The heat is on

Pramod Pillai is working on a technology to efficiently convert the Sun's energy to electricity. However, unlike photovoltaic (PV) technology, which utilises light energy to generate electricity, Pillai is looking to harness the Sun's heat. "If you can design a material that can absorb the UV and visible parts and convert it into heat, you will get more electricity: you can utilise the sunlight better," says Pillai, Associate Professor at the Indian Institute of Science Education and Research (IISER) Pune.

There are three steps to this process: absorb maximum sunlight, convert it into heat and use this heat to produce electricity. Heat is derived from the Sun, and a thermoelectric device is used to convert heat to electricity. Such a device can generate an electric current when there is a temperature difference between the two ends of the device. In order to generate electricity, one end needs to be hot, the other cool. The higher the temperature difference, the more the electricity generated. One end can be kept hot by channelling waste heat from industries, power plants or from the Sun. Pillai, a nanoscientist, has demonstrated that photonic gold nanoparticles can absorb maximum incident sunlight and efficiently convert it into heat. This can increase the efficiency of solar-to-electricity conversion.

Decades ago, the quest to produce electricity from the Sun had led to research on photovoltaics. The first modern solar cells were invented in 1954, but their low efficiency (around 6%) impeded commercialisation for years. However, as research in the area progressed, the efficiency of solar cells improved, and the cost of solar panels dropped – which catalysed the widespread adoption of the renewable, clean and off-grid method of electricity production. Today, 1.6 terawatts of solar PV capacity has been installed worldwide, according to the International Energy Agency.

Ironically, however, climate-change-induced rising temperatures have impaired the efficiency of photovoltaics. Their intermittency also limits their applicability, particularly since energy storage systems are costly. Given this, a complementary technology that uses the Sun's heat to produce electricity should have an advantage, to the extent that solar heat can be harvested even on cloudy days; if thermal energy is stored and supplied to such systems, they can be operated at night. However, this alternative technology currently suffers from issues of low efficiency; scientists are looking to improve the efficiency so that it can be used either alone or together with photovoltaics to enhance electricity production.

Solar thermoelectric generators (STEGs) that generate electricity from sunlight consist of a thermoelectric device sandwiched between a solar collector and absorber, and a heat sink to cool the other end. This simple structure is low-maintenance, performs noiseless operation and has a longer lifespan.

Pillai's team added a thin layer of photonic gold nanoparticles on a commercially available TEG device. It found that this coating enhanced the power output by nine times and yielded an overall solar-to-electricity conversion efficiency of 9.6% at ambient conditions, better than the earlier reported conversion efficiencies (bit.ly/gold-nanoparticle). The team also showed that two such STEGs powered a calculator and a timer, and three connected STEGs were able to light up 120 LEDs.

This high conversion efficiency was possible due to the excellent sunlight-absorbing properties of gold nanoparticles and their ability to convert 95% of the light absorbed into heat. "There is a very high conversion of sunlight to heat, and metals like gold are very good conductors of heat. So, whatever heat is generated, it conducts it to the thermoelectric device," explains Pillai.

Although gold is an excellent solar absorber, its high price inhibits commercialisation, concedes Pillai. He is therefore working on copper nanoparticles; even if they are not as good as gold nanoparticles, they will be a game-changer as copper is abundant and much cheaper.

Early solar absorbers were mostly black coatings or paints, which had high absorption but lost heat easily. Advances in nanotechnology and photonics have led to the development of solar absorbers that capture sunlight and convert it to heat with minimal heat loss.

Other scientists are using different approaches to increase solar absorbance on thermoelectric devices. Sendhil Kumar Natarajan, Associate Professor at the Puducherry-based National Institute of Technology, used a solar parabolic dish concentrator device to focus sunlight on the solar absorber, increasing the temperature on the hot side of the thermoelectric device. However, he was able to get a conversion efficiency of just 2.76% (bit.ly/STEG-efficiency) in his set-up.

"We have to maintain the temperature on the cold side. Only then will delta T (temperature difference) be higher. The more the temperature difference, the greater the efficiency," explains Natarajan.

While Pillai and Natarajan were raising the temperature of one side of the thermoelectric device, Jeyashree Y. was trying to keep the other side cool. An Associate Professor at the SRM Institute of Science and Technology, Kattankulathur (in Chennai), Jeyashree became interested in off-grid technology due to the frequent power cuts in her city. She likes the STEG technology as it absorbs heat – unlike photovoltaics, which create a heat island effect.

Given her experience of working on thermoelectric devices, she reasoned that thermal stabilisation is the key to ensuring that the electricity production does not vary with varying sun intensity during the day. "If the hot side temperature as well as the cold side temperature are maintained constant, we will get constant voltage or electricity," she explains. An efficient heat sink that dissipates heat effectively on the cold side and contributes to establishing the temperature gradient across the thermoelectric device is therefore an important component of STEG technology.

Jeyashree's team used a phase change material (PCM) for thermal management on the cold side. PCMs absorb heat at a constant temperature, helping in temperature control. In the experimental set-up, the team used a Fresnel lens to concentrate sunlight to the hot side of a thermoelectric device coated with a carbon nanotube solar absorber. The other (cool) end of the device was provided with aluminium fins, which act as heat sinks. Additionally, the aluminium fins were dipped in a PCM composite, which consisted of aluminium oxide nanoparticles in a commercial PCM. The study showed that PCM and fins helped in maintaining the cold side temperature at 40-45°C (bit.ly/thermal-stabilisation).

While it kept the temperature constant, the nano-PCM also acted as a thermal energy storage, releasing the heat at night. This additional heat was used to drive the thermoelectric device at night. Through this one TEG set-up, the team got a maximum output of 0.615V and believes that integrating a large number of such TEG devices can help power devices. For example, 44 TEGs in series and 66 in parallel with a length of 330 cm and breadth of 220 cm can provide 76.5 watts of power. In the future, the team wishes to incorporate PCMs on both the hot side and the cold side of the STEG to maintain a constant temperature on the hot side as well.

Apart from maintaining thermal difference, scientists are also trying to increase the efficiency of heat-to-electricity conversion by developing new and more efficient thermoelectric materials. Advanced optimisation and system design of STEG systems are also being tried to make the whole system efficient. As the low efficiency of STEG is a major bottleneck in its deployment currently, scientists are combining thermoelectric devices with photovoltaic modules to make hybrid systems. In such integrated systems, the heat that is not utilised by photovoltaics is used by the TEG to produce some additional electricity. The added benefit is the removal of heat, which would otherwise have decreased the performance of PV panels. Scientists like Pillai, Natarajan, and Jeyashree hope that the STEG technology also follows the model of photovoltaics, and that breakthroughs in material science will make this technology commercially viable in the coming decades.

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