Renewables TCASE

TCASE 7: Scaling up Andasol 1 to baseload

Andasol 1 is Europe’s first parabolic trough solar thermal power station, which went online in Nov 2008. It is located on a high desert site in Granada, Spain, which enjoys a high level of direct insolation – an average of 2,136 kWh / m2 / year. The mirror field — turbine infrastructure can yield a peak electricity generation capacity of 49.9 MWe (20 MWe average, see below). It also has a thermal storage system using molten salt.

The purpose of this post is to consider how one might scale up an Andasol 1 type plant in order to meet a rated power demand for 8,000 hours per year — thereby giving it a capacity factor of ~90%, similar to a baseload coal or nuclear power stations. This is a first attempt to improve the comparisons first given in TCASE 4.

But first, let’s look at the technology and current numbers. Here’s a good summary of its main features:

The Andasol 1 storage system absorbs part of the heat produced in the solar field during the day. A turbine produces electricity using this heat during the night, or when the sky is overcast. This process almost doubles the number of operational hours at the solar thermal power plant per year, the company said.

The heat generated in the solar field will be stored in a molten mixture of 60% sodium nitrate and 40% potassium nitrate. Both substances are used in food production as preservatives and are also used as fertilizer. The storage tank consists of two, 14-meter high tanks with a diameter of 36 meters and a capacity of 28,500 tons of molten salt. During the pumping process from the cold to the hot tank, the molten salt absorbs additional heat at an outlet temperature of approximately 280°C, reaching a temperature of 380°C.

A fully loaded storage system can keep the turbine in operation for 7.5 hours, which means almost 24-hour operation of the power plant in during high sunshine periods.

More technical details, including some useful illustrations of the storage system, can be found here and here. In summary, the solar collectors for the existing plant add up to a total of 510,120 square metres (0.51 km2), consisting of 209,664 mirrors along 312 rows with a total length of 24 km, with 90 kilometres of absorption pipes. The total physical area occupied by the plant (after appropriate collector spacing, and allowing for the storage and turbine housing, etc.) is 1.95 km2.  The estimated energy yield is 178 GWh / year (I haven’t seen reports of actual performance data), at a capacity factor of 40.7%, and an average power yield of 10.4 W/m2. It will use 560 million litres/year of fresh water, mostly for cooling the steam circuit, drawn from local ground water (a plant using air cooling would have a lower efficiency and would have to be larger to compensate).  The lifespan of the plant is estimated to be 30 — 40 years.

Precise construction costs are hard to come by, but it seems to have been about €300 million ($AUD 500 million). This works out to be $25 billion per GWe of average power, but this is clearly a first-of-a-kind cost that can be expected to fall with replicated builds. The levelised cost of energy (including the energy storage) is estimated to be 45 c/kWh (in Australian cents) — which is about the size of the Spanish feed-in tariff which is set to run for 25 years. Including its charge for electricity to customers, the maximum cost has been capped at 58 c/kWh.

Some of the above data were already summarised in TCASE 4, and cooling water requirements were covered in TCASE 6.

The crucial data for construction material requirements for Andasol 1 is found in the NEEDS report 2008, “Final report on technical data, costs, and life cycle inventories of solar thermal power plants” – specifically, Table 7.3, page 88. Early in the report (page 28), they calculate costs for a solar thermal power station, located in the Sahara (with better insolation than Spain, but let’s skip this detail for simplicity), generating for 8000 hours per annum — close enough to 90%. They base this on 16 hours storage per day, which they project can be achieved by 2020. The value of 16 seems to be an average number of hours per year, rather than the crucial minimum delivery. Given that the time in winter that is suitable for generating with solar thermal technology is about 5 or 6 hours per day (on clear sunny days), the power station would need to have 18 to 19 hours storage to allow it to have a capacity factor of 90% (excluding bad weather).

The base figures for material inputs for the current plant works out to be 1,303 tonnes of concrete, 406 tonnes of steel, and 133 tonnes of glass, per average MWe. To increase the capacity factor from 40% to 90%, one would have to roughly increase the size of the mirror field by a factor of 2.25 (90/40) and the thermal storage facilities by 2.5 (18.5/7.5). The larger mirror field can be rationalised on two fronts: (1) more collecting area is required to recharge the larger volume of storage salts, and (2) the solar multiple for winter will be about twice that of summer.

Let’s use a half-way figure from above — 2.4 — as a scaling constant. This gives 3,127 tonnes of concrete, 974 tonnes of steel, and 300 tonnes of glass per MWe delivered at a 90% capacity factor. Scaled up to the size of an AP-1000 reactor (1,154 MWe at 90 % CF), this is 3.61 million tonnes of concrete, 1.12 million tonnes of steel, and 0.34 million tonnes of glass, with the total plant covering ~101 km2 of desert. By comparison, the reactor would require 0.24 million tonnes of concrete and 0.015 tonnes of steel, and occupy 0.04 km2 of land. So, the comparative solar : nuclear ratios comes out as follows:


Ratio of materials/land requirements, for equivalent solar thermal : nuclear (both calculated at 90 % capacity factor):

Concrete = 15 : 1; Steel = 75 : 1; Land = 2,530 : 1


The conclusion? When energy storage is properly accounted for, the material and land requirements for solar thermal vs nuclear power area appallingly lop-sided. Further, if the solar plant doesn’t end up lasting 40 years, and the AP-1000 lasts 60 years (nearly half of the US reactor fleet is now licensed to run for this long), then the numbers get even more skewed.

Needless to say, for concrete and steel — two of the most carbon-intensive products embedded in any power generation facility — this amounts to a large difference in the embodied energy and associated greenhouse gas emissions of the capital infrastructure. As such, the additional mining, required to deliver the limestone and iron ore needed to produce the construction materials for solar thermal versus nuclear, must be set against uranium mining (until Generation IV reactors are standard, that is). Anti-nukes who raise the mining objection against nuclear power, but ignore the mining associated with solar (or wind) construction, are presenting a false comparison. They can’t have it both ways.

Although I have been careful in my calculations, the above figures are nevertheless a first attempt. As such I’m happy to entertain challenges from commenters, and if these criticisms prove to be right, then I’ll happy adjust my comparative figures accordingly.

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By Barry Brook

Barry Brook is an ARC Laureate Fellow and Chair of Environmental Sustainability at the University of Tasmania. He researches global change, ecology and energy.

121 replies on “TCASE 7: Scaling up Andasol 1 to baseload”

Why are Andasol 1 and Andasol 2 trough type solar plants? From what I have read, it looks as though the power tower system is better. Because it can produce higher temperatures, it is more efficient and therefore requires less cooling water and less land space. The reduced shading effect is also an advantage. In addition, it doesn’t require the land to be as level as a billiard table. Probably it is easier to wash the dust of off flat mirrors than to wash troughs.

It would be interesting to have firm numbers to compare the trough system with the power tower system.


Hi Frank Eggers,

Needs (2008) compares solar trough and and solar tower.

Solar trough is more developed at the moment. Solar tower may take over in the future. However, there appears to be little difference between them in the projected cost of electricity. Both are totally uncompetitive. See herte for more, and take notice of what would be involved in building sufficent to make a significant contribution to our electrcity supply.


Frank Eggers,

Sorry, the first line of the second paragraph should say
“Solar trough is more developed at the moment. Solar tower may take over in the future.”


From what I have seen in various places, your are right; solar power cannot adequately provide for the electrical needs of a large industrialized country, although there are certain situations where it is useful, such as in remote areas where grid connections would be impractical or where little power is required. Solar water heaters are commonly used by the few people in third-world countries who can afford them because it is less expensive than using other means to heat water.

Recently, Senator Feinstein objected to a solar PV installation in the (California) Mojave desert because the installation would cover 60 square miles of desert. The Sierra Club and other environmental organizations object to having trails in the desert for off-road vehicles because of the fragile nature of the desert environment. Thus, even if covering large desert areas with solar installations would provide sufficient power, it is likely that environmental groups would object. It is unclear just what means they would accept to provide sufficient power.

India has a population density 11 times greater than the U.S.; China has a population density 4.3 times greater than the U.S. In fact, our population density is lower than average. Considering that, even if we could collect sufficient solar power for our needs, it would be totally impossible for more densely populated countries to do so, a fact which is generally ignored. I wonder how the pro-solar crowd would expect Alaska to receive sufficient solar power.

The pro-solar crowd wants solar installations in the African Sahara Desert to provide power for the whole continent of Africa. The area available and the climate might make it possible, but I wonder whether they are aware of the huge sand dunes which continually move around, or where they expect to get the water to keep the mirrors clean, or who would keep the roads clear to provide access for maintenance, etc.

One would suppose that before embarking on a mission to install large solar plants and wind farms, careful calculations would be done to determine whether it is practical. It is difficult to understand why that has not been done.


Interesting to read this debate.

Noticed a lot of references to Gen III+ AP1000 reactors, however as far as i’m aware these have failed to obtain NRC approval in the US due to “safety issues” with “sheild building”.

See here:

If solar thermal with baseload is commerically operating and AP1000 plants are yet to even be approved for construction in the US, wouldn’t it make sense to go for the first option?

Also good article about Obama’s plans for nuclear power in US here:

Reality is that most talk of Gen III+ and Gen IV reactors is just cover for politicians to pursue a nuclear agenda on the basis of outdated technology.


If AP1000 plants have not yet been approved, wouldn’t it make sense to get them approved as quickly as possible?

How many solar thermal plants with adequate storage would have to be built to provide as much power as a single AP1000 plant?

Could the solar thermal plants be built where they could tie into the existing grid, or would it be necessary to spend huge amounts of $$ to accommodate the solar thermal plants?

Environmental organizations are concerned about the fragile desert environment, which is why senator Feinstein (D, CA) objected to a solar plant which would have covered 60 square miles of Mojave desert. Can we be sure that they wouldn’t mind covering tens of thousands of desert with solar thermal plants?

In my opinion, pressurized water reactors of the AP1000 type are obsolete; they require enriched uranium of which they waste about 99% and also require a large reactor vessel which is pressurized to more than 2000 psi. We should be designing and building LFTR reactors which use thorium as fuel instead of uranium. If that cannot be done quickly, we should be building CANDU reactors. They can use existing nuclear waste and natural uranium as fuel, thereby solving the existing waste problem.


“If AP1000 plants have not yet been approved, wouldn’t it make sense to get them approved as quickly as possible?”

Not if they haven’t been approved due to safety concerns and its a nuclear power station.


And how long would it take to get approval from the NRC compared to how long it would take to make major grid changes to support solar?

What is the largest solar plant which has ever been built?

How many solar plants would it take to deliver as much continuous and reliable power as one AP1000 plant? How long would it take to build them?


Why store power as heat and not use water storage in 2 dams 1 at the top of the hill and another at the bottom. A Hydro Power station in the middle. Use Water pumps to pump water from the bottom dam to the top dam when the sun shines. This energy can be stored for hours or months depanding on the DAM size. Also provides water for usage for consmption. The storage doens’t need to reside near the Solar power station as the solar energy can be transferred by power lines to the pumps.


[…] As our alternative extreme scenario, suppose India opted for concentrating solar thermal power stations similar to the Spanish Andasol system to supply 14 million GWh annually. Each such unit supplies about 180 GWh per year, so you would need at least 78,000 units with a solar collector area of 3.9 million hectares, equivalent to 13 of our hypothesized exclusion zone wildlife parks from the accidents. But, of course, these 3.9 million hectares are not wildlife parks. I say “at least 78,000″ units because the precise amount will depend on matching the demand for power with the availability of sunshine. Renewable sources of energy like wind and solar need overbuilding to make up for variability and unpredictability of wind and cloud cover. The 78,000 Andasol plants each come with 28,000 tonnes of molten salt (a mix of sodium nitrate and potassium nitrate) at 400 degrees centigrade which acts as a huge battery storing energy when the sun is shining for use when it isn’t. Local conditions will determine how much storage is required. The current global production of ordinary sodium chloride is about 210 million tonnes annually. Producing the 2.1 billion tonnes of special salt required for 78,000 Andasols will be difficult, as will the production of steel and concrete. Compared to the nuclear reactors, you will need about 15 times more concrete and 75 times more steel. […]


[…] Andasol 1 is worth revisiting. This has a peak power output of 50 MW. To build it required 65,150 tonnes of concrete, 20,300 tonnes of steel and 6650 tonnes of glass sitting on close to 200 hectares with its power plant and appropriate spacing between the 50 hectares of mirrors. Efficiency gains are limited by the diffuse nature of the energy it is harvesting. […]


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