Rolf Hanitsch

Numerical simulation of current-voltage characteristics of photovoltaic systems with shaded solar cells

Abstract In photovoltaics the actual curve of the current-voltage characteristics of a solar generator is often needed, for example if the maximum power point is to be determined. It is convenient to use a model for the description of the relationship between current and voltage, which is able to describe the solar cell in the generation region as well as the breakdowns at positive and negative voltages. As the mathematical expression cannot be given in an explicit form, a set of suitable numerical algorithms to compute the currents for a given voltage and vice versa is given. For more complex solar generators, it can neither be assumed that the solar cells have identical electrical characteristics, nor that they are evenly illuminated. A general model for the description of solar generators is proposed that gives all voltages and currents as well as the voltage and current at the outputs of the solar generator itself. This new model is also implemented in a numerical algorithm. The characteristics of partially shaded solar generators were calculated and discussed using the proposed models.

Combined photovoltaic and solar thermal systems for facade integration and building insulation

Abstract Most photovoltaic (PV) facades are built as curtain facades in front of thermally insulated buildings, with air ducts in between. This causes additional costs for support structure and installation, while heat dissipation from the solar cells is often not optimal. Measurements carried out are facing both concerns: integration of a thermal insulating layer (which meets the latest German heat-preserving regulation, WSV 95) into the PV facade, plus additional cooling by active ventilation or water flow. Active ventilation at conventional curtain PV facades allows a reduction of cell operating temperatures of 18 K, resulting in an 8% increase in electrical energy output at an airspeed of about 2 m/s. Cell temperatures increase by 20.7 K at thermal insulating PV facade elements (TIPVE) without cooling, which causes a 9.3% loss of electrical yield, but installation costs can be reduced by 20% (all related to a conventional PV curtain plus a heat-insulating facade at a building). HYTIPVE, a hybrid thermal insulating PV facade element combined with a water cooling system, which could also serve for heating up water, lowers the operating cell temperature by 20 K and increases electrical yield by 9% (compared with conventional curtain PV facades). Further economic investigations of such a HYTIPVE, including its operational costs and substitution effect, related to the electrical and thermal yield, are in progress.

Actual optical and thermal performance of PV-modules

Abstract Actual efficiency of photovoltaic generators is often lower than predicted by standard test conditions (STC) or standard operating conditions (SOC). This is mainly caused by an underestimation of reflection losses and solar cell temperature in the module. To get more accurate results in predicting the actual performance of PV-modules, the parameters influencing incoming (optical parameters) and outgoing power flow (electrical and thermal parameters) were investigated by simulation and some verifying experiments at the University of New South Wales and the Australian desert.

Irradiance calculation on shaded surfaces

Operating results of recently built photovoltaic systems have shown that shading can cause high losses in performance. We need methods for calculating the reduced irradiance to minimize the shading losses in the planning process. There are well-known models to estimate the irradiance received by a tilted surface. However, these methods normally do not include shading reduction caused by objects in the surroundings. Therefore, methods are proposed to include shading in calculations of direct and diffuse irradiation. Before these methods can be used, the surroundings have to be surveyed. Hence, how to survey objects in an easy way is discussed. Using the proposed methods and calculations it is possible to include shading reduction in the prediction of the irradiance on a tilted surface. Finally, the influence of shading on the irradiance is shown by an example.

Influence of shading on electrical parameters of solar cells

Shading of solar cells not only reduces the cell power P/sub MPP/, but also changes the open-circuit voltage V/sub OC/, the short circuit-current I/sub SC/, the fill factor FF, and the efficiency /spl eta/. The authors introduce a shading factor S describing the amount of shading. From the well-known solar cell equations, the authors have calculated approximate equations for the solar cell parameters as a function of the shading factor S. They then compare these equations with simulated values. The approximate equations can also be used for the description of equally shaded solar modules. Furthermore, they propose a simple algorithm for the calculation of the performance of a partially shaded standard PV-generator.

Power conditioning of a 132 MW wind farm

A possible reason for a low power quality at the output of a 132 MW wind farm is investigated. A resonant circuit formed by the wind farm components is found to be responsible for a high level of the dominating 7th current harmonic. Various compensation strategies are proposed and analyzed.

Electromagnetic machines with Nd-Fe-B magnets

Abstract Permanent magnet motors are now becoming more accepted for general use in industrial fixed and variable speed drives. With the application of high-energy permanent magnets, such as Nd-Fe-B, the new motors offer higher efficiency and reduced size and weight compared with wound field energy converters of the same rating.

Investigation of the current harmonic parameters of wind energy converters

Current harmonic parameters of six grid connected wind energy converters (WECs) with a power of 1.5 MW and 1.8 MW were investigated. The evaluation was done according to the basic guidelines for harmonic limits EN 61000-3-2 and IEC 1000-3-3. Focus was on the definition and explanation of a typical shape of the standard parameters total harmonic distortion (THD), partial weighted harmonic distortion (PWHD) and the self-defined parameter total harmonic distortion with consideration of interharmonics (THDz) over the relative power of the WECs. Caused by the working principle of the power electronic voltage-source-inverters (VSI) used, the harmonic parameters show a clearly definable pattern, depending on the relative power of the WECs. The context between the power electronic converter operation and the format of the power quality curves over the relative power is given.

Comparison of high-speed induction motors employing cobalt-iron and silicon electrical steel

This paper presents the design of 20 kW cage rotor induction machines operating at speeds above 20.000 rpm employing different kind of electrical steels. One machine is designed using cobalt-iron laminations of 0.2 mm thickness. For the second machine 0.35 mm silicon electrical steel of low-loss grade has been used. Both machines are compared in terms of the materials influence on lamination layout, mechanical behavior and thermal loading. Measurements of both machines underline the presented facts.

Design, construction and test of a permanent-magnet prototype machine for wind energy applications

A low-speed radial-flux permanent-magnet (PM) electrical generator with new topology was designed, optimized, built and tested. The equivalent magnetic circuit approach together with the permanent magnet load line method were used as pre-design tools. The finite element technique (FET) was used for detailed characteristics and final adjustments. The design was carried out with high-energy NdFeB magnets with flux concentration arrangement. The topology allows for toroidal (torus) windings placed in flat slots with short ends, which contributes to higher efficiency and higher power to weight ratio. The flux concentration of the permanent magnets is tangential on the rotor support structure, which can be made from light nonmagnetic material. The cogging torque of the machine is estimated using the flux-MMF technique together with the help of FET. Good agreement between theoretical and experimental results has been achieved. Compared with other machine types of close power ratings, the developed prototype machine exhibits relatively high efficiency.