Technology Review has an article on solar thermal power company Brightsource and its molten salt energy storage technology - Energy Storage for Solar Power.

BrightSource Energy has become the latest solar thermal power company to develop a system for generating power when the sun isn't shining. The company says the technology can lower the cost of solar power and make it more reliable, helping it compete with conventional sources of electricity.

The company, based in Oakland, California, is building one of the world's largest solar thermal power plants. The 392-megawatt solar plant in Ivanpah, California, however, will not include the storage technology. Instead, BrightSource is working with utilities to determine which future projects could best benefit from storage.

Solar thermal systems use mirrors to focus sunlight, generating temperatures high enough to produce steam to drive a turbine. One of the advantages of the solar thermal approach, versus conventional photovoltaics that convert sunlight directly into electricity, is that heat can be stored cheaply and used when needed to generate electricity. In all solar thermal plants, some heat is stored in the fluids circulating through the system. This evens out any short fluctuations in sunlight and lets the plant generate electricity for some time after the sun goes down. But adding storage systems would let the plant ride out longer periods of cloud cover and generate power well into, or even throughout, the night. Such long-term storage could be needed if solar is to provide a large share of the total power supply.

BrightSource is using a variation on an approach to storage that's a decade old: heating up a molten salt—typically, a combination of sodium and potassium nitride—and then storing it in a tank. To generate electricity, the molten salt is pumped through a heat exchanger to generate steam. BrightSource CEO John Woolard says one big factor in making this technology economically attractive is the use of power towers—in which mirrors focus sunlight on a central tower—that generate higher temperatures than other solar thermal designs. That higher temperature makes it possible to store more energy using a smaller amount of molten salt. "It's a much more efficient system and much more cost effective, overall," he says.

Storage allows a thermal power plant to run more hours in the day, so they can more quickly recover the cost of expensive steam turbines and generators. Woolard says that while a solar thermal plant without storage can generate electricity about 2,700 hours a year, BrightSource's storage system increases that to 4,300 hours. The increased output more than offsets the added cost of storage. A study from the National Renewable Energy Laboratory (NREL) in Golden, Colorado, estimates that storage in a power tower system could cut costs per kilowatt hour by 25 to 30 percent.

At least two other companies are pairing power tower technologies with molten salt storage. Torresol Energy has built such a system at a 19.9-megawatt solar thermal power plant near Seville, Spain, and demonstrated that it can run the power plant through the night using stored heat. In the United States, Solar Reserve plans to build a power tower with molten salt storage in Riverside County, California.

Carbon Commentary has a post on the new solar thermal power demonstration plant that has started operation in Italy -
The world’s first molten salt concentrating solar power plant.

On 14 July 2010 the Italian utility Enel unveiled “Archimede”, the first Concentrating Solar Power (CSP) plant in the World to use molten salts for heat transfer and storage, and the first to be fully integrated to an existing combined-cycle gas power plant. Archimede is a 5 MW plant located in Priolo Gargallo (Sicily), within Europe’s largest petrochemical district. The breakthrough project was co-developed by Enel, one of World’s largest utilities, and ENEA, the Italian National Agency for New Technologies, Energy and Sustainable Economic Development.

Several CSP plants already operate around the world, mainly in the US and Spain. They use synthetic oils to capture the Sun’s energy in the form of heat, by using mirrors that beam sunlight onto a pipe where pressurised oil heats up to around 390°C. A heat exchanger is then used to boil water and run a conventional steam turbine cycle. Older CSP plants can only operate at daytime – when direct sunlight is available -, an issue that has been dealt with in recent years by introducing heat storage, in the form of molten salts. Newer CSP plants, as the many under construction in Spain, use molten salts storage to extend the plants’ daily operating hours. Archimede is the first plant in the world to use molten salts not just to store heat but also to collect it from the sun in the first place.

This is a competitive advantage, for a variety of reasons. Molten salts can operate at higher temperatures than oils (up to 550°C instead of 390°C), therefore increasing efficiency and power output of a plant. With the higher-temperature heat storage allowed by the direct use of salts, the plant can also extend its operating hours well further than an oil-operated CSP plant with molten salt storage, thus working 24 hours a day for several days in the absence of sun or during rainy days. This feature also enables a simplified plant design, as it avoids the need for oil-to-salts heat exchangers, and eliminates the safety and environmental concerns related to the use of oils (molten salts are cheap, non-toxic common fertilizers and do not catch fire, as opposed to synthetic oils currently used in CSP plants around the World). Last but not least, the higher temperatures reached by the molten salts enable the use of steam turbines at the standard pressure/temperature parameters as used in most common gas-cycle fossil power plants. This means that conventional power plants can be integrated – or, in perspective, replaced – with this technology without expensive retrofits to the existing assets.

The author Carlo Ombello has some additional commentary about expected levelised pricing for solar thermal power in the comments, which is estimated to be 4,5 €cents/kWh:

I have published some more data from ENEA on my blog, regarding their expectations from this technology once it’s fully developed: http://www.opportunityenergy.org/?p=94 :

For a typical 100MW stand alone plant in Egypt, it should look like this:

Annual direct solar radiation: 2.900 kWh/(m2 year)
Total area occupied by the solar collectors: 67 ha
Total area occupied by the solar field: 134 ha
Nominal power output: 100 MW (Peak 485 MW)
Thermal storage capacity: 1.800 MWh
Net annual electricity produced: 369 GWh/year
Plant load factor: 42 %

Total Cost : 157 M€
Specific cost : 1.570 €/kWe
Service life: 25 years
Interest rate: 7%
Annual operating (O&M) costs: 2% of investment cost
Levelized Cost Of Electricity (LCOE): 4,5 €cents/kWh

Obviously, this can be scaled down or up according to needs, as much as the technical data can be changed. Some clients might want more storage to achieve a higher load factor, some might prefer a lower storage capacity, and just modulate the plant according to peak prices (which tend to be matched quite well by nature in this case).

Grist has an article on some DOE research funding for energy storage for solar thermal power - A hot technology: Feds push solar solution to coal addiction.

The Obama administration last week gave a $62 million boost to efforts to make solar power truly competitive with coal.

"The projects announced today will seek to improve component and system designs to extend operation [of concentrated solar power projects] to an average of about 18 hours per day, a level of production that would make it possible for these plants to displace traditional coal-burning power plants," the Department of Energy said in a statement announcing cash grants that are being doled out over the next five years.

The recipients are companies developing technology to store energy generated by solar thermal plants so that it can be used at night or when the sun doesn't shine. In the utility biz, that's called baseload power. (Solar thermal plants typically use vast arrays of mirrors to focus the sun on a liquid-filled boiler to create steam that drives an electricity-generating turbine.)

It's hardly a huge amount of cash. But it's going to a mix of startups and big old-line tech companies -- many in California -- that are working on some potentially game-changing technology.

But how much of the game needs to be changed? That question seems heretical -- we'll have achieved renewable-energy nirvana when solar farms grow electrons 24/7, right? But it was raised by J.D. Sitton, chief executive of Infinia, a solar startup backed by prominent green-tech venture capitalist Khosla Ventures as well as eSolar founder Bill Gross' Idealab and Vulcan Capital, the Seattle investment firm run by Microsoft cofounder Paul Allen.

"There's a raging debate in the solar thermal business about how much is storage worth and how much it matters," says Sitton, whose, Kennewick, Wash., company, scored $3 million from the Department of Energy to create storage technology for its Stirling solar dish.

Resembling a large mirrored satellite receiver, Infinia's 21-foot-tall PowerDish focuses the sun on a Stirling engine suspended on an arm over the center of the device. The heat causes a gas inside the engine to expand and drive a piston that generates electricity.

The DOE grant -- and others Infinia has received from the federal government -- will allow the company to integrate storage capacity into the dish apparatus. Sitton says that will involve some form of molten salt that will store PowerDish-generated heat that can be released to drive the Stirling engine when the sun is not shining.

TreeHugger has a look at how storing energy with molten salt works.

The perennial complaint by renewable energy skeptics about wind and solar power is, that despite the fact that they produce clean power, they are intermittent in nature and require some sort of storage technology to fully exploit the power generated. A recent article at Renewable Energy World spotlights one solution to this very real issue: Molten Salt.

Basically the set-up works like this: Heliostats (large concentrating mirrors) focus the sun onto a central tower, which heats a liquid inside. The liquid in question is molten salt—in this case a combination of sodium and potassium nitrate—which is heated, transferred to a storage tank, and then fed through a steam generator to turn a turbine and generate electricity. The cooled salt is fed back into the tower to repeat the process. The difference between the molten salt and other liquids that are used in this sort of system is that the molten salt retains heat for a long enough time that it can effectively time-shift the stored solar energy from when it is most efficiently generated to when it is most needed.

The original article quotes Terry Murphy, CEO of SolarReserve who helped develop the technology. "Molten salt is a heat storage medium that that retains thermal energy very effectively over time and operates at temperatures greater than 1000°F, which matches well with the most efficient steam turbines. Second, it remains in a liquid state throughout the plant's operating regime, which will improve long-term reliability and reduce O&M costs. And third, it's totally 'green,' molten salt is a non-toxic, readily available material, similar to commercial fertilizers."

Personally, I wouldn’t go around touting commercial fertilizers or anything similar to them as being “totally ‘green’”, but as a way to time-shift renewably generated energy—this really isn’t long term storage in the sense of a conventional battery or fuel cell—it certainly is a solution.

Scientific American has a reasonably ambitious plan to derive a large part of the energy required by the US from solar power by 2050, which they deem "A Solar Grand Plan". The point out this could end US dependence on foreign oil and slash greenhouse gas emissions (though there would still be more work required to slash greenhouse emissions by the required amount). They put more emphasis on photovoltaic power plants compared to CSP power plants than I would prefer - I would have thought getting PV on as many rooftops as possible (at least in the sun belt) would be the best bet and leave CSP for large scale generation out in the deserts.

Key Concepts

* A massive switch from coal, oil, natural gas and nuclear power plants to solar power plants could supply 69 percent of the U.S.’s electricity and 35 percent of its total energy by 2050.
* A vast area of photovoltaic cells would have to be erected in the Southwest. Excess daytime energy would be stored as compressed air in underground caverns to be tapped during nighttime hours.
* Large solar concentrator power plants would be built as well.
* A new direct-current power transmission backbone would deliver solar electricity across the country.
* But $420 billion in subsidies from 2011 to 2050 would be required to fund the infrastructure and make it cost-competitive.

Introduction


Well-meaning scientists, engineers, economists and politicians have proposed various steps that could slightly reduce fossil-fuel use and emissions. These steps are not enough. The U.S. needs a bold plan to free itself from fossil fuels. Our analysis convinces us that a massive switch to solar power is the logical answer.

Solar energy’s potential is off the chart. The energy in sunlight striking the earth for 40 minutes is equivalent to global energy consumption for a year. The U.S. is lucky to be endowed with a vast resource; at least 250,000 square miles of land in the Southwest alone are suitable for constructing solar power plants, and that land receives more than 4,500 quadrillion British thermal units (Btu) of solar radiation a year. Converting only 2.5 percent of that radiation into electricity would match the nation’s total energy consumption in 2006.

To convert the country to solar power, huge tracts of land would have to be covered with photovoltaic panels and solar heating troughs. A direct-current (DC) transmission backbone would also have to be erected to send that energy efficiently across the nation.

The technology is ready. On the following pages we present a grand plan that could provide 69 percent of the U.S.’s electricity and 35 percent of its total energy (which includes transportation) with solar power by 2050. We project that this energy could be sold to consumers at rates equivalent to today’s rates for conventional power sources, about five cents per kilowatt-hour (kWh). If wind, biomass and geothermal sources were also developed, renewable energy could provide 100 percent of the nation’s electricity and 90 percent of its energy by 2100.

The federal government would have to invest more than $400 billion over the next 40 years to complete the 2050 plan. That investment is substantial, but the payoff is greater. Solar plants consume little or no fuel, saving billions of dollars year after year. The infrastructure would displace 300 large coal-fired power plants and 300 more large natural gas plants and all the fuels they consume. The plan would effectively eliminate all imported oil, fundamentally cutting U.S. trade deficits and easing political tension in the Middle East and elsewhere. Because solar technologies are almost pollution-free, the plan would also reduce greenhouse gas emissions from power plants by 1.7 billion tons a year, and another 1.9 billion tons from gasoline vehicles would be displaced by plug-in hybrids refueled by the solar power grid. In 2050 U.S. carbon dioxide emissions would be 62 percent below 2005 levels, putting a major brake on global warming. ...

The Energy Blog points out that solar cell production increased by 50% last year.

According to The Earth Policy Institute production of photovoltaics (PV) jumped to 3,800 megawatts worldwide in 2007, up an estimated 50 percent over 2006. At the end of the year, according to preliminary data, cumulative global production stood at 12,400 megawatts, enough to power 2.4 million U.S. homes. Growing by an impressive average of 48 percent each year since 2002, PV production has been doubling every two years, making it the world’s fastest-growing energy source.

A key force driving the advancement of thin-film technologies is a polysilicon shortage that began in April 2004. In 2006, for the first time, more than half of polysilicon production went into PVs instead of computer chips. While thin films are not as efficient at converting sunlight to electricity, they currently cost less and their physical flexibility makes them more versatile than traditional solar cells. Led by the United States, thin film grew from 4 percent of the market in 2003 to 7 percent in 2006. Polysilicon supply is expected to match demand by 2010, but not before thin film grabs 20 percent of the market.

MSNBC has an article on using molten salt to store energy - a storage mechanism that reportedly only loses 1% of the energy stored per day. There is a discussion at Slashdot

An aerospace manufacturer is developing a way to use molten salt to store solar energy that could produce enough electricity to power 250,000 to 500,000 homes a year. Hamilton Sundstrand, a subsidiary of United Technologies Corp., announced last week that it is working with a California venture capital firm on the project.

The partnership between Hamilton Sundstrand, which makes power systems for the Space Station and Boeing's Dreamliner aircraft, and US Renewables Group in Santa Monica, Calif., would use molten salt to store the sun's heat, which will then be converted to electrical power that could be added to utilities' grids at times of peak demand. ...

Molten salt, a mixture of sodium and potassium nitrate, circulates through a central receiver, is heated by sunlight to more than 1,000 degrees, stored in a tank and dispatched into a steam generator. The steam drives a turbine that generates electricity. The cooled salt re-circulates and the process begins again. The salt loses only 1 percent of its heat per day, which is far better than water and other materials, said Dan Coulom, a spokesman for Hamilton Sundstrand.

Popular Mechanics has an article on the Super Soaker inventor who "aims to cut solar costs in half". His invention is more like a stirling engine than a photovoltaic or CSP set up - but has no moving parts and a claimed conversion efficiency of over 60%. It will be interesting to see how this technology fares out in the wild.

Solar energy technology is enjoying its day in the sun with the advent of innovations from flexible photovoltaic (PV) materials to thermal power plants that concentrate the sun’s heat to drive turbines. But even the best system converts only about 30 percent of received solar energy into electricity—making solar more expensive than burning coal or oil. That will change if Lonnie Johnson’s invention works. The Atlanta-based independent inventor of the Super Soaker squirt gun (a true technological milestone) says he can achieve a conversion efficiency rate that tops 60 percent with a new solid-state heat engine. It represents a breakthrough new way to turn heat into power.

Johnson, a nuclear engineer who holds more than 100 patents, calls his invention the Johnson Thermoelectric Energy Conversion System, or JTEC for short. This is not PV technology, in which semiconducting silicon converts light into electricity. And unlike a Stirling engine, in which pistons are powered by the expansion and compression of a contained gas, there are no moving parts in the JTEC. It’s sort of like a fuel cell: JTEC circulates hydrogen between two membrane-electrode assemblies (MEA). Unlike a fuel cell, however, JTEC is a closed system. No external hydrogen source. No oxygen input. No wastewater output. Other than a jolt of electricity that acts like the ignition spark in an internal-combustion engine, the only input is heat.

Here’s how it works: One MEA stack is coupled to a high- temperature heat source (such as solar heat concentrated by mirrors), and the other to a low-temperature heat sink (ambient air). The low-temperature stack acts as the compressor stage while the high-temperature stack functions as the power stage. Once the cycle is started by the electrical jolt, the resulting pressure differential produces voltage across each of the MEA stacks. The higher voltage at the high-temperature stack forces the low-temperature stack to pump hydrogen from low pressure to high pressure, maintaining the pressure differential. Meanwhile hydrogen passing through the high-temperature stack generates power.

“It’s like a conventional heat engine,” explains Paul Werbos, program director at the National Science Foundation, which has provided funding for JTEC. “It still uses temperature differences to create pressure gradients. Only instead of using those pressure gradients to move an axle or wheel, he’s using them to force ions through a membrane. It’s a totally new way of generating electricity from heat.”

The bigger the temperature differential, the higher the efficiency. With the help of Heshmat Aglan, a professor of mechanical engineering at Alabama’s Tuskegee University, Johnson hopes to have a low-temperature prototype (200-degree centigrade) completed within a year’s time. The pair is experimenting with high-temperature membranes made of a novel ceramic material of micron-scale thickness. Johnson envisions a first-generation system capable of handling temperatures up to 600 degrees. (Currently, solar concentration using parabolic mirrors tops 800 degrees centigrade.) Based on the theoretical Carnot thermodynamic cycle, at 600 degrees efficiency rates approach 60 percent, twice those of today’s solar Stirling engines.

This engine, Johnson says, can operate on tiny scales, or generate megawatts of power. If it proves feasible, drastically reducing the cost of solar power would only be a start. JTEC could potentially harvest waste heat from internal combustion engines and combustion turbines, perhaps even the human body. And no moving parts means no friction and fewer mechanical failures.

Gizmag has a post on solar power that works at night using "nanoantennas" - and this technology is claimed to have a conversion efficiency of 80%. Cryptogon also has some comments - both point to this article from the Idaho National Laboratory.

Idaho National Laboratory (INL) reports that research conducted in conjunction with partners at Microcontinuum Inc. (Cambridge, MA) and Patrick Pinhero of the University of Missouri is promising a method for developing cheap solar energy technology that could be imprinted on flexible materials and still draw energy after the sun has set. The technology uses a special manufacturing process to stamp tiny square spirals, or “nanoantennas”, of conduction metal onto a sheet of plastic and the team estimates individual nanoantennas can absorb close to 80 percent of the available energy in comparison to current commercial solar panels which usually transform less that 20 percent of the usable energy that strikes them into electricity – this is even more impressive than the 30% conversion rate offered by the recently discussed development of nano flakes.

Due to their size – each interlocking spiral nanoantenna is as wide as 1/25 the diameter of a human hair - the nanoantennas absorb energy in the infrared part of the spectrum, just outside the range of what is visible to the eye. Since the sun radiates a lot of infrared energy, some of which is soaked up by the earth and later released as radiation for hours after sunset, nanoantennas can take in energy from both sunlight and the earth's heat, with higher efficiency than conventional solar cells. The new approach, which garnered two 2007 Nano50 awards, was made possible by the boom in nanotechnology, but finding an efficient way to stamp out arrays of atom-scale spirals took a number of years. The INL team says that the antennas might one day be produced like foil or plastic wrap on roll-to-roll machinery and so far they have demonstrated the imprinting process with six-inch circular stamps, each holding more than 10 million antennas.

The nanoantennas could prove to be a more efficient and sustainable alternative to current commercial solar panels, which are made of processed silicon – the supply of which is lagging – and doped with exotic elements to boost efficiency. In contrast the nanoantenna circuits can be made of a number of different conducting metals, and the nanoantennas can be printed on thin, flexible materials like polyethylene. By focusing on readily available materials and rapid manufacturing the team’s aim is to make nanoantenna arrays as cheap as inexpensive carpet. The team says nanoantenna collectors might be used to charge portable battery packs, coat the roofs of homes or even be integrated into polyester fabric.

As exciting as the potential of the technology is, not all the hurdles have been passed yet. While the nanoantennas are easily manufactured, the problem of creating a way to store or transmit the electricity is yet to be solved. Although infrared rays create an alternating current in the nanoantenna, the frequency of the current switches back and forth ten thousand billion times a second - much too fast for electrical appliances, which operate on currents that oscillate only 60 times a second. The team is exploring ways to slow that cycling down and has a patent pending on a variety of potential energy conversion methods. They anticipate they are only a few years away from creating the next generation of solar energy collectors.