Cycle cut-off criterion effect on the performance of cascaded, sensible, combined sensible-latent heat storage tank for concentrating solar power plants

Highlights

• The thermal performance of the different thermocline tank configurations is analyzed.

• The SHS configuration demonstrated the shortest charge/discharge time of 239/207 min.

• At a higher charge cut-off value, the CSLHS case has the highest capacity ratio of 90.55%.

• The MLPCMs-4 case has the highest recovered energy of 183.2 MWh with a lower charge cut-off value.

Abstract

Thermocline thermal energy storage (TES) technology that uses the molten salt as heat transfer fluid is cheaper than a traditional two-tank structure, owing to its composite design and use of inexpensive filler materials. Nevertheless, this TES type suffers from the significant disadvantage of low capacity and utilization ratio when it is integrated with concentrating solar power plants due to moderate charge/discharge cut-off values. The primary purpose of the current research is to investigate and evaluate the impact of the change charge/discharge cut-off values on the thermal performance of six different TES tank configurations. First, the equations for the dispersion-concentric model were approached through the MATLAB program, and then the present research results are authenticated. The results show that the thermocline zones shrink when the charge cut-off value rises, and the discharge cut-off value reduces because a higher charge cut-off value and a smaller discharge cut-off value permit a longer charge/discharge time. Moreover, when the cut-off value of the charge/discharge changes simultaneously, the combined sensible-latent heat storage configuration has the highest capacity ratio, utilization ratio, recovered energy, and overall efficiency, which equals 90.55%, 83.3%, 147.2 MWh, 91.3%, respectively, at low charge cut-off value.

Introduction

Thermal energy storage (TES) that is used to store thermal energy for future use [1] offers an inexpensive and convenient way to overcome the imbalance between energy demand and supply in different energy sectors [2]. There are many applications for the use of TES, for instance, in high-temperature fields, which include concentrating solar power (CSP) plants [3], adiabatic compressed air energy storage [4], and high-temperature waste heat recovery in manufacturing operations [5]. TES is of great importance as it enhances the flexibility of CSP plants and improves the thermal stability of the power block [6].

There are currently numerous high temperature TES technologies available; for instance, two TES reservoir of molten salt have been commonly used in commercial CSP plants [7]. One layer thermocline TES tank utilizing phase change materials (PCMs) attracts growing interest because of the advantages of being cheap and having a large energy storage density [8]. Amongst all those high-temperature TES innovations, a packed bed of solid particles that enables the use of air as heat transfer fluid encourages lower-cost and higher temperature storage and has already been widely studied and analyzed in recent decades [9]. A variety of studies have explored numerous approaches to enhance the charge and discharge performance of the thermocline TES tank by utilizing various forms of specific heat materials [10], a one PCM TES tank with various melting points [11], multilayers PCMs [12], and integrated sensible-latent heat thermocline TES system [13]. Various laboratory testing has been performed to check the operational viability and merits of the TES tank system. Elfeky et al. [14] investigated the thermal and economic performance of the three-layer thermocline TES tank by changing the volume fraction of each PCM layer. Bruch et al. [15] designed an oil/rock thermocline TES tank and conducted several charge/discharge cycles. Bellan et al. [16] proposed a laboratory-scale experimental configuration of a high-temperature latent TES tank to utilize sodium nitrate as an encapsulated PCM and air as a heat transfer fluid. Wang et al. [17] have developed a detailed parametric analysis of the molten-salt thermocline tank's combined thermal and mechanical efficiency. The findings demonstrated that the molten salt inlet velocity, cold molten salt temperature, porosity of the porous bed, capsule shell thickness, thermal conductivity, and the particular latent heat of the storage filler all have reasonably clear implications on the discharge efficiency of the tank.

Lately, due to the superior thermal performance and energy efficiency of cascading TES configuration over the single-stage latent heat TES, the term of the latent heat cascaded TES with multiple PCM has intrigued widespread attention among the energy scientists sector. In our previous work [11], the thermal efficiency of one layer at several melting temperatures and the architecture of three layers were examined. The characteristics of the thermal performance of a three-layer tank at various filling ratios of the encapsulated PCMs were studied by Zhao et al. [18]. Li et al. [19] examined a conventional single-layer and modern compacted multi-layered molten-salt TES tank thermal efficiency. Cheng et al. [20] established a comparison of a three-layer and one-layer structure by evaluating the thermal performance of the TES tank system. The thermal analysis of cascading latent heat storage systems in CSP plants was studied by Chirino et al. [21]. The effect of various alignments as well as the significance of multiple porosity coefficients and the height of the layer on the overall thermal efficiency of the thermocline tank was explored by Mohammadnejad et al. [22]. Khor et al. [23] tested the thermal performance of a multi-layer TES tank to use the PCMs in the tank to generate high storage volumetric potential. Li et al. [24] studied the thermal performance characteristics of the combination Brayton-Rankine system power plants attached to the multilayers TES tank system.

In spite of the benefit of cost savings, the molten-salt thermocline TES tank experiences a significant disadvantage when incorporated into CSP plants because the outlet temperature variation near the end of the charges and discharges may disrupt the normal operation of the receiver and steam turbine components of the system. Unique cut-off temperatures are defined as termination criteria for both operations to address this issue. González et al. [25] contrasted the thermal efficiency of a responsive thermocline TES tank system with different charging cut-off temperatures. Zhao et al. [26] explored the possibility of taking less restrictive cut-off temperatures to boost the efficiency of the TES tank. Modi et al. [27] examined the storage ability of sensible heat TES tank systems with various temperature fluctuations (100 °C and 280 °C) within various cut-off temperature variations (10 °C, 20 °C, and 30 °C). In general, the cut-off temperatures for the TES tank incorporated in a molten-salt CSP turret plant have been selected as a 320 °C during charge and 540 °C during discharge [28]. Such conservative cut-off temperatures result in low capacity factors approximately 50–80% for different thermocline TES tank configurations [29]. Biencinto et al. [30] specified a charge and discharge cut-off temperatures of 350 °C and 430 °C, respectively, for the thermocline TES tank. Liao et al. [31] examined the overall efficiency of rock alone and the mixture of rock/PCM capsules utilizing air as a heat transfer fluid. Fasquelle et al. [32] tested the principle of complex cut-off temperatures to improve the activity of the sensible heat thermocline TES tank by experimental work on a 150 kWt parabolic trough mini power plant.

A literature survey previously conducted showed that limited studies are focusing on evaluating the thermal performance of the thermocline TES system under the influence of different charge-discharge cut-off temperatures for various tank configurations, especially for multilayer phase change materials (MLPCMs) configuration with various PCM layer thickness. Nevertheless, the thermal performance of the TES tank consisting of three identical layers of PCM has been comprehensively investigated in previous research, using a different numerical strategy. In contrast, the performance of this system has not been studied with variable PCM layer thickness, sensible heat storage (SHS), and combined sensible-latent heat storage (CSLHS) TES system under the influence of different charge/discharge cut-off values together. To address these shortcomings, the present study is conducted to investigate the impact of many different charge/discharge cut-off values on the dynamic performance of the thermocline TES tank through six arrangements, depending on a one-dimensional dispersion-concentric (D-C) numerical model which has been advanced in our preceding researches [33]. In the current study, six different TES tank configurations are investigated. After completing the charge/discharge cycles, a comparison will be made between the molten salt temperature distribution, the charge time, discharge time, the capacity ratio, the utilization ratio, the recovered energy, and the overall efficiency of all the studied configurations for three different stages as follows: (1) studying the impact of various values of charge cut-off with a constant value of discharge cut-off; (2) investigating the effect of multiple values of discharge cut-off with a constant value of charge cut-off; and (3) studying the impact of changing both charges and discharge cut-off values simultaneously. We think that the current research could participate in strengthening the knowledge of the thermal performance improvement of the thermocline TES tank technology incorporated into CSP plants.