Storing solar and wind power is a base-load conundrum

solarhbj.com
13 January 2011

Here in Colorado we have an abundance of both sun and wind, so it's no surprise that our state Renewable Portfolio Standard was recently raised to 30% by the year 2020, from the 10% that was enacted in 2004 by constitutional amendment.

So with the excellent solar and wind resources here, why not increase our RPS MetOcean to 50%, 80% or even 100%? The problem is "base load". That's the minimum amount of power that must be available from the utility at all times, and it changes with daily and hourly predictions based on historical data and current trends. Unfortunately, the sun isn't always shining and the wind isn't always blowing, so 24/7/365 energy sources are always required.

Hydroelectric generation is a renewable base-load option in some areas, but Colorado doesn't have much of a hydro resource. Coal- and nuclear-fired power plants are the most common base-load generation sources throughout the United States, because they are cheap to operate and consume inexpensive fuel.

When energy needs spike (such as during a hot summer day when everyone turns on their air conditioners, or on a cold winter night when electric heaters are running full blast) utilities turn to "peaking" generation systems that can be spun up and generating within minutes.

You can't just flip the "on" switch to get a coal or nuclear generator working -- they take hours or even days to fire up from cold to producing energy. Hydroelectric plant operators can quickly increase the water flow and the number of generators operating, but only if there's enough water available at the time. In most areas, natural gas turbines are used to handle peaking loads, but the fuel is too expensive to use for providing constant base-load capacity.

Energy Storage
The answer to using solar or wind for utility base load seems obvious. Just store any excess energy produced and release it when the renewables are not generating, just like in an off-grid home. All you need is a really big battery! Therein lies the rub.

Until recently, America's biggest battery was located in Fairbanks, Alaska. Called "BESS" (Battery Electric Storage System), it was built and installed by the ABB Group for the Golden Valley Electrical Association in 2003 at a cost of $35 million. This half-acre, 1,435-ton battery bank provides 27MWs of base-load backup power for GVEA's 90,000 customers in the event that grid power from Anchorage is interrupted. Unfortunately, BESS can only do that for 15 minutes -- just long enough to get Fairbanks' diesel backup generators spun up and on line.

The town of Presidio, Texas, also had a problem -- frequent blackouts due to the now-undersized 1940s-vintage transmission lines still feeding the town. Extra energy could be purchased from Mexico, but with hours-long blackouts common, changes were made. In April 2010, Electric Transmission Texas put "BOB" (Big Ol' Battery) into commission. BOB weighs over 320 tons, costs about $25 million, and can provide 4MWs of base-load power to Presidio for about eight hours.

Those battery numbers are all pretty depressing when viewed in a cost-vs.-storage capacity perspective. Right now, the only other real storage option on a utility scale is hydroelectric -- excess generation powers giant pumps that move massive amounts of water up a steep hill to a large reservoir, to be released through hydro turbines to generate electricity again when peaking generation is needed. The problem with this method is that the efficiency is dismal, with close to 70% of the incoming energy wasted.

Battery technology is rapidly improving, thanks in part to greater public interest in plug-in electric vehicles. BOB in Texas employs new technology, sodium-sulphur cells, instead of the more traditional nickel-cadmium cells used in BESS. But the cost per kW of electrochemical energy storage remains tremendously high, especially when compared to current methods that store the most amount of energy for the least cost -- a lump of coal, uranium pellets, or a barrel of oil. Are there other options? The future

Certain recent industrial-scale solar power projects employ some energy storage. These solar thermal plants use mirrors to heat molten salts, which are used to flash water into steam and power conventional steam turbine generators. The salts hold heat for quite some time, and the turbines continue to run after the sun sets. While this represents great progress, it's still tricky. The storage part of the system is far more complex and expensive than the generation side, and heat cannot be stored long enough to provide a source of base-load capacity.

Of course hydrogen production, storage and generation have been touted for many years, and these might well be our future for energy storage. The "hydrogen economy" hype of the past few years has died down, though, mostly because hydrogen is notoriously difficult and inefficient to produce, store and use. It can escape through the tiniest gaps, it turns regular steel unsafely brittle and is explosive when mixed in the right proportion with air.

These problems will all be solved -- eventually -- but at great cost and over a long time frame. Compressed air energy storage on a large scale has been discussed too, but suffers similar technology cost and efficiency problems.

Flywheel energy storage has recently received media attention, and with good reason. Spin a heavy wheel up to tens of thousands of RPM in a near vacuum to reduce air friction and float it on magnetic levitation bearings, and you have a formidable energy storage system. But flywheel storage remains a very high-tech and expensive possibility on a utility scale for base-load storage. It's currently used only in specialized applications.

A better mousetrap?
As I sit here 11 miles off the grid and gaze at my (literally) ton of home backup batteries, the thought crosses my mind that I'm looking at 1915-era technology -- and that it's still the most efficient, cost-effective choice for me. And when I float in Pinewood Reservoir with my fly rod each summer, it occurs to me that this massive, inefficient pump, water storage and hydro turbine system is about the most efficient and cost-effective way for the utility to store its energy, too.

It's a good bet that future utility-scale, renewable energy storage systems will use some variation of the methods we have already imagined. Scientists understand the basic laws of physics pretty well these days. The winner won't be the company that invents the most high-tech and efficient energy storage mousetrap -- it will be the company that builds the one that stores the most energy at the lowest cost.