The latest in the series of University At Buffalo—NYSERDA Directed Energy workshops was on Energy Storage in Amherst, New York, on May 10. The workshop had an excellent slate of speakers covering pumped water storage, compressed air storage and the role of the ISO (independent service operator) in balancing supply and demand.

Martin Casstevens of Directed Energy and Karen Parysek of Praxair outlined the issues of balancing renewable energy resources including solar PV. Load balancing is excruciatingly difficult. Efficiencies of coal powered and hydro generation plants are highest in a relatively small operating range. Peaking supply and demand cycles are both long term and extremely short term, and response times for different systems vary widely with combined cycle natural gas stations having the fastest response. Nuclear plants have a slow response and coal plants can only be turned down by about 40%.

Night usage is about 40% of daytime use and winter usage is about half of summer usage except in Florida during cold spells, where many houses have electric heating used infrequently. Wind power generally peaks at night whereas solar (naturally) peaks during daylight hours.

The economic storage limit for batteries, flywheels and super capacitors is generally less than 10 MW; superconducting magnets may one day take us to 50 MW but the only two GW capable technologies on the horizon are pumped water and air storage.

We had presentations on the Niagara and Blenheim-Gilboa (30 miles south of Albany, New York) pumped storage projects by Joseph Kessler and Brian Saez of the New York Power Authority. There are approximately 40 pumped storage facilities in the USA of varying sizes. The Niagara Power Project pump-generating plant is 240 MW (and originally designed to regulate the flow over Niagara Falls by treaty with Canada), Blenheim-Gilboa is an 1160 MW unit with 12000 MWh storage and the Bath County VA plant is the largest in the world at 2770 MW. The pumping and regeneration process is about 75% efficient and the turbines have traditionally operated in a restricted speed range to avoid cavitation damage. New turbine designs by Hitachi have widened this range considerably and upgrades to both turbine systems are underway. Response time to load at Blenheim-Gilboa is approximately 5 minutes.

Karen Parysek of Praxair outlined the proposed Seneca Advanced Compressed Air Energy Storage (CAES) facility near Watkins Glen, New York, just undergoing final environmental and other reviews. This is a depleted salt mine which will be filled with 0.5 billion cubic feet of compressed air at 800-1100 psi pumped using a natural gas powered compressor plant. Overall efficiency of the compressed air part of the storage process is approximately 80%. Initially designed to balance wind generation, this type of storage is also amenable to solar energy storage but depends on geological availability. There are two compressed energy plants in existence and a further one planned. Interestingly, if you do a little reading on this, compressed air energy storage on a somewhat smaller scale was used by cities including Paris in the 19th century! Why not liquefied air, with a volume 700 times smaller than compressed air? Higher costs rule this out.

Esther Takeuchi of University of Buffalo outlined the challenges and opportunities in battery technology. Surprisingly the requirements for implantable batteries and batteries for energy storage are quite similar, including long life and high pulse power. Lithium air batteries show promise for high storage density, but as with all batteries, the cost is still too high for volume use—in a previous issue we mentioned that the installation planned in Hawaii is approximately $1 per watt hour—and the sheer volume of materials needed for industrial scale storage and the setting up of a decommissioning and recycling process is daunting.

So how is all this power generation, consumption and storage managed without frying the transmission lines? Mike Swider from the New York Independent Service Operator (ISO) outlined the processes and challenges. They run a market to balance power. Their balancing resources include battery and flywheel storage. Solar power is not a big factor in New York State yet but wind is, with 1.3 GW installed and another 7 GW proposed. One challenge is the grid layout—typically point to point rather than a mesh, which is needed with distributed generation. Market prices can go negative short term when supply exceeds demand in spring and fall, encouraging storage suppliers. In a recent case in the UK, there was a storm of protest when wind generators were paid to stop power generation because of grid concerns—obviously not the best solution!

At the Water Innovations Alliance meeting in Dayton, Ohio, May 16-17, we also heard from Paul Gagliardo of American Water how water utilities are now working with energy utilities to balance load by timing pumping and other operations (water towers, waste treatment etc). These utilities are big power users and pumps are often oversized to meet peak demand—so are suitable for intermittent operation—and this gives another tool for load balancing.

So what does this mean for solar PV? It means that a lot of work is being done behind the scenes to balance the characteristics of renewable power generation and power usage. We heard how this is being done on a large scale but it is also being done on a small scale, with the emergence of solar powered residential hybrid air conditioner systems as one example. As our industry develops this work is critical to the success of future solar PV deployment.