Converting waste heat to electricity with better and cheaper thermoelectric materials

 
With the advancements in technology and increase in population, the consumption of energy will inevitably go up. Regardless of the used method, the efficiency of current energy production is not at a sustainable level on the global scale. A vast majority of the energy is produced by combustion, and these processes induce huge losses as waste heat. If this waste heat can be harvested, e.g. with the utilization of the thermoelectric effect, the efficiency of energy production can be improved.

The greatest obstacles in thermoelectrics at the moment are the relative ineffectiveness and the scarcity of the raw materials needed in the best-performing devices. In order to improve efficiency on the global scale, we need to produce thermoelectric modules from cheap materials. One of the themes in the CloseLoop project is to find and synthesize effective thermoelectric oxide materials using metals that are abundant in the Earth’s crust, such as zinc and copper.

Thermoelectric efficiency of simple compounds can be, to a certain degree, systematically improved. A common strategy is to “dope” the thermoelectric material with some guest atoms, usually another metal. This is common practice from the semiconductor industry, where it is used to modify the electronic properties of semiconductors in a detailed and controlled fashion. Recently, it was discovered that the thermoelectric efficiency of materials can be improved also by nanostructuring metal oxides with organic compounds, such as hydroquinone (Figure 1). With this approach, not only can the efficiency be increased, but also the amount of metal needed to produce the thermoelectric devices is decreased.

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Figure 1. A structural model of thermoelectric zinc oxide with organic layers.

 

One goal for this project is also to produce functioning thermoelectric modules with the new materials, and perform a life-cycle assessment. To reach a full closed loop of raw materials, a complete evaluation of all the components in the devices and their lifecycles must be available. This includes information all the way from mapping the abundance and availability of raw materials for extraction to the recycling process at the end of the product’s lifetime. In fact, the lifetime of thermoelectric modules is one of their biggest strengths. With no moving parts, wearing from mechanical stress is practically non-existent. Perhaps the brightest example of long lifetimes is in the space probe Voyager 2, which is still powered by the same thermoelectric devices it had when it was launched more than 40 years ago.

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Figure 2. Example of a module architecture. A single module consists of several thermoelectric elements connected in series.

MSc Jarno Linnera (jarno.linnera@aalto.fi)
Prof. Antti Karttunen (antti.karttunen@aalto.fi)
Aalto University, Department of Chemistry and Materials Science

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