Investigation into the use of thermoelectric modules as an alternative to conventional fluxmeters: Application to convective and radiative heat flux in buildings
Introduction
In various thermal engineering fields, there is an increasing need for predictive models to provide a more precise estimate of the real thermal stress values (superficial heat transfer and/or absorbed heat flux) to verify model-measurement reliability when accounting for such thermal stresses. These needs may arise, for instance, when calculating cooling or heating needs in order to maintain the internal thermal comfort in buildings, in the characterization of walls to determine the incoming and outgoing heat flux [[1], [2], [3], [4], [5], [6]], when estimating convective and radiative heat transfer in heat exchangers [7], and also when estimating convective heat transfer in drying ovens [2].
In this context, thermal engineers proposed a widely used technique for heat flux measurement more than 30 years ago, which is able to estimate heat flux through the surface (conduction) and at the surface (convection and radiation). This technique is based on the use of heat flux sensors (HFS), a conventional heat fluxmeter in the shape of a square flat-plate with black and shiny coatings. The black coated sensor for total heat flux (radiation + convection), and the shiny coated sensor for convective heat flux [7]. Note here that, in literature, such HFS have been identified by other names: Tangential gradient fluxmeter (TGF) [7].
The HFS was proven to perform sufficiently well in heat flux measurement, and in estimating convective and radiative heat fluxes. The use of a shiny HFS coated with thin aluminum foil was proposed for measuring the convective heat transfer coefficient on heavyweight walls [8]. These results agreed with values reported in the literature for the case of laminar free convection along a vertical and isothermal heated plate [8]. According to an experimental and numerical study carried out within a heat exchanger using black and shiny HFSs, a heat flux balance equation over the surfaces of each sensor along with the classical radiosity method was able to separate the convective and radiative parts [7]. 5% of relative error was found when comparing the experimental procedure to numerical calculations, concluding that by applying the experimental procedure it was possible to uncouple the convective and radiative parts from the heat flux measurement.
The use of thermoelectric modules (TEMs), also known as Peltier modules, for heat flux measurement, has become more common recently, basically due to the desirable characteristic of being far cheaper than HFSs. TEMs are composed of several thermocouple junctions connected electrically in series and connected thermally in parallel, integrated between two ceramic plates [9] (see Fig. 1). These thermocouple junctions consist of n- and p-type semiconductor materials connected by small, thin copper tabs; the most common semiconductor materials used are quaternary alloys of bismuth, tellurium, selenium, and antimony, e.g., Bi2Te3.
The thermoelectric properties of a TEM vary with the average temperature of the thermoelectric n-p junctions (usually called ‘elements’); generally, a polynomial correlation with second-order temperature terms is used. For a thermoelectric material of n- and p-type, the average value of the properties is used (value of n + value of p)/2 [9].
Although TEMs have not been employed as widely as HFSs for measuring heat flux, according to the literature, reported results show that the TEM works sufficiently well when estimating Solar radiation heat flux (in W·m−2), after finding a relative error of 4.8 ± 3.9% when comparing a black-coated TEM measurement with that of a pyranometer [10]. Conversely, it has been found that the TEM overestimates convective heat flux by a factor of about two [5,11]. Also, after comparing experimental results from heat flow detection through an external wall with a simulation, it was shown that the amount of heat flow detected through the wall was not identical to the amount that would be detected without the use of a TEM [12]. Two reasons were given to explain this difference: changes in the wall boundary conditions, and the effect of thermal contact resistance between the TEM and the wall surface.
However, since HFSs are more expensive than TEMs of the same size (for example, a 3 × 3 cm2 Captec® type HFS costs around 10 times more than a TEM of the same size), researchers have proposed an alternative technique to measure heat flux based on the use of a single-stage TEM. Although TEMs are not directly designed to measure heat flux [13], it is very interesting to use them because they present stronger thermoelectric power than a HFS of the same size and even than one of a bigger size. This therefore means, in principle, that weak thermal loads can be measured more accurately.
The response time of a TEM (about 1 min) is relatively slow compared to a HFS, due principally to the materials used in their manufacture. This duration can limit TEM implementation in slow processes. In buildings, however, since a significant change in the evolution of the envelope and indoor air temperatures can be detected in a couple of hours, indicating that significant variation in the thermal stresses also takes place over a similar duration, a heat flux sensor with a shorter response time is not essential.
There are still TEMs without ceramic plates, which have the advantage of eliminating the thermal resistance of the ceramic plate. However, they also have the disadvantage of mechanical fragility and they require electrical insulation. In contrast, the ceramic plates serve as good electrical insulation and provide high thermal conductance [9]. In the case of HFSs, their active zone of measurement and electrical connections is highly suggestive to oxidation of the external copper layers and it is fragile if not treated carefully, which can affect the heat flux measurement data adversely. Table 1 summarizes some advantages and disadvantages when comparing TEM with HFS.
Analyzing these relevant pros and cons for using the TEM instead of the HFS (Table 1) clearly indicates the use of the TEM as the best alternative to HFS for heat flux measurement, especially for estimating the convective and radiative heat flux on rigid surfaces. One of the most relevant pros is that the TEM fits the search for low-cost experimental procedures. This is closely followed by the greater robustness of the sensor manipulation, extremely low fragility, less need for special cleaning products or special technical assistance for their installation and finally, the ability to measure lower heat flux levels due to their high sensitivity.
Since in estimating the radiative and convective heat flux using a TEM an overestimation has been reported only on the convective part, a more detailed study is needed to understand the implications of this overestimation and to provide solid explanations. Thus, the aim of the present work is to propose a methodology for using a TEM to estimate the convective and radiative parts at the same time, and to correct the corresponding estimated convective part. For this correction, three critical TEM parameters are studied here, and compared with HFS, to look for an explanation for this overestimation: the influence of the TEM thickness, the influence of the energy stored in the TEM, and the influence of the calibration method employed. The next section presents the experimental setup and methods used in our study for the TEM characterization and calibration.
Section snippets
Experimental setup for heat flux measurements
First, to compare the TEM and HFS the TEM was adapted for the heat flux measurement by adding the elements presented in Table 1:
- i.
A type T thermocouple was installed inside the TEM, introduced between the semiconductor junctions. This was to measure the sensor temperature. This addition was made since the HFS is able measure the surface temperature.
- ii.
This thermocouple was coated with nail polish to electrically insulate its measurement point, thus avoiding any electrical perturbation.
- iii.
Silicon paste
Experimental and analytical approaches for decoupling the convective and radiative parts
The approach used to estimate the convective and radiative heat exchanges consists, on the one hand, of a combination of two sensors, capable of measuring both heat exchanges along with the air and surface temperatures, and on the other hand, of an analytical model which requires these measurements to estimate both heat exchanges.
Based on the experimental setup proposed by former researchers [2,7,8], the experimental setup used here consists of the following components (see Fig. 4):
- i.
A couple of
Validation of the use of the TEM
This section deals with the implementation of the proposed experimental setup for the heat flux measurement described in section 2 above. First, the case study where the TEMs are tested is described. Here, both the TEM and the HFS are installed in an in situ weak heat flux environment. Next, the measurements from the TEM and the HFS are compared.
Application of TEM in a controlled and strong heat flux environment
After verifying the proposed experimental and modeling approaches presented before, when using the TEM in the case of a weak convective and radiative heat flux, another experiment was performed to test our approaches in the case of a strong radiative heat flux environment. This strong heat flux environment is produced by a set of six halogen lamps, with available radiation intensity from 0 to 1380 W m-2. A black TEM and a shiny TEM (109 ± 7 V/Wm−2 of size 3 × 3x0.37 cm3) were placed on the
Conclusions
An experimental investigation to propose the use of a TEM as an alternative to a HFS to estimate the convective and radiative parts was conducted in weak and strong heat flux environments. The heat flux measurement from the TEM and the HFS are compared and analyzed, under the premise that the performance of the HFS in measuring heat flux has been proven to be largely acceptable. Commercial HFSs were used as conventional heat flux sensors. A comparison of the properties and functionalities of
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
A part of this project has received funding from the European Union's Horizon 2020 research and innovation program under grant agreement No. 637221. The sole responsibility for the content of this paper lies with the authors. It does not necessarily reflect the opinion of the European Union. Neither the EACI nor the European Commission is responsible for any use that may be made of the information contained therein. The authors would also like to extend their gratitude to IFARHU, a Panamanian
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2022, International Journal of Thermal SciencesCitation Excerpt :Referred to the measuring equipment, the thermal evaluation methods consist of contact methods and non-contact methods. The contact equipment covers thermocouples, heating plates, heat flux sensors [108], etc. The non-contact equipment consists of the infrared camera, heating lamps, etc.