Improving thermoelectric energy harvesting efficiency by using graphene

This study is aimed at enhancing the efficiency of a thermoelectric (TE) energy harvesting system by using a thick graphene layer. This method is a simple yet effective way to increase the temperature gradient across a conventional TE module by accelerating heat dissipation on the cold side of the system. Aqueous dispersions of graphene were used to prepare a 112-μm thick graphene layer on the cold side of the TE system with aluminum as the substrate material. The maximum efficiency of the proposed system was improved by 25.45 %, as compared to the conventional TE system, which does not have a graphene layer. Additionally, the proposed system shows very little performance deterioration (2.87 %) in the absence of enough air flow on the cold side of the system, compared to the case of the conventional system (10.59 %). Hence, the proposed system, when coupled with the latest research on high performance TE materials, presents a groundbreaking improvement in the practical application of the TE energy harvesting systems.

This study is aimed at enhancing the efficiency of a thermoelectric (TE) energy harvesting system by using a thick graphene layer.This method is a simple yet effective way to increase the temperature gradient across a conventional TE module by accelerating heat dissipation on the cold side of the system.Aqueous dispersions of graphene were used to prepare a 112-µm thick graphene layer on the cold side of the TE system with aluminum as the substrate material.The maximum efficiency of the proposed system was improved by 25.45 %, as compared to the conventional TE system, which does not have a graphene layer.Additionally, the proposed system shows very little performance deterioration (2.87 %) in the absence of enough air flow on the cold side of the system, compared to the case of the conventional system (10.59%).Hence, the proposed system, when coupled with the latest research on high performance TE materials, presents a groundbreaking improvement in the practical application of the TE energy harvesting systems.C 2016 Author(s).All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).[http://dx.doi.org/10.1063/1.4953237]Thermoelectric (TE) energy harvesting is one of the most promising energy harvesting technologies because of its several attractive features such as durability, low maintenance, no moving part and high reliability. 1Researchers have even proposed to harvest human body heat with TE systems, which in itself highlights the practicability of these systems. 2 However, the practical application of TE modules for energy harvesting has been limited owing to its limited efficiency.For most of the temperature ranges, the TE efficiency of commercially available systems is considerably less than the Carnot efficiency. 1 A lot of researches are done on improving the efficiency of TE materials in the last few decades; 3 but most of the reported TE efficiencies are significantly less than other energy harvesting systems.Therefore, more research is needed to improve the environmental and temperature conditions of the TE systems, so that the TE energy harvesting could be made more efficient.
The concept of increasing heat spread by using the graphene was first proposed by Subrina et al. 4 and it was also later applied for thermal management of high-power GaN transistors. 5On the other hand, we propose the use of a graphene layer on the heat dissipation surface in order to enhance the heat transfer across the TE module.The proposed system basically improves the heat flow from solid medium to the gaseous medium (i.e., the air) which in turn increases temperature difference across the TE module.Since the temperature conditions have a prominent effect on the system performance and efficiency, this increased temperature gradient should improve the system efficiency significantly, which is shown in this study.
A graphene layer basically helps to enhance heat dissipation, by spreading the heat in lateral direction. 6It means that even if inward heat flux to the graphene layer is located at the specific location; the heat energy is very quickly distributed across the area of the graphene layer, which in a Electronic mail: kih119@kaist.ac.kr b Electronic mail: hjung@kaist.ac.kr turn allows heat energy to be dissipated more quickly as compared to the case with no graphene layer.Figure 1 explains this phenomenon by showing the heat flow.Figure 1(a) shows a TE module sandwiched between aluminum layers with no additional layer on the cold side.As shown in figure 1(b), it is evident that as the heat flow touches the graphene layer, it is evenly distributed to the complete surface area of the graphene layer; in contrast to no graphene case, where the heat does not travel to the farther end of the aluminum plate.Hence, the total area of the hot surface exposed to the air is significantly increased, which in turn leads to increased heat dissipation from the cold side of the TE module and also improves the TE efficiency of the system.In figure 1(a), the color of heat dissipation arrows represents the magnitude of heat dissipation; red representing the most heat dissipation and yellow the least.Similarly, in figure 1(b) the color of arrows is the same, highlighting the fact that heat dissipation is the same across the total area of the graphene layer, due to its high thermal conductivity.
The nano-scale materials such as single layer graphene (SLG) are quite popular among researchers for various properties. 7,8Graphene laminates are specifically known to have exceptionally high thermal conductivity. 9According to initial investigations for properties of suspended graphene, the thermal conductivity of graphene was much higher than the known value for the solid graphite. 10

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Usman, Kim, and Jung AIP Advances 6, 055027 (2016) Moreover, graphene is quite favored for practical applications, due to the possibility of inexpensive production as liquid phase exfoliated (LPE) graphene and flexibility in its use as filler material.
The researchers have spent a lot of time and efforts trying to develop the large-scale graphene films while maintaining the high thermal conductivity. 6Li et al. 11 developed aqueous dispersions of chemically converted graphene sheets without using any polymeric or surfactant stabilizers.A similar water based product, developed by MExplorer Co., Ltd., by the name of adhesive conducting graphene paste (AC G-paste), 12 is used in this study to make the graphene layer.The AC G-paste basically contains graphene flakes of thickness less than 10 nm and lateral size ranging from several µm to tens of µm.
The TE effect, more commonly known as the Seebeck effect, is defined by the generation of electric potential across two points with a significant temperature gradient, when they are connected by two different metals.Seebeck coefficients of different materials differently vary with increasing temperature.Hence, a suitable pair of materials is crucial for high efficiency of the TE system.However, the major hurdle in the way of its more extensive application is its limited efficiency.For TE systems, the maximum efficiency is given by where T H and T C are the temperatures at the heat source and at the heat sink, respectively, and T is the mean temperature.Here, Z, the non-dimensional figure of merit (FOM), is given by where σ is the electric conductivity, S is the Seebeck coefficient and κ is the thermal conductivity of the TE module.Conventionally, the FOM has been considered as a substitute for the system efficiency.
However, the previous study 3 shows that this FOM is not the exact measure of the system performance, instead the maximum efficiency was recommended as a critical parameter.This is because FOM only considers the material properties of the TE material, whereas maximum efficiency also includes more important factors such as temperature gradient and absolute temperature etc.A heat transfer simulation was carried out using a commercial software COMSOL for both the cases of conventional system and graphene coated surface on the colder side of the system.During the simulation, the graphene layer of of 112-µm thickness was considered from figure 4 and the thermal conductivity of graphene of 1100 W/m − K as reported by Ahn et al. 13 was used.The values of temperature on the cold and hot sides of the TE module, were then substituted in equation ( 1) to calculate the maximum efficiency of the TE system with module properties from table I. Figure 2  shows the comparison of efficiencies for the conventional TE system and the proposed system with graphene coated surface.As shown in the graph, the maximum efficiency is significantly increased by addition of a graphene layer on the cold side of the TE module.A lab scale test was carried out, in which on one side of the TE module, an electric heater was installed to provide the inflow of heat, while on the other side, an aluminum plate was used.This aluminum plate was kept uncoated for conventional system and was coated with graphene layer for the proposed system.The experimental setup is shown in figure 3. The inset shows the detailed view which is hidden in the main picture.The TE module (PGM-15-15-220) developed by HTRD Ltd. 14 was used and the AC G-paste was used to prepare a graphene layer on the aluminum surface.The graphene layer was then dried at 90 • C for 6 hours as specified by the manufacturers.This caused the liquid matrix, i.e., water, to dry out and leave pure graphene in the layer.Figure 4 shows the SEM images of the graphene layer, whose thickness was 112-µm.The figure depicts the uniform graphene layer which is well attached to the aluminum substrate.Inset shows the compactness of the graphene layer, whereas continuity is evident in the cross section, which suggests it will exhibit good thermal conductivity.K-type temperature sensors were used to measure the temperature on both sides of the TE module.The temperatures measured on the cold and hot sides of the TE module and the equation (1) were used to calculate the system efficiency with constants taken same as those used for simulations.Figure 5(a) shows the effect of the proposed system and the absence of a fan on the maximum system efficiency.It is evident that the efficiencies of the proposed system (red lines) are much higher than those of the conventional system (blue lines).Additionally, the effect of the absence of a fan on the cold side was studied for both the conventional and proposed systems.Here, the presence of a fan signifies the importance of air flow which spreads the heated air to accelerate the heat dissipation. 15However, as explained earlier in figure 1, the graphene coating itself helps to spread the heat over the entire surface area; therefore, the independence from the flowing air is another advantage of the proposed system which has been ratified by the results shown in the figure 5(a).The absence of a fan has no major effect on the proposed system while it reduces the efficiency of the conventional system significantly.For the conventional system without a fan, we can see the peak efficiency at much lower temperature than the proposed system, which means that at that point the heat dissipation has stopped because of no air flow on the cold side and deceleration of heat flow across the TE module.There is a discrepancy in the efficiency improvement of the experiment results compared with those from simulation, which can be attributed to the fact that air flow conditions could not be simulated.Additionally the temperature difference for the proposed system without a fan is marginally higher than with fan case, but this doesn't affect the efficiency because of higher absolute temperature.
Table II shows the maximum values of the efficiency and temperature gradient, for the system with and without graphene.It is evident from table II that the maximum efficiency and maximum temperature gradient were increased significantly for the system with graphene.The percentages in brackets show the percentage improvement compared to the conventional system.By comparing the values of efficiencies with and without a fan, it is clear that the percentage improvement for the case without a fan is significantly larger than the case with a fan.The percentage of reduction in maximum efficiency because of the absence of a fan in the conventional system is 10.59 %, whereas for the proposed system it is only 2.87 %.This again shows that in the proposed system, the absence of blowing air on the cold side of the system does not have any significant effect on the system efficiency.Hence, we can conclude that the proposed system is more efficient and stable as  compared to the conventional system, because the availability of air flow is not possible in most of the practical systems.As explained earlier in equation ( 1), the maximum efficiency of the TE system is dependent upon the TE FOM, which is characterized by the material properties of the TE module.Since the proposed system lies outside the TE module, the most important parameters is the temperature gradient across the TE module.Figure 5(b) shows the improvement in this particular parameter as well.This increase in temperature gradient is mainly because of increased heat dissipation on the cold side of the system.The temperature reduction on the cold side of the TE module for the with fan case is around 15.9 • C, while the temperature gain on the hot side is 34.5 • C, resulting in an increased temperature difference of 50.4 • C across the TE module.The similar applications in the previous studies 4,5 had shown the temperature reduction of around 20 • C to 40 • C.
In summary, we have reported a TE energy harvesting system, with added graphene layer, on the cold side of TE module.In addition to improved system efficiency, the simplicity, ease of application and independence from air flow are the main attractive features of the proposed system.As we have used the aqueous processable graphene to prepare the graphene layer, it is easy to apply in systems with more complicated geometry.The improved system efficiency can be attributed to the quicker heat flow, caused by the increased heat dissipation, on the surface of the graphene film.The proposed system increased the temperature difference across the TE module, by at least 50 • C and the maximum system efficiency was improved by 24.45 %.The effect of air flow on the cold side of the system was also studied for the proposed and the conventional systems.The improvement in proposed system efficiency was not affected by the absence of air flow, where the absence of air flow affected the conventional system, much more than the proposed system.
FIG. 5. (a) Maximum thermal efficiency against temperature (b) Temperature difference across the TE module against time.

TABLE I .
Properties of TE module and AC G-Paste.

TABLE II .
Effect of the proposed system and the deterioration of performance because of the absence of a fan.