Roadmap identifies sustainable materials for energy harvesting

Researchers have put together a roadmap for sustainable materials for energy harvesting systems, from solar cells to thermal energy and even RF energy capture.

Ravi Silva at the Advanced Technology Institute at the University of Surrey in the UK with over 100 collaborators, including Prof Vincenzo Pecunia at Simon Fraser University in British Columbia, looked at the current sate of materials development and lifecycle analysis (LCA) to determine their overall sustainability.

Compact energy harvesters will be key to powering the exponentially growing smart devices ecosystem that is part of the Internet of Things, from smart homes and smart cities to smart manufacturing and smart healthcare. Innovative materials are needed to efficiently convert ambient energy into electricity through various physical mechanisms, such as the photovoltaic effect, thermoelectricity, piezoelectricity, triboelectricity, and electromagnetic power transfer.

Charting the path toward sustainability of energy harvesting materials is not straightforward. One reason for this is due to the diversity of components such as transducers, power management and energy storage making up the harvesters.

The 225 page roadmap covers the state of research into various classes of materials being developed for photovoltaic, piezoelectric, triboelectric, thermoelectric and radiofrequency energy capture and the environmental impact.

One major challenge of sustainability of energy harvesting materials is the difficulty in synthesizing materials that are simultaneously efficient for maximum energy conversion and environmentally benign. In cases where such materials are needed, the sustainability of the manufacturing route is not guaranteed.

Nonetheless, an evaluation of the environmental profile of energy harvesting materials and technologies at the design or pilot stage before expensive investments and resources are committed is essential. Such evaluations can aid the communication of key findings to materials developers and policymakers.

Energy harvesting is critically dependent on the availability of suitable materials for devices to convert ambient energy into usable electric energy. Research requires a broad, cross-cutting effort, ranging from the discovery of new materials to the study of energy harvesting properties, the engineering of their compositions, microstructure, and processing, and the integration into devices and systems.

A major challenge faced by all energy harvesting technologies is the limited power density available from ambient energy sources, which makes it essential to develop energy harvesting materials and devices that can efficiently convert such energy. Current energy harvesting technologies typically deliver electric power densities well below the mW cm-2 when harvesting ambient energy.

Several types of novel solar photovoltaic materials including organic, dye sensitized, colloidal quantum dot, and perovskite solar cells (PSCs) have emerged in recent years and have been touted to be economically and environmentally viable option to traditional silicon-based technology. To verify this assertion, several comparative LCA studies have been conducted for two PSC types across sixteen impact categories in comparison to other photovoltaic types concluding that not only the PSC demonstrated environmental edge, but they also have a low energy payback period. This reduced energy payback period is attributed to a reduction in the energy intensive processes required for PSC manufacturing due to the elimination of silicon and rare earth element processing.

However the use of gold in the material architecture of PSC caused significant environmental impact due to the enormous energy required for its production from its ore. In the context of ambient energy harvesting for IoT applications, more LCA work is required.

The toxicity of materials and global policy initiatives and legislation such as the Waste Electrical and Electronic Equipment (WEEE) and Restriction of Hazardous Substance (RoHS) have called for the prohibition of lead in piezoelectric materials. This has reinvigorated the race to develop lead-free alternatives based on potassium sodium niobate (KNN) and sodium bismuth titanate (NBT) for lead zirconate titanate (PZT).

To ascertain the environmental benefits of lead-free alternatives over lead-based ones, the LCA showed KNN is environmentally worse than PZT with respect to climate change and eco-toxicity due to the presence of the niobium pentoxide whose mining and milling, through hydro- and pyro-metallurgical processing, for refining niobium has substantial adverse impacts across numerous environmental indicators.

At the same time the lower energy consumed by NBT during synthesis yielded a lower overall environmental profile, based on primary energy consumption and toxicological impact, in comparison to PZT and KNN. When NBT is compared with PZT, the former has higher environmental impact compared to lead oxide, because of the additional processing and refining steps which pose extra challenges in metallurgical recovery.

Triboelectric nanogenerators (TENGs) offer the potential to generate energy in self-powered devices at low cost but their environmental impact was previously unknown. An LCA and techno-economic analysis of two TENG modules compared a thin-film-based micro-grating TENG with its electrode arrays arranged linearly generates enough energy to power standard electronics (Module A) and Module B, which uses a planar structure based on electrodes generating periodically charged triboelectric potential, yielding energy from water and air flow and bodily movement. The results showed Module A has a better environmental profile, lower production costs, lower CO2 emissions and shorter energy payback period (EPBP).

Module B’s higher environmental impact is due to its higher content of acrylic in its structure and higher electrical energy requirements during fabrication. Acrylic can however be recycled or reused at the end-of-life stage releasing no toxic gases during combustion, thus improving its overall profile. Nonetheless, when compared with emerging solar PV materials technologies,

TENG modules offer better environmental profile and shorter payback period, although Module B is marginally higher than that of a PV technology based on perovskite structured methyl ammonium lead iodide. Future work on TENG on lifetime and efficiency improvements is more important than the identification of cheaper materials and manufacturing processes.

This is because the embodied energy (i.e., the emissions across the entire value chains) of ambient energy harvesters can be potentially high. As such, a better figure of merit would be the net environmental gain, which is a function of how the embodied energy compares with the operational energy saved over their lifetime as well as the energy that would otherwise be required if, for instance, batteries were employed as the power source.

Due to the presence of heavy metals and rare-earth elements in the constituent materials, attention has been paid to their toxicity, with less work focusing on their supply-chain environmental profile.

A comparative lifecycle impacts of inorganic, organic and hybrid thermoelectric materials at their production stage, across multiple environmental indicators concluded that the inorganic type yielded significantly higher environmental impacts. Bi2Te3 was identified as the only inorganic exception whose environmental impact was the lowest compared to all the thermoelectric materials studied. For organic and hybrid types, issues around raw material supply risks was identified as the main sustainability issue.

Source: Nick Flaherty, eeNews Europe