Feature

CSP Power Tower technology


Stewart Taggart

Stewart Taggart investigates the rise of the Power Tower, a Concentrating Solar Thermal Power generating technology which many experts think could be the way forward.

Background to Concentrating Solar Power (CSP)

Parabolic trough and fresnel reflector CSP plants concentrate sunlight onto a thin pipeline to warm a heat liquid to around 400 deg. celsius (c).

Solar dishes focus sunlight on a single point to create temperatures of about 750 deg. c.

But Power Towers surround and bathe an elevated tower with reflective light from all sides, to generate temperatures around 1,000 deg. c. In all the cases, the heat is then used to drive traditional steam turbines, which in turn create electricity. In the case of Towers, whether or not the higher temperatures will offset the higher costs is the key question. Deep pocketed, risk-tolerant investors are now stepping up to the plate to find out.

Fresh investment momentum

Probably the deepest pocketed are the green sheiks of Abu Dhabi. In mid-March 2008, the Emirate's renewable energy company MASDAR announced a joint venture (JV) with Spanish engineering group SENER, to roll out three 50MW parabolic trough CSP plants with molten salt storage in Spain, to be followed by Tower projects. MASDAR and SENER, through a JV called Torresol, aim to get 320MW of CSP plants up and running by 2012. They aim to get 1,000MW of CSP up and running by 2018.

To date, MASDAR/Sener/Torresol hasn't announced the specifics of how many Troughs and Towers it may build. It's quite possible they don't know yet. But with US$15 billion to spend, MASDAR's entrance into CSP has sent Richter-scale-sized reverberations throughout the industry.

And in the Solar Concentration Off-Tower (SCOT) concept, from Tokyo Institute of Technology's Professor Yutaka Tamaura – that is to be tested in Masdar – so-called cutting-edge “beam-down” technology will be tested. Experts believe this could be very exciting for CSP Tower technology; mirrors send the sunlight up to a concentrator that then reflects it back to a ground collector, which heats the liquid to make the steam to drive the turbine. According to the experts, this results in the highest MW potential for solar energy generation.

Established expertise

But money isn't everything. Elsewhere, two hardened veterans of the early CSP days in California have also tossed their hats into the solar tower ring.

One is privately-held Brightsource Energy, which was founded by Israeli experts who in the 1980s built the world's first Parabolic Trough CSP plant in California. Brightsource now plans to build 400MW of Towers in California, scaling up technology now being tested on a pilot basis in Israel's Negev desert.

The second is SolarReserve, a joint effort between New York Stock Exchange-listed United Technologies Corp., and NASDAQ-listed US Renewables Group. They plan to commercialise solar Tower and molten salt technologies developed in the 1980s and 1990s by onetime Boeing subsidiary Rocketdyne.

SolarReserve's aim is to build Tower plants capable of producing up to 500MW of afternoon high-priced peaking power – through combining real-time afternoon solar energy generation, doubled up with energy generated from molten salt heat reserves built up during the morning hours.

All the companies are looking to pluck low hanging fruit. That's because most fundamental research into solar Towers to date occurred in the 1980s and 1990s in California. During that period, two flagship 10MW research projects, Solar One and Solar Two, proved the technology worked, albeit at a high cost.

But that's as far as things got. Low oil prices caused Government support to erode and industry players shelved the technology. Early in the new millennium, after 10 years of narcolepsy, EU-funded research into CSP started up in Spain.

As a result of this renewed interest and EU inducements, Madrid-listed Abengoa now operates a commercial 11MW solar Tower near Seville, Spain called PS10. A 17MW tower called Solar Tres is under development by privately-held SENER. Abengoa Solar is also building PS20, which it says will produce 20 MW.

So far, so good. But whether these new solar Tower bets provide long-term returns more akin to an oil strike or a subprime mortgage, remain to be seen.

What questions need to be answered?

As with troughs and dishes, issues surrounding Towers revolve around how soon, how big and how cheap?

Regarding how soon, 2011–2012 seems a reasonable guess for plants from MASDAR, Brightsource or SolarReserve.

How big? Given that a major advantage of Towers is their scale and ability to generate high temperatures, it pays to build them big. But this leads to system complexity. It also vastly increases the importance of proper site selection, since small amounts of additional sun can have exponentially compounded impacts on the lifetime economics of a Tower plant. At present, optimum sizes are believed to lie somewhere between a rather vague 50MW-400MW.

How cheap? In a 2003 study, Chicago consultants Sargent and Lundy estimated solar Towers could become the lowest-cost form of CSP. They estimated that Towers might be able to generate electricity at a levelised cost of energy around US$0.04 cents/kWhr by 2020.

What happens next?

In the coming years, the pace of the Tower industry will be set in California and Spain, so these two markets are the ones to watch, as are any developments or announcements from MASDAR/Sener, Brightsource and SolarReserve.

It will also pay to keep an eye on society at large, for signs of community opposition to large, new and unfamiliar solar complexes. Further, given that Towers benefit from very strong sun found in isolated places, developments regarding extending grid access to remote generation sites will be important to monitor. Lastly, given that molten heat storage is a key element in the economics of Towers, research and deployment developments in this area also should be closely watched.

As money flows into solar towers, the biggest, most expensive and arguably most risky of CSP technologies, the entire industry is reaching a durable, economic take-off phase.

With solar Towers now in the game, the CSP industry now has a complete suite of commercial technologies, all with an exciting future of technological and economic discovery ahead. Even though Towers appear to hold the greatest long-term promise for huge, new, low-cost electricity supplies, having an unambiguous technology winner in CSP may be just what the industry doesn't need. The best outcome may be a suite of technologies, engaged in long-term economic competition with each other. That way, the market can most fully do its work.

Were one CSP technology to emerge an unambiguous winner, it could foster a monoculture industry. This could, in turn, prematurely choke off research in Troughs, Dishes or Fresnel Reflectors, or even in Concentrating PV (CPV) and idiosyncratic cousins like solar chimneys.

However, a long-term, neck-and-neck race among stable mates would foster faster innovation in the industry. It would also be more fun to watch. In any event, the competitors are now all at the starting line. And that's good news.

Further information:

Power Towers – technology in brief

In tower systems, a heliostat field comprising movable mirrors is oriented to reflect the solar radiation, and concentrate it up to 600 times on a receptor located on the upper part of the tower. This heat is then transferred to a fluid, with the purpose of generating steam that expands on a turbine coupled to a generator to produce electricity.

Tower technology operation is based on three main features: heliostats, receptor, and tower.

  • Heliostats are designed to capture solar radiation and direct it to the receiver. They are composed of a reflective surface, a supporting structure and mechanisms used to orientate them, following the sun's movement (this involves systems for the heliostats' movement as well as control systems). The most widely-used reflective surfaces today are glass mirrors;
  • The receiver transfers the heat to an operating fluid (which could be water, molten salts, etc). This fluid transmits heat to other parts of the CSP plant, generally to a water store, in order to obtain high-temperature steam to produce electricity through a turbine. The latest R&D is aimed at creating high-temperature Towers, with heat transporting fluids such as air and salts;
  • The tower acts as support for the receiver, which should be located at a certain height above the heliostats to avoid – or at least reduce – shading and blocking.

To install Tower technology CSP plants, there are certain requirements, such as:

  • The site needs to be level;
  • The direct normal insolation (DNI) should be as high as possible;
  • Water is needed for cooling in the power block;
  • Electric lines and transmission capacity are needed to convey solar power from the plant to the consumer.

Source: Abengoa Solar

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