Abstract
Picture a perfect city: safe, self sustaining, and pollution-free. In the year 2247, this is not a fantasy; it is a reality for Mirai. Located in a massive high-tech pyramid over Tokyo Bay, Mirai (Japanese for “Future Life”) has all of these qualities and more.
Mirai’s technologically advanced infrastructure is unrivaled. The pyramid design is not only stable enough to sustain Japan’s devastating earthquakes and tsunamis, it also efficiently utilizes vertical space rather than spreading out horizontally, answering Tokyo’s longstanding overpopulation problems. The pyramid is divided into layers. The lower layers are devoted to residential space, office buildings, and industry. The upper layers are reserved for research, recreation, and entertainment, providing a breathtaking view of Tokyo Bay.
Mirai utilizes the latest, most environmentally friendly power available. Each building is self-powered by Bio-electrochemical Microbial Reactors (BEM-R’s) that run on the organic materials in wastewater. Every building is also equipped with energy curtains, which capture solar energy during the day, and provide light at night. Areas not connected to buildings, including transportation, are powered in two ways. The trusses of the pyramid are coated in a photovoltaic film that converts the sun’s energy to electricity. Beneath the pyramid lies a “fence” of underwater turbines that harness the extensive power of water currents.
The hollow supports provide an excellent means of transportation for Mirai’s citizens. The large tubes, constructed from carbon nano-tubes, feature three levels. The top layer is an accelerated walkway. FlexiTaxi pods are whisked along an automated conveyor in the middle level, while the lower level is used for shipping goods throughout the city.
The chemical engineers in Mirai have designed a more efficient method of obtaining fresh water. Wind traps, located along the outer edge of the pyramid, are placed in the path of prevailing winds. A sharp drop in temperature causes water in the air to condense. This water is filtered to remove any traces of salt, providing clean drinking water. Additional water is recycled, within each building, using the BEM-R power system.
To fix the past problems of cell phones (slow service and dropped calls), Mirai uses free-space lasers instead of radio waves. Light waves transmit data one hundred times faster than any previous technology. A modulated laser beam is sent through the air to an optical receiver. The messages appear on roll-up displays that can re-size at will. These screens are made of several layers. The top layer is coated with plastic; the middle layers are made up of liquid crystals and color filters. The backing is a transistor with a semi-conductor. These screens are flexible so they can roll up and transport easily.
The tourism and entertainment industries keep Mirai’s economy healthy. Underwater resorts and virtual recreation facilities attract visitors from the world over. Mirai also exports excess the power produced by the tidal fence to generate additional revenue.
With Mirai’s perpetual power, terrific transportation, wonderful water system, and connective communication, this city will be ready for whatever the future may hold.
Essay
From Wasteland to Wonderland
Imagine a fuel cell so innovative it reliably produces enough electricity to power the industrial zones in a large city, and also eliminates wastewater, producing fresh, clean drinking water. A microbial fuel cell feeds on waste to create electrons for electricity with a by-product of clean water. This system is reliable because each building has its own on-site plant so when a facility fails; only one building is affected rather than an entire grid.
Chemical engineers in Mirai have made this dream a reality. This groundbreaking fuel cell system uses a chain of Bio-electrochemical Microbial Reactors (BEM-R’s). As long as there is a steady supply of waste, electricity is produced. Wastewater from factories enters a storage facility beneath each building. From there, it is pumped into one of many six-meter long cylinders containing an iron rod covered in a bio-film of microbes. The microbes Geobacter metallireducens feed on waste and “digest” electrons and positively charged hydrogen ions. “Wastewater and sewage contain a slurry of bacteria and undigested food” (Biever, 2004). The microbes process the organic matter.
As the microbes devour the waste, they deposit electrons onto the iron rods in the anode. The hydrogen ions pass through a cellophane-like proton exchange membrane to the cathode. The electricity exits through copper wires and travels to a metal loop on each floor. The electric current causes the loop to resonate at its natural frequency, creating a magnetic flux. Wireless energy is transferred to other loops with the same frequency, much like tuning forks. Upon returning to the cathode, the electric charge combines the hydrogen with oxygen, producing water.
On-site distribution has made power outages a thing of the past. Power lines are costly. They lose up to 20% of the energy produced in a power plant from heat loss and resistance. Using on-site distribution is extremely efficient and reliable, eliminating factory shutdowns due to grid blackouts.
Mirai’s population is just under 500,000. The average energy consumed in an industrial zone in a city of that magnitude is 1,250,000 kW. Sewage processing plants are some of the largest consumers of power in a city’s industrial zone. The bacteria break down the waste contained in the sewage, so they eliminate the need for water treatment facilities. On-site distribution requires each factory to produce only enough energy for itself. The power requirement for an industrial building ranges from 250 kW to 2 MW. The larger the factory, the more fuel cell cylinders required. Each cylinder can generate 250 kW. The more energy-consuming buildings may require as many as eight to ten fuel cell cylinders.
The BEM-R system’s efficiency is extremely high (85%), which means that the microbes can transfer 85% of the electrons in the wastewater to an electrode. Compare that to the maximum efficiency of a steam generator, which is only 48%. The by-products of the BEM-R system are heat and water. They are pumped into boilers, and recycled for heat and freshwater in the factories, making the BEM-R system renewable.
There are other advantages of incorporating microbial fuel cells into Mirai’s energy strategy. They run at room temperature so they do not require heat to generate energy, like other fuel cells. Microbes produce electrons as a part of their natural digestive processes, and deposit them directly onto iron. A costly platinum catalyst is unnecessary. These fuel cells run on common, inexpensive materials. Wastewater is readily available in factories, especially in food and agricultural industries. The BEM-R system not only produces electricity, but also eliminates much of the city’s waste.
The only disadvantage of microbial fuel cells is finding an electrode with sufficient surface area to allow enough microbes to attach to the material. A porous material with an abundant surface area will rarely conduct electricity well. To fix this problem, materials engineers designed iron rods with ridges along the entire surface. The iron provides good electrical conductivity, while the ridges offer additional surface area upon which the microbes can attach.
In conclusion, the BEM-R system is not only environmentally friendly, but also an extremely efficient method of generating electricity. Thanks to the brilliant minds of engineers, we can now keep our future from going down the drain.
References
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