3.1. Power Load Feature
External electronic devices and batteries have their rated input voltage, and 5.0 V is used widely in small electronic devices. Therefore, the output voltage of the proposed SPTEG was set to 5.0 V. The maximum power point tracking (MPPT) DDC is well recognized to be important in SPV and has also been used in SPTEG studies [
24,
25,
26]. In the present work, a regular DDC was selected instead of an MPPT DDC for the following reasons: (1) The present experiments provide experimental data to select a mating combination of the MPPT DDC and battery. (2) A tunable MPPT DDC can be obtained widely in open market, and the proposed SPTEG is prepared to adopt a mating combination of the MPPT DDC and battery. However, the present work does not focus on the MPPT DDC.
The power load feature test helps to reveal the power generating performance, that is, the potential of the designed SPTEG. The hot end temperature (
Th), temperature difference (Δ
T), input voltage (
Uin), load voltage (
Uld), load current (
Ild), load electric power (
Pld), and total electric power (
Ptot,), under different external load resistances are exhibited in
Figure 3. The electric power output was stable for 5 min before recording. The temperature difference ranges from 117 °C to 119 °C during the experiments, thereby resulting in the input voltage that lies between 13.1 V and 18.3 V. The decreasing trend of the input voltage while the load resistance decreases is caused by the ratio of the external load resistance to the total electrical resistance, whereas the open-circuit voltage of the SPTEG is unchanged. This result will be explained mathematically in
Section 3.3.
The load and total electric power increase while the external load resistance decreases. However, the external load resistance should not be significantly small enough to cause unstable output voltage, that is, the output voltage cannot be maintained at 5.0 V. In
Figure 3, the maximum total electric power was12.9 W when the load resistance was fixed at 3.5 Ω, whereas the corresponding load electric power was6.9 W (
Uld =4.92 V,
Ild =1.41 A). The difference between the total and the load electric power (6.0 W) comprise the electric power consumed by the water pump, DDCs, EETs, blowers for the radiator, and all other line losses. Most small electronic devices have a rated current of 1.0 A at 5.0 V (5 W). Consequently, an hour running of the proposed SPTEG with an appropriate DDC and a battery ensures 1.38 h of charging at the charging power of 5 W. The decreasing trend of output power while the load resistance increases is also explained in
Section 3.3.
A few studies have conducted a power load test [
14], and real-time measurements of electric power output are also rare [
23,
24]. In general, the power generation should be based on measurements directly on the external loads, such as electronic devices, batteries, and electrical loads. Moreover, the electric power output should be held stable for a sufficiently extended time considering the popular Peltier effect. The transform efficiency of the DDC or MPPT DDC should be measured before further characterizing the SPTEG performance. If a battery is used, then proper caution should be taken because the battery may not accept all the electric power despite incorporating an MMPT DDC. The power load test that uses an electronic load is suggested to measure the SPTEG performance because the electronic load accepts all the provided electric power.
The experimental data can be used to select MPPT DDCs and batteries after performing the power load test on the basis of an electronic load. Furthermore, several previous SPTEGs have operated at a relatively high operating temperature (higher than 200 °C) [
25,
26]. The wearing out of the Bi
2Te
3 material should be addressed.
The electric power output generated by each TE module per unit temperature difference (
PTE/Δ
T) is compared with that of the available previous studies, displayed in
Figure 4. The water-cooled SPTEGs provide a larger
PTE/Δ
T than the air-cooled SPTEGs. However, this result should be evaluated carefully because the heat flux from the cold end must dissipate into the surrounding air. In particular, the water-cooled SPTEG can be cooled by air eventually while the cooling water is circulating. All the reported results in
Figure 4 were measured with air-cooled SPTEGs or circulated water-cooled SPTEGs.
In
Figure 4, the maximum record of
PTE/Δ
T is 0.019 W/K for an air-cooledSPTEG [
15]. However, the minimum datum of
PTE/Δ
T is 0.027 W/K for a water-cooledSPTEG [
25,
26], which is obviously larger than that by an air-cooled SPTEG. This divergence should be evaluated sensibly because the Seebeck coefficient and electrical resistivity for Bi
2Te
3 are nearly the same for available TE modules. This relationship will be discussed in detail in
Section 3.3, which focuses on the theoretical aspect of the proposed SPTEG. The water-cooled SPTEGs presented in
Figure 4 use water tanks and circulating water pumps, but did not utilize a blower. Therefore, the heat dissipation into the surrounding air is the natural air convection from the walls of the water tank and pipes. The divergence illustrated in
Figure 4 is unknown and requires further studies. A possible reason for the divergence may be caused by the large volume of the water tank, that is, the area for natural convection is large, whereas the heat flux from the cold end is limited (limited TE modules are incorporated). Therefore, the water-cooled SPTEG works similar to the SPTEG based on no water circulation. In particular, the circulated water is sufficiently cold given the large water tank. If the volume and mass weight of the water-cooled SPTEG are considered, then radiators and blowers should be used, and the present works should act as an initial attempt. In
Figure 4, the power generation capability, that is,
PTE/Δ
T is only 0.016 W/K for the proposed SPTEG, must be downgraded to be compact and light for the water-cooled SPTEG. The proposed SPTEG demonstrates advantages, such as avoiding installation of the heavy and large finned heat sinks on the TE modules, over the air-cooled SPTEGs.
Another advantage is the possible CHP. The heated air from the radiator supplies clean heat. Moreover, the radiator can be placed indoor, while the stove is running outdoor, thereby ensuring no pollution indoor. However, the CHP is not focused on in the present work.
3.2. TE Efficiency
The TE efficiency can be determined by the following equation:
where
Qout is the heat flux from the cold end. The TE efficiency measures the electric power conversion ratio with respect to the heat energy absorbed by the TE modules. Therefore, the TE efficiency is not based on the heat energy released by the fuel. This condition implies that the heat collector is vital in a TEG. In Equation (1), the essential problem becomes the determination of the heat dissipation rate by the cooling water, which can be derived in accordance with the following equations:
where
m is the mass flow rate of cooling water and is measured using the weighing method, that is,
m = 0.0195 kg/s on average (0.950 kg, 1.195 kg, and 1.380 kg of cooling water in 50, 60, 70 s, correspondingly). The inlet and outlet water temperatures are 31.5 °C and 38.0 °C, respectively. Consequently, the heat dissipation rate by the cooling water is 532.4 W. The natural convection and thermal radiation heat loss to the surroundings can be estimated by:
where the natural convection heat transfer coefficient (
h) is estimated to be 4.5 W/m
2·K [
30], whereas the emissivity of the aluminum alloy (
ε) is estimated to be 0.25 [
30].The effective area of the heat sink is 0.0128 m
2, and the heat loss is estimated to be 5.34 W. The total heat flux to the cold end can be obtained as follows:
The total electric power, that is,
Ptot =12.9 W, is demonstrated in
Figure 3. Therefore, the TE efficiency is calculated to be 2.34% at a temperature difference of 119 °C. The measured data for the TE efficiency are displayed in
Table 3.
The TE efficiency can be predicted theoretically by assuming that the figure-of-merit is independent of temperature [
31,
32].
where the figure-of-merit (
Z) is defined in Equation(6).
where
α is the Seebeck coefficient,
ρ is the electrical resistivity, and
k is the thermal conductivity.
r is the thermal contact ratio,
w is the ratio of ceramic thickness to TE leg, and
n is the electrical resistivity ratio. For the TE module adopted in the present work,
w=0.516.
r and
n are estimated to be 0.2 and 0.1, respectively and are used widely for the TE modules with ceramic substrates and type-A configuration [
32,
33].
L is the length of the TE leg. The figure-of-merits of the P- and N-type TE legs are 1.62 × 10
−3 K
−1 and 1.36 × 10
−3 K
−1 for the present TE module, respectively.
Figure 5 presents the influence of
Z and
L on the TE efficiency. As shown in
Figure 5, the length of thermoelectric leg has a minor influence on the TE efficiency, while the figure-of-merits is very important to the TE efficiency. In the present work, the TE efficiency is calculated to be 2.57%, which is consistent with the experiment data (2.34%). There are several possible ways to improve the TE efficiency. First, thermoelectric materials with higher
ZT value should be developed [
4,
5,
6,
7]. Second, increasing the hot end temperature to 200 °C while maintaining the cold end temperature unchanged results in the TE efficiency of 3.26% (an improvement of 26.8%). However, the aging problem has to be taken into consideration. Third, increasing the radiator size can lower the cold temperature which helps to improve the TE efficiency, yet the additional electric power consumption has to be balanced due to the added flow resistance and cooling fans.
In Equation(5), the temperature difference is an important parameter for increasing the TE efficiency in addition to the figure-of-merit, thereby implying that the hot end (heat collector) and the cold end (heat sink) should be optimized. In the present work, the heat collector performs well, and its features are as follows:
- (1)
The heat collector can incorporate a relatively large number of TE modules (eight TE modules in the present work) into a stove while controlling the hot end temperature within a reasonable range, distributing the temperature evenly, and avoiding possible gaps between the TE modules and the heat collector [
16].The present heat collector is already in a plate shape, which indicates that no heat spreading plate is required and that gaps can be eliminated from the design. The temperature distribution and interface heat flux are guaranteed by the large thermal conductivity of copper and the symmetry design.
- (2)
This device avoids large differences of power generation in different TE module groups. This function is important to exert the potential of every TE module. The present heat collector is straightforward but is an appropriately designed copper plate, which ensures that the power generation performance from the two TE module groups is close to each other.
The heat sinks in the present work are regular multi-channel (M shaped channel) heat sinks, which remain to be optimized. However, decreasing the cold end temperature to a low value is unnecessary because the cooling water can be used in a CHP application (supplies clean warmed airs). Furthermore, a multi-stage arrangement of the TE modules [
34,
35,
36] is another means of increasing the TE efficiency.
Limited SPTEG studies have performed TE efficiency testing.
Table 4 displays the details of the TE efficiency in the SPTEG studies. In
Table 4, the TE material is clearly the major parameter (
ZT) to determine the TE efficiency. All SPTEGs have low TE efficiency. However, only a few previous SPTEG studies have conducted such a test in addition to Montecucco’s work [
26].
In the present work, the hot end temperature (159 °C) is 41 °C lower than the long-term working limit of the TE module (200 °C), which is the result of thorough optimizations. The experiments reported in the present work were conducted on cold winter days, but the SPTEG should work normally on hot summer days. The ambient air temperature affects the hot end temperature because the heat dissipation is achieved by the radiator, which works with ambient airs. On hot summer days with the air temperature at approximately 40 °C, the hot end temperature of the SPTEG reaches the long-term working temperature limit of the TE module.