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Bertram E.,Institute for Solar Energy Research Hamelin
Energy Procedia | Year: 2014

Different concepts of solar assisted heat pump systems with ground heat exchanger are simulated according to IEA SHC Task44/HPP Annex38 reference conditions. Two aspects of the concepts are investigated using TRNSYS simulations. First, the solar impact on system efficiency is assessed by the seasonal performance factor. Second, the solar impact on the possible shortening of the ground heat exchanger is evaluated by the minimum temperature at the ground heat exchanger inlet. The simulation results reveal diverging optimums for the concepts. The direct use of solar energy clearly achieves the best effect on the efficiency improvement. A simple domestic hot water system reaches a seasonal performance factor of 4.5 and solar combi-systems seasonal performance factors up to 6. In contrast, the use of solar energy on the cold side of the heat pump achieves the best effects on the shortening of the ground heat exchanger of up to 20%. Two highly sensitive influences are investigated with the developed transient system model. First, the minimum allowed heat source temperature is varied. Here 1 K equals a variation of 0.25 in the seasonal performance or of around 10% ground heat exchanger length. Second, the ground heat exchanger model is simulated without and with a pre-pipe that improves the transient model behavior. The influence of this pre-pipe on the SPF is small for conventionally designed ground heat exchangers, but of around 2 K for the minimum inlet temperature. Therefore, the dynamic model quality reveals potential to reduce the size of the ground heat exchanger corresponding to investment costs. Source


Brendel R.,Institute for Solar Energy Research Hamelin | Brendel R.,Leibniz University of Hanover
Progress in Photovoltaics: Research and Applications | Year: 2012

Modeling of transport and recombination of charge carriers in solar cells is useful for understanding and improving the device performance. We implement the fully coupled transport equations for electrons and holes into the finite-element partial differential equation solver COMSOL. The dopant-diffused surface regions such as junctions, floating junctions, or back surface field layers are treated as conductive boundaries of the volume in which the semiconductor equations are solved. This so-called conductive boundary (CoBo) model characterizes diffused layers by their sheet resistances and diode saturation current densities. Both are directly experimentally accessible. The CoBo model exhibits excellent numerical stability and enables two-dimensional simulations on a laptop. We find agreement when testing the two-dimensional COMSOL implementation of the CoBo model for one-dimensional devices against simulations using the code PC1D. We apply the CoBo model to elucidate how the sheet resistance of diffused vias impacts the power conversion efficiency of emitter wrap through solar cells. Copyright © 2010 John Wiley & Sons, Ltd. Source


Glembin J.,Institute for Solar Energy Research Hamelin | Rockendorf G.,Institute for Solar Energy Research Hamelin
Solar Energy | Year: 2012

This simulation study investigates different discharging and charging strategies and their effects on the performance of a combined solar thermal system in a single family house. Based on the system according to Task 32 of the Solar Heating and Cooling Program of the International Energy Agency (IEA SHC) the system layout and in specific the storage connections are modified to investigate different stratified charging and discharging concepts realized by external valves or devices within the storage tank.A broad literature overview shows that a comprehensive comparison of charging and discharging strategies under the same boundary conditions and after optimization of the storage connection heights is not available. The results of this study show that a good thermal stratification within the storage and thus higher energy savings can be reached by both a stratified charging and discharging. Depending on the system size and the design of charging and discharging connections the stratified discharging leads to the same or even higher energy savings than a stratified charging. Already one single four-way mixing valve in the space heating flow (i.e. two tapping points) leads on the one hand to more than 80% of the advantage of an idealized discharging with seven tapping points, and on the other hand in all cases to significantly higher energy savings if compared to two charging devices with three-way valves. The relative energy savings increase with increasing solar fraction, e.g. with larger dimensions and better insulated buildings. The best option with the highest benefit depends on the system design like storage in- and outlet positions, system size and load conditions. Therefore, the decision for the best suited strategy can only be determined by simulations representing the respective system. The results presented in this paper, however, allow deriving general dependencies. © 2011 Elsevier Ltd. Source


Werner F.,Institute for Solar Energy Research Hamelin | Schmidt J.,Institute for Solar Energy Research Hamelin
Applied Physics Letters | Year: 2014

We manipulate the negative fixed charge density Qf at the c-Si/Al2O3 interface by applying a bias voltage in a metal-oxide-semiconductor configuration or by depositing corona charges onto the Al2O3 film. A significant increase of the negative fixed charge density from |Qf| = 4 × 1012 cm-2 to values above 1013 cm-2 is observed for surface Fermi energies close to or within the silicon conduction band. The additional charges are shown to be partly unstable under annealing or changing the polarity of the bias voltage. Our experimental data are best described by assuming at least three different types of charge traps responsible for the formation of the negative fixed charge density at the c-Si/Al2O3 interface. © 2014 AIP Publishing LLC. Source


Jung V.,Institute for Solar Energy Research Hamelin | Kontges M.,Institute for Solar Energy Research Hamelin
Progress in Photovoltaics: Research and Applications | Year: 2013

Rear sides of crystalline silicon solar cells are usually covered with aluminum on which it is difficult to solder. To ease soldering, we present a durability study for a Ni: V/Ag stack on evaporated Al as rear-side metallization. We adapt this cost-effective metallization stack from the microelectronic industry and investigate it as metallization for silicon solar cells. Here, a long-term stability of the metallization and of the solder joint must be guaranteed for 25 years and is therefore evaluated in detail by thermal aging experiments. During this experiment, the mechanical stability of the solder joints is measured. The chemical stability and the intermetallic compound (IMC) growth within the solder joints are examined by secondary electron microscopy, backscattered electron imaging, and energy dispersive X-ray analysis. Experiments with either a Sn-Ag-coated copper tab or pure Sn-Ag solder show two different sorts of IMCs at the Ni: V/Solder interface. With the copper tab, a Cu-Ni-Sn compound, presumably (Cu1 - xNix) 6Sn5, grows at the Ni/solder interface, whereas in case of a pure Sn-Ag solder, a Ni-Sn compound grows, which is likely to be Ni 3Sn4. Analysis of the reaction kinetics leads to activation energies of 77 and 42 kJ/mol, respectively, for a diffusion-controlled IMC growth. By using temperature histograms of PV modules in the field, the necessary minimum Ni: V layer thickness is estimated: without a copper tab up to 1.6 μm Ni and with a copper tab less than 0.2 μm may be consumed by IMC formation during 25 years of lifetime. Copyright © 2012 John Wiley & Sons, Ltd. A Ni: V/Ag stack on evaporated Al is investigated as rear-side metallization for silicon solar cells by a thermal aging of solder joints on this metallization. Depending on the presence of Cu on top of the solder, varying intermetallic compounds are formed at the Ni: V/solder interface. By analyzing the reaction kinetics and taking module temperature histograms into account, the necessary minimum Ni: V layer thickness for 25 years lifetime is estimated. Copyright © 2012 John Wiley & Sons, Ltd. Source

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