Towers versus troughs?
The moment for solar power tower technology has arrived. But in the solar tech race, is there room for more than one leader; or is trough technology about to be sidelined?
By Rajesh Chhabara
Parabolic troughs have dominated the concentrated solar thermal power industry for the last two decades, winning the confidence of utilities and investors.
But the technology landscape is about to change, with solar power tower technology promising unique advantages.
Parabolic troughs, currently the most proven technology, concentrate sunlight onto thin tubes carrying thermal oil. The heat from the hot oil is transferred to the water in a heat exchanger to produce steam, which then is used to drive turbines to generate power.
With solar power tower technology, a large number of heliostats or mirrors concentrate sunlight onto a boiler atop a tower. The heat is used to produce very hot steam from water, which rotates the turbines to generate power.
Towers have the potential to be much more efficient than troughs, because they have far higher concentration ratios (300 to 800 suns vs. 80 or so for troughs), according to Craig Tyner, senior vice president of engineering at e-Solar.
While troughs produce heat at around 400 degree Celsius, towers can produce up to 550 degree Celsius. “Higher temperatures allow use of more efficient turbines, reducing energy costs.
Towers also have the potential for more efficient storage using molten salt as their working fluid, as well as the storage fluid,” says Tyner.
Tyner notes that while troughs have been built with salt storage, their lower temperature differential and need for oil/salt heat exchange make their storage much more costly and somewhat less efficient.
In solar thermal energy, the most cost-effective way to drive efficiencies is by taking advantage of more efficient, higher temperature steam turbines - which is possible using tower technology, according to Keely Wachs, senior director of communications, Bright Source Energy.
It is in this sense that the evolution of tower technology promises to resolve some of the efficiency and cost limitations of trough technology. “If you look at the history of solar thermal power plant performance, the single greatest driver of cost reductions is by increasing a plant’s efficiency. A slight increase in efficiency can lead to a significant cost decrease,” says Wachs.
BrightSource Energy’s Luz Power 550 (LPT 550) solar thermal power tower system is designed to match steam turbines that run at 550 degrees centigrade. LPT 550’s solar-to-steam approach allows the system to reach higher temperatures and also avoid parasitic losses endemic in parabolic trough plants, which use a transfer fluid to create steam, according to the company.
More efficient storage
Storage is another key advantage of the tower technology. Valerio Fernandez, operations manager of Abengoa Solar, says the ability to generate higher temperatures improves the efficiency of the storage system.
“The plant can operate longer when solar resources are not available, as high temperature allows storing more energy using the same amount of heat storage media,” he says.
A further advantage of tower technology is its flexibility when it comes to siting. Whereas an even or leveled land area is needed for parabolic troughs, heliostats do not need to be sited on an even surface. Tower technology can even be deployed in a moderately hilly area.
Wachs highlights that recent technological innovations have advanced solar thermal power technology to give towers the edge over troughs.
“Progress in the software industry has made it possible to accurately track and control hundreds of thousands mirrors, which reflect the sun’s energy to a boiler atop a tower.”
A separate area of development has been in the design of more efficient steam turbines, which run off steam at 550-650 degrees Celsius.
Although tower technology promises a host of advantages, the main barrier to its promotion comes from the fact that only a handful of tower plants are currently operational.
More commercial tower plants need to come online before the technology can be considered ‘proven’.
But Fernandez is convinced that tower technology’s moment has arrived. “If towers were considered a demonstration technology some years ago, at this moment it is considered as commercial as any other CSP technology,” he says.
In addition to existing solar tower projects, including Abengoa Solar’s 11MW PS10, and 20MW PS20 solar towers in Sevilla, Spain, and the 10MW Solar One tower, (later expanded and renamed as Solar Two) in California, developed by the US Department of Energy and other government agencies, there are a number of ambitious projects in the pipeline that promise to bring towers into the mainstream.
Bright Source has 2.6 GW of contracts with Pacific Gas & Electric and Southern California Edison. The company is working on permitting and financing Ivanpah, Brightsource’s first plant in the US, which will have a capacity of 440 MW. Brightsource has already signed on Bechtel as the engineering contractor and equity partner for the Ivanpah project.
It is also constructing a 29 MW thermal plant for Chevron in Coalinga, California. “When completed, the plant will help Chevron demonstrate the value of solar-generated steam in the enhanced oil recovery process. By replacing natural gas with a steam production fuel source, we are also helping Chevron reduce their carbon footprint,” says Wachs.
Using Solar One and Two’s technology, Torresol Energy is building the Solar Tres Power Tower in Spain, which will have the capacity to produce 15MW.
US-based e-Solar, which unveiled the company’s first tower plant- a 5MW project- in Sierra, California this August, has announced an agreement with New Jersey-based NRG Energy to develop three plants in California and New Mexico that will generate up to 465 MW of electricity using eSolar technology. The first plant is expected to be online by 2011.
In March, eSolar licensed its technology to India-based ACME Group for up to one gigawatt of eSolar solar thermal capacity to be built over the next 10 years.
Banks also appear to be taking a more positive view of tower technology. Fernandez says that both PS-10 and PS-20 have been constructed with project finance from banks.
“This is true that convincing banks on the technical feasibility of tower technology needed great efforts a few years ago, but with the operational experience of PS-10 and PS-20, this not a problem at the moment.”
Some observers, however, say that banks are still cautious in financing tower plants. “Tower technology is certainly nowhere near as bankable now as trough technology,” says Tyner of e-Solar.
He says that each tower technology provider will need to build a few plants with special financing, for example with government loan guarantees, before the technology will be routinely bankable.
The upcoming tower projects by Bright Source, Torresol and others will be watched closely and their success will likely trigger investment into tower plant project development around the world.
Will towers replace parabolic troughs in future? Observers say both technologies will co-exist as both have their own unique advantages. While towers offer higher efficiencies leading to reduction in costs, parabolic troughs come with their own set of strengths.
“A key advantage of parabolic troughs is that it is a very modular technology. Just by adding more loops in parallel, the capacity of the plant can be expanded,” says Fernandez.
He explains that this lends itself to the incremental development of very large-scale plants. When it comes to scale, however, tower technology has yet to prove itself.
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