HydroVoltaics – Harnessing Electricity from Evaporation

New research shows that nanoscale devices can be used to harvest electricity from fluid evaporation, revealing a large energy potential that is yet to be properly utilized.

Evaporation is a natural process. In simple terms, it is a form of vaporization that happens on a liquid’s surface as it turns into gas. It is present all around us, but we don’t really pay attention to it. 

Interestingly, about half of the solar energy reaching the earth drives evaporative processes. Evaporation, which enables a continuous exchange of energy in the global water cycle, is a renewable source of energy that remains currently untapped, as per the study.

Water, which is available in abundance given that it covers over 70% of our planet’s surface, contains tons of energy in the form of chemical, thermal, and kinetic energy, but not much of it is harnessed. When it comes to the total power generation potential of natural evaporation from lakes and reservoirs, the US alone has an estimated power generation potential of 325 GW. This represents over 69% of the US electrical energy production rate in 2015, notes the study.

Therefore, over the past many years, researchers have been working on harnessing evaporation’s energy potential. This is being explored through many devices, such as self-powered generators, hybrid systems, and tandem devices. Research over the years has drawn a lot of attention to evaporation-driven hydrovoltaic devices, especially with the progress in nanomaterials and nanotechnology.

For that, let’s get into the HV effect, which allows nanostructured materials to generate electricity in interaction with water. Water here can be anything from droplets, moisture, and liquid to evaporation. So, in the HV effect, evaporation produces a constant flow within nanochannels inside these devices. This effect is also seen in plants’ microcapillaries, where water transport happens due to a combination of capillary pressure and natural evaporation.

Four mechanisms are primarily seen as responsible for this HV effect: streaming, pseudo-streaming, ion gradient diffusion, and the electron drag effect. The hydrovoltaic effect potentially extends the technical capability of harvesting water energy and enables the creation of self-powered devices. 

Now, researchers at the Swiss-based public research university EPFL (Swiss Federal Institute of Technology Lausanne) received funding support from the Swiss National Science Foundation (SNSF) via the Korean-Swiss Science and Technology Cooperation Fund and the Swiss Government Excellence Fellowship. EPFL’s experimental facilities, the Interdisciplinary Centre for Electron Microscopy (CIME) and Center for MicroNanoTechnology (CMi), are also involved in harvesting power from fluid evaporation that contains higher concentrations of ions than those found in water that is purified.

The thing is hydrovoltaic devices already exist, however, not much is known about the physical phenomena governing HV production at the nanoscale. There is also a lack of functional understanding of the conditions, something this study aims to rectify. 

Published earlier this month in the Cell Press journal Device, the study from the Laboratory of Nanoscience for Energy Technology (LNET) called “Salinity-dependent interfacial phenomena toward hydrovoltaic device optimization” is conducted by Giulia Tagliabue and Tarique Anwar.

The study experimented with multiphysics modeling to specify fluid flows, ion flows, and electrostatic effects due to solid-liquid interactions, with the aim of optimizing HV devices.

According to Tagliabue, who’s the head of the LNET at the School of Engineering, this is the first study that quantifies hydrovoltaic phenomena thanks to their new and highly controlled platform. These hydrovoltaic phenomena are measured by showcasing the importance of various interfacial interactions. 

During this process, the team discovered a big finding of hydrovoltaic devices’ ability to operate over a wide range of salinities. This, Tagliabue noted, contradicts the earlier belief that highly purified water is necessary for optimal performance.

The Multiphysics Model

Recently, studies of evaporation-driven fluid flow in materials that are microstructured at the nanoscale have given rise to a new way to generate renewable or green energy, which is by converting thermal energy into electrical energy through an electrokinetic pathway. Improvements related to performance here are led by the optimization of the electrode contacts. 

However, the lack of modeling tools and questions regarding the geometrical and chemical characteristics of the system limits both the performance and application range of this emerging technology, Hydrovoltaic (HV) devices, for sustainable energy generation.

So, the researchers developed a quantitative multiphysics model and leveraged ordered arrays of silicon nanopillars (Si NPs) that brought to the surface what was previously unexplored. Notably, the study found that the surface charge, which is dependent on ion concentration along with the mobility of ions, is what directs multiple local maxima in open-circuit voltage (VOC), with ideal conditions diverging from traditional expectations of low concentration.

Moreover, structural asymmetries can generate an electrostatic potential that enhances HV performance. On top of it, the study affirms ion adsorption and inversion of charge for many monovalent cations, allowing such devices to operate even at high concentrations. 

The HV device developed by the research team consists of centimeter-scale regular arrays of Si NPs etched in a p-type Si wafer. The team then made use of a combination of colloidal lithography and metal-assisted chemical etching (MACE) to fix the pitch of the hexagonal array of NPs while varying their length and diameter in the range of 1.23–4.4 μm and 420–560 nm, respectively. By changing the Si NP dimensions, they were able to directly control the nanochannel geometry and the solid-liquid surface area as needed.

The hydrovoltaic device in this study represents the first application of the nanosphere colloidal lithography technique, allowing the researchers to form a hexagonal network of precisely spaced Si NPs. The spaces between these silicon nanopillars created the perfect passage to evaporate fluids. This can further be finely tuned to better understand the effects of fluid confinement and the solid/liquid contact area.

Anwar, who’s a PhD student at LNET, explained that most fluidic systems with saline solutions have an equal number of positive and negative ions. But by confining the fluid to a nanochannel, we can get only those “ions with a polarity opposite to that of the surface charge.” So, by allowing liquid to flow through the nanochannel, we can generate current and voltages, he added.

Understanding the fundamental mechanisms of voltage and current generation in HV devices meanwhile requires control over the solid-liquid interface properties and the liquid nanoconfinement, which is nanochannel geometry.

In conclusion, the study showed a high power density output of 8 μW/cm2 at 0.1 M, using a power output comparable to that of tap water-operated devices but with “two orders of magnitude higher concentrations” than ever reported before. This paves the way for the broader applicability of HV systems across salinity scales, with optimal operating conditions being dictated by distinct interfacial phenomena.

As Tagliabue explained, the chemical equilibrium for the surface charge of the nanodevice can be exploited to extend the operation of HV devices across the salinity scale. So, as the concentration of fluid ion rises, the nanodevice’s surface charge increases as well, as such, allowing us to “use larger fluid channels while working with higher-concentration fluids. This makes it easier to fabricate devices for use with tap or seawater, as opposed to only purified water,” Tagliabue said.

This way, the study argues that it offers key insight and a design tool for optimizing evaporation-driven hydrovoltaic (EDHV) devices, along with pointing toward broader application opportunities for these self-powered systems.

The study states that the performance metric of their open-circuit voltage (VOC) can be augmented by improving the rate of evaporation. Depending on the surface charge and nanoconfinement’s geometry, VOC can be doubled by a 5x increase in the evaporation rate, which means that power can be increased up to four times. 

This is mainly because of the enhancement in streaming current but will require more in-depth knowledge of the fluid dynamics in these devices in order to be confirmed.

Exciting Potential Applications of Hydrovoltaic Devices

Water vapor is everywhere on earth, hence providing a big opportunity, especially for electricity generation and alleviating the shortage of global energy. For this, hydrovoltaic devices can be used to extract energy through different means, such as constructing an asymmetric structure. 

However, power generation depends on the properties of the liquid, including its type and the concentration of solute in the solution. This is because the creation of electric energy in the HV effect is connected with the accumulation of charge carriers caused by an electrical double layer (EDL) forming on the liquid-solid interface.

Despite these challenges, hydrovoltaic devices have huge potential due to continuous evaporation over a wide range of temperatures and humidities. This makes HV devices primed for many exciting applications, including power supply, all-weather power generation, and water collection and desalination.

The study found that for freshwater conditions, EDL overlap is a requirement that can be realized under low total surface charge and high confinement, i.e., small nanochannel size. Still, increasing the surface area or charge makes larger nanochannel sizes viable. 

In seawater conditions, an optimum can be created at large pore diameter (Dp values >100 nm) by controlling the surface charge, which suggests that geometrical confinement at the nanometer scale can be prevented, streamlining the scalability of these devices. 

Meanwhile, at high salinity levels, charge inversion can be used by minimizing the solid-liquid interface and initial surface charge. However, operation at such high concentrations and in the long term can be a challenge because of ion adsorption and salt crystallization. This is due to its direct effect on the properties of the surface and nanostructure’s geometry. Hence, the researchers wrote that further investigation is needed.

The researchers of the study also hope to explore the diverse potential of the device, with the support of the organizations that gave them grants, to understand and analyze this emerging technology. 

One of the supporting organizations, the Swiss National Science Foundation (SNSF), which is mandated by the federal government, aims to harness energy from evaporation and develop an entirely “new paradigm for waste-heat recovery and renewable energy generation at large and small scales.” This includes a prototype module in real-world situations on the largest lake in central Europe, Lake Geneva, to gather data on hydrovoltaic generation on alpine lakes.

SNSF has been supporting energy-related projects for many years now to contribute to the development of knowledge and expertise for future use. These projects cover energy production, storage, distribution and management, and efficiency.

Tagliabue received the SNSF Starting Grant in 2022, under which CHF 1.8 million is provided over five years. This funding helps the team “expand our effort towards the nanoengineering of hydrovoltaic devices for renewable energy generation,” said Tagliabue.

She said at the time:

“By establishing the necessary fundamental understanding, modeling tools, and engineering strategies, this project will provide a disruptive contribution towards making evaporation a widespread, safe, and continuous renewable energy source for both large and small scales.” 

HV devices offer a big opportunity, and because they can operate anywhere there is liquid, which can even be sweat, they also hold potential for use in sensors. This covers everything from health and fitness wearables to smart TVs.

Tagliabue is further interested in understanding just how light and photothermal (related to electromagnetic radiation) effects could be used to control surface charges and evaporation rates in HV systems.

Finally, researchers see a symbiosis between HV systems and clean water generation, with Anwar pointing out how natural evaporation is used for desalination processes. Condensing the vapor produced by an evaporative surface allows fresh water to be harvested from saltwater.

“Now, you could imagine using an HV system both to produce clean water and harness electricity at the same time.” 

– Anwar

Conclusion 

With the energy and environmental problems constantly rising, there is a need for efficient, flexible, and eco-friendly solutions. Here, hydrovoltaic devices can be of immense help as they generate electric energy by using ubiquitous water evaporation. 

However, this innovation does not come without its challenges. They manifest as generated power that falls short of practical application requirements, uncertainties about stability and durability in real-world conditions, and hurdles in achieving large-scale integrated applications. Despite these obstacles, the technology and research into hydrovolcanic materials and devices are still in their infancy, and there is a need for more advanced technologies and more research for their wide usage.

To put it simply, hydrovoltaic devices have great advantages which can’t be ignored. In contrast to complex, expensive, and environmentally damaging solutions, this presents a novel and promising method for power generation in the future. And with time, hydrovoltaic devices can be expected to grow into a viable and broad industry.

Click here for a list of the top ten renewable energy stocks to invest in.