Sisyphus in Greek Mythology was condemned to eternally roll a large rock up a hill only to watch it crash to the bottom. The myth lives on today with the term Sisyphean that refers to tasks that are never ending, thankless and frustrating.
Today, large Utilities contend with the Sisyphean task of meeting the ever-changing, daily demands of users who require different amounts of electric power at different times. The task is further complicated by the growing availability of renewable energy feeding into the grid which is not necessarily synchronized with demand. The disconnect between capacity and usage has led the Utilities to seek efficient ways of storing the power generated by renewables and not consumed. The stored energy can then be returned to the grid when the electric power demand is the highest, and frankly, when the user is willing to pay more for the electrical power to avoid brown-outs. Large scale methods for storing renewable energy have long been suggested but rarely deployed prior to the widespread wind turbine and photovoltaic installations.
The electric storage solution is simple: store the energy in electric batteries that can maintain conversion efficiencies over many thousands of cycles and without depleting the world’s noble metals and the Utilities bank accounts.
Another option is to store energy by mechanical means. Two of the most common methods are to store energy by either compressing air (by what has become called: Compressed Air Energy Storage CAES) into a very large vessel or pumping liquid to very tall heights. The recovery of this power is through any means that can rotate a shaft and thus generate electric power for general usage. The methods for turning a generator shaft using this potential thermal or mechanical energy is very straightforward and typically involves turbines or expanders; devices that can expand the high pressure of the compressed air or high pressure water back to atmosphere. These turbines can be optimized for efficiency and manufacturability using sophisticated analysis and design tools, such as Concepts’ Agile Engineering Design System® which is a specialized suite of software for Computer-Aided Engineering (CAE) and Computer–Aided Manufacturing (CAM) of turbomachines. Specialized turbomachinery software helps enable the distinctly different air foil profiles that are being designed to maximize efficiency and match the needs of the energy storage system.
The need for high efficiency conversion of this stored energy begins with the storing of the energy which can be done by compressors and or pumps for the compressed air energy or pumped water storage systems. An improvement in efficiency of 5% for the compression (energy storage sequence) as well as the expansion (energy recovery sequence) process translates into a net 10% increase in the amount of energy that can be recovered.
Compressed air and pumped water energy storage systems are only the two most common mechanical means of storing the energy from a renewable energy source. A start-up company named ARES, named after its technology, Advanced Rail Energy Storage, plans to push freight train cars up a hill on tracks to increase the trains’ potential energy. The trains are then released when power is needed. This company will start laying track in 2017/18.
If the rail car mechanical energy storage system is successful, I would like to suggest that utilizing existing structures can help improve the overall cost for such a system. For example, the corkscrew roadways in decommissioned open pit mines can be retrofitted with rail track and multiple train cars wound up the face of the mine during energy storage and then allowed to wind down the face during energy recovery. These open pit mines have vertical depths of over 2,000 feet and thus there can be much more potential energy stored in the trains at these heights. Of course there is less control of the maximum power that could be recovered during the energy recovery phase; being that the acceleration of gravity is now in control, but you knew that. To put an even more positive spin on the idea (pun intended), the helical wound tracks lengthens the time that the train is in motion and so lower power can be generated longer.
I bet Sisyphus would have rather pushed a train up a track; at least then he could have hopped on board and enjoyed the ride down.