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Thermal Energy Storage

With the exception of dish parabolic reflectors, concentrated solar power systems generally separate the production of heat from the production of electricity. In so doing, they have available to them a means of storage that cannot be used by photovoltaic systems, namely thermal storage.

Types of Thermal Storage

Water

Water is the traditional heat storage medium because it is abundant and relatively easy to work with. It is widely used in Scandinavia where a great deal of energy is created through solar and wind. The hot water is used to supply continuous power and meet peak demand.

Despite the fact that water is easy to work with, it has several drawbacks. Chief among those problems is that it boils at a relatively low temperature to produce steam. Managing any gas requires a pressurized system to reduce volume. Pressurizing adds cost and complexity, making a liquid solution more favorable and hence the movement away from water in most solar applications.

Molten Salt

The use of molten salt to store heat is a relatively new application to CSP though it has been used in the past to cool nuclear reactors that operate at exceptionally high temperatures. It was first demonstrated to be feasible for solar at the U.S. research power tower named Solar Two in 1995. There are several things that make molten salt attractive.

Molten salt solutions are usually eutectic solutions, which means two or more substances have been mixed together but have a uniform chemical composition. These solutions will solidify at a point lower and boil at a higher point than that of any of their constitute components. Thus, eutectic solutions increase the thermal capacity of a liquid solution.

Most current salt solutions are a mixture of 60% sodium nitrate and 40% potassium nitrate. The combined solution has a boiling point above >1400 degrees Celsius. This means that even at the high temperatures generated by concentrated solar power designs, molten salt remains a liquid. In most systems, the salt is maintained between 280 and 570 degrees Celsius and can be stored for up to 1 week without freezing.

When compared to water, the major drawback of molten salt is that it freezes at very high temperatures. This means that the system is vulnerable in colder environments and that it must always be maintained above the critical 221 degree Celsius freezing point. There is current research into salt solutions that do not freeze until reaching 100 degrees Celsius that is being conducted at the Sandia National Laboratories in the United States.

Major Advantages and Drawbacks

What make thermal storage attractive are its simplicity, longevity, and cost in comparison to batteries. The most advanced battery currently available for installation is a sodium-sulfur battery made by NGK Insulators in Japan. Each battery can store roughly 7 megawatt-hours of power, which means 20 batteries could power 500 average homes for 7 hours. The cost is $3 million per megawatt! By contrast, a 10 meter tall tank that is 24 meters in diameter could store 400 megawatt-hours of energy or more than 3 times what 20 sodium-sulfur batteries could store at a cost of less than $1 million total.

In stationary applications, thermal storage has a number of advantages over battery storage, with one exception. Batteries are generally capable of operating at much colder temperatures than are thermal systems. In most cases, if the temperature of a thermal system drops below 220 degrees Celsius (for salt), the solution will solidify and clog the system, rendering it useless. While insulation can help ensure freezing does not occur, the lower bound of temperature on the system does limit the amount of useful energy that can be extracted.

Where batteries excel, in terms of energy, is density. That is to say, the 20 batteries mentioned above store far more energy per unit area than molten salt does. This is what makes batteries so useful for portable energy needs and is the reason that, as of yet, thermal solutions have not been implemented in transportation. There is ongoing investigation into the storage of heat energy in molecular bonds, which would allow for energy densities comparable to that of lithium ion batteries.

Small Scale Thermal

Before leaving the concept of thermal storage it is worth noting that thermal storage does not have to be confined to large scale application. In fact, thermal storage and thermal inertia are critical in passive solar design.

Concrete, water, and insulation are the keys to thermal storage on a smaller scale. Many homes designed for passive solar efficiency use water features to regulate heat exchange and maintain constant temperatures. As a compound, concrete is one of the most studied substances on the plant. It has a high thermal inertia, which means it resists changes in temperature, but is also susceptible to heating via photons. When exposed to the sun in the winter, concrete will absorb heat to be reradiated at a later point. When shaded from the sun, even during high ambient temperatures, concrete will remain cool and absorb heat from the environment.

The concept behind small scale thermal has more to do with inertia than large scale thermal. The idea is not to superheat a substance, like molten salt, but more to maintain the status quo in terms of temperature.
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