• Energy Research

The exponential growth of global capacity along with a reduction in manufacturing costs in the last two decades has caused photovoltaic (PV) energy technology to reach a high maturity level. As a consequence, currently, researchers from all over the world are making great efforts to analyse how different types of degradation impact this technology. This study provides a detailed review of the impact of different optical degradation mechanisms, which mainly affect the transmittance of the top-sheet encapsulant, on the spectral response of the PV modules. The impact on the spectral performance of PV modules is evaluated by considering the variations of the short-circuit current since this is the most widely used parameter to study the spectral impact in outdoors. Some of the most common types of optical degradation affecting the performance of PV modules worldwide, such as discoloration, delamination, aging and soiling have been addressed. Due to the widely documented impact of soiling on the spectral response of modules, this mechanism has been specially highlighted in this study. On the other hand, most of the publications analysed in this review report optical degradation in PV modules with polymeric encapsulant materials. Furthermore, an innovative procedure to quantify the spectral impact of degradation on PV devices is presented. This has been used to analyse the impact of two particular cases of degradation due to soiling and discoloration on the spectral response of different PV technologies.

Due to its special features, one of the problems of high concentrator photovoltaic (HCPV) technology is the estimation of the electrical output of an HCPV module. Although there are several methods for doing this, only some of them can be applied using easily obtainable atmospheric parameters. In this paper, four models to estimate the maximum power of an HCPV module are studied and compared. The models that have been taken into account are the standard ASTM E2527, the linear coefficient model, the Sandia National Laboratories model, and an artificial neural network-based model. Results demonstrate that the four methods show adequate behavior in the estimation of the maximum power of several HCPV modules from different manufacturers.

Abstract The electrical output of a photovoltaic system is often related to the plane-of-array global irradiance and the cell operating temperature. However, the spectral distribution of the irradiance also influences the system electrical behaviour, through its coupling with the spectral response of the photovoltaic material employed. Many authors have evaluated the spectral performance of different types of semiconductors under real operating conditions worldwide by using different methods and spectral indexes. However, there are not comparative studies on the accuracy of these methods in the literature. In this paper, two simplified existing indexes and a novel introduced index are compared to the spectral factor method in order to assess their relative accuracy for the climate of Granada, Southern Spain, and considering the most widespread used photovoltaic materials. Results show the election of a suitable index depends on the type of material, availability of spectral response information of the material, computing capabilities and required accuracy. The maximum annual relative error with respect to the spectral factor considering every index and material is 1.77%, while the maximum monthly relative error can reach 4.22%.

Abstract The measurement of the cell temperature in HCPV modules, due to the special features of these types of modules, is a very complex task. Nowadays, some methods to calculate the cell temperature are available. These methods have turned out to be very exact, but are usually too complex or require the use of data which are difficult to get. The aim of this paper is to find a simple expression that allows easy calculation of the cell temperature of a HCPV module, without very high error, by using readily available parameters. To achieve this objective, several atmospheric parameters and the cell temperature of a HCPV module have been measured for a year in a research center located in the south of Spain. Results show that the proposed expression could be used for estimating the cell temperature of a HCPV module from atmospheric parameters with a root mean square error of 7.4%.

Abstract The prediction of the energy yield of HCPV (high concentrator photovoltaic) systems is crucial to evaluate the potential and promote the market expansion of HCPV technology. Currently, there is a lack of experience in the modelling of these kinds of systems due to the special features of such technology. In this work, a practical model based on simple mathematical expressions and atmospheric parameters is introduced. The proposed model takes into account the main important parameters which influence the output of a HCPV system such as cell temperature, spectrum and efficiency of the inverter and other losses of the BOS (balance of system). The results obtained are validated using the data of a HCPV installation located at the University of Jaen in southern Spain and monitored daily every minute since 2011. The model accurately predicts the monthly energy yield with a deviation ranging from 4.07% to −0.47% and the annual final energy yield with a deviation of 0.9%.

Bifacial technology is attracting the attention of the photovoltaic community. Although considered premature, research and development activities still need to be carried out to improve bPV performance. In addition, the need for a standard test reference will aid bankability and increase confidence in this technology. This article describes the state of the art of bifacial technology, going through the bPV cell and its difference compared to conventional monofacial cells and listing the different sources of limitations, with an identification of different parameters that characterize the performance of the bifacial. Then, the paper reviews the different modeling methods that allow predicting the performance of bPV systems, and ends with the most important applications, whether for dual use of land to produce energy and food (agrivoltaic) or for placing bPV modules on water bodies instead of on the ground (aquavoltaics), or for vertical use as solar fences, acoustic barriers, or building-integrated photovoltaic modules.

Abstract In this paper, the soiling impact on photovoltaic systems in Aguascalientes, in central Mexico, an area where 1.4GWp of new photovoltaic capacity is being installed, is characterised experimentally. A soiling rate of −0.16%/day in the dry season for optimally tilted crystalline silicon modules, and a stabilization of the soiling losses at 11.2% after 70 days of exposure were observed. With these data, a first of its kind novel method for determining optimum cleaning schedules is proposed based on minimising the levelised cost of energy. The method has the advantages compared to other existing methods of considering the system investment cost in the determination of the optimum cleaning schedule. Also, it does not depend on economic revenue data, which are often subject to uncertainty. The results show that residential and commercial systems should be cleaned once per year in Aguascalientes. On the other hand, cleaning intervals from 12 to 31 days in the dry season were estimated for utility-scale systems, due to the dramatic decrease of cleaning costs per unit photovoltaic capacity. We also present a comparative analysis of the existing criteria for optimising cleaning schedules applied to the same case study. The different methods give similar cleaning intervals for utility-scale systems and, thus, the choice of a suitable method depends on the availability of information.