What if you could run your air conditioner not on conventional electricity, but on the sun’s heat during a warm summer’s day? With advancements in thermoelectric technology, this sustainable solution might one day become a reality. Thermoelectric devices are made from materials that can convert a temperature difference into electricity, without requiring any moving parts — a quality that makes thermoelectrics a potentially appealing source of electricity.
The phenomenon is reversible: If electricity is applied to a thermoelectric device, it can produce a temperature difference. Today, thermoelectric devices are used for relatively low-power applications, such as powering small sensors along oil pipelines, backing up batteries on space probes, and cooling minifridges. But scientists are hoping to design more powerful thermoelectric devices that will harvest heat — produced as a byproduct of industrial processes and combustion engines — and turn that otherwise wasted heat into electricity.
However, the efficiency of thermoelectric devices, or the amount of energy they are able to produce, is currently limited. Now researchers at MIT have discovered a way to increase that efficiency threefold, using “topological” materials, which have unique electronic properties. While past work has suggested that topological materials may serve as efficient thermoelectric systems, there has been little understanding as to how electrons in such topological materials would travel in response to temperature differences in order to produce a thermoelectric effect.
In a paper published this week in the Proceedings of the National Academy of Sciences, the MIT researchers identify the underlying property that makes certain topological materials a potentially more efficient thermoelectric material, compared to existing devices. “We’ve found we can push the boundaries of this nanostructured material in a way that makes topological materials a good thermoelectric material, more so than conventional semiconductors like silicon,” says Te-Huan Liu, a postdoc in MIT’s Department of Mechanical Engineering. “In the end, this could be a clean-energy way to help us use a heat source to generate electricity, which will lessen our release of carbon dioxide.” Liu is first author of the PNAS paper, which includes graduate students Jiawei Zhou, Zhiwei Ding, and Qichen Song; Mingda Li, assistant professor in the Department of Nuclear Science and Engineering; former graduate student Bolin Liao, now an assistant professor at the University of California at Santa Barbara; Liang Fu, the Biedenharn Associate Professor of Physics; and Gang Chen, the Soderberg Professor and head of the Department of Mechanical Engineering.
A path freely traveled
When a thermoelectric material is exposed to a temperature gradient — for example, one end is heated, while the other is cooled — electrons in that material start to flow from the hot end to the cold end, generating an electric current.
The larger the temperature difference, the more electric current is produced, and the more power is generated. The amount of energy that can be generated depends on the particular transport properties of the electrons in a given material. Scientists have observed that some topological materials can be made into efficient thermoelectric devices through nanostructuring, a technique scientists use to synthesize a material by patterning its features at the scale of nanometers.
Scientists have thought that topological materials’ thermoelectric advantage comes from a reduced thermal conductivity in their nanostructures. But it is unclear how this enhancement in efficiency connects with the material’s inherent, topological properties. To try and answer this question, Liu and his colleagues studied the thermoelectric performance of tin telluride, a topological material that is known to be a good thermoelectric material.
The electrons in tin telluride also exhibit peculiar properties that mimick a class of topological materials known as Dirac materials. The team aimed to understand the effect of nanostructuring on tin telluride’s thermoelectric performance, by simulating the way electrons travel through the material. To characterize electron transport, scientists often use a measurement called the “mean free path,” or the average distance an electron with a given energy would freely travel within a material before being scattered by various objects or defects in that material.
Nanostructured materials resemble a patchwork of tiny crystals, each with borders, known as grain boundaries, that separate one crystal from another. When electrons encounter these boundaries, they tend to scatter in various ways. Electrons with long mean free paths will scatter strongly, like bullets ricocheting off a wall, while electrons with shorter mean free paths are much less affected.
In their simulations, the researchers found that tin telluride’s electron characteristics have a significant impact on their mean free paths. They plotted tin telluride’s range of electron energies against the associated mean free paths, and found the resulting graph looked very different than those for most conventional semiconductors. Specifically, for tin telluride and possibly other topological materials, the results suggest that electrons with higher energy have a shorter mean free path, while lower-energy electrons usually possess a longer mean free path.
The team then looked at how these electron properties affect tin telluride’s thermoelectric performance, by essentially summing up the thermoelectric contributions from electrons with different energies and mean free paths. It turns out that the material’s ability to conduct electricity, or generate a flow of electrons, under a temperature gradient, is largely dependent on the electron energy. Specifically, they found that lower-energy electrons tend to have a negative impact on the generation of a voltage difference, and therefore electric current.
These low-energy electrons also have longer mean free paths, meaning they can be scattered by grain boundaries more intensively than higher-energy electrons.
Going one step further in their simulations, the team played with the size of tin telluride’s individual grains to see whether this had any effect on the flow of electrons under a temperature gradient. They found that when they decreased the diameter of an average grain to about 10 nanometers, bringing its boundaries closer together, they observed an increased contribution from higher-energy electrons. That is, with smaller grain sizes, higher-energy electrons contribute much more to the material’s electrical conduction than lower-energy electrons, as they have shorter mean free paths and are less likely to scatter against grain boundaries.
This results in a larger voltage difference that can be generated. What’s more, the researchers found that decreasing tin telluride’s average grain size to about 10 nanometers produced three times the amount of electricity that the material would have produced with larger grains. Liu says that while the results are based on simulations, researchers can achieve similar performance by synthesizing tin telluride and other topological materials, and adjusting their grain size using a nanostructuring technique.
Other researchers have suggested that shrinking a material’s grain size might increase its thermoelectric performance, but Liu says they have mostly assumed that the ideal size would be much larger than 10 nanometers. “In our simulations, we found we can shrink a topological material’s grain size much more than previously thought, and based on this concept, we can increase its efficiency,” Liu says. Tin telluride is just one example of many topological materials that have yet to be explored.
If researchers can determine the ideal grain size for each of these materials, Liu says topological materials may soon be a viable, more efficient alternative to producing clean energy.
“I think topological materials are very good for thermoelectric materials, and our results show this is a very promising material for future applications,” Liu says.
Eskom chairperson Zethembe Khoza confirmed to Fin24 on Saturday that he submitted his resignation to Minister of Public Enterprises Lynne Brown on Friday and he’s waiting for her to accept it. “I think it’s in [the] good interest of the country…after all the noise, it’s better for me,” he told Fin24 by phone. He said that his resignation was “not necessarily” due to pressure from Finance Minister Malusi Gigaba, who told journalist this week that the financial situation at the state-owned company (SOC) requires urgent attention.
“I was putting out fires on a daily basis, so I think it’s good to give new people a chance,” Khoza said. Both Brown and her spokesperson Colin Cruywagen could not be reached for comment. Rumours were rife on Saturday that former finance minister Nhlanhla Nene was the favourite to replace Khoza, but the outgoing chair said he was unaware who had been chosen as his replacement.
ANC national executive committee (NEC) member Enoch Godongwana told Fin24 on Saturday he was unable to comment on changes to the Eskom board. When asked by Fin24 if changes needed to be made to the top structures at the power utility, he answered “of course, yes”. Godongwana previously chaired the NEC sub-committee for economic transformation.
Khoza is the third Eskom chairperson to have tendered his resignation in as many years. He was appointed in an acting capacity in June 2017, after Ben Ngubane quit. Ngubane replaced Zola Tsotsi in March 2015 who later told the Eskom parliamentary inquiry he was forced to exit.
Brown appointed Khoza as Eskom chairperson on 8 December 2017, despite the controversies around him, for a period of three years, subject to annual review. Khoza’s appointment as permanent chair in December earned the ire of Business Unity South Africa (BUSA) who said the retention of 7 Eskom directors on the new board, including Khoza, together with the two new appointment was a blow to the parastatal’s sustainability Lobby group Organisation Undoing Tax Abuse (OUTA) also pointed out that the board, including Khoza had approved former CEO Brian Molefe’s controversial reappointment and R30m pension payout.
The Sunday Times reported they’d seen a signed affidavit in which evidence leader at the Parliamentary Eskom Inquiry, Advocate Ntuthuzelo Vanara claims that Minister of State Security Bongani Bongo was instructed by Khoza to derail the probe. Khoza denied the allegations. Former acting CEO Matshela Koko and suspended CFO Anoj Singh were set to testify before MPs next week.
Government under pressure to act
Eskom’s financial woes have mounted in recent months and Standard and Poor’s Global Ratings warned this week that it was in ‘clear danger of default’ on its government backed debt.Finance Minister Malusi Gigaba hinted that a shakeup at Eskom was on the cards at a pre-World Economic Forum breakfast on Thursday, where he said some board managers and senior officials did not grasp the seriousness of the situation.
- ^ Fin24 (www.fin24.com)
- ^ “Eskom wants to fire anyone who is fair and ethical” (mybroadband.co.za)
Telegram recently announced it would launch an ICO for its blockchain platform. The blockchain- Telegram Open Network (TON) – will feature a cryptocurrency called Grams. TON aims to deliver a fast, scalable blockchain with support for decentralised applications, distributed file storage, and micropayments.
Telegram aims to raise £1.2 billion from its ICO, where it will distribute Grams to investors following the launch of its wallet.
TON is an ambitious project which is more than a cryptocurrency integrating within the Telegram app. In its whitepaper for TON, Telegram outlined the focusses of the blockchain’s development and showed that it plans for TON to be a powerful and versatile platform. The blockchain will focus on decentralised applications, micropayments, and anonymity – taking on cryptocurrencies like Ethereum, Bitcoin, and Monero – and will deliver the following:
- Storage – Distributed file-storage (similar to IPFS) will allow for torrent-like peer-to-peer file transfers.
- Proxy – TON Proxy hides the identity and IP addresses of TON nodes and will be used to create decentralised VPN services.
- Third-party apps – TON will include support for third-party decentralised applications built on smart contracts, as well as a web browsing experience.
- DNS – TON’s DNS will allow users to assign human-readable names to addresses, similar to Ethereum’s ENS project.
- Payments – Telegram will implement micropayment channels and off-chain scaling solutions.
To support these features, TON uses a dynamic sharding scaling model, consisting of a master chain with primary consensus and up to 292 side-chains.
These side-chains can split and merge dynamically to spread the transaction load evenly over the network. The blockchain will eventually be fully decentralised, with validator nodes maintaining consensus through a proof-of-stake mechanism and earning Grams for their contribution to the network. A major advantage for the adoption of TON is the integration of the blockchain into Telegram’s Messenger app.
TON light wallets will be built into Telegram mobile applications, allowing users to store their funds on the blockchain with minimal hassle. The platform will also allow Telegram users to transfer and spend cryptocurrency.
The Gram cryptocurrency will be the central medium of exchange on the TON blockchain. Grams will be used for the following transfers on TON:
- Commission paid to TON validator nodes.
- Stakes deposited by validators to be eligible to validate transactions.
- Capital lent to validators in exchange for a share of their reward.
- Voting power required to support or oppose changes in the parameters of the TON protocol.
- Payment for services provided by apps built on the platform.
- Payment for storing data securely in a decentralized way.
- Payment for registering blockchain-based domain names.
- Payment for using the TON-based proxy.
- Payment for bypassing censorship imposed by local ISPs.
If application owners opt to adopt a freemium or advertisement-based model, these services could be made free to users.
Telegram will distribute Grams following the launch of its TON main network in Q4 2018, and the implementation of its light wallet client. The development roadmap for the TON blockchain is outlined below.
Telegram’s ICO will take place in Q1 2018, and the company aims to raise £1.2 billion from the endeavour.
If it succeeds, the ICO will be the biggest ever by a factor of five. During the first stage of the platform’s development, Telegram will hold the majority of the Gram tokens to prevent speculative trading and manipulation. Telegram will the release Grams to network participants at a later stage.
The ICO will see 2.2 billion tokens distributed among investors, with the price of a token starting at £0.1.
The funds raised by the token sale will be used for the development of TON, said the company.
By 2021, Telegram aims to make the project truly decentralised and lose the Telegram element in its name, eventually becoming “The Open Network”.
- ^ recently announced (mybroadband.co.za)
- ^ Big debt problems for South African cryptocurrency investors (mybroadband.co.za)