04-02-2013, 11:32 AM
Thermoelectric Power Generation Using Waste-Heat Energy as an
Alternative Green Technology
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Abstract:
In recent years, an increasing concern of environmental issues of emissions, in particular global warming and
the limitations of energy resources has resulted in extensive research into novel technologies of generating electrical
power. Thermoelectric power generators have emerged as a promising alternative green technology due to their distinct
advantages. Thermoelectric power generation offer a potential application in the direct conversion of waste-heat energy
into electrical power where it is unnecessary to consider the cost of the thermal energy input. The application of this
alternative green technology in converting waste-heat energy directly into electrical power can also improve the overall
efficiencies of energy conversion systems. In this paper, a background on the basic concepts of thermoelectric power
generation is presented and recent patents of thermoelectric power generation with their important and relevant
applications to waste-heat energy are reviewed and discussed.
INTRODUCTION
A thermoelectric power generator is a solid state device
that provides direct energy conversion from thermal energy
(heat) due to a temperature gradient into electrical energy
based on “Seebeck effect”. The thermoelectric power cycle,
with charge carriers (electrons) serving as the working fluid,
follows the fundamental laws of thermodynamics and
intimately resembles the power cycle of a conventional heat
engine. Thermoelectric power generators offer several
distinct advantages over other technologies [1-4]:
• they are extremely reliable (typically exceed 100,000
hours of steady-state operation) and silent in operation
since they have no mechanical moving parts and require
considerably less maintenance;
• they are simple, compact and safe;
• they have very small size and virtually weightless;
• they are capable of operating at elevated temperatures;
• they are suited for small-scale and remote applications
typical of rural power supply, where there is limited or
no electricity;
• they are environmentally friendly;
• they are not position-dependent; and
• they are flexible power sources.
BASIC THEORY OF A THERMOELECTRIC
POWER GENERATOR
The basic theory and operation of thermoelectric based
systems have been developed for many years. Thermoelectric
power generation is based on a phenomenon called
“Seebeck effect” discovered by Thomas Seebeck in 1821 [1].
When a temperature difference is established between the
hot and cold junctions of two dissimilar materials (metals or
semiconductors) a voltage is generated, i.e., Seebeck voltage.
In fact, this phenomenon is applied to thermocouples that are
extensively used for temperature measurements. Based on
this Seebeck effect, thermoelectric devices can act as
electrical power generators. A schematic diagram of a simple
thermoelectric power generator operating based on Seebeck
effect is shown in Fig. (1). As shown in Fig. (1), heat is
transferred at a rate of H Q
from a high-temperature heat
source maintained at TH to the hot junction, and it is
rejected at a rate of L Q
to a low-temperature sink maintained
at TL from the cold junction. Based on Seebeck effect, the
heat supplied at the hot junction causes an electric current to
flow in the circuit and electrical power is produced.
THERMOELECTRCI MATERIALS FOR POWER
GENERATORS
Among the vast number of materials known to date, only
a relatively few are identified as thermoelectric materials. As
reported by Rowe [7], thermoelectric materials can be
categorized into established (conventional) and new (novel)
materials, which will be discussed in the next sections.
Today's most thermoelectric materials, such as Bismuth
Telluride (Bi2Te3)-based alloys and PbTe-based alloys, have
a ZT value of around unity (at room temperature for Bi2Te3
and 500-700K for PbTe). However, at a ZT of 2-3 range,
thermoelectric power generators would become competitive
with other power generation systems [1,15].
Conventional Thermoelectric Materials
Rowe [7] reported that established thermoelectric materials
(those which are employed in commercial applications)
can be conveniently divided into three groupings based on
the temperature range of operation, as shown in Fig. (6).
Alloys based on Bismuth (Bi) in combinations with
Antimony (An), Tellurium (Te) or Selenium (Se) are referred
to as low temperature materials and can be used at temperatures
up to around 450K. The intermediate temperature
range - up to around 850K is the regime of materials based
on alloys of Lead (Pb) while thermoelements employed at
the highest temperatures are fabricated from SiGe alloys and
operate up to 1300K.
Micro-Scale Waste Heat Applications
Growing applications like autonomous micro-systems or
wearable electronics urgently look for micro-scale power
generators. One possibility is to convert waste heat into
electrical power with a micro thermoelectric power
generator. Micro thermoelectric power generators can be
fabricated using integrated circuit technology [8]. For
example, in [18], alternate n- and p-type thermoelements are
ion implanted into an undoped silicon substrate. A
photograph of the miniature thermoelectric generator
developed by [18] is shown in Fig. (9) [18]. Metalisation of
thermoelement connecting strips and output contacts enables
several hundred thermocouples to be connected electrically
in series and occupy an area approximately 25 mm2. The
miniature generator in [18] was designed specifically to
provide sufficient electrical power to operate an electronic
chip in a domestic gas-monitoring system. In excess of 1.5
volts could be produced when a temperature difference of a
few tens of degrees was established across the module. In
this case, any available waste heat source, such as the surface
of a hot water pipe would provide sufficient heat flux to
thermoelectrically generate the required chip-voltage [8]. In
[19], waste human body heat is used to power a thermoelectric
‘watch battery’. In this application, thermocouples
were prepared by depositing germanium and indium
antimonide on either side of a 1 mm thick insulator which
served as a simulated watch strap. It was estimated that 2875
thermoelements connected in series would be required to
obtain the 2V required to operate the watch.
CURRENT & FUTURE DEVELOPMENTS
Recently, an increasing concern of environmental issues
of emissions, in particular global warming and the
constraints on energy sources has resulted in extensive
research into innovative technologies of generating electrical
power and thermoelectric power generation has emerged as a
promising alternative green technology. In addition, vast
quantities of waste heat are discharged into the earth’s
environment much of it at temperatures which are too low
(i.e. low-grade thermal energy) to recover using conventional
electrical power generators. Thermoelectric power
generation offers a promising technology in the direct
conversion of waste-heat energy, into electrical power. In
this paper, a background on the basic concepts of
thermoelectric power generation is presented and recent
patents of thermoelectric power generation with their
important and relevant applications to waste-heat energy are
reviewed and discussed. Currently, waste heat powered
thermoelectric generators are utilized in a number of useful
applications due to their distinct advantages. These applications
can be categorized as micro- and macro-scale
applications depending on the potential amount of heatwaste
energy available for direct conversion into electrical
power using thermoelectric generators.