|Fig. 1: Schematic representation of a typical personalized energy system. The arrows indicate the flow of energy during daytime (red arrows) and nighttime (blue arrows) conditions.|
While many attempts to solve the global energy problem focus on scaling up existing technologies and creating large, centralized power sources, the "personalized energy" (PE) philosophy proposes a drastically different approach. One of the most well-developed examples of a PE paradigm is being advocated by MIT's Dan Nocera. In Nocera's vision, individual homes and businesses would be powered by solar panels during the day. The excess energy produced would power a water splitting catalyst, generating fuel (nominally hydrogen, but methanol and other fuels could also be generated) to store the energy for use at night and during inclement weather. The system would also provide energy for both electric and hybrid vehicles.  Fig. 1 summarizes the energy flow in a typical personalized energy system during both daytime and nighttime conditions.
The PE model offers several advantages over our current energy system. It is carbon-neutral and indefinitely sustainable, requiring water and sunlight as its only inputs. Additionally, it is reliable and secure, with individual consumers controlling their own energy production instead of relying on governments or corporations. Finally, since power generation is distributed, large-scale transmission lines are unnecessary, reducing infrastructure costs and making power more accessible to the developing world.
While an appealing concept, there are many technical requirements for realizing a large-scale personalized energy economy. Nocera has overcome the first major hurdle, developing a cheap, efficient water splitting catalyst made from earth-abundant materials. This cobalt/phosphate catalyst self-assembles in neutral aqueous solutions on an inert anodically polarized electrode and can run on water from a variety of sources, including waste water; later use of the hydrogen in a fuel cell would allow for the production of clean drinking water in tandem with electricity generation. This catalyst, when coupled with an appropriate proton reduction catalyst, would allow for the storage of solar energy as molecular hydrogen with roughly 80% efficiency .
In addition to an efficient and robust catalyst, the PE model would require extremely efficient solar energy harvesting. Nocera often touts the fact that only 5.5 L of water would need to be need to store 20 kWh (72 MJ), the amount of energy used by a typical American family each day, and that a 8 m2 solar panel operating at 20 mA cm-1 would produce the energy to split that amount of water in 2.5 hours.  However, this logic is fairly naïve, ignoring the substantial energy losses in the water-splitting catalysis, hydrogen compression, and in the fuel cell's conversion of stored hydrogen back into electricity, all of which would significantly increase the required efficiency of the panels. Furthermore, collecting the energy needed to power personal vehicles would be an enormous burden. The average individual travels 12 miles each way to get to work; even in a vehicle averaging 50 miles per gallon, this commute would still require approximately 0.25 gallons of gasoline worth of energy each day.  Since the energy density of hydrocarbon fuels is around 45 MJ/kg, and the density of gasoline is approximately 0.75 kg/L, this would require the generation of another 60 MJ (4.2 kWh) of energy, increasing the demand by nearly 25%.  Overcoming this barrier would require either a significant increase in the conversion efficiency of the solar panels or a considerable reduction in the energy costs associated with storing the fuel. While both of these issues are the topic of significant research, they represent significant technical barriers that will require substantial technological innovation to overcome.
While the technical barriers to personalized energy are numerous, the practical barriers are perhaps fundamentally more significant. While Nocera's water splitting catalyst provides a method of storing solar energy, the PE model still relies on a surfeit of sunlight to power the operation. In regions where significant sun exposure is consistent, bright, and year-round, the PE model could likely be implemented with fairly inexpensive low-efficiency silicon solar panels. However, many regions of the world simply do not receive consistent amounts of sunlight, and while the PE model would allow for energy storage during times of surplus, the compressors needed to store enough energy to last a cold, dark winter would substantially increase the costs of such a system. Furthermore, storing such large quantities of compressed hydrogen and oxygen would represent a significant explosive risk . Aside from these problems, perhaps the most limiting constraint is cost. Even with optimistic estimates, solar panels cost at least $2 per watt; if a home receives 5 hours of sun a day, a fairly typical value for the US, this would require 4 kW of solar generating capacity to provide Nocera's stated 20 kWh of energy demand, a total cost of $8,000 for the solar panels alone. [7,8] Additionally, the costs of the necessary electrolyzers, fuel compressors, storage tanks, and fuel cells substantially increase the overall bill. Even though the PE model would provide essentially free energy after purchase, the large initial price tag would make such a system infeasible for implementation in developing countries such as Eritrea, where the average GDP per capita was only $900 in 2009.  Thus, although the PE model is technically promising, significant advances must be made before it can compete with traditional generation techniques in terms of price.
© Andrew Moir. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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