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AbstractBlack TiO2 has demonstrated a great potential for a variety of renewable energy technologies. However, its practical application is heavily hindered due to lack of efficient hydrogenation methods and a deeper understanding of hydrogenation mechanisms. Here, a simple and straightforward hot wire annealing (HWA) method is presented to prepare black TiO2 (H–TiO2) nanorods with enhanced photo‐electrochemical (PEC) activity by means of atomic hydrogen [H]. Compared to conventional molecular hydrogen approaches, the HWA shows remarkable effectiveness without any detrimental side effects on the device structure, and simultaneously the photocurrent density of H–TiO2 reaches 2.5 mA cm−2 (at 1.23 V vs reversible hydrogen electrode (RHE)). Due to the controllable and reproducible [H] flux, the HWA can be developed as a standard hydrogenation method for black TiO2. Meanwhile, the relationships between the wire temperatures, structural, optical, and photo‐electrochemical properties are systematically investigated to verify the improved PEC activity. Furthermore, the density functional theory (DFT) study provides a comprehensive insight not only into the highly efficient mechanism of the HWA approach but also its favorably low‐energy‐barrier hydrogenation pathway. The findings will have a profound impact on the broad energy applications of H–TiO2 and contribute to the fundamental understanding of its hydrogenation.

AbstractAn innovative method for heating proton exchange membrane fuel cell (PEMFC) stacks during start‐up by direct heating of the cells is presented and investigated. By imposing an alternating current on the stack, heat is generated locally depending on the internal cell resistance. It is shown, that an alternating current (AC) perturbation with a suitable high frequency mainly heats the ohmic resistors which are membrane and contact interfaces. The electrodes are protected from voltage cycling, due to (de)charging currents of the double layer capacitances at high frequency. This AC heating technique is applied on a 6‐cell low temperature (LT) PEMFC stack and a 30‐cell high temperature (HT) PEMFC stack, both with an active area of approximately 30 cm2. Both stacks are heated under realistic temperature conditions, the LTPEMFC stack from sub‐zero temperatures to 40 °C and the HTPEMFC stack from room temperature to 120 °C. Heating times are shown and discussed for different stack geometries and voltages. In both cases, this strategy leads to a short and efficient heating, as the cells are heated directly and not externally by coolant or other stack components.

In recent years, a wide range of strategies has been implemented in different EU-countries to increase the share of electricity generation from renewable energy sources. This paper evaluates the success of different regulatory strategies. The most important conclusions of this analysis are: (i) regardless of which strategy is chosen, it is of overriding importance that there should be a clear focus on the exclusive promotion of newly installed plants; (ii) a well-designed (dynamic) feed-in tariff system ensures the fastest deployment of power plants using Renewable Energy Sources at the lowest cost to society; (iii) promotion strategies with low policy risks have lower profit requirements for investors and, hence, cause lower costs to electricity customers.

Small scale (solar-) thermally driven cooling systems suffer from two important drawbacks: firstly, the systems usually offer no means of adapting the chilling capacity to the actual load; secondly constantly running pumps and fans lead to high auxiliary electricity consumption even when the available driving and cooling water temperatures only allow a reduced chilling capacity. To solve these problems a generic approach for controlling the main parasitic electrical devices – the cooling water pump and the heat rejection fan - as a function of the actual boundary conditions was developed. Different variants of control strategies are analyzed in different system configurations under a variety of climates and load conditions by means of dynamic system simulations in TRNSYS. The most typical combinations of ab- and adsorption chillers with dry cooler and wet cooling tower are covered. The results show that capacity modulation can be realized well by this approach. Additionally electricity savings of up to 25% can be achieved for reasonably sized systems compared to a reference control strategy with fixed pump speed and fixed cooling water set temperature. Yet it becomes obvious that the concrete savings depend strongly on the system configuration and boundary conditions.

Abstract Titanium-doped and undoped CuCoMnOx spinel films were deposited on Al substrates from sols which were made from the following: Co-acetate, Cu-chloride and Mn-acetate (Ti:CoCuMnOx-I); and Co-acetate, Cu-nitrate and Mn-acetate (CoCuMnOx-II). The precursors’ ratio Co:Cu:Mn was equal to 1:3:3. The solar absorptance (αs) and the thermal emittance (eT) of the films, which were annealed at 450°C for 15 or 30 min, were determined from the corresponding diffuse reflectance spectra in the 0.32–20 μm range. The results show that the CoCuMnOx-II films with SiOx protective over-coatings exhibited values of αs=0.85–0.91 and eT below 0.036 after just a single dipping/annealing cycle. The structure of the films was studied with X-ray diffraction (XRD), infrared (IR) absorbance and near-grazing incidence angle (NGIA) reflection-absorption spectroscopy. Our results suggested that the films have a spinel structure with the composition CoCuMnOx. The stability of the films was tested by soaking them in boiling water for 2 h. NGIA IR spectra of the treated films confirmed the formation of the hydrated mixed oxide (Mn-, Co-, Cu-) phases. To improve the stability of the films two kinds of protective over-coatings were tested: one over-coating was based on polysiloxane resin and the other on high-density silica (T-resin). Films that were resistant to boiling water were obtained by applying the high-density silica protective over-coating, which was cured at 140°C for 30 min.

Abstract A method to quantify spectral effects on the electric parameters of multi-junction solar cells is presented. The method is based on measuring the short circuit current of at least two monitor cells. Ideally these monitor cells have the same spectral responses as the subcells in the investigated multi-junction solar cell. In contrast to the subcells, the current of the individual monitor cells can be measured separately. This allows conclusions to be drawn about the spectral impact on the current mismatch of the multi-junction solar cell. A spectrometric evaluation method is then applied. The method has been tested experimentally with three concentrator modules using III–V triple-junction solar cells. These modules were measured outdoors for several months under variable solar spectral conditions. In parallel, the IV curves of the modules and the current of two component cells were measured. A spectral parameter Z was derived from the monitor cell current signals, which was correlated to the short circuit current and the fill factor of the modules. A linear correlation was found between Z and the normalized short circuit current of the concentrator modules. Translation equations were derived from the linear correlation. These enable the calculation of a module’s short circuit current under any spectral conditions. In particular, the short circuit currents of the modules were derived for direct normal irradiance of 850 W/m 2 and spectral conditions corresponding to the AM1.5d low AOD spectrum. This is an important step towards comparing the performance of modules which show strong spectral sensitivity. Future rating methods can benefit from the presented simple method for quantifying spectral impacts on multi-junction solar cells. Furthermore, the method can be of interest for tuning the spectrum of pulsed solar simulators.

Adsorption heat transformation (AHT) is an environmentally friendly energy-saving process applied for air conditioning purposes, that is, either for cooling (including also ice making and refrigeration), or heating. AHT is based on the cycling adsorption and desorption of a working fluid in a porous material. When the working fluid is driven to evaporation by the active empty sorbent material, the required heat of evaporation translates into useful cooling in thermally driven adsorption chillers. Driving heat regenerates the empty sorbent material through desorption of the working fluid. The heat of adsorption in the sorbent material and the heat of condensation of the working fluid can be used in the adsorption heat-pumping mode. Thus, adsorption heat transformation contributes to energy-saving technologies. Adsorbent development plays a critical role for the improvement of AHT technologies. Besides silica gel and zeolites as adsorbent materials, which are up to now used in the commercially available AHT devices; especially metal-organic frameworks (MOFs) are getting more attentions in recent years. Composite materials from salts with silica gels, zeolites and MOFs as well as activated carbons have also been researched to contribute to AHT technologies. Reduction of installation/production cost and enhancement of the efficiency of AHT devices need to be achieved to increase the wider usage of AHT.

Abstract With this work, we introduce numeric three-dimensional simulation of metal spiking into highly boron-doped surfaces of n-type silicon solar cells, which is moreover performed with a simulation of the quasi-steady-state photoconductance technique. This setup serves as a virtual experiment to simulate the dark saturation current density j 0,met of metallized boron-doped emitters with respect to metal spikes originating from the silver–aluminum (Ag–Al) contact. With the results obtained from this simulation model we approach quality and quantity of increased j 0,met and give detailed insight to which degree a solar cell’s performance is possibly harmed by this effect. We show that metal spikes penetrating into boron-doped emitters are of harmless nature concerning j 0,met until their tips reach depths where boron doping concentration is lower than approximately 10 18 cm −3 . Deeper spikes then lead to an exponential increase in j 0,met as more and more carriers from emitter and also the base are able to diffuse to its tip and recombine there. With the help of j 0 -results obtained experimentally in combination with the simulation results, we discuss the influence of spikes on emitter recombination, the benefits that can be achieved with deeper emitter doping profiles, and suggestions for the further development of pastes to contact boron-doped surfaces.