The industrial engineer Miguel Araiz, a researcher at the Smart Cities Institute (ISC) of the Public University of Navarra (UPNA), has designed a thermoelectric generator that takes advantage of the residual heat of a thirty-meter industrial chimney, which dissipates gases at 250 ° C of temperature, to generate a total of 363 MWh of electricity per year, equivalent to the annual electricity consumption of 111 Spanish households. The system would produce such energy at a cost of 14.6 cents per kilowatt.

The thermoelectric generator devised by the industrial engineer Miguel Araiz (Estella-Lizarra, 1991) is the result of the doctoral thesis he has defended at UPNA under the direction of ISC researchers David Astrain Ulibarrena and Álvaro Martínez Echeverri. The research, which has been rated with outstanding “cum laude”, has obtained funding from the central government for R & D & I projects within the framework of the State Plan for Scientific and Technological Research and Innovation.

"The current energy situation - and all the environmental, political and economic problems associated with it - make it increasingly necessary to optimize power generation systems and incorporate energy saving measures into processes," says Miguel Araiz. " In this sense, different investigations have focused on the recovery of waste heat, an energy that is produced in different processes and that is not usually used, but is released into the environment or dissipates. ”

In this way, Miguel Araiz's doctoral thesis has studied the use of residual energy through thermoelectric generators, based on the Seebeck effect, devices capable of producing electrical energy from a heat source. “For this, it is enough to achieve a temperature difference between the faces of the thermoelectric modules, the essential element of these devices. The greater this temperature difference, the greater the electrical power generated, ”says the researcher.

Greater efficiency

Among the advantages of these devices, are “the absence of moving parts, which reduce and even eliminate maintenance; the possibility of taking advantage of any temperature jump to produce electricity; and the scalability and modularity, which allows a better adaptation to the heat source ”, explains Araiz. However, the efficiency of the heat-electricity conversion of these systems is low, and one way to increase it is through the optimization of the heat exchangers that accompany the modules in the thermoelectric generators.

And that is what Araiz has done. "The mission of these exchangers is to bring the temperature of the faces of the modules closer to that of the respective bulbs, thus increasing the temperature difference between the faces of the modules and achieving an increase in electrical production." 

In this sense, Miguel Araiz proposed, in his work, to use, as heat exchangers, on the cold side of thermoelectric generators, thermosyphons with phase change or biphasic. “They are responsible for dissipating heat that is not transformed into thermoelectric modules into the environment. In addition, they do not need auxiliary equipment such as fans or pumps, so that passive and autonomous cooling is achieved, unlike the usual systems, such as fins or water exchangers, ”he says.

Specifically, the researcher developed a computational model capable of simulating the behavior of these biphasic thermosipons in prototypes of heat exchangers. With this tool, he designed and built a thermoelectric generator prototype that included a two-phase thermosiphon on its cold side. This prototype was installed in the outlet duct of a combustion boiler located in the UPNA laboratories and, after experimentation under different working conditions, a maximum of 240 watts per square meter of chimney was generated.

Rehearsal in a real industry?

Given the results obtained in the experimental tests in the laboratories, Miguel Araiz then carried out a computational study of the implementation of this technology in a real industry, where the residual heat of an industrial process can be recovered. Thus, he analyzed the production process of a company where there are several heat sources that were not being used.??

"After selecting the point that had the best thermal characteristics, a thermoelectric generator was designed to be placed on the outer surface of a thirty-meter-high chimney in which hot gases are currently dissipated at a temperature of 250 ° C," he says. "The proposed system includes heat exchangers on both sides of the thermoelectric modules. Specifically, it has a two-phase thermosiphon, in the cold part, and fin dissipators, in the hot part. In this way, the thermoelectric generator developed is completely passive and all the electric power generated could be used by the company. In addition, installing it would cost ten euros per watt. ”