Energy Yield and Electricity Management of Thin-Film and Crystalline Silicon Solar Cells:from Devices to Systems

In the case of high photovoltaic (PV) penetration into the electricity grid, the energy produced by a PV system that is effectively used (useful energy) depends on the energy yield and on how this energy is managed to avoid detrimental effects occurring at high PV injection, e.g. during the midday peak. The overall goal of this thesis is to provide guidelines for maximizing the useful energy of a PV system by quantifying losses incurred during operation at both the solar cell device and the system levels. Solar cells are usually optimized for the standard test conditions (STC). However, the conditions are generally different during operation. This work assesses how solar cell materials and designs can be optimized to maximize the energy yield for specific operating condition. We mainly focus on thin-film silicon solar cells because of their challenging metastable behavior. The temperature dependence of the performance of thin-film amorphous silicon (a-Si:H) and microcrystalline silicon solar cells is thus measured for different deposition parameters and cell designs. We observe that, by tuning the intrinsic layer thickness of a-Si:H cells, the cells with the best (STC) efficiency do not necessarily provide the highest energy output. We also explain the presence of a maximum in the value of the fill factor as a function of temperature. The temperature dependence study is then extended to thin-film silicon multi-junction, crystalline silicon heterojunction (SHJ) and other crystalline silicon solar cells. For thin-film silicon solar cells, spectral effects and degradation or recovery effects due to the metastable character of a-Si:H (due to the Staebler-Wronski effect) significantly impact the energy yield. Based on indoor and outdoor degradation/recovery experiments, we show that it is challenging to describe this metastability with a diode model. However, such a model with a current loss term and an additional temperature dependence for the saturation current and ideality factors accurately reproduces the current-voltage characteristics of a-Si:H solar cells over a wide range of irradiance levels and operating temperatures. On the system level, we model a PV system with local storage to evaluate several strategies to reduce the detrimental midday injection peaks. The impact of such measures on the useful energy is also investigated. We develop a simple control algorithm that minimizes the losses due to a feed-in limit and maximizes self-consumption without the need of a production forecast. We show that heat storage using a boiler or a heat pump performs as well as battery storage. In general, a feed-in limit reduces significantly peak injection but only a relatively small storage capacity is needed to reduce losses (due to this limit). Changes in tilt and orientation of modules also reduce losses resulting from feed-in limits and shrink the winter/summer production ratio by more than a factor of two. We also develop a statistical method that estimates - from loads measured every 15 min - when different electrical appliances in a household are commonly used. This model indicates that about 8% of the total load could be shifted easily to the midday period, thereby reducing the midday injection peak. Finally, we combine device and system aspects to show that varying cell technology (e.g. with different temperature response) has a limited but not negligible impact on system output

Control algorithm for a residential photovoltaic system with storage

High penetration of photovoltaic (PV) electricity could affect the stability of the low-voltage grid due to over-voltage and transformer overloading at times of peak production. Residential battery storage can smooth out those peaks and hence contribute to grid stability. A feed-in limit allows for the easy setting of a maximum power injection cap and motivates PV owners to increase their self-consumption. A simple control strategy for a residential battery system coupled with a PV system that maximizes selfconsumption and minimizes curtailment losses due to a feed-in limit is presented. The algorithm used in this strategy does not require a forecast of insulation conditions. The performance of this algorithm is compared to a second algorithm—a control strategy based on linear optimization using a forecast. Assuming an exact forecast, this second algorithm is very close to the maximum self-consumption and minimum curtailment losses achievable and can be used to benchmark the simple strategy. The results show that the simple strategy performs as well as the second algorithm with exact forecasts and performs significantly better than the second algorithm using real forecasts. Moreover, it is shown that this result is valid for a large range of storage capacities and PV sizes. Furthermore, it is shown that with a time resolution of 15 min for the input data (the resolution of most PV production and load data) selfconsumption is overestimated by about 3% and curtailment losses are underestimated by the same amount. Load sensitivity simulations show that different load curve shapes do not fundamentally change the results. Finally, to assess the effect of load aggregation, the case where the strategy is applied separately to 44 households with storage is compared to the case where it is applied to a centralized storage system of the same size as the total storage of the 44 households. The reduction of the curtailment losses with the number of aggregated houses is showed

Effect of the Fluctuations of PV Production and Electricity Demand on the PV Electricity Self-Consumption

The electricity self-consumption level of a family household is determined both experimentally and by modelling. The effect of rapid fluctuation in PV production and electricity demand, and especially the effect of the temporal resolution of their measurements, are studied. For accurate determination of the self-consumption level, temporal resolution of at least 30 s for the load profile and at least 10 min for the PV production measurements are suggested. The effect of temporal resolution becomes negligible when a local electricity storage system is added. Keywords: Small Grid-connected PV Systems, storage, gird integratio

Peak Shaving Capability of Household Grid-Connected PV-System with Local Storage: A Case Study

Effect of grid feed-in curtailment of a PV system with heat or electricity storage (battery) is simulated as a function of system dimensioning with a focus on the induced PV power losses (due to this limit) and on cost balance. Heat storage is provided by a domestic hot water tank heated with heat pump or electrical water heating system. The case studied is based on a Swiss household with an annual electricity energy consumption of about 5000 kWh (without thermal loads). The effect of electricity generation forecast imprecision on cost balance optimization is also evaluated. The simulations showed that only relatively small electrical storage capacities or controllable thermal loads are sufficient to reduce considerably the PV-losses. Forecast inaccuracies have a non-negligible detrimental impact on cost balance with the designed algorithm

High Spatial Resolution of Thin-Film-on-ASIC Particle Detectors

Thin-film-on-ASIC (TFA) detectors are monolithic pixel devices that consist of amorphous silicon detecting diodes directly deposited on readout electronics. This paper presents a characterization of the TFA spatial resolution using the electron-beam-induced current (EBIC) technique, in which pixel pads patterned in microstrips were swept by the beam. We measured the spatial resolution for different configurations and thicknesses of the TFA active layer with different beam energies, currents and sweep speeds. We observed that the generated electron-hole pairs are collected most efficiently when the beam is over the microstrips. This better collection efficiency gives a larger signal than off the strips, and thereby enabled us to distinguish strips as small as 0.6 wide which are spaced by 1.4 gaps. This high spatial resolution was obtained even though microvoids in the amorphous silicon layer—induced by an ASIC morphology as rough as 2 —were observed in the detector cross section, thus demonstrating the potential of the TFA architecture even with non-planar readout electronics

Charge collection in amorphous silicon solar cells: Cell analysis and simulation of high-efficiency pin devices

The drift length Ldrift=μτE within the i layer of a-Si:H solar cells is a crucial parameter for charge collection and efficiency. It is strongly reduced not only by light-induced reduction of μτ, but also by electric field deformation ΔE by charges near the p–i and i–n interfaces. Here, a simple model is presented to estimate contributions of free carriers, charges trapped in band tails and charged dangling bonds to ΔE. It is shown that the model reproduces correctly trends observed experimentally and by ASA simulations: charged dangling bonds contribute most to ΔE of meta-stable cells. Electrons trapped in the conduction band tail near the i–n interface lead to the strongest field deformation in the initial state, while positively charged dangling bonds near the p–i interface get more important with degradation under AM1.5g spectrum. The measurable parameter Vcoll is proposed as an indirect parameter to estimate the electric field, and an experimental technique is presented that could enable the distinction of defects near the p–i and the i–n interfaces

Light-induced Voc increase and decrease in high-efficiency amorphous silicon solar cells

High-efficiency amorphous silicon (a-Si:H) solar cells were deposited with different thicknesses of the p-type amorphous silicon carbide layer on substrates of varying roughness. We observed a light-induced open-circuit voltage (Voc) increase upon light soaking for thin p-layers, but a decrease for thick p-layers. Further, the Voc increase is enhanced with increasing substrate roughness. After correction of the p-layer thickness for the increased surface area of rough substrates, we can exclude varying the effective p-layer thickness as the cause of the substrate roughness dependence. Instead, we explain the observations by an increase of the dangling-bond density in both the p-layer—causing a Voc increase—and in the intrinsic absorber layer, causing a Voc decrease. We present a mechanism for the light-induced increase and decrease, justified by the investigation of light-induced changes of the p-layer and supported by Advanced Semiconductor Analysis simulation. We conclude that a shift of the electron quasi-Fermi level towards the conduction band is the reason for the observed Voc enhancements, and poor amorphous silicon quality on rough substrates enhances this effect

Demand Side Management for Enhanced Integration of Photovoltaics into Grid

Demand side management could be a very effective way to shift electric consumption of households to the maximum production hours of photovoltaic (PV) systems. A field study on more than 100 households were performed to evaluate the share of electric consumption that could be easily shifted. Assuming a fixed time window for the PV production (namely 11:00 to 15:00) the share of consumption already in this window was found to be around 20 % while the flexible consumption outside of that window was ca. 5% (not including hot water heating). Information to households was found insufficient to push households to change behaviour and shift consumption demonstrates that information is not sufficient. However, financial incentive households were able to shift ca. 3% of their consumption, Hot water heating at mid-day (rather than during the night) is shown to be a very effective demand side management measure