The Sun Shines at Night: Solar Thermal Power with Storage

Summary.  The Solana Generating Station, a solar thermal electric generating facility in Arizona, has begun operation in October, 2013.  Solar thermal plants capture energy from the sun as heat, which is then used to form steam that drives a turbine generator.  Solana additionally features a thermal storage capability, based on heating a molten “salt”, to buffer extra heat energy for use when the sun does not shine.  The buffer provides up to six hours of thermal operation.  Solana’s electricity is contracted to Arizona Public Service, an electric utility company.

Solana is one of several commercial-scale solar energy facilities in the U. S. currently under construction or newly operating.  Its financing included major support from the U. S. Department of Energy Loan Guarantee Program.  This program supports many projects that commercialize new or unconventional technologies that provide energy for the U. S. economy.  Only 6% of its guarantee funds were granted to projects that have been “discontinued”.  This indicates its funds have a highly creditable success rate of 94%.  The program  fulfills an important governmental function, that of supporting projects that would have difficulty attracting private investment.

Introduction.  Incoming solar radiation generates electricity on a commercial scale using two different technologies.  First, solar photovoltaic power uses semiconductor light-sensitive panels directly to generate electric current.  These include the familiar solar panels used on rooftops for local generation.  Photovoltaic power is not considered here.

The second technology is solar thermal power generation.  The heat contained in sunlight is captured in a circulating fluid that heats water to steam.  The steam is then used in a conventional turbine to generate electricity.  An industrial scale solar thermal facility with the added feature of storing heat has just become operational in Arizona; it is described here.

The Solana Generating Station has begun operation near Phoenix, AZ.    This station a) focuses the sun’s energy using mirrors to heat an oil fluid flowing through the black horizontal pipes in the photo below.

Thousands of curved mirrors focus sunlight onto black pipes to heat the oil circulating in them.

Source: http://www.nytimes.com/2013/10/18/business/energy-environment/arizona-utility-tries-storing-solar-energy-for-use-in-the-dark.html

 

The heat stored in the oil is used either to b) heat water to make steam, which then drives turbines to generate electricity, or c) heat a high temperature molten “salt” (not table salt) which stores the transferred heat in a hot salt tank.  This is shown in the image below for daytime operation of the station.

 

At night the station d) transfers the heat stored in the molten salt back to the oil, which is then used to heat water to steam, driving the electric turbines.  This is shown in the image below.



 

Enough heat is stored in the molten salt during the day for about six hours of generating service after dark.

 

Other solar energy installations have tried storing excess energy in electric batteries.  But these are expensive, and so not suitable for large installations.

 

Industrial scale electricity generation.  The Solana Generating Station operates two generating turbines to provide a peak power (power is the rate of generating electrical energy) of 280 megawatts (MW; millions of watts; a watt is a unit of power).  The parabolic mirror-circulating oil facility includes thousands of individual mirrors covering about three square miles of land.  The station was built by the Spanish electricity company Abengoa using primarily materials sourced in the U. S. 

 

The electricity provided by Solana is being sold under a long term contract to Arizona Public Service (APS), providing electricity sufficient for 70,000 customers.  In addition to the Solana station, APS will have a total installed solar power capacity of 750 MW by the end of 2013, enough to serve 185,000 customers.

 

The Solana Generating Station represents a capital investment of about US$2 billion.  The U. S. Department of Energy (DOE) Loan Guarantee Program supported US$1.45 billion of this amount.  The utility customer base and long service lifetime of the station provide reasonable assurance that the loan guarantee will have been a successful venture.

 

During construction, the project provided 1,500 jobs in the local community.  Abengoa will require a small number of local permanent positions for maintenance and service of the station.

 

Discussion

 

The Solana Generating Station is but one of several industrial-scale renewable energy generating facilities being developed in the U. S.  A previous post summarizes some of the solar projects included in this category.  These too have reached fruition or are about to begin operation.

 

General descriptions of solar energy, be it photovoltaic or solar thermal, have been concerned with intermittency of service since the sun’s energy is available only during daylight hours.  The Solana Generating Station overcomes this criticism by use of its heat storage system based on molten salt heat reservoirs.  Although inevitable engineering losses arise during heat transfer into and out from the reservoirs, the station’s design permits using the sun’s energy, in the form of the heat stored in the reservoirs, to generate electricity during dark hours of the daily cycle, and/or if clouds excessively obscure sunlight.  Additionally the stored heat complements portions of the daily light cycle when other renewable energy sources, such as solar photovoltaic generators or wind turbines, are poorly effective.

 

The DOE Loan Guarantee Program (LGP) provided valuable financial support for this project.  The objectives of the program are summarized as “guarantee[ing] loans that support early commercial use of advanced technologies, if there is reasonable prospect of repayment by the borrower.” DOE loan guarantees are intended to promote commercial use of innovative technologies, but not to support energy research, development, and demonstration programs.

 

The LGP has provided loans totaling US$24.2 billion to 29 renewable energy and advanced technology commercial projects other than loans supporting nuclear energy.  They are summarized as having saved jobs or provided construction jobs totaling about 50,000 (although 33,000 of these are ascribed to jobs saved by loans to Ford Motor Co.).  Most of the loans are listed as “closed”.  Only a few, totaling US$1.5 billion, are listed as “discontinued”, including a loan to Solyndra Inc. which gained unfavorable political attention two years ago. 

 

In other words, only 6% of the funds guaranteed under the LGP have performed unsatisfactorily.  This is a highly positive outcome for a program intended to encourage novel or unproven technologies.  Thus the LGP has been remarkably successful in supporting the commercialization of new or unconventional technologies that promote expansion of the energy mix, and greater efficiency in transportation, for the U. S.  It is concluded that the LGP fulfills an important governmental function, that of supporting projects that would have difficulty attracting private investment.

 

  

© 2013 Henry Auer