Changes in the serial resistance Of a- Si, a-/μc-Si and mono-crystalline PV modules during the year
The changes in the serial resistance Rs of amorphous (a- Si) and amorphous/microcrystalline (a-/μc-Si) tandem silicon-based solar cells is a criterion to analyze the process of light -induced degradation and thermal healing of defects described by the Staebler - Wronski effect. Measurements under laboratory conditions show that the serial resistance increases during the light-induced degradation. Our annual measurement of a- Si, a-/μc-Si and mono-crystalline (c-Si) solar cells under field test conditions will verify this and it will improve the possibility of predicting the behavior of thin-film silicon photovoltaic cells. For a year, the complete current-voltage curve was measured every minute, recorded and analyzed. This data has been transferred into the equivalent circuit diagram for solar cells.
Defect relaxation in amorphous silicon: Stretched exponentials, the Meyer-Neldel rule, and the Staebler-Wronski effect.
Annealing and production of metastable defects in disordered solids is explained quantitatively with a model in which defect relaxation is a local phenomenon. The stretched-exponential time dependence of defect relaxation and the Meyer-Neldel rule for the relaxation-time constant are natural consequences of this model. The results are obtained by using an exponential distribution of activation barriers for transitions between the two states of the local defect. The model, applied to data in hydrogenated amorphous silicon, a-Si:H, gives an exponential distribution of barriers with a characteristic temperature of 220 \ifmmode^\circ\else\textdegree\fi{}C, roughly equal to the accepted freeze-in temperature for defect distributions in a-Si:H. The model explains that long degradation times convert defects with higher barriers and this results in longer annealing times. The microscopic models of the metastable defects in a-Si:H, weak-bond breaking and carrier trapping by charged dangling bonds, are discussed in the framework of this defect-relaxation model.
Evaluation of CuAl2O4 as an Oxygen Carrier in Chemical-Looping Combustion
The chemical-looping combustion (CLC) process is a novel solution for efficient combustion with intrinsic separation of carbon dioxide. The process uses a metal oxide as an oxygen carrier to transfer oxygen from an air to a fuel reactor where the fuel, or gasification products of the fuel, reacts with the solid oxygen carrier. In this work, copper(II) aluminate (CuAl2O4) was assessed as a potential oxygen carrier using methane as fuel. The carrier particles were produced by freeze–granulation and calcined at 1050 °C for a duration of 6 h. The chemical-looping characteristics were evaluated in a laboratory-scale fluidized-bed reactor in the temperature range of 900–950 °C during 45 alternating redox cycles. The oxygen carrier exhibited reproducible and stable reactivity behavior in this temperature range. Neither agglomeration nor defluidization was noticed in any of the cycles carried out at 900–925 °C. However, after reactivity tests at 950 °C, soft agglomeration and particle fragmentation were observed. Systematic phase analysis of the Cu–Al–O system during the redox cycle was carried out as a function of duration of reduction and oxygen concentration during the oxidation period. It was found that the CuAl2O4 is reduced to copper(I) aluminate (CuAlO2; delafossite), Cu2O, and elemental Cu. The CuAlO2 phase is characterized by slow kinetics for oxidation into CuO and CuAl2O4. Despite this kinetic limitation, complete conversion of methane with reproducible reactivity of the oxygen carrier is achieved. Thus, CuAl2O4 could be a potential oxygen carrier for chemical-looping combustion.