3. Thermodynamic modelling

#### 3.1 Thermodynamic modelling of the advanced micro-gasifier cookstove

The graphical illustration of the advanced micro-gasifier cookstove with the ATEG is presented in Figure 4. The thermal resistance network of the combined advanced cookstove ATEG is exposed in Figure 5.

By using the first law of thermodynamics, the energy balance equation of the biomass advanced cookstove ATEG can be written as

$$Q\_{gen} = Q\_{loss} + Q\_{useful} \tag{1}$$

3.1.1 Conductive resistance

3.1.2 Convective resistance

surface finned annulus):

written as

183

material used is Vermiculite composite:

DOI: http://dx.doi.org/10.5772/intechopen.84237

Conductive resistance from a CC to ATEG (radial direction) is determined considering one-dimensional (1D) with steady-state conductive heat transfer via composite cylindrical walls. Three different types of materials, namely, steel-1 and steel-2 made up of extraordinary carbon steel, are used; the thermal insulation

Energy and Exergy Analysis of an Advanced Cookstove-Based Annular Thermoelectric…

Rs<sup>1</sup> <sup>¼</sup> <sup>1</sup>

Rins <sup>¼</sup> <sup>1</sup>

Rs<sup>2</sup> <sup>¼</sup> <sup>1</sup>

Rteg <sup>¼</sup> <sup>1</sup>

Convective resistance from ATEG to combustion air (radial direction) is considered as 1D steady-state convective heat transfer for ATEG (through extended

Rair <sup>¼</sup> <sup>1</sup>

3.1.3 Heat transferred at hot and cold side junction of ATEG

Qh <sup>¼</sup> <sup>α</sup>IT<sup>h</sup> � <sup>I</sup>

Qc ¼ αIT<sup>c</sup> þ

3.2 Thermodynamic modelling of ATEG

deliberated for the study.

<sup>2</sup>πks1<sup>L</sup> ln <sup>r</sup><sup>2</sup>

<sup>2</sup>πkinsL ln <sup>r</sup><sup>3</sup>

<sup>2</sup>πks2<sup>L</sup> ln <sup>r</sup><sup>4</sup>

<sup>2</sup>πktegL ln <sup>r</sup><sup>5</sup>

2πhairLr<sup>5</sup>

τI Tð Þ <sup>h</sup> � Tc 2

τI Tð Þ <sup>h</sup> � Tc 2

Waste heat from the outer surface of CC is absorbed by the ATEG (Qh) at the

<sup>2</sup> <sup>þ</sup> K Tð Þ� <sup>h</sup> � Tc

The transmitted heat through the ATEG is released at the cold side junction (annular fins) of the ATEG by preheating the combustion air. The equation is

<sup>2</sup> <sup>þ</sup> K Tð Þþ <sup>h</sup> � Tc

A cross-sectional observation of the thermoelement of an ATEG is depicted in Figure 6. The cross-sectional area A(r) of the ATEG thermoelectric pair increases in a radial direction (r). The exhaustive thermodynamic modelling and energy and exergy analysis of the ATEG in view of the Thomson effect have been deliberated in the following section, as studied by Kaushik and Manikandan [18]. The assumptions

• 1D steady-state heat transfer equation of ATEG alongside the radial path is

used for the thermodynamic modelling and investigations of ATEG are:

hot junction can be deliberated by the energy balance equation given as

2 R

I 2 R r1

r2

r3

r4

(7)

(8)

(9)

(10)

(11)

� n (12)

� n (13)

$$Q\_{gen} = m\_{fuel} \times CV\_{fuel} \tag{2}$$

$$Q\_{loss} = Q\_{rad} + Q\_{cond} + Q\_{conv} \tag{3}$$

$$Q\_{rad} = \sigma \varepsilon\_{pan} A\_{pan} \left( T\_{pan}{}^4 - T\_{air}{}^4 \right) \tag{4}$$

$$Q\_{cond} = Q = \frac{T\_{cc} - T\_h}{R\_{i1} + R\_{im} + R\_{i2} + R\_{\text{leg}}} \tag{5}$$

$$Q\_{conv} = \frac{T\_h - T\_c}{R\_{air}}\tag{6}$$

Figure 4. Schematic illustration of the advanced micro-gasifier ATEG.

Figure 5. The thermal network of advanced micro-gasifier ATEG.

Energy and Exergy Analysis of an Advanced Cookstove-Based Annular Thermoelectric… DOI: http://dx.doi.org/10.5772/intechopen.84237

#### 3.1.1 Conductive resistance

3. Thermodynamic modelling

Figure 4.

Figure 5.

182

Schematic illustration of the advanced micro-gasifier ATEG.

The thermal network of advanced micro-gasifier ATEG.

advanced cookstove ATEG is exposed in Figure 5.

Biomass for Bioenergy - Recent Trends and Future Challenges

biomass advanced cookstove ATEG can be written as

3.1 Thermodynamic modelling of the advanced micro-gasifier cookstove

Qrad ¼ σεpanApan Tpan

Qcond <sup>¼</sup> <sup>Q</sup> <sup>¼</sup> Tcc � Th

Qconv <sup>¼</sup> Th � Tc Rair

The graphical illustration of the advanced micro-gasifier cookstove with the ATEG is presented in Figure 4. The thermal resistance network of the combined

By using the first law of thermodynamics, the energy balance equation of the

Qgen ¼ Qloss þ Quseful (1) Qgen ¼ mfuel � CVfuel (2)

<sup>4</sup> (4)

(5)

(6)

Qloss ¼ Qrad þ Qcond þ Qconv (3)

<sup>4</sup> � Tair

Rs<sup>1</sup> þ Rins þ Rs<sup>2</sup> þ Rteg

Conductive resistance from a CC to ATEG (radial direction) is determined considering one-dimensional (1D) with steady-state conductive heat transfer via composite cylindrical walls. Three different types of materials, namely, steel-1 and steel-2 made up of extraordinary carbon steel, are used; the thermal insulation material used is Vermiculite composite:

$$R\_{s1} = \frac{1}{2\pi k\_{s1}L} \ln \frac{r\_2}{r\_1} \tag{7}$$

$$R\_{\rm ins} = \frac{1}{2\pi k\_{\rm ins}L} \ln \frac{r\_3}{r\_2} \tag{8}$$

$$R\_{i2} = \frac{1}{2\pi k\_{i2}L} \ln \frac{r\_4}{r\_3} \tag{9}$$

$$R\_{\rm teg} = \frac{1}{2\pi k\_{\rm leg}L} \ln \frac{r\_5}{r\_4} \tag{10}$$

#### 3.1.2 Convective resistance

Convective resistance from ATEG to combustion air (radial direction) is considered as 1D steady-state convective heat transfer for ATEG (through extended surface finned annulus):

$$R\_{air} = \frac{1}{2\pi h\_{air} L r\_5} \tag{11}$$

#### 3.1.3 Heat transferred at hot and cold side junction of ATEG

Waste heat from the outer surface of CC is absorbed by the ATEG (Qh) at the hot junction can be deliberated by the energy balance equation given as

$$Q\_h = a\Gamma T\_h - \frac{I^2 R}{2} + K(T\_h - T\_c) - \frac{\pi I(T\_h - T\_c)}{2} \times n \tag{12}$$

The transmitted heat through the ATEG is released at the cold side junction (annular fins) of the ATEG by preheating the combustion air. The equation is written as

$$Q\_{\varepsilon} = a\Gamma T\_{\varepsilon} + \frac{I^2 R}{2} + K(T\_h - T\_c) + \frac{\pi I(T\_h - T\_c)}{2} \times n \tag{13}$$

## 3.2 Thermodynamic modelling of ATEG

A cross-sectional observation of the thermoelement of an ATEG is depicted in Figure 6. The cross-sectional area A(r) of the ATEG thermoelectric pair increases in a radial direction (r). The exhaustive thermodynamic modelling and energy and exergy analysis of the ATEG in view of the Thomson effect have been deliberated in the following section, as studied by Kaushik and Manikandan [18]. The assumptions used for the thermodynamic modelling and investigations of ATEG are:

• 1D steady-state heat transfer equation of ATEG alongside the radial path is deliberated for the study.

The thermal properties and electrical properties of a TE material combined together are referred to as figure of merit (FOM). Dimensionless FOM has been generally used to measure the desirability of TE materials for devices by multiplying

Energy and Exergy Analysis of an Advanced Cookstove-Based Annular Thermoelectric…

The power output (W) produced, electrical energy efficiency (%) and exergy efficiency (%) of an advanced micro-gasifier-based ATEG system can be considered

Eq. (19) designates that the Thomson effect will decrease the power output of the ATEG. The energy efficiency (electrical) of advanced cookstove-assisted ATEG

<sup>Q</sup> <sup>¼</sup> ð Þ <sup>α</sup> � <sup>τ</sup> ð Þ Th � Tc <sup>I</sup> � <sup>I</sup>

The exergy efficiency (electrical) of the advanced cookstove-assisted ATEG is

<sup>¼</sup> ð Þ <sup>α</sup> � <sup>τ</sup> ð Þ Th � Tc <sup>I</sup> � <sup>I</sup>

Hence, the combination of potential energy and exergy efficiencies of the

The energy as well as exergy analysis of the micro-GATEG is analysed via

Thermal efficiency (%) is defined as the fraction of heat energy given off by the biomass fuel that is successfully transported to the water in the cooking vessel. The remaining unrecovered heat energy is dissipated into the largest heat sink of an atmosphere. The method used to assess the thermal energy efficiency is specified in

The maximum possible work, which can be created by a system for a particular environmental condition, is generally taken as the Carnot hypothetical maximum

mfw � CVfuel ( ) (23)

<sup>η</sup>th <sup>¼</sup> <sup>4</sup>:<sup>186</sup> � mwi � mwf � � � Twf � Twi � � � � <sup>þ</sup> ð Þ Wv � <sup>2257</sup>

advanced micro-gasifier cookstove ATEG system can be written as

engineering equation solver (EES) for different operating conditions.

Qcond <sup>1</sup> � Tair

Th

Combined efficiency %ð Þ¼ ηel þ ηth (22)

Qcond

ð Þ Th � TC

2 R ¼ I 2

2 R

> 2 R

� � � <sup>n</sup> (21)

<sup>2</sup> (18)

RL (19)

� n (20)

ZTm <sup>¼</sup> ∝ ∝ð Þ � <sup>τ</sup> ρk

Pout ¼ Qh � Qc ¼ ð Þ α � τ ð Þ Th � Tc I � I

from the altered work done by Manikandan and Kaushik [1]:

<sup>η</sup>el <sup>¼</sup> Pout

<sup>ψ</sup>el <sup>¼</sup> Pout EQ

with mean operating temperature (Tm) [1]:

DOI: http://dx.doi.org/10.5772/intechopen.84237

is given as

derived as

3.3 Cookstove performance

3.3.1 Energy efficiency

Eq. 23, as follows:

3.3.2 Exergy efficiency

185

Figure 6. Cross-sectional view of ATEG [1].


For the study, it has been assumed that Qstorage ¼ Qg,loss ¼ 0; however Qr and Qr+dr are the heat input (from waste heat) supplied to the ATEG from outside the CC and heat output dissipated from the ATEG into the secondary air, respectively, whereas Qgen can be the addition of Thomson and Joule's heat produced in the element (dr) [1]. The cross-sectional region of the thermoelement is established on the study conducted by Shen et al. [19].

Based on the assumptions, the cross-sectional area, length (L) and thickness (δ) of the p-type and also n-type thermoelectric (TE) leg are the same; the dispersal of temperature in the p-type and n-type leg of the ATEG is also assumed to be the same. Shen et al. [19] have studied the thermal conductance (K) and electrical resistance (R) of the ATEG are as given below:

$$K = \left(K\_n + K\_p\right) = \frac{\Delta\rho\delta}{\ln(r\_5/r\_4)}\left(k\_n + k\_p\right) \tag{14}$$

$$R = \left(R\_n + R\_p\right) = \frac{\ln(r\_5/r\_4)}{\Delta \rho \delta} \left(\rho\_n + \rho\_p\right) \tag{15}$$

The only difference is the value of K and R in the thermodynamic modelling of the FTEG and ATEG; the rest of the equations for the GATEG and GFTEG are comparable with Shen et al. (2015):

$$I = \frac{(\infty - \pi)(T\_h - T\_C)}{R + R\_L} \tag{16}$$

$$R\_L = \sqrt{\mathbf{1} + Z T\_m R} \tag{17}$$

Energy and Exergy Analysis of an Advanced Cookstove-Based Annular Thermoelectric… DOI: http://dx.doi.org/10.5772/intechopen.84237

The thermal properties and electrical properties of a TE material combined together are referred to as figure of merit (FOM). Dimensionless FOM has been generally used to measure the desirability of TE materials for devices by multiplying with mean operating temperature (Tm) [1]:

$$ZT\_m = \frac{\mathfrak{\alpha}(\mathfrak{\alpha} - \mathfrak{\tau})}{\rho k} \frac{(T\_h - T\_C)}{2} \tag{18}$$

The power output (W) produced, electrical energy efficiency (%) and exergy efficiency (%) of an advanced micro-gasifier-based ATEG system can be considered from the altered work done by Manikandan and Kaushik [1]:

$$P\_{out} = Q\_h - Q\_c = (a - \pi)(T\_h - T\_c)I - I^2R = I^2R\_L \tag{19}$$

Eq. (19) designates that the Thomson effect will decrease the power output of the ATEG. The energy efficiency (electrical) of advanced cookstove-assisted ATEG is given as

$$\eta\_{el} = \frac{P\_{out}}{Q} = \frac{(\alpha - \pi)(T\_h - T\_c)I - I^2R}{Q\_{cond}} \times n \tag{20}$$

The exergy efficiency (electrical) of the advanced cookstove-assisted ATEG is derived as

$$\mathcal{W}\_{el} = \frac{P\_{out}}{E\_Q} = \frac{(a - \tau)(T\_h - T\_c)I - I^2R}{Q\_{cond}\left(1 - \frac{T\_{air}}{T\_h}\right)} \times n \tag{21}$$

Hence, the combination of potential energy and exergy efficiencies of the advanced micro-gasifier cookstove ATEG system can be written as

$$\text{Combined efficiency} \ (\%) = \eta\_{el} + \eta\_{th} \tag{22}$$

The energy as well as exergy analysis of the micro-GATEG is analysed via engineering equation solver (EES) for different operating conditions.

#### 3.3 Cookstove performance

#### 3.3.1 Energy efficiency

• The thickness (δ) of the ATEG module is constant throughout.

combustion chamber).

Cross-sectional view of ATEG [1].

Figure 6.

actual inbuilt electrical resistance.

Biomass for Bioenergy - Recent Trends and Future Challenges

the study conducted by Shen et al. [19].

comparable with Shen et al. (2015):

184

resistance (R) of the ATEG are as given below:

K ¼ Kn þ Kp

R ¼ Rn þ Rp

� � <sup>¼</sup> <sup>Δ</sup>φδ

� � <sup>¼</sup> ln rð Þ <sup>5</sup>=r<sup>4</sup>

The only difference is the value of K and R in the thermodynamic modelling of the FTEG and ATEG; the rest of the equations for the GATEG and GFTEG are

> <sup>I</sup> <sup>¼</sup> ð Þ <sup>∝</sup> � <sup>τ</sup> ð Þ Th � TC R þ RL

RL <sup>¼</sup> ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

• Convection losses and radiation losses from the sides of the thermoelectric modules to the atmosphere are negligible (as heated air is recirculated into the

• The electrical resistance of the contact is presumed to be about 10% of the

For the study, it has been assumed that Qstorage ¼ Qg,loss ¼ 0; however Qr and Qr+dr are the heat input (from waste heat) supplied to the ATEG from outside the CC and heat output dissipated from the ATEG into the secondary air, respectively, whereas Qgen can be the addition of Thomson and Joule's heat produced in the element (dr) [1]. The cross-sectional region of the thermoelement is established on

Based on the assumptions, the cross-sectional area, length (L) and thickness (δ) of the p-type and also n-type thermoelectric (TE) leg are the same; the dispersal of temperature in the p-type and n-type leg of the ATEG is also assumed to be the same. Shen et al. [19] have studied the thermal conductance (K) and electrical

ln rð Þ <sup>5</sup>=r<sup>4</sup>

kn þ kp

<sup>1</sup> <sup>þ</sup> ZTmR <sup>p</sup> (17)

<sup>Δ</sup>φδ <sup>ρ</sup><sup>n</sup> <sup>þ</sup> <sup>ρ</sup><sup>p</sup> � �

� � (14)

(15)

(16)

Thermal efficiency (%) is defined as the fraction of heat energy given off by the biomass fuel that is successfully transported to the water in the cooking vessel. The remaining unrecovered heat energy is dissipated into the largest heat sink of an atmosphere. The method used to assess the thermal energy efficiency is specified in Eq. 23, as follows:

$$\eta th = \left\{ \frac{\left[4.186 \times \left(m\_{wi} - m\_{wf}\right) \times \left(T\_{wf} - T\_{wi}\right)\right] + \left(W\_v \times 2257\right)}{m\_{fw} \times CV\_{fuel}} \right\} \tag{23}$$

#### 3.3.2 Exergy efficiency

The maximum possible work, which can be created by a system for a particular environmental condition, is generally taken as the Carnot hypothetical maximum

relating to the ambient temperature. The thermal exergy input supplied to the pot for water boiling can be stated as below [13, 14, 20]:

$$\text{Exin} = (m\text{TP} \times \text{C1} \times \eta c + \text{x} \times d \times \text{C2}) \left(1 - \frac{\text{Ta}}{T \text{fuel}}\right) \tag{24}$$

The exergy output of the ATEG attached advanced micro-gasifier cookstove is the quantity of energy spent by the boiling water times the Carnot factor as follows [20]:

$$\text{Exo} = \left\{ mw \times \text{Cp} \times (T\text{fw} - \text{Tiw}) \left( 1 - \frac{Ta}{T\text{fw}} \right) + \text{mpot} \times \text{Cp}, \text{pot} \times (T\text{fp} - \text{Tip}) \left( 1 - \frac{Ta}{T\text{p}} \right) \right\} \tag{25}$$

Generally, lowering heat source or raising heat sink lowers exergy. The exergy efficiency (ψ) is well defined as the fraction between the output exergy and the input exergy as shown below:

$$\eta \text{th} = \frac{\text{Ex}\rho}{\text{Exin}} \tag{26}$$

influence of hot side and cold side junction temperature and the influence of the number of thermocouples and operating electric current (A) on electric power output (W), thermal output (of cookstove) and energy and exergy efficiencies (%)

Energy and Exergy Analysis of an Advanced Cookstove-Based Annular Thermoelectric…

The influence of CC temperature outside the insulation on the power output as well as the energy and exergy efficiencies (%) of the ATEG have been studied. The combustion air temperature passed over the annular fins of the TEG varies with time so the cold junction temperature is also varied between 30 and 150°C in this

The effects of various hot junction temperatures on the power output of the ATEG are shown in Figure 8(a–f). The various zones of the CC temperature determine the hot side temperature of the ATEG. During combustion of the advanced micro-gasifier cookstove, the flame front propagates downwards with respect to the fuel bed density and heat transfer rate. Hence, heat is not uniform throughout from top to bottom. The uniform temperature is reached only after the conversion takes place from volatile combustion mode to char combustion mode, almost at the end (after 70% of weight loss in fuel). Therefore, when there is a variation in hot junction temperature, like an increase from a smaller value to higher, the power output (W) and the optimum current (A) value for maximum power output also increase. The reason is that when the temperature inside the combustion zone increases, a respective temperature of the outside chamber after insulation increases, thus increasing the power output of the TEG. It is also clear from Figure 8(a–f) that the power output of the micro-gasifier annular thermoelectric generator is maximum when the outside combustion temperature is at a maximum of 275°C. At a current flow rate of 0.8 A, the actual power output of GATEG is

10.05 W, electrical energy efficiency is 6.76% and exergy efficiency is 15%.

Similarly, from Figure 8(a–f), it is clear that the power output of the microgasifier ATEG is minimum when the outside combustion temperature is at a minimum of 150°C. At a current flow rate of 0.4 A, the power output of GATEG is 2.414 W, electrical energy efficiency is 1.31% and exergy efficiency is 4.62%.

of the GATEG system are studied.

DOI: http://dx.doi.org/10.5772/intechopen.84237

Figure 7.

187

4.2.1 Effect of change in hot junction temperature

Energy and exergy efficiencies of the advanced micro-gasifier cookstove.

study. The atmospheric temperature is considered as 30°C.

A small number of essential parameters like the mass of water (kg), the weight of fuel (kg), the volume of the kerosene sample (for the ignition of fuel) and the weight of unfilled Al vessel were computed before starting the test. Readings of water temperature (°C) and pot temperature (°C) were taken on a minute-tominute basis. The reference conditions taken for exergy analysis are To=303 K and Po=101.325 kPa.

### 4. Results and discussions

## 4.1 Analysis of energy and exergy efficiencies of the advanced micro-gasifier cookstove

The performance of the ACS cookstove is assessed in terms of energy efficiency (%) and exergy efficiency (%) using Prosopis juliflora, coconut shells and tamarind seed pellets as fuel. It is perceived that the thermal efficiencies of the stove are 36.7�0.4, 37�0.4 and 35�0.4% for coconut shell, Prosopis juliflora and tamarind pellets, respectively, after four repetition tests. The exergy efficiencies (%) of the cookstove are 15.6�0.45, 17.5�0.45 and 15�0.45% for the discussed three different fuels. The uncertainties for energy and exergy efficiencies are established as 0.43% and 0.48%, respectively.

The comparison on the energy efficiency and exergy efficiency of the advanced micro-gasifier cookstove is illustrated in Figure 7 for a distinct set of operational constraints. It is also witnessed that the energy efficiency (%) performance of the ACS cookstove is considerably higher than that of exergy efficiency (%) performance. This is due to the extent of energy extracted in the hot water for ACS cookstove being much less than the worth of energy extracted due to temperature constraint; this phenomenon is common for all cookstoves.

#### 4.2 Conceptual modelling results of GATEG

In this investigative study, the energy and exergy analysis of an advanced microgasifier cookstove ATEG is studied under various operating circumstances. The

Energy and Exergy Analysis of an Advanced Cookstove-Based Annular Thermoelectric… DOI: http://dx.doi.org/10.5772/intechopen.84237

#### Figure 7.

relating to the ambient temperature. The thermal exergy input supplied to the pot

Exin <sup>¼</sup> ð Þ <sup>m</sup>TP � <sup>C</sup><sup>1</sup> � <sup>η</sup><sup>c</sup> <sup>þ</sup> <sup>x</sup> � <sup>d</sup> � C2 <sup>1</sup> � Ta

Tfw 

The exergy output of the ATEG attached advanced micro-gasifier cookstove is the quantity of energy spent by the boiling water times the Carnot factor as follows

Generally, lowering heat source or raising heat sink lowers exergy. The exergy efficiency (ψ) is well defined as the fraction between the output exergy and the

<sup>η</sup>th <sup>¼</sup> Ex<sup>o</sup>

4.1 Analysis of energy and exergy efficiencies of the advanced micro-gasifier

The performance of the ACS cookstove is assessed in terms of energy efficiency (%) and exergy efficiency (%) using Prosopis juliflora, coconut shells and tamarind seed pellets as fuel. It is perceived that the thermal efficiencies of the stove are 36.7�0.4, 37�0.4 and 35�0.4% for coconut shell, Prosopis juliflora and tamarind pellets, respectively, after four repetition tests. The exergy efficiencies (%) of the cookstove are 15.6�0.45, 17.5�0.45 and 15�0.45% for the discussed three different fuels. The uncertainties for energy and exergy efficiencies are established as 0.43%

The comparison on the energy efficiency and exergy efficiency of the advanced micro-gasifier cookstove is illustrated in Figure 7 for a distinct set of operational constraints. It is also witnessed that the energy efficiency (%) performance of the ACS cookstove is considerably higher than that of exergy efficiency (%) performance. This is due to the extent of energy extracted in the hot water for ACS cookstove being much less than the worth of energy extracted due to temperature

In this investigative study, the energy and exergy analysis of an advanced microgasifier cookstove ATEG is studied under various operating circumstances. The

constraint; this phenomenon is common for all cookstoves.

4.2 Conceptual modelling results of GATEG

A small number of essential parameters like the mass of water (kg), the weight of fuel (kg), the volume of the kerosene sample (for the ignition of fuel) and the weight of unfilled Al vessel were computed before starting the test. Readings of water temperature (°C) and pot temperature (°C) were taken on a minute-tominute basis. The reference conditions taken for exergy analysis are To=303 K and

Tfuel 

<sup>þ</sup> mpot � <sup>C</sup>p; pot � ð Þ <sup>T</sup>fp � Tip <sup>1</sup> � Ta

Exin (26)

(24)

Tfp

(25)

for water boiling can be stated as below [13, 14, 20]:

Biomass for Bioenergy - Recent Trends and Future Challenges

Ex<sup>o</sup> <sup>¼</sup> mw � Cp � ð Þ <sup>T</sup>fw � Tiw <sup>1</sup> � Ta

input exergy as shown below:

4. Results and discussions

Po=101.325 kPa.

cookstove

and 0.48%, respectively.

186

[20]:

Energy and exergy efficiencies of the advanced micro-gasifier cookstove.

influence of hot side and cold side junction temperature and the influence of the number of thermocouples and operating electric current (A) on electric power output (W), thermal output (of cookstove) and energy and exergy efficiencies (%) of the GATEG system are studied.

#### 4.2.1 Effect of change in hot junction temperature

The influence of CC temperature outside the insulation on the power output as well as the energy and exergy efficiencies (%) of the ATEG have been studied. The combustion air temperature passed over the annular fins of the TEG varies with time so the cold junction temperature is also varied between 30 and 150°C in this study. The atmospheric temperature is considered as 30°C.

The effects of various hot junction temperatures on the power output of the ATEG are shown in Figure 8(a–f). The various zones of the CC temperature determine the hot side temperature of the ATEG. During combustion of the advanced micro-gasifier cookstove, the flame front propagates downwards with respect to the fuel bed density and heat transfer rate. Hence, heat is not uniform throughout from top to bottom. The uniform temperature is reached only after the conversion takes place from volatile combustion mode to char combustion mode, almost at the end (after 70% of weight loss in fuel). Therefore, when there is a variation in hot junction temperature, like an increase from a smaller value to higher, the power output (W) and the optimum current (A) value for maximum power output also increase. The reason is that when the temperature inside the combustion zone increases, a respective temperature of the outside chamber after insulation increases, thus increasing the power output of the TEG. It is also clear from Figure 8(a–f) that the power output of the micro-gasifier annular thermoelectric generator is maximum when the outside combustion temperature is at a maximum of 275°C. At a current flow rate of 0.8 A, the actual power output of GATEG is 10.05 W, electrical energy efficiency is 6.76% and exergy efficiency is 15%.

Similarly, from Figure 8(a–f), it is clear that the power output of the microgasifier ATEG is minimum when the outside combustion temperature is at a minimum of 150°C. At a current flow rate of 0.4 A, the power output of GATEG is 2.414 W, electrical energy efficiency is 1.31% and exergy efficiency is 4.62%.

Figure 8.

Power output (W) with respect to current (A) for (a) 275°C, (b) 250°C, (c) 225°C, (d) 200°C, (e) 175°C and (f) 150°C.

efficiency of the ACS cookstove annular/flat thermoelectric generator cogeneration system is studied. With an increase in the number of thermocouples, there is a rise in the heat transfer area. Hence, heat transfer between a hot side and cold side junction of the ATEG system is improved, as deliberated by Manikandan and Kaushik [1] and He et al. [21]. Figure 11 shows the effect of numbers in thermocouples on the power output (W) of GATEG, with a clear indication of the number of thermocouples being directly proportional to the power output of GATEG, as

Electrical energy efficiency (%) with respect to current (A) for (a) 275°C, (b) 250°C, (c) 225°C, (d) 200°C,

Energy and Exergy Analysis of an Advanced Cookstove-Based Annular Thermoelectric…

DOI: http://dx.doi.org/10.5772/intechopen.84237

The hot junction temperature is considered as 275°C, and the cold side temperature is retained at 30°C. The losses in the systems are considered as negligible. Figure 11 demonstrates the influence of the number of TEG modules on the electric power generation for variation of current (A) levels. Figure 11 clearly indicates the power produced; it is maximum as a result of the addition of/rises in the number of

Eventually, the addition of modules leads to overall thermal resistance causing a

reduction in the combined modules, Rtem, which leads to a fall in temperature difference, rapidly offsetting any further rise in voltage output. The plots shift to small current range slowly as "n" increases primly due to the interior electrical resistance upsurges steadily with the number of modules. The maximum power

proposed by Manikandan and Kaushik [1].

TEG modules.

189

Figure 9.

(e) 175°C and (f) 150°C.

The influence of variation of hot lateral temperature on the electrical energy efficiency (%) of GATEG is shown in Figure 9(a–f). It can be observed from Figure 9(a–f) that the electrical energy efficiency of GATEG is for the range of heat input considered, for the maximum hot side temperature of 275°C and an operational current flow of 0.8 A; the electrical efficiency (%) of GATEG is 6.76%.

The variation of exergy efficiency of GATEG for changing cold junction temperatures at maximum hot side temperature of 275°C is shown in Figure 10(a–f). It is seen that the exergy efficiency of the GATEG is high for all working conditions. It is obvious that the exergy efficiency of GATEG obtained for the heat input of 149 W at a working current of 0.8 A is 6.76%. This is due to the power output (i.e., exergy output) of the GATEG that is marginally greater because of superior heat transfer rates.

#### 4.2.2 Effect of the number of thermocouples

The influence of the number of thermo-plates (i.e., thermocouples) on the performance variance like power output (W), energy efficiency and exergy

Energy and Exergy Analysis of an Advanced Cookstove-Based Annular Thermoelectric… DOI: http://dx.doi.org/10.5772/intechopen.84237

#### Figure 9.

The influence of variation of hot lateral temperature on the electrical energy efficiency (%) of GATEG is shown in Figure 9(a–f). It can be observed from Figure 9(a–f) that the electrical energy efficiency of GATEG is for the range of heat input considered, for the maximum hot side temperature of 275°C and an operational current flow of 0.8 A; the electrical efficiency (%) of GATEG is 6.76%. The variation of exergy efficiency of GATEG for changing cold junction temperatures at maximum hot side temperature of 275°C is shown in Figure 10(a–f). It is seen that the exergy efficiency of the GATEG is high for all working conditions. It is obvious that the exergy efficiency of GATEG obtained for the heat input of 149 W at a working current of 0.8 A is 6.76%. This is due to the power output (i.e., exergy output) of the GATEG that is marginally greater because of superior

Power output (W) with respect to current (A) for (a) 275°C, (b) 250°C, (c) 225°C, (d) 200°C, (e) 175°C

The influence of the number of thermo-plates (i.e., thermocouples) on the performance variance like power output (W), energy efficiency and exergy

heat transfer rates.

188

Figure 8.

and (f) 150°C.

4.2.2 Effect of the number of thermocouples

Biomass for Bioenergy - Recent Trends and Future Challenges

Electrical energy efficiency (%) with respect to current (A) for (a) 275°C, (b) 250°C, (c) 225°C, (d) 200°C, (e) 175°C and (f) 150°C.

efficiency of the ACS cookstove annular/flat thermoelectric generator cogeneration system is studied. With an increase in the number of thermocouples, there is a rise in the heat transfer area. Hence, heat transfer between a hot side and cold side junction of the ATEG system is improved, as deliberated by Manikandan and Kaushik [1] and He et al. [21]. Figure 11 shows the effect of numbers in thermocouples on the power output (W) of GATEG, with a clear indication of the number of thermocouples being directly proportional to the power output of GATEG, as proposed by Manikandan and Kaushik [1].

The hot junction temperature is considered as 275°C, and the cold side temperature is retained at 30°C. The losses in the systems are considered as negligible. Figure 11 demonstrates the influence of the number of TEG modules on the electric power generation for variation of current (A) levels. Figure 11 clearly indicates the power produced; it is maximum as a result of the addition of/rises in the number of TEG modules.

Eventually, the addition of modules leads to overall thermal resistance causing a reduction in the combined modules, Rtem, which leads to a fall in temperature difference, rapidly offsetting any further rise in voltage output. The plots shift to small current range slowly as "n" increases primly due to the interior electrical resistance upsurges steadily with the number of modules. The maximum power

(W) is obtained once the load resistance (RL) matches with the system resistance according to maximum power transfer theorem. A similar curve with a notable difference in power output indicates that the maximum power output is attained for an increasing number of thermoelectric modules. The increase in efficiency by increasing 100 numbers into 1000 numbers is 100%. Thus, from the power output point of view, using an increased number of modules produces more power. This observation is similar to that of a steady state conducted by Jie Chen et al. [22]. A further intensification in the number of thermocouples results in an increase in surface area and volume which offers more resistance, thereby increasing the temperature of combustion air and consequently reducing the power output of TEG (refer Figure 11) and its electrical energy efficiency. These outcomes are compara-

Energy and Exergy Analysis of an Advanced Cookstove-Based Annular Thermoelectric…

An investigation of the micro-gasifier ATEG is conducted based on the first law and second law of thermodynamics, and its performance factors are investigated for varying hot and cold side temperature conditions and by varying number of ther-

• The power output (W), electrical energy efficiency (%), exergy efficiency (%) and combined system energy and exergy efficiencies (%) of the GATEG are 10.05 W, 6.76%, 15.12% and 43.46% and 30.72%, respectively, calculated for the maximum temperature difference of 275°C across the TEG, with the help of

• The same modelling is repeated for a low surface temperature of 150°C outside the combustion chamber. The power output (W), electrical energy efficiency (%), exergy efficiency (%) and combined system energy efficiency (%) of the GATEG are 2.41 W, 1.31%, 4.63% and 38% and 20.23%, respectively, for the

• The advanced micro-gasifier cookstove annular thermoelectric generator is a suitable option since it has many advantages like enhanced heat transfer characteristics in the hot side and cold side of the ATEG due to the greater heat

The fixing of the ATEG with the cylinder-shaped CC will be very easy, and the facility to arrange for thermal insulation to the cold lateral of the TEG will become easier if the ATEG has been adopted. The conceptual model analysis untaken in this study may be supportive in designing actual GATEG systems for electric power production from engine exhaust heat (flue gas), other heat pipes, etc. As the power

transfer area. The diameter (D) of the CC (based on the cooking load requirement) can be increased if necessary, to provide more heat transfer

output (W) and overall exergy efficiency (%) of the GATEG are low, these arrangements are improvident, but with the improved/novel TEG materials with the higher ZT, this type of concepts will gain more significance in the near future.

mocouples. From the conceptual modelling, the following conclusions are

maximum temperature difference of 150°C across the TEG.

ble to those recorded by He et al. [21, 23].

DOI: http://dx.doi.org/10.5772/intechopen.84237

5. Conclusions

summarised:

191

EES software.

surface area to the ATEG.

Figure 10.

Electrical exergy efficiency (%) with respect to current (A) for (a) 275°C, (b) 250°C, (c) 225°C, (d) 200°C, (e) 175°C and (f) 150°C.

Figure 11. Influence of the number of TEG modules on the power output.

Energy and Exergy Analysis of an Advanced Cookstove-Based Annular Thermoelectric… DOI: http://dx.doi.org/10.5772/intechopen.84237

(W) is obtained once the load resistance (RL) matches with the system resistance according to maximum power transfer theorem. A similar curve with a notable difference in power output indicates that the maximum power output is attained for an increasing number of thermoelectric modules. The increase in efficiency by increasing 100 numbers into 1000 numbers is 100%. Thus, from the power output point of view, using an increased number of modules produces more power. This observation is similar to that of a steady state conducted by Jie Chen et al. [22]. A further intensification in the number of thermocouples results in an increase in surface area and volume which offers more resistance, thereby increasing the temperature of combustion air and consequently reducing the power output of TEG (refer Figure 11) and its electrical energy efficiency. These outcomes are comparable to those recorded by He et al. [21, 23].
