Crescent Dunes CSP was developed by the Santa Monica-based SolarReserve and features the company’s market leading molten salt power tower technology with fully integrated energy storage.

The $1 billion Crescent Dunes project near Tonopah in the Central Nevada Desert, some 300kms north of Las Vegas, was developed by the Santa Monica-based SolarReserve and features the company’s market leading molten salt power tower technology with fully integrated energy storage.

The 110MW Crescent Dunes Solar Energy Plant, a concentrated solar power project due to be completed in Nevada early next year, will not just be the largest solar power tower plant with fully integrated energy storage built – it could also challenge the way the world thinks about renewable energy.

What makes it unique and a potential game changer in the electricity industry is the flexibility and dispatchability of its power, meaning that it can deliver electricity whenever it is needed by customers; and its cost, which already beats diesel, is competitive with new build coal and gas generation.

The Crescent Dunes concentrating solar power facility will have 10 hours of molten salt storage, which on average will allow it to deliver 110 MW of baseload capacity to Las Vegas between the hours of 12 noon and midnight each day, when the city needs it most to power the lights and air conditioning of its casinos and entertainment palaces. It has signed a 25-year power contract with NV Energy, Nevada’s largest utility, to  do that.

Tom Georgis, SolarReserve’s senior vice president of development, says the unique capabilities of the technology means that the solar thermal plant could have been configured in any number of ways. With a 180MW turbine, for instance, it could have produced power for 10 hours each day, which was the original intention. With a smaller turbine, and more than 20 hours storage, it could have delivered 50MW of base-load power 24/7.

In the end, Nevada pitched for midday to midnight to suit its needs. In effect, the plant is providing baseload power for a fixed period each day – delivering the benefits of coal-fired power without the downsides, which is of course heavy pollution and an ability to be switched off at will or at regular intervals.

‘You can’t do that with a coal fired facility,” Georgis says of the Nevada contract. But the technology also allows it to compete with gas-fired generation, both in the ability to provide baseload and as a peaking plant.

Next year, SolarReserve begins construction of the 150 MW Rice Solar Energy Plant in Southern California, which will act more like a peaking power station to suit that state’s needs. Proposals the company will take to Chile, Australia and the Middle East will likely be for baseload power. (We will explain more about those plans in the next two days).

“This should be the winning technology. It has all the attributes you looking for to displace conventional generation,” Georgis says. “It’s not just fulfilling renewable energy targets, you are displacing any new build fossil plants – from nuclear, gas and coal. This is going to change the discussion in energy markets, certainly around the idea that renewables are variable.”

Georgis says there is a lot of confusion about storage and what it means. He says the way to think about it is in the amount of electricity produced by a solar tower plant over a year.

With the standard “S-Class” configuration of the plant, the design being built at Tonopah, the plant can produce over 500GWh of electricity a year in strong solar locations such as Western Australia and the Southwest USA. That can be sliced and diced whichever way a customer – be it a utility, industrial group or a miner – chooses. And that can be in 24/7 base-load, delivering electricity at specific times of the day as in Nevada, or as a peaking plant.

“We are not just a renewable energy generator, we can integrate and help firm and shape other variable energy sources,” Georgis says. ”It will run in summer for 16-18 hours a day. But I can turn down the turbine and run it 24/7 if the utility asks me to do so.“

To emphasise the point, Georgis points to the following graphics that show the output from a solar PV plant, and a solar tower plant without storage. The big square blocks – coloured green and yellow – are two output scenarios from the tower with storage, but it can be reshaped and timed to suit the customer’s needs. Grid operators and many customers like big square blocks.

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The other key point, and this is the critical one, is in price.

The Nevada project has a publicly disclosed power purchase agreement of $US135/MWh with NV Energy. The project is supported by cheaper finance from the Department of Energy Loan Guarantee Program, and tax incentives, but Georgis says it is also the first of its kind to be built at this scale and has extra margins and contingencies typical of a first plant.

For this reason, Georgis says $135/MWh should be viewed as a reasonable estimate of costs, because by the time the fourth or fifth plant has been constructed, the capital costs will have come down dramatically.

Georgis says that even without incentives, the LCOE would be “well south” of $200/MWh. In Chile, where the excellent solar resources means that the output is 40 per cent greater (around 700GWh a year), the price is already at $135/MWh without any incentives.

“By 2020, we should be south of  $100/MWh (in the US, Australia and elsewhere) and not reliant on any type of government subsidy or incentive program”, he says. That is key, because as Bloomberg New Energy Finance noted, new coal fired and gas fired plants are already more expensive, and will be well north of that figure by 2020.

And Georgis says the generation facility can participate in any kind of energy market. “If you have a flexible dispatch market, a storage market, or a capacity market, it can participate in all of those. It can even play in the merchant market with a robust price,” he says.

The Crescent Dunes plant will not be the first of its type, but it will be the biggest to date, and the first built to what Georgis describes as “utility scale”. The 18MW Gemasolar plant (pictured) has been operating in Spain for the last 18 months, and the 10MW Solar Two demonstration facility near Barstow in California’s Mojave Desert was operated by the Department of Energy in the 1990s.

SolarReserve has the exclusive worldwide license to the technology which was developed by Rocketdyne, a subsidiary of Aerojet, and perfected during various space programs. These include the algorithms for the solar trackers that move the heliostats, and the receiver, which uses proprietary metallurgy technology that allows it to expand and contract, and to resist melting.

The company currently has 7 projects in various stages of development the US, two of which have power purchase agreements (Crescent Dunes and Rice), and six projects in Spain, including one (the 50MW Cinco Casas project) with a PPA, although the Spanish projects are not progressing at the moment because financing is difficult to obtain in the current market. SolarReserve is pursuing contracts in Chile, South Africa, North Africa, the Middle East, China and Australia.

The 110MW Crescent Dunes Solar Energy Plant, located near the town of Tonopah in the Nevada desert, will be the largest solar tower plant with integrated energy storage facility built to date.

But what exactly is it? What does storage actually do? And how does it work?

Let’s start with the tower.  There are a bunch of different technologies that come under the umbrella of solar thermal, or concentrated solar power. These include compact linear Fresnel reflectors, and parabolic troughs.

Solar towers use heliostats (or dual-axis sun-tracking mirrors) to reflect the sun’s heat onto a single receiver point.  This technology is  favoured because it can generate more heat than other technologies, has great economies of scale, and can integrate storage.

That heat could be used for industrial processes, such as steam production, as well as generating electricity. Generally, the more heat that is created, the more efficient the plant.

The heliostats track the sun‘s movements through the day. At SolarReserve’s Crescent Dunes facility, the plant will comprise 600 hectares of land, approximately 10,340 heliostats (each one 115 sq metres) with a total of approximately one million square metres of glass.

In the case of Crescent Dunes, the receiver (and the solar tracking algorithms) was derived from rocket engine propulsion technology developed by Rocketdyne, now a subsidiary of Aerojet.

Unlike other solar towers, which heat water directly to create steam and drive a turbine, the Crescent Dunes facility will heat molten salt, which is piped through the receiver located at the top of a tower, which is 180m high.

Two storage tanks are used. A cold tank stores the salt at 280C, pumps it up to the top of the tower where it circulates through the receiver, where the salt’s temperature is taken to 565C and it is then piped back down to the hot storage tank.

There, the energy is stored for use at a later time or released immediately into a heat exchanger that produces steam that powers a standard steam generator.

Here is a schematic of the plant.

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Tom Georgis says that in Nevada, the Crescent Dunes plant will capture and store the sun’s energy throughout the day because the utility does not want the plant to generate electricity in the morning.

This is the key to the technology’s strengths – it can be configured in any way that the customer wants. To underline the point, and the value of dispatchable, emissions free energy, SolarReserve show what output from a solar PV farm looks like, then wind, which varies from day to day, and what a solar tower power plant with storage can deliver.

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At Tonopah, the plant will have 10 hours storage at 110MW capacity, and will deliver under contract between the hours of noon and midnight on average.  An “hour” of storage means that the plant can run for one hour at full output using only stored energy.

It could have installed a bigger turbine with less storage capability, or a smaller turbine with more storage. A 50MW turbine would mean storage for 20-24 hours, and the ability to produce baseload power 24/7. Although, Georgis points out that you don’t need 24 hours storage to run baseload 24 hours, because when the sun is shining, energy can be stored and generated at the same time.

Georgis says that there are three key phases to the plant:

In phase 1, in the morning, it will collect and store the sun’s energy; in phase 2 (in the afternoon) it will start to generate, releasing salt to the heat exchangers and continuing to release cold salt to be recirculated. It is producing electricity, but it still collecting and storing.  In phase 3 (at night), it will not collect any more energy, but will generate electricity from the energy stored in the salts.

This is how SolarReserve explain the technology on the website (click here if you want to see the original with pictures).

SolarReserve’s technology, typically referred to as Concentrated Solar Power (CSP), uses thousands of mirrors to reflect and concentrate sunlight onto a central point to generate heat, which in turn is used to generate electricity.

More than 10 thousand tracking mirrors called heliostats reside in a 1,500 acre field, where they reflect and concentrate sunlight onto a large heat exchanger called a receiver that sits atop a 550-foot tower.

Within the receiver, fluid flows through the piping that forms the external walls; this fluid absorbs the heat from the concentrated sunlight. In SolarReserve’s technology, the fluid utilized is molten salt, which is heated from 500 to over 1,000 degrees Fahrenheit.

Molten salt is an ideal heat capture medium, as it maintains its liquid state even above 1,000 degrees Fahrenheit, allowing the system to operate at low pressure for convenient energy capture and storage. After passing through the receiver, the molten salt then flows down the piping inside the tower and into a thermal storage tank, where the energy is stored as high-temperature molten salt until electricity is needed.

SolarReserve’s technology leverages liquid molten salt as both the energy collection and the storage mechanism, which allows it to separate energy collection from electricity generation. When electricity is required by the utility, day or night, the high-temperature molten salt flows into the steam generator, as water is piped in from the water storage tank, to generate steam.

Once the hot salt is used to create steam, the cooled molten salt is then piped back into the cold salt storage tank where it will then flow back up the receiver to be reheated as the process continues.

After the steam is used to drive the steam turbine, it is condensed back to water and returned to the water holding tank, where it will flow back into the steam generator when needed. After the molten salt passes though the steam generator, it flows back to the cold tank and is re-used throughout the life of the project. The hot molten salt generates high-quality superheated steam to drive a standard steam turbine at maximum efficiency to generate reliable, non-intermittent electricity during peak demand hours.

The steam generation process is identical to the process used in conventional gas, coal or nuclear power plants, except that it is 100 percent renewable with zero harmful emissions or waste. SolarReserve plants provide on-demand, reliable electricity from a renewable source—the sun—even after dark.

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