New operational data from utility scale solar towers will provide firms with a clearer Levelized Cost of Energy to compare against the risks of the relatively new technology.

By the end of 2015, data from SolarReserve’s 110 MW Crescent Dunes tower in Nevada will reveal more on whether more attractive LCOE outweighs innovation’s higher risk.

Trough is the best-known Concentrated Solar Power (CSP) technology - proven over twenty years, most widely adopted and easiest to finance, with the low risk of proven technology.

However, publicly available PPA prices and tariffs paid for trough (with and without storage), versus for tower with storage, indicate towers offer lower costs.

PPAs signed since 2010 for tower with molten salt storage are in the 11-13 cent range, compared to 20 - 21 cents per kWh for current trough projects.

Tower CSP with integrated molten salt storage is the technology most likely to compete against PV in the solar plus storage field which mitigates intermittency and responds to evening peak demand.

Currently, adding energy storage to Concentrated Solar Power (CSP) costs about $30 per KWh of capacity, contributing between 15 and 20% of the project cost, but tower CSP with integrated molten salt storage is still much cheaper than PV-plus-batteries at utility-scale.
“Currently, battery storage costs several hundred dollars a kilowatt hour up into the thousands,” said Dr. Ranga Pitchumani, Chief Scientist for the SunShot Initiative at the US Department of Energy (DOE). “This is the primary reason that thermal energy storage is an attractive option.”

In tower with molten salt storage, heliostats are deployed to heat salts to 1,050°F in the receiver atop a central tower. The salt flows into a hot tank where thermal energy is available to superheat steam on demand to power a steam turbine.

Once heated, molten salt could be stored for up to two months without losing appreciable heat - though normally the heat would be extracted for use within each night. The now “cold” (550°F) salts are recycled up the tower again to be reheated repeatedly, not just in full sun, but even under semi-cloudy conditions.

Keeping molten salts hot enough to power a steam turbine after 12 hours of darkness is not as straightforward as burning coal overnight, however. Focusing mirrors to reflect sunlight to heat a liquid in a tower up to several thousand feet away requires 21st century technology. Sunlight reflected by mirrors is completely emission-free, based on a scientific principle known for centuries; but making it power turbines at utility-scale is literally rocket science.
 

Data power

Remote controlling tens of thousands of heliostats with electronic software would not have been possible before the connectivity of the internet of things. California-based SolarReserve has over 100 patents on its technology acquired from
NASA contractor Aerojet RocketDyne, and like ACWA Power, it has two CSP towers.

“Our heliostat control system hardware and software platform is derived from the technology used to develop power system controls deployed on the International Space Station. Because it orbits the earth 17 times a day it must continually articulate its solar panels to capture energy from the sun to keep its power systems operational,” said Tim Connor, SolarReserve VP of Engineering & Technology.

“Most of the CSP players in the market right now - Abengoa, SolarReserve, BrightSource - are developing and operating their own platforms for performance modelling, controlling and monitoring their operations,” said Gwendalyn Bender, Assessment Services Product Manager at Vaisala 3TIER.

At SolarReserve’s Crescent Dunes power tower with molten salt storage plant in Nevada, which is currently in final testing, proprietary software running on a set of dedicated high-end servers controls and monitors the position of each of 10,347 heliostats throughout the day.
The software also calculates aiming points and positioning coordinates every 10 seconds using the latest computing capabilities.

“Each of the 10,347 heliostats also have on-board computers that resolve the azimuth and elevation position commands and use closed loop algorithms to maintain pointing accuracy to within 0.5 milliradians,” said Connor.

“In a given day, millions of individual pieces of data are analyzed to ensure optimal performance. In addition, offline computers and highly sophisticated computer programs process heliostat images and positioning data collected throughout the day for every day in the year. Optimization routines generate offsets to heliostat aim points regularly fed from the offline computers to the heliostat control system.”

Gemasolar, the first to operate commercially, (albeit at just 20 MW) only began generating in 2011. Pre-commercial pilots like the DOE 10 MW Solar Two plant were operated only as R&D vehicles, and have been followed by a few number of projects under 5 MW.

But now, four 100 megawatt-plus power towers with molten salt storage are underway. They are: Abengoa’s Atacama1 in Chile, ACWA Power’s 150 MW Noor III in Morocco, SolarReserve/ACWA Power’s 100 MW Redstone in South Africa and Crescent Dunes in US.