Latent heat augmentation of thermocline energy storage for concentrating solar power – A system-level assessment

Abstract

Molten-salt thermocline tanks are a low-cost energy storage option for concentrating solar power plants. Despite the potential economic advantage, the capacity of thermocline tanks to store sufficient amounts of high-temperature heat is limited by the low energy density of the constituent sensible-heat storage media. A promising design modification replaces conventional rock filler inside the tank with an encapsulated phase-change material (PCM), contributing a latent heat storage mechanism to increase the overall energy density. The current study presents a new finite-volume approach to simulate mass and energy transport inside a latent heat thermocline tank at low computational cost. This storage model is then integrated into a systemlevel model of a molten-salt power tower plant to inform tank operation with respect to realistic solar collection and power production. With this system model, PCMs with different melting temperatures and heats of fusion are evaluated for their viability in latent heat storage for solar plants.Thermocline tanks filled with a single PCM do not yield a substantial increase in annual storage or plant output over a conventional rock-filled tank of equal size. As the melting temperature and heat of fusion are increased, the ability of the PCM to support steam generation improves but the corresponding ability of the thermocline tank to utilize this available latent heat decreases. This trend results from an inherent deconstruction of the heat-exchange region inside the tank between sensible and latent heat transfer, preventing effective use of the added phase change for daily plant operations. This problem can be circumvented with a cascaded filler structure composed of multiple PCMs with their melting temperatures tuned along the tank height. However, storage benefits with these cascaded tank structures are shown to be highly sensitive to the proper selection of the PCM melting points relative to the thermocline tank operating temperatures.3

Macroencapsulation and characterization of phase change materials for latent heat thermal energy storage systems

“…discharging is a major concern for LHS systems since most of the PCMs have very low thermal conductivity [2]. A number of methods have been proposed to increase the thermal conductivity of PCMs [3][4][5][6][7][8][9]. The finned tube configuration [9][10][11][12], metal structure insertion into the PCMs [13,14], and dispersion of thermally conductive nano or micro particles (graphite flakes, CNTs, graphene, etc.)…”

Encapsulated High Temperature PCM as Active Filler Material in a Thermocline-based Thermal Storage System

“…They act as a sacrificial layer which is removed by heat treatment and leads to a void space between the PCM and the shell that will allow the expansion of the PCM when melting and protect the PCM from dissolving in water as well as in several organic solvents. This route has been also found in the state of the art analysis by University of South Florida [13,14].…”

“…A recent publication [13] shows the simulation of tank with a cascaded filler structure composed of multiple encapsulated PCM with their melting temperatures tuned along the tank height. It has been concluded that modification of the porous bed with a cascaded PCM structure increases the utilization of the latent heat and can provide significant improvement over conventional quartzite rock filler.…”

Compatibility tests between Solar Salt and thermal storage ceramics from inorganic industrial wastes

“…Yang and Garimella [15] proposed a comprehensive, two-temperature model developed for the cyclic operation of a thermocline operating with a commercially available molten salt as the heat transfer fluid and quartzite rock as the filler. In order to increase the overall energy density, Fluckiger and Garimella [16] studied a promising design modification using an encapsulated phase-change material (PCM) as filler material. The utilization of latent heat can provide significant improvement over conventional quartzite rock filler.…”