# Comparative Analysis of Common MPPT Techniques for Thermoelectric Generators in Small-scale Energy Harvesting Applications

Mashiul Huq, Slavisa Jovanovic, Philippe Poure

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Mashiul Huq, Slavisa Jovanovic, Philippe Poure. Comparative Analysis of Common MPPT Techniques for Thermoelectric Generators in Small-scale Energy Harvesting Applications. 2024 31st IEEE International Conference on Electronics, Circuits and Systems (ICECS), Nov 2024, Nancy, France. pp.1-4, 10.1109/ICECS61496.2024.10848770 . hal-04955237

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# Comparative Analysis of Common MPPT Techniques for Thermoelectric Generators in Small-scale Energy Harvesting Applications

Mashiul Huq, Slavisa Jovanovic, and Philippe Poure

Université de Lorraine, CNRS, IJL, F-54000 Nancy, France
{mashiul.huq, philippe.poure, slavisa.jovanovic}@univ-lorraine.fr

**Abstract**—The thermoelectric generator (TEG) is a very useful energy harvesting device that operates on the basis of the Seebeck Effect and can convert thermal energy into electrical energy. Due to its compact size and usefulness in small-scale applications, it has lots of potential in self-powered internet of things (IoT) devices. These devices are in high demand, and to be sustainable, they must be in line with green IoT principles. To cater to a wide range of green IoT devices, an array of TEGs can be used. The main problem that can occur is temperature mismatch within the TEGs of the array. If not suitably managed, the maximum potential of this energy source cannot be utilized. To compensate for this, an electronic interface with maximum power point tracking (MPPT) can be utilized. This study will propose a potential electronic interface and compare two commonly used MPPT techniques to judge their suitability for small-scale energy harvesting applications.

**Index Terms**—energy harvesting, MPPT, hysteresis control, electronic interface, Fractional Open Circuit Voltage, Perturb and Observe

## I. INTRODUCTION

The projected forecast of the IoT (Internet of Things) market is massive (1). This rapid growth entails a large amount of energy to fulfil this demand. This is not an easy task in itself, but what makes this even more difficult is that many of these IoT devices are used in remote locations with limited or no access to the electrical grid. Therefore, there is a need for self-powered, autonomous IoT devices. The concept of green IoT was made in response to this need. An energy harvesting device that has shown promise due to its innate characteristics and also supports the goals of green IoT is the thermoelectric generator (TEG) (1).

The TEG is a device that can convert thermal energy into electrical energy where the magnitude of the energy generated is proportional to the temperature gradient across the device and the material the TEG is made from (2). It is currently being applied in many applications (3), varying in degree of scale, but it has shown tremendous promise in small-scale applications such as energy harvesting to power IoT devices (1). To expand the capability of the TEG, it can be connected in an array to be able to power more demanding small-scale applications.

This may seem like a straightforward solution, but one problem that may occur within the array is that the temperature gradient of each TEG within the array may differ. This will

cause a temperature mismatch and cause the maximum power point (MPP) of some of the TEG modules within the array to move. This manifestation is illustrated in Fig. 1. It can be observed that even a small difference in temperature gradients can produce a significant change in the MPP.

<figure><img src="image_2.png" /><figcaption>Fig. 1. MPPs of Temperature Mismatched TEG Array</figcaption></figure>

To counter this, an electronic interface is necessary for global maximum power point tracking (MPPT). The major components of this electronic interface are the type of DC/DC converter (4) and the MPPT technique being utilized with its associated control mechanism (5). Additionally, the type of electrical connection applied within the array will also have an impact on the electronic interface’s design.

In this study, an electronic interface will be proposed applying an electrically isolated connection in Section II. In Section III, the specific MPPT techniques under investigation will be defined and clarified. Next, a comparison between the examined MPPT techniques via simulations will be carried out in Section IV. Finally, the study will conclude in Section V with a summary of the findings from the analysis.

## II. ELECTRONIC INTERFACE

In this section, the various types of possible electrical connections will be briefly discussed, followed by a more detailed explanation regarding the electrically isolated connection. The specific advantages and disadvantages of the electrically isolated connection will be highlighted and will make up the basis of the proposed electronic interface. Finally, the proposed electronic interface will be discussed along with its working principle.
A. *The Electrically Isolated Electrical Connection*

Along with the electronic interface, there are three possible
electrical connections to implement. These are the series,
parallel, and electrically isolated connection. In the series
connection, the current through the sources is the same. In
the parallel connection, the voltage across the sources are
the same. In case of a temperature mismatched TEG array,
this is a problem because if either the voltage or current is
constant then not all of the TEGs can operate at their MPP
(see Fig. 1) (6). Multiple modifications and additions would
be needed to an electronic interface to allow MPPT with these
electrical connections. On the contrary, this is not the case for
the electrically isolated connection.

For the electrically isolated connection, each TEG in the
array is connected to its own dedicated converter. This is
beneficial as each TEG, regardless of where their MPPs
are located, can operate at its MPP. The drawback of this
connection is that if there are n number of TEGs, then n
number of converters are required. In terms of small-scale
applications, the electronic interface may become too large
if there are many TEGs in the array.

B. *Proposed Electronic Interface*

There may be a way to harvest energy from all TEGs in an
array at their MPPs, by applying time division multiplexing
(TDM). TDM is a method of transmitting independent signals
across a common signal path. The topology would look similar
to Fig. 2. The multiplexer would be used for source selection,
and one TEG at a time would be connected to the boost
converter where MPPT can take place and operate at its
specific MPP. While this transpires, the other TEGs transfer
energy to the capacitors placed in parallel with them until their
corresponding switch is closed. Therefore, energy harvesting
occurs the same way for each TEG in the array.

<figure><img src="image_3.png" /><figcaption>Fig. 2. Proposed Electronic Interface with a Single Converter</figcaption></figure>

Then, if energy harvesting by one TEG is considered, the
circuit of the proposed electronic interface can be reduced to
that shown in Fig. 3. The parameters of the electronics within
the circuit are given, as well as the characteristics of the TEG.
These parameters were used in the simulations that will be
discussed in Section 4.

<figure><img src="image_4.png" /><figcaption>Fig. 3. Equivalent Circuit of Proposed Electronic Interface for a Single TEG</figcaption></figure>

III. MAXIMUM POWER POINT TRACKING

In this section, the MPPT techniques being compared in this
study will be introduced and their operation will be clarified.
Their respective control mechanisms will also be elaborated
upon.

A. *Perturb and Observe*

The first MPPT technique under investigation regarding
its suitability to be applied to a temperature mismatched
TEG array is the perturb and observe (P&O) method. Fig. 4
illustrates how P&O works using the characteristic curve of
a TEG as an example. The ultimate goal is to operate at the
MPP, but the first step would be to identify where on the
curve the operating point is. Depending on where that point
is, the voltage is adjusted to slowly pull the operating point
toward the MPP. Fig. 5 shows the operational flowchart of

<figure><img src="image_5.png" /><figcaption>Fig. 4. Working Principle of P&O (black) and FOCV (color)</figcaption></figure>

P&O. The first step is to measure the voltage and current.
This is needed to calculate power. Afterwards, the change
in power (ΔP) is calculated by subtracting its present value
with its previous value. The same process is done with voltage
(ΔV). Depending on these results, the location of the operating
point as well as its necessary direction of movement can be
identified. The movement is carried out by adjusting the duty
cycle by a fixed variation (ΔD) and applying it to the switch.
This process is repeated until the operating point reaches the
MPP. The time required to reach the MPP will depend on the
value of ΔD.

B. *Fractional Open Circuit Voltage*

The other MPPT technique under examination is the frac-
tional open circuit voltage (FOCV) method. FOCV takes
advantage of a linear relationship between the open circuit
<figure><img src="image_6.png" /><figcaption>Fig. 5. Working Principle of P&O (7) (8)</figcaption></figure>

voltage (OCV) and the voltage at the MPP ($V_{mpp}$). Due to the symmetrical nature of the characteristic curve, the $V_{mpp}$ is equivalent to half of the open circuit voltage for a TEG. This relationship is displayed in color in Fig. 4. The upper operational band (UB) and the lower operational band (LB) are also shown and are important in the control mechanism which is hysteresis control (9).

Fig. 6 showcases how hysteresis control is applied. In the first step, the OCV is first measured. Following this, the UB is formed by multiplying the OCV with 0.5, which is $V_{mpp}$, and then multiplying that with 1.03. Therefore, the UB is located at a voltage level just 3% higher than $V_{mpp}$. The LB is made in a similar way but is located at a voltage level 3% below $V_{mpp}$ by multiplying the $V_{mpp}$ with 0.97 instead of 1.03. This percentage can be altered if desired, but 3% was selected as a compromise between limiting switching losses and not deviating too far from the MPP.

Once the UB and LB have been established, the operational points will only be within these two thresholds. To maintain operation within these two bands, a simple comparison is made with the voltage across the capacitor (refer to Fig. 3) $V_{TEG}$ and with the UB and LB respectively. If $V_{TEG}$ becomes equal or greater than UB, the switch at the converter (MSc) is closed. If $V_{TEG}$ becomes equal or less than LB, MSc is opened.

<figure><img src="image_7.png" /><figcaption>Fig. 6. Control Flowchart for FOCV</figcaption></figure>

## IV. COMPARISON BETWEEN P&O AND FOCV

This section will compare the MPPT techniques P&O and FOCV with the assistance of simulations made in MATLAB/SIMULINK. The parameters set to the TEG were based on Fig. 3. For easier comparison of voltage waveforms, the

UB and LB normally meant for hysteresis control in FOCV, were kept with the voltage waveforms for P&O. They have no influence on the simulations for P&O.

### A. Operational Comparison

Fig. 7 shows the voltage waveform at steady state when using FOCV and P&O respectively. From the voltage waveforms, it can be seen that the voltage has a similar shape. The only noticeable difference is that for P&O, it operates slightly lower than the UB and slightly higher than the LB. The waveforms shown for P&O applied a $\Delta D$ value of $0.1\mu$ and $1\mu$. Both methods produced the same average power of 33.9mW. This is the same as the theoretical power calculated and are consistent with the characteristic curve shown in Fig. 4.

<figure><img src="image_8.png" /><figcaption>Fig. 7. Comparison of P&O and FOCV</figcaption></figure>

### B. Settling Time

It has already been observed that both MPPT techniques behave in a very similar manner when they are in steady state, but there are other noticeable differences. One difference that can be perceived is the settling time necessary to reach steady state which is shown in Fig. 8. In case of FOCV, the settling time was 23ms. This time is necessary as the capacitor voltage must reach the open circuit voltage value as sampling the $V_{oc}$ is a necessity to initialize the UB and LB. In the case of P&O with a $\Delta D$ of $0.1\mu$, the settling time was 28ms which is longer than that of FOCV. For P&O, there is no need for the voltage to rise to the $V_{oc}$, therefore the voltage begins its decent toward the MPP much quicker than FOCV. The settling time of P&O can be reduced by increasing the value of $\Delta D$, as shown for $\Delta D$ with a value of $1\mu$. However, there is a trade-off, as the value $\Delta D$ becomes larger, the voltage waveform losses more of its stability and begins to distort even at steady state which may result in a loss of power.
<figure><img src="image_9.png" /><figcaption>Fig. 8. Comparison of P&O and FOCV in terms of Settling Time</figcaption></figure>

### C. Possible Losses

The last observation to be noted is the frequency of operation which is important to consider in case of switching losses. Fig. 7 also displays the number of complete voltage cycles for the same time period for both FOCV and P&O ($\Delta D=0.1\mu$ and $\Delta D=1\mu$). The number of complete cycles is lower for FOCV and becomes higher for P&O with a lower value of $\Delta D$. As a result, switching losses for P&O are more substantial compared to FOCV.

## V. DISCUSSION AND CONCLUSION

In this study, the possible electrical connections for an electronic interface designed for small-scale energy harvesting applications were first briefly discussed. From this discussion, it was determined that the electrically isolated connection had the most potential but had a drawback of requiring a DC/DC converter for every TEG in an array. Therefore, an electronic interface that only required one DC/DC converter applying TDM was proposed.

Next, two MPPT techniques (P&O and FOCV) were defined and explained. P&O required both current and voltage readings to calculate power its opertional rate and accuracy are dependent on the $\Delta D$ value. In case of FOCV, only the OCV value is required. The UB and LB are made correspondingly and operation only persists within these thresholds ensuring that operation only takes place near or at the MPP.

Afterwards, both MPPT techniques were compared with each other. They both had average powers corresponding to the theoretical MPP based on the conditions of the simulation (Fig. 3). In case of P&O, operation was slightly closer to the MPP compared to FOCV, but due to the flat shape of the characteristic curve of the TEG near the MPP, it produced a negligible benefit in terms of power. In case of settling time

for both techniques, FOCV had a slightly shorter settling time compared to P&O. However, the settling time for P&O can be shortened if $\Delta D$ is increased, but there is a compromise with stability and precision which may decrease power.

Finally, in terms of possible losses, the most apparent based on the waveforms shown are potential switching losses. It was showcased that in the same time period, there are more complete cycles for P&O. Therefore, it operates at higher frequency compared to that of FOCV. The difference is even more significant applying a higher value of $\Delta D$.

These losses in addition to the computational demand of constantly comparing the present and previous values for both power and voltage become significant for P&O. Additionally, P&O requires both voltage and current values to function. While for FOCV, only voltage values are needed, specifically OCV, FOCV also does not require constant computation, only the initial computation to establish the UB and LB.

In conclusion, considering the data presented, FOCV seems to be a more suitable choice compared to P&O for small-scale energy harvesting applications. Although there are several merits to P&O, the losses within P&O are much more detrimental and cannot be ignored at small-scale.

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