# Analysis of the Behavior of the Series Connection in Small-scale Hybrid Photovoltaic-Thermoelectric Energy Harvesting Systems

Mashiul Huq, Slavisa Jovanovic, Philippe Poure

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Mashiul Huq, Slavisa Jovanovic, Philippe Poure. Analysis of the Behavior of the Series Connection in Small-scale Hybrid Photovoltaic-Thermoelectric Energy Harvesting Systems. JNRSE 2025 - 13th National Days on Energy Harvesting and Storage, Jun 2025, Le Bourget Du Lac, France. hal-05448792

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# Analysis of the Behavior of the Series Connection in Small-scale Hybrid Photovoltaic-Thermoelectric Energy Harvesting Systems

Mashiul HUQ¹, Slavisa JOVANOVIC¹*, Philippe POURE¹,

¹Institut Jean Lamour, Campus Artem - 2 allée André Guinier - BP 50840 54011 Nancy Cedex

*Corresponding author e-mail: philippe.poure@univ-lorraine.fr

***Abstract***—Photovoltaics (PV) cannot utilize the entire solar spectrum to produce electrical energy. As a result, there are losses in the form of heat energy. Valorization of this heat energy can be carried out through thermal coupling of the photovoltaic cell with a thermoelectric generator (TEG), creating a hybrid PV-TE device. To fully maximize the potential of this PV-TE device, an electronic interface must be incorporated with the device to harvest energy at the maximum power point. The design of this interface is influenced by the electrical connection applied between the two sources. The possible electrical connections that can be applied are the parallel, electrically isolated, and series connection. A detailed analysis of a series connected PV-TE device is performed. The results indicated that the series connected PV-TE device could only be useful at low irradiance levels.

***Index Terms***—energy harvesting, hybridization, electrical connection, series connection, maximum power point tracking

## I. INTRODUCTION

Photovoltaics (PV) operate on the basis of the photoelectric effect [1]. Therefore, electrical energy can be generated through the interactions of photons and the material of the PV. Not all of these interactions are successful, and losses in the form of heat are inevitable. This heat can be utilized with the help of a thermoelectric generator (TEG), where its hot side is thermally coupled with the PV. The TEGs function on the basis of the Seebeck effect [2], which converts heat energy to electrical energy. To maximize the potential of this energy conversion at small-scale, the PV and TEG can be coupled into a hybridized PV-TE device. To optimize the performance of this PV-TE device, it must be combined with an electronic interface as shown in Fig. 1

<figure><img src="image_2.png" /><figcaption>Fig. 1. PV-TE System [3]
Funded by ANR Project HYDRES (ANR-21-CE50-0003)</figcaption></figure>

The electronic interface is responsible for the maximum power point tracking (MPPT), but the design of this interface depends on the electrical connection between the two sources. The possible electrical connections are the parallel, electrically isolated, and series. In this study, these possible electrical connections will be briefly analyzed in Section II. As this study will focus more on the series connection, a deeper analysis of the series connection will be conducted in Section III. Following this, Section IV will discuss simulations done in the MATLAB/SIMULINK environment based on the findings of the previous sections. Finally, key points and conclusions of the study will be elaborated in Section V.

## II. POSSIBLE ELECTRICAL CONNECTIONS

In this section, possible electrical connections for hybrid PV-TE devices are analyzed.

### A. *Parallel*

In the parallel connection, the voltage across each parallel branch must be the same. With this constraint, both PV and TEG will not be able to operate at their maximum power point (MPP) due to their very different characteristics, considering that each source would have similar dimensions if coupled. Naturally, the PV acts like a current source, while the TEG acts like a voltage source. The literature also has stated the difficulty of use of the parallel connection [4].

### B. *Electrically Isolated*

The electrically isolated connection is where the PV and TEG are not electrically connected with each other. This makes the control of the electronic interface very simple as both sources can operate at their respective MPP at the same time, but requires two DC-DC converters. This can be considered as a reference scenario when compared to the other connections and will be considered as such in the simulations that will be discussed later in this study. As a result, this type of connection for hybrid devices can be found in the literature [3].

### C. *Series*

The series connection has a similar drawback to the parallel connection, but instead of voltage being equal, the current in this connection is equal. Therefore, like the parallel connection, both sources cannot operate at their MPPs at the same time. Regardless of this glaring setback, the series connection
is commonly found in the literature [3]. Perhaps, this is because of the benefit of requiring just one DC-DC converter in its electronic interface compared to the multiple converters required for the parallel and electrically isolated connection.

### III. ANALYSIS OF THE SERIES CONNECTION

One may expect that a combination of two sources would always produce more power compared to a single source, but this is not the case with the series connection. This phenomenon has been highlighted by experimental results in the paper presented by Park et al. [5].

It can be observed in Fig. 2 that the PV and TEG are connected in series. The current produced by the PV flows through the internal resistance of the TEG. This will create an unavoidable voltage drop at this resistance. The implications of this voltage drop on the overall system can be seen on the right side of Fig. 2.

The right side of Fig. 2 illustrates the percentage difference in power produced by the PV-TE and a single PV. It can be seen, based on the components used in their experiment and the experimental conditions, that with irradiance values less than 600W/m² the hybrid device will perform better than a single PV, but will perform worse than a single PV for irradiance values greater than 600W/m².

<figure><img src="image_3.png" /><figcaption>Fig. 2. Electrical modeling of the Hybrid PV-TE (left) & Percentage Change in Power compared to a Single PV (right) [5]</figcaption></figure>

### IV. SIMULATIONS

To illustrate what was in practice in the paper presented by Park et al., simulations were carried out in the MAT-LAB/SIMULINK environment. The simulations were performed using the parameters of actual commercially available components, but were designed for small-scale applications. This illustrates that this phenomenon can occur for both large and small-scale implementation.

The PV was modeled based on the component SM141K10LV by ANYSOLAR, and the TEG was modeled based on the component GM250-241-10-16 by European Thermodynamics. The temperature gradient maintained across the TEG was 5°C. Simulations were carried out with different irradiance values, and the corresponding behavior of the PV-TE device was observed compared to a single PV, as well as the reference case where both sources can operate at their MPP at the same time. The results at 400W/m² (left) and 800 W/m² (right) can be seen in Fig. 3.

It can be observed that at an irradiance level of 400W/m², the PV-TE device (122.2mW) performs slightly better than

<figure><img src="image_4.png" /><figcaption>Fig. 3. Power vs Voltage at 400W/m² (left) & Power vs Voltage at 800W/m² (right)</figcaption></figure>

the single PV (118.8mW) at their respective MPP, but just below the reference value (122.6mW). At an irradiance level of 800W/m², the reverse can be seen. The single PV (237.7mW) produces more power compared to the PV-TE device (231.8mW), and both still produce less than the reference case (241.5mW).

### V. DISCUSSION AND CONCLUSION

In this study, the potential and working principle of a hybrid PV-TE device was discussed. The necessity of an electronic interface and how the electrical connection between the sources affects its design was also emphasized. Following this, the three different possible electrical connections (parallel, electrically isolated, and series) were briefly explained with a particular focus on the series connection. After the effects of the series connections were expressed in more detail, the results of simulations articulating the behavior the series connections were shown.

It is clear from the work of Park et al. [5] and the simulations that the series connection has very apparent disadvantages. A series connected PV-TE device can produce more power compared to a single PV at low levels of irradiance but never at the level of the reference case as both sources can never operate at their respective MPP at the same time. The PV-TE device performs worse than a single PV at higher irradiance levels. This is due to the voltage drop at the internal resistance of the TEG. It is much more significant at high irradiance levels, compared to the output power the TEG produces. Therefore, further research on the electronic interface is necessary to make the series connection suitable for PV-TE devices at all levels of irradiance.

### REFERENCES

[1] A. O. Ali et al., "Advancements in photovoltaic technology: A comprehensive review of recent advances and future prospects," Energy Convers. Manag. X, vol. 26, p. 100952, Apr. 2025

[2] N. Jaziri et al., "A comprehensive review of Thermoelectric Generators: Technologies and common applications," Energy Rep., vol. 6, pp. 264-287, Dec. 2020

[3] M. Huq et al., "Critical analysis of optimized energy harvesting at small-scale by thermally coupled photovoltaic-thermoelectric systems," Renew. Sustain. Energy Rev., vol. 195, p. 114331, May 2024

[4] J. Zhang and Y. Xuan, "The electric feature synergy in the photovoltaic-thermoelectric hybrid system," Energy, vol. 181, pp. 387-394, Aug. 2019

[5] Y. Park et al., "Effect of net voltage of thermoelectric generator on performance of hybrid energy device," Energy Rep., vol. 6, pp. 2836-2840, Nov. 2020