Christian Reise

Multiresolution analysis of satellite-derived irradiance maps—An evaluation of a new tool for the spatial characterization of hourly irradiance fields

Satellite derived irradiance maps with a high temporal and spatial resolution are a valuable data source for the analysis of large scale grid intergrated solar energy systems. For an appropriate use of this huge data base convenient methods for the characterization of the spatial and temporal structures of the satellite derived irradiance field are necessary. In this paper we introduce the application of a multiresolution decomposition for the structure analysis of irradiance maps. Using this tool the maps are decomposed into local averages (context) and gradients (detail) on various spatial scales. The information on the context and the detail may be directly interpreted in terms that are of interest, e.g. for the anlaysis of PV-systems implemented on a very large scale.

Spectrally selective sensors for PV system performance monitoring

The main purpose of PV system performance monitoring is to determine whether the system is operating as expected. This requires measuring the actual output of the system as well as the conditions under which it is operating. Solar radiation intensity in the plane of the array (POA) is by far the most important operating condition and is the basis for calculating performance ratios (PR). However, differences in spectral and directional response between pyranometers and PV modules lead to intraday as well as seasonal fluctuations in the performance ratio, even though the system may be operating without faults or degradation. For c-Si PV systems, a reference cell can be used instead of (or in addition to) a pyranometer to measure POA irradiance. Since reference cells have spectral and directional responses that are similar to PV modules, their output signals correlate better with system output. As a result, it is becomes easier to identify abnormal system operation. In this paper, we present a practical alternative to using a spectrally matched reference cell for measuring POA irradiance. The method uses dual sensors with different spectral responses whose outputs are combined to produce a composite signal. That composite signal can effectively match the spectral response of any PV module type over a wide range of irradiance conditions. As a result, this method makes possible the rapid and accurate detection of abnormal system operation for thin-film systems.

Investment risks of utility-scale PV: Opportunities and limitations of risk mitigation strategies to reduce uncertainties of energy yield predictions

Investment risks of utility-scale PV systems may arise from a wide range of sources: political stability in a region, interest rate levels and currency exchange rates or future energy price. However, the presence of stable political and economic conditions and feed-in tariffs or power purchase agreements may limit interest and price risks to acceptable levels. The technical risk of deviations between expected and actual life-time energy yield of a PV power plant is mostly influenced by the quality of energy yield predictions in case that system components correspond to their datasheet and guaranteed values and the maintenance concept is applied as expected. Recent publications estimate the standard uncertainty of life-time energy yield predictions to about 8%, which directly contributes to overall investment risk. In this paper we analyze two different strategies to reduce the influence of uncertainties of energy yield predictions on investment risks. The first strategy is diversification of risk, i.e. investing in a portfolio of systems. The second strategy is related to adjusted investment periods. It is concluded, that both strategies as well as the combination of these strategies are able to significantly reduce uncertainties. The resulting uncertainty of the lifetime energy yield for the combination of both approaches is estimated to about 3%.