Thermophotovoltaics (TPV) is a method of converting heat to electricity using infrared light as an intermediary. Combustion heats an emitter to incandescence and the resulting thermal radiation is converted to electricity by a photovoltaic cell.

What are Hot Solar Cells?

Len Calderone for | AltEnergyMag

What are Hot Solar Cells? A new solar device, developed by a team of MIT scientists, converts heat into focused beams of light to create inexpensive and continuous power.

Existing solar panels are bulky, expensive, and inefficient. These conventional photovoltaics are limited and can only absorb a fraction of the energy from sunlight. The new device turns sunlight into heat and then converts it back into light, which is focused within the spectrum that solar cells can use.

The silicon solar cells, which are used today, capture the visual light from violet to red. This is a limiting factor, which means that they can never generate more than 32 percent of the energy in sunlight into electricity. The new design could lead to inexpensive solar power that keeps working after the sun sets.

The average house in Arizona needs about 574 square feet of solar to meet its daily energy needs. In Vermont, the same house would need 861 square feet. That’s a lot of roof space.

The new solar power device could hypothetically double the efficiency of conventional solar cells, but don’t expect to see this technology anytime soon. It may take 10 or more years to get it to market.

So, how will it work? The first step in creating the device is the development of mechanism called an absorber-emitter, which acts as a light funnel above the solar cells. It uses solid black carbon nanotubes that captures all of the energy in sunlight and converts most of the energy into heat. When temperatures reach about 1,800 °F, the adjoining emitting layer radiates that energy back out as light, which is focused to bands that the photovoltaic cells can absorb.

Black carbon nanotubes (MIT)

It’s known that for total heat radiation, the ratio of emissive power to absorptive ratio was the same for all bodies emitting and absorbing thermal radiation in thermodynamic equilibrium. This means that a good absorber is a good emitter.

A black carbon nanotube is a perfect physical body that absorbs all incident electromagnetic radiation, regardless of frequency or angle of incidence. It is an ideal emitter. At every frequency, it emits as much or more thermal radiative energy as any other body at the same temperature.

Black, as we know it, might not be the darkest shade. The British tech company Surrey Nanosystems says that it developed the world's blackest material, which is made of carbon nanotubes.  It can absorb 99.96 percent of light that hits it. The developers say that to the human eye, the material — called Vantablack — completely erases any features on a surface, becoming basically a void.

This material is black like a hole, because of the dense coating of carbon nanotubes. Rolled sheets of carbon atoms are used to create a lattice that absorbs virtually all light as it's refracted around the tubes.

The emitter is made from a photonic crystal, which is a structure that can be fabricated at the nanoscale to manage which wavelengths of light flow through it. A highly specialized optical filter that transmits the converted light while reflecting most of the unusable photons back was added. This produces more heat, which generates more of the light that the solar cell can absorb, increasing the productivity of the system.

Photonic crystals are intermittent nanostructures that are designed to affect the motion of photons by defining both acceptable and prohibited electronic energy bands. Generally, photonic crystals are composed of recurring dielectric, or metallo-dielectric nanostructures, which have alternative lower and higher dielectric constant materials in one, two and three dimensions to influence the spread of electromagnetic waves inside the structure. As a result, the transmission of light is absolutely zero in certain frequency ranges which is known as Photonic Band Gap.

Photonic crystals are classified into three categories according to the nature of structure periodicity: One Dimensional (1D), Two Dimensional (2D), and Three Dimensional (3D) Photonic crystals.

 

In 1D photonic crystals, the periodic modulation of the refractive index occurs in one direction only, while the refractive index variations are uniform for the other two directions of the structure.

Photonic crystal structures that are periodic in two different directions and homogeneous in third direction are called 2D. In most of the 2D photonic crystals, the photonic band gap occurs when the lattice has sufficiently larger index contrast.

A 3D photonic crystal is a dielectric structure, which has periodic modulation along three different axes, provided that the conditions of sufficiently high dielectric contrast and suitable periodicity are met. A photonic band gap appears in all directions. Such 3D Photonic Band Gaps, unlike the 1D and 2D ones, can reflect light incident from any direction.

Using a hot absorber-emitter, sunlight can be converted into thermal emissions tuned to energies directly above the photovoltaic bandgap. Solar thermophotovoltaics promises to leverage the benefits of high efficiency, by harnessing the entire solar spectrum; scalability and compactness, because of their solid-state nature; and dispatchability, and because of the ability to store energy using thermal or chemical means.

Thermophotovoltaics (TPV) is a method of converting heat to electricity using infrared light as an intermediary. Combustion heats an emitter to incandescence and the resulting thermal radiation is converted to electricity by a photovoltaic cell. The difference between a solar photovoltaic system and a TPV system is that a TPV system produces its own light.

TPV offers some advantages over other microgenerator technologies. A static conversion process allows favorable scaling down to the millimeter scale, the high-power density of combustion and thermal radiation results in a compact microgenerator.

The drawback for efficient collection of sunlight in the absorber and spectral control in the emitter is predominantly challenging at high operating temperatures. Limited experimental demonstrations of this approach to conversion efficiencies are around or below 1%.

Thanks to the nanophotonic properties of the absorber–emitter surface, efficiencies of 3.2% can be achieved. The device integrates a multiwalled carbon nanotube absorber and a one-dimensional Si/SiO2 photonic-crystal emitter on the same substrate, with the absorber–emitter areas optimized to tune the energy balance of the device. The device is flat and compact and could become a viable option for high-performance solar thermophotovoltaic energy conversion.

Technologies for harnessing the thermal energy in sunlight are continually growing with solar poised to become the dominant source of power in the future.

 
 
 
The content & opinions in this article are the author’s and do not necessarily represent the views of AltEnergyMag
Len Calderone - Contributing Author

Len Calderone - Contributing Author

Len contributes to this publication on a regular basis. Past articles can be found with an Article Search and are listed below. He also writes short stories that always have a surprise ending. He has also written a book on wedding photography on a budget. These can be found at http://www.smashwords.com/profile/view/Megalen

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