Nanomaterials on Earth and Beyond Series
Sun is the ubiquitous source of energy in our solar system, and solar cell applications for space date back to 1950. Traditional solar cells made of mostly silicon or germanium are bulky and have very poor efficiency (efficiency is the ratio of the electrical output to the incident energy from sunlight and calculated based on a cells power output, surface area of the cell and the input light). Because of poor efficiencies (Silicon cells have 18% efficiency) (33) solar cells need to be arranged in large arrays to generate enough power. In fact the mars rover currently has multi-panel solar array for power. The solar panel is subjected to degradation due to anticipated dust coverage on the solar panels. EcoSolargy (34) is attacking this problem on earth using nano- coating techniques mentioned in previous sections to fill tiny holes that typically would accumulate dirt, dust, or water, and has increased efficiency by 35 percent over a 20-year period. This is a huge step for space installations where sand storms and space dusts are common.
Nanotechnology is also paving way for flexible, low cost and light weight solar cells especially with organic photovoltaic cells made of graphene and zinc oxide nanowires. Princeton university researchers recently demonstrated 3 times higher efficiency than conventional solar cells by applying a “nano-mesh” to plastics (35). The nano-mesh dampens reflection, traps light and converts them into electrical energy (existing technologies cannot fully capture all the light entering the cell). While these solar cells are marketed towards flexible chargers for laptops and cellphones, “Wall-E” like robots roaming the extra-solar planets’ surfaces or mining asteroids for minerals are not too far in the future.
On the storage side, small amounts of CNT powder (1% by weight) are blended with active materials and a polymer binder (such as LiCoO2 cathodes and graphite anodes) that have wide usage in lithium ion batteries for mobile computers and phones (36). CNTs enhance the capability and cycle life by providing increased electrical connectivity and mechanical integrity. CNTs are also used as catalyst support in fuel cells, which can potentially reduce platinum (Pt) usage by 60% compared with carbon black (37).
Furthermore doping i.e. intentionally introducing impurities into CNTs may enable fuel cells that do not require platinum (Pt) (38). But the most promising of energy storage for space applications would be in ultracapacitors. Ultracapacitors can store significantly more charge than regular capacitors, they can be recharged within seconds over a million times, have 10 to 100 times greater power densities, and can hold that charge over a very long period (39). CNT used in a 40-F ultracapacitors (40) produced an energy density (amount of energy stored per unit volume) of 16 Wh/Kg and a power density (amount of power stored per unit volume) of 10 KW/kg (a standard Li-Ion battery has a power density of 250-340 W/kg at 3.5 V).Accelerated testing forecasted a 16-year lifetime for these batteries. Ultracapacitors could be applied to many emerging technologies such as electric vehicles (NASA has been testing this technology on their light hybrid electric transit buses), satellite propulsion and pulse power applications (41).
An important application where all the technologies mentioned earlier in this article will have a huge impact is in the realization of a “space elevator”.