Performance investigation of single-tank thermocline storage systems for CSP plants

Highlights

• Numerical analysis of single tank thermocline system with different HTFs.

• Effect of porosity on effectiveness of the tank with different HTFs.

• Study of charging and discharging characteristics on the performance of the tank.

• Study of operation and design parameters of the tank.

Abstract

To address the intermittency in solar energy system due to unforeseen weather conditions and to improve dispachability, the requirement of Thermal Energy Storage (TES) system is becoming increasingly inevitable. A single tank packed bed thermocline TES system has potential to provide an effective solution. A comprehensive one-dimensional non-thermal equilibrium model is considered and is solved using method of characteristics to investigate energy storage in a single tank packed bed thermocline storage system. Thermal characteristics including temperature profiles and discharge effectiveness with different commercially available Heat Transfer Fluids (HTFs) are explored and analyzed. A tank size of 12.5 m in height and 8 m in diameter is considered with a capacity of 40 MW h_t for the present study. The effect of porosity on discharge effectiveness of the storage tank with different HTFs is investigated with porosity, ε varied from 0.1 to 0.7. It is found that the discharge effectiveness consistently drops when ε value increases from 0.1 to 0.7 for all the HTFs. Two-dimensional numerical simulation is also carried out to explore the transient temperature distribution inside the tank for different HTFs with a porosity of 0.2. It is observed that as the operation time of the tank increases, filler material eventually loses heat to adjacent fluid and attains equilibrium. The thermocline moves at a faster rate in Solar Salt than the HITEC due to its higher viscosity.

Introduction

With proliferation of problems associated with the energy crisis, global warming and other environmental issues, surge of opinions and agreements leading to the development and utilization of nascent renewable energies have become issues of pressing concern. Concentrating Solar Power (CSP) technology is among the front-runners in main stream technologies developed so far. CSP power plants also named Solar Energy Generating Systems (SEGS) are turning out to be one of the most entrusted technologies for transforming solar energy into electricity on a large scale (Joseph et al., 2013, Kuravi et al., 2013). TES is a pertinent element in solar energy applications, enabling the power plant to be self-reliant, independent of the existing sunlight conditions and unforeseen weather conditions (Izquierdo et al., 2010). It has been an undisputed fact that a substantial cost reduction in concentrated solar thermal power plants can be achieved by incorporating TES system which facilitates heat for extended duration and henceforth multiplying the operational capacity of the power plant and at the same time augmenting the ability of power dispatch (Sanderson and Cunningham, 1995, Mawire and McPherson, 2009, Singh et al., 2010, Stekli et al., 2013). In a CSP plant, another salient feature is that the TES system makes the power plant more versatile by allowing the power plant to distribute power to meet the peak demand. Hence an extensive investigation of TES is of profound importance to the solar thermal applications (Gil et al., 2010). The current contemporary design for SEGS utilizes synthetic oil (Therminol VP -1) as HTF in the collector field. Existing CSP plants uses an expensive sensible heat-based TES system widely known as two-tank storage, where hot and cold fluid is stored in two separate tank volumes (Herrmann et al., 2004). A frugal system replaces the two-tank concept (Pacheco et al., 2002) to a single tank, where the hot and cold fluid is contained in a single tank, in a so called thermocline. Physical separation of the hot and cold region is maintained via buoyancy force (cold salt is denser than hot salt) which induces stratification leading to two isothermal regions along the vertical direction. A narrow layer of substantial temperature gradient grows at the interface between the hot and cold region known as the thermocline or heat transfer region. In addition to the HTF, thermocline tank is also filled with an inexpensive filler material that diminishes the volume of expensive HTF required for storage and enhances the fluid mixing that is conducive for thermal stratification.

In case of parabolic troughs and power towers, the heat output to the power block is limited to the selection of the HTF materials, i.e., synthetic oils and molten salts that remains in liquid state at very high temperatures. The operating temperatures of different commercially available HTFs are illustrated (Kearney et al., 2003). Synthetic oils such as Therminol VP-1 are a promising option as they remain in liquid state under ambient temperatures. However, due to high vapor pressures the usage of such oils is restricted to temperatures below 400 °C, truncating the performance of the plant. Substantial increase in thermal efficiency can be established with a transition to molten nitrate salt mixtures, which is stable in liquid states up to a temperature of 600 °C (Kearney et al., 2003). Furthermore, molten salts are low-cost, non-flammable, and non-toxic. Commercial salt mixtures include Solar Salt (60 wt% NaNO₃, 40 wt% KNO₃), HITEC (53 wt% KNO₃, 40 wt% NaNO₂, 7 wt% NaNO₃), and HITEC XL (48 wt% Ca(NO₃)₂, 45 wt% KNO₃, 7 wt% NaNO₃. Concrete is considered to be a desirable material selection because of its low cost, but its relatively low thermal conductivity poses a threat to an effective heat exchange between the HTF (Laing et al., 2006). Py et al. (2011) used the idea of recycled asbestos containing waste for solid storage. A comparison made found a similar heat capacity with an order-of-magnitude reduction in cost.

In order to ensure the thermal stability of rock candidates Pacheco et al. (2002) immersed candidates in HITEC XL molten salt for several hundred thermal cycles in the range of 290 °C and 400 °C. Out of the candidate’s quartzite, taconite, and silica sand were found to be the most resilient, with quartzite rock and silica sand being most promising due to low material costs. Pomeroy (1979) developed a procedure to dampen de-stratification in a porous bed of liquid sodium HTF and iron spheres. Bharathan and Glatzmaier (2009) performed a computational fluid dynamics (CFD) analysis of forced convection between molten salt and a solid quartzite sphere. Afrin et al. (2013) performed a parametric to investigate flow mal-distribution inside the porous bed. Bayón and Rojas (2013) recently did analysis on one-dimensional single-phase model, taking into account thermal equilibrium between the liquid and solid filler, and developed a design equation in order to optimize thermocline tank height. Van Lew et al. (2011) developed one-dimensional numerical solutions using the method of characteristics with minimal computational times. It was found from the study that the storage efficiency of cyclic charge and discharge processes increased with tank height and reduced filler size. Valmiki et al. (2012) used a model to study tank performance by integrating with a parabolic trough solar collector. Xu et al. (2012) analyzed the validity of a lumped capacitance assumption for the solid filler and introduced effective convection coefficients to correct for large Biot numbers.

Yang and Garimella (2010) did an extensive analysis using a comprehensive model of mass, momentum, and two-temperature energy transport inside the thermocline porous bed. The investigation used multidimensional tank geometries and non-adiabatic tank wall boundary conditions. The authors carried out a parametric study of thermocline with parameters tank height, filler diameter, and molten-salt discharge power in order to optimize tank discharge efficiency in case of both adiabatic and non-adiabatic tank boundary conditions. Then the authors developed a general correlation of storage efficiency for varying Reynolds number and cycle duration by extending the boundary conditions to an adiabatic wall. Xu et al. (2012) studied a multidimensional tank geometry and performed CFD analysis to study forced convection and thermal diffusion inside the thermocline porous bed. It was found that implementation of multiple correlations for convection and effective thermal conductivity did not substantially affect tank discharge behavior. An updated energy transport equations with a dispersion concentric model was later used by eliminating the assumption of lumped capacitance in the solid filler particles. As anticipated the resultant temperature distributions inside the solid were found to be negligible for small particle sizes. Kolb and Hassani (2006) studied performance of thermocline tank at a system level using TRNSYS model for a 1 MWe parabolic trough plant. The authors did an analysis on annual plant performance without storage and with the addition of a 30 MW h_t synthetic oil-filled thermocline tank. Integration of thermocline storage was found to almost double annual plant capacity factor from 23% to 42%. Powell and Edgar (2013) used a novel; adaptive-grid one-dimensional model for representing thermocline thermal energy storage systems. They concluded that adaptive-grid model has an improved accuracy compared to standard, fixed-grid one-dimensional models. So far studies carried out were mostly based on the filler material and thermal analysis is performed using the same. The studies with HTFs were found to be scarce which is equally important. The present work is intended to investigate the TES thermocline system for SEGS with different HTFs namely therminol oil, Solar Salt and HITEC. In this present work different HTFs are studied by taking discharge effectiveness as a pivotal parameter to analyze the influence of bed porosity, HTF, design and operating parameters. Furthermore, a transient behavior of the tank is studied to get a better understanding using different HTFs.