Direct Energy Conversion

Transcription

1 Direct Energy Conversion Energy Conservation & Waste Heat Recovery IIT Kharagpur NPTEL 2017

2 Thermoelectric Generation - video

3 Thermoelectrics - Seebeck effect Discovered by Thomas Johann Seebeck in Àccidentally found that a voltage existed between two ends of a metal bar when a temperature gradient existed within the bar. The voltage is proportional to the temperature difference

4 Peltier Effect When current I flows through junction of two dissimilar materials, amount of heat (over and above Joule heat) which has to be removed/absorbed at the junctions to keep them isothermal is called Peltier heat (Q A-B ) and given by Peltier coefficient is related to Seebeck coefficient by

5 Thermoelectric Module Watch the video link:

6 Thermoelectricity & Energy Conversion Engine Refrigerator n p Q in n p T hot n p Q in n p T cold Q out T cold I Q out T hot R V Arun Majumdar (ISHMT 2007)

7 Applications to WHR Exhaust gas in automobiles Thermoelectric Generator Development for Automotive Waste Heat Recovery - Gregory P. Meisner, GM Global R&D,

8 Derivation Refer to class notes

9 Stack of a steam power plant Stack of Steam power plant Source: Block Diagram of a Thermal Power Station, Quora Hot gas Power Plant Technology P.K. Nag

10 Magneto Hydro Dynamics (MHD)

11 What is MHD? Based on Faraday s Law of electromagnetic induction when a conductor and a magnetic field move relative to each other, an electric voltage is induced in the conductor. Image: Wikipaedia

12 MHD Generator Uses hot ionized gas as the moving conductor gas heated to high temperature valence electrons of the excited atoms move to higher quantized orbits ultimately at certain energy levels, they fly off to become free electrons Gas becomes conducting Normal gas would need very high temperatures to reach ionization potential hot gas seeded with alkali/metal such as Cesium (Cs) or Potassium (K) Adequate electrical conductivity of about 10mho/m can thus be realised at temperature C

13 Application in WHR Power Plant Technology P.K. Nag

14 Other points MHD power output restricted At T<2000K, electrons combine with ions to form neutral atoms electrical conductivity becomes very low Current produced is DC Need inverter for conversion to AC Oxides and hydroxides of seeding element cause severe air pollution electrostatic precipitator helps recover the seed for re-use

15 Thermionic Generation

16 Thermionic Generator Direct conversion of thermal to electrical energy Two ELECTRODES EMITTER heated to sufficiently high temperature to EMIT electrons COLLECTOR at a significantly lower temp receives emitted electrons Space between the electrodes normally filled with a vapour or gas at low pressure

17 Thermo Photovoltaic (TPV) Generation

18 Thermophotovoltaic Generator Works on the same principle as traditional Photovoltaics (PV) P-N junction absorbs optical energy, generates and separates electron/hole pairs to generate electricity Difference source of optical energy is emission of photons from an emitter surface at high temperature TPV system consists of Emitter at elevated temperature (900 to 1300 C) Photovoltaic power converter Additional components filters, concentrators, reflectors, etc.

19 TPV additional points Emitter solid material or engineered surface Thermal emission characteristics (wavelength, polarization, direction) tend to be matched to the absorption characteristics of the PV cell to minimize losses Filters and surface treatement to get selective emission often used for optimization Low efficiencies hence ideal for auxiliary power generation from waste heat