Dr. Anna Chiara Tizzoni

1. Storage for parabolic CSP plants, two tanks vs. one tank configuration. Innovations expected in OPTS project Dr. Anna Chiara Tizzoni Rome, 15 September 2014

2. PTSP configuration scheme Where HTF and the HSM are the same material, there is no need of an intermediate heat exchanger (HX) between them. This configuration is employed by the ENEA developed plants and facilities, in particular, it is present in the Archimedes solar power plant located in the south eastern part of Sicily . Storage for parabolic CSP plants

3. Two tanks systems • The HTF exiting the parabolic trough receiver is fluxed into a “hot tank” (usually presenting a temperature around 550 °C), then the HTF/HTM flows into a “cold tank”, feeding a HTM/steam HX placed in between the two tanks. The “cold” HTM/HTF (at a temperature about 290 °C) is introduced into the parabolic trough heating system, in order to have it heated up again. • At night, the temperature difference between the two tanks is maintained by the sensible heat stored into the “hot tank” during the daytime. Storage for parabolic CSP plants

4. One tank: OPTS project • OPTS project aims at developing a new Thermal Energy Storage (TES) system based on single tank configuration using stratifying Molten Salts (MS) as heat storage medium at 550°C maximum temperature, integrated with a Steam Generator (SG). • The system is intended to provide efficient, reliable and economic energy storage for the next generation of trough and tower plants. Storage for parabolic CSP plants

5. One tank: OPTS project • The three year experimental program will be focused to the full development of the integrated system (TES-SG) up to demonstration level. • The SG, with natural recirculation of the MS, can be positioned either directly into the tank (Pool-type) or as an external shell-and-tube once-through SG (Loop-type) with piping system and pump. Storage for parabolic CSP plants

6. Concepts • Among the renewable energies technologies, a distinct advantage of CSP is the possibility of using relatively cheap Thermal Energy Storage (TES) systems. The TES concepts highly depend on the daily/yearly variation of solar radiation and on the power load profile. • Each TES concept aims to collect energy in order to shift its delivery to a later time, or to smooth out the plant output during intermittently cloudy weather conditions. Storage for parabolic CSP plants

7. Concepts • In this way the dispatchability of energy by a CSP system can be extended beyond periods of no solar radiation minimizing the need of burning fossil or renewable fuels in hybrid or backed-up systems. • Various are the TES options considered worldwide today; it is necessary to point out the ones having the best potential to provide efficient, reliable and economic energy storage for CSP plants. Storage for parabolic CSP plants

8. Concepts The different TES options can be obtained by crossing the mechanism of storage (sensible heat, latent heat, chemical state/solution) with the suitable combination of different constructive characteristics (design concept, HSM and HTF). Storage for parabolic CSP plants

9. Concepts • The design of an effective TES system shall have to be based primarily on its thermal capacity and tailored on the features of the various CSP technologies , including the storage temperature range (midrange 200-400 °C, high: 400-600 °C, highest: >600°C). • Moreover every TES system has to be evaluated by cost-benefit criteria in the respect of all the parameters that affect its design, operation, maintenance and performances. Storage for parabolic CSP plants

10. …and objectives • OPTS will focus on the study of an innovative concept of a TES system based on the single tank configuration using stratifying Molten Salts (MS) as both HSM and HTF and with integrated Steam Generator (SG). • The general objective of the present proposal is to assess and to develop this TES concept, chosen among all the best concepts able to fit to all the fundamental aspects resumed above, without trying to say that this concept is the best among all. Storage for parabolic CSP plants

11. These SGs, of both types, are foreseen to be of modular types and they can be used in n-multiple units, inserted in larger tanks for the Pool-type, or distributed around them for Loop-type, to fit the rated power of bigger solar CSP plants, e.g. 125 MWth/50MW. • Another proposed innovation consists in the possible presence of a dedicated back-up MS heater, able to heat directly the salts mixture in the absence of solar insolation, possibly using renewable energy sources (e.g., gasified biomass). Storage for parabolic CSP plants

12. Study of HTF/HTM • A key point for the assessment of the TES system is a complete knowledge of the thermophysical and chemical properties of the employed HTF/HSM. Storage for parabolic CSP plants

13. The experimental program The integration with the SG can be realized in two different ways: • The first one is very innovative: the SG is positioned directly into the tank (Pool-type) where it carries out the MS stratification, and it operates at natural recirculation of the MS shell-side with a drastic simplification of the process loops and with a more compact and manageable device, so avoiding external piping systems and pump.  • The second one is an external (Loop-type) shell-and-tube once-through SG, with respective piping system and pump, always working with natural recirculation of the MS so helping to determine the stratification of the salts mixture in the tank, even if it gives fewer advantages in terms of reduction of pipes and components. Storage for parabolic CSP plants

14. Thank you for your attention!