Sustainability

Renewable energy storage goal

As we have seen in the previous paragraphs, one of the main drawbacks in the diffusion of renewable energy sources is the fact that they are not constant. Therefore their widespread use depends on the need to find storage systems to guarantee the energy supply also when the renewable source is not available. Energy storage is convenient when net recovered energy (meaning the amount of usable energy minus the energy used to operate the storage system) is above 70% of the energy produced.
Compressed air as an energy carrier
Energy storage with the use of compressed air is not only an efficient and clean, but also low cost way to solve the inconstancy of certain renewable sources. This system uses electric energy produced during low demand times (for instance at night) to power some special compressors to store air in caves, underground tunnels and other natural reservoirs. Compressed air is then used to power a turbine and generate energy at peak demand times.
Extra large ice cubes for environmental cooling
A Californian company sells ice block making machines that run on the energy surplus which is produced at low demand times. These ice blocks which are produced in building basements are then used to cool the buildings in the day time. This is a way to use intermittent energy sources such as solar and eaolian efficiently, making them more feasible and competitive compared to traditional sources.
From photosynthesis to energy storage
Professor Daniel Nocera, with his research team from the Massachusetts Institute of Technology (MIT) has developed a procedure which is similar to plant photosynthesis that should help solve the problem of renewable source intermittence. This method is based on the use of surplus energy produced by a photovoltaic cell, or wind turbine or any other energy source, to break down water into gaseous hydrogen and oxygen. The hydrogen and oxygen would then be used by a fuel cell to produce electric power for homes and cars. “Solar energy use has always been limited and far from being the final solution” says Daniel Nocera, “But now we can begin to think seriously about unlimited solar energy use”. Further studies are necessary in order to apply this new scientific discovery in present day photovoltaic systems, but Nocera hopes that buildings will be able to accumulate more and more solar energy through photovoltaic cells and use the surplus amounts for hydrogen production.
Batteries and energy storage systems
The first aeolian battery in the world has been tested in an aeolian park in Minnesota. It is an 80-ton sodium sulfur battery, consisting of twenty 50 KW modules with an overall storage capacity of 7.2 MWh which, when fully charged, would be able to supply electricity for up to 7 hours to 500 homes. If this result should be confirmed, it could be the beginning of large scale production with a valid energy storage solution. Efficient energy surplus exploitation ENEA is working on a project to use renewable source energy surplus amounts to produce hydrogen (H₂) which, reacting with carbon dioxide (CO₂), is then converted to methane (CH₄). In fact, by applying the Sabatier reaction it is possible to produce methane starting from molecular hydrogen and carbon dioxide: CO₂ + 4H₂ = CH₄ + 2H₂O With combustion, methane releases into the environment the same amount of carbon dioxide that has been absorbed in the process, thus avoiding a CO₂ emission increase. Furthermore, methane use allows us to use the systems which are currently being used, which are cheaper than the fuel cells that would be necessary to exploit hydrogen.