The aluminum-powered electric vehicle was developed as part of two courses offered by MIT’s Mechanical Engineering Department, 2.013 (Engineering Systems Design) and 2.014 (Engineering Systems Development). Previous iterations of these classes characterized the production and use of a novel aluminum fuel treated with a gallium eutectic. When combined with water, this treated aluminum fuel undergoes an exothermic reaction that generates hydrogen and inert aluminum hydroxide. During the reaction, 45% of the total energy is released in the form of hydrogen gas and the other 55% is released as thermal energy. The hydrogen’s energy is easily harnessed by a fuel cell, allowing this treated aluminum fuel to be used as an efficient source of clean electricity.
One of the most compelling properties of the treated aluminum fuel is its energy density; at 83.8 MJ/L, aluminum fuel is unmatched for energy density by volume.
A particularly striking illustration of this energy density is the potential use of aluminum fuel as a replacement for batteries in electric vehicles. The battery of the 2015 BMW i3 electric vehicle (EV) has a mass of 230 kg, and the i3’s battery-only range is 81 mi. If one were to replace the 230 kg battery in the i3 with the same mass of treated aluminum fuel, it would be possible to drive from New York City to Los Angeles without stopping to refuel.
Designing a car capable of carrying 230 kg of treated aluminum fuel would involve building a car and fuel system from the wheels up, which is unfeasible for a small team to complete within the scope of a class. However, our team still wanted to demonstrate the incredible potential of treated aluminum fuel for large-scale power generation. To this end, we chose to retrofit an existing EV to run on treated aluminum fuel. An EV is an ideal platform for the aluminum fuel power system because, as mentioned above, the hydrogen produced by the aluminum reaction can be converted to electricity using a fuel cell, and then used to charge an EV battery.
This aluminum fuel system was specifically designed around the BMW i3 with Range Extender (REX). Most EVs have software failsafes in place to prevent the car from driving while the battery is charging; however, the i3 REX is unique among EVs because it includes a small internal combustion engine (the range extender) that can be used to charge the battery while the car is driving. Despite the addition of this engine, the i3 REX is a fully-electric vehicle. The engine is only used to drive a generator to charge the car battery, and only turns on when the battery state of charge drops below 6%. The i3 REX is otherwise operated using power from the battery alone.
Because the i3 REX is already configured for charging while driving, we are able to remove the range extender and install an aluminum fuel system, essentially tricking the car into believing the range extender is still in place. This allows us to use our aluminum fuel system to charge the car while driving and extend its range compared to that of battery power alone.
In order to demonstrate the potential of aluminum fuel as a power source for a car, our team decided to drive across the United States in a modified i3 REX powered exclusively by our aluminum fuel system. The general public interacts with cars on a daily basis, which provides an intuitive understanding of the power required to drive a car. The cross-country trip is particularly appropriate because, in addition to being an exciting demonstration with broad public appeal, the trip is difficult in most EVs due to their limited range.
The power demands of a cross-country trip in a BMW i3 motivate the specifications of the aluminum fuel power system. Based on EPA specifications, the i3 consumes 300 Wh/mi at average highway speeds of 62 mi/h. This leads to a steady-state power consumption of 18.6 kW. Meeting this power demand with our system would enable us to fully power the car while driving without depleting the battery. This would allow for a cross-country endurance run that only requires stops to refuel the system and switch drivers. However, this power output is only achievable at high temperature and pressure (325C and 135 barg). A system operating at these conditions was deemed unfeasible given the allotted time and budget.
The system scale is already constrained by the operating conditions of the reaction, so our system is sized around the next most critical system component: the fuel cells. Most fuel cells on the kilowatt scale are sold without control systems, which significantly increases their complexity of integration. Fuel cells with built-in controllers are available off-the-shelf at a maximum of 5 kW; we use a pair of these 5 kW fuel cells for a total of 10 kW power output.
The rest of the system was likewise scaled down to be compatible with a pair of 5 kW fuel cells. After an estimated 0.8 kW loss from the low-voltage system (fans, pumps, and fuel cell controls) and a further 0.6 kW loss from the power electronics, the 10 kW output from the fuel cell produces 8.6 kW net power. This 8.6 kW net output is sufficient to power the car for two hours, but the reserves in the battery will be depleted. After two hours of driving with the aluminum fuel power system running, the car must be stopped for an additional two hours and and fifteen minutes to recharge the battery (still using the treated aluminum fuel.) The aluminum fuel power system therefore substitutes for the i3’s existing gasoline-powered range extender. The system allows for a 117-mile range, a significant improvement on the range of the i3 on batteries alone; additionally, it reduces the 3-hour charging time of a Level 2 EV charger by 45 minutes.
To fit the aluminum fuel power system inside the BMW i3, the overall volume of the system must be less than the total trunk and rear seat capacity of 1050 L. The system mass, including the fuel and water required for a two-hour driving cycle, must be less than 315 kg so that a driver and a passenger can be in the vehicle without exceeding the maximum payload. Three primary subsystems comprise the aluminum fuel range extender: the water system (which includes the reaction chambers), the hydrogen system (which includes the fuel cells), and the electronics. The aluminum fuel reaction is monitored by a family of sensors, including a pressure transducer, a thermocouple, and a hydrogen flow meter. Because the aluminum fuel produces hydrogen when it reacts with water, the reaction rate (and thus the rate of hydrogen production) can be controlled by adjusting the flow of water into the reaction chamber. A Raspberry Pi monitors the feedback from the sensors and regulates the water flow into the reaction chamber to produce hydrogen at a steady rate.
The hydrogen leaving the reaction chambers is mixed with saturated steam, and the fuel cells require pure dry hydrogen to operate reliably. The output hydrogen is therefore directed through a purification system: a condenser removes the steam, an oxygen scrubber extracts any remaining gaseous oxygen, and a pressure regulator steps down the pressure of the hydrogen to match the rated intake pressure of the fuel cells. The output voltage of the fuel cells is significantly lower than the rated voltage of the i3 REX battery, so a boost converter steps up the voltage to something the i3 battery will accept.