How Thermophotovoltaic system works?

Introduction : Thermophotovoltaic cell

Thermophotovoltaic cells are Static energy converters that convert thermal radiation into electricity by means of photo voltaic diode. These cells are used to convert solar energy and heat energy in to electrical energy. Now a days these Thermophotovoltaic cells are used for combined power generation.

Henry Kolm had constructed an elementary TPV system at MIT in 1956 and Pierre Aigrain is widely cited as the inventor of TPV.

Why we need thermophotovoltaic System?

• The available energy from the sun is as low as 1000 w/m2
• No solar energy is available at the night and in winter.
• It can be operated on cloudy and rainy days
• The efficiency of solar systems are very less(12-20%)
• It can also be used as a heat recovery system

Major system component of the Thermophotovoltaic system

1. Heat source

2. Emitter

3. Filter

4. Array of photo voltaic diode

How does it work?

The heat emitted from the heat source is converted to the radiation by the emitter and it is selectively filtered by the filter (optical filter). Filter transmits only the desired wavelength radiation. The part of radiation is transmitted to the photovoltaic cell and the rest of radiation is reflected to the emitter. The PV diode converts the transmitted photons with energies in excess of the diode energy band gap into charge carriers. PV cells are cooled using separate cooling system. The detail working of TPC is presented in the fig below.

fig:  Working of thermophotovoltaic system

Heat source

The primary source is of heat for thermophotovoltaic system is the sun. Heat from a boiler, waste heat from plants, chimneys of factories, waste heat from auto mobiles are commonly used heat sources.

Emitter

While selecting emitter we have to consider efficiency, temperature resistance and the cost of the material. Polycrystalline silicon carbide (SiC) and tungsten are major material used as emitter materials. They are thermally stable up to 1700C, radiates in the visible and near IR range. SiC are cheaper and used in commercial application while tungsten is used un burner TPVs. Other selective materials used for emitters are Rare earth oxides (Yb₂ O₃ & Er₂ O₃ ) and photonic crystals. Recently at Sandia Labs have demonstrated a high-efficiency (34%) TPV emitter using tungsten photonic crystals.

Basic properties of Emitter are thermal stability, corrosion resistant, shock resistant, and High thermal conductivity. Deviations from perfect absorbing and perfect black body behavior makes emitter inefficient.

Filter

Filter prevent the entry of photons without a specified energy. For blackbody emitters or imperfect selective emitters, filters are needed to reflect non-ideal wavelengths back to the emitter. Any light that is absorbed or scattered and not redirected to the emitter or the converter is lost and both can lead to inefficiencies. Practical filters often reflect a small percentage of light in desired wavelength ranges or transmit light of non-ideal wavelengths which reduces the efficiency.

Photovoltaic Diode

Photovoltaic diode converts the photon in to charge carrier. While selecting the photovoltaic diode material; low bandgap, low cost, ease of manufacture and availability of the material should be considered. Currently, widely used PV diodes are:

• Gallium antimonide (band gap of 0.72)
• Indium gallium arsenide antimonide (band gap of 0.55 eV)
• Indium gallium arsenide (band gap of 0.75 eV)

The major reason of inefficiency associated are non-radiative recombination and ohmic losses.

Efficiency of the TPV

The absolute upper limit for efficiency in TPVs is the Carnot efficiency that of an ideal heat engine

Tcell is the temperature of the converter

Temit is the temperature of the emitter

For a practical TPV maximum efficiency is 83% for Tcell = 300 K and Temit =1800 K.

Advantages of the TPV

1. High fuel utilization factor (close to the unity)
2. TPV system as a combined heat and power system
3. Low produced noise levels (due to the absence of moving parts)
4. Easy maintenance(similar to a common domestic boiler)
5. Great fuel flexibility(natural gas ,oil ,coke , nuclear fuels)
6. TPV system usually allows very low pollutant emissions (e.g. CO and NOx)
7. Low installation & maintenance cost
8. It can be the best alternative if available space is less 
9. Can generate current densities 300 times that of conventional photovoltaic cells

Application of TPV

TPV can be used as off grid generators and best alternative to the present-day system and flue flexibility when compared to the other system. It can provide uninterrupted power supply in winter and night time. Other application of TPV are:

• Distributed combined heat and power generation

• In the automotive sector in case of hybrid vehicles

• Glass or other high temperatures industries

• Co-generation systems

• Military application

•In Space

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