INTRODUCTION

An estimated 840 million people in India use a solid-fuel cookstove (chulha) powered off wood or biomass, and forecasts indicate that by 2040 over 400 million people will still lack access to clean cooking facilities. Chulhas emit harmful pollutants and particulates that contribute to respiratory diseases, including chronic obstructive pulmonary disease and lung cancer. India has the second highest air pollution level worldwide and the largest number of people exposed to high levels of indoor air pollution (IAP) caused by solid-fuel cookstoves (Heath Effects Institute, 2019. State of Global Air 2019, Special Report, Boston). IAP, which disproportionally affects women and young children, was responsible for 482,000 deaths in India in 2017 alone, the largest number of deaths attributable to IAP worldwide. Furthermore around 100 million people in rural India do not have access to electricity. Despite a massive program of electrification of villages that has seen the number without electricity halve in the past five years, those without access remain among the most disadvantaged members of society. The average power consumption per capita of 50.5 W, remains well below that of developed countries. An Indian village is classed as electrified when only 10% of households and civic buildings have access to electricity. When electricity reaches a village, the cost of connection is often prohibitive, and the supply is unreliable. Chulhas operate at temperatures up to 600 °C, providing a ready supply of waste heat. A TE device is capable of converting this waste heat into useful electricity to power a fan for forced air circulation to improve combustion in the cookstove. In addition, a TE device affords a localized power source to fulfill basic needs in lighting, communication and mobile device charging.

We seek to bring the collective expertise of UK and Indian materials researchers to bear on this problem by developing materials for a TE generator that will use the heat from a cookstove to create 10 W of off-grid electrical power, sufficient to drive a 1.5 W fan for forced-air circulation and to power low-energy LED lighting (typically 5- 7 W). The project directly addresses UN Sustainable Development Goals (SDGs) 3 (Good Health and Well-Being) and 7 (Affordable and Clean Energy) as it seeks to facilitate cleaner combustion and therefore reduce IAP, whilst simultaneously providing a large number of the population with a straightforward and accessible supply of electricity, without huge infrastructure costs. It offers the potential to make progress against Goal 3.4 (By 2030, reduce by one third premature mortality from non-communicable diseases...), 3.9 (By 2030, substantially reduce the number of deaths and illnesses from hazardous chemicals and air, water and soil pollution...), 7.1 (By 2030, ensure universal access to affordable, reliable and modern energy services) and 7.3 (By 2030, double the global rate of improvement in energy efficiency). In its potential to produce a step change in living conditions for those in rural communities in India, the project also impacts on other SDGs, in particular creating more sustainable communities (SDG 11) and resilience to environmental issues like climate change (SDG 13). There is also potential to create a new market for indigenous copper ores, thereby stimulating economic development.

A TE device consists of an array of n- and p-type semiconductors (Figure below) connected electrically in series and thermally in parallel. Performance is directly related to that of the component semiconductors, expressed in a dimensionless figure of merit, ZT = S2σT/κ, where S, σ and κ are the Seebeck coefficient, electrical and thermal conductivities, and S2σ is the power factor. The challenge in designing high-performance TE materials is the interdependence of each of the key parameters, S, σ and κ. The cost, abundance and safety of the materials are also important factors to consider. Proof-of-principle TE power generation in a (laboratory-constructed) cookstove using commercial Bi₂Te₃ devices has been demonstrated, but Bi₂Te₃ suffers from major disadvantages:

i. Scarcity of tellurium. Identified as one of the top 9 ‘at risk’ elements2 and on the US Government critical mineral list,3 tellurium is expensive ($82/kg). The amount of Te in the Earth crust (1 ppb) is insufficient for large-scale adoption of TE technology based on Bi₂Te₃.

ii. Inappropriate Temperature Range. The ZT of Bi₂Te₃ approaches unity around 100 °C but falls off sharply with increasing temperature, with material melting occurring at 580 °C. Consequently in a cookstove the device needs to be located where the heat is attenuated, meaning that the high-grade waste heat is not fully utilized.

iii. Toxicity. Tellurium and tellurium compounds are categorized as being of acute toxicity, which poses risks in a domestic setting.

The substantial improvements in ZT values of non-telluride TEs achieved in recent years have been realized at temperatures well above those generated in a cookstove. We will apply design principles, formulated in conjunction with computer modelling, to create TE materials specifically tailored to the 200 – 400 °C range, appropriate to chulhas. Target materials are related to Earth abundant sulphide minerals, including chalcopyrite, bornite and chalcocite, substantial reserves of which exist in the Indian copper belts including the Khetri, Singhbuhum, and Malanjkhand copper belts. The program of work is thus well-aligned with the natural resources of the country. Unlike tellurides, sulphur and sulphides do not have associated problems of acute toxicity.