MIT scientists have demonstrated a system on the basis of a method that allows solar cells to break through a hypothetically predicted ceiling on the amount of sunlight they can transform into electricity.
Since 1961, it is known that there is an absolute hypothetical limit - referred to as the Shockley-Queisser Limit - to how competent conventional solar cells could be in their energy conversion. For a sole-layer cell constructed using silicon — the kind used for the majority of present solar panels — that upper limit is around 32%. However, it is also known that there are some possible ways to raise that total efficiency, like utilizing numerous layers of cells, a technique that is being extensively studied, or by transforming the sunlight initially to heat before producing electrical power. It is the second process, using devices called solar thermophotovoltaics (STPVs), that the group has currently demonstrated.
The results of the study have been published in the Nature Energy journal, in a research paper by MIT doctoral student David Bierman, Professors Evelyn Wang and Marin Soljačić, and four other people.
While all of the research in conventional photovoltaics encounters identical underlying hypothetical limitations, Bierman says, “with solar thermophotovoltaics you have the possibility to exceed that.” Actually, theory predicts that hypothetically this technique, which involves coupling traditional solar cells along with extra layers of advanced materials, could well exceed the hypothetical limit of efficiency, making it possible to distribute double the power from a specified area of panels.
We believe that this new work is an exciting advancement in the field, as we have demonstrated, for the first time, an STPV device that has a higher solar-to-electrical conversion efficiency compared to that of the underlying PV cell.
Evelyn Wang, Professor, MIT
During the demo, the group used a comparatively low-efficiency photovoltaic (PV) cell, so that the system’s overall efficiency was just 6.8%, however it clearly indicated in direct comparisons the enhancement facilitated by the STPV system.
The fundamental principle is easy: rather than dissipating unused solar energy in the form of heat in the solar cell, the entire energy and heat is initially absorbed by an intermediary component, to temperatures that would permit that component to release thermal radiation. By modifying the materials and configuration of these supplementary layers, that radiation can be released in the form of appropriate wavelengths of light for the solar cell to absorb. This enhances the efficiency and at the same time decreases the heat produced in the solar cell.
The solution is utilizing advanced materials referred to as nanophotonic crystals, which could be made to release accurately determined wavelengths of light once heated. In this analysis, the nanophotonic crystals are incorporated into a system containing vertically aligned carbon nanotubes, and work at a high temperature of 1,000°C. Upon heating, the nanophotonic crystals persist to release a thin band of wavelengths of light that accurately matches the band that a neighboring PV cell can absorb and transform to an electric current. “The carbon nanotubes are virtually a perfect absorber over the entire color spectrum,” says Bierman, enabling it to absorb the entire solar spectrum. “All of the energy of the photons gets converted to heat.” Then, that heat is re-emitted as light but, due to the nanophotonic structure, is transformed into just the colors that corresponds with the peak effectiveness of PV cell.
In effect, this method would use a traditional solar-concentrating system, WITH mirrors or lenses that focus the sunlight, in order to sustain the high temperature. An extra component in the form of a superior optical filter allows all the preferred wavelengths of light to the PV cell, although reflecting back unnecessary wavelengths, as even this superior material is not ideal in restraining its emissions. The reflected wavelengths are then re-absorbed, helping to maintain the photonic crystal heat.
According to Bierman, a system like this can offer numerous benefits over traditional PV, either based on silicon or other materials, but, the fact that the photonic device produces emissions depending on heat and not light, indicates that it would not be affected by short environmental changes, like clouds passing before the sun. Actually, it could in principle, pave a method to utilize solar power on a round-the-clock basis, if combined to a thermal storage system.
For me, the biggest advantage is the promise of continuous on-demand power.
David Bierman, Doctoral Student, MIT
Additionally, due to the way the system absorbs energy that would be wasted as heat otherwise, it could decrease excessive production of heat that can destroy certain solar-concentrating systems.
To establish that the technique indeed works the group ran analysis using a PV cell with the STPV components, initially under direct sunlight and then by completely blocking the sun so that only the secondary light emissions of the photonic crystal were lighting up the cell. The conclusions proved that the actual performance were in accordance with the projected improvements.
A lot of the work thus far in this field has been proof-of-concept demonstrations. This is the first time we’ve actually put something between the sun and the PV cell to prove the efficiency [of the thermal device. Even with this comparatively basic initial-stage demonstration] we showed that just with our own unoptimized geometry, we in fact could break the Shockley-Queisser limit.
David Bierman, Doctoral Student, MIT
In principle, this system can reach efficiencies more than that of a perfect solar cell.
The subsequent steps include discovering ways to develop bigger versions of the small, lab-scale experimental unit, and formulating methods of manufacturing such systems in a cost-effective way.
This indicates a “significant experimental advance,” says Peter Bermel, Assistant Professor of Electrical and Computer Engineering, Purdue University, who was not involved with this work.
To the best of my knowledge, this is a new record for solar TPV, using a solar simulator, selective absorber, selective filter, and photovoltaic receiver, that reasonably represents actual performance that might be achievable outdoors. It also shows that solar TPV can exceed PV output with a direct comparison of the same cells, for a sufficiently high input power density, lending this approach to applications using concentrated sunlight.
Peter Bermel, Assistant Professor of Electrical and Computer Engineering, Purdue University
The research group also included Andrej Lenert PhD ’14, MIT alumnus, currently a research fellow at the University of Michigan; Walker Chan and Bikram Bhatia, MIT postdocs; and Ivan Celanovic, research scientist.
The study was supported by the Solid-State Solar Thermal Energy Conversion (S3TEC) Center, financed by the U.S. Department of Energy funded this project.