Instrumentation for accelerated life tests of concentrator solar cells

Concentrator photovoltaic is an emergent technology that may be a good economical and efficient alternative for the generation of electricity at a competitive cost. However, the reliability of these new solar cells and systems is still an open issue due to the high-irradiation level they are subjected to as well as the electrical and thermal stresses that they are expected to endure. To evaluate the reliability in a short period of time, accelerated aging tests are essential. Thermal aging tests for concentrator photovoltaic solar cells and systems under illumination are not available because no technical solution to the problem of reaching the working concentration inside a climatic chamber has been available. This work presents an automatic instrumentation system that overcomes the aforementioned limitation. Working conditions have been simulated by forward biasing the solar cells to the current they would handle at the working concentration (in this case, 700 and 1050 times the irradiance at one standard sun). The instrumentation system has been deployed for more than 10 000 h in a thermal aging test for III-V concentrator solar cells, in which the generated power evolution at different temperatures has been monitored. As a result of this test, the acceleration factor has been calculated, thus allowing for the degradation evolution at any temperature in addition to normal working conditions to be obtained.

Real-time reliability test for a CPV module based on a power degradation model

Models based on degradation are powerful and useful tools to evaluate the reliability of those devices in which failure happens because of degradation in the performance parameters. This paper presents a procedure for assessing the reliability of concentrator photovoltaic (CPV) modules operating outdoors in real-time conditions. With this model, the main reliability functions are predicted. This model has been applied to a real case with a module composed of GaAs single-junction solar cells and total internal reflection (TIR) optics

Thermal behaviour of Sunrise, a balloon-borne solar Telescope

Sunrise is a solar telescope, successfully flown in June 2009 with a long duration balloon from the Swedish Space Corporation Esrange launch site. The design of the thermal control of SUNRISE was quite critical because of the sensitivity to temperature of the optomechanical devices and the electronics. These problems got more complicated due the size and high power dissipation of the system. A detailed thermal mathematical model of SUNRISE was set up to predict temperatures. In this communication the thermal behaviour of SUNRISE during flight is presented. Flight temperatures of some devices are presented and analysed. The measured data have been compared with the predictions given by the thermal mathematical models. The main discrepancies between flight data and the temperatures predicted by the models have been identified. This allows thermal engineers to improve the knowledge of the thermal behaviour of the system for future missions.

Grid-connected roof solar plants in educational institutions for testing and electricity generation: a case in Spain.

The use of photovoltaic experimental plants in engineering educational buildings contributes to an increase in acceptance of this technology by future engineers. There are some photovoltaic (PV) systems in educational buildings in Spain, but they are usually limited to buildings in relation to electrical technologies or research areas. They are not common in other educational or official buildings. This paper presents the project of a grid-connected solar plant with two main objectives. First, different PV module technologies will be compared. Second, an emphasis on agronomical areas in educational settings will be reviewed in an attempt to facilitate student engagement in the use of the power plant. The system is grid-connected in order to pay-back the investment in the plant. In fact the electricity generated by the plant will be used by the installations of the building, as it is the closest consumer. This work intends to approximate photovoltaic technology to university degrees not directly related with it and at the same time research in comparison of systems with different technologies. This is a good example of an solar plant for both optimum production and educational purposes.

One-watt fiber-based power-by-light system for satellite applications

In this work, a fiber-based optical powering (or power-by-light) system capable of providing more than 1 W is developed. The prototype was used in order to power a shunt regulator for controlling the activation and deactivation of solar panels in satellites. The work involves the manufacture of a light receiver (a GaAs multiple photovoltaic converter (MPC)), a power conditioning block, and a regulator and the implementation and characterization of the whole system. The MPC, with an active area of just 3.1 mm2, was able to supply 1 W at 5 V with an efficiency of 30%. The maximum measured device efficiency was over 40% at an input power (Pin) of 0.5 W. Open circuit voltage over 7 V was measured for Pin over 0.5 W. A system optoelectronic efficiency (including the optical fiber, connectors, and MPC) of 27% was measured at an output power (Pout) of 1 W. At Pout = 0.2 W, the efficiency was as high as 36%. The power conditioning block and the regulator were successfully powered with the system. The maximum supplied power in steady state was 0.2 W, whereas in transient state, it reached 0.44 W. The paper also describes the characterization of the system within the temperature range going from -70 to +100?°C.

Indoor characterization at production sccale: 200 kWp of CPV solar simulator measurements

In order to complement ISFOC’s characterization capabilities, a Helios 3198 CPV Solar Simulator was installed in summer 2010. This Solar Simulator, based on a parabolic mirror and a high-intensity, small area Xenon flash lamp was developed by the Instituto de Energía Solar in Madrid [1] and is manufactured and distributed by Soldaduras Avanzadas [2]. This simulator is used not only for R&D purposes, but as a quality control tool for incoming modules that are to be installed in ISFOC’s CPV plants. In this paper we will discuss the results of recent measurements of close to 5000 modules, the entire production of modules corresponding to a small CPV power plant (200 kWp). We scrutinize the resultant data for signs of drift in the measurements, and analyze the light quality before and after, to check for changes in spectrum or spatial uniformity.)

The unstoppable rise of photovoltaic solar energy: PV vs. PV

Photovoltaic (PV) solar energy has been growing during the last decade an explosive rate. Last year (2011) the solar cell production amounted to more than 37 GW. It is the energy technology most installed nowadays. The power generated by the 37 GW is similar to the one generated by about 7 nuclear units of 1 GW each. The solar industry is already a huge industry dominated by Asian countries led by China. It is not anymore a promise. It is just a reality.

Electrical Power Fluctuations in a Network of DC/AC inverters in a Large PV Plant: relationship between correlation, distance and time scale

This paper analyzes the correlation between the fluctuations of the electrical power generated by the ensemble of 70 DC/AC inverters from a 45.6 MW PV plant. The use of real electrical power time series from a large collection of photovoltaic inverters of a same plant is an impor- tant contribution in the context of models built upon simplified assumptions to overcome the absence of such data. This data set is divided into three different fluctuation categories with a clustering proce- dure which performs correctly with the clearness index and the wavelet variances. Afterwards, the time dependent correlation between the electrical power time series of the inverters is esti- mated with the wavelet transform. The wavelet correlation depends on the distance between the inverters, the wavelet time scales and the daily fluctuation level. Correlation values for time scales below one minute are low without dependence on the daily fluctuation level. For time scales above 20 minutes, positive high correlation values are obtained, and the decay rate with the distance depends on the daily fluctuation level. At intermediate time scales the correlation depends strongly on the daily fluctuation level. The proposed methods have been implemented using free software. Source code is available as supplementary material.

VentanaTM power train features and performance

Most CPV systems are based on Fresnel lenses. Among these, LPI-patented Fresnel-Köhler (FK) concentrator outstands owing to performance and practical reasons. The VentanaTM power train is the first off-the-shelf commercial product based on the FK and comprises both the primary (POE) lenses (a 36-units 1×1 m2 acrylic panel manufactured by EVONIK and 10×) and glass (or Savosil) secondary optics (SOE). This high concentration optical train (Cg=1,024×, ~250mm optical depth) fits with 5×5 mm2 (at least) solar cells. The optical train is the fruit of a 1-year development that has included design, modeling, prototyping and characterization, and through the process LPI had the opportunity to find out how well the actual performance correlates with models, but also learned practical aspects of a CPV system of this kind, some of which have very positive impact on system performance and reliability.

Hydrogenated amorphous silicon deposited by high pressure sputtering for HIT solar cells

Hydrogenated amorphous silicon thin films were deposited using a high pressure sputtering (HPS) system. In this work, we have studied the composition and optical properties of the films (band-gap, absorption coefficient), and their dependence with the deposition parameters. For films deposited at high pressure (1 mbar), composition measurements show a critical dependence of the purity of the films with the RF power. Films manufactured with RF-power above 80W exhibit good properties for future application, similar to the films deposited by CVD (Chemical Vapor Deposition) for hydrogenated amorphous silicon.

The intermediate band approach in the third solar cell generation context

Hydrogenated amorphous silicon thin films were deposited using a high pressure sputtering (HPS) system. In this work, we have studied the composition and optical properties of the films (band-gap, absorption coefficient), and their dependence with the deposition parameters. For films deposited at high pressure (1 mbar), composition measurements show a critical dependence of the purity of the films with the RF power. Films manufactured with RF-power above 80W exhibit good properties for future application, similar to the films deposited by CVD (Chemical Vapor Deposition) for hydrogenated amorphous silicon.

Development and experimental evaluation of a complete solar thermophotovoltaic system

We present a practical implementation of a solar thermophotovoltaic (TPV) system. The system presented in this paper comprises a sunlight concentrator system, a cylindrical cup-shaped absorber/emitter (made of tungsten coated with HfO2), and an hexagonal-shaped water-cooled TPV generator comprising 24 germanium TPV cells, which is surrounding the cylindrical absorber/emitter. This paper focuses on the development of shingled TPV cell arrays, the characterization of the sunlight concentrator system, the estimation of the temperature achieved by the cylindrical emitters operated under concentrated sunlight, and the evaluation of the full system performance under real outdoor irradiance conditions. From the system characterization, we have measured short-circuit current densities up to 0.95 A/cm2, electric power densities of 67 mW/cm2, and a global conversion efficiency of about 0.8%. To our knowledge, this is the first overall solar-to-electricity efficiency reported for a complete solar thermophotovoltaic system. The very low efficiency is mainly due to the overheating of the cells (up to 120 °C) and to the high optical concentrator losses, which prevent the achievement of the optimum emitter temperature. The loss analysis shows that by improving both aspects, efficiencies above 5% could be achievable in the very short term and efficiencies above 10% could be achieved with further improvements.

Global optimization of solar thermophotovoltaic systems

n this paper, we present a theoretical model based on the detailed balance theory of solar thermophotovoltaic systems comprising multijunction photovoltaic cells, a sunlight concentrator and spectrally selective surfaces. The full system has been defined by means of 2n + 8 variables (being n the number of sub-cells of the multijunction cell). These variables are as follows: the sunlight concentration factor, the absorber cut-off energy, the emitter-to-absorber area ratio, the emitter cut-off energy, the band-gap energy(ies) and voltage(s) of the sub-cells, the reflectivity of the cells' back-side reflector, the emitter-to-cell and cell-to-cell view factors and the emitter-to-cell area ratio. We have used this model for carrying out a multi-variable system optimization by means of a multidimensional direct-search algorithm. This analysis allows to find the set of system variables whose combined effects results in the maximum overall system efficiency. From this analysis, we have seen that multijunction cells are excellent candidates to enhance the system efficiency and the electrical power density. Particularly, multijunction cells report great benefits for systems with a notable presence of optical losses, which are unavoidable in practical systems. Also, we have seen that the use of spectrally selective absorbers, rather than black-body absorbers, allows to achieve higher system efficiencies for both lower concentration and lower emitter-to-absorber area ratio. Finally, we have seen that sun-to-electricity conversion efficiencies above 30% and electrical power densities above 50 W/cm2 are achievable for this kind of systems.

Optimization of Pumping System in Solar Pumped Laser.

The coupling between solar light radiation and laser rod medium in a solar pumped laser affects the efficiency of the laser. To optimize the pumping system, simulation of the two-stage pumping system with a Fresnel lens and conic pumping cavity is carried out with Tracepro software. According to the power density distribution along the axis at focal place of the Fresnel lens, the diameter and position of the pumping cavity window and the distance of the window from the Fresnel lens are optimized. The power density distributions along the laser rod axis of different cavity lengths and different cavity tapers are also analyzed. The optimal structure of taper cavity is obtained. The mirror relecting cavity and ceramic cavity are introduced in detail.

Evaluation of the reliability of high concentrator GaAs solar cells by means of temperature accelerated aging tests

ABSTRACT Evaluating the reliability, warranty period, and power degradation of high concentration solar cells is crucial to introducing this new technology to the market. The reliability of high concentration GaAs solar cells, as measured in temperature accelerated life tests, is described in this paper. GaAs cells were tested under high thermal accelerated conditions that emulated operation under 700 or 1050 suns over a period exceeding 10 000 h. Progressive power degradation was observed, although no catastrophic failures occurred. An Arrhenius activation energy of 1.02 eV was determined from these tests. The solar cell reliability [R(t)] under working conditions of 65°C was evaluated for different failure limits (1–10% power loss). From this reliability function, the mean time to failure and the warranty time were evaluated. Solar cell temperature appeared to be the primary determinant of reliability and warranty period, with concentration being the secondary determinant. A 30-year warranty for these 1 mm2-sized GaAs cells (manufactured according to a light emitting diode-like approach) may be offered for both cell concentrations (700 and 1050 suns) if the solar cell is operated at a working temperature of 65°C.