DJ’s house since he has a smaller parallel system with battery storage, but that’s another story.) When it comes down to it, storing intermittent energy is very costly and often quite inefficient. It’s currently the big downside to using intermittent flow-conversion mechanisms. Energy Density : Energy density refers to the quantity of potential energy derivable from an energy carrier per unit mass or volume. Human history has been one of progressively moving up the energy density ladder – from animal protein to wood, from wood to coal, and coal to gasoline. It should be obvious, if the number of joules that can be released by an energy carrier is low, it’s le ss useful. Hydrogen, potentially created via electrolysis and much touted as a future energy carrier, has very low energy per unit volume – about ¼ as much as natural gas. 181 Such low energy density poses significant technical challenges to any large-scale production, transport, and storage for commercialization of hydrogen as a fuel. Energy Surplus : Energy surplus is the energy return (ER/EI) of a process multiplied by the scale of the operation. The unprecedented expansion of our population, economy, and per capita living standards in the last 200 years was powered by high ER/EI, high energy surplus fossil fuels. With high ER/EI fuels we must reinvest only a little and then allocate the rest of the energy surplus to infrastructure, hospitals, schools, and everything else. With lower ER/EI sources, a greater portion stays within the energy sector itself, meaning other aspects of society get less, or none at all. This goes back to the description of the energy return for a lion chasing a gazelle, a rabbit, or a mouse. Though new electric technology like wind and solar photovoltaic (PV) have decent energy returns, most alternatives to conventional liquid fuels have very low or unknown ER/EIs. Portability : How hard is it to move the energy carrier? Is it bulky, volatile, dangerous, stable at ambient temperatures, etc.? For instance, a modern jet can stay aloft more than 12 hours fueled with kerosene. If it were powered by a less-portable substance, the jet would have to have a less-aerodynamic shape and waste more energy lugging the energy carrier around. Transformity : How easily, and with what efficiency, can one form of potential energy be transformed into a different form of potential energy? This is of crucial importance going forward, in ways we’ll discuss. For instance, we seem to have a whole lot of ways of making baseload electricity, but that’s only about 20% of the energy we use. 182 Storability : What are the costs and special considerations needed to store this energy carrier? Some can be stored in a simple tank, others require cryogenic refrigeration, are poison gas, require high-pressure vessels, or
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