Heat storage in solar plants

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Tim Bruewer details optimal thermal management for the use of molten salt in solar thermal power plants with concentrated radiation

The demand for environmentally friendly and low-CO2 forms of energy generation has been increasing in Europe since the ‘Green Deal’ was presented by the EU Commission in 2019, defining the climate neutrality of 26 member states up to the year 2050. Among other methods, the focus is on solar energy as an important representative in this area. 

Solar thermal power plants with concentrated solar power (CSP), in particular, often offer higher efficiency than photovoltaic systems. To compensate for the fluctuating production of electricity due to changing solar radiation, molten salt is often used for heat storage in such plants. However, if the temperature of the melt falls below a limit of approximately 228°C, conventional salt compounds “freeze,” which can block lines. On the other hand, if the temperature is too high, at around 585°C, the salt dissolves and can no longer be used as a heat carrier. In addition, leakage can occur at the valves, reducing efficiency and increasing the likelihood that the melt will freeze. Therefore, extensive heat management is necessary to ensure that the temperature of the salt is stable. This can be achieved by a heating system consisting of electrical heating modules, sensors, and control units to stabilise the temperature of the molten salt at any point in the plant.

In recent years, social and political movements have led to an increased focus on renewable forms of energy in the countries of the European Union. According to data from the Energy Industries Council (EIC), more renewable energy projects are currently being planned and implemented in the EU area than in any other region in the world. The same applies to solar thermal generation with concentrated radiation (CSP) in particular, which, in contrast to now controversial wind energy, is characterised by particularly low environmental impact. The European Solar Thermal Electricity Association (ESTELA) puts the number of ongoing systems in Europe at 2,385, with a further 588 systems being planned. Spain, for example, as the European pioneer of solar thermal energy, is working on new legislation that aims to launch new projects by 2024. However, to make this form of energy generation sustainable and at the same time economical, it is necessary to optimise efficiency on the one hand, and minimise disruptive factors in operation on the other.

Molten salt as standard solar energy storage

An essential starting point for the efficient and cost-saving use of CSP systems is the heat carrier circulating in the system. In the example of a CSP plant used here, sunlight is concentrated by mirrors onto a central tower, which absorbs the light and thus transfers the heat to a liquid energy source inside the tower. Here, the medium often used is molten salts, which are superior to thermal oil because of their properties: while the oil can be used only up to about 400°C, molten salt is stable up to about 565°C. By this means, steam can be generated at a higher temperature, which has a positive effect on the efficiency of the steam turbine and thus on the energy generated in the power generator. For this reason, chemical compounds such as NaNO3 and KNO3, which must first be preheated to a temperature of about 265°C to be able to circulate, have proven themselves for some time. After the melt has been further heated by the solar heat in the central absorber to about 565°C, the salt first flows into a storage tank where it is kept at a constant temperature. Depending on the system, it can remain there for several hours in order to provide heat or energy at night or during cloud cover. The plant then pumps the salt to a steam generator where the heat of the salt is used to produce steam from water. During this process, the salt cools down and is then fed back into the cycle. The resulting steam in turn operates a steam turbine and an electricity generator, which ultimately generates energy.

Freezing and leakage are primary risks in the use of molten salt for solar energy

However, in this complex process, which is characterised by very large differences in temperature, difficulties arise that can affect both the efficiency and the condition of the system. As the melt makes its way back to the central tower from the steam generator, there is a risk that the temperature of the salt will fall below a specific limit of about 228°C, and the salt will solidify (known as “freezing”). This presents a great risk for the plant, as the salt can clog pipes and consequently shut down the entire process. At the same time, a significant amount of energy is required to reliquefy the solidified salt. This results in a poor energy balance and endangers the profitability of the plant. A further risk is that leaks may occur at the valves in the pipes used. This in turn reduces the temperature of the melt, and the probability of freezing increases. The loss of salt also has a negative effect on the efficiency of the plant and may lead to downtime if the valves need to be repaired. Excessive heating of the salt is also critical: if a temperature limit of about 585°C, which varies depending on the molten salt, is exceeded, the salt dissolves and can no longer be used.

Temperature management ensures stable molten salt circulation

To counter these problems, the temperature is constantly monitored and regulated by a heating system. This requires sensitive temperature sensors, which are installed in the storage tanks as well as in the inlet and outlet tanks of the central tower. For a constant temperature of the melt in the storage tanks, powerful heating elements are also required. This task is usually performed by six to eight immersion heating elements, which are mounted in an additional cladding tube, and each has a length of about 5m (16ft). The materials used in the tanks must also be corrosion resistant and suitable for high temperatures. The austenitic iron-nickel-chromium alloy Alloy 800 or the special steel SS347H, for example, are suitable for the shells of the heating elements. This means that temperatures up to 600°C are no problem, the material is corrosion resistant and stable even at low temperatures. Alternatively, parts exposed to media may also be made of chrome-nickel stainless steel AISI 347H, which also tolerates high temperatures and is resistant to intergranular corrosion.

But comprehensive temperature management is required not only in the storage tanks: numerous temperature sensors and controllers must be installed in the absorbing tower also, to ensure a uniform flow of the melt. To ensure that the temperature remains constant there, all lines carrying medium are fitted with high-temperature, tubular heating elements, which are characterized by a particularly short heating time and are themselves heat resistant up to 982°C, thanks to the use of Alloy 800 or special steel SS347H. At the same time, the temperature of the melt can be controlled with the aid of control technology in such a way that no locally limited cold zones occur. As an option, mineral-insulated cables can also be used to heat the lines. Overall, the heat management system ensures that the temperature is monitored without interruption, and thus the molten salt can be used without any costly downtimes or loss of efficiency.

Tim Bruewer is with Watlow

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