Fonteret, E. (2013). Modelling of vaporisation and condensation processes in a latent heat storage plant [Diploma Thesis, Technische Universität Wien]. reposiTUm. http://hdl.handle.net/20.500.12708/160663
latent energy storage; PCM; heat transfer correlations; APROS; six-equation model; two-phase flow
en
Abstract:
The increase of the use of renewable energy sources seems to be inevitable in order to address the climate problem. Their main drawback though is the common temporal mismatch between energy generation and use, which is aggravated by the fact that efficient storage solutions are still under development. This thesis is incorporated within the framework of Thermal Energy Storage systems, whose aim is to store and release energy during successive cycles in order to allow the decoupling of the energy generation and its transmission on the grid. The work presented here is based on the example of the 22 MWel solar thermal power plant detailed in [1]. The goal of this thesis is to model and simulate both charging and discharging processes in the latent heat storage system of the plant, using the process simulation software APROS and its six-equation model. Sodium nitrate NaNO3 is used as PCM (Phase Change Material). In charge mode, condensation of the steam occurs and the PCM melts; in discharge mode, liquid water vaporises and the PCM solidifies. The non-uniformity of the melt (respectively, the solidification) along the height of the heat exchanger has been taken into account in the model, as well as the strong dependence of the heat transfer on the salt fraction being already frozen or molten. The heat transfer calculation is based on the evaluation of the inner (water/steam side) and the outer (salt side) heat transfer coefficients, which are both variable over the height of the heat exchanger. Different scenarios in an assisted circulation configuration have been investigated (steam injection below the heat exchanger or into the drum), as well as a once-through mode for charging. The latter proved to be less advantageous. On the contrary, the effect of a sliding circulation mass flow turned out to be interesting to limit the pressure increase. The effect of the variation of the entry velocity of steam (slip ratio) has also been investigated.