*Please note that some of this stuff has been changed and may not accurately reflect the current system, but overall, it's pretty close. Please don't steal our information and ideas. Thanks!
Introduction to MHD
Magnetic fields can produce an electric current in a closed lip, but only if the magnetic flux linking the surface area of the loop changes with time.
This project is a variation of the initial Faraday’s Law. A conductor is moved in a static magnetic field. Consider a wire of length l moving across a static magnetic field B (which the stationary magnets are going to be creating in our system) at a velocity of v. The conducting wire contains free electrons. The magnetic force F acting on a any charged particle q moving with a velocity v in a magnetic field B is given by
F=q(v x B).
Since magnetic force is equivalent to the electrical force
E = F/q
The field E generated by the motion of the charged particle is called a motional electric field, and it is in a direction perpendicular to the plane containing v and B. The magnetic force acting on the electrons in the wire causes them to move in the direction of –E. This in turn induces a voltage difference between the two ends.
From this theory of how the conductive fluid flowing through a magnetic field will in turn produce a voltage from the above equation we will need to design all the above parameters to be the maximum of what we can design. The velocity will need to be as large as possible; the magnetic field will need to be large and so will the cross sectional length of the conductive fluid will also need to be as large as possible
Current Solutions
Currently, there are no solar MHD products available. Most MHD systems in existence involve superheated plasma as its conductive fluid. With them, issues of acquiring such fluids and harnessing solar power to get temperatures high enough become concerns. Such tasks would likely be economically impractical.
There are also many forms of alternative energy power generation. In terms of solar power, photovoltaic cells and concentrating solar thermal power plants are in existence. In addition, there are numerous other forms of alternative and renewable energy as well as traditional fossil fuel burning systems in existence. It should be noted that such systems doing mechanical and chemical work operate under different principles than MHD.
Problem/Goals
While the mission statement guides the group towards a working prototype, the project is being treated more as a feasibility study of the solar MHD concept, not necessarily to offer competition to other solar power systems already in existence. Inducing a voltage potential within the system would be highly desirable as it would help with the electricity generation. Current would be drawn through an outside resistive circuit integrated with the generator and detected with readily available measuring equipment.
Ideal Final Result
If there were no restrictions on resources and laws of physics could be bent, the IFR would be a cheap, sturdy, and environmentally safe system that collects and converts solar energy into electricity with 100% efficiency.
In reality, the idea of the Carnot efficiency, a number based off the maximum and minimum temperatures of the system, limits efficiency of the work done in a thermodynamic cycle.
Functional Requirements
The top level functional requirement for the system is to produce electricity. Within that requirement, other requirements must be met. It must draw power. A fluid density gradient must be produced by containing pressure, converting solar energy to heat on one side, and dissipating the heat on the other side.
Constraints
In addition to fulfilling the functional requirements, there are other restrictions governing the design, manufacturing, and use of the system.
Financially, the team has a limit of $10,000. This is the amount of money provided by the EPA for the project. Additional funds may be pursued to cover the travel and shipping costs for the team to present the system in Washington DC in the spring.
A related constraint is that the system must be transportable. Weight is to be taken into consideration as through the life of the system, it will be primarily moved by the team, which at most has six people together at any given time. A similar constraint is the system must be small enough to fit in elevators and get through doors.
Limiting negative environmental impacts is also a notable constraint. With the goals of the P3 project and the fact that it is sponsored by the EPA, it is desirable to do more good to the environment than to risk damaging it more. These constraints are constantly taken into consideration during the design of the system.
Final Design
The basic design was created by the faculty advisers/sponsors. Different kinds of analyses were performed by the student group to optimize the performance of individual components within the design. There are six fundamental component subsystems that make up the whole system; conductive fluid, a reservoir, a solar trap, a magnetic field, an electrode assembly, and heat sinks.
Conductive Fluid
The first choice the group made was deciding on a fluid that would conduct electricity. Websites were available that had large lists of fluids and their properties. While conductivity was of significant weight, so were coefficients of thermal expansion, heat transfer coefficients, viscosity, and cost.
Salt water was selected as it has very low electrical resistance with respect to the load resistor, its viscosity allows for it to flow freely, it expands quickly and is very inexpensive since it only cost the price of salt.
Other fluids taken into consideration were mercury and different kinds of acids, however, these fluids can get expensive and there is significant risk of harming people and the environment in the event of a leak. This is where the true Wow Factor of using salt water is taken into consideration.
Reservoir
To store the fluid, a reservoir will be built. Before the dimensions were set, computer simulations were performed using a FORTRAN program written by Dr. Li. The simulations were run with scenarios representing various lengths, widths, and depths of the channel. Numbers associated with the simulations are used to determine the relation between buoyant and viscous forces, or the Grashoff number. It was determined that length (or height) increases velocity and efficiency, depth increases velocity and output, but with no effect on efficiency, and channel width should be minimized but keep laminar flow. Laminar flow is of particular importance in this project as the turbulence means there will be wasted energy and the velocity would be reduced.
Solar Trap
The purpose of the solar trap is to collect and retain uniform heat. This will allow distribution of heat into the reservoir. A large Fresnel lens will be used to focus sunlight onto the heat trap. With the large concentration of sunlight in a small area of the solar trap, there will be a temperature gradient compared to the rest of the system.
Sunlight will hit aluminum mirrors angled to reflect a maximum amount of sunlight into holes in the trap. They are 8.5 inches long on the solar trap providing a field of view of 140 degrees. The holes will serve as blackbodies to keep heat in. Finite element simulations were performed using ABAQUS for various geometries. The actual velocity of the fluid was still a changing variable.
Magnetic Field
To create the magnetic field required for Faraday’s law application, magnets will be purchased and used in the system.
While researching magnet sellers online, it was found that there are many choices. Knowing the details for size and performance specifications, a 3” by 0.5” by 0.125” Neodymium Rare Earth Magnet, with 0.313 Tesla, and 105.67 lb pull force will be the primary type of magnet being used. The cost of these Neodymium Rare Earth Magnet are going to be $5.00 each and we are going to buy 12 of them for a total of $60 for the magnets.
So far in the laboratory, readily available magnets from the Home Depot have been used to preliminary tests. However, they did not come with detailed product specifications and therefore are unusable to quantitative analysis.
Computer simulation for magnetic performance was not easily available, so trial and error will be the method used in the lab to test for the optimal configuration for the system. The system has been design so magnets are easily accessible, with movable brackets and insulation so that the magnets do not just attract each other and get stuck.
Electrode Assembly
To actually draw current out of the reservoir, electrodes will be embedded into the aluminum shell. This will allow for connection to an external resistive circuit. Conductivity is an important factor in selection of electrodes. So is resistance to corrosion when under constant exposure to salt water. A solution that meets those requirements is gold electrodes. While they are significantly more costly than everyday wire, the expected performance cannot be sacrificed for cost. Additionally the price for such electrodes is still within budget. The solid gold sheet is going to cost $318.50 and will be split into two pieces.
Outside of the reservoir, four feet of 14 ga copper wiring can handle 20 amps as well as give room to maneuver any external circuitry that may be attached to the reservoir.
Heat Sinks
In order for there to be a temperature gradient, the heat that enters the system must leave. The design specifies that heat sinks will be placed on the side opposite the heat trap. Heat gets conducted through the fins and natural convection from the air cools the fins. Heat transfer and cost analyses were done assuming that the team would have to machine its own heat sinks. Heat sinks were offered by the Aerospace and Mechanica Engineering/College of Engineering office of Computing, but the sepcifications were not known, and it was decided that the team would buy heat sinks.
A benefit of using many small heat sinks rather than one or a few large ones is that they can be added or removed fairly easily, with the use of screws, thermal pastes, etc.