of the PV panels and the TE generator, it was found that
the cost per watt of the TE module is very competitive
with PV panels. The objective of this work was to study
the feasibility of using TE modules coupled with a rice
husk gas gasifier in order to generate electricity. A more
reasonable model of a TE generator has been adopted for
system analysis. Testing results showed promise of using
a TE generator for waste heat recovery in a gasifier stove.
M
ETHODS
A. Calculating Methods
The equations used to model the behavior of the TE
power generator are based on the Seebeck, Fourier and
Joule effects. Using the standard model [7] and assuming
one dimensional conduction through the module, the rate
of heat supply (Qh) and heat removal (Qc) can be
estimated at the hot and cold junctions as
c
h
h
h
T TKR 0.5I
αIT Q
2
(1)
c
h
c
c
T TKR 0.5I
αIT Q
2
(2)
where
is the Seekeck coefficient, R is the electric
resistance of the TE module, K is the thermal
conductivity of the TE module, I is the input electric
current, T
c
and T
h
are cold side and hot side temperatures,
respectively.
Theoretically, the maximum power output (P) of a
realistic TE modules takes into account the contact
resistance as given by [8]
2
c
2
c
h
2
L
rL2
1nL
TTNA
R2
P
(3)
where A is the aera of thermoelement , L is the length
(also termed height) of thermoelement, Lc is the length
(thickness) of solder/contact in module, N is the number
of thermoelements per module, n is the contact parameter
and r is the contact parameter, dimensionless
Typically, n = 0.1 mm, r = 0.2, L= 1.2 mm, Lc = 0.8 mm,
α =3.92 x 10
-4
V/K, N= 127 couples, R = 1.48 × 10
-5
Ωm,
K = 1.63 W/mK and A = 1.96 mm
2
.
The electrical output of the TE modules (P) is also
calculated from the measured data as follows:
VI P
(4)
where I is the current of the TE modules at a matched
load. V is the voltage of the TE modules at a matched
load.
Miller et al. [9] suggested that the conversion
efficiency is as follows
h
c
c
e
T
T
M
1M
(5)
where
m
ZT 1 M
which
c
h
m
TT5.0 T
T
h
and
T
c
are the hot and cold side temperatures
of the TE module, respectively.
T
m
is the average temperature
Z is the figure of merit of the
TE material (
Z
=1.6 × 10
-3
1/K)
Note that
ZT
m
is a characteristic parameter of the
thermoelectric element and essentially governs its
internal conversion efficiency. It is well known that the
value of
Z
can have strong variations in temperature. In
this study, in order to gain insight into the optimal
collector operating temperatures, the value of
Z
is
assumed to be constant. Although this may be an over
simplification of the actual situation, it provides tractable
solutions for the solar collector temperature and operating
efficiency of the thermoelectric element.
c
is the Carnot efficiency;
h
c h
c
T
T T
(6)
The thermal efficiency of the gasifier is defined as the
ratio of the energy entering the pot to the energy content
of the fuel consumed. The standard water boiling test
(WBT) [10] is used for testing the efficiency of the
gasifier. The thermal efficiency (
th
) can be calculated
using the formula:
f f
evap ,w i
e
pw w
th
Hm
L mTTCm
(7)
where m
w
is the mass of water initially in the pot, m
w,evap
is th mass of water evaporated, m
f
is mass of rice husk,
C
pw
is the specific heat of water, H
f
is the calorific value
of rice husk (higher heating value), L is the latent heat of
vaporization, T
i
is the initial temperature of water in the
pot and T
e
is the temperature of boiling water.
It is a given that electrical energy is higher in grade than
thermal energy. Therefore, Ji et al. [11] used the overall
efficiency
o
for a TE-RSG as.
power
e
th
o
(8)
where
power
, often assigned a value of 38%, is the
electrical-power generation efficiency of a conventional
thermal power plant.
B. Experimental Apparatus
The TE-RSG was a batch fed up-draft gasifier. It
consists of two main parts: a thermal part (gasifier) and
an electrical part (TE). The gasifier basically consists of a
reactor, a burner and a blower. The reactor is a concentric
cylinder in tube having an inside tube diameter of 17 cm
and a height of 70 cm. The reactor is made of 2 mm thick
galvanized iron sheet. The outer cylinder has a diameter
of 21 cm. An annular space between the inside and
outside cylinder holds a ceramic fiber insulator to prevent
heat loss from the reactor. The inner cylinder is fixed to
the top flange. The flange, together with the inner
2013 International Conference on Alternative Energy in Developing Countries and Emerging Economies
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