Single-tank thermal energy storage systems for concentrated solar power: Flow distribution optimization for thermocline evolution management

Abstract

Concentrated Solar Power (CSP) technology captures solar radiation and converts it into heat for electricity production. It has received an increasing attention because integrated thermal energy storage (TES) systems can largely enhancing the reliability and the dispatchability. Over the last decade, low-cost single storage tank based on the thermocline technology becomes an alternative to commonly-used two-tank TES system. However, the improper inlet/outlet manifolds may cause the strong mixing of hot and cold fluids and disturb the temperature stratification, resulting in reduced thermal performances of the storage tank. This study aims at solving the flow maldistribution problem in the single-tank thermocline storage system by appropriately structuring the inlet/outlet manifolds. The technical solution is based on the insertion of optimized perforated baffles in the manifolds. 2D Computational fluid dynamics simulations were performed to calculate the transient flow and temperature profiles in the storage tank during the charging and discharging operations. The optimal size distribution of orifices on the upper baffle has been determined for homogenizing passage times of the thermal front, so as to enhance the temperature stratification. A novel intermediate evaluation indicator was introduced to characterize the real-time thermal behavior, which could reduce the computational cost of the optimization problem by a factor of 6 at least. Numerical results shown that the proposed optimization algorithm could significantly improve the thermal performances, indicated by the increased values of charging/discharging efficiency, the capacity ratio and the overall efficiency, ex., the fully charging efficiency be increased by 29% by comparing the unstructured manifold geometry and the one with optimized baffles. The parametric study on certain geometry and operating factors also demonstrated that the proposed method for flow distribution optimization was robust, effective and efficient.

Tailoring the fluid flow distribution in a parallel mini-channel heat sink under multiple-peak heat flux

“…Under these circumstances, the intentional flow non-uniform distribution may be a better option to be adapted to decrease the local maximum temperature, as pointed out by Kumar and Singh [13]. This is actually in line with some observations reported in the literature [30][31][32] in that the optimal flow distribution is usually not uniform but obeys certain trends subjected to a defined optimization objective and constraints. In this regard, Kumar and Singh [13] indicated that the flow arrangement and the actual flow distribution should fit the heat flux shape to achieve a better thermal performance, i.e., lower maximum temperature and thermal resistance.…”

A Review on the Performance Indicators and Influencing Factors for the Thermocline Thermal Energy Storage Systems

“…The capacity ratio (σ) is the ratio of the actual stored thermal energy to the maximum energy storage capacity [12],…”

“…In practice, a high-steep slope for the outlet temperature curve is expected to have a positive effect on the capacity ratio. Thus, as a data analysis, a dimensionless indicator called the tail factor is calculated from the dimensionless outlet temperature profile against the dimensionless time [12]. The tail factor is independent on the thermal properties of HTF, the geometries, and the operating conditions.…”

“…Furthermore, an original CFD-based optimization algorithm was developed by Lou et al [12] to determine the optimal flow distribution and restricted thermocline propagation manner using a SMT tank at high temperature as an example. A practical method for homogenizing residence times of the thermal front in order to flatten the thermocline zone was proposed, based on the insertion of a geometrically optimized perforated.…”

“…Note that the performance indicators of Category #1 can be defined based on the first law of thermodynamics (e.g., the charging and discharging efficiencies), the second law of The evaluation criteria of the performance (i.e., the performance indicators) are of importance for guiding the design of thermocline TES systems. In fact, researchers have proposed many performance indicators [4], including the charging and discharging efficiencies [11], the capacity ratio [12], the exergy efficiency [13], the tail factor [12], and the stratification number (Str) [14]. Nevertheless, a clear classification and analysis on these performance indicators are lacking among the existing literature.…”