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Engineering LibreTexts Pumped Heat Energy Storage and Liquid Air Energy Storage

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    In the United Kingdom, researcher developed two entirely new technologies of thermal energy storage.

    One of these ideas is a radical remedy for the low efficiency of energy recovery in molten salt technology and the technology developed by SiemensGamesa (SG) described in the previous section. The conversion of thermal energy into electricity in both methods is the same as in conventional coalfired power plants – which is shown schematically in Figure X. As discussed in Chapter 3, from a portion of heat Q using an ideal heat engine using the famous Carnot Equation one can get the maximum mechanical energy W1 amounting to:

    \[W=\left(1-\frac{T_{\mathrm{c}}}{T_{\mathrm{h}}}\right) \times Q=\epsilon_{\mathrm{i}} \times Q \notag\]

    where Th is the absolute temperature of the “hot source”, and Tc is the absolute temperature of the “heat sink”. The temperature of the latter in the case of using steam turbines is roughly 100C = 373 K. The highest Th in the S-G method is, as mentioned, 800C = 1073 K, and then, as heat is drawn from the hot reservoir, it drops to 400C = 673 K – which corresponds, respectively, to E equal 0.65, and 0.45. Yet, as discussed in Chapter 3, for real thermal engines a more realistic is the modified Carnot formula:

    \[W=\left(1-\sqrt{\frac{T_{\mathrm{c}}}{T_{\mathrm{h}}}}\right) \times Q=\epsilon_{\mathrm{r}} \times Q \notag\]

    from which, for the two temperatures –the initial and the final – we obtain, respectively, the Er values of 0.41 and 0.26, with an average of 0.34

    Figure \(\PageIndex{1}\): A general scheme of a heat engine, which can also be an energy storage device if the “hot source” is a reservoir, “charged” with thermal energy which is produced from surplus energy from wind or solar farms.

    In the molten salt storage method the figures are even lower, because the the highest value of Th is 550C = 823 K, and if it drops to 250C = 523 K in the course of the heat extraction process, the average value of Er is only about 0.24. In other words, in these methods from 2 to 3 of the energy storedis deposited to the heat sink – which simply means “lost”.

    Figure \(\PageIndex{1}\): The idea of the pumped heat energy storage system. Plot (a) is essentially a heatpump system in which mechanical work (from an electric motor) drives a heat pump, i.e., a heat engine acting “in revers”. Plot (b): the thermal energy that has been transferred by the heat pump to a reservoir playing the role of a “hot source”, now is used for operating a heat engine. In an ideal case such a storage system makes it possible to recover 100% of the energy used to drive the heat pump.

    The idea of the new British PHES method is shown in the Fig. 11.16. The point is that the electrical energy to be stored is not deposited directly to the “hot source” – instead, a heat pump is used. A heat pump, as was discussed in Chapter 8, is a thermal engine working “in reverse”. In normal operating mode, a thermal engine withdraws a portion of heat Q from the “hot source”, converts E Q to mechanical work W , and deposits (1 E) Q in the heat sink. In the pump mode (Fig. 11.16 (a)) , things get reversed – the input of a mechanical work portion Win into the pump causes that it “withdraws” from the sink (now called the “cold reservoir”) a portion of heat equal:

    \[Q_{\text {sink } \rightarrow \text { pump }}=W_{\text {in }} \times\left(\frac{1}{\epsilon}-1\right) \notag\]

    In the pump, this portion of thermal energy adds up to the mechanical energy input and this sum:

    \[W_{\text {in }}+Q_{\text {sink } \rightarrow \text { pump }}=W_{\text {in }}+W_{\text {in }} \times\left(\frac{1}{\epsilon}-1\right)=\frac{W_{\text {in }}}{\epsilon} \notag \]

    is now transferred “up” to the hot source (now called the “hot reservoir”).

    Now, in order to withdraw energy, the pump is switched over to the “heat engine” mode (Figf. 11.16 (b)). If the same amount of heat Q = Win/E that was stored in the hot reservoir is now withdrawn from it and passed to the heat engine, the work output yielded by the engine will be:

    \[W_{\text {out }}=Q \times \epsilon=\frac{W_{\text {in }}}{\epsilon} \times \epsilon=W_{\text {in }} \notag \]

    i.e., 100% of the stored energy is recovered! But only if ideal heat pumps and heat engines are employed – which, of course, do not exist. However, it should be expected that a PHES system built even from existing materials and components will provide much higher energy recovery efficiency than the S-G technology and molten salt technology. The whole idea outlined above is so simple that it must have been known for a long time the only problem was that it was not really known how to implement it and create a working installation. And finally, the British researchers managed to do that! Here one can find a more detailed description of the design of their prototype and of the physical processes used in it. This is another article worth reading. It turns out that the sophisticated PHES technology offers an efficiency of 60-65%, which is indeed a substantial gain in comparison with the other methods. Also interesting are the economic aspects of PHES technology, its cost-effectiveness compared to other methods – such an analysis is presented in this Sciencedirect report.

    The other highly interesting new British idea is the thermal storage technology using – paradoxically! – not a very hot medium to store energy, but a very cold one! Yes, very cold, because we are talking of liquid air, the temperature of which is -194, or 79 K. How can energy be stored in a cold medium? It may come as a surprise, but the answer is pretty simple. Consider the basic theory of heat engines. What does a heat engine need to work? Well, a “hot source” and a “heat sink”, yes? And in order to get a decent efficiency E, what is needed? A temperature difference between the two, of course. So, why don’t we treat the liquid air as the “heat sink”, and the ambient environment, with its temperature of 20C, or 293 K, as the “hot source”? Then, we get the realistic efficiency of a heat engine:

    \[\epsilon=1-\sqrt{\frac{T_{\mathrm{c}}}{T_{\mathrm{h}}}}=1-\sqrt{\frac{79 \mathrm{~K}}{293 \mathrm{~K}}}=0.48\]

    The efficiency of 48% is definitely not a bad result!

    More technical details about the liquid air energy storage (LAES), a.k.a. “cryogenic” energy storage (CES), are given in the Web page of Highview Power the company who has pioneered this unusual technology – and their YouTube video.

    The technology of liquefying air is well established and is widely used by many industries. The process of storing energy is thus nothing else than running a liquefying machinery and saving the output in tanks – there are so-called “Dewar vessels” in which liquid air can be stored for prolonged periods with minimal losses. However, the overall efficiency will be lower than 48% because our estimate did not include the energy initially needed for liquefying the air. On the other hand, there are possible ways of increasing the efficiency: above, we used the “ambient temperature” of 20C = 293 K as Th, but there are many industrial facilities that “dump” large amounts of “waste heat”. So, if a liquid air storage installation could use such heat to augment its “hot source” by, say, 100 K, the efficiency would increase from 48% to 55%. The aim of the Highview Power engineers, as Javier Cadava, the COE of Highview Power, declares in this article, is to reach 70-75% of overall efficiency (or “roundtrip efficiency”, as Cadava calls it).

    There are certainly many advantages of LAES – to list a few, the medium needed for implementing this method is available everywhere in the world in unlimited quantities; it is patently non-toxic; and it is 100% recyclable without leaving any nasty byproducts. So, let’s keep our finger crossed and let’s wish the Highview Power all possible successes in further development of LAES and in making it one of the leading energy storage technologies on a global scale.


    1The output mechanical energy from a thermal engine still needs to be converted to electrical energy, but the efficiency of modern generators used for this conversion is very high and can be taken with a good approximation as 100%. Pumped Heat Energy Storage and Liquid Air Energy Storage is shared under a CC BY 1.3 license and was authored, remixed, and/or curated by Tom Giebultowicz.

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