Advancement in the field of science can be realized in two ways: the first is by a piecemeal approach, and the second by a complete revolution. In terms of developments in the field of solar energy, the piecemeal approach ensures a concept or idea (the product) organically develops to produce the required improvements. In the case of solar energy technology, improvement is sought in cost effectiveness and electricity conversion efficiency rates. On the other hand, the revolutionary approach focuses on out-of-the-box concepts and completely re-thinking (and re-designing) the product with an aim to improving the efficiency rate and lowering the manufacturing cost.
The solar energy industry has seen progressions both piecemeal and revolutionary. Step-by-step developments include the progress from first-generation monocrystalline silicon solar cells to second-generation thin film solar cells (thin films made up of cadmium telluride (CdTe) or copper indium gallium selenide (CIGS) thin films).
Re-designed solar cells include third-generation and fourth-generation solar cells. For example, solar cells utilizing multi-junctions (third generation) and solar cells composed of carbon nanotubes (third generation) are constantly pushing the boundaries of solar cell technology in order to produce a better solar cell. Fourth-generation solar cells have been designed using principles of quantum science to achieve improvements to the efficiency of the solar cell and a reduction in the associated manufacturing costs. Both third-generation and fourth-generation solar cells show the promise of revolutionary thinking.
Another addition to this out-of-the-box concept focuses on solar cells not designed to use sunlight to generate electricity. In a conventional solar cell, the device would absorb sunlight to begin the process that eventually results in electricity being generated. The time has come for a redesign on how to exploit this abundant natural resource from the sun.
Deriving Electricity On Not-So-Sunny Days
Current energy demands have placed a significant strain on the earth’s resources. As such, diversifying the energy mix to include sources that are renewable and clean is a sensible approach. With more sunlight striking the earth in one day than what is needed to sustain this energy requirement, developing solar energy technologies to harness this resource and meet the demand is important. With issues such as diffuse skylight and energy storage affecting the rapid deployment of solar energy technology (not to mention the efficiency rate and cost), creative ways will be required to overcome these barriers and assist in the widespread use of solar energy technology.
One such creative way is to harness solar energy without actually utilizing direct sunlight, achieved by using thermophotovoltaic (TPV) solar cells. TPV solar cell systems, composed of a thermal body (thermal radiation source) and a photovoltaic device, work by utilizing thermal radiation and not direct sunlight. TPV solar cell systems emit in the near infrared (0.78 – 3 micrometers), whereas the sun emits radiation that spans the entire electromagnetic (EM) spectrum; an interesting point to note is that the majority of the sun’s irradiance is in the infrared region, making exploitation of this region – and this technology – extremely worthwhile.
This unique way to generate electricity makes possible the idea that electricity could be generated from any source, as all that is required is a thermal body source radiating at a temperature higher than that of the photovoltaic device. This gives rise to the prospect of generating electricity on a not-so-sunny day. This also gives rise to the possibility of generating electricity from waste heat, such as the heat given off by a large industrial machine in operation.
Benefits to Using TPV Systems in Generating Electricity
TPV solar cells work by generating photons from a thermal radiation source and then transferring these photons to a photovoltaic diode cell, kick-starting the process of generating electricity. Therefore, a typical TPV solar cell basically consists of only a thermal radiation source and a photovoltaic diode cell; such a low number of non-movable parts allows for a reduction in operating costs, as the cost of maintenance would be lower.
In addition to lower operating costs, TPV solar cell systems offer other advantages. For example, TPV solar cells possess a higher power density and TPV solar cells are portable, silent, and operate independently of sunlight. Such factors make TPV systems highly attractive for use by the solar industry but the drawback of efficiency conversion has prevented such a roll out. However, with recent developments concerning improvements to the efficiency rates and a reduction in the cost of the raw materials required (for example, by using III – IV photovoltaic devices), TPV solar cell systems once again are in the frame to realize their potential and transform the solar industry.
The Essentiality of Improving the Components of the TPV System
TPV systems, that are practically made up of two parts (thermal radiation source and a photovoltaic device), possess numerous advantages and further progress is largely dependent on developing the system on a holistic scale. However, current research has focused on improving each component individually and not as a whole.
[i] Thermal Radiation Source
With respect to the thermal body (the thermal radiation) source, TPV solar cell systems focus on the cost, the efficiency conversion rate, and the temperature resistance of the material selected to be the thermal radiation source. While all three factors are individually important, the most important factor is the temperature resistance of the material as temperature resistance also affects the efficiency: at higher temperatures, shorter wavelengths are used by the radiating body and this shift towards shorter wavelengths allows for better photon absorption.
The typical material selection includes polycrystalline silicon carbide, tungsten, some oxides of rare-earth metals, and photonic crystals. Of this group, and with temperature resistance being a crucial factor, polycrystalline silicon carbide is the most popular choice with a temperature resistance of 1,700 degrees Centigrade.
The choice of polycrystalline silicon carbide is not a problem-free one. Most of the energy emitted is in the long wavelength of the EM spectrum, resulting in a high degree of wasted radiation. By modifying the TPV solar cell system set-up, by either adding mirrors at the back end of the photovoltaic device to reflect back onto the emitter or by using special non-absorbing filters in front of the photovoltaic source, this high degree of wasted heat can be reduced. The effect of such an addition would be to boost the efficiency conversion rate. Recycling the unconverted energy is just one aspect that makes polycrystalline silicon carbide an ideal choice for use in a TPV solar cell system; the other feature is the low cost to manufacture polycrystalline silicon carbide.
[ii] Photovoltaic Device
In terms of the photovoltaic device, quite naturally, silicon is the most common choice for TPV systems. With economic attributes such as fairly low extraction costs, low manufacturing cost and low processing costs, and chemistry-related attributes such as a relatively small band gap, silicon is a rather ideal choice. Whilst the relatively small band gap of 1.1 electrovolts (eV) is an acceptable choice for conventional photovoltaic devices, for thermal radiation set-up, the 1.1 eV is not very suitable. As such, silicon needs to be replaced with photovoltaic material with lower band gaps. For example, current research is focused on introducing material such as germanium and gallium antimonide, material more commonly associated with third-generation photovoltaic solar cells. Other choices include indium gallium arsenide and indium gallium arsenide antimonide.
As the band gaps for the material listed are 0.72 eV and below, they make for better photovoltaic devices than the traditional silicon-based photovoltaic systems. In fact, the resurgence in the use of TPV solar cell systems could be put down to the introduction of material such as indium gallium arsenide and indium gallium arsenide antimonide in photovoltaic devices. Using the elements found in Groups III – IV of the periodic table led to an increase in the efficiency conversion rate, reaching a high of 30% for certain configurations.
Constraint of Applying TPV Technology in the Solar Energy Industry
Engineering is the biggest obstacle preventing the TPV systems from becoming a mainstream form of solar energy technology. As the scientific principles behind TPV are not new, progress can be viewed as being piecemeal in nature. A considerable amount of research has been carried out in pursuit of realizing higher efficiencies using these III- IV photovoltaic devices, achieving the aim of reducing the manufacturing costs and / or improving the efficiency conversion rates. Thankfully, this area has also experienced its moment of ‘revolution’
Shifting TPV Solar Cell Systems to a Higher Level
Quite recently, a new method to engineer TPV solar cell systems has taken TPV systems one-step closer to being commercialized as a mainstream piece of solar energy technology.
In July 2011, researchers at the Institute for Soldier Nanotechnologies (ISN) at the Massachusetts Institute of Technology (MIT), led by Ivan Celanovic, created a tailor-made TPV energy conversion system. Similar to a standard TPV solar cell system, the material used to perform the photovoltaic function has etched onto its surface billions of pits or indentations that radiate heat at precise (and pre-selected) wavelengths.
As thermal radiation occurs mostly in the infrared region of the EM spectrum, and as low- band gap photovoltaic material is able to absorb more of the infrared region than first-generation photovoltaic material, the solution would be to use material able to operate in the thermal region. However, such a native material would be difficult to come by, so the solution had to be engineered.
INS researchers engineered photonic crystals of the sample material (in this case, tungsten) and etched pits along the surface, creating a slab of tungsten with a regular and repeating pattern of indentations. Figure 1 shows the electron micrograph of the material tungsten after etching.
Figure 1: Electron micrograph showing a section of tungsten with pits etched onto the surface after the etching process. Credit: MIT Energy Initiative
When the tungsten slab was heated, the material gave off a bright light with a very different emission spectrum; this difference in spectral emissions was due to the billions of pits acting as resonators. These pits only gave off thermal radiation at pre-selected wavelengths, offering the prospect of a controlled TPV solar cell system; choosing the design of the nanostructure allows for selection of material with the desired optical properties, such as only emitting at certain wavelengths.
Two prototype models highlight the potential of this bespoke TPV solar cell system. The first model was the size of a button and used butane as a fuel supply. This model provided power three times longer than a lithium-ion battery. And once in need of recharging, the process simply involved removing the extinguished power supply and placing a new fuel supply, instantly recharging the device. The second model used a radioisotope and is predicted to generate power (through the slow decay of the isotope) for thirty years. Such developments indicate the potential advantages of TPV solar cell systems, reflecting the promise this technology holds.
Industrial Outlook of TPV Solar Cell Systems
TPV solar cell systems offers the entire solar industry an opportunity: whether the individual business is concerned with the emitter side or the photovoltaic side, the development and roll out of TPV solar cell systems will be of benefit.
On the emitter side, companies that provide the fuel supply (such as butane gas) for gas-burner TPV systems will benefit, as will businesses that are concerned with sourcing and providing raw materials used for the thermal emitter. And when the prototype from MIT becomes commercialized, the material for etching such as tungsten will be in great demand, thus benefitting those companies who are raw material providers.
On the photovoltaic side, silicon raw material providers will be in demand as will companies who supply material such as indium gallium arsenide for multi-junction (triple layer) solar cells, as conventional and recently-developed versions of TPV solar cell systems utilize these materials. In addition, as TPV systems use a photovoltaic diode, all solar photovoltaic manufacturers will be interested in the development of TPV systems in order to diversity their product portfolios. Most solar photovoltaic manufacturers focus on producing first-generation solar cells and second-generation thin-film solar cells. By integrating thin-film technology into the TPV set-up, costs may be reduced further.
Future Outlook of TPV Solar Cell Systems
Currently, there is no single piece of solar energy technology that could fully harness the power of the sun in providing and sustaining the power demands of society. However, through the intelligent utilization of the technology available, it is indeed feasible that solar energy technology could meet the energy demands of the world.
Using conventional solar cell technology for when sunlight is unobstructed, and using thermal photovoltaic solar cell systems on diffuse days, and by using slow-decaying radioisotopes or versions of the MIT device for periods with no sunshine, then a solar-derived and thermally derived source of constant electricity could be viable and worthwhile.
