Getting serious about solar

One of the ideas that always amused me was hippie belief that power companies didn't like solar because there was "no way to meter the sun."  I have even heard that nonsense from highly educated people.

Of course, by the time solar energy has been captured and transformed into the electrical current that can run your computer or television set, it is just as easy to meter as any electricity created by burning coal or using nuclear fission.  What the power companies really disliked about solar was its unreliability.  And that's what their customers weren't going to like about it either.

Without some major technological breakthroughs in energy storage, solar is going to be very unreliable indeed.  Powering one's life with solar without storage will be, at best, akin to sailing--which is a highly enjoyable sport but not something anyone does for a living anymore.  Just remember, when steam power came to sea transportation, the sailing fleets that were displaced included the clippers--the most magnificent sailing vessels ever built.

Catching the sun’s heat

Storing thermal energy in chemical form has the potential to make it indefinitely storable and transportable.

David L. Chandler, MIT News Office

October 26, 2010

Broadly speaking, there have been two approaches to capturing the sun’s energy: photovoltaics, which turn the sunlight into electricity, or solar-thermal systems, which concentrate the sun’s heat and use it to boil water to turn a turbine, or use the heat directly for hot water or home heating. But there is another approach whose potential was seen decades ago, but which was sidelined because nobody found a way to harness it in a practical and economical way.

This is the thermo-chemical approach, in which solar energy is captured in the configuration of certain molecules which can then release the energy on demand to produce usable heat. And unlike conventional solar-thermal systems, which require very effective insulation and even then gradually let the heat leak away, the heat-storing chemicals can remain stable for years.

Researchers explored this type of solar thermal fuel in the 1970s, but there were big challenges: Nobody could find a chemical that could reliably and reversibly switch between two states, absorbing sunlight to go into one state and then releasing heat when it reverted to the first state. Such a compound was discovered in 1996, but it included ruthenium, a rare and expensive element, so it was impractical for widespread energy storage. Moreover, no one understood how the compound worked, which hindered efforts to find a cheaper variant.

Now researchers at MIT have overcome that obstacle, with a combination of theoretical and experimental work that has revealed exactly how the molecule, called fulvalene diruthenium, accomplishes its energy storage and release. And this understanding, they said, should make it possible to find similar chemicals based on more abundant, less expensive materials than ruthenium.

Essentially, the molecule undergoes a structural transformation when it absorbs sunlight, putting the molecule into a higher-energy state where it can remain stable indefinitely. Then, triggered by a small addition of heat or a catalyst, it snaps back to its original shape, releasing heat in the process. But the team found that the process is a bit more complicated than that. more

Meanwhile, the Spaniards have built a solar plant that actually produces power around the clock.

Solar plant keeps working, even after sundown 

Published Monday, 4th July 2011

A concentrated solar power plant in Spain has just supplied its first uninterrupted day of electricity to the network, providing energy to the grid even after sundown thanks to molten-salt storage.

Located in Fuentes de Andalucía (Seville), the Gemasolar plant is property of Torresol Energy, a joint venture between Masdar — Abu Dhabi’s clean-energy initiative — and SENER, a Spanish engineering and construction company.

The plant, barely one month into commercial operation, can store solar energy in molten salt using a thermal-transfer technology developed by SENER. The system enables the facility to deliver 15 hours of electricity production without solar radiation, helping to overcome fluctuations in the energy supply.

“Gemasolar achieved optimal performance in its systems in the last week of June,” said Diego Ramírez, director of production at Torresol Energy. “The high performance of the installations coincided with several days of excellent solar radiation, which made it possible for the hot-salt storage tank to reach full capacity. We’re hoping that in the next few days our supply to the network will reach an average of 20 hours a day.”

The salt storage system allows the plant to stretch its electrical production hours to beyond sunset, regardless of the cloud cover. With a 19.9-megawatt steam turbine, Gemasolar is able to supply electricity to some 25,000 households.

Eventually, the plant is expected to be able to supply 24 hours of uninterrupted production per day on most summer days, providing a higher annual capacity factor than most baseload plants such as nuclear power plants.

“The prove-out of both this technology and the commercial approach we have taken to funding and operating the facility are of huge significance to the solar industry,” said Frank Wouters, director of Masdar Power.

SENER has applied its solar technology with thermal-storage capacity in plants throughout Spain, some of which are already in commercial operation. This system significantly improves performance compared to plants with no storage capacity. In addition, it makes it possible to manage the supply of electricity sent to the network and respond to spikes in demand. In this way, the reliability of solar energy becomes comparable to that of conventional fossil-fuel power plants, which is decisive as the demand for renewable energy increases.

Gemasolar is the first plant to apply the thermal-storage system in a configuration with a central tower and an array of heliostats. The main difference between it and plants with parabolic-trough technology is its ability to reach a much higher operating temperature (over 500 degrees C) by dispensing with oil and directly using salts as a transfer fluid. The molten salts make it possible to generate hotter steam at higher pressures, which significantly boosts the plant’s efficiency. more