- Mixing solutions of different salinity (salinity gradient) generates renewable energy that can be harvested.
- Capacitive mixing is a promising method of salinity gradient energy harvest, using electrodes to exploit the energy generation due to mixing.
- Professor Tianwei Ma of the College of Engineering and Computer Science at Texas A&M University-CorpusChristi, USA and fellow researchers have collaborated to investigate capacitive mixing, specifically how the structure of electrodes can improve blue energy harvest.
- Their latest work shows using surface-modified nanoporous electrodes could lead to improved capacitive mixing performances, making this method comparable to traditional membrane-based methods.
Did you know that you can harvest energy from mixing water streams? In an estuary, where a river meets the ocean and the two water bodies merge, the physical and chemical processes occurring can lead to energy generation. This is a renewable source of energy called ‘blue energy’. Blue energy can be produced from water streams spinning a wheel, or from harnessing wave energy. It can also be produced due to physicochemical phenomena, such as the difference of the composition between two water streams that are merging.
Chemically-speaking, mixing two solutions with different salt concentrations produces a novel solution (thermodynamic system) with a new uniform concentration and equilibrium state. During the process of reaching the new equilibrium, water and salt molecules rearrange themselves to ensure the new solution has a uniform concentration. The process of mixing releases energy, known commonly as the ‘Gibbs free energy’.
However, mixing two solutions, such as two water streams, in a controlled way requires energy, such as that required by pumping and pre-treatment of waters. In this case, the Gibbs free energy that is released is more than the energy required to mix the waters. This ‘leftover’ Gibbs free energy is available for harvesting and use.
The mixing of a river (low salt content) and the sea (high salt content) water tends to generate a much higher amount of energy than can be harvested and used. This process, known as ‘salinity gradient energy harvest’, has been gaining a lot of attention lately. Professor Tianwei Ma of the College of Engineering and Computer Science at Texas A&M University-Corpus Christi (TAMU-CC), USA is one of many on the scientific quest to harvest blue energy and move away from fossil fuel energy.
Salinity gradient energy harvesting – how is it done?
The most well-known method for salinity gradient energy harvesting is the use of semi-permeable membranes that separate two bodies of water with different salinity, exploiting the naturally occurring osmosis. Osmosis requires a concentration equalisation between two solutions with varying salinity content when separated by a semi-permeable membrane. To exploit osmosis for energy generation, we can amplify the natural ion movement by applying pressure either on the low salt concentration side of the membrane (pressure retarded osmosis) or electrical current (electrodialysis).
The most well-known method for salinity gradient energy harvesting is the use of semi-permeable membranes that separate two bodies of water with different salinity, exploiting the naturally occurring osmosis.
While both processes are well-studied and industrially applied, they have serious drawbacks – mainly related to the use of membranes, the external energy required, and the maintenance needs. A novel method has thus been gaining attention in this quest for efficient salinity gradient energy harvest.
This involves capacitive mixing using porous electrodes. Capacitive mixing employs electrodes immersed in a salty solution to directly store the energy generated due to change of salt concentration, from higher to lower, when freshwater is added.
Capacitive mixing: Nanoporous electrodes
The electrodes used for capacitive mixing have unique porous surfaces that allow for better energy storage due to the cavities that enhance the micro-climate where concentration gradient occurs. Ma has conducted extensive research in this area, exploring the effect of nanoporous electrode structures. In a recent publication, Ma and fellow researchers show that the smaller the pore structure of the electrode, the better the result when it comes to harvesting the energy generated due to salinity gradient.
The findings illustrate that using nanoporous electrodes with specific pore density can generate comparable or better results compared to the traditional salinity gradient energy harvest methods.
The team specifically looked at nanoporous electrodes, meaning pore structures of only a few nanometres, and how pore size and pore distribution can affect energy harvest. Their findings illustrate that using nanoporous electrodes with specific pore density can generate comparable or better results compared to the traditional salinity gradient energy harvest methods (osmosis, electrodialysis) without the need for membranes or application of external energy.
Nanoporous electrodes with surface functionalisation
Having identified nanoporous electrodes as a promising method to harvest blue energy, the team looked into optimising the electrodes by tailoring the surface of the pores. This could be done by adding ‘chemical helpers’, via a process called surface functionalisation. In their most recent publication, they revealed that nanoporous electrodes with surface functionalisation led to better salinity gradient energy harvesting compared to bare nanoporous electrodes.
The team used oxygen- and nitrogen-based compounds at various concentrations, to explore both negative (oxygen) and positive (nitrogen) charge functionalisation. The presence of specific chemicals on the electrode surface when immersed in a salty solution led to protonation or deprotonation (transfer of proton). Depending on the nature of the chemical and whether it accepts (oxygen) or donates (nitrogen) a proton, this led to a positive or negative charge change, respectively.
As a result of the natural energy generation caused by the salt ion movement along with the addition of the nanoporous electrode and the charge change caused by the charge functionalisation, more energy is generated and stored on the electrodes. Results also showed that the higher the concentration of functionalisation agents, the higher the effect they had on capacitive mixing.
The future of salinity gradient energy harvesting
Compared to existing blue energy harvesting methods, the team’s work using nanoporous electrodes with surface functionalisation showed a similar or higher performance, with the added benefit of electrode durability for over 54,000 cycles. These results demonstrate a promising and sustainable method to harvest blue energy through capacitive mixing, which does not require membrane use, high maintenance costs, or external power used in current industrially applied methods. Ma maintains that future work is required to show the scalability and industrial applicability of this method.
Can this process be scaled up for industrial application under ‘real-life’ conditions?
We are developing a containerised clean energy generator from mixing waters of different concentrations (eg, ocean water and river water). It is estimated that over 5000 terawatt-hours (TWh) of energy is abundantly available from the natural mixing of river water and seawater each year, which is about 20% of global electricity consumption. America has tremendous blue energy source as the total length of shoreline is over 95,000 miles. Unlike solar and wind energy, blue energy can be harvested under almost all weather conditions, which renders it a reliable and clean energy source.
This process can be scaled up for real-life applications. For example, containerised energy harvesters with a size of 10 ft x 10 ft x 40 ft can be fabricated to generate about 160 kW, which provides electricity for normal household usage in remote, coastal areas. We envision it being connected to the grid to provide a supplemental energy supply when other renewable energy sources are not available. The electrodes can be mass-produced and arranged in stacks or spiral-wound systems, which will maximise the volumetric power density of the energy harvester to achieve the performance required by real-world applications.
How does the capital and operational cost of this process compare to costs associated with reverse osmosis or reverse electrodialysis?
The capital and operational cost of these processes depend on various factors, such as the design, scale, location, technology, energy source, maintenance, and environmental impact of each system.
For Pressure Retarded Osmosis (PRO), the capital cost of PRO plants ranges from $0.38/KWh to $0.56/KWh. The operational cost can vary from $0.16 to $0.24/KWh, depending on the system efficiency and performance. The membranes account for more than 40% of equipment costs. (Loeb, S, 2002, doi.org/10.1016/S0011-9164(02)00233-3).
For our process, the capital cost of the systems ranges from $0.2/KWh to $0.28/KWh, depending on the type and quality of activated carbon used. The operational cost can vary from $0.03 to $0.05/KWh. Additionally, the material is 10 times less than membranes. The membrane cost is about 50% of total cost.
Apart from being a sustainable alternative for blue energy harvesting, what are the additional benefits of your generator?
There are several advantages of the blue energy generator compared to Pressure Retarded Osmosis (PRO) and Reverse Electrodialysis (RED):
1. Simplicity of design and scalability: The blue energy generator has a simple design, utilising a pair of functionalised carbon electrodes to directly convert Gibbs free energy into electricity. In contrast, PRO and RED involve additional equipment for the conversion of Gibbs free energy to an intermediate form before electricity generation. The simplicity of the design and the use of functionalised carbon materials also make the system easily scalable.
2. Cost-effectiveness: Core materials in the blue energy generator, carbon materials, are relatively cheap and durable. The reduced material costs, combined with the simple design, scalability, and durability, contribute to cost-effectiveness compared to membrane-based methods.
3. Durability and maintenance: The blue energy generator prototype shows excellent durability, retaining 90% capacity after 54,000 charging-discharging cycles. This contrasts with potential fouling and durability issues associated with membranes in PRO and RED. The reduced maintenance requirements and longer lifecycle could contribute to cost savings.
4. Reduced fouling potential: Studies suggest that porous carbon materials used in the blue energy generator show reduced fouling potential compared to membranes in PRO and RED when natural water with organic matter is used. This may simplify operational processes and reduce the need for frequent cleaning and pre-treatment of source water.
