Inside-outside temp differences power smart building IoT

Published in

Purdue Engineering Review

4 min readMar 24, 2023

Understanding materials and how they transport themselves from one state to another at the nano- and micro-scale can help researchers tailor their physical, chemical, thermal and mechanical properties for specific engineering applications. This knowledge base can enable a broad array of research advances, including renewable-energy-harvesting devices and innovative Internet of Things (IoT) sensing technology for the environment and human health monitoring.

Fossil fuels are a finite resource, cause pollution, and are not available everywhere on Earth. As global warming issues affect more people’s lives, the importance of conserving energy and developing a sustainable energy strategy is attracting growing public attention. Some two-thirds of worldwide CO2 emissions come from energy systems; any increase in the concentration of CO2 and other greenhouse gases results in Earth’s temperature rising.

Sustainable energy is a form of energy that does not cause any harm to the environment. Most people are familiar with energy sources like solar, wind, geothermal, hydropower and ocean energy, which are sustainable because they are stable and plentiful.

Another source of sustainable energy is thermoelectric technology, which converts heat energy into electricity. The benefits of this technology stem from the fact that as long as there is a temperature difference (heat flux), electricity will be an output. Temperature affects how electrons travel in an electrical circuit — the hotter it is, the more resistance, and vice versa. Electrons move toward where the heat is lower, enabling direct current to flow through the circuit.

Despite advancements in developing energy-efficient materials, a significant portion of energy in building environments is lost due to inside-outside temperature differences. In principle, this lost heat could be converted to electricity to power IoT sensors or small electronics using thermoelectric generators (TEGs). These devices take advantage of the Seebeck effect, named after the German physicist Thomas Seebeck, who in 1821 discovered that energy potential is present when there is a temperature variance between different electrical conductors.

TEGs are convenient, contain no moving parts, and can be used across a large temperature range. They also can integrate with different energy-harvesting devices, like solar cells, to enhance their efficiency.

However, current TEGs are hindered by such issues as rigid geometry, toxicity, expensive materials, and labor-intensive fabrication. What’s needed are novel nanostructured oxides that can adopt different geometries, as well as scalable manufacturing methods to enable production of non-toxic, large-scale, cost-effective flexible TEGs.

My research goal is to enable sustainable built environments and human-environmental health through multidisciplinary research into materials and related device technology. I have discovered several non-toxic oxide materials for TEG applications, and developed a novel method to produce flexible TEGs on a roll-to-roll platform.

These flexible TEGs can be a reliable power source for IoT sensors and small electronic devices for control systems in smart-built environments. The roll-to-roll manufacturing method makes device fabrication possible via a non-polluting, energy-conserving, economically-sound process.

Using a building with a TEG as an example, the device could utilize the difference between indoor and outdoor temperatures to generate the electricity to both power and provide a battery backup for IoT sensors. The TEG can last 10 to 20 years, not only conserving energy but also reducing maintenance-related labor costs.

Schematic of proposed roll-to-roll production line of conformal TEG (Purdue University image/Yining Feng)

Currently, the most widely used materials in TEGs are telluride-based, which are toxic and expensive; some TEGs also employ rare-earth elements. The device fabrication process is labor-intensive and difficult to scale. We’re working to discover additional nanomaterials that are non-toxic, Earth-abundant, and cheaper than telluride-based versions. We want to use additive manufacturing to decrease labor costs, and develop a platform for larger-scale manufacturing.

Challenges remain, as the technology is very customized. It is hard to develop one product to fit all purposes, so TEGs need to be modified based on each application. That’s why the manufacturing process is so crucial, and we have two provisional patents in this area.

But as IoT sensors in smart-built environments proliferate — automating service management by sensing and responding to changes in temperature and humidity, air quality, lighting, and the like — their power source has become a bigger question mark.

We need to develop more novel renewable-energy-harvesting devices to meet that energy demand. Such devices will benefit society by enabling a cleaner environment and greater choice among energy sources.