Compressed sensing current mapping spatial characterization of photovoltaic devices

In this work a new measurement technique for current mapping of photovoltaic (PV) devices is developed, utilising the compressed sensing (CS) sampling theory. Conventional current mapping measurements of PV devices are realised using the light beam induced current (LBIC) measurement method. For its realization, a light beam scans a PV device and the induced current is measured for every point, generating the final current map of the device. Disadvantages of the LBIC method are the low measurement speed, the complicated and usually expensive measurement layouts and the impractical application of the method on PV modules. With the development of CS current mapping in this work, the above issues can be mitigated. Instead of applying a raster scan, a series of illumination patterns are projected onto the PV sample, acquiring fewer measurements than the pixels of the final current map. The final reconstruction of the current map is achieved by means of an optimisation algorithm. Spatially resolved electrical simulations of CS current mapping demonstrate that theoretically the proposed method is feasible. In addition, it is shown that current maps can be acquired with even 40% of the measurements a standard LBIC system would require, saving a significant amount of measurement time. The performance of CS current mapping is the same, regardless of the features a sample may contain and measurements can be applied to any type of photovoltaic device. The ability of the method to provide current maps of PV modules is demonstrated. The performance of several reconstruction algorithms is also investigated. An optical measurement setup for CS current mapping of small area PV devices was built at the National Physical Laboratory (NPL), based on a digital micromirror device (DMD). Accurate current maps can be produced with only 40% of the measurements a conventional point by point scan would need, confirming simulation results. The measurement setup is compact, straightforward to realise and uses a small number of optical elements. It can measure a small area of 1cm by 1cm, making it ideal for current mapping of small research samples. A significant signal amplification is achieved, since the patterns illuminate half of the sample. This diminishes the use of lock-in techniques, reducing the cost for current mapping of PV devices. Current maps of an optical resolution up to 27μm are acquired, without the use of any demagnification elements of the projected pattern that the DMD generates. v A scale up of this new current mapping method is demonstrated using Digital Light Processing (DLP) technology, which is based on DMD chips. A commercial DLP projector is utilised for building a proof of concept CS current mapping measurement system at the Centre of Renewable Energy Systems Technology (CREST). Current maps of individual PV cells in encapsulated modules can be acquired, something that is extremely difficult to achieve with conventional LBIC systems. Direct current mapping of a PV module with by-pass diodes is successfully applied for the first time. Specific shading strategies are developed for this purpose in order to isolate the cell under test. Due to the application of compressive sampling, current maps are acquired even if the signal-to-noise-ratio levels are so low that a point by point scan is not possible. Through the above implementations of CS current mapping of this work, the proposed technique is studied and evaluated. The results demonstrate that this novel method can offer a realistic alternative approach for current mapping of PV cells and modules that can be cost effective and straightforward to implement. In addition, this work introduces the application of the CS theory and DLP technology to PV metrology in general

Accessing the performance of individual cells of fully encapsulated PV modules using a commercial digital light processing projector

Accessing the electrical parameters of individual cells in fully encapsulated photovoltaic (PV) modules can be a cumbersome and time-consuming procedure. It usually requires mechanical shading, which is achieved by using meshes. This limits the control and variability of shading, as there is always a limited variety of mesh patterns available. In this work digital projection technology is utilised as the light source to achieve this. Partial shading can be applied rapidly and performance parameters of individual cells in fully encapsulated modules can be acquired. This is demonstrated in this work using a custom mini module. Individual cells can be accessed even in the case that bypass diodes are included. Performance information of individual cells acquired with such a system can be used for studying upscaling losses or degradation mechanisms for commercial or research PV modules

Utilising digital light processing and compressed sensing for photo-current mapping of encapsulated photovoltaic modules

Photocurrent mapping can provide useful spatial information about the electrical and optical properties of a photovoltaic (PV) device under actual operating conditions. Although it is a well-established technique for PV cells, direct current mapping measurements of PV modules is impractical and time-consuming to be applied. One has to mechanically shade specific cells of the PV module or destructively access the cell to be measured. In this work, non-destructive, automated current mapping of encapsulated PV modules is demonstrated. A commercial Digital Light Processing (DLP) projector is utilised in order to apply compressive sampling for current mapping of PV modules. This method is non-destructive, cost effective and significantly fewer measurements are needed for acquiring a current map compared to raster scanning methods. When applying compressive sampling, a series of patterns is projected on the sample, the current response is measured for each pattern and the current map is acquired using an optimisation algorithm. Specific shading strategies, voltage bias settings and I-V curve details are investigated for optimised compressive sampling

A simple optical setup for current mapping of small area PV devices using different sampling strategies

An optical setup for current mapping of photovoltaic devices is presented. It is based on a digital micro-mirror device (DMD) and a small number of additional optical elements making the implementation simple and cost effective. The specific properties of the DMD chip enable the application of two different sampling methods; point by point sampling and compressive sampling. Both sampling strategies are compared and cases when each one of them performs better are investigated. It is shown that compressive sampling can significantly enhance weak current signals and provide current maps in the cases when the point by point current signal is below the noise threshold

Uncertainty contributions in photocurrent linearity measurements of PV devices using a flash solar simulator

Especially for reference devices, the linearity of photocurrent over irradiance is an important characteristic that requires a low measurement uncertainty. This work investigates the uncertainty contributions when using a typical flash solar simulator with attenuation masks to determine the linearity characteristics of a device. Due to the complexity in measurement correlations, a Monte-Carlo simulation model was developed to estimate the final uncertainty. Results show that attenuation masks are not necessarily spectrally neutral and, if left uncorrected, this can significantly impact the measurement results. Furthermore, uncertainty in linearity is also dependent on the linearity of the sample under test itself. A shunted, non-linear device can have double the linearity uncertainty in low light conditions than a similar, linear sample