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Clean Power Screen-Shot-2013-07-02-at-2.31.57-AM

Published on July 6th, 2013 | by Giles Parkinson

7

How Does Integrated Solar Work?

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July 6th, 2013 by  

This article first appeared on RenewEconomy

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.

Screen-Shot-2013-07-02-at-2.31.57-AM

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.

Screen-Shot-2013-07-04-at-11.20.20-AM

Screen-Shot-2013-07-04-at-11.20.31-AM

Screen-Shot-2013-07-04-at-8.53.00-AM

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:

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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About the Author

is the founding editor of RenewEconomy.com.au, an Australian-based website that provides news and analysis on cleantech, carbon, and climate issues. Giles is based in Sydney and is watching the (slow, but quickening) transformation of Australia's energy grid with great interest.



  • Breath on the Wind

    “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.”

    This is poorly worded and a bit confusing. The “tower” has nothing to do with fresnel reflectors and parabolic troughs. Later heliostats are described as working in two directions, however heliostats for parabolic troughs only need to work in one direction due to site orientation and the nature of troughs. An interesting article but the quality could have been better.

  • Others

    For those who celebrated Germany’s Solar Power Increase, please read the other side of it

    http://www.eia.gov/cfapps/ipdbproject/IEDIndex3.cfm?tid=1&pid=7&aid=1

    Germany increased the Coal consumption, why shut down nuclear power plants and consume more Coal. They can build more Solar and Wind Plant, but not shut down the nuclear power.

    • Bob_Wallace

      Germany is going through a temporary increase in coal use.

      I think we should respect the feelings of German citizens who decided that they no longer want to live with the risk of a nuclear meltdown. And since Germany is so far ahead of most of the rest of the world in installing renewables and implementing efficiency measures, I think they have earned a bit of latitude.

    • http://zacharyshahan.com/ Zachary Shahan

      I don’t really get how people can criticize Germany. Germany has been a leader in cutting greenhouse gas emissions. If it decides to shut down nuclear power in order to avoid a disaster in its front yard, i think it has every right to… at least until others catch up with its climate leadership.

  • Ivor O’Connor

    All this pumping of molten salt seems error prone. I’d much rather they over engineer the CSP so http://en.wikipedia.org/wiki/Thermosiphon can be passively used. I wouldn’t want a pump or controlling circuitry or an earthquake to be responsible for a phase transition somewhere that brings the whole system to a halt.

    What are the recovery steps needed after a failure when the molten salt is now frozen salt? Will the solid salt destroy the pipes? If not has the heating mechanism to change the solid salt back into molten salt already been installed? Have these steps been tested? Will they be tested on a regular basis?

    It seems like the people that have designed passive fail safe nuclear power plants could be employed in the construction of passive CSP plants…

    We need robust CSP and we need it now!

    • Bob_Wallace

      The “salt” is basically fertilizer, IIRC. A spill wouldn’t be an environmental disaster.

      The plumbing is done with corrosion-resistant materials.

      At least one system has heating coils attached to the plumbing so that the system can be warmed back up if necessary.

      I’m not convinced that it’s time to start building CSP on a grand scale. Let’s see how this generation works and what the costs actually are.

      • Jared Conner

        Although, CSP does have a huge potential for replacing fossil fuels in industry for heat production. Have you seen this video on Youtube?
        http://www.youtube.com/watch?v=svAPyyUJUCo And you could even sequester carbon with concentrated sunlight by using it to do pyrolysis of organic matter, and capturing the compounds off-gassed, to keep it carbon negative.

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