Mind Map

For the nanoporous disc, the team investigated multiple options: the purchase of a manufactured ceramic disc, manufacturing of a disc by 3D-printing or sintering, or direct deposition of a nanoporous film on our scaffold using a Chemical Vapor Deposition (CVD) machine. After communicating with a representative from the Oak Ridge National Laboratory, where we have potential access to a CVD machine, we learned that this process could only practically be applied to a material with holes or pores on a microscopic scale, which might limit the concept’s compatibility with different scaffold options. This contact did recommend that we deposit a layer of nanopores on a commercially available metal microporous mesh to get around that complication, so we added a new concept based on this design approach.

For the scaffold connecting the nanoporous disc to the tubes, our advisor originally suggested a microporous ceramic or metal cylinder into which we would machine nubs and a heating channel, sealed to the nanoporous disc with simple silicone sealant. To these concepts we added a solid metal scaffold into which we would drill channels directly from the tube nubs to the nanoporous disc. This would reduce the complexity of the system by removing the need for sealing the outer edge of the scaffold (potentially reducing the risk of failure of the seal), and it should allow for greater heat transfer to the transpiration surface as compared to a purchased ceramic microporous material. It would also be less brittle than a purchased ceramic microporous material. However, because we were concerned about uneven filling of the nanoporous disc during the charging process, we also considered a similar concept that included an interconnection between the parallel channels near the nanoporous layer. Depending on which combination of nanoporous disc and scaffold material we choose, silicone sealant alone could be used to join the two layers. Our other concept for joining the layers, a clamp combined with rubber O-rings and sealant, could provide increased security with regard to the airtightness of the system, but might be incompatible with more brittle materials required for some nanopore or scaffold concepts.

C. Solar Concentrator Concept Generation

In natural trees, there are hundreds, if not thousands of leaves that pull water up the tree compared to our design which only has one. Given that all of the water within our system was concentrated in this leaf, introducing a method for increasing evaporation from the nanoporous medium would be highly beneficial and could be done by using a solar concentrator. A solar concentrator is a device that reflects or absorbs sunlight into a designated surface area to be used for heating purposes. For our system, it was proposed that conductive fluid be pumped through the microporous scaffold of the lead. The fluid would be heated by the sun and focused on a tube, either plastic or metal, that is fixed in the scaffold to the leaf. It was crucial to use a solar concentrator in this year’s system, as we intended to show the scalability of flow rates within the tubes with a higher transpiration rate.

We determined that two methods of solar concentration would be examined: reflection and absorption. Utilizing reflective mirrors to concentrate heat onto a specific surface area could  increase the total heat absorbed, however it would be quite costly and difficult to construct. Considering this, we produced a new concept that would use a black body to absorb sunlight and radiate heat over a surface. To transfer the heat, we found that using a high temperature-rated tube would be the most straightforward and effective solution, as we have to heat the interior of our microporous scaffold. In the reflective mirror design, the tube would be fixed at a calculated distance from the reflective surface, while for the black body design, the tube would be fixed inside and allowed to heat up. To transport the heat to the tube, we evaluated multiple materials and found that a glycol solution would be appropriate due to its high thermal conductivity and resistance to boiling. Car engine coolant uses a 50/50 mixture of glycol and water and has a high temperature rating, so we determined that it would be the best commercially available option. We also concluded that a low maintenance, high temperature chemical pump would need to be purchased to circulate the fluid. As will be discussed in the “Detailed Design” section, the team chose to forego the incorporation of a heating element within the leaf scaffold as we found this would complicate our design and was not practical for our purposes.

A parabolic concentrator as well as a trough reflector were evaluated as potential design solutions. The two concepts function quite similarly; the sun is reflected off of the mirrors and concentrated into a horizontal running tube that carries a thermal fluid. These designs are highly efficient as they produce large amounts of energy, yet they are both costly and difficult to maintain. Because the parabolic and trough reflectors need to always be directly facing the sun to achieve favorable efficiency, we considered a parabolic concentrator that tracks the sun throughout the day. It would utilize either a timer or optical device that notes the time of day and accordingly adjusts the angle of the device. A conical, or vertical cylinder design was also proposed; this device would work well at peak day, but again, the geometry limits the efficiency of the device significantly. Taking these different findings into account, we developed a new design that we call the planar solar concentrator. It was a black box that contained a coiled tube through which a pump would drive the thermal fluid. This system does not require tracking of the sun and is not limited in its shape. It is also very cost effective as well as simple to use, and demonstrates similar thermal results to the others due to its excellent average energy production. It was also proposed that adding reflective mirrors on the outside at both the east and west positions of the box would increase the amount of energy that could be absorbed by the system. To quantify the concepts and determine which would be best for further improvements, we performed concept down selection by weighting engineering characteristics using pairwise comparison and finding the concept with the best overall score. 

Mdot is the mass flow rate of water, Wsolar is the power generated by the solar concentrator, and l is the latent heat of evaporation. Wsun is the solar power exerted on the surface of the Earth, on average, Ar is the reflector area, Ө is the reflector angle, and As is the surface area of the box surface. Based on our current solar box design, we expect to generate and direct 200 W of heat onto the leaf scaffold during daytime hours. This is based on the solar power concentrator directly into the box from the sun and from each of the reflectors. Assuming most of this heat is absorbed by the water, we would expect a mass flow rate of about 2.7 * 10-5 kg/s. Once again, the viscous pressure generated by this flow rate is lower than the maximum Laplace pressure, so it can be supported by the design.

The main advantages of this concept were that heat was introduced into the leaf in a direct fashion, as opposed to running tubes through the concentrator and then circulating the fluid in the tubes from the concentrator to the leaf. For this particular design, the size of the concentrator was smaller, making it optimal for mounting the leaf inside. This also streamlined the design as the leaf and solar concentrator mounting structures were combined into one. The only drawback to this design was that it was less effective as the design scaled. If more heat was needed it would have been more ideal to revert to the design with the tubes running through the box. On the other hand, the solar concentrator wasn’t exactly a key component of the long term design of the tree, but rather a mechanism for its function. It was present to provide heat, meaning that it could be replaced by waste heat from a power generator. 

Conclusion

The year largely consisted of design work, and culminated in a nearly fully functional final product. We generated customer needs and target specifications based on the requirements provided by our faculty advisor. Next, we generated initial concepts and performed down selection to ensure that the generated specifications were properly met. Then, we performed detailed analysis utilizing the driving equations of the overall system and generated CAD of the sub systems. After finalizing the CAD of the system and locating specific part numbers and prices of components, we ordered the necessary parts. Finally, we constructed and attempted to test our system. All components and subsystems were functional except for the nanoporous mesh, and we determined the most likely cause for the failure of this component. Our work this year should provide future teams with a platform from which to test a new nanoporous mesh and expand the design to generate hydropower. We have discussed potential additions, such as a solar still that would condense water vapor from the leaf and transport the water back downwards over a turbine to generate electricity.