One of the most interesting sessions at the recent NY-BEST Technology Conference was the one on Thermal and Mechanical Storage. It was a good reminder that there is more going on in the energy storage world than batteries. Ultracapacitors and fuel cells are also areas of significant activity, but electrochemical technologies are not the only technologies in play. Indeed, pumped hydro energy storage is still the largest capacity form of grid energy storage with over 127 GW worldwide. However, pumped hydro is by no means the only game in town.
Another example of a non-battery grid storage technology was presented by Expansion Energy, whose VPS Cycle uses liquid air as an energy storage medium. The liquid air is produced using an original power source (such as wind or solar) and then stored in cryogenic containers. When power is needed, the liquid air is compressed, heated and expanded to run turbine generators. The overall round-trip efficiency of the process is claimed to be greater than 95%. The process is particularly intriguing in that air liquefaction is already well established and in place on a large scale. In addition, existing gas-fired power plants can actually be retrofitted with VPS technology.
There were also several presentations about thermal storage technologies. The most familiar of these is ice storage. During the hot summer months, space cooling is generally the most expensive usage of electricity, particularly when daytime rates are higher. Using cheaper electricity at night to freeze water into ice and then using the cooling power of the ice during the day rather than running the compressors when electricity is expensive can reduce the cost of air conditioning buildings. Quite a number of buildings in New York City take advantage of ice storage as a way to shift demand to off-peak and manage their demand charges.
A more uncommon application of ice storage is at the power plant level rather than at the building level. Third Power, a NY-BEST member, has pioneered this approach. The concept is based on the fact that certain power plants lose substantial amounts of capacity in hot weather. In particular, river-cooled plants are restricted to a maximum temperature of their outflows but the output of steam-cycle power plants depends on the temperature difference between inflow and outflow. By the use of an ice/water/glycol coolant, outflow temperatures can be reduced resulting in a 4-20% capacity boost for the plant even when taking into account the energy used to freeze the coolant. Storing energy in the form of ice leads to increased energy output for the power plant.
Yet another version of thermal energy storage makes use of phase change materials (PCMs) whose thermal properties permit chillers to store as much as four times more energy than water ice in the same volume. Such materials have the potential to greatly increase the efficacy of thermal storage systems provided that the applications warrant the additional costs.
Without a doubt, there were many exciting talks about battery technology at our recent conference. But as the sessions on ultracapacitors and on thermal and mechanical storage demonstrated, batteries are not the only exciting topic in energy storage.