The world’s first molten salt concentrating solar power plant

On 14 July 2010 the Italian utility Enel unveiled “Archimede”, the first Concentrating Solar Power (CSP) plant in the World to use molten salts for heat transfer and storage, and the first to be fully integrated to an existing combined-cycle gas power plant. Archimede is a 5 MW plant located in Priolo Gargallo (Sicily), within Europe’s largest petrochemical district. The breakthrough project was co-developed by Enel, one of World’s largest utilities, and ENEA, the Italian National Agency for New Technologies, Energy and Sustainable Economic Development.

Several CSP plants already operate around the world, mainly in the US and Spain. They use synthetic oils to capture the Sun’s energy in the form of heat, by using mirrors that beam sunlight onto a pipe where pressurised oil heats up to around 390°C. A heat exchanger is then used to boil water and run a conventional steam turbine cycle. Older CSP plants can only operate at daytime – when direct sunlight is available -, an issue that has been dealt with in recent years by introducing heat storage, in the form of molten salts. Newer CSP plants, as the many under construction in Spain, use molten salts storage to extend the plants’ daily operating hours. Archimede is the first plant in the world to use molten salts not just to store heat but also to collect it from the sun in the first place.

Image: Archimede Solar Energy

This is a competitive advantage, for a variety of reasons. Molten salts can operate at higher temperatures than oils (up to 550°C instead of 390°C), therefore increasing efficiency and power output of a plant. With the higher-temperature heat storage allowed by the direct use of salts, the plant can also extend its operating hours well further than an oil-operated CSP plant with molten salt storage, thus working 24 hours a day for several days in the absence of sun or during rainy days. This feature also enables a simplified plant design, as it avoids the need for oil-to-salts heat exchangers, and eliminates the safety and environmental concerns related to the use of oils (molten salts are cheap, non-toxic common fertilizers and do not catch fire, as opposed to synthetic oils currently used in CSP plants around the World). Last but not least, the higher temperatures reached by the molten salts enable the use of steam turbines at the standard pressure/temperature parameters as used in most common gas-cycle fossil power plants. This means that conventional power plants can be integrated – or, in perspective, replaced – with this technology without expensive retrofits to the existing assets.

 

Carlo Rubbia

So why hasn’t this technology come before? There are both political and technical issues behind this. Let’s start with politics. The concept dates back to 2001, when Italian nuclear physicist and Nobel prize winner Carlo Rubbia, ENEA’s President at the time, first started Research & Development on molten salt technology in Italy. Rubbia has been a preminent CSP advocate for a long time, and was forced to leave ENEA in 2005 after strong disagreements with the Italian Government and its lack of convincing R&D policies. He then moved to CIEMAT, the Spanish equivalent of ENEA. Under his guidance, Spain has now become world leader in the CSP industry. Luckily for the Italian industry, the Archimede project was not abandoned and ENEA continued its development till completion.

There are also various technical reasons that have prevented an earlier development of this new technology. Salts tend to solidify at temperatures around 220°C, which is a serious issue for the continuous operation of a plant. ENEA and Archimede Solar Energy, a private company focusing on receiver pipes,  developed several patents in order to improve the pipes’ ability to absorbe heat, and the parabolic mirrors’ reflectivity, therefore maximising the heat transfer to the fluid carrier. The result of these and several other technological improvements is a top-notch world’s first power plant with a price tag of around 60 million euros. It’s a hefty price for a 5 MW power plant, even compared to other CSP plants, but there is overwhelming scope for a massive roll-out of this new technology at utility scale in sunny regions like Northern Africa, the Middle East, Australia, the US.

The Italian CSP association ANEST claims Italy could host 3-5,000 MW of CSP plants by 2020, with huge benefits also in terms of jobs creation and industrial know-how. A lot more can be achieved in the sun belt south of the Mediterranean Sea, and in the Middle East. If the roll out of solar photovoltaics in Italy is to offer any guidance (second largest market in the World in 2009), exciting times are ahead for Concentrating Solar Power.

By Carlo Ombello. Carlo’s web site is at www.opportunityenergy.org

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  1. Rod Adams’s avatar

    How much of the $60 million can be attributed to first of a kind costs? Are there any projections for the cost of the 5th such facility? How about the 20th?

    How much storage is part of the system? There has to be a finite limit on the number of BTUs or kilowatt-hours that can be withdrawn from the storage system without new heat input from the sun before the temperature of the salt reaches some predefined limit. What is that number?

  2. Carlo Ombello’s avatar

    Hi Rod,

    I have seen a lot of concerned comments over the price of this first plan, which surprised me quite a lot. If I hadn’t stressed myself the high price of this first experiment, I guess we wouldn’t have had as much discussion, so I’m glad I did. Anyway, I have published some more data from ENEA on my blog, regarding their expectations from this technology once it’s fully developed: http://www.opportunityenergy.org/?p=94

    For a typical 100MW stand alone plant in Egypt, it should look like this:

    Annual direct solar radiation: 2.900 kWh/(m2 year)
    Total area occupied by the solar collectors: 67 ha
    Total area occupied by the solar field: 134 ha
    Nominal power output: 100 MW (Peak 485 MW)
    Thermal storage capacity: 1.800 MWh
    Net annual electricity produced: 369 GWh/year
    Plant load factor: 42 %

    Total Cost : 157 M€
    Specific cost : 1.570 €/kWe
    Service life: 25 years
    Interest rate: 7%
    Annual operating (O&M) costs: 2% of investment cost
    Levelized Cost Of Electricity (LCOE): 4,5 €cents/kWh

    Obviously, this can be scaled down or up according to needs, as much as the technical data can be changed. Some clients might want more storage to achieve a higher load factor, some might prefer a lower storage capacity, and just modulate the plant according to peak prices (which tend to be matched quite well by nature in this case).

  3. Rod Adams’s avatar

    Hi Carlo:

    In your response above, you stated that a 100 MWe plant in Egypt would require a total of 134 ha to be occupied by the solar field.

    134 ha is equal to 1.34 square kilometers. (Please correct me if I am wrong on my unit conversion.)

    According to the Archimede Solar web site technology page at
    http://www.archimedesolarenergy.com/solar_receiver_tube.htm

    a system that uses 1 square kilometer of land would produce between 100-130 gigawatt-hours of solar electricity each year.

    Using that figure, a plant occupying 134 ha of land would produce between 134-174 gigawatt-hours of solar electricity each year.

    If it produces at a 42% load factor that would give it a peak output of just 47 MWe, not 100 MWe.

    Also, for a high temperature-high pressure steam power plant a total O&M budget of just 3.2 M€ (2% of 157 M€) is optimistic – especially if the operators need to buy enough fresh water to keep the Egyptian dust off of 67 ha of polished mirrors.

  4. Carlo Ombello’s avatar

    Rod,

    I have reported figures from ENEA, so I am not the man behind those numbers, particularly the financial projections. As to the technical data, I would tend to agree with the ENEA figures from a 42 page report thou, rather than to a website page by a private company briefly illustrating the technology. Could well be a typo.

    ENEA assumes a surface of 67 hectares for the solar collectors of a 100MW plant. The Archimede Plant collectors are about 3 hectares and 5 MW, the proportion is therefore consistent.

  5. Rod Adams’s avatar

    Carlo:

    The factor that the ENEA does not take into account that the Archimede web site does is the specific environmental conditions that developers of concentrating solar power systems will face in their target areas like North Africa. Here is the quote from that web site.

    “Other promising areas of the world to apply CSP technology include Southern Europe, Northern Africa and the Middle East, parts of India, China, and Australia. These regions have peculiar territorial features as large amounts of atmospheric humidity, dust and fumes so that 1 sq km of land is enough to generate 100-130 gigawatt hours of solar electricity a year, using solar thermal technology.”

    I have spent some time in a few of the areas mentioned and recognize that the vendors of this technology are being realistic. The ENEA may not be so realistic and may not be factoring in such inevitable influences as dust, humidity, and air pollution. Anything exposed to the open weather in a North African desert has a high probability of being covered with a thick layer of powdery dust in just a few weeks.

  6. Carlo Ombello’s avatar

    Hi Ron, I don’t see your point.

    Archimede plant, the first of its kind, indeed follows ENEA’s figures, and it does so at a tiny scale. But let’s say, regardless of this, that you are right and ENEA are wrong. Let’s say that it takes 2, even 3 times as the surface they claim (and granted, a real case scenario is already proving different). We have millions of square kilometres of unused land available in North Africa, Middle East, and elsewhere. Big deal! We will still require just a tiny fraction of the land available. And not all of this land is battered by constant sand storms.

    But again, I don’t see how we can deny what the Archimede plant already achieves in terms of design figures. There is available literature to prove the amount of surface required for a given output. Let’s not confuse the need for maintenance to the output capabilities, related to collectors’ surface and storage volume (which is the main parameter to determine the load factor, along with the latitude of the chosen location).

    Incidentally, I found another example of ENEA design, for a 400MW power plant with a high 80% load factor to deliver 2.8TWh per year (basically, bigger heat storage, should it ever be required even in a hot desert area). Again, the surface required for solar collectors is assessed at around 3.3km2, that is 82.5 hectares per 100MW, slightly more than the 67 reported above.

    The document (in italian) is available here: http://www.enea.it/produzione_scientifica/pdf_enea_attiivita/Calore_alta_temp.pdf

    Regards

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