Lithium in thermal energy storage: A state-of-the-art review
Introduction
Thermal energy storage (TES) is used to keep thermal energy to be used at a later time. A complete TES process involves at least three steps: charging, storing and discharging (Fig. 1) [1]. The most important part of the storing step is the storage media. A wide variety of choices exists, depending on the temperature range and the application. There are several types of TES methods, shown in Fig. 2. Basically one can use a physical process or a chemical process. Physical processes are sensible heat storage and latent heat storage. For sensible heat storage, water is a common choice because, among its other positive attributes, it has one of the highest specific heats of any liquid at ambient temperatures. Solids have the advantage of higher specific heat capacities, which allow for more compact storage units. TES using latent heat change can also be used; the most common example of latent heat storage is the conversion of water to ice. The other category of storing heat it is considered through the use of reversible endothermic chemical reactions.
With the use of TES, systems achieve benefits by fulfilling one or more of the following purposes: increase the generation capacity, enable better operation of cogeneration plants, shift energy purchase to low cost periods, increase system reliability, or integrate other functions.
Lithium is recognized as a “critical material”, that is, a material important to the clean energy economy and with risk of supply disruption [2] (Fig. 3). Materials are deemed important or have a high impact based on the particular properties that make them well suited for applications in which they are used. For photovoltaics, this might be the improvement of photovoltaic performance of electrochemical cells [3], [4], [5]; for magnetic materials it might be the magnetic flux density [6]; for thermal energy storage it might be the specific heat capacity [7], [8], [9], [10], [11] or the phase change enthalpy [12], [13]. Lithium is always listed as a critical material in electric batteries [2], [14]. Some of the materials are simply rare in their overall abundance in the earth’s crust, or do not commonly occur in single deposits with significant concentrations; others are difficult to recover economically; others are byproducts of primary production of other materials.
Lithium is produced mainly in Canada, Brazil, Australia, some areas of Africa and Russia as mineral, and in China, USA, Argentina and Chile from brines. 61.8% of the total world lithium resources come from brines, around 26.9 Mt [14], [15], [16]. The abundance in Earth’s crust is 19–21 ppm and in seawater 0.17–0.18 ppm [16]. The annual production in 2010 was 25,300 t; its distribution and the reserves by country are shown in Table 1 [15]. When lithium ore is exploited, its typical grade is 0.57–0.3%, with a minimum economic ore grade of 0.2–1% [16]. When comparing the productions costs of the lithium compounds from minerals and brines, countries producing lithium compounds from brines have lower production costs than those that produce them from minerals (2–3 $/kg vs. 6–8 $/kg) [17].
Lithium is mainly used in electrical energy storage, as the development of the electric car industry is based in lithium-ion batteries performance [15]. Lithium-ion batteries are also used for a wide range of electrical storage applications, from computers to video cameras. Lithium compounds are used in pharmaceuticals, as a mood stabilizer, and as an alloying agent to lighten and increase the strength of a number of metals, especially those used in the aerospace industry.
On the other hand, there are estimations of the lithium demand in the world, which give data on lithium demand by compound (Table 2) and by application (Table 3) [14]. It should be highlighted that meanwhile the demand of lithium compounds is expected to increase dramatically within the next few years, the application reviewed in this paper, thermal energy storage, is not even mentioned. Therefore, any deployment of lithium compounds for TES would need to be studied within this demand in other applications.
With known resources of lithium in the world of over 43 million tons, there is an abundance of physical lithium available to meet the demands estimated and shown above. The price of one kg of lithium metal is reported to be from 46 to 74.8 € [16].
The objective of this paper is to review the role that has played lithium compounds up to now in the different technologies of thermal energy storage, to evaluate the opportunities of lithium in them.
Section snippets
Sensible heat storage
The most common method for TES is sensible heat storage (Fig. 4). Typical materials for sensible heat storage are solids such as stone and bricks, or liquids such as water. The amount of energy that can be stored in a sensible heat storage process can be calculated with:where ΔQ is the amount of energy stored, m is the mass of storage material involved, cp is the heat capacity of the storage material, and ΔT is the temperature change occurred during the process.
For a proper
Latent heat storage
Fig. 4 presents a latent heat storage process, showing that in this case the storage happens while the phase change takes place, therefore it happens at constant temperature. Materials used in latent heat storage are usually referred to as phase change materials (PCM). The amount of energy stored is calculated with the following equation:where ΔQ is the amount of energy stored, m is the mass of storage material involved, and Δh is phase change enthalpy. PCM can store about 3 to 4 times
Thermochemical storage
Thermochemical energy storage happens when gas is absorbed in an absorbent, adsorbed in an adsorbent, or when there is a chemical reaction. Absorption happens with a gas being absorbed in a liquid absorbent; the heat involved is generally similar to the condensation heat of a gas, around 41 kJ/mol for water. Adsorption happens with a gas being adsorbed on a solid adsorbent; the heat involved is the condensation heat of a gas plus a light chemical bonding heat with solid surface, around 50–60
Conclusions
Lithium is mainly used in electrical energy storage, as the development of the electric car industry is based in lithium-ion batteries performance, it is recognized as a “critical material” and, on the other hand, it is studied to be used in thermal energy storage in several applications. The main producer countries are Canada, Brazil, Australia, some areas of Africa and Russia as mineral, and China, USA, Argentina, and Chile from brines. But due to its specific characteristics, the production
Acknowledgements
This work was partially funded by the Spanish Project ENE2011-22722 and ENE2011-28269-C03-02. The research leading to these results has received funding from the European Union׳s Seventh Framework Programme (FP7/2007-2013) under grant agreement no. PIRSES-GA-2013-610692 (INNOSTORAGE). Dr. Luisa F. Cabeza would like to acknowledge the Generalitat de Catalunya for the quality recognition 2014-SGR-123 and Dr. A. Inés Fernández for 2014-SGR-1543. The authors acknowledge to FONDECYT (grant no.
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