Friday, September 27, 2013

How can Robo Raven “feed” itself in jungles?

Our previous version of Robo Raven needs to be plugged into an electric socket for charging the battery. Ultimately, we envision Robo Raven flying deep into jungles, far away from civilizations, and hence electric sockets. To do this, Robo Raven needs to figure out a way to “feed” itself to keep going during long missions.

Real ravens are omnivorous and are happy to eat whatever is available. Unfortunately, mimicking this feat in Robo Raven is not practical at this point in time because the equipment necessary to convert biomass into 30 W of electrical power would make Robo Raven too heavy to fly. Since it is not practical to build a flying platform that can directly convert the biomass into energy needed to flap wings at the moment, we had to come up with a different option to “feed” Robo Raven.

From an energy perspective, ravens are constantly converting biomass into mechanical energy to flap their wings. Common sources of biomass that ravens consume in the wild are carcasses of dead animals. The dead animals accumulated their biomass by consuming plants which converted solar energy to biomass. Here is a high level summary of the energy conversion process at work behind the flight of ravens. Solar energy is converted to biomass (i.e., plants). One type of biomass (i.e., plants) is converted into another type of biomass (i.e., meat). Finally, ravens convert the biomass (i.e., meat) into mechanical energy needed to flap wings. So, ravens ultimately derive their energy from the sun.

We decided to bypass the multi-step energy conversion process used by ravens and instead have Robo Raven harness solar energy directly. Robo Raven features sufficiently large wings, so we decided to make wings out of flexible solar cells since there would be enough surface area for solar cells to generate a usable amount of power. The underlying material of the flexible solar is different from the material used in the previous version of Robo Raven, so we needed to design new wings. Additionally, we had to develop a new additive manufacturing process for making these wings. Solar cells on Robo Raven do not produce enough power to directly drive the motors (they produce around 3.6 W while we need around 30 W). So we decided to charge the battery using the solar cells. I am happy to report that thanks to the hard work of Savannah Nolen, Ariel Perez-Rosado, and Luke Roberts, students in our lab (co-advised by Hugh Bruck and I), we have developed Robo Raven III, the first flapping wing micro air vehicle that flies with solar cells. Please see below for a video.

So how good is the performance? Solar cells currently cover less than half the wing area in Robo Raven III. These solar cells produce 3.6 W of power during a sunny day. The efficiency of these solar cells appears be around 6%, and the combined efficiency of batteries and motors is somewhere between 25 to 50%.  We hope that the performance will go up significantly as more efficient solar cells become available and we cover more of Robo Raven III’s wing and body area with solar cells in future versions.

So how does this compare with energy conversion efficiency found in the nature? Plants are able to convert less than 10% of available solar energy into biomass. Plant-based biomass to meat-based biomass conversion is not very efficient either. It takes around ten gram of plant based biomass (e.g., corn) to produce 1 gram of meat if you ignore the energy needs of other body parts and metabolism. In other words, you need to eat at least 10 lbs. of corn to gain 1 lb. of body weight. Finally, converting energy stored in biomass into mechanical energy is also not very efficient.  Animals use the aerobic respiration to derive energy from the food, and typically less than one fourth of the energy available from respiration is converted into mechanical energy. Animals also lose a lot of energy due to metabolism.

As described in the paragraphs above, using solar cells to convert solar energy directly into mechanical energy for flapping wings is an order of magnitude more efficient when compared to conversion via the biological path. This advantage will magnify as solar cell technology improves, thus allowing conventional engineering to beat nature in terms of the solar energy conversion efficiency.

However, nature has a significant edge over engineered system in other areas. For example, one gram of meat stores 20 times more energy than one gram of the current battery technology. So in terms of the energy density, we engineers have a lot of catching up to do. In nature, solar energy collection devices (e.g., trees) are not on-board ravens. Hence, ravens ultimately utilize a large collection area to gather energy into highly a dense storage source (e.g., meat), giving them a much longer range and better endurance than Robo Raven III.

We still need to make significant improvements in solar cell efficiency and battery energy density to replicate the endurance of real ravens in Robo Raven III, but the good news is that Robo Raven III has already demonstrated that we can fly with a solar cell and battery combination. Now that we’ve successfully taken this step, swapping new technologies that are more efficient should be relatively simple!