Converting Waste Heat Into Power With Wood

Researchers at the University of Limerick (UL) and the University of Valencia have developed a method to convert waste heat into electricity using lignin, a byproduct of the paper industry, offering a sustainable alternative to conventional energy recovery methods. 

What Is Energy Recovery? 

Energy recovery (heat recovery in this case) is the process of capturing and reusing waste heat generated by industrial processes, machinery, or natural sources. Instead of allowing this energy to dissipate, heat recovery repurposes it to perform useful work, such as generating electricity or heating systems. By improving energy efficiency and reducing reliance on non-renewable resources, heat recovery offers real value in lowering operational costs and minimising environmental impact. 

What Inspired the Research? 

Every day, industries, homes and even our bodies produce vast amounts of waste heat, with approximately 66 per cent of industrial energy output lost as heat, much of it below 200°C. Recognising this untapped potential, researchers at UL, supported by Irish Government funding and led by Muhammad Muddasar, PhD candidate at the School of Engineering, focused on developing a sustainable solution for harnessing this energy. 

Traditional thermoelectric materials, such as bismuth telluride, have been used for heat-to-electricity conversion but are costly, toxic, and reliant on finite resources. Seeking a greener alternative, the researchers identified lignin \9a renewable, abundant byproduct of the paper industry) as an ideal, eco-friendly candidate for creating efficient thermoelectric materials. 

The Thermoelectric Effect 

At the heart of this discovery lies the ‘thermoelectric effect’, where a temperature difference across a material generates an electrical potential. The UL and Valencia team engineered lignin-based membranes infused with a salt solution to exploit this phenomenon. 

When a temperature gradient was applied to the lignin membrane, ions within the salt solution migrated i.e., positively charged ions moved towards the cooler side, while negatively charged ions gravitated towards the warmer side. This ion separation generated an electric potential across the membrane, which could be harnessed as electricity. 

Lignin-Based Membranes Are Great at Converting Low-Grade Heat to Electricity 

The researchers developed membranes from lignin that can turn low-temperature heat (below 200°C) into electricity. This type of heat is commonly wasted in industrial settings, such as manufacturing plants and power stations, so these membranes could help capture and reuse it. 

The study showed that lignin membranes performed well for this purpose, with a figure of merit (ZTi) of 0.25, measuring their ability to convert heat to electricity effectively. They also achieved an ionic Seebeck coefficient of 5.71 mV K⁻¹, demonstrating a strong electrical response from temperature differences. 

The lignin-based membranes are lightweight, safe for biological environments, and eco-friendly, making them suitable for applications ranging from industrial energy recovery to sustainable energy solutions. 

Practical Applications and Benefits 

The implications of this discovery could extend across industries and everyday scenarios. For instance, manufacturing facilities generate vast amounts of waste heat during production processes. Integrating lignin-based thermoelectric systems could allow these facilities to recover and reuse energy, reducing operational costs and environmental footprints. 

Remote and off-grid locations could also benefit significantly. Lignin membranes could power sensors, communication devices, and small-scale lighting systems, eliminating the need for traditional fuel-based generators. Wearable technologies could also leverage the discovery, e.g. membranes could enable self-powered fitness trackers, medical monitors, and GPS devices that utilise body heat for continuous energy supply. 

In buildings and infrastructure, lignin membranes could be integrated into heating, ventilation, and air conditioning (HVAC) systems to recapture waste heat and offset energy consumption. Their eco-friendly nature aligns perfectly with green building standards and sustainability goals. 

A Green Alternative to Supercapacitors? 

Beyond energy harvesting, the UL team explored the use of lignin-based materials in energy storage. Traditional supercapacitors, which rapidly charge and discharge energy, often rely on carbon derived from fossil fuels. The researchers developed porous carbon electrodes from lignin, creating a sustainable alternative. 

These lignin-based supercapacitors demonstrated exceptional performance in storing and delivering energy generated from waste heat. Their rapid charge-discharge capability makes them ideal for applications requiring quick bursts of power, such as electric vehicles and renewable energy systems. 

Broader Context and Similar Research 

The study adds to a growing body of research exploring sustainable materials for energy generation. In recent years, cellulose-based membranes and ionic gels have gained attention for their thermoelectric properties. However, lignin offers the unique advantage of being a byproduct of an existing industrial process, requiring minimal additional processing, and making it highly cost-effective. 

For example, a 2021 study by researchers at Chalmers University of Technology in Sweden highlighted the potential of cellulose membranes for thermoelectric applications. While these membranes demonstrated impressive performance, their mechanical fragility posed challenges for practical use. By contrast, the UL team’s lignin-based membranes are mechanically robust and suitable for real-world applications. 

Environmental and Economic Impact 

Lignin-based thermoelectric materials offer clear environmental benefits. By converting waste heat into electricity, these membranes could reduce reliance on fossil fuels, lower greenhouse gas emissions, and enhance energy efficiency across sectors. Harnessing lignin can thus transform what was once industrial waste into a valuable resource, contributing to a circular economy. 

Cost Savings 

Economically, lignin-based technology could drive significant cost savings. The pulp and paper industry produces an estimated 50 million tonnes of lignin annually, much of which is discarded or burned for low-value energy recovery. Redirecting this lignin towards high-value applications, such as thermoelectric energy harvesting, could represent a win-win for industries and the environment. 

Key Challenges and Future Directions 

Despite its promise, the technology is not without challenges. Scaling up lignin membrane production while maintaining consistent quality will require further research. Also, optimising the membranes’ performance under varying environmental conditions (such as humidity and prolonged heat exposure) remains a focus area. 

Looking Ahead 

The researchers envision extending lignin-based materials to other forms of energy harvesting, such as solar thermal systems. Enhancements in membrane design, such as incorporating nanoscale channels for improved ion transport, could further boost efficiency and broaden applications. 

What Does This Mean for Your Organisation? 

The development of lignin-based membranes could represent an exciting leap forward in sustainable energy technology. By converting waste heat (a largely untapped resource) into electricity, this innovation addresses both energy inefficiency and industrial waste. It is a clear example of how a circular economy can transform byproducts like lignin from the paper industry into valuable resources, paving the way for more environmentally responsible and economically viable solutions. 

The potential value to industries could be significant. For example, in manufacturing facilities and power plants, where vast amounts of low-grade heat are routinely wasted, integrating lignin-based thermoelectric systems could reduce operational costs and improve energy efficiency. These membranes offer a way to recover lost energy and transform it into an asset, potentially reshaping markets that rely heavily on energy-intensive processes. Similarly, the transportation sector, including electric vehicles, could benefit from this technology’s ability to power auxiliary systems using heat generated during operation, improving overall efficiency and sustainability. 

For businesses, the membranes present multiple opportunities. Industries involved in energy-intensive processes could achieve cost savings and reduced emissions, aligning with growing regulatory and public demands for sustainable practices. Furthermore, the eco-friendly nature of lignin membranes may open new markets, as green building standards and sustainability certifications increasingly influence decisions in sectors such as construction, infrastructure, and electronics. Companies that adopt and invest in this technology early could gain a competitive advantage in these evolving markets. 

The implications for off-grid and remote locations are equally compelling. Lignin membranes could power devices and systems in areas where traditional energy infrastructure is lacking or expensive e.g., communication systems and wearable technologies. This could reduce reliance on fossil fuels and support the global push for decentralised, renewable energy solutions. 

Although challenges remain in scaling production and optimising performance, the potential economic and environmental benefits of lignin-based membranes are undeniable. By offering a cost-effective, sustainable alternative to conventional thermoelectric materials, this innovation could revolutionise energy recovery across industries and inspire a shift in how businesses approach waste, sustainability, and energy use.