Stefan Krauter

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.

Considerations for the calculation of greenhouse gas reduction by photovoltaic solar energy

A CO2 comprehensive balance within the life-cycle of a photovoltaic energy system requires careful examination of the CO2 sinks and sources at the locations and under the conditions of production of each component, during transport, installation and operation, as well as at the site of recycling. Calculations of the possible effect on CO2 reduction by PV energy systems may be incorrect if system borders are not set wide enough and remain on a national level, as can be found in the literature. For the examples of Brazil and Germany, the effective CO2 reductions have been derived, also considering possible interchange scenarios for production and operation of the PV systems considering the carbon dioxide intensity of the local electricity grids. In the case of Brazil also off-grid applications and the substitution of diesel generating sets by photovoltaics are examined: CO2 reduction may reach 26,805 kg/kWp in that case. Doing these calculations, the compositions of the local grids and their CO2 intensity at the time of PV grid injection have to be taken into account. Also possible changes of the generation fuel mix in the future have to be considered: During the operation time of a PV system, different kinds of power plants could be installed that might change the CO2 intensity of the grid. In the future also advanced technologies such as thin films have to be considered.

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.

Modelling of the Encapsulation Factors for Photovoltaic Modules

The actual silicon crisis in photovoltaic industry forces the whole value chain to realize more installed Wp photovoltaic power out of every kg silicon. This is approached through thinner wafers by the wafer producers, higher cell efficiencies by the cell manufacturers, and efficiency enhancements via improved encapsulation schemes at module production. Recently, some alternative encapsulation materials came in consideration, e.g. EVA replacements and cover glass with anti-reflective coatings. Cell technologies are changing as well. Some innovations are already adopted in standard products, but need to be tuned to each other. While the number of parameters involved has increased, it became cumbersome to minimize the optical losses experimentally. Modelling and variation of those parameters is performed to understand their interdependence and to propose optimal parameters sets, which can be used as good starting parameters for experimental probing

Integrated solar home system

To date, many traditional solar home systems (SHS) have consisted of separate components which required assembly by trained individuals and were also more susceptible to failure and maintenance. As a result, many SHSs in remote areas have not fulfilled their expected lifecycles or simply have not functioned at all. Thankfully, a solution to these problems has arrived - the newly developed integrated solar home system (I-SHS). Within this new system all components such as the support structure, foundation, PV modules, charge controller, DC-AC converter and wiring are pre-assembled by the manufacturer. This eases installation and maintenance resulting in a reduction of cost and failure. Additionally, electrical yield was increased by 9% by a significant reduction of operating cell temperature. This was achieved by an integrated water tank, serving as a cooling unit and also providing the systems foundation. This measure is neither expensive nor energy intensive, improves output of the system in an unproblematic way, and also allows use of the heated water.

Comparison of module temperature measurement methods

Aim of this study is to improve the prediction of electrical energy yield especially for PV modules based on thin film technologies. Problems with deficient power prediction of PV power plants have manifold reasons: One of the easy accessible is the measurement of NOCT (nominal operation cell temperature). Deviations of NOCTs measured have a direct influence of the yield predicted (1–5%). The deviations due to different measurement methods and different temperature sensors have been discussed in theory (systematic error) and have been investigated in laboratory tests as well as under real world conditions in an outdoor lab. The thermal behavior is studied and the measurement results are compared to theoretical models. As a result the theoretical yield as a function of the measured NOCT is compared for different installation sites and for different technologies.

New optical and thermal enhanced PV-modules performing 12% better under true module rating conditions

Improvements paying regard to actual operating conditions of PV-modules have been investigated. A result is the TOEPVIS-device (thermal and optical enhanced PV-module with integrated standing). It shows significantly lowered cell temperatures and higher efficiencies during operation by the use of an attached heat capacity built as a water tank which serves as the module foundation. Reduced reflection losses especially for nonperpendicular incidence have been achieved by an anti-reflective-coating, better matched refractive indices of the encapsulation layers and a selective V-structure on the front-surface. An additional feature-paying regard to BOS-is the substitution of the module framing, the support structure and the concrete foundation by the water tank, which allows implementation of all enhancements without extra costs. As a positive side-effect TOEPVIS devices have a very similar performance of actual measured results compared to results at STC (PR: 90%). Tests have been carried out in Berlin, Rio de Janeiro and Bulawayo (Zimbabwe) and showed gains in electricity yield of up to 11.7% for sub-optimal devices.

Energy-economic comparison of photovoltaic modules equipped with a layer of conventional and improved phase-change material

Cooling of photovoltaic devices leads to an increase in voltage, power output, and yield. However, an appropriate cooling measure is only beneficial if the additional costs are lower than the cumulated profit. For that reason an energy-economic analysis of a passive cooling measure based on phase-change materials was conducted. Measurement data from two photovoltaic modules each equipped with a phase-change material were studied. Related investigation considers the variation at the electricity spot market during the time of day, while additional PV power is being supplied in the forenoon - due to cooling. This becomes particularly interesting in electrical grids that already contain a considerable amount of (uncooled) PV (Germany as an example).

THE INFLUENCE OF THE CAPSULATION ON THE EFFICIENCY OF PV-MODULES

Performance of PV-modules is often in contradiction to a forecast made on the basis of bare cells. An overview on the factors influencing the efficiency, like optical and thermal characteristics, show that insufficient heat-transfer result in a low efficiency caused by high cell temperatures and increasing reflection losses due to a temperature dependency of material properties. In order to enhance performance some efficient cooling strategies should be taken into account.

Inaccuracies of input data relevant for PV yield prediction

Accuracy of the PV yield prediction process, including meteorological data (direct and diffuse irradiance with its actual spectral composition and spatial distribution), material properties of encapsulation (refractive indices, absorption coefficients, thermal properties), parameters relevant for heat transfer, PV conversion parameters of the cell (temperature coefficients, spectral response, weak light performance, degradation) considerably depends on the quality of the input data applied (derived from literature, data sheets, norms, software tools, or own measurements). The contribution gives an overview of the processes involved, the relevant parameters, the accuracy achievable and the impact on yield prediction.