Rubén Núñez

Characterization of CPV arrays based on differences on their thermal resistances

Thermal characterization of Concentrating Photovoltaics (CPV) modules and arrays is needed to determine their performance and modelling of energy forecast. Module-ambient thermal resistance is easily obtained from its definition but the cell-module thermal resistant needs to be estimated from indirect procedures, two of them are presented in this paper. In addition, an equivalent parameter is defined, the Concentrator Nominal Operating Module/Cell Temperature (CNOMT/CNOCT), the temperature at Concentrator Standard Operating Conditions (CSOC). Definitions and expression to relate (CNOMT/CNOCT) to thermal resistances are presented, plus several examples of estimations from real operating arrays.

Spectral classification of worldwide locations using SMR indexes

Spectral variation of solar irradiance can be analyzed by the means of component cells, being useful for multi junction (MJ) and concentrator photovoltaics (CPV) characterization by the use of component cells derived spectral indexes, namely, spectral matching ratios (SMRs). When enough spectral data is available, a location can be also characterized and a MJ solar cell can be locally tuned. The AERONET network of ground-based sun photometers is employed to obtain the atmosphere parameters for 34 sites worldwide. To conclude, the SMR indexes are obtained for every location, and a comparative analysis is completed for four architectures of MJ solar cells technologies (two lattice match and two metamorphic). Based on this information, some cluster of locations that share similar spectral features arise.

Hybrid lighting-CPV, a new efficient concept mixing illumination with CPV

Hybrid Lighting-CPV concept mixes illumination using low-cost fiber optics for light transmission with conventional Concentrating Photovoltaic (CPV) technology. This approach may offer an important increase in the value of a CPV system. Direct illumination from sunlight with fibers does not require the double light-electricity-light energy conversion allowing for efficient lighting using just a small area of the CPV system, preferably those areas with lower CPV efficiency conversion. Simultaneously the need of concentration and tracking required is completely covered by the infrastructure of the CPV system. In this work, we present a theoretical study of the losses in optical fibers in order to establish the design criteria for the hybrid system. Losses caused by chromatic aberration increase with f-number while transmission losses in the fiber decrease.

Spectral Impact on Multijunction Solar Cells Obtained by Means of Component Cells of a Different Technology

Spectral indices can be defined to characterize how the solar spectrum affects photovoltaic devices. The use of such indices is particularly important in technologies which are highly dependent on spectral variations, such as tandem, or multijunction (MJ), solar cells. In this paper, we analyze the capability of three effective irradiance values, given by the photocurrents of the components of triple-junction solar cells, and covering different spectral bands, to define the spectrum condition. Even though these parameters will be based on a particular type of MJ solar cell, they provide enough information to accurately assess the spectral impact on other types of MJ solar cell technologies with significantly different spectral responses. Consequently, specific component cells for each MJ solar cell technology may not be necessarily needed and a reference technology could be used for all of them, providing a way for the future standardization of these devices as a mean of defining the solar spectral condition.

Spectral network based on component cells under the SOPHIA European project

In the frame of the European project SOPHIA, a spectral network based on component (also called isotypes) cells has been created. Among the members of this project, several spectral sensors based on component cells and collimating tubes, so-called spectroheliometers, were installed in the last years, allowing the collection of minute-resolution spectral data useful for CPV systems characterization across Europe. The use of spectroheliometers has been proved useful to establish the necessary spectral conditions to perform power rating of CPV modules and systems. If enough data in a given period of time is collected, ideally a year, it is possible to characterize spectrally the place where measurements are taken, in the same way that hours of annual irradiation can be estimated using a pyrheliometer.

SOPHIA CPV MODULE ROUND ROBIN: POWER RATING AT CSOC

In the frame of the European project SOPHIA a concentrator photovoltaic (CPV) module measurement round robin has been initiated. The round robin includes measurements of four CPV modules at seven different test laboratories located in Europe. IV curves of the modules are measured with different measurement equipment under various climatic conditions. The aim of this activity is to perform at each site a rating of the modules at concentrator standard operating conditions CSOC according to IEC 62670-1. The outcome of the round robin is intended for direct feedback to the current draft standard IEC 62670-3 “Concentrator Photovoltaic (CPV) Performance Testing - Performance Measurements and Power Rating”. The paper discusses initial results from the first three partners that have already finished the measurements up to now.

Atmospheric parameters, spectral indexes and their relation to CPV spectral performance

Air Mass and atmosphere components (basically aerosol (AOD) and precipitable water (PW)) define the absorption of the sunlight that arrive to Earth. Radiative models such as SMARTS or MODTRAN use these parameters to generate an equivalent spectrum. However, complex and expensive instruments (as AERONET network devices) are needed to obtain AOD and PW. On the other hand, the use of isotype cells is a convenient way to characterize spectrally a place for CPV considering that they provide the photocurrent of the different internal subcells individually. Crossing data from AERONET station and a Tri-band Spectroheliometer, a model that correlates Spectral Mismatch Ratios and atmospheric parameters is proposed. Considering the amount of stations of AERONET network, this model may be used to estimate the spectral influence on energy performance of CPV systems close to all the stations worldwide.

Hybrid lighting-CPV, a new efficient concept combining illumination with CPV

A combination of CPV module with lighting guides is presented as Hybrid Lighting-CPV. A significant improvement in total efficiency can be achieved by this new approach in terms of electricity and luminous flux obtained, without modifying or increasing considerably the cost of a classical CPV. In order to characterize the lighting subpart some experiments are carried out. It is determined the optical fiber transmission as function of optical fiber length, diameter and material properties and f-number of the lens. An increase on f-number reduces the reflections inside the fiber and so the transmission increases. Additionally are checked the characteristics of fibers as light lamps what it confirms the need of a diffuser. Measurements shows that the light provided by optical fibers is at least as good as typical fluorescent lamps in terms of color correlated temperature and color rendering index.

Impact of the temperature dependence of CPV optics transmittance on the current mismatch of multi-junction solar cells

The influence of spectra and temperature dependence of concentrator photovoltaics (CPV) optics is investigated by modelling yearly series of ambient conditions, such as typical meteorological year (TMY) data, and simulating the response of relevant CPV technologies. Results show the influence the optics have on the effective spectra available at the solar cell level. This allows us to find the limiting factors of a CPV module performance and permits energy yield predictions.

From component to multi-junction solar cells for spectral monitoring

Concentrator photovoltaic usually embeds multi-junction solar cells, which exhibit high spectral sensitivity due to the internal series connection of the sub-cells. The use of so-called isotype or component cells with the same spectral response as the corresponding sub-cell, is widely applied for characterizing the spectral content of the impinging irradiance. These isotype sensors can be substituted by the multi-junction cells themselves, which are inherently spectrally tuned to any evolution of the multi-junction technology. To convert a multi-junction cell in a spectral sensor, it is necessary to add bias light within the spectral response of all but one of the sub-cells to saturate the corresponding junctions, so the non-saturated limits the current under any specified impinging spectrum. This paper shows indoor and outdoor side-by-side comparison of the so-called pseudo isotypes, based on a triple-junction solar cell, and genuine isotypes. The conditions to ensure an accurate spectral response, particularly for the bottom pseudo-isotype, are presented and discussed.Concentrator photovoltaic usually embeds multi-junction solar cells, which exhibit high spectral sensitivity due to the internal series connection of the sub-cells. The use of so-called isotype or component cells with the same spectral response as the corresponding sub-cell, is widely applied for characterizing the spectral content of the impinging irradiance. These isotype sensors can be substituted by the multi-junction cells themselves, which are inherently spectrally tuned to any evolution of the multi-junction technology. To convert a multi-junction cell in a spectral sensor, it is necessary to add bias light within the spectral response of all but one of the sub-cells to saturate the corresponding junctions, so the non-saturated limits the current under any specified impinging spectrum. This paper shows indoor and outdoor side-by-side comparison of the so-called pseudo isotypes, based on a triple-junction solar cell, and genuine isotypes. The conditions to ensure an accurate spectral response, parti...