As an abundant and clean resource, solar energy has a role to play as a reliable component of the energy mix of a country, especially for those countries located along the ‘Sunbelt’. With more than enough insolation striking the earth in a single six-hour time period than what is required to meet the current energy demands of the earth for a whole year, the potential of solar energy is unmatched. However, the utilization of solar energy is not without complication. From a technology point of view, the solar energy industry has always tried to enhance the electrical energy conversion rate (i.e. the efficiency) and lower the manufacturing costs associated with solar cell photovoltaic (PV) production.
In addition to these two industry-related points, a wholly non-technical and non-industry issue affects the full development and deployment of solar energy technology: solar availability. While it is certainly true that the sun is always shining somewhere in the world, it is of little use to urban and rural areas who require electrical power between sunset and sunrise. Therefore, to be a truly essential element of the energy mix, electricity from solar energy requires a better system for storage.
The Current Development of Electricity Storage
An important aspect of using solar energy concerns the storage of the electricity produced. Unfortunately, electricity cannot be directly stored using the technology of today (but indirect, creative ways do exist). Two recent developments have offered the opportunity to overcome this hurdle, to overcome this fact of nature that the sun only shines in one place for a set amount of time on any given day.
One of the two recent developments is actually based on revisiting an old development that was abandoned many years ago, that of solar thermochemical technology. The other recent development, using carbon nanotubes to store electricity obtained from the conversion of sunlight, is an extension of the results of research into solar thermochemical technology and carries on the work of using carbon nanotubes in solar energy technology development.
By making progress in the area of solar thermochemical technology, and by further pushing the boundaries of using carbon nanotubes in the field of solar energy, researchers at Massachusetts Institute of Technology (MIT) in America have offered the prospect of harnessing solar energy during the day and storing the electricity for use for when the sun does not shine: in short, the development offered the possibility of on-demand solar-derived electricity.
Using Thermochemical Technology for Energy Storage
As mentioned, the proposal to use thermochemical reactions in storing electricity is not a new one. The investigations into using thermochemical reactions to store electricity began in the 1970s, but were later abandoned due to constraints such as finding a suitable material to efficiently carry out the storage requirement.
Whilst initial research was unsuccessful, an ideal candidate for the solar thermochemical storage device was found in 1996: fulvalene diruthenium. Even though the compound was ideal in performing the role of energy storage, a drawback concerned the use of ruthenium. Ruthenium is an exorbitantly expensive material, and quite rare. This uncertain availability of ruthenium coupled with the cost, made further development of this storage device unattractive to industry. This prevented a full understanding of how the compound works to be determined.
The Mechanisms of Fulvalene Diruthenium in Storing Heat
A solar thermochemical storage device involves the use of an appropriate chemical compound/molecule that will absorb incoming insolation. On absorbing this insolation (effectively storing the energy), the chemical compound will change state. When required, the chemical compound will release the stored heat and change state once more.
In December 2010, Professor Jeffrey Grossman, in the Department of Materials Science and Engineering at MIT,continued investigating fulvalene diruthenium and determined the chemical structuring processes that the compound undergoes. Figure 1 is an illustration of a molecule of fulvalene diruthenium.
Figure 1: Computational schematic of a molecule of fulvalene diruthenium.The blue spheres represent ruthenium atoms, the white spheres represent hydrogen atoms, the green spheres represent carbon atoms, and the red spheres represent oxygen atoms. Credit: MIT News
The chemical compound absorbs heat in the form of sunlight and changes structurally. This change to a higher-energy state allows for the chemical compound to remain as a stable storage unit, storing the heat indefinitely. The structurally-altered chemical molecule, on application of either heat or by the application of a selected catalyst, will return to its original structure and in the process, release the trapped heat. The released heat can get up to 200 degrees Centigrade, which is more than sufficient to power a turbine to provide electricity.
However, the discovery and subsequent understanding of the intermediate step in this chemical process was key to determining the principles of how this chemical compound behaves as a storage device. The intermediate step in this process involves fulvalene diruthenium forming a semi-stable arrangement, in-between the two energy states of the ‘resting’ structure (original configuration) and the ‘active’ structure (heat-trapping configuration).
This semi-stable structure explains why the process is reversible, going from trapping heat to releasing heat on-demand. However, the discovery of this intermediate step also indicated as to why other material candidates had not worked: the intermediate step was specific to ruthenium. Selecting chemical compounds not containing ruthenium failed, as perhaps those chemicals did not posses the semi-stable intermediate step inherent to the use of ruthenium.
Further Development in Research on Electricity Storage
While the idea of using fulvalene diruthenium is theoretically sound, the prohibitive cost and the rare nature of the material makes this a non-starter for industry. In addition, a replacement to ruthenium would require a similar intermediate step to be able to reproduce the results of using fulvalene diruthenium.
So, the next step for this research will focus on taking the fundamental understanding of how the molecule works and searching for alternative candidates that could perform just as well but are more commonly available and inexpensive.
Such a search, performed by the Grossman Group at MIT, recently proposed an ideal alternative using a hybrid complex of azobenzene and carbon nanotubes (CNTs): azobenzene-functionalized carbon nanotubes.
Applying CNTs and Azobenzene to the Solar Industry
CNTs are exceptional, and have been widely heralded since their discovery. The elongated version of the third form of carbon (after graphite and diamond), CNTs possess a wealth of interesting electrical and mechanical properties that make CNTs attractive for many applications. For the solar energy industry, frontier research on the next generation of solar cells involve integrating CNTs into solar cells to improve the efficiency and to reduce costs.
Azobenzene is a compound that is made up of two benzene rings (a ring of six carbon atoms and six hydrogen atoms) linked by two nitrogen atoms connected by a double bond. One interesting property of azobenzene concerns photo-induced isomerization (the changes to the structure on the application of heat).
When heat in the form of light is applied, between 300 nanometers (nm) to 400 nm, the molecule of azobenzene will change state from trans-azobenzene (where the two benzene rings are diagonally opposite each other) to cis-azobenzene (where the two benzene rings are adjacent to each other). The process is reversible on the application of light at a wavelength greater than 400 nm or by allowing the molecule to thermally relax. This reversible process highlights the molecule’s suitability as a replacement for ruthenium.
The Role of Azobenzene-functionalized CNTs in Electricity Storage Units
Professor Grossman, with postdoctoral research associate Alexie Kolpak, revealed at the end of July 2011 a possible replacement to fulvalene diruthenium: CNTs functionalized with a molecule called azobenzene. Figure 2 shows an illustration of the azobenzene-functionalized CNT compound.
Figure 2: Computational schematic showing the atomic arrangement of a molecule of azobenzene-functionalized CNT. Credit: MIT News
The entire process is practically identical to when using fulvalene diruthenium, as shown in Figure 3.
Figure 3: Illustration showing the photoinduced process of storing and releasing heat using azobenzene-functionalized CNT. Credit: MIT News
Upon a light source striking the device, the molecule will absorb the light and change structurally, changing to a photo-excited state and storing the energy as heat. With some sort of trigger being applied, the photo-excited state will revert to the original ground state and release heat in the process. This released heat can then be used, with some modification, to provide power in the form of electricity.
The Potential of Using of Azobenzene-functionalized CNTs in the Solar Industry
By functionalizing CNTs with molecules of azobenzene,Grossman and Kolpak were able to alter the shape and structure of azobenzene and in so doing, realized new properties. Compared to fulvalene diruthenium, the azobenzene-functionalized CNTs was less expensive and significantly better at heat storage, two important factors that will appeal to manufacturers of solar thermal devices. For its size, the CNT-azobenzene compound was 10,000 times better at storing heat (technically, the volumetric energy density) than for fulvalene diruthenium, effectively making this compound comparable to lithium-ion batteries.
Furthermore, by introducing nanofabrication methods for production, the team was able to control both how much energy the compound could store and also control the interactions allowing for a high degree of control on how long the chemical compound remained stable. Both are also independently controlled, allowing for the prospect of a solar thermal device that can store electricity and release electricity with one process not effecting the other. From an industrial point of view, this development is the solution for providing inexpensive and round-the-clock electricity, but for consumers, this may bring an additional concern.
An Issue of Azobenzene-functionalized CNT Solar Storage Device
The concept of the azobenzene-functionalized CNT for use in storing heat is very appealing, but a certain issue does exist. The device can absorb heat, store heat, and release heat when required but it cannot absorb sunlight and utilize sunlight immediately for electricity production. As such, this device will be a standalone device, installed in conjunction with solar photovoltaic (PV) cells. Some consumers may be reluctant to install two separate devices.
Industrial Outlook for Solar Thermochemical Storage Units
For manufacturers, this recent advancement will appeal as the material is inexpensive and the manufacturing technique is controlled, helping to lower costs. Furthermore, no modification to existing production processes will be required by manufacturers, helping to maintain and possibly lower costs.
In addition to solar energy technology manufacturers, raw material suppliers will also benefit due to the predicted surge in demand for CNTs and azobenzene, as well as for silicon solar cells; the prospect of this development allows for every link in the supply chain to experience increased business.
Future Prospect of Thermochemical Technology for Solar Storage
Further developments to this incredible technology might see a PV cell integrated with the solar thermochemical storage unit. This will allow for one part to absorb sunlight and generate electricity during the day and the other part to absorb sunlight and store it as heat, releasing heat to power homes when required during the night.
As a standalone device, this technology is also very attractive and paves the way for future materials to be investigated. The possibility of utilizing other photoactive compounds for use in absorbing and storing heat allows for the hope that electricity derived from solar power can indeed sustain the demands of our society, providing energy that is clean, renewable, inexpensive, and available all day and night.
