The power of water

Water, the elixir of life, evaporates from oceans and land, forms clouds and returns as rain or snow. This movement governs weather patterns, regulates the Earth's temperature and shapes its mineral cycle. Now, scientists are harnessing it to generate clean and renewable energy in an emerging field called hydrovoltaics. Hydrovoltaic devices generate electricity when water flows through a charged porous material carrying ions along the surface, creating a small electrical current. These devices are small, simple, low-cost, flexible and portable. A floating hydrovoltaic material placed above lakes and oceans can harness electricity when water evaporates; such devices on rooftops can generate electricity when rainwater falls on them; those fixed to walls or embedded in clothes can harvest electricity by tapping moisture from the air.

The technology, however, faces an obstacle: it yields low power. "One metre of a hydrovoltaic device produces just 4-5 watts of power... You cannot even light an 8-watt bulb," says Dhirendra K. Rai, Associate Professor at the Indian Institute of Technology (IIT) Indore. But while such low-power technology was not useful two decades ago, the rise in the use of smart devices that require only microwatts of power has put the focus on this energy-harvesting technology. Rai points out that hydrovoltaic devices can power sensors, Internet of Things (IoT) devices and wearables.

Freshwater, apart from water molecules, has positively charged hydronium ions and negatively charged hydroxide ions. Natural or seawater also consists of positively charged sodium ions and negatively charged chloride ions. Water flows when evaporation creates a suction pressure, and, in this movement, the negatively charged material attracts positively charged ions. The negative ions are left behind, creating a charge imbalance at the two ends, generating current. An ideal hydrovoltaic material has high porosity, high surface charge and a large surface area, enabling interaction between the water and the material. Scientists are developing such materials and demonstrating their use.

In 2025, a team led by Dipak Kumar Goswami at IIT Kharagpur developed a biocompatible hydrovoltaic material by embedding tin disulphide in a gelatin film, and showed that it generated a power density of 358.6 microwatts per square centimetre at 90% relative humidity. He demonstrated that the device could generate electricity even from moisture in human breath and suggested its application for monitoring breathing patterns or conditions such as sleep apnoea (bit.ly/Breath-Device). Similarly, in 2019, Chinese researchers developed a self-powered air-quality monitoring system that uses a porous carbon film to generate voltage as rainwater evaporates. The surface charge in the carbon film generates current and binds to various gas molecules, as detected by changes in the current, making it useful for monitoring air quality (bit.ly/Rainwater-Fuel). At IIT Hyderabad, a team has developed a hydrovoltaic cell that can harvest 37.5 microwatts of power per square metre from moisture, sweat, and rainwater by coating borophene nanosheets onto carbon cloth and creating a moisture gradient structure with hydrophilic and hydrophobic coatings. They also demonstrated its application in powering LEDs and ethanol sensors and predicted its use in wearable electronics, health-monitoring devices, and environmental sensors (bit.ly/Power-Harvest).

Apart from nanomaterials, scientists are using natural materials such as wood, cellulose, nutshells and biochar, which are water-loving, have natural porosity and surface charge, and are inexpensive. A team from Huazhong University of Science and Technology, China, electrospun cellulose fibres, adjusting their pore size to less than 25 nanometres and their porosity to 52.6% by compressing the spun fibres. They showed that by coupling three such cellulose-based hydrovoltaic devices, they could power an electronic calculator (bit.ly/Powering-Calculator).

In India, the first evidence of harnessing electricity from water movement was observed in 2003, when researchers found that voltage was generated when a liquid flowed over bundles of carbon nanotubes. The field of hydrovoltaics, however, grew slowly, gaining momentum only in the last decade as advances in materials sciences enabled scientists to engineer efficient hydrovoltaic materials.

Rai of IIT Indore, for instance, has improved the hydrovoltaic potential of graphene oxide, which is water-loving and has a naturally high surface charge and nanochannels. But it swells and dissolves in water, distorting porosity and reducing voltage generation. To prevent swelling, his team intercalated graphene oxide with zinc imidazole, which increased the interlayer spacing, creating better channels for water, and also improving the surface charge density. It was found that intercalation increased the voltage from 0.1 volt (V) to 0.75V and the current from 0.06 to 0.4 milliamperes. By combining three such devices in a series, it lit a 2-volt commercial LED (bit.ly/Hydrovoltaic-LED).

Scientists are employing different strategies to enhance the power of hydrovoltaic devices. Sudip Kumar Batabyal at the Amrita Vishwa Vidyapeetham moved from a 2-D to a 3-D structure, increasing the surface area and evaporation rate for better results. Instead of a thin film, he and his team moulded it into a tubular structure, which increased the voltage from 1V to 9V.

Batabyal makes the hydrovoltaic material perform other tasks as well. In 2025, his team developed a material which could generate both freshwater and electricity. To develop this, they attached sulphonic acid (SO₃H) groups to graphene oxide, increasing its water-absorbing capacity. This graphite material also raises its temperature instantly in response to light, thereby increasing the rate of evaporation of the absorbed water. When humidity is high, the material absorbs moisture, and absorbed water dissociates the sulphonic group into positively charged hydrogen ions and negatively charged sulphite ions, creating a charge difference and generating current. The vapours are directed to a condenser, where they cool, producing pure freshwater. The team found that this material captured 0.3 grams of water per gram of material in one hour and released purified water with 25 ppm and a pH of 6.32 after 30-120 minutes of sunlight, while producing a voltage of 0.25V and a current of 0.16 milliamperes (bit.ly/Power-Water).

His team has also developed a porous cement-carbon composite that, when floating on water, absorbs sunlight, driving water evaporation and generating electricity. The evaporated water can be condensed and purified. Batabyal believes this may be a future solution for desalination.  "(There is) no need for an expensive membrane or electrical heater. By solar energy itself, I think 5-10 litres of water per square metre can easily be generated in a very cost-effective way," he adds.

Because hydrovoltaics produce little power on their own, scientists are also combining them with other energy-generation technologies, such as photovoltaics (electricity generation from sunlight) and thermoelectricity (electricity generation from a temperature difference). In 2026, scientists from Saudi Arabia and the U.S. developed a hybrid system with solar photovoltaics on top, followed by a hydrogel and a cellulose hydrovoltaic device at the bottom. The hydrogel absorbs waste heat from solar panels and passively cools the solar photovoltaic system via evaporative cooling, enhancing its efficiency. This evaporated water powers the hydrovoltaic devices at the bottom, generating power from both photovoltaics and hydrovoltaics. Through such a system, photovoltaic efficiency increased by 15% and the power output of hydrovoltaic devices by 150% (bit.ly/Hybrid-Methods). In 2024, researchers at Tiangong University, China, developed a fabric that generates power from both temperature differences and evaporation. The fabric has three layers: the first layer of polypyrrole absorbs sunlight and passes it to a thermoelectric bilayer, and the third layer of water-loving polypropylene nonwoven fabric acts as a hydrovoltaic material. While the thermoelectric set-up alone produced only 6.6 millivolts, the hybrid set-up increased the current to 73 millivolts (bit.ly/Three-Layers).

Scientists hope this low-power technology will follow the trajectory of photovoltaics, which is now a clean, green technology but took decades to develop. One day, perhaps, billions of hydrovoltaic sensors will work together to keep the world green and connected.