New challenges for space-application cells
The aforementioned ni-cad cells, etc., are excellent cells that have been researched over a long history. The energy storable per 1kg of cell is called the mass energy density (unit: Wh/kg). This value of the above-mentioned cells ranges from 40 to 60 Wh/kg. With the high integration of cellular phones and computers, a higher-density cell was required and eventually the lithium-ion secondary cell was developed.
The biggest feature of this cell is its lightness. It has a mass energy density of over 100 Wh/kg (some recently reached 160 Wh/kg), more than twice the figure of conventional secondary cells. The development of the lithium-ion secondary cell started around 1990 and it now has become an essential technology for mobile appliances.
If we install ni-cad cells on, for example, a 4ton-class earth-observation satellite, about 7% of the total mass, or 250 to 300 kg, is the weight of battery. If lithium-ion secondary cells are used, the weight of battery will decrease to less than 150 kg, or to about half of the ni-cad, allowing us to install more sensors and/or to save launch cost. For this reason, the development of the lithium-ion secondary cell for space is being actively implemented around the world.
Japan leads the world in the development of large lithium-ion secondary cell for satellites. On the explorer “HAYABUSA” launched in May 2003, high-capacity (about 13Ah) lithium-ion secondary cells for space were installed for the first time in the world. Currently, HAYABUSA is flying smoothly. HAYABUSA, its lithium-ion secondary cells and battery are shown in the photograph.
Expectations for fuel cells
Aside from the secondary cells outlined above, the other technology supporting mankind’s space quest is the fuel cell. This is the power-generation system that generates electricity by the electrochemical reaction of hydrogen and oxygen, not by combustion. Because the reaction produces water in addition to electricity, this technology has progressed mainly with manned space activities.
Most cells are first developed for ground applications and are then used in space. The fuel cell, however, was born in space. Needing both electricity and water in space, mankind decided to develop practical-use fuel cells when it began planning to go to the Moon. At the time, the fuel cell was in the research phase. The fuel cell developed for practical use in the Gemini Project in the 1960s has been employed for subsequent projects including the Apollo spaceship and the Space Shuttle. Even today, the fuel cell supports manned space activities.
Today, the fuel cell is attracting attention as an automotive and household power source. Its basic technology was born in previous space-development efforts. Expectations are growing for the development of less expensive, more reliable fuel cells adopting the latest material technology. From the same request, we are trying to develop a new fuel cell for the aerospace field.
Fig. 3 shows a trial model of a fuel cell developed by JAXA. The model is designed to operate in a closed environment in space, unlike ordinary fuel cells that are intended for use in an atmospheric environment. The model is now undergoing operation tests under the same conditions required for the Space Shuttle and stable electrical performance has been confirmed. Once a prototype has been completed, many research directions are opening up.
First, we intend to modify this closed-type fuel cell to a more compact and lightweight one so that it can be installed on a spacecraft. In case of HTV (H-II Transfer Vehicle), a supply vehicle for the International Space Station currently under development by JAXA, the lithium primary cells will weigh around 1ton of the vehicle’s total weight of about 16ton and payload weight of about 6ton. If the fuel cell can be employed, the weight of the vehicle’s power-supply system will be reduced by half and allow us to improve the space-transfer mission capability.
ISAS/JAXA has balloon and vertical liftoff/landing rocket technologies. Balloon flights in the stratosphere are used to conduct observations and experiments. If we were able to use a cell as the balloon’s power source, long-duration observation would be possible even in regions without sunlight, such as northern Europe or Antarctica in winter. The vertical liftoff/landing rocket’s fuel is hydrogen and oxygen. If it were possible to utilize even a few kilograms of the rocket’s surplus fuel, it could drive a generator to provide the electricity needed for the rocket.
By extending this approach, a breakthrough technical innovation is possible in deep-space explorers or earth-orbiting satellites. Because satellites carry fuel and oxidizer for their attitude control in orbit, if that fuel and oxidizer were shared with the fuel cell, then it should be possible to reduce the weight of the satellite’s power supply. Furthermore, when a failure occurs in the satellite’s power system, data transmission concerning the failure would be possible if power could be generated using any remaining fuel. It would also be possible to change the satellite’s orbit to prevent it from becoming space debris. Thus, we expect the cell under study to have a technological future of promise and significance.