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Getting More for Less: Improvements to Concentrated Photovoltaic Systems

published: 2012-06-21 15:38

By Dr Dino R Ponnampalam

The advancement of our civilization has been marked by numerous seminal moments; one such event was the Industrial Revolution.  This point in history allowed for the development and introduction of machines to make life easier by performing certain labor-intensive tasks, initiating great change worldwide in the fields of agriculture and manufacturing to name but two.

The mechanization of life resulted in a great demand for energy, and power sources were utilized to meet this demand.  One primary energy source was, at the time and even now, coal.  The rate of usage of this primary source increased dramatically as the demand for energy grew, due to a rising population growing more affluent.  Flash forward to the present and supplies of this polluting – but nonetheless primary – energy source are dwindling. 

Introduction to the Primary Energy Source Transition

A new movement has begun, one that will seek to harness natural sources of energy to power our energy-hungry societies. And with a nod to the Industrial Revolution, technology has been developed to make use of these relatively pollution-free energy sources. 

This transition from coal and oil, energy constituents that are finite in volume and polluting in character, will incorporate a return to millennia-old energy sources, which are significantly more abundant in volume and practically pollution-free in character.  For example, utilizing hydroelectric power and wind power to generate electricity are two techniques being touted as alternatives to this current coal-and-oil model. 

In addition to hydroelectric power and wind power, a significant component of this monumental drive to a low-carbon society is solar energy.  In terms of a power resource, no other technology can compete: in a single day more sunlight is received that, once converted to electricity, could easily meet our energy demand for an entire year. 

The inference from this staggering revelation is that solar energy technology is essential in making the transition to using renewable energy sources.  And with the rapid advances being made in this field, the concept of a clean energy future is indeed not far away.

Conventional Technological Approaches Employed to Generate Electricity

Silicon-based solar cells, also known as first generation solar cells, form the backbone of the solar industry.  Conceived on the deep understanding of silicon in the semiconductor industry, silicon was chosen to act as the raw material for solar cells.  In terms of efficiency rates, which measure the rate at which sunlight is converted into useable electricity, first generation solar cells reach efficiency rates of around 20%.  

Second-generation solar cells, commonly referred to as thin film solar cells due to their thin dimensions, were developed with a focus on reducing the manufacturing costs by using a different type of silicon (amorphous silicon) or by replacing silicon with alternatives such as cadmium telluride (CdTe) and copper indium gallium selenide (CIGS).

Improvements to conventional solar cells focus on either reducing the manufacturing costs or improving the efficiency rate; the ideal situation would be both, reducing the costs and improving the efficiency rates.  Usually, the reality of industry is that one feature can be achieved but not both.  Further developments to the solar cell will probably lead to the ideal solar cell one day, with low manufacturing costs and a high efficiency rate. 

Concentrated Solar Technology: An Alternative

Solar cells are generally considered as being installed as modules that are either roof-mounted or ground-based.  However, alternatives do exist for solar energy technology that go against the notion of individual and independent solar cells.

Concentrated solar energy technology is an idea based on using mirrors to direct sunlight to produce heat that will power a steam turbine engine (Concentrated Solar Power, CSP) or directed via optics to a highly efficient solar cell array (Concentrated Photovoltaics, CPV).

The technology behind the rapidly growing area of CPV has been introduced previously (reference: Magnifying Solar Energy: CPV Technology, EnergyTrend).  As such, this article will focus on recent improvements to CPV technology, and how these improvements are further helping solar energy technology become a powerful component of this change to using electricity derived from natural resources.

In its simplest form, concentrated solar technologies scale up the power of the sun and direct the power of this multiplied sunlight on to a small piece of semiconducting material.  For CPV, the apparatus includes an optic system fixed to a tracking system to track the position of the sun.  By doing so, CPV systems maximize direct sunlight exposure that generally ensures a healthy efficiency rate. 

However, by scaling up the power of the sun, the temperature of the module system shoots up and this hinders the operation of the CPV module.  This hindrance can be reduced and even eliminated by including a heat sink on the module.  This will allow for the module to be kept cool, maintaining the efficiency rate.  

Recent advances made by a commercial entity in the United States of America have indicated that by modifying the arrangement of the CPV module previous constraints have been eliminated or minimized. 

Unique Features of the Improved CPV Systems

[i] HCPV Solar Technology Innovation

Semprius, a high concentrating photovoltaics (HCPV) solar technology player based in Durham (North Carolina, USA), designed an innovative and patented printing technique that ensures the manufacture of low-cost and high performance CPV modules. The patented micro-processing printing technique allows for the direct transfer of pre-formed circuits on a semiconductor base to be transferred to any substrate. 

There are many advantages to this micro-processing technique, such as precise control of the formation of the circuits and also the transfer of the circuits from the semiconductor source wafer.  Two significantly important benefits of this technique are the low cost feature and the yield.  The printing yield is more than 99%, meaning an extremely high output of CPV-ready solar material.  When coupled with a low operating cost and a high output, CPV modules with high efficiencies can further be used as a source of generating electricity from sunlight, making CPV technology an attractive option.

Another benefit to this micro-processing technique concerns heat dissipation.  In the field of CPV, temperatures can reach extreme highs and this affects the operating efficiency of the CPV module.  In normal practice, a heat sink is added and this helps to reduce the temperature of the cell and maintain its efficiency.  With the micro-processing technique used by Semprius, tens of thousands of very small cells are produced and this innovative twist provides the cell device with a very high tolerance for high temperature.  The absence of a temperature limitation removes one of the challenges of operating an efficient CPV module. 

[ii] The Novel Material Choice of Gallium Arsenide

In addition to the micro-printing technique, Semprius changed the semiconducting material from conventional silicon to a relatively new material choice: gallium arsenide (GaAs).  There are some drawbacks to using GaAs, such as the toxicity of the material and the rarity of the compound, but GaAs has a band gap of 1.43 electron volts (eV) which is fairly close to the band gap of silicon (1.1 eV), making GaAs a suitable replacement.  Silicon is a typical single junction solar cell, but GaAs is a multijunction solar cell but still able to mimic a single junction solar cell due to the fairly similar band gap figure. 

Drawbacks aside, GaAs is a suitable replacement to silicon due to its high absorption rate and tolerance to high temperatures; characteristics that would be ideal for use in concentrator technologies such as CPV.  The material is extremely rare but as only a fraction would be required, especially for application in CPV modules, the costs will be low and manageable. 

In a traditional silicon-based solar cell, the single p-n junction leaves no room for modification.  Furthermore, absorption takes place in a narrow region of the solar spectrum.  In a multijunction solar cell, such as a GaAs solar cell, there is room for modification.  Semprius manufactured a bespoke GaAs solar cell containing three layers of GaAs, with each layer available for modification. This enables each layer to be fine-tuned to a different part of the solar spectrum, offering a wider range than that of a single junction solar cell.

Efficiency Rate Improvements for CPV Systems

Two aspects are key when discussing renewable energy technologies: the manufacturing costs, and the electrical energy conversion efficiency rate.  By using their patented micro-processing printing technique to manufacture tens of thousands of small cells to dissipate heat efficiently, and by using GaAs as a triple junction multilayer solar cell with a wider absorption range in the solar spectrum, Semprius produced a CPV module with a higher efficiency than other commercial products.

In terms of the manufacturing costs, Semprius devised a way to reduce operating costs and material costs.  By developing their patented micro-processing technique, manufacturing costs were reduced as the technique allowed for controlled printing of solar elements on any substrate, minimizing waste in the process.  In addition, this technique allows for scalability, which ensures cost savings.  By using GaAs wafers, costs were reduced even further, as only a fraction of GaAs material was required to achieve the desired leveraging effect. 

With regards to the electrical energy efficiency rate, the CPV module from Semprius was independently tested and rated at 33.9%.  This impressive achievement is made so by this being the first time a solar module had converted one-third of incoming sunlight into electricity.  Generally, solar cells are able to convert more sunlight into electricity but this efficiency rating is restricted to solar cells under laboratory conditions.  The 33.9% efficiency rating achieved by Semprius was for their commercially available CPV module, indicating the real output if it were to be installed. 

Compared to silicon-based first generation solar cells with a module efficiency rating of around 15%-20%, or even compared to second-generation solar cells with a module efficiency rating of around 12%-15%, the CPV module from Semprius simply exceeds both first-generation and second-generation photovoltaic cells.  With respect to other CPV modules on the market, the previous best has an efficiency rating of 32%.  Therefore, the rating of 33.9% is a slight increase but this 1.9% increase will result in significant cost savings.    

The CPV Prospect

As solar energy is going to be a significant part of the energy mix for selected countries, adopting technologies that are efficient and low-cost will be essential to ensure widespread adoption.

Within the solar energy technology family, first and second generation solar modules will primarily be used for households and small businesses.  However, for larger businesses with available land, CPV technology offers a cost-effective solution to generate clean electricity. 

As attractive as CPV technology is, there is point that should be made concerning location.  CPV works best in locations that receive lots of direct sunlight, so installations in countries located in the Sunbelt (+/- 35 degrees of the equator) might be of more benefit.

With innovative developments further pushing this technology into the mainstream, such as the new record-breaking CPV module from Semprius, the concept of a solar power-derived future is indeed within our grasp.  

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