An industry perspective on a new portable solar powered energy generation system, the “Storm Cell” has been developed by Institute for NanoEnergy (INE) at the University of Houston.
Authors and Affiliations : Nigel J. Alley1, Seamus A. Curran1, Rene Rosas1, Kang-Shyang Liao1, Johnny Rodrigues2, Kenneth E. Russell3.
As we now proceed into the 21st century, we are seeing changes in our everyday lives that are driven by technology. Video calls, once the purview of ‘Star Trek’ on TV is now so common we have it on our mobile phones. All of these changes, whether in medicine, electronics, telecoms or even agriculture depends on one thing, power. The rise in demand for energy is apparent and inevitable given the pace of development across the globe. It is non-trivial to ensure an adequate infrastructure which requires access to electricity in the most remote areas. Standard power grids associated with industrialized nations are aligned with the larger population centers but have been significantly developed to service the less densely populated rural, agricultural and non-suburbanized areas. In remote regions however, the costs associated with providing electricity to small numbers of inhabitants is prohibitively expensive and for the most part economically not a viable option. New technologies employed in the development of smart-grids and improving energy efficiency may contribute to alleviate the strain already felt on the standard energy grid but the use of fossil fuels will eventually come to an end whether it is by choice or not is the pertinent question.
When we define nanotechnology, people forget it’s firstly a dimensional measurement of physical space. However, a crossover occurs between what is a dimensional measurement to a probabilistic understanding which is introduced to us by quantum mechanics. This is where the divergence in classical mechanics occurs as we go to smaller systems, even though intuitively we still think in a classical way. Quantum mechanics is the defining component in looking at energy levels, atomic orbitals and confinement effects. A simple example of this is that we can transform a common metal such as gold into nanoparticles that interact with light of varying frequencies. The use of quantum mechanics in everyday life is becoming more apparent through the manufacturing of products at the nanoscale.
Having greater control of the structure of the silicon in terms of the lattice and doping, we can now produce thinner and thinner sheets of silicon solar cells with much higher efficiencies. However, there are limitations in how far we can expect silicon to perform. The maximum theoretical efficiency of a solar cell was first calculated by William Shockley and Hans Queisser in 1961 and is known as the Shockley-Queisser (SQ) detailed balanced limit. According to the SQ limit, the maximum efficiency obtainable for a p-n junction solar cell made using silicon is ~ 30 % where the energy band gap is 1.1 eV. So, even as we get better performance from silicon, which has dominated the solar market three decades there is a well-defined limit to the possible photovoltaics module output. In order to go beyond the SQ limit, we need to look at the use of additional and alternative materials and structures to allow us to harvest more of the solar spectrum.
Although there has been a divergence, three directions dominate the field of photovoltaic research. Firstly, we have seen the emergence of thin film solar cells such as cadmium telluride (CdTe), which are not designed to beat the efficiency of silicon but due to their unique thin film nature can perform better in diffuse light conditions. The consequence of this is that there are a number of companies, such as First Solar that are dominating the thin film market, but that is not yet at a saturated level. The photovoltaic module efficiencies are not yet level with those developed in silicon, but this is in part due to structural and excitonic problems. As processing parameters become better defined, and our understanding of defects and defect states where trapping becomes better understood and controlled at the nanoscale, we will see these thin film cells dramatically improve their efficiencies. Other thin film solar cells also include copper indium gallium selenide (CIGS). These are somewhat behind the emergence of CdTe but also hold a lot of promise. Their defining problems are in part due to the availability of materials, the structural defects and issues at managing device interfaces. Aside from availability of materials, other scientific and engineering issues will be managed and overcome by using various analytical techniques such as Atomic force microscopy (AFM), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS).
Another area of interest, organic photovoltaic (OPV) devices, is often coined as the third generation of solar (the first being silicon and the second being thin film inorganic PV). For many years polymer scientists have been working on materials whose very structures are on the nanoscale. Carbon based polymers are an intriguing form of materials, and clearly driven by the nature of carbon itself where it can form different bonding structures. We now know that by altering carbon single and double bonds can result in a plastic semiconductor or synthetic metal, and it is this very nature that we now explore. OPVs are very thin, the active layer commonly used in our labs are less than 100 nm in thickness. Generally, all resonant light that is absorbed by the active layer can be turned into an exciton, which is a bound electron-hole pair that under the right conditions facilitates the generation of a useable current. The materials used, in comparison to inorganic materials require less energy intensive processing and can be fabricated much cheaper when scaled up to high volumes. The first successful photoactive organic device that consisted of a bilayer of an electron transporting perylene tetracarboxylic derivative and hole conducting copper phthalocyanine was demonstrated by C.W. Tang in 1986. The efficiency of the reported devices was around 1%. However, the major expansion in OPV research was after the introduction of the concept of the bulk heterojunction (BHJ). The development of new polymer materials has seen OPV efficiencies increase steadily. By 2006, OPV efficiency reached 5.2 % using an electron donating polymer, poly-3-hexylthiophene (P3HT), and electron accepting molecule, phenyl-C61-butyric acid methyl ester (PCBM). More recently, the best laboratory OPV devices or champion cells have reached efficiencies of ~10%. One could imagine that this is a major success story about to unfold, but unfortunately using organic semiconductors introduces new challenges. By the very nature of the OPV structure, the movement of the exciton inside an organic bulk involves a series of transfer processes within a characteristic diffusion length. If the exciton encounters a dissociation site with this distance useable charges can be realized. This actually means that the exciton cannot travel very far before decaying resulting in sub-optimal device performance. So, the concept of BHJ is that we have p and n regions (dissociation sites) very close together to get the charges out they are lost.
Another problem typically encountered with the use of organics is the limitation on the charge carrier mobility, i.e. the ease of charge extraction for the active layer. Organic semiconductors are notorious due to their poor carrier mobility and conductivity. While this is now improving, the issue is still a significant problem, so once the carriers are generated the very necessity of having a thin film is to have the electrodes very close to where the exciton is generated and have the charge carriers retrieved. The use of the standard flat photovoltaic architecture designed originally for inorganics may not be the best approach to achieve efficiencies of 10% and above which can compete with fossil fuels. The use of intrinsic light trapping designs may provide some solutions to the aforementioned problems with organics. Device structures based on optical fibers such where light trapping occurs due to the very nature of an optical fiber makes use of a waveguide as the substrate for the solar cell into which the incident light is coupled. Light is essentially trapped and travels within the device along the fiber axis making multiple passes through the active layer thus significantly increasing the amount of incident light absorbed. The cylindrical geometry of a fiber makes it useful in a number of ways — if the cell is fabricated so that light enters from the side walls it may then efficiently couple diffuse light (light over a range of incident angles) into the active layer, reducing the problems seen with conventional cells due to partial shading and possibly removing the need to incorporate solar tracking. Vertically aligned devices also show promise where the thickness of the active layer is maintained within the optimum range whilst trapping the incident light inside the optical cavity until all available resonant light is absorbed. The methods discussed for improving flat panel devices are indeed a step in the right direction for improving efficiency.
A new portable solar powered energy generation system, the “Storm Cell” has been developed by Institute for NanoEnergy (INE) at the University of Houston. The purpose of the design is to tackle many of the issues with regards solar integration. Adding solar power is a time consuming, costly process and being able to determine the optimum system (surge and power consumption issues) is always a quandary that experts generally can only answer. The Storm Cell is designed as a compact and portable unit that uses solar panels to extract electricity from the sun, store the power in a specialized chassis and deliver power as required, and can be designed to manage different surge requirements. The system is a combination of sophisticated mechanical and electronic engineering coupled with nanomaterials to enhance performance of the units. The environmental aspects of the widespread use of fossil-fuel based generators are simply immeasurable considering the lack of regulation across the globe concerning emissions and associated pollutants. Understandably, the provision of a back-up power supply is of paramount importance when the conventional infrastructure fails. The stresses on the energy supply due to increased power consumption at peak times effectively resulting in brown-outs can also now be provided at a cost that is the equivalent of one diesel or propane generator in one year. In contrast, the Storm Cell has a far longer lifetime without the need for regular servicing. The initial Storm Cell units will have an electrical power generation capacity of 6 – 10 kW which is sufficient to power a small home indefinitely.
The continuous application of materials science to enhancing the operation of the Storm Cell is resulting in more lightweight, user-friendly and durable products. The vast research field born only in the latter half of the 20th century involving carbon allotropes such as the nanotube and graphene has yielded some of the most important findings for materials science and is credited with the birth of the world’s fascination with nanotechnology. Fullerenes are another carbon based conjugated system that has intrigued nanotechnologists since their discovery. They may be divided into the Buckyball such as C60 or C70 and the nanotube such as the single walled nanotube and multi-walled nanotube. Carbon nanotubes have demonstrated interesting properties including high mechanical strength and unblemished conjugation with high electrical and thermal conductivities. Initial studies of nanotubes involved the use of microscopy and spectroscopy, while STM was used to study the individual electronic properties. Combining nanotubes with polymers (conjugated and non-conjugated) has led to significant development where it was realized that the combined properties could actually influence the prospect of using both materials in organic nano-electronics. The use of nanotubes as filler materials has a huge potential to serve as an external coating for thermal dissipation as well as heat sinks in the electronics. The potential of using these nanocomposite coatings for electronic enclosures has the other benefit of acting as a faraday cage. In addition, the historical and recent novel methods of composite fabrication ensures that lower loadings are needed but actually provides more pronounced electronic and phonic properties.
The use of solar panels in their current form to achieve this goal is not without drawbacks, people rarely think about the operations and management costs associated with the maintenance of renewables. In the case of solar, the energy produced diminishes dramatically when the transparent protective casings of the solar panels become dirty and blocking the sunlight. By studying materials science and in particular the unique properties of the surfaces at nano-scale become apparent as they yield answers to overcome these obstacles. The development of nanocoatings for solar panels allowing them to stay cleaner and clearer for much longer between maintenance cycles is hugely important in terms of long term sustainability. Recently researchers at INE have developed a proprietary ultra-thin hydrophobic “self-cleaning” coating that is optically transparent and highly resistant to rubbing. This coating have the unique surface structure where water drops, such as rainwater from the outdoor environment, can move by sliding when the solar panel is tilted. Due to the greatly reduced surface tension between water and the “self-cleaning” surface, the water droplet slides down leaving no water droplets trail behind them. In this process, the dirt particles are completely washed away by sliding water droplets due to the reduced adhesion of dirt to the coating. C-Voltaics has licensed this technology and further developed scale-up methods for applying this coating to glass which may yield better performing solar panels. Based on similar principles, this coating can also be applied to metals and plastics which will definitely broaden up their applications.
Changing the paradigm of energy by incorporating nanotechnology with mechanical systems to alter how we address energy use and demand. It is clear that the amalgamation of materials science and nanotechnology to improve the prospects of renewable energy sources is already helping to turn the tables on our unbalanced energy portfolio. If embraced at a policy level by the fastest growing nations, these solutions could be part of the many innovations that help us see the end of the correlation between economic growth and the rise in fossil-fuel consumption. This deviation from the historic development of industrialized nations may accelerate improvements to society as a whole.
1 Institute for NanoEnergy, University of Houston, Houston, Texas 77204, USA
2 C-Voltaics Inc. – 1 Dundas Street West, Suite 2500, Toronto, ON. M5G 1Z3
3 Cisco Systems, Inc. 1900 South Boulevard, Charlotte, NC 28203-4732
