Dr. Frauke Küsgen and Daniel Kueser, MAN Ferrostaal, Germany
Concentrating Solar Power (CSP), and the parabolic through technology in particular, have come to attract ever more public interest in the past few years. Although the first solar-thermal power plants of this type have been in operation (in California) from as early as 1985, the CSP market has not seen sustained growth rates until recently. Some technologies are now technologically mature, but others are in advanced stages of development. Legislative frameworks are providing necessary incentives for the solar industry as a whole. Meanwhile, the price of fossil fuels and the costs of conventional power generation keep rising.
Concentrated solar power, demonstrated with the Fresnel collectors.
While total global CSP capacity tallied a mere 354 MW worldwide in 2006, additional capacities of 100 MW went into operation in the last two years. Currently, plants with a combined output of more than 700 MW are under construction in Spain, and another 25 MW in Italy. More projects in Abu Dhabi, Egypt, Algiers, Morocco, Libya, Jordan, Iran, China, Malta and various other countries, are being tendered or are already in the early works stage.
In its 2008 World Energy Outlook, the International Energy Agency (IEA) predicts worldwide CSP capacities of 2 GW until the end of 2010. By 2020, the globally installed capacity will amount to ten times that much 20 GW.
Middle Eastern Promise
According to the German Aerospace Centre (DLR), there is great potential for solar-thermal power generation in the Middle East. Calculations by the institute suggest that the insolation on just one per cent of the square footage of the Sahara or less than one-third of one per cent of the expanse of the deserts in the MENA region would supply enough energy to cover the global energy demand completely.
The functional characteristics of FresDemo.
With state subsidies in the low Euro billions, the DLR believes it is possible to support the market entrance of solar-thermal power generation sufficiently enough to end subsidization before 2020. Any increase in fossil fuel prices until then, as likely as they may be, have not been factored into the calculation.
As the leading CSP technology, parabolic trough collectors already have established themselves on the marketplace. With Fresnel, a new type of power plant is now coming close to the point of marketability.
With the joint project FresDemo, the technology supplier Solar Power Group (SPG), the general contractor MAN Ferrostaal, the DLR and Fraunhofer Institute commissioned a prototype in Almeria, Spain, in 2006, in order to test the capabilities of the Fresnel technology in actual operation. “FresDemo was meant to verify the technical feasibility of CSP technology,” says Dr. Reiner Kistner, senior vice president of solar power at MAN Ferrostaal, which was acquired by Abu Dhabi’s International Petroleum Investment Company in March. “We achieved this goal. In 18 months of operation, the technology delivered the expected performance in practice.”
The general approach of the Fresnel collector is the simplification of design and functionality to keep the “levelized cost of energy” (LCOE) as low as possible. A steel construction made of standard components supports several rows of planar primary mirrors. The so-called “receiver unit” is placed about ten metres above the primary mirrors and consists primarily of a secondary mirror and an absorber tube.
The primary mirrors reflect the sunlight onto absorber tubes of the receiver unit, to directly heat the medium in the absorber. As some sunlight scatters and doesn’t meet the absorber tube naturally, a secondary mirror captures and refocuses most of the dispersed light. By using the secondary mirror, it is possible to heat up the medium to temperatures of over 420 °C.
Heat transfer medium
The Fresnel collector uses water as a heat transfer medium and allows direct steam generation. This has several advantages. Water has the property of allowing higher process temperatures as it isn’t subject to the limiting factor of pyrolysis, a thermal decomposition which affects organic media at specific critical temperatures. The limit for thermo oil, the medium of parabolic trough collectors, for example, is reached at approximately 400 °C. It can be said in general that the overall efficiency of a system increases with higher process temperatures which can most likely lower the LCOE. In any case a heat exchanger between oil and steam flow is not required, hitherto a component of solar-thermal power plants and parabolic trough collectors facilitating the thermal transfer from the solar field to the power cycle.
During the FresDemo project, operation took place under process conditions of 450 °C at 100 bar which constitutes a real breakthrough competitors on the market operate in temperatures of approximately 280 °C with their Fresnel collectors.
With the exception of the receiver unit, none of the components of the FresDemo collector requires sophisticated manufacturing or elaborate treatment. In order to keep the mirrors flexible and resistant against weather conditions, flat and heat-treated glass is used. The silvering is applied in a two-step process, yielding in a reflexivity of up to 90 per cent. Only the coating of the secondary mirror and the absorber tube is undertaken under vacuum conditions in a physical process called sputtering. This particular improvement of the receiver unit’s properties increases the efficiency of the whole system.
Parabolic trough versus Fresnel technology
Due to the mass manufacturing of components, the accumulated operational knowledge, as well as the material usage of only 18 kilograms of steel and 11 kilograms of glass per square metre, parabolic trough collectors have the competitive edge over other CSP technologies at the moment.
The Fresnel design, with very few parts, provides excellent opportunities for standardization and mass manufacturing of nearly all parts. In plants with an effective output of 50 MW, the costs of one square metre reflector surface typically ranges between €200 and €250 ($259 to $324).
While it is true that plants on the basis of Fresnel technology theoretically have a lower optical efficiency than the parabolic trough, the much simpler design of the Fresnel collector compensates this advantage by reducing investment and operational costs to such a degree that the LCOE benchmark of the parabolic trough can be met. The steel construction consists of standard parts entirely, all of which can be manufactured cheaply almost anywhere with local added value.
The planar mirrors of the Fresnel collector are cheaper to produce than concave mirrors, and it is possible to set up a reflector array three times the width of a parabolic trough. The architecture offers less contact surface for winds and fits a larger reflector surface on a smaller space.
Automated cleaning process
Another advantage of planar mirrors is that they can be surface cleaned in an automated process. The tracing system that positions the mirrors is less intricate because of the collector design. While parabolic trough mirrors trace the sun by means of powerful hydraulics, the light-weighted construction of the Fresnel collector requires only small engines.
The fixed absorber tube does not need heat or pressure resistant joints. Furthermore, because of direct steam generation, thermo oil as a heat transfer medium and the heat exchanger itself are dispensed with. All factors combined lower investment costs up to thirty per cent compared to other CSP technologies.
“Since we use water as heat transfer medium, there are numerous applications for the technology,” explains Kistner. “Of course, the generation of electricity is at the top of the list. Cogeneration of heat and power is possible just as well as the combination with a seawater desalination plant to produce drinking water.
“It’s also possible to generate nothing but industrial process steam. In connection with an absorption chiller, the system can be used for air conditioning. Another interesting area of application is tertiary oil production, commonly referred to as Enhanced Oil Recovery (EOR). The process steam of the plant flushes out crude oil from conventionally exploited wells. High-temperature steam is pumped into the borehole and increases the viscosity of remaining oil reserves that otherwise couldn’t be extracted easily. Large international oil companies are very interested in the topic.”
With regard to power generation, one of the greater technical challenges lies in the keeping of stable steam parameters. While water as a heat transfer medium has the advantage of better temperature gradients, the favourable thermal inertia of thermo oil is lost. Even brief clouding may affect the steam water mixture. Input and output temperatures, pressure and mass flow must be kept steady.
Upon completion of the trial run at the end of last year, it had become clear that the Fresnel collector stood up to the technical test. “This is of great importance for future tenders and financing models,” stresses Kistner. “The performance of FresDemo matched our calculations. The praxis has proven that the theory was right. Now we can take the next step.”
Currently, the FresDemo project partners are preparing for a second round of trials to be conducted on Plataforma Solar de Almeria. The foremost intention is to gain more experience in plant operation and maintenance and to improve the technology further. The new tests are also meant to maximize the efficiency of single components, such as the absorber tube and the secondary reflector.
Meanwhile, other plans for a new and larger-scaled prototype, also to be realized as joint project, are under way. Accordant application procedures have been initiated.
“The development of the Fresnel technology isn’t finished,” says Kistner. “With the next prototype we’re going to reduce the costs. For any commercial use, we’ll have to prove that the costs can compete with the parabolic trough. The immediate goal is then to arrive at €140 to €150 per square metre reflector surface. And we’re very optimistic about reaching this benchmark.
“Mass manufacturing and low assembly costs will make the Fresnel technology substantially cheaper. We have seen the same with other technologies in the past. At locations with an excellent mean of daily insolation in the US Southwest, Northern Africa and the Middle East, for example the LCOE will be about €0.15/kwh.”
Kistner continued: “In meeting this benchmark the Fresnel technology will absolutely become competitive with the parabolic trough. Compared to fossil fuelled power plants, solar-thermal power generation can compete with oil fired power plants when the price for a barrel of crude oil is at $70 (€54).
“With a barrel price of $130 (€100), CSP is economically more viable than natural gas fired open cycle plants. We will build a fully operational, semi-commercial plant with our partners next year, and we will produce electricity with it. The first fully commercial plant will follow in 2011. There are already plenty of interested parties approaching us.”
SOLAR POWER IN THE MIDDLE EAST
Dr. Frauke Küsgen
1. Can solar power compete with fossil fuelled power generation in the Middle East?
Dr. Frauke Küsgen
There has never been greater interest in the Middle East for solar power than as we are now seeing. The demand for power plants is high, but the dependence on oil and gas should be strongly reduced.
Nowadays the protection of the environment plays a major role in all regions. Generally one may also state that the attractiveness of renewable energies in general increases with an increasing oil price. According to a study by Boston Consulting Group, CSP can compete at an oil price of $70 (€54) with oil fired power plants. At an oil price of $130 (€100), solar thermal plants operate more economically than gas fired open cycle power plants.
2. What kind of interest in solar power from Middle East governments and power utilities is there?
There is generally a big interest, but not only in the solar area. However, we want to build up the solar area as our own field of expertize because we offer a great deal of local value creation here, as well as opportunities for a diversification of the economy and the strategic development of the industry. Against this background it is exclusively about power generation. However, the solar segment currently is still in its infancy.
3. What will solar power be used for in the Middle East?
Power and water are the most important application areas. In some areas in the Middle East air conditioning units account for 80 per cent of the power consumption. Solar cooling can offer a good alternative here. Currently, some North African countries are also working on the construction of so-called integrated solar combined cycle systems (ISCCS). Here, solar fields are integrated into conventional gas fired power plants. By adding solar energy, conventional fuel can be saved here.
4. Will CSP be more viable than solar PV in the Middle East?
CSP and PV both have their markets. PV is very successful in decentralized applications, whereas CSP offers advantages for central and large-scale applications. CSP power plants are the most cost-efficient way to generate and to store dispatchable CO2-free electricity. However, there is no competition between both. Rather, they have to be seen as complementary technologies.