Increasing solar cell competitiveness with add-on components

Energy, Resources & Utilities

CONTENTS

Modern Solar Concentrators:

The Next Innovative Phase

SUMMARY

Silicon remains the most widely-used material for solar cells largely due to its low cost. However, silicon-based  solar cells are inherently inefficient in converting solar energy to electricity, with a maximum theoretical efficiency of only 30%. However, technological measures can be taken to improve the energy conversion efficiency of silicon solar cells, and in some cases, even exceed this theoretical limit. One method involves using competitively priced 'add-on' components, which are affixed directly to the solar cell, and increase the solar energy-to-electricity conversion ratio dramatically, without significantly increasing the solar cell’s $/Wh ratio. In this paper we discuss the use of 'solar concentrators', which are 'add-on' components that increase the amount of sunlight that strikes a solar panel. In the past, solar concentrators were expensive and cumbersome, and primarily suitable for commercial installations and solar farms. Modern concentrator products have addressed these issues with innovative design solutions, such as parabolic mirror concentrators, light tracking concentrators, and integrated solar energy and water heating systems. This paper also highlights several cutting-edge developments in the design of solar concentrators from university labs, including 3D printed solar concentrators, biomimetic concentrators, and luminescent solar concentrators. 

 
 

Introduction
As the global demand for electric energy increases, scientists and engineers are working hard to meet this demand through innovative, clean sources of energy generation. Solar power has shown promise, but significant progress will be required for it to become economically competitive with fossil fuel-based electricity generating sources. As the world needs solar cells to be competitive today, research has focused on the development of  ‘add-on components’ which can be affixed to solar panels and instantly improve their efficiency. Add-on components include solar concentrators, which increase the amount of the sunlight that strikes a solar panel, and additional solar-to-electricity conversion layers, which allow panels to use a greater amount of the available sunlight for energy generation. 
Here we discuss the use of solar concentrators in improving the efficiency of solar installations. Previous generations of solar concentrators were only suitable for use in large-scale commercial and utility installations, as the concentrators were expensive, bulky, and heavy. Due to their size, these types of solar concentrators often shaded adjacent solar panels. Another technical hurdle related to solar concentrator technology is the increased amount of waste heat that solar cells absorb. By directing more sunlight onto the solar panel, solar concentrators also cause more heat to build up within the panel. This excess heat can eventually damage the solar cells, and care must be taken to safely dissipate it. Much effort has gone into eliminating the shortcomings of early solar concentrator models. Consequently, modern solar concentrators have dramatically improved characteristics and represent a useful interim solution for increasing solar cell performance.

Parabolic Mirror Concentrators
A parabolic mirror concentrator is a curved mirror that, due to its geometry, generates several reflections and combines them into one intense beam of light. As a result, the amount of sunlight striking the solar panel is increased, improving its ‘solar energy to electricity’ conversion efficiency from 21% to 26%. The company Solartron has used solar concentrators of this type in combination with expensive, high-performance multijunction solar cells (cells made up of several light absorbing layers). Solar concentrators managed to increase the efficiency of these high-performance solar cells to 46%–the world record. Skyfuel are currently designing next generation parabolic mirror concentrators that use reflective thin films rather than glass. This reduces the likelihood of mirror breakage as well as the weight and bulk of the concentrator, and allows for the production of extremely precise mirrors which maximize light concentration.

Light Tracking Concentrators
Combining solar concentrators with light tracking devices is also an attractive approach, as it allows each panel to harvest more sunlight throughout the day by tracking the sun’s position in the sky. Suntrack Pro offer solar tracking modules which can be integrated onto existing installations of solar cells affixed with concentrators. Solar tracking modules consist of a light sensor and an ‘inclinometer’ that tilts the solar concentrator depending on the position of the sun in the sky. This type of tracker is known as a mechanical tracker because it moves the entire solar concentrator. Glint Photonics are developing an optical tracker as an alternative, which moves a liquid layer in the solar concentrator to redirect the light striking the unit, rather than the unit itself. Compared to mechanical light trackers, this technology has the advantage of being much less bulky and heavy.

Integrated Solar Energy And Water Heating System
As mentioned above, solar concentrators increase the risk of solar panels becoming damaged by waste heat produced in the light conversion process. This damage can be avoided by using systems that dissipate the heat using water. This approach has the additional benefit of producing heated water that can be used for other purposes. Absolicon currently offer this type of system in their product line, and IBM are also developing a solar concentrator system that is currently in pilot stage and scheduled to become commercially available in late 2017. IBM’s system efficiently converts sunlight to energy and heats water, which is channelled into an attached desalinisation system to produce clean drinking water. Their unit is unsuitable for domestic installations due to its size and weight, however, but given the functionality of the system, the current target market will be installations in rural locations or at large commercial complexes (i.e. shopping malls). 

Cutting-Edge Research In Solar Concentrators
While many innovative solutions are already commercially available, solar concentrators are also an active field for cutting-edge research. Here, we highlight three technologies that appear to be the most promising solutions currently under development.
3D printed solar concentrators have been demonstrated by Lourens van Dijk (Utrecht University, Netherlands). This development was made possible by recent improvements in 3D printing technology, which is now precise enough to produce an optical layer that can act as a solar concentrator. Such optical layers consists of a sheet of tiny lenses coated with silver ink. Like conventional solar concentrators, the sheet creates several reflections and combines them into one intense beam of light.  A 3D printed concentrator can reportedly increase the energy conversion efficiency of silicon solar cells to 34%, and could prove to be a smaller, lighter weight alternative to conventional solar concentrators. This method of fabrication has great potential for commercial success, as it makes use of inexpensive, readily-available plastics that are commonly used in 3D printing. Additionally, it allows for the design of lens sheets that can be easily customised to fit any solar cell with the help of standard software.
Biomimetic solar concentrators, which are designed to imitate certain types of biological phenomena, are also being investigated. Structures occurring in biological systems have already been optimised by millions of years of evolution to perform their function extremely well, and they can offer inspiration for technological innovations.  For example, Katie Shanks (Exeter University, UK) has shown that the microstructure of white butterfly wings is capable of concentrating sunlight, which heats up the butterflies’ bodies and enables them to fly. Attaching the wings of this type of butterfly to a silicon solar cell increased its efficiency to 32%. Thus, the wings’ unique microstructure could be used as a starting point for designing new types of solar concentrators. 
Luminescent solar concentrators are completely different from conventional light concentrators, consisting of a layer of plastic embedded with luminescent material, and address a different inherent constraint of solar cells. Solar cells can only absorb light at a specific energy, or more precisely, at a specific wavelength. The sun, however, emits light at various wavelengths, most of which cannot be absorbed by a solar cell, and are thus wasted.  Certain luminescent materials absorb most of the available sunlight and in response emit light of the wavelength a solar cell can use. Thus, a luminescent solar concentrator can allow a solar cell to receive much more sunlight than it would normally absorb, thereby greatly increasing its efficiency. One example of this technology was demonstrated by Francesco Meinardi (University of Milan, Italy), who created a transparent luminescent solar concentrator. In the future, this kind of device could be integrated with transparent solar cells to make a smart, energy-harvesting window

Conclusion
This insight has discussed the modern design of commercially-available solar concentrators, as well as new solar concentrator technologies that are at the pilot or research stage. Parabolic solar concentrators have been proven to have excellent performance, and current work in this field intends to make these even smaller and lighter. Light-tracking concentrators are continually being improved, so much so that they may now have the potential to replace mechanical trackers. Integrated solar power and heating systems are gaining popularity due to their dual functionality, with IBM currently trialling their combined system in the field. A great deal of cutting-edge academic research is also focused on exploring novel solar concentrator designs. In summary, though solar concentrators have already been adopted by the solar industry, further improvements to their design and efficiency will result in wider use and, perhaps most importantly, could make expensive non-silicon based solar technologies more accessible.

"A 3D printed concentrator can reportedly increase the energy conversion efficiency of silicon solar cells to 34%, and could prove to be a smaller, lighter weight alternative to conventional solar concentrators."

 
 
 
 
 

"Attaching the wings of this type of butterfly to a silicon solar cell increased its efficiency to 32%."

"Solar concentrators managed to increase the efficiency of these high-performance solar cells to 46%–the world record."

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