Tech Briefs: What got you started on this project?
Dr. Zhiqun (Daniel) Deng: I'm a mechanical engineer. My research focuses on developing integrated sensing systems to understand and mitigate the environmental impacts of renewable energy systems. So, our goal is to help employ more renewable energy systems — and ocean energy has great potential. I also work on other renewable energy systems as well, like hydropower and wind, which share similar challenges. You need to monitor their operations once they're deployed. So, I focused a lot on developing advanced sensing systems, including for some established renewable systems — hydropower, for example. We tie our measurements to the operations of the hydropower systems, so we can better understand them and either improve the turbine design or operate them to both maximize the power generation and minimize their environmental impact.
That's a key part of my research, but another aspect of it, as part of developing sensors, deals with the question of how to power them — especially these days when autonomous real-time sensing has become increasingly important. Our system for powering the sensors has the added bonus that we can potentially scale it up for power generation systems that can be part of our renewable energy supply.
We are a Department of Energy (DOE) lab and this project is funded by the DOE Water Power Program Office. Our ultimate goal is to sustainably generate electricity on a large scale. But we’re not there yet — similar to wind, where it took two or three decades from prototype to large-scale deployment. I think the same goes for marine energy. Right now, it’s still an emerging market so we’ve identified some short term and intermediate term applications for marine energy. This is a part of the DOE’s goals under their Powering the Blue Economy initiative — instead of going right to a large utility-scale system, our goal is to identify immediate applications. That's why we came up with a technique to power small sensors as our first application. Since developing small sensors had already been a main research focus for my team, this was a natural fit for us. That's why are trying to demonstrate this with a generator that can power acoustic transmitters and other small sensors.
What we have right now is very small — milliwatts. But many acoustic sensors can run on less than milliwatts. And, some water temperature sensors, and quite a few water quality sensors can run on milliwatts, as well.
Tech Briefs: What kinds of water quality sensors are there?
Deng: Water temperature, salinity, and conductivity are some examples. And you want your measurements to be autonomous and in real time, which requires remote communication. Typically, if it's deep in the ocean, you have to use acoustic communication to the surface, then from the surface, satellite communication.
So, we identify sensors and communications as the immediate applications, because of their low power levels. Although we’re now at the milliwatt level, we hope to get to the watts level soon, but that’s still a work in progress.
Tech Briefs: Wouldn't the communications require higher power?
Deng: That depends on your range. A lot of acoustic transducers we work with are very low power. We have developed small transmitters for example, to localize marine animals or freshwater fish. We are also developing some low power satellite transmitters as well. Based on our calculations, that’s feasible because they're not continuous — we might take a reading once or twice an hour, which can be sufficient for some applications. Once you can power some sensors and their satellite communications, they can be used for ocean buoys that deploy in remote locations such as the Arctic environment, which is really expensive and challenging to travel to. If you have self-powered autonomous remote monitoring capability, you can reduce the frequency at which you have to go there to service the buoys.
Tech Briefs: Tell me a little bit about the device.
Deng: We use the triboelectric nanogenerator approach. The triboelectric effect is what you experience when you wear a sweater and rub across it. For wave harvesting, you can use contact and separation mode, where you have two materials that you can separate and contact and are connected to electrodes. You can also use a sliding mode.
We used a free-standing mode for this study. Our device has two main parts — an external cylinder and an internal cylinder. We didn't invent this technology, but we have improved and adapted it for our application. Prof. Zhong Lin Wang of the Georgia Institute of Technology pioneered triboelectric nanogenerators (TENGs) a few years ago and he is recognized as its inventor.
Our innovation is we have a pair of magnets. Our device goes up along a wave, but the repellent force between the two magnets holds the relative position between the external cylinder and the internal cylinder so they don't move. Because of the added mass on the internal cylinder, once the device starts to rotate the potential energy gets bigger and bigger, but the repellent force of the two magnets holds the rotor in place until it cannot hold anymore as it starts to move past the crest of the wave. Then the internal cylinder will start to rotate very fast. If you didn’t have the repellent magnets, there would always be a slow rotation during the whole cycle.
Although the rotor only moves during a small fraction of the wave motion, because of the quick sliding motion between the internal and external cylinders, it generates more electricity and has higher efficiency — that's our main contribution.
Because we can hold the rotor in place, it can even be used for waves that are less than one Hertz or that have low wave heads. That's why we can convert low wave head, slow waves into a motion that can be efficiently harvested. And even though the size of our device is pretty small, we were able to generate enough power for the sensors we demonstrated.
There are a series of challenges: converting the wave motion to mechanical motion, mechanical motion to electrical energy, and then storing the electrical energy.
Tech Briefs: Isn’t the static electricity generated by the triboelectric nanogenerator at a high voltage?
Deng: That's a key challenge because it's hundreds of volts, but at a very small current. So, we have to use an efficient power management circuit.
Tech Briefs: Wouldn’t you need electricity to operate the power management electronics?
Deng: Yes, there are a couple of ways to do that — some people use a supercapacitor and others use a rechargeable battery. There are advantages and disadvantages to both, so we are evaluating them. If you use a rechargeable battery, you can still power your sensors when there are no waves. However, you have to consider leakage current, charging efficiency, and other factors. The power management circuit design is critical.
Tech Briefs: If you run your power management circuit with a battery, then aren’t you losing the advantage of your system because you'll have batteries anyway.
Deng: If you use a primary battery, you will have a limited service-life for your sensing system or for your buoy system. However, there are still some advantages even with rechargeable batteries. You can either extend the service life of your platform or reduce the frequency of your servicing. Ocean observing platforms are very challenging to service in general.
Tech Briefs: Would the power management circuit be up on the surface of the water?
Deng: Ours is an integrated system — all in one package. We're hoping that eventually It can be integrated with whatever the application platform is. For example, if it is a big buoy, it can be either external or inside the housing of the system platform. That's why we need to make all the parts as small as possible so it can be more adaptable for all of its applications.
Tech Briefs: Is there anything you want to add?
Deng: We still have a long way to go. We can power small sensors but our goal is to increase our output to the watts-level so we can power larger systems. In the future, we would like to develop our technology to be used as a charging station for underwater vehicles at the 100-Watt level to be placed in the open ocean — that's a long-term goal.
Tech Briefs: How long term, would you guess?
Deng: I would think 5 to 10 years. I envision a lot of challenges to go with that. Not just for the power generation. Other challenges will include reliability in harsh environments. We are doing reliability testing right now — accelerated testing to test fatigue and all of the moving parts under environments such as extreme temperatures and salt water — to see how long they can last.
Tech Briefs: Are you also working on the energy storage now?
Deng: We're working on energy storage and increased efficiency. We are also working on scaling up for different applications, one of which is for an Arctic sensing platform.
Finally, we hope to deploy our nanogenerator system in the field in a couple of years.