Home' Technology Review : November December 2012 Contents S2
in the process of building a 280-megawatt plant in Arizona that
will incorporate storage in the form of molten salts, and has con-
tracts for two more plants in California, while Pamplona-based
Acciona built a solar thermal plant in Nevada that began oper-
ations in 2007, and, together with Barcelona-based Sener, was
recently awarded a contract for a new plant in South Africa. Sener
holds additional contracts for systems totaling 200 megawatts in
India and for projects in the United States and Morocco.
Spain leads in solar thermal in part because its government
retained a research facility in the hot, sunny south, long after
the world’s first rush of interest in the technology, sparked by the
energy crisis of the 1970s, had waned. That center, along with
other research and pilot sites that companies have built, provides
the necessar y facilities to investigate advanced technologies. Many
solar thermal experts believe that while trough systems are a proven
technology, the CSP systems of the future will eventually focus
on the newer tower technology.
In a tower system, a field of heliostats (blocks of mirrors that
change angles over the course of the day to catch the light) shine
all the sunlight they reflect at the top of a tower. Since the sun-
light is concentrated at one spot instead of along miles of tubes,
the system can reach much higher temperatures than troughs do
and is more efficient. The tower system’s mirrors are flat instead of
cur ved, and so are simpler to manufacture. Company representa-
tives say this also means that tower systems built to the same scale
as existing trough ones will be less expensive.
Only t wo companies in the world have so far built commercial-
scale tower systems, both of them in southern Spain: Abengoa and
Sener. (Other companies are now in the process of developing
similar systems.) Abengoa built the first t wo commercial systems,
PS10 and PS20. And in 2012, Sener’s Gemasolar began operations.
Sener’s 19.9 -megawatt tower system is the first commercial-scale
tower system to incorporate storage in the form of molten salts
(a combination of potassium nitrate and sodium nitrate). The
salts absorb excess heat during the day and release it slowly into
the evening hours.
While Sener had already employed molten salts in some of its
trough systems, a tower receiver presented additional challenges.
The salts had to be pumped up to the top of the tower, which had
never been accomplished before. So Sener helped a pump manu-
facturer design a new pump that can withstand corrosive salts
and high temperatures.
The focal point of a tower system is subjected to a bruising
range of temperatures that can span hundreds of degrees in a
single day. Jorge Sendagorta, Sener’s president, explains that the
company’s experience in aerospace engineering proved critical,
as satellites in space can also be exposed to temperature ranges of
hundreds of degrees.
Based on the success of Gemasolar, Sener is now scaling up and
designing systems that are two and three times that system’s size,
at a cost of about 40 percent less.
Abengoa’s experience enabled the company to win the bidding
for two plants in South Africa, one trough and one tower. For
the 50-megawatt tower, it is employing a new technology called
overheated steam, where water is subjected to temperatures of
nearly 500 °C , 100 degrees hotter with the current technology.
José Domínguez Abascal, chief technology officer of Abengoa,
says the South African tower plant will be 25 percent more effi-
cient than the company’s towers in Spain. The overheated steam
will also serve as a storage medium, because, like molten salts, it
retains heat into the evening hours.
Domínguez-Abascal says that Abengoa is now investigating the
use of heated gas, where compressed air is sent to the top of the tower
and heated in place of steam. That high-pressure, high-temperature
gas would operate a turbine. Such a system could be combined with
a natural gas plant, also run by gas (as opposed to steam) turbines.
Natural gas could be employed when solar power does not suffice;
the two together would consume perhaps 30 percent of the natural
gas required by a conventional natural gas plant, at a comparable
operating cost. Abengoa recently unveiled a pilot plant in Seville
that operates with heated compressed gas, and the company expects
to offer a commercial-scale plant in only a few years.
Links Archive September October 2012 January February 2013 Navigation Previous Page Next Page