Renewable energy has an important contribution to make in protecting the environment and climate. Solar energy plays a particularly significant role here and has become more widespread in recent years due to photovoltaics. However, this technology does not yet offer an efficient means of energy storage. An alternative is concentrated solar power (CSP) plants with heat storage systems. Heat is stored in molten salt so that electricity can be generated during the night or in bad weather conditions. Despite advancements in this technology, the investment costs in building a CSP plant are still very high. Therefore, it is all the more crucial to run the power plants, including the heat storage systems, as efficiently, economically and for as long as possible. BASF has developed a new software solution to help with challenges in this area. The tool focuses on the stability of the storage medium (i.e. the molten salt), which includes the simulation of gas emissions, equipment corrosion and the estimation of the plant life span.
BASF plays a key role in the CSP industry: The German chemical company produces inorganic salts, which are used as an energy storage medium in concentrated solar power plants. As such, the company is one of the suppliers of sodium nitrate for Noor Energy 1 in Dubai – the largest solar project to date. As a producer of salts for such a long time, BASF has gained expertise through intensive research and can use this knowledge to support the planning, construction and operations of CSP plants. “Over the last few years, BASF has intensively examined the chemical reactions and equilibriums of molten salts at high temperatures. We have now incorporated this knowledge in a software solution so that we can make it available to the CSP industry,” said Christian Schütz, research engineer in process development at BASF. “Thanks to our extensive research, we have a very good knowledge of salt chemistry at temperatures of over 400°C.” This has a significant meaning especially for CSP tower plants, which operate up to 565°C.
The high temperatures in CSP plants are a challenge here: The salt is heated to over 400 °C using concentrated sunlight and then, when needed, emits the thermal energy again using a steam generator. The advantage of the high temperatures is that the same volume of salt can absorb more energy, while the efficiency of the cyclical process for converting the steam into electricity also increases along with the temperature. Basically, this means the higher the temperature, the more heat is stored, and the more energy is generated. “However, different reactions in the salt become evident above 400°C,” explained Schütz. For instance, the equilibrium of sodium nitrate and sodium nitrite shifts, as individual components of the salt decompose at higher temperatures, releasing different types of gases. The loss of gases translates into a loss of salt. “This is especially apparent the first time the plant is heated up and the salt melting process starts. This is when the greatest mass loss occurs for the plant operator.”. If it is not prevented, this mass loss must be compensated at regular intervals.
The new simulation tool from BASF allows mass loss to be predicted and counteracted by adjusting parameters such as temperature or additives. “We calculated this extremely accurately using equations,” commented Schütz, adding that those equations are derived from lab work of the last 6 years. Tests were conducted to establish how the salt mixture behaves under a wide range of temperature and pressure conditions and how the repeated heating and cooling affects long-term stability. These changes that take place over the long term, i.e. the “aging” of the salt over a number of years, can also be simulated using the software. A further function is the calculation of the corrosion rate of the power plant components, as the salt reacts with metals at high temperatures. As the corrosion rate has a direct influence on the life span of the plant, the tool can also be used to provide information on the predicted life span of the plant. As such, the tool offers operators an overview of the wear and efficiency of their power plant with respect to the salt cycle. There are default parameters for each individual module of the power plant, which can each be adjusted to the power plant in question. “The aim of our simulation tool is to control the stability of the salt in such a way that they run efficiently and for as long as possible,” emphasized Erik Wiegert, digital expert in the Inorganics division at BASF.
The software is already available as a fully functional prototype. If there is sufficient interest from the industry, BASF aims to add more functions to the program at a later stage and also offer it alongside its salt business. As well as offering added value in plants that are already up and running, the tool can also be used when designing new power plants, added Erik Wiegert: “We use the tool to pass on our salt expertise to operators.”