EXECUTIVE SUMMARY (download or view as a PDF)

Product Description
            The product designed for this project is a human powered water distillation system.  The purpose of the product is to take a polluted water source, heat it to a boil using only human input power, and then to condense the steam produced into clean distilled water.
            A bicycle connected to a DC generator was used to produce electrical power.  The input pedaling motion rotates a chain system, consisting of two gear reductions, which in turn rotates the generator shaft.  This rotation induces a voltage across the generators wires.  These wires are connected to a resistive heating element mounted within an insulated container. This heating element heats the water inside the container by dissipating the electrical energy. Upon boiling, steam flows through piping out of the container and into a coil of copper tubing. This copper tubing, contained within a reservoir of room temperature polluted water, condenses the steam back to water.  This condensate flows out the bottom of the tube and into the final collection vessel. This distilled water should be devoid of any contaminants present in the original water. The condensation process heats the water in the condenser reservoir, and a return valve system allows pre-heated water to flow into the boiler chamber, thus “recycling” the pedaling energy that went into the steam’s latent heat of vaporization.
            The first primary system is the Electrical Current Generation System. A human being sits on a standard bicycle and pedals to provide the input power. The largest gear on the pedal sprocket hub drives the smallest gear on the rear wheel sprocket hub by means of a chain.  The largest gear on the rear hub then drives a gear connected to the generator shaft, which is mounted near the base of the rear wheel.  The purpose of this power train system is to provide a double gear ratio in order to create a higher RPM.  The generator shaft gear is threaded onto an aluminum cylinder with a half inch bore through its center.  This aluminum cylinder is fixed to a half inch steel shaft with set screws, and this shaft is supported by two roller bearings mounted in aluminum pillow blocks.  The bearings are contained within the pillow blocks by two shaft collars.  The end of this shaft assembly is connected to the generator shaft with a shaft coupler.  The roller bearings and this coupler allow the generator shaft to be driven while ensuring that no lateral forces are placed on the generator shaft during operation.  The pillow blocks and the generator itself are mounted to a sheet metal base using L-brackets and machine screws.  When its shaft is rotated, the generator provides electrical power to the heating element.
            The second primary system is the Heating System. The generator wires are attached to a heating element made of resistive NiChrome wire contained within a U-shaped glass tube.  The heating wire is covered with sand to facilitate conductive heat transfer from the wire through the glass tube into the water. The NiChrome wire must be inside the glass tube to avoid direct contact between the wire and the salt water in the boiling container to prevent a corrosive chemical reaction from taking place. The heating element assembly is placed in a vacuum insulated heating container. This container holds enough polluted water to surround the heating element (NiChrome wire) section of the heating element tube. The boiler container also contains a plastic insert placed at the bottom which reduces the volume of the container and thus the amount of water required to maintain the water level above the heating region (to prevent heating element over heating).  The lid of the heating container is made of High Density Polyethylene (HDPE). There are two holes machined into the lid that allow the heating element to be mounted. There are also holes for the steam exit tube, the water return tube, and the temperature sensor. This lid must be sealed to prevent steam from escaping but cannot be permanent, so a rubber gasket is placed on the inner lip of the thermos to seal off the lid. In order to place the necessary amount of pressure on the gasket for an actual seal to be made, a ring of HDPE is placed around the outside of the lid and it is pressed down by an elastic strap.
            The third primary system is the Condenser System.  A piece of copper tubing five feet long is coiled and immersed in a large cylindrical plastic reservoir container full of polluted water.  A lid machined for the reservoir has holes in it for the steam inlet flow, return flow tubing, and a graduated cylinder.  The top of the copper coil is connected with PVC pipe to the steam exit connector on the boiling chamber.  Steam flowing from the boiler condenses within the copper coils, and the liquid water flows downward through the coil into a collection vessel positioned below the condenser.  The condenser return flow connector is attached to the return flow hole on the boiler chamber, with a manual valve between them to regulate flow.  A graduated cylinder mounted to the condenser lid allows the amount of water flowing back to the boiler to be measured.  The pressure head from the water in the graduated cylinder causes water to flow into the return tubing when the return valve is opened.  Copper tubing connections are soldered together and all other connections are sealed with a waterproof adhesive sealant.
            The final primary system is the Structural System. The bicycle is mounted on two upright 4x4 wood columns. One holds up the front end of the bike and the second supports the midsection of the bike below the pedal cranks. The front column is mounted on a 1x4 piece of wood that runs perpendicular to the length direction of the bicycle using L-brackets. In a similar manner, the back column is mounted on a 1x6 that runs parallel to the length direction of the bicycle. The generator and shaft assembly are mounted on a piece of sheet metal, which is mounted to the wooden support base underneath the rear wheel.  A secondary structure supports the condenser and boiler assemblies. This structure has a seat cut into a base piece of wood for the boiler container to be placed in. A vertically-mounted piece of wood supports the return valve assembly.  This return valve assembly uses a bicycle gear shifter to allow the user to open the valve from a riding position by shifting up.  When the gear is shifted down, a counter force from a spring pulls the valve back to its closed position. On the opposing side of the vertical piece, the condenser rests in a machined seat on an elevated platform.  A hole is cut in this platform to allow distilled water from the end of the copper condenser coil flow to a collection container positioned below the condenser platform.

Design Relevance
            The concept of a human powered distillation apparatus has significant real world design relevance.  Maintaining adequate levels of personal hydration becomes increasingly difficult in emergency situations, when normal clean water supplies are not functioning.  A recent exhibit of this is the situation of Hurricane Katrina, where the only available water source was not potable due to salt content and other contaminants, and traditional purification devices (filters, chemicals, electric boilers, etc) were not available or could not be powered.  The ability to purify one’s own water in such a situation could prove invaluable to the survival of an emergency victim.
            An even more socially relevant application of a human powered water still is in remote areas and third world countries, where an infrastructure of purified water distribution simply does not exist, much less an electrical supply network to use to power a filtration device.  A 2006 report from the United Nations lists world water and sanitation as a major crisis that requires urgent action.  This report estimates that this crisis leads to as many as 2 million child deaths per year, and claims 1.1 billion people worldwide have no access to safe water, and have come to accept the use of contaminated water as a standard practice.  This contaminated water serves as a breeding ground for disease and sickness.  As water is a vital component to life, access to clean water should be considered the right of any living being.  A portable purification still powered solely by human energy could supply remote, rural areas with a continuous and sustained clean water source, reducing disease and death while improving total quality of life.  For this reason, the development of such an apparatus has very significant cultural, social, and economic relevance.
            Direct competition within this market is virtually non-existent, as no other similar device exists commercially.  The closest competition comes in the form of filtration devices and chemicals that can be used to purify water.  The aim of this product is not to enter into direct competition with manufacturers of these products; rather, it is to serve as a compliment to them to provide maximum opportunities for those in rural, distant areas to have access to continuous purified water and improve quality of life.

Design Process Used
            During the early stages of the project, it was necessary to make decisions regarding the design concept the group would pursue.  The first step taken in this process was to define a set of design requirements (based primarily on ASME contest and course requirements).  Design options from the concept proposals (developed individually by the group members) were evaluated based on the extent to which each option or idea satisfied these design requirements.  To accurately assess the design ideas, research was necessary to gain a better understanding of the details and feasibility of each option or component.  Technical research, consultation with professors and experienced persons, research into product/component specifications and availability, quantitative engineering calculations (thermodynamic, electrical, etc.), and experimentation/testing were used as sources of information.  After using all of the available data to effectively evaluate each design option, the group, through general consensus, identified the design options which had the highest value (that is, those which best satisfied the design requirements).  By selecting the design options with the highest value for each of the various parts of the system, the overall design concept was formulated.
            As the project progressed, the necessary decisions became less conceptual and instead required quantitative design values and selection of specific components.  Focused investigations of four critical elements of the system were conducted in engineering trade studies, which utilized various forms of engineering technical analysis (such as heat transfer and statics/solids analysis).  The quantitative information gained from these studies directly influenced decisions about the design of the support structure, the condenser, the boiler chamber, and the power train system.  Further experiments were performed to test the performance and feasibility of certain components (see Heating Element Tests).
            Information provided by manufacturers and vendors on product availability, specifications, and cost became a critical factor in decisions as specific components were selected for use in the prototype. When selecting components, the performance requirements were defined by the group (for example, for the generator, performance was based on power rating, RPM, cost, and efficiency), the respective values of all available options were evaluated based on these requirements, and the group selected the components with the highest value for use in the prototype.
            The group schedule was set by establishing internal group deadlines that were either in line with or ahead of the class schedule.  These deadlines focused primarily on the specific action items planned for the upcoming week or two, but the group periodically performed a broader progress evaluation to ensure that the work being done was keeping the team on track to meeting all longer term deadlines.  Group meeting schedules were set during the regular class periods by considering the personal schedules of all group members.  In addition to set class times, all group members met regularly on Sunday afternoons, as well as at other times during the week as schedules permitted and as the project required.
            To effectively allocate the time resources of the group, the all-group meeting times were used to focus on tasks of planning and decision making, since these tasks could be better performed with all members present.  Additional tasks related to the current action items were divided amongst the group members by first considering the individual strengths and the preferences of each team member, and then assigning tasks appropriately.  The monetary resources of the group were carefully allocated by keeping an up to date budget sheet of all expenses and by always considering cost as a very important parameter in the selection of any prototype component.

Options Considered
            As the design concept for a human powered distillation system was formulated, it was necessary to make decisions between the following possible design options. 

For sketches of design options, see Design Options Considered

Solutions Selected with Rationale
            A foot-pedal system was selected over a hand crank system as the method of human power acquisition because research into basic human strength and endurance suggested a far greater amount of power could be obtained from a human’s leg muscles than from their arms.  Although a custom built pedal system could be more compact than an entire bike frame, incorporating a standard bike into the system was selected because it would allow for a comfortable upright riding position (sitting or lying down would be a less natural position), the existing sprocket/gear/shifting system could be easily utilized (without requiring extensive construction work for a custom system), and the existing handle bars could be used for rider stability support.
            The option of using the mechanical input power to rotate a generator and power a heating element was selected over using friction to dissipate energy as heat.  Both options involved similar challenges in transferring the rotational motion of pedaling, but using friction would require maintaining proper pressure between contacting friction materials.  Although dissipating energy through friction would result in a direct conversion of almost all input power to heat, a primary disadvantage of this option is the significant difficulty of effectively transferring this heat to the water.  A generator has some inefficiency in the conversion from mechanical to electrical energy, but the electrical power produced could then be efficiently utilized to heat the water with a resistive heating element (with much less heat loss to the environment than could easily be accomplished with friction).
            To use the input mechanical pedaling rotation to rotate the generator shaft, the option of having a double chain system to drive a small sprocket on the generator shaft was chosen.  One reason for this decision was that the manner in which a rear wheel sprocket hub is mounted within a bike frame makes it somewhat difficult to transfer the rear wheel shaft rotation directly to the generator shaft.  A second, more important reason was that, based on common generator specifications, it was clear that the generator shaft would need to be rotated at a higher RPM than the rear wheel rotated.  Allowing the largest pedal gear to drive the smallest rear wheel gear, and then using the largest rear wheel gear to drive a small generator shaft sprocket created a double gearing ratio, which could achieve appropriate rotation rates for the generator shaft (see Power Transfer Trade Study).
            Wood was chosen as the material for the support structure instead of metal primarily because of its lower cost and ease of machining and connection.  Although this framework results in less robustness under multiple hours of use than would a similar metal structure, it is suitable for a proof of feasibility prototype (see Structural Support Statics Trade Study).
            A custom built submersible NiChrome heating element (contained within a U-shaped glass tube) was selected over other options.  Direct contact between water and an immersed heating element within an electrically-insulated container would result in much greater heat transfer efficiency and heat containment than an element external to the container could achieve.  Additionally, a custom built heating element could be designed to prevent the salt water from contacting the heating wire, which could result in corrosive chemical reactions.  Another advantage of this option was that the cost of fabricating a custom heating element was significantly less than purchasing a commercially manufactured one.  Finally, the resistance of a custom built heating element can be easily adjusted by changing the length of NiChrome wire, allowing the element to be specifically tailored to fully utilize the power and current output capabilities of the generator (see Boiler Vessel Trade Study).
            A condenser made of coiled metal tubing was selected over a fin surface design primarily because a tubing coil would allow for large surface areas for heat transfer but would be significantly easier to fabricate than a custom built fin apparatus.  The option of having the condenser surrounded with room temperature water was selected because, based on heat transfer analysis of a condenser tube, a tube immersed in water was found to be capable of dissipating far more heat than a tube of similar length and size surrounded by ambient air (see Condenser Apparatus Trade Study).  This design also inventively optimizes system performance because the reservoir of polluted water surrounding the condensing coils allows a large volume of water (over five times the basic ASME allotment) to experience thermal gain while requiring a much smaller quantity to be heated and boiled before distilled water is obtained.  Additionally, the water flowing from the reservoir to the boiler to replace boiled water is preheated, thus recycling thermal energy.

Implementation Details and Lessons Learned
            Upon completion, testing, and operation of the prototype, the group realized several aspects of our design that had potential for improvement.  The first is the addition of an operational fly wheel to our prototype.  While the user is pedaling the bicycle at approximately 120 rpm, there is significant disparity in the resistance he encounters during the “power” (down) stroke of the pedaling motion and the “return” (up) stroke.  This causes an inconsistency in rpm, which is multiplied by 7 at the generator shaft due to the gear ratio.  This causes a significant fluctuation in generator shaft speed, leading to inconsistent voltage and power output to our heating element.  An operational flywheel would stabilize the rotation of the shaft and lead to a smoother, continuous rpm over time.  This flywheel idea was considered by the design group throughout the semester, but was abandoned due to lack of budget for the necessary components.  However, we now feel that it would significantly improve the performance of our system, and should be included.
            A second “lesson-learned” from the prototype that would influence design concerns the gear ratio.  We incorporated a dual-stage gear reduction to achieve a ratio of 7.  While this meets our design criteria of allowing a generator shaft speed of 900 rpm, we now realize that a higher rpm would be very advantageous for multiple reasons.  It would allow higher rpm at the generator shaft for the same human input, translating to higher voltage.  It would also augment the torque and resistance that the user feels when pedaling, increasing the force that the user can exert on the pedals.  This would lead to more human power input into the system.  Our group would add an additional gear (for a three-stage gear reduction) to achieve a higher rpm at the generator shaft.
            A third “lesson-learned” deals with the insulation of the piping interface between the boiler and condenser.  As steam travels up this tube, the group found that it was partially condensing within the tube, releasing thermal energy into the environment.  This is unrecoverable energy, and decreases the efficiency of our system.  To fix this, the group could make the connection tubes shorter and increase their insulation.
            A final improvement that the group would make after having tested the prototype is to incorporate a simpler support structure that would allow for easier assembly and disassembly.  Currently, our structure consists of many semi-permanent connections that cannot be easily taken apart.  We would change these to easily-detachable fasteners, replacing screws with nuts and bolts, etc.  This would allow for quicker assembly, disassembly, more convenience, and increased portability.

Conclusions Reached and Feasibility Assessment
            The Home Brew Water Crew human-powered water still had the primary purpose of performing in the ASME contest, with real-world application as a secondary goal.
            The prototype system meets all of the ASME guidelines and requirements. It operates at approximately 840 rpm input to the generator, produces 20-25V output from the generator, and delivers 70-125 W of power to the boiler. The boiler heats with a thermal efficiency of 82% (the total thermal energy gain of the water divided by the total electrical energy input to the boiler) and the condenser produces distilled water at a rate of 50 mL/hr. Also, the system dissipates heat to a 2.7 L reservoir of water, which gains additional points in the contest. This performance data indicates that the system is very feasible to enter in the design contest.
            The still also serves as a prototype of a final product that could aid people in disaster situations and remote areas that are in need of clean water. This final product would be built to maximize robustness and reliability for repeated use in harsh conditions. There is a notable limitation to the practicality of this device based on the large amount of human work required to produce fairly small volumes of purified water; an adult would clearly need to drink more than 50mL of water after an hour of vigorous pedaling. However, in situations where well water may have a level of bacteria safe for adults but dangerous to infants, an adult could operate a system similar to the prototype to produce clean distilled water for an infant, and could then refresh themselves with well water after the pedaling effort.