be purged with dry or humidified streams of inert gas (Ar), O2, and CO2, or mixtures thereof while monitoring in situ the chemical changes of the LSPS solid electrolyte and the formed gaseous species by DRIFTS. Figure 1 shows stacked IR spectra of LSPS after exposure to different gasses, stating no reactivity with O2 or CO2 but noticeable changes in the IR spectrum for exposure to moisture and a combination of CO2 and moisture, respectively, indicating decomposition reactions.The spectral analysis of the decomposition process is supported by ex situ X-Ray Diffractometry (XRD) analysis of the LSPS powder after exposure to the different gas streams. Concluding, the impact of exposing LSPS to the above mentioned gas mixtures on the ionic conductivity of the solid electrolyte will be demonstrated. Acknowledgements: This work was carried out as part of the research project ASSB coordinated by ZAE Bayern. The project is funded by the Bavarian Ministry of Economic Affairs, Regional Development and Energy.

Identification of wavelength regions for non-contact temperature measurement of combustion gases at high temperatures and high pressures

Stationary gas turbines are still an important part of today’s power supply. With increasing temperature of the hot combustion gas inside a gas turbine, the efficiency factor of the turbine increases. For this reason, it is intended to operate turbines at the highest possible gas temperature. Therefore, in the combustion chamber and especially at the position of the first stage guide vanes the gas temperature needs to be measured reliably. To determine the gas temperature, one promising approach is the application of a non-contact measurement method using a radiation thermometer. A radiation thermometer can measure the gas temperature remotely from outside of the harsh environment. At ZAE Bayern, a high temperature and high-pressure gas cell has been developed for this purpose in order to investigate gases and gas mixtures under defined conditions at high pressures and high temperatures. This gas cell can be placed in a FTIR-spectrometer in order to characterize the infrared-optical properties of the gases. In this work the measurement setup is introduced and gas mixtures, which are relevant for gas turbine applications are analyzed thoroughly. The derived results are presented and discussed in detail. To identify suitable wavelength regions for non-contact gas temperature measurements, first tests have been performed. Based on these tests, an appropriate wavelength region could be chosen, where future gas temperature measurements can be carried out.

Energetic performance of two different PCM wallboards and their regeneration behavior in office rooms

In order to increase energy efficiency and thermal comfort in lightweight buildings like offices, PCMs (Phase Change Materials) can be integrated within building components like walls or ceilings. The aim of this research is to investigate the thermal behavior of two different PCM wallboards, the Knauf Comfortboard-23® and the DuPont Energain® Board. These wallboards are equipped in the office rooms of the Energy Efficiency Center of ZAE Bayern in Würzburg, Germany. It is evident that the regeneration behavior of PCM wallboards critically influences their thermal performance. Therefore, monitoring data of the years 2015 and 2016 have been evaluated to investigate the regeneration behavior of PCM wallboards under real life conditions. The results show that a sufficient regeneration of PCM wallboards is an issue which has to be addressed to improve the thermal performance. However, PCMs are still well suited to achieve thermal comfort for lightweight buildings in moderate climate zones (e.g. Germany); the room temperature is mostly kept between 22°C to 26°C during summers.

Experimental efficiency of a low concentrating CPC PVT flat plate collector

PVT collectors aim to solar co-generation of electricity and heat. An new concept raises thermal efficiency by concentrating sunlight with CPC reflectors, in order to access a higher number of solar thermal applications. Losses in PV efficiency due to a higher operating temperature are accepted with regard to a higher overall collector efficiency. As a side effect, the concentration lowers the material usage of PV and enables a high efficient thermal coupling of the PV cell to the heat carrying fluid.The work focuses on the construction, as well as on the angle dependent electrical and thermal measurement of the real-size CPC PVT collector prototype (1460mm×600mm×150mm). In previous publications, calculations and experiments studied the influence of the CPC reflectors on the PV efficiency. Further, the thermal coupling between PV cell and heat carrier fluid has been measured in a lab experiment. These results, together with transient annual simulations, were considered in the design process of the CPC PVT prototype with an angular acceptance range of ±25°.The experiments were conducted on the outdoor solar test facility at ZAE Bayern in Garching, Germany. The thermal and PV efficiency has been measured with MPP tracking, as well as for open circuit voltage for fluid temperatures up to 107°C.It could be shown, that the thermal efficiency while MPP tracking is elevated to 34% compared to a glazed flat plate PVT with 17% for collector temperatures 60K over ambiance. At the same time, the electrical efficiency drops from 15% cell efficiency to an overall collector efficiency of 9%, due to the optical setup, temperature effects and a non-uniform flux distribution caused by the reflectors.

PCM cooling ceilings in the Energy Efficiency Center—passive cooling potential of two different system designs

In the Energy Efficiency Center—the new R&D building of the ZAE Bayern—two different PCM cooling ceiling types are installed and monitored in two office rooms. The ceilings are connected to a water circuit and can be used for heating and cooling. The PCM ceilings are prototypes which differ in the positioning of the PCM layer: ceiling type 1 with the PCM on top of and ceiling type 2 with the PCM below the water pipes. It was found, that cooling ceiling type 2 had a better thermal connection between PCM and room. Nevertheless, the passive cooling power of the two different system designs was quite similar when measured as a function of the globe temperature. Passive cooling powers of 10–15W per m2 PCM cooling ceiling area were measured for a globe temperature of 26°C.

Experimental characterization and theoretical modeling of the infrared-optical properties and the thermal conductivity of foams

In order to understand and improve the thermal insulation performance of cellular polymeric foams, two different polymers and corresponding foams, extruded polystyrene foam (XPS) and Polyurethane foam (PUR) were selected and thoroughly characterized at ZAE Bayern. At first radiative properties, cell morphology and thermal conductivity were characterized. The measured results were then compared to the theoretical model developed at ZAE Bayern. The validated model allows then the prediction of the insulation performance of an arbitrary XPS or PUR foam with given density and geometrical structure. It also allows defining the optimal structure (resin, cell morphology, density) to achieve the desired thermal conductivity.

Vacuum Insulation Panels – Exciting Thermal Properties and Most Challenging Applications

Vacuum insulation panels (VIPs) have a thermal resistance that is about a factor of 10 higher than that of equally thick conventional polystyrene boards. VIPs nowadays mostly consist of a load-bearing kernel of fumed silica. The kernel is evacuated to below 1 mbar and sealed in a high-barrier laminate, which consists of several layers of Al-coated polyethylene (PE) or polyethylene terephthalate (PET). The laminate is optimized for extremely low leakage rates for air and moisture and thus for a long service life, which is required especially for building applications. The evacuated kernel has a thermal conductivity of about 4 x 10 -3 W·m -1 ·K -1 at room temperature, which results mainly from solid thermal conduction along the tenuous silica backbone. A U-value of 0.2W·m -2 ·K -1 results from a thickness of 2cm. Thus slim, yet highly insulating facade constructions can be realized. As the kernel has nano-size pores, the gaseous thermal conductivity becomes noticeable only for pressures above 10 mbar. Only above 100 mbar the thermal conductivity doubles to about 8 × 10 -3 W·m -1 ·K -1 , such a pressure could occur after several decades of usage in a middle European climate. These investigations revealed that the pressure increase is due to water vapor permeating the laminate itself, and to N 2 and O 2 , which tend to penetrate the VIP via the sealed edges. An extremely important innovation is the integration of a thermo-sensor into the VIP to nondestructively measure the thermal performance in situ. A successful "self-trial" was the integration of about 100 hand-made VIPs into the new ZAE-building in Wurzburg. Afterwards, several other buildings were super-insulated using VIPs within a large joint R&D project initiated and coordinated by ZAE Bayern and funded by the Bavarian Ministry of Economics in Munich. These VIPs were manufactured commercially and integrated into floorings, the gable facade of an old building under protection, the roof and the facades of a terraced house as well as into an ultra-low-energy "passive house" and the slim balustrade of a hospital. The thermal reliability of these constructions was monitored using an infrared camera.

Silica aerogel granulate material for thermal insulation and daylighting

Silica aerogel granulate is a nanostructured material with high solar transmittance and low thermal conductivity. These properties offer exciting applications in building envelopes. One objective of the joint R&D project ISOTEG at ZAE Bayern was to develop and characterize a new glazing element based on granular silica aerogel. Heat transfer coefficients of less than 0.4 W/(m 2 K) and a total solar energy transmittance of 35% for the whole glazing unit were achieved. The glazing has a thickness of less than 50 mm. Another application for granular silica aerogel is, for example, in solar collectors. The thermal properties of the glazing as well as the optical and thermal properties of the granular aerogels are presented here. The solar transmittance of a 10 mm packed bed of silica aerogel was 53% for semi-translucent spheres and 88% for highly translucent granulate. In our heat transfer experiments the gas pressure, external pressure load, temperature and gas filling were varied. The various thermal conductivity values measured for the glazing and collector applications were compared to the values calculated using two different packed bed models. For the gas-dependent measurements the intergranular voids in the granulate were 1.0 ± 0.1 mm before loading the packed bed, 0.3 ± 0.1 mm at an external load of 3.2 bar (3.2 × 10 5 Pa) and 0.6 ± 0.1 mm after release. A direct radiative conduction of λ direct = 4.5 ± 0.5 × 10 −3 W m −1 K −1 was obtained.

Thin-film crystalline silicon mini-modules using porous Si for layer transfer

Layer transfer processes yield Si films of high electronic quality on low-cost non-Si carriers such as glass: a crystalline Si film grows on a Si–substrate wafer, is detached from that substrate and transferred to a low-cost non-Si device carrier. The substrate wafer is re-used for further growth cycles. Electrochemical etching of a porous Si (PSI) layer system into the substrate wafer enables homoepitaxial growth of monocrystalline Si films and facilitates the detachment of the film. We discuss the potential of crystalline thin-film cells from layer transfer and review the layer transfer work conducted at ZAE Bayern. The sevenfold use of a substrate wafer and the transfer of a 10 × 10 cm 2 epitaxial film from a 6″-wafer is demonstrated. A new module process that permits an integrated series connection by a single metallization step is demonstrated to yield a module efficiency of 10%.

Comparison of the optics of non-tracking and novel types of tracking solar thermal collectors for process heat applications up to 300 °C

Evacuated CPC (compound parabolic concentrator) collectors with non-tracking reflectors are compared with two novel tracking collectors: a parabolic trough and an evacuated tube collector with integrated tracking reflector. Non-tracking low concentrating CPC collectors are mostly mounted in east–west direction with a latitude dependent slope angle. They are suitable at most for working temperatures up to 200–250 °C. We present a tracking evacuated tube-collector with a trough-like concentrating mirror. Single-axis tracking of the mirror is realized with a magnetic mechanism. The mirror is mounted inside the evacuated tube and hence protected from environmental influences. One axis tracking in combination with a small acceptance angle allows for higher concentration as compared to non-tracking concentrating collectors. Ray-tracing analysis shows a half acceptance angle of about 5.7° at geometrical concentration ratio of 3.2. Losses of well constructed evacuated tube collectors (heat conductivity through the manifolds inside the thermally insulated terminating housing are low) are dominated by radiation losses of the absorber. Hence, reducing the absorber size can lead to higher efficiencies at high operating temperature levels. With the presented collector we aim for operating temperatures up to 350 °C. At temperatures of 300 °C we expect with anti-reflective coating of the glass tube and a selective absorber coating efficiencies of 0.65. This allows for application in industrial process heat generation, high efficient solar cooling and power generation. A first prototype, equipped with a standard glass tube and a black paint absorber coating, was tested at ZAE Bayern. The optical efficiency was measured to be 0.71. This tube-collector is compared by ray-tracing with non-tracking market available tube-collectors with geometrical concentration ratios up to 1.1 and with a low cost parabolic trough collector of Industrial Solar Technology (IST) with an acceptance half angle about 1.5°, a geometrical concentration ratio of 14.4 and a measured optical efficiency of 0.69.

Angular-dependent measurements of the thermal radiation of the sky

Measurements were carried out to determine the incoming longwave radiation of the sky as a function of the zenith angle at the ZAE Bayern in Wurzburg. The obtained data allowed the adjustment of an existing model to regional climatic conditions. This model was used to compute the thermal heat losses for various buildings with respect to variations of the emissivities of outer wall surfaces as well as their orientation. Condensation of water vapour was also taken into account. It was found that the application of wall paints with emissivities of e = 0.5 instead of normal paints (e 0.9) would reduce the thermal heat losses by about 5%-25%. Poorly insulated buildings and south-facing walls show the greatest energy savings.

Highly insulating aerogel glazing for solar energy usage

Granular silica aerogels have been integrated into highly-insulating translucent glazing. This work was performed within the large R&D project ISOTEG pursued by the ZAE Bayern. To avoid settlement of the granules, which often occurred in earlier glazing concepts and even caused destruction of the glazing, the granules were sandwiched between a double skin sheet made of PMMA. The sheet was mounted between two low-e coated glass panes. To optimize the thermal insulation, krypton was used as filling gas. This construction allows to achieve heat transfer coefficients of less than 0.4 W/(m 2 K). Optimized granular layers provide high solar transmittance of 65% for a thickness of 20 mm. Thus a total solar energy transmittance of 35% for the whole glazing unit is achieved. The glazing has a thickness of less than 50 mm. Such aerogel glazings can be integrated into solar wall systems or used as lightscattering daylighting elements with vanishing energy losses over the heating period even for north facade integration. Optical and thermal properties of the developed granular aerogels as well as the thermal properties of the whole glazing unit are reported.

Small-scale thermal seawater desalination simulation and optimization of system design

The Bavarian Center of Applied Energy Research (ZAE Bayern) and T.A.S. GmbH & Co. KG, Munich, have optimized a distillation method which was originally developed at the at the University of Munich and which is protected by patent. The method — developed for small-scale production (0.5–2 m 3/d) — is ideally suited for the use at decentralized stand-alone operation because it is nearly maintenance free and needs no electrical grid connection. It can be run either by solar energy or by waste heat from a diesel generator. The thermodynamic principle of multi-effect humidification (MEH) is based on the evaporation and condensation of water inside a thermally insulated box at ambient pressure. Air transports water vapour from the evaporator to the condenser where the distilled water is collected. A very efficient construction of evaporator and condenser allows high heat recovery ratios. The gained output ratio (GOR) (performance ratio) quantifies the multiplicity of thermal energy use for evaporation of the brine in the MEH system compared with evaporation without heat recovery. The process has been technically optimized so that the air circulation is driven entirely by natural convection. The produced distillate is of pure water quality. In order to make the proven reliable desalination process needing low maintenance competitive from economic point of view, 24-h operation of the desalination system is necessary. Thus, if the system is driven solar thermally, a thermal storage tank has to be implemented.

Solarthermal seawater desalination systems for decentralised use

The performance of a pilot solar Multi Effect Humidification (MEH) Desalination system in Fuerteventura, Canary Islands, had been measured and analysed in detail by the ZAE Bayern since 1992. The investigated distillation units showed constant performance over several years without extensive maintanance. However the efforts towards further efficiency improvement by economic means pointed out the need of supplementing the system with a thermal storage tank. The distillation unit being the most expensive part of a desalination system has to be run 24 hours a day in order to be economic. A cost estimation for storage implementation yields the result, that cost reduction for the produced water by more than a half is possible. In April 1997 a desalination system with 24 hour thermal storage was built up in Sfax / Tunisia. The results of a short term measuring campaign at this site are presented here. For optimizing the ratio of sizes of thermal storage, collector field and distillation module a simulation tool for collector field and storage is developed at the ZAE Bayern. The simulation results of a combined laboratory distillation unit and storage system are presented.

New development in the theory of heat and mass transfer in solar stills

The Bavarian Center of Applied Energy Research (ZAE Bayern) and T.A.S. GmbH & Co. KG, Munich, have optimized a distillation method which was originally developed at the at the University of Munich and which is protected by patent. The method — developed for small-scale production (0.5–2 m3/d) — is ideally suited for the use at decentralized stand-alone operation because it is nearly maintenance free and needs no electrical grid connection. It can be run either by solar energy or by waste heat from a diesel generator. The thermodynamic principle of multi-effect humidification (MEH) is based on the evaporation and condensation of water inside a thermally insulated box at ambient pressure. Air transports water vapour from the evaporator to the condenser where the distilled water is collected. A very efficient construction of evaporator and condenser allows high heat recovery ratios. The gained output ratio (GOR) (performance ratio) quantifies the multiplicity of thermal energy use for evaporation of the brine in the MEH system compared with evaporation without heat recovery. The process has been technically optimized so that the air circulation is driven entirely by natural convection. The produced distillate is of pure water quality. In order to make the proven reliable desalination process needing low maintenance competitive from economic point of view, 24-h operation of the desalination system is necessary. Thus, if the system is driven solar thermally, a thermal storage tank has to be implemented.