Solar For After Dark: How CSP Works

October 31st, 2016 by Susan Kraemer

Concentrated Solar Power (CSP) is the form of solar that can store the sun’s energy thermally for on-demand electricity generation at any time.

CSP is clean energy, like solar PV or wind, but the back end or the power block works like any thermal energy power station (coal, natural gas, and nuclear power plants are thermal energy power stations). The difference is that instead of digging up a finite fuel from the earth, CSP harvests sunlight to make thermal energy.

Because of its ability to store solar energy thermally, CSP can be fully dispatchable. In other words, it can be switched on when needed — in the evening, before sunrise, or at whatever time the utility needs power. It can respond to new demand within the same day. The speed of start-up is limited only by the time it takes to start the turbines, about half an hour.

How CSP Works

In general, all CSP plants operate by aiming mirrors (heliostats) to reflect the sun’s light onto a receiver which then heats a fluid that ultimately generates thermal electricity like any other thermal power plant. That is where the similarities among the various types of CSP plants end.

There are four types of CSP, with the oldest (Trough) being the most widespread, the newest (Tower) being the most likely to succeed, and two lagging technologies (Stirling Dish and Fresnel) that have yet to break more than a few megawatts of installed capacity.

Why the Need for Storage Means Tower CSP is the Future

Tower CSP is most likely to be the CSP of the future because it is the most economical technology to incorporate storage. And increasingly, storage is becoming a requirement of CSP.

For utility-scale applications, the thermal storage possible with tower CSP is typically cheaper, more durable, and longer lasting than battery storage, rivaling the almost limitless recycling capability of pumped hydro.

At grid scale, CSP thermal storage is cheaper than batteries, depending on how long the storage is — a half hour is better served by fast-acting batteries. Two tower CSP projects in Chile actually have both battery and thermal storage in order to combine the speedy reaction time of batteries with cheap long-term thermal storage.

(Behind the meter, of course, the uncertainty of net metering makes battery storage sensible financially for solar homeowners — and in such applications, a battery is the only option.)

How Tower CSP Works (with Storage)

In tower CSP with storage, molten salt is heated by reflecting sunlight with mirrors onto a receiver atop a tower. The heated fluid then descends the tower and can either be used right away or be stored thermally in a “hot” tank for use later.

When the thermal energy in the molten salt is to be used, it is sent to a heat exchanger where the heat is extracted. Once the heat has been extracted, the now cooled molten salt is stored in a “cold” tank ready to be sent up the tower to be heated again by the sunlight reflected onto the receiver.

Whether sent immediately or stored first, the extracted heat is then used to make steam to run a steam turbine in a power block that is the same as any other thermal power plant (like a combined cycle natural gas, coal, or nuclear plant). Basically, electricity is generated from steam that turns a turbine.

If the heated molten salt is left in the hot storage tank, it is stored as thermal energy that can be tapped later to generate electricity, and it loses only about 1% of heat a day. Normally, of course, it will be used the same evening or next day, but it is possible to size the storage capacity so that it could supply a longer period if desired.

Unlike batteries that can only be cycled so many times, molten salt thermal energy storage be heated and cooled daily for at least 30 years. At that point, the tanks might need corrosion repair, so the molten salt would be emptied and then returned to the tanks to supply another 30 or more years.

How Tower CSP Works (without Storage)

One of the earliest tower CSP plants was direct steam. BrightSource Energy’s Ivanpah (2014) used the reflected sunlight to simply boil water in the tower and operate directly off the steam produced. In 2008 when they negotiated their PPAs, California utilities were not interested in the inclusion of storage.

Direct steam tower CSP like Ivanpah does not lend itself to storage (because steam just doesn’t hold its heat as long as molten salt). It is not economically feasible to add storage after a project has been permitted and constructed as a non-storage project.

The only CSP plant to try storage with steam, Abengoa’s Shams I, did include 2 hours of steam storage on a trough plant, but most industry insiders do not consider it a success. It is telling that, when faced with having to sell off assets in its bankruptcy restructuring, the only CSP Abengoa sold, despite being a global leader in CSP, was Shams 1. (The other assets that Abengoa put on the chopping block were not its dozen or so trailblazing CSP plants, but ethanol and other projects.)

How Trough CSP Works

In trough CSP, the receiver is not atop a tower, but is simply a small pipe running continuously in front of the rows of the parabolic mirrors throughout the solar field. A heat transfer fluid (HTF) runs through the piping and is heated by the sunlight that is focused by the reflection on this continuous point in front of the rows of parabolic trough-shaped mirrors. This HTF traverses the solar field and when it gets to the power block goes through a heat exchange system where the hot oil is ultimately converted to steam to make electricity thermally. (Luz’s SEGS I-IX, 1980–1990)

Trough CSP is not ideally paired with storage because trough operates at a lower temperature and adding storage makes it more expensive than the gravity-fed storage tanks of a molten salt tower CSP project. But when trough CSP includes storage there is an extra step. The HTF is a hot oil, and then the heat is transferred to molten salt for storage there, and when the heat is needed to generate electricity, steam is run through the turbine to generate electricity (Abengoa’s Solana, 2013).

One possible idea for reducing the extra step of converting from hot oil to molten salt is this idea from Chiyoda, to try molten salt as the HTF throughout the solar field. Despite papers from as far back as 2003, this has not been used commercially, however.

Why the Changing Grid Needs Dispatchable Clean Energy

Solar PV and wind are intermittent renewables. As more PV and wind have come online, the grid has changed. The old model — a baseload resource that is always on, like a coal plant — has now even become a liability. Baseload energy has gone from being a feature to a bug. In a grid with various renewables, an unresponsive base load, always on, regardless of grid saturation with solar or wind, is out of sync.

So, as the world adds more PV and wind, the need is shifting towards a dispatchable “switch-on” supply to fill in the gaps, rather than a baseload resource that is always on, regardless of whether it’s needed or not. CSP solar generation can be switched on quickly on demand, because of its thermal storage.

Why CSP is Controversial: It Displaces Gas- & Coal-Fired Electricity

CSP is the renewable that most directly threatens natural gas–fired electricity, because of its ability (with storage) to deliver power on demand. With its ability to store its solar energy, CSP can extend clean solar into the evening, overnight (if the grid needs that!), before sunrise the next day, at times when plants are down for maintenance, and when there are some cloudy mornings or days.

CSP is perfectly suited to smoothing out the increasingly fat “duck curve” of new demand as the load is time shifted by the solar power generated by ever-increasing daytime PV. In most places, as more solar PV comes on line in the daytime, evening is when there is the need for new generation — to smooth out the ramped-up demand that builds up as the sun goes down (i.e., the duck curve).

Smoothing the duck curve is a job now performed by very dirty natural gas plants in states like California, and by coal plants in states like Arizona. All this fossil-fueled dirty energy gets switched on when the sun goes down to cover the evening peak till bedtime.

So CSP is an important renewable because it can actively edge dirty energy off the grid.











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