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| Fig. 1: Although this silo is not used for a sand battery, similar silos are used to store the sand. (Source: Wikimedia Commons) |
With a global push to limit greenhouse gas emissions, many countries and companies have begun to integrate renewables into their energy grids. Renewables like wind and solar photovoltaic are subjected to environmental conditions for energy production. At times, a mismatch between energy production and energy demand can drive the grid to experience negative net load. This means that at peak renewable production time, the total energy demand can be less than the total energy produced from renewables, resulting in excess energy. Then, at peak demand or off-hour times when renewables are not producing at their maximum, the energy supply needs to be supplemented using fossil fuels. This mismatched demand and supply can be addressed by using a good storage solution to store the excess energy during excess production times, shifting when the energy is useful.
There are various energy storage solutions. Commonly discussed are lithium-ion batteries, which are nearly found everywhere from handheld electronics, electric vehicles, and excess energy storage. However, they do have downsides, including, but not limited to: aging causing decreased performance, fire, and material sourcing concerns. [1] Since not all battery storage will excel in all metrics, it is valuable to explore other options. Various other forms of energy storage precede lithium-ion batteries by over 150 years. The first fuel cell was created in 1839 and pumped hydro was first used in 1907. [2] Over the last century and a half, many energy storage solutions have been proposed. They can be categorized into thermal energy storage (TES), mechanical energy storage, chemical energy storage, electrochemical energy storage, and electrical energy storage. [2] One TES subcatergory is sensible heat storage (SHS). In SHS heat raises the temperature of a well-insulated solid or liquid (without inducing a phase change), such that the now hot material can be used at a later time for heat energy. One recent example of SHS is using sand as the medium for heating, resulting in sand batteries.
First proposed by researchers at the University of California in 2014, the first sand battery has made it way into residential use in Finland in 2022. [3] A typical sand battery installation, shown in Fig. 1, consists primarily of a large, well-insulated silo that contains the sand and embedded piping. Sand batteries are heated to 600 - 1000⁰C using hot air pumped through pipes. [3] When at maximum operating temperatures, sand batteries have the benefit of staying hot for months. The long storage duration makes them particularly useful for northern countries where renewables such as photovoltaic become less efficient in the winter. The general limiting factor of the temperature of sand batteries is not the sand itself, but the insulating material. [3] Typically, due to its large specific heat capacity and low cost, cement can be used as a thermal barrier, keeping the sand temperature high for months.
In practice, compaction and moisture content can result in sand with specific heat capacities of 0.8-1.1 J/(kg K) and densities of 1300-2000 kg/m3. [4] Since specific heat tells us how much heat is stored per kilogram, and density tells us how much mass fits into the same volume, both a high specific heat and a high density are ideal. In choosing a sand, both these properties and availability are worthwhile considerations. We will assume sand has a specific heat capacity of 850 J/(kg K) and a density of 1620 kg/m3. [4] To calculate the sensible heat stored in a material, we will use the sensible heat storage equation
Where q is sensible heat (J), V is volume (m3), ρ is density (kg/m3), cp is specific heat (J/kg ⁰C) and dT is temperature change (°C). Assuming an increase of sand temperature of 580°C (20°C to 600⁰C), we can calculate that per 1 m3, sand holds approximately 0.798 GJ/m3 of energy. By comparison, lithium-ion batteries can store 1.08 GJ/m3 of energy. [5] Using the 2020 lithium-ion battery price of $132/kWh, we find that the cost of lithium-ion batteries is roughly $39600/m3. [6] Compared to the 2020 cost of sand, $16/m3, lithium-ion batteries are 2500 times the cost of sand batteries with respect to the storage medium, not shipping or other construction costs. [7] Furthermore, as dirt can take the place of sand (albeit with a decrease in sensible heat capacity per m3), materials for sand batteries could be considered free and abundant. With these costs, the 0.2 GJ of energy mismatch between lithium-ion and sand batteries could easily be compensated for with a corresponding size increase. However, sand batteries are not overtaking the energy storage market, and their other associated energy cost and monetary costs can give us an indication of why.
Converting the heat energy of sand batteries back into useful energy remains problematic. Currently, systems pipe cool air through the hot sand to convectively heat the cool air to be used directly for heating purposes. A practical limitation of sensible-heat storage in solids is the need for embedded piping. Conductive heat transfer through sand is too slow for useful power output, so forced convection through pipes is required. There is also an unavoidable thermodynamic penalty; transferring heat from a hot medium to cooler air increases entropy according to dS = dQ/T. Because of this, heat extracted from the sand is always delivered at a lower temperature than the charging temperature, reducing its quality as an energy source. The useful work potential available is known as exergy. Exergy efficiency is defined as [8]
In practice, the overall exergy efficiency is 12% for a well-mixed sensible thermal storage system. [9] This underscores the challenge of using sensible heat systems for dispatchable energy storage.
We assumed that sand is cheap and abundant. However, there is more nuance to it. Although globally abundant, not all sand is created equal, with differences stemming from size, density, and composition. [10] To meet standards, industry-grade sand is typically sourced from river beds, not deserts, as many may assume. This sand is used in many industries such as glass, metal, chemical, oil and gas, and paint, to name a few. The cost of sand has risen from $9.95/metric ton to $13.90/metric ton from 2020 to 2024, slightly outpacing inflation during the same period, although not significantly. [7] Although sand may not be scarce; it is being used at a rate faster than it can replenish. [10] If sand batteries were to meet the 2024 global energy demand of 650 EJ, and using our value of 0.798 GJ/m3 of energy storage for sand batteries, and our assumed average density of 1620 kg/m3, roughly 1.319 × 1015 kg of sand would be needed. [11] Using the 2019 sand extraction weight of 9.371 × 1012 kg, we would need roughly 140.8 years of sand extraction to match the world energy needs in sand batteries. Of course, the full energy demand would not need to be stored in batteries, and this does not take into account other construction limitations, such as piping. This also ignores the fact that soil could be used for the same purpose with diminished returns. Nevertheless, industry-grade sand is a finite resource using today's technology, and it is important to consider these material implications during energy development.
Sand batteries will not be the solution for energy storage at all locations globally. However, they do offer another possible strategy to limit the use of fossil fuels and store excess energy from production, all using an abundant material. With the demand and supply energy mismatch, batteries in any form are worth exploring.
© Erik Schreiner. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute, and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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