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Solar Power When the Sun Goes Down


By Todd Woody, New York Times Green Inc., 3 November 2009 --

The holy grail of renewable energy is a solar power plant that continues producing electricity after the sun goes down. A Santa Monica, Calif., company called SolarReserve has taken a step toward making that a reality, filing an application with California regulators to build a 150-megawatt solar farm that will store seven hours’ worth of the sun’s energy in the form of molten salt.

Heat from the salt can be released when it’s cloudy or at night to create steam that drives an electricity-generating turbine. The Rice Solar Energy Project, to be built in the Sonoran Desert east of Palm Springs, will “generate steady and uninterrupted power during hours of peak electricity demand,” according to SolarReserve’s license application.


Molten Salt Flow Diagram

The perennial complaint about wind and solar power is, that despite the fact that they produce clean power, they are intermittent and require storage technology to fully exploit the power generated. Molten salt could provide the necessary storage capacity. ©2009 United Technologies

So-called dispatchable solar farms would in theory allow utilities to avoid spending billions of dollars building fossil fuel power plants that are fired up only a few times a year when electricity demand spikes, like on a hot day.

SolarReserve is literally run by rocket scientists, many of whom formerly worked at Rocketdyne, a subsidiary of the technology giant United Technologies. Rocketdyne developed the solar salt technology, which was proven viable at the 10-megawatt Solar Two demonstration project near Barstow, Calif., in the 1990s.

United Technologies has licensed the technology to SolarReserve and will guarantee its performance — a crucial advantage for the startup when it seeks financing from skittish bankers to build the Rice solar farm.

As many as 17,500 large mirrors — each one 24 feet by 28 feet — will be attached to 12-foot pedestals. The mirrors, called heliostats, will be arrayed in a circle around a 538-foot concrete tower.

Atop the tower will sit a 100-foot receiver filled with 4.4 million gallons of liquid salt.

The heliostats will focus the sun on the receiver, heating the salt to 1,050 degrees Fahrenheit. The liquefied salt flows through a steam-generating system to drive the turbine and is returned to the receiver to be heated again.

SolarReserve isn’t the only developer planning to tap molten salt to store solar energy. Abengoa Solar, for instance, intends to use salt storage at its 280-megawatt Solana solar trough plant outside Phoenix.

That project, however, will heat tubes filled with synthetic oil to create steam and transfer some of the heat to salt-filled storage tanks.

By using salt for both steam and storage, SolarReserve can generate higher-temperature steam, which will allow the Rice power plant to operate much more efficiently, according to Kevin Smith, SolarReserve’s chief executive.

“Consequently, our system can capture three times the energy for the same pound of salt,” Mr. Smith wrote in an e-mail message. “Plus they have additional ‘bolt on’ equipment, plus multiple heat transfer steps to go from oil to salt to oil and then to steam for electricity generation.”

SolarReserve’s plant will be built on private land — the site of a former World War II-era Army airfield — near the desert ghost town of Rice. The company will air-cool the power plant, avoiding controversies over water use that have dogged other solar projects.

But the height of the solar tower — 653 feet when a maintenance crane is attached to the top — could generate resistance from conservationists worried about the impact of the project on desert vistas. A proposed SolarReserve power plant in Nevada ran into resistance from Air Force officials concerned that the tower would interfere with radar at a nearby military base.

The company said it is negotiating with California utilities to buy the electricity generated from the Rice project and expects the solar farm to go online in October 2013, barring unforeseen delays.


Additional Article follows:
Rice Solar Energy Molten Salt Project

By Brian Merchant, 4 November 09, New York

The problem with solar power plants is, of course, that they're rendered useless on cloudy days and at night. But news broke a while back that a process of storing solar energy in molten salt was being considered to help make solar power plants more commercially viable. Now, it looks like that technology is about to be put into action: a California company has filed an application to build a 150 megawatt solar plant that will store seven hours worth of the sun's energy in molten salt--allowing it to provide power nearly around the clock.

According to Green Inc, the Rice Solar Energy Project is going to be built in the Sonoran Desert near Palm Springs. And according to the license application from Solar Reserve, the company helming the project, it will "generate steady and uninterrupted power during hours of peak electricity demand."

The prospect of storing energy eliminates the need to rely on using fossil fuel plants for backup during dark days and peak hours.

Here's how the Rice Solar Energy Project will be set up:

17,500 large mirrors - each one 24 feet by 28 feet -- will be attached to 12-foot-hight pedestals. The mirrors, called heliostats, will be arrayed in a circle around a 538-foot-tall concrete tower. Atop the tower will sit a 100-foot-tall receiver filled with 4.4 million gallons of liquid salt. The heliostats will focus the sun on the receiver, heating the salt to 1,050 degrees Fahrenheit. The liquefied salt flows through a steam generating system to drive the turbine and then is returned to the receiver to be heated again.

The 'molten salt solution' is a promising idea in the quest to make continuous power production from solar a reality--and the success of this project could be huge news for the solar industry.

Additional Article follows
from the California Energy Commission
Rice Solar Energy, LLC, (RSE) a wholly owned subsidiary of SolarReserve, LLC, proposes to construct, own, and operate the Rice Solar Energy Project (RSEP or project).

The RSEP will be a solar generating facility located on a privately owned site in unincorporated eastern Riverside County, California.

The proposed project will be capable of producing approximately 450,000 megawatt hours (MWh) of renewable energy annually, with a nominal net generating capacity of 150 megawatts (MW).

The RSEP will be located in an unincorporated area of eastern Riverside County, California. Land surrounding the project site consists mostly of undeveloped open desert that is owned by the federal government and managed by the U.S. Bureau of Land Management (BLM).

The proposed facility will use concentrating solar power (CSP) technology, with a central receiver tower and an integrated thermal storage system. The RSEP's technology generates power from sunlight by focusing energy from a field of sun-tracking mirrors called heliostats onto a central receiver. Liquid salt (The salt is a mixture of sodium nitrate, a common ingredient in fertilizer, and potassium nitrate, a fertilizer and food additive. These mineral products will be mixed onsite as received directly from mines in solid crystallized form and used without additives or further processing other than mixing and heating.), which has viscosity and appearance similar to water when melted, is circulated through tubes in the receiver, collecting the energy gathered from the sun. The heated salt is then routed to an insulated storage tank where it can be stored with minimal energy losses. When electricity is to be generated, the hot salt is routed to heat exchangers (or steam generation system). The steam is then used to generate electricity in a conventional steam turbine cycle. After exiting the steam generation system, the salt is sent to the cold salt thermal storage tank and the cycle is repeated. The salt storage technology was demonstrated successfully at the U.S. Department of Energy-sponsored 10-MW Solar Two project near Barstow, California, in the 1990s.

According to the applicant, this unique CSP technology offers several important benefits. Because liquid salt has highly efficient heat transfer and storage properties, it is used as the heat transfer medium in the cycle. Natural gas heating is therefore not required for startup or for operating stability during routine cloud cover. Second, the stored energy in the salt can be extracted upon demand and produce electricity even when there is no sunlight. Finally, the output from the RSEP will produce a stable electricity supply, compensating for potential impacts on the electricity grid from other intermittent energy sources having less predictable operating characteristics.

The solar facility will have the following key elements:

• A large circular field of mirrors (heliostats) that reflect the sun's energy onto a central receiver tower

• A conventional steam turbine generator to produce electricity
• Insulated tanks to store the hot and cold liquid salt heat transfer fluid

• An air-cooled condenser (ACC) to eliminate water consumption for cooling the steam turbine exhaust


Rice Solar Energy Project (California, USA)

Solar Reserve