Hussam Khonkar

Degradation in PV encapsulation transmittance: An interlaboratory study towards a climate-specific test

Reduced optical transmittance of encapsulants resulting from ultraviolet (UV) degradation has frequently been identified as a cause of decreased PV module performance through the life of service in the field. The present module safety and qualification standards, however, apply short UV doses only capable of examining design robustness or “infant mortality” failures. Essential information that might be used to screen encapsulation through product lifetime remains unknown. For example, the relative efficacy of xenon-arc and UVA-340 fluorescent sources or the typical range of activation energy for degradation is not quantified. We have conducted an interlaboratory experiment to provide the understanding that will be used towards developing a climate- and configuration-specific (UV) weathering test. Five representative, known formulations of EVA were studied in addition to one TPU material. Replicate laminated silica/polymer/silica specimens are being examined at 14 institutions using a variety of indoor chambers (including Xenon, UVA-340, and metal-halide light sources) or field aging. The solar-weighted transmittance, yellowness index, and the UV cut-off wavelength, determined from the measured hemispherical transmittance, are examined to provide understanding and guidance for the UV light source (lamp type) and temperature used in accelerated UV aging tests.

High-Concentration Photovoltaics—Effect of Inhomogeneous Spectral Irradiation

At high solar concentration, subtle optical and electrical effects in combination can have a substantial impact on photovoltaic power (PV) generation. We have identified such an effect through its clear signature: a “ripple” in the output current with respect to the pointing angle of the concentrated PV (CPV) system to sun direction. At small angular misalignment, this effect can lower cell current by as much as 15% at 1600x concentration in full sun. At medium concentration between 500 and 1000x, while not as clearly visible in single cells, the effect also reduces output by a smaller amount. The disappearance of the “ripple” signature at low concentrations below 300x indicates that the effect is not a linear effect, such as a light loss. We attribute the pronounced angular sensitivity of power output at high concentrations to a combination of inhomogeneous spectral irradiation incident on the multijunction solar cell and of the impact of the finite lateral resistance of the cell.

Exploring the limits of concentration for UHCPV

Practical multi receiver ultra high (1000+ Suns) concentration photovoltaic (UHCPV) systems experience large radiation, thermal and electrical loads in addition to large power density transients under routine operation. This report is a summary of the issues involved in determining the practical limits to concentration. How high is too high? Explorations into UHCPV have both theoretical and experimental aspects. Understanding the theoretical device physics and circuit limitations is often essential to determining which experiments to do and in interpreting results. On the experimental side the work can be divided into two fields depending on the type of light source. The first is artificial or simulated sources and the second is working in the field with direct solar irradiation. Both fields have advantages and disadvantages. Direct solar radiation was selected for the current experiments due to the low cost and ability to produce ultra high concentrations (4000+) over relatively large areas (25+ mm2). Several experimental examples from these direct solar measurements shed light on some of the basic theories of how concentrated light affects the performance of multi junction photovoltaic cells. Out of these examples and theoretical foundations we conclude that for practical devices the first order constraint to optimum efficiency at ultra high concentrations is the series resistance. We also present a simple model based on published data and our results that can be used to predict the total system series resistance needed to optimize a system for a particular concentration.

Multi receiver concentrator photovoltaic testing at extreme concentrations

Practical multi receiver concentrator photovoltaic systems operating at high solar concentration levels up to 2000 suns experience large radiation, thermal and electrical loads in addition to large power density transients under routine operation. These systems require efficient cooling to manage the associated incident power densities between 100 to 200 W/cm2. Photovoltaic cells and thermal interface materials experience considerable stress under these load conditions. Their assembly is sensitive to contamination and process optimization. Efficient optical coupling of light at high concentration requires precise component alignment and tracking. We will discuss high power testing of single and multi receiver, high concentration systems comprising commercial triple junction cells, Fresnel optics, electric actuators, and cooled through a metal thermal interface using active and passive cooling methods.

Concentrator photovoltaic reliability testing at extreme concentrations up to 2000 suns

Practical concentrator photovoltaic systems operating at high solar concentration levels up to 2000 suns experience large thermal and electrical loads in addition to large power density transients under routine operation. These systems require efficient cooling systems to manage the associated incident power densities up to 200 W/cm2. Photovoltaic cells and thermal interface materials experience considerable stress under these load conditions. Reliability data is sparse for operation above 500 suns. We present high power test results for a commercial triple junction cell cooled through a high performance metal thermal interface using active liquid cooling methods for power densities up to 200 W/cm2.

Optimizing defocus to increase efficiency in concentrator photovoltaic modules

We describe a process for increasing power efficiency of concentrator photovoltaic systems by optimizing the lens-to-cell spacing. We find that there is an optimum defocus position with improved power output and reduced sensitivity to pointing errors, which in combination can result in a more than 10% enhancement. The improvement can be realized by minor changes to module cases which should not require changes to other manufacturing, installation, or component costs. In fact optimizing the defocus position allows for lower costs per unit power due to increased power and relaxed system tolerances. The paper focuses on detailed data illuminating the behavior of ultra high concentration photovoltaic modules. While one can look forward to optimizing defocus through sufficiently detailed simulation, at present, we find that an empirical determination of optimum defocus is necessary. The data reveals that even without design parameters changing, supply chain changes can have a significant impact on the optimum defocus - data from five different module configurations with components from different manufacturing lots are presented. These different configurations serve to illustrate the consequences of component changes and the importance of verifying the optimum defocus. A discussion of the effects that are important to determining the optimum defocus and which underlie these differences is included.

Ultra-high CPV system development and deployment in Saudi Arabia

This paper discusses the development and deployment of an ultra-high concentrating PV module that utilizes concentration above 1400X on multijunction solar cells. The development process included the selection of cell assemblies, primary and secondary optics, and focal distance. The systems were deployed in Saudi Arabia inside the Solar Village near Riyadh and in Khafji near the border of Saudi and Kuwait, following the deployment of first prototype in Yorktown, NY. Data from operation in those areas are shown here, and next steps of optimizing the module performance are discussed.

Degradation in PV encapsulant strength of attachment: An interlaboratory study towards a climate-specific test

Reduced strength of attachment of the encapsulant resulting from the outdoor environment, including ultraviolet (UV) radiation, may decrease photovoltaic (PV) module lifetime by enabling widespread corrosion of internal components. To date, few studies exist showing how the adhesion of PV components varies with environmental stress. We have conducted an interlaboratory experiment to provide an understanding that will be used to develop climatic specific module tests. Factors examined in the study included the UV light source (lamp type), temperature, and humidity to be proposed for use in accelerated aging tests. A poly (ethylene-co-vinyl acetate) (EVA) formulation often used in veteran PV installations was studied using a compressive shear test - to quantify the strength of attachment at the EVA/glass interface. Replicate laminated glass/polymer/glass coupon specimens were weathered at 12 institutions using a variety of indoor chambers or field aging. Shear strength, shear strain, and toughness were measured using a mechanical load-frame for the compressive shear test, with subsequent optical imaging and electron microscopy of the separated surfaces.

The effects of modified production method and new parts integration on CPV modules

King Abdulaziz City for Science and Technology (KACST) in collaboration with IBM has developed the first manually manufactured concentrator photovoltaic (CPV) modules. A single CPV module is rated at 120 W at nominal operating test condition (NOTC) of 1000 W/m2 direct normal insolation (DNI) and 25°C ambient temperature. Two trackers are installed and operated in the solar village outside Riyadh and one located in the eastern coastal city of Alkhafji. Figure 1 shows the trackers in Solar Village and Alkhafji. Valuable lessons were learned after three years of operating the systems on sites with different environmental characteristics. A production process flaw that caused cell material weakening and falling of light pipes from the cell have caused huge output degradation. Other factors that improved module performance are modification of production methods, integrating and replacing new parts. The Fresnel lens was replaced with a higher transmission efficiency lens of the same make as the former. Focal le...

Two year performance of a 10 kW CPV system installed in two areas of Saudi Arabia

The three year KACST/IBM collaboration in solar technology research led to the design and development of a 10kW CPV system. The system is comprised of 81 PV modules, inverters and a tracking system and is grid connected. A primary and secondary optics were employed to reach 1600x concentration on multijunction solar cells. Two CPV trackers were installed in the city of Riyadh and one in the eastern coastal city of Al Khafji. These two areas differ in climatic conditions. Riyadh is mostly dry and very often hit by very strong sand storms while Al Khafji is very humid with sand storms. Very fine dusts and dirt carried by the storms hits the surface of the primary optics, Fresnel lens, of the system. In Riyadh, the particles stick to the lenses but accumulation in the surface is not much since it is blown away by wind. However, the humid condition of the coastal areas wets the dusts and makes it sticky, cumulating more dusts and dirt. This paper discusses in details the parts of the 10kW CPV system. It presents a comprehensive analysis of the systems performance since the time they were installed and operated. CPV systems are operated with the least number of personnel and supervision. However, dust and dirt lessens the amount of sunlight passing through the primary optics. It requires periodic cleaning of the Fresnel lens. Different methods of cleaning were tried to identify the efficient way to clean the system that results to a higher power generation. Corrections and modifications of the system to further increase power production are presented.The three year KACST/IBM collaboration in solar technology research led to the design and development of a 10kW CPV system. The system is comprised of 81 PV modules, inverters and a tracking system and is grid connected. A primary and secondary optics were employed to reach 1600x concentration on multijunction solar cells. Two CPV trackers were installed in the city of Riyadh and one in the eastern coastal city of Al Khafji. These two areas differ in climatic conditions. Riyadh is mostly dry and very often hit by very strong sand storms while Al Khafji is very humid with sand storms. Very fine dusts and dirt carried by the storms hits the surface of the primary optics, Fresnel lens, of the system. In Riyadh, the particles stick to the lenses but accumulation in the surface is not much since it is blown away by wind. However, the humid condition of the coastal areas wets the dusts and makes it sticky, cumulating more dusts and dirt. This paper discusses in details the parts of the 10kW CPV system. It prese...