THERMOELECTRIC WASTE HEAT RECOVERY AS A RENEWABLE ENERGY SOURCE


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ABSTRACT

A thermoelectric converter is a solid-state heat engine
in which the electron gas serves as the working fluid
and converts a flow of heat into electricity. It has no
moving components, is silent, totally scalable and
extremely reliable.

Introduction

Thermoelectrics revolves around modern-day
thermocouples and their application in power generation
and refrigeration. This technology is thoroughly covered
in a number of texts and review articles[1-16] and
current progress is reported in Proceedings of
International Conferences[17-19]. Conventional
metallic thermocouples as shown in Figure 1(a) are
made from metal or metal alloys. They generate small
voltages, typically tens of microvolts per degree
temperature difference by the Seebeck effect, when the
junction is located in a hot or cold environment
compared to the ambient, and generate small amounts of
electrical power when connected to a resistive load.
They are used extensively in the measurement of
temperature or as sensors to operate control systems
such as those in domestic air conditioning equipment,
refrigerators and central heating units. However,
probably less familiar is the use of thermocouples made
from modern semiconductors whose material properties
and geometry has been tailored specifically to meet the
intended application requirements and possess Seebeck
coefficients of hundreds of micro volts per degree.

Thermoelectric materials

Among the vast number of materials known to date,
only a relatively few are identified as thermoelectric
materials. In Figure 3 is displayed the figure-of-merit Z
of a number of thermoelectric materials together with
potential generating applications.

Large scale waste heat recovery

In 1994 a £1.8M research project sponsored by the
Japanese New Energy and Technology Development
Organisation (NEDO) commenced in the School of
Engineering at Cardiff University, UK with the
objective to convert low temperature waste heat into
electrical power. A series of WATT (Waste heat
Alternative Thermoelectric Technology) prototype
generators have been constructed and identified as
WATT-X where X denoted the power output in watts.
Basically the generator consists of an array of modules
sandwiched between hot and cold water–carrying
channels. Some of the heat flux which is established by
the hot and cold temperature difference between the hot
and cold water flows is converted into electrical power.
When operated using hot water at a temperature of 90C
and cold flow at ambient Watt-100 generates 100 watts
at a power density approaching 80kW/m3.