Effect of material properties on the performance of TEG module

In Figure 5.13 a significant reduction in the open circuit voltage is observed for the device affected by thermal contact resistance, which is a slight improvement from the device without a heat sink. Experimental findings in the literature [81], has suggested that the thermal contact resistances can be minimized by the application of thermal grease. This will aid in filling up the air gaps between the heat sink and the bottom of the TEG module. Another approach to minimizing the contact resistance is to insert a soft metallic foil such as Tin, Silver, Copper, Nickel, or Aluminum between the two surfaces [81].

5.9. Effect of the material properties, electrical contact resistances and load resistance on the performance of Ge/SiGe-based TEG module

The properties of the thermoelectric material in a TEG module play a significant role on the conversion efficiency and power generated by the TEG module. A high Seebeck coefficient will lead to increased voltage in the circuit, a low resistivity (or high electrical conductivity) will minimise the Joule heating losses and a low thermal conductivity will aid in maintaining a high temperature difference between the hot and cold junctions of the TEG module [3].

5.9.1. Effect of material properties on the performance of TEG module

Table 5.3 presents the properties of two sets of thermoelectric material used for building a TEG.

Table 5.3: Properties of two types of thermoelectric materials

Property / Material (W/mK)  (V/K) S/m

p-type n-type p-type n-type p-type n-type

Material 1 (M1) 5.5 26.1 112 269 4099 9400

Material 2 (M2) 5.0 6.0 394.4 455.4 8633 1834.6

The properties of M 1 and M 2 were obtained via material characterization techniques of reference [6]. M1 refers to the first set of p- and n- type materials that were used in fabricating the TEG module that was discussed in chapter 4 of this work, while M2 refers to the material that is used subsequently to build an improved TEG module. Both M1 and M2 refer to the building of a TEG module that is void of thermal and electrical contact resistance

By inspection of Table 5.3, it can be seen that the Seebeck and thermal conductivity values are more preferable in M2 than in M1. However, the electrical conductivity of M2 for the n-type material is less preferable than M1.

Still, M2 is considered to be more efficient than M1 because of the overall ZT of the p and n-type material combinations. The overall ZT is defined as [4]:

(√ √ ) whereby . Thus, at 300 K, M1 has a ZT of 0.0055, while M2, which is more efficient, has a ZT of 0.03288.

The performances of M1 and M2 are assessed based on the Seebeck voltage in open circuit, load current, generated output power for an external load resistance of 1.0  and efficiency. Also, the hot side temperature is varied from ambient temperature of 25 C (or 298.15 K) to 100 C (or 398.15 K), the upper limit being determined by the melting point of the Indium bond (i.e. 120C).

Figure 5.13 (a – d) show the comparisons of the output results for M1 and M2.

Figure 5.14: Comparison of M 2 and M 1 for (a) open circuit voltage (b) load current (c) output power and (d) efficiency

It is observed from Figure 5.14 (a) that the open circuit voltages for M2 are much larger in magnitude than that of M1. This is because of the significant difference in the total Seebeck values of the p- and n- type materials for M2 as compared to that of M1. Moreover, M2 has a much lower thermal conductivity than M1 and (see Table 5.3), which helps to further increase the open circuit voltage based on the Seebeck effect.

A similar trend is observed for the load currents in Figure 5.14 (b). It also indicates the magnitude of the load voltage, which is the same as the load current, because a load resistance of 1 Ω was connected to the device in close circuit. The plots further suggest that by connecting an external load resistance of 1 Ω, there is minimal voltage drop between the open circuit and load voltage for M2 and M1. Recall from the circuit theory analysis discussed in Section 4. 2.

3 of Chapter 4, that the load voltage can be determined from the voltage division rule:

If r << 0, then VL  Voc.Therefore, for the present analysis the magnitude of the parameters Voc, VL and IL are equal. Hence, from Table 5.3, it can be seen that M2 and M1 have a high electrical conductivity. Thus, the internal resistance of M2 and M1 is very small so that the open circuit and load voltages are almost equal. In reality, it is difficult to obtain such a low internal resistance that is negligible compared to the external load resistance. Moreover, there are issues of electrical contact resistance that affect the performance of the TEG, and this will be discussed in Section 5.9.2. It is also unreasonable to increase the external resistance indefinitely because it will affect the performance of the device. Further explanation on the effect of external load resistance on the performance of the TEG module is discussed in the Section 5.9.3. A significant drop in the output power, (Figure 5.14 (c)), is observed for M1 as compared to M2. From Table 5.3, M2 is expected to have a higher output power as compared to M1 as defined by Equation 5.17 [4]:

A similar trend is observed for the efficiency plots of Figure 5.14 (d). Given the same amount of heat input, Q represented by the heater temperature Th (which

was varied from 298.15 K to 398.15 K, it is expected that M2 will have a higher efficiency. Given the output power P, the efficiency is defined as:

5.9.2. Effect of electrical contact resistance on the performance of Ge/SiGe-based TEG module

In order to illustrate the effect of contact resistance on the device performance, the results obtained from the analyses of the first experiment conducted in Chapter 4 will be used. Let M1 refer to the building of a TEG module that is void of thermal and electrical contact resistance while M1 + contact refers to the building of the TEG module that is affected by contact resistance. Table 5.4 is used to show the material properties obtained for M1 and M1 + contact

Table 5.4: Properties of M1 and M1+contact

Property / Material (W/mK)  (V/K) S/m

p-type n-type p-type n-type p-type n-type

Material 1 (M1) 5.5 26.1 112 269 4099 9400

Material 1 + contact 5.5 26.1 90 90 15.5 15.5

The properties of M1 + contact was obtained from fabrication and simulations of the TEG module discussed in the previous sections (i.e. Sections 5.2.1, 5.6.1 and 5.6.2). The electrical conductivity for M1+ contact is said to fall within the range of 1 – 40 S/m based on previous discussions (Refer to Table 5.1). However, a specific value within this range (i.e. 15.22 S/m) is set for the subsequent analyses that will be discussed shortly.

Figure 5.15: Comparison of M 1 and M 1+ contact for (a) open circuit voltage (b) load current (c) output power and (d) efficiency

The plots in Figure 5.15 (a), suggest that the open circuit voltage of the TEG module was reduced by less than half, due to the presence of contact resistance in the device. The consequence of the contact resistance is also seen in the other figures (i.e. (b)-(d)).

The figure in (b) shows that the load voltage for M1 + contact drops by approximately half of its open circuit voltage displayed in (a). Recall, for M1 + contact, the internal resistance was measured to be approximately 1.2 Ω which is approximately the same in value as the external load resistance. This explains the half drop in voltage for M1 + contact as illustrated by Equation 5.17.

Figures 5.14 shows the comparison of M2 and M1, thereby showing the effect of using a more efficient material in building the TEG, while Figure 5.15 shows the comparison of M1 and M1 + contact, thereby showing the effect of contact resistance on the device performance. Thus there is no need to include M2 + contact as this will show a similar effect as M1 + contact. Based on the above analyses in Sections 5.9.1 and 5.9.2, it can be deduced that the performance of TEGs to a large extent are affected by both the type of material being used as

0

well as the contact resistances. The contact resistance will significantly reduce the performance of the TEG. Hence, measures need to be taken to improve the efficiency of the thermoelectric material as well as reduce the contact resistance to its barest minimum for improved performances.

5.9.3. Effect of load resistance on the performance of