Solar Salt at 620°C - Is it Worth it?

Abstract

The generation of dispatchable electricity from CSP, uses energy storage in the form of molten nitrate salt heated to up to 565 °C. This type of storage has become state of the art in most newly built CSP plants, offering a cost-effective way to extent power production even until the next morning. Although working after the same principle as modern fossil fired power plants, CSP plants are not as efficient. One challenge is that the maximum steam temperature is limited by the maximum temperature of the salt. Hence, the power block operates at sub-critical steam values in CSP. Raising the temperature of the molten salt to 620°C could offer a considerable efficiency improvement ultimately allowing for supercritical steam temperatures. Besides solar power towers there are two more applications where this approach is considered to play a substantial role: - One promising application is in the area of hybrid CSP-PV plants with electric heaters where increasing the salt temperature can result in an increase in efficiency and possibly even reduce the cost of electricity. Here the molten salt storage is virtually used as a battery for the PV-part of the plant. - Another promising implementation is in existing fossil-fired power plants, where thermal energy storage can be used to convert them into thermal storage power plants capable of storing excess electricity from wind and PV on a large scale and independent of geography [1]. With an increased salt temperature, even highly efficient supercritical steam power plants can be retrofitted with low-cost molten salt storage. To answer the question of "is it worth it?" with respect to a temperature elevation of solar salt it is looked at various aspects: - What happens to the solar salt itself when operated constantly at 620°C maximum temperature or exposed temporarily to an even hotter surface? While the problem of an increased permanent temperature is quite well understood, the influence of a hot wall is currently still investigated experimentally. Thermal stability of molten nitrate salt can be maintained by providing specially composed gas phases in the storage tanks [2,3]. - What are the challenges for molten salt components at 620°C? In various nationally funded projects pumps, valves, tanks and gas systems are developed and investigated. The current status of recent project activities at DLR will be presented in this work. - How big is the efficiency increase and how does this compare to potential increase in investment costs? A case study for a solar power tower with molten salt storage at 620°C reveals the potential that comes with increasing the salt temperature. This work gives an overview on the latest findings in the field of material stability, component development and system integration for molten nitrate salt storage systems with an increased temperature of 620°C.