With some new ideas, the Green Machine's overall design has seen some great improvements. Now, we'll go over some of the reasoning behind our choices for the Green Machine's parts and other ideas we've considered.
Keeping in mind that this project’s ultimate goal is to create a device that produces biodiesel fuel run solely on solar-powered energy, it was determined that there are four essential components to the design. First, some type of infusion mechanism is needed to pump the reactants through the microreactor. Second, there must be an element used to heat the reaction. Third, a solar power source must be implemented to power the infusion mechanism and the heating element. And fourth, a microreactor configuration must be chosen to provide for optimal mixing of the reactants.
Discussion and research regarding the infusion mechanism focused on two main delivery systems: a gravity feed and metering pumps. A gravity feed arrangement would allow for a relatively inexpensive, easy to use method of infusion that would not require any power to run. The main issue with this mechanism however, is that as gravity is the only force acting on the reactants, it would be extremely difficult to control the flow rates of the reactants. Additionally, this method would require consideration of the viscosity of both fluids and the required pressure for the reactants to enter the microreactor. These properties would determine the length of the tubes and their diameters. Though possible, this method was found to be impractical for the two very different reactants that are being used.
Using a metering pump for the delivery method proved to be more promising and practical. There are various different kinds of metering pumps with varied limits to flow rates and pressure. After examining a detailed chart that compares the different types of metering pump specifications, which can be found in Appendix B, the syringe pump was determined to be the best alternative. Syringe pumps have several benefits. First and foremost, this project has a limited budget, and consequently any equipment that can be used without making new purchases is invaluable. As it turns out, the Physics Department has generously donated a syringe pump, Pump 33, to the Green Machine. Second, in terms of specifications, the syringe pumps handle the lowest flow rates. In the performance specifications section, it is shown that flow rates below 0.01 mL/min are needed. The syringe pump is actually the only metering pump that can handle flow rates this low (see Appendix B) [ref 6]. Additionally, as the desired pressure for the microreactors is still unknown, the Pump 33 Syringe Pump allows the operator to simply input the desired flow rates for each reactant, and the pump mechanism pushes the syringes at whatever rate is necessary to achieve the flow rates.
For heating the microreactor, several different alternatives were considered. In preliminary laboratory experiments, heating tape has been used by placing it under the microreactor and controlling it with a variac. A multimeter was connected to a calibrated thermistor which was in turn attached to the heating tape. The multimeter provided readings of resistance as a result of the heat through the thermistor. These resistances are shown in Appendix B, and they indicated the temperature that the heating tape was at. Adjustments were made using the variac to provide more or less power through the heating tape which kept the temperature at about 60ºC. This safely allowed for heating of the reactants to achieve mixing while avoiding the boiling point of methanol, which is roughly 64.5ºC. Though this method allowed maintenance at a constant temperature, there was a big safety issue regarding the use of heating tape that could not be ignored. If the heating tape were to fold over onto itself, it would burn the fabric and could potentially start a fire. Thus, the heating tape could not be left unattended at any time and required extremely careful placement on safe surfaces. Therefore, heating tape is not practical for use in the final design.
Next, a heating lamp was considered to heat the reactants. It was difficult to concentrate the heat directly on just the reactants, and even more difficult to monitor the temperature. One consideration made was the possibility of incorporating the heating lamp into a circuit with a built in temperature sensor, wherein the sensor would turn the lamp on and off depending on the need for more or less heat. However, this would run the risk of burning out the bulb too quickly. Next, a hot water bath was considered to heat the microreactor. Specifically, it was determined that the use of a miniature immersion heater would allow for a constant temperature reading, and would result in very evenly distributed heating throughout the reaction. Because the main purposes of the heating element are to heat at a constant temperature, monitor and control that temperature, and avoid temperature gradients, the hot water bath immersion heater was chosen for the final design.
Although the hot water bath was ultimately chosen, it should also be mentioned that using solar power to heat the microreactor via focused solar radiation with a lens was an option that was considered. However, this would be a completely different type of solar power than what is used throughout the rest of the Green Machine system. That is, the use of this focused lens requires concentrating solar power energy, but the Green Machine makes use of photovoltaics. Precisely what that means and why that type of solar power energy was chosen will not be addressed in greater detail.
The concept of solar power, as in the conversion of sunlight into electricity, is anything but new, dating back at least to the late 19th century. Paving the way for alternative sources of energy, solar power use has seen dramatic rise in popularity and effectiveness from home usage to megawatt-scale power plants. For the generation of electricity, there are two methods that we briefly just mentioned: photovoltaics (PV) or concentrating solar power (CSP). CSP uses lenses, mirrors, and other systems to focus a wide area of sunlight into a narrow beam, which is then used as a heat source for a power plant similar to coal-burning. The other method, PV, converts light directly into electric current by making use of the photoelectric effect. In addition to these two methods, solar energy (the term used to broadly describe the energy from the sun that reaches Earth) is harnessed using solar thermal systems which convert the energy from the sun to heat. This method is most commonly used as an alternative water heating system.
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