STORING SOLAR ENERGY

“Ah but I may as well try and catch the sun…” Donovan might just as well have lamented in his iconic 1960s ballad about wind and lost love. Lament has long been the mood of solar energy enthusiasts seeking to compensate for the sun’s intermittency. In search of what to do when "sundown pales the sky" and rain has "hung the leaves with tears" (Donovan again, sorry), solar engineers have for decades coupled photovoltaic panels with batteries and stored a frustratingly small part of the electricity generated.

There was never a need to lament, solar power plant designers now think. There was only the need to focus on storing the sun’s heat rather than its electricity. John S. O’Donnell, executive vice president, Ausra: “The thermos costs about $5 and the laptop battery $150…”

There are presently 2 competing concepts for solar energy storage. One involves fields of parabolic mirrors and storing the sun’s heat as pressurized steam. The other uses fields of flat mirrors with a power tower and would store the heat as molten salts.

The power tower concept is being developed by, among others, eSolar under a $10 million Google Foundation grant and is said to have the potential, in conjunction with storage capacity, to bring solar energy costs down to the same range as the cheapest coal- and natural gas-fired power plant electricity.

Storage concepts have been around for some time. Solar thermal power plants were built in the California deserts in the early 1990s and could have tested the storage ideas but investment fell away when natural gas prices dropped very low.

Terry Murphy, president/chief executive, SolarReserve: “There were not renewable portfolio standards…Nobody cared about global warming, and we weren’t killing people in Iraq.”

Catch the sun? Wouldn't that be "mellow yellow?" As Donovan said, it "...would be the sweetest thing, t'would make me sing..."

A NY Times schematic of the molten salts storage concept. (click to enlarge)

New Ways to Store Solar Energy for Nighttime and Cloudy Days
Matthew L. Wald, April 15, 2008 (NY Times)

WHO
John S. O’Donnell, executive vice president, Ausra; Terry Murphy, president/chief executive, SolarReserve; Larry Stoddard, manager of renewable energy consulting, Black & Veatch

From Ausra. (click to enlarge)

WHAT
Storage of solar energy is now possible because solar power plant developers have shifted from the storage of the sun’s electricity to the storage of the sun’s heat.

WHEN
- Storage is still conceptual. There is no working solar power plant presently capable of storing and dispatching solar energy.
- Storage allows solar energy to be dispatched (1) at times of peak demand, (2) on cloudy days and (3) after dark.

click to enlarge

WHERE
- Storage concepts are being developed at solar power plants presently being planned, approved and/or constructed in California, Arizona, Spain and North Africa.
- Storage also allows solar power plants to operate at higher latitudes and in places with lower insolation.

Schematic of the molten salts storage concept. (click to enlarge)

WHY
- Solar thermal power plants generally use heat to boil water to drive traditional turbines that generate electricity.
- One type of solar thermal plant has fields of parabolic mirrors that concentrate the sun’s light and heat on pipes running the length of the mirrors through their focal point. In the pipes is a glycerin/water mixture that heats to 500 degrees F. or more. The heated liquid flows to the boiler to create the steam that drives the turbine. The steam, though, can be captured under pressure and held for when power is needed.
- An alternative power tower concept focuses fields of flat mirrors that track the sun and keep its maximum light and heat on a point at the top of a tower where salts are heated to molten temperatures near 1000 degrees F. The salts can be stored without pressure and used the same way as the stored steam. A power tower that generates 540 megawatts of heat can produce 250 megawatts of electricity.

Schematic of the power tower concept. (click to enlarge)

QUOTES
- Murphy, SolarReserve: “You take the energy the sun is putting into the earth that day, store it and capture it, put it into the reservoir, and use it on demand…”
- Larry Stoddard, manager of renewable energy consulting, Black & Veatch, on the molten salt concept: “…your turbine is totally buffered from the vagaries of the sun…[whereas without it] if I’ve got a 50 megawatt photovoltaic plant, covering 300 acres or so, and a large cloud comes over, I lose 50 megawatts in something like 100 to 120 seconds…That strikes fear into the hearts of utility dispatchers.”