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7: Direct Solar Power
7.3: The Concentrated Solar Power (CSP) Technology

7.3: The Concentrated Solar Power (CSP) Technology

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The simplest method of heating a boiler of in such a way is to put the boiler at the top of a high tower surrounded by a number of mirrors, called heliostats. A heliostat is a mirror, usually a flat one, which can be rotated and the tilt of which can be changed – so that, when Sun moves across the sky, it always sends the reflected beam of sunlight to the predetermined target.

Figure $\PageIndex{1}$: A scheme of a CSP power plant. A tower is surrounded by fields of heliostats – e.g., of large mirrors, the individual orientation of each is computer-controlled so that the reflected beams is always sent towards the “receiver” at the tower’s top – no matter what’s the position of Sun in the sky (aop).

An example: In order to make a quick estimate of how many heliostats are needed in such a power plant. Suppose it’s noon on a sunny day. Suppose that the effective Total Solar Irradiance in the area where the plant is located is 0.8 kW/m², and each heliostat carries a mirror of the $ 3\; \times \; 3 \; m $ size, i.e., with the surface area of $ 9 \;m^{2} $. But a single heliostat does not send a beam of $9 \; m^2 \times \;0.8 \;kW/m^{2} = 7.2 kW $ towards the tower, because the sunlight does not fall on the heliostats at a right angle. Actually, the incident angle is different for each individual heliostat. In order to make our life simpler, let’s assume that the average power sent by an individual heliostat is just one-half of the power we have calculated above, i.e., 3.6 kW (perhaps it’s too conservative a value, but, remember – we want to make an estimate only). So, for depositing the power of 1 MW = 1000 kW at the boiler, we need to set up a field of 1000/3.6 = 278 heliostats.

Let’s take a look at an existing CSP plant – for instance, at the Crescent Dunes Solar Energy Project, a facility capable of generating 110 MW of electric power. After clicking on the link provided, one will see the aerial photograph of the heliostat field. A big one, isn’t it? It’s difficult to count the heliostats, but their number is given in the article: 10,347. OK, in the example we worked on above we found that 278 heliostats are needed for 1 MW. So, 10,347 of them should yield 10347/278 = 37.2 MegaWatts. Why not 110 MW? Well, if we keep reading the article, we find that they use much larger heliostats than in our example, not of $9\;m^{2} $, but of $115.7\;m^{2} $. 115.7/9 = 12.9 times larger! So, we have to multiply the 37.2 MW by this number and we get... 37.2 MW 12.9 = 480 MW. Now, much to large a power! What’s going on?

Well, we have to keep in mind that 480 MW we have gotten is the thermal power delivered to the boiler. And the steam from the boiler is sent to a turbine + generator system, like in a conventional thermal power plant.

Typically, as was discussed in Chapter X – the efficiency of converting steam thermal power to electric power in such a system is 30%. So, 30% of 480 MW is 144 MW – closer to the figure of 110 MW. Considering that what we got from our example was an estimate only, we can conclude that the 144 MW result is ”reasonably close” to the real value of 110 MW.

But there is another possible explanation for the difference. There is one more piece of important information in the article about he Crescent Dunes facility: namely, it can generate power for several hours after the sunset. How? During the daytime hours some of the sunshine energy “captured” is not sent to the turbine, but is stored and later reused for electricity generation, when there is no sunshine. So, we probably had to use not 480 MW in our calculations, but 480 MW minus the amount of power sent to a reservoir to be used later. And how such a reservoir works is explained in the Subsection that follows.

Molten Salt Heat Storing Technology

In the Crescent Dunes power plant there is huge dual-chamber reservoir containing M = 32 000 000 kg of molten salt – a mixture of 54% of KNO₃ and 46% of NaNO₃ (sometimes referred to as the “solar salt”). Both components are inexpensive fertilizers. In the 54/46 weight proportion they form a so-called Eutectic system with the lowest melting temperature (131^◦C). The reservoir consists of two thermally-insulated tanks: the “cold one”, which stores molten salt of 288^◦C temperature, and the “hot one”, to where the molten salt is transferred after being heated during the “storing” phase – and where it’s stored at temperature of 566^◦C.

During the “generating” phase, the salt travels the other way: it’s pumped to a “steam generator”, which is nothing else than a heat exchanger ¹ in which the molten salt is cooled from 566^◦C down to 288^◦, in the process heating up water and making it a 566^◦C steam.

The heat capacity ${ }^{2}$ of the molten salt mixture is about $c=1.5 \mathrm{~J} / \mathrm{kg} 1^{\circ} \mathrm{C}$. In the steam generator the molten salt is cooled down from $566^{\circ} \mathrm{C}$ to $288^{\circ} \mathrm{C}$, i.e., by $\delta t=566^{\circ} \mathrm{C}-288^{\circ} \mathrm{C}=278^{\circ} \mathrm{C}$. Hence, the total amount of heat $\Delta Q$ given away by the $M=32 \cdot 10^{6} \mathrm{~kg}$ of the "solar salt" is:

\[ \Delta Q=M \cdot c \cdot \Delta t=32 \cdot 10^{6} \mathrm{~kg} \times 1.5 \cdot 10^{3} \mathrm{~J} / \mathrm{kg} \cdot 1^{\circ} \mathrm{C} \times 278^{\circ} \mathrm{C}=1.33 \cdot 10^{13} \mathrm{~J} \]

If this thermal energy is released over the period of 10 hours $=36000 \mathrm{~s}$, the thermal power released is $1.3310^{13} \mathrm{~J} / 36000 \mathrm{~s}=36910^{6} \mathrm{~W}=369 \mathrm{MW}$. Of thermal power - but if we assume $30 \%$ efficiency in conversion to electric power, we obtain... 0.3\$69 MW = 110.7 MW!

So, it seems that we got too low a result (144 MW) because it was based on estimates that had been "too conservative". Probably, the real story is such that the thermal power deposited at the tower top in Crescent Dunes is not $480 \mathrm{MW}$, but rather $740 \mathrm{MW}$ - of which one half, $370 \mathrm{MW}$ is converted to electricity right away, and the other $370 \mathrm{MW}$ is stored in the molten salt tanks.

The example of the Crescent Dunes solar power plant points out to a considerable advantage of the CSP technology - the capability of generating power after sunset. With the molten saltstorage reservoir, a power plant may be even able to generate electricity at the same power level for full 24 hours. However, the largest tower-type CSP power plant in the US, the Ivanpah Solar Power Facility with three heliostat fields a net power of $392 \mathrm{MW}$, does not use thermal storage. Other US solar power plants do, although they are not tower-type plants (for the list of the largest CPS plants in the world, please see this Web document).

It's worth watching a short but highly instructive YouTube video explaining how the molten salt energy storage system works.

If this thermal energy is released over the period of 10 hours = 36 000 s, the thermal power released is 1.33 10¹³J/36000s = 369 10⁶ W = 369 MW. Of thermal power – but if we assume 30% efficiency in conversion to electric power, we obtain... 0.3 369 MW = 110.7 MW!

So, it seems that we got too low a result (144 MW) because it was based on estimates that had been “too conservative”. Probably, the real story is such that the thermal power deposited at the tower top in Crescent Dunes is not 480 MW, but rather 740 MW – of which one half, 370 MW is converted to electricity right away, and the other 370 MW is stored in the molten salt tanks.

The example of the Crescent Dunes solar power plant points out to a considerable advantage of the CSP technology – the capability of generating power after sunset. With the molten salt storage reservoir, a power plant may be even able to generate electricity at the same power level for full 24 hours. However, the largest tower-type CSP power plant in the US, the Ivanpah Solar Power Facility with three heliostat fields a net power of 392 MW, does not use thermal storage. Other US solar power plants do, although they are not tower-type plants (for the list of the largest CPS plants in the world, please see this Web document).

It’s worth watching a short but highly instructive YouTube video explain- ing how the molten salt energy storage system works.

Now, the question is, why do we want to use liquid salts, and not, for instance, metals? The answer is simple – metals which are liquids at the same temperature range as the “solar salt” have much lower specific heat. For instance, for lead (Pb) in its liquid state c = 0.14 J/kg 1^◦C, over ten times less than the c of “solar salt”, which explains why it’s not a good heat storage agent. But the popularity “solar salt” comes from the fact that there exist a huge fertilizer industry producing millions of tons of nitrates per year – therefore, KNO₃ and NaNO₃ are widely available. But the “solar salt” is not the only possible mixture of salts with good heat storage properties – there are many other possible combinations, and their properties are reported in many Web publications (e.g., in this one).

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1. Heat exchangers are described in greater detail in the Geothermal Energy Chapter.

2. Heat capacity, commonly denoted as c, is the amount of heat needed to raise the temperature of a mass unit of a given substance by 1^◦C (or, equivalently, by 1 K), or the amount of heat given away by the mass unit of this substance when it’s cooled down by 1^◦C/1 K.