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Harvesting Heat: Converting Waste Heat into Usable Electricity
September 25, 2012
This is the first of a series of guest posts from winners of Dow Chemical’s Sustainability Innovation Student Challenge Award (SISCA) program. The program recognizes and rewards students and universities for research and innovation to promote sustainable solutions to the world’s most pressing challenges.
When materials such as glass and steel are produced, a huge source of energy — waste heat — is lost into the environment. Excess heat from these and other industrial processes has often been recaptured and used to warm associated buildings. However, industrial waste heat is an energy source with potential beyond heating, and has become the focus of sustainability research. With improvements in various energy-conversion technologies, there has been increased interest in utilizing this waste heat as electricity.
In the Tufts School of Engineering's Renewable Energy and Applied Photonics Laboratories, we conduct research on a technology called thermophotovoltaics (TPVs) to directly convert heat into electricity. Thermophotovoltaic devices use the same physics as the solar panels (photovoltaics) typically used on rooftops — they both capture light in a semiconductor material that excites electrons, which then can flow out of the device as current. One major difference is that TPV devices can convert infrared light; this is the portion of the electromagnetic spectrum that is felt as heat.
Currently, for TPVs to work efficiently, heat sources have to be in excess of 1000°C. The glass and steel industries with their large, hot furnaces are low-hanging fruit best suited to TPV waste-heat harvesting technology. For example, in the glass industry furnaces must be maintained at no less than 1575°C to keep glass in a molten state. In steel production, iron ore must first be heated in blast furnaces up to 2300°C. Studies have shown that the technology implemented to capture industrial waste heat can pay for itself in at least three to five years, after which any additional electricity produced is essentially a free source of energy.
One of our projects involves extending the operating range of these thermophotovoltaic devices, giving them the ability to convert lower-temperature heat sources as well as converting the higher-temperature sources more efficiently. Traditionally, this is done using exotic alloys of different semiconductor materials; however, this approach can only go so far. We are presently working on using nanostructures, quantum effects, and novel device designs to push the technology to new boundaries. Initial results are promising, showing electricity generation from thermal sources at 500°C and below.
Now is the time for adoption of this technology. Anything with a hot, constant heat source is a potential candidate for TPV technology, and there are many chemical and industrial factories and processes that meet these requirements. Beyond industrial applications, if TPV devices were made efficient enough at lower temperature operation, they could be used to harvest heat in electronic devices, car engines, or battery packs. Even the human body could be used as a heat source, if our research is successful in allowing TPV devices to operate with temperatures as low as 37°C. If the TPV devices could harvest heat in these ways, batteries could last longer, farther distances could be traveled with fewer emissions, and huge advances could be made in bioengineering and medicine.
We will be using the cash prize from the Dow Sustainability Innovation Student Challenge Award (SISCA) for our research and it will help us continue to find impactful solutions to these and other energy and sustainability problems. For more information on this project, please visit our online project profile.