One of the main drivers in photovoltaics (PV) has been increasing conversion efficiency. Solar PV is limited by the fact that it cannot use the full spectrum of energy available in sunlight. Thermal PV was originally developed to overcome this limitation, but has also found an application in the conversion of high temperature stored heat to electricity, opening new windows for thermal energy generation and storage.
The primary and initial interest in photovoltaics was the conversion of sunlight to electricity, but a recent development has been the conversion of high temperature stored thermal energy to electricity. Traditional solar PV cells have an inherent limit on the efficiency at which they can convert sunlight into energy. This limit, based on the bandgap of the material used, and known as the Shockley-Queisser limit, is about 33,7% for standard solar cells, and 40% for concentrated solar PV . It is essentially due to a PV material’s inability to respond to all wavelengths of sunlight.
The response band of conventional PV to sunlight is shown Fig. 1. The area in blue represents the portion of the spectrum that can be captured by a single junction silicon PV cell. The remainder of the energy is converted to heat and other losses.
Low energy photons in the longer wavelength region do not have sufficient energy to be absorbed, while shorter wavelength higher energy photons have too much energy to be absorbed. More than 67% of the energy impinging on the PV cell surface will be converted to heat .Thermal PV technology
In thermal PV technology, most of the energy in the sunlight is converted into heat via an absorber. The heat is re-radiated to the PV cell within a narrow bandwidth to which the PV cell is tuned.
Concentrated sunlight is focussed on the absorber raising the temperature to between 800 and 1000°C. The absorber is thermally coupled to an emitter which radiates heat in a narrow bandwidth to the PV cell, which is bandgap-tuned to convert all the energy in the spectrum to electricity. Efficiencies in the range of 80% have been predicted. The principle of the thermo-solar PV cell appears simple, but the construction and design are far more complex.
Concentrating the sunlight is the first problem, although this has been solved in many available concentrated PV (CPV) systems. A concentration ratio of greater than 750 times is required to raise
the temperature to the 1000 to 1500°C range.
The second consideration is the absorber material. The materials should be able to absorb solar energy within the full solar spectrum and also be able to withstand high temperatures. Practical devices will require materials and structures capable of withstanding extended operation times at temperatures in excess of 2000°C and frequent thermal cycling.
Research has been conducted on a class of materials called metallic dielectric photonic crystals, which exhibit both of these properties. The material considered is a two-dimensional metallic dielectric photonic crystal, and has the additional benefits of absorbing sunlight from a wide range of angles and withstanding extremely high temperatures. Perhaps most importantly, the material can also be made cheaply at large scales. It has also been reported that these materials can be prepared from any metal capable of withstanding high temperatures, using conventional standard manufacturing processes .Tungsten and other high melting-point materials have been investigated . The collector material and the structure of the collector must be such that it is possible to absorb the full spectrum of sunlight. One design uses tungsten photonic crystals.
It is common practice to integrate the emitter with the collector and structures based on nanotechnology structures seem to be the dominant feature. In one design the absorber outer layer uses an array of multi-walled carbon nanotubes, and the emitter portion is a photonic crystal layer made of silicon and silicon dioxide. With the right choice of structural parameters, such as sample thickness, surface topography, and metal filling fraction, an emitter structure can be realised that emits most of its power right above the bandgap of the PV cell .
Some designs incorporate a photonic filter between the emitter and PV cell to ensure that only energy within the appropriate bandwidth reaches the cell. Energy outside of the bandwidth is reflected back to the emitter.
Low bandgap materials are required for thermal-photovoltaic (TPV) systems. Typical materials that have been investigated include InGaAsSb, InGaAs and GaSb, but recent research has focused on Ge based structures as well. The emission peak of selective emitters typically used in TPV systems is close to the bandgap of germanium.
Therefore, germanium photovoltaic devices are well suited as TPV cells. Moreover, this semiconductor has a relatively low cost compared to other TPV materials such as InGaAs or GaSb. An important theme of the TPV research community is cost reduction. For germanium-based TPV cells, excellent results were obtained demonstrating both cost reduction and improved conversion efficiency.  Laboratory demonstration units using available materials have achieved conversion efficiencies of 20%, which is a far shot from the 80% predicted.Thermo-photovoltaic (TPV) devices
Conventional solar thermal photovoltaics can require temperatures of between 1000 and 1500°C. Devices have been developed which well at less than 1000°C, and, in theory, the technology could economically generate electricity at temperatures as low as 100°C . This large temperature range could make the technology attractive for generating electricity from heat from a variety of sources, including automobile exhaust, which would otherwise be wasted .
The principle can be applied to capturing energy from any source of heat, where the heat source is the absorber or is placed close to the absorber. The heat source may be used directly as the radiator as well. A typical device is shown in Fig. 3. Development of thermal PV devices is much further advanced than solar thermal PV, and there are a number of products available that use the principle to capture waste heat from various sources, or provide portable sources of electricity powered by heat sources.
TPV energy storage systems
Thermal storage of energy is one of the simplest and most efficient means of energy storage, but has been limited in the past by the means of recovering stored energy. Most systems use stored heat to drive rankine cycle generators, which is inefficient and loses a significant portion of the energy. TPV offers a potentially much higher conversion rate of stored heat to electricity, and if developments yield expected results, could open a new sector for energy storage.
Nuclear reactor coupled TPV systems
TPV devices have been considered as a power conversion technology for converting heat from nuclear sources in the past, but limitations in efficiency kept it from being a viable choice. However, new advances in the photovoltaic and compound semiconductor wafer industries have substantially increased cell performance to give rise to renewed interest in TPV devices as a viable passive energy conversion technology.
There are already devices available that convert heat directly into electricity using TPV technology, such as chargers for EV batteries, and other small devices. Although inefficient and really in the gadget class they are nonetheless working devices which go further than proving the concept.
 D Levitan: “Thermophotovoltaic Device Has Potential to Reach Huge Solar Efficiencies”, IEEE spectrum, 21 November 2016.
 S Fan: “Ultra-High Efficiency Thermophotovoltaic Solar Cells Using Metallic Photonic Crystals as Intermediate Absorber and Emitter”, Stanford University Global Energy and Climate project, September 2008.
 VL Teofilo, et al: “Thermophotovoltaic Energy Conversion for Space”, J. Phys. Chem. C, 2008.
 Solarcellcentral: “Solar efficiency limits”, www.solarcellcentral.com/limits_page.html
 E Parton, et al: “How to make thermoPV cost affordable”, INTERPV, June 2012.
 K Bullis: “Better thermal photovoltaics”, MIT Technology review.
Send your comments to firstname.lastname@example.org