Liquid Thermal Storage Research Articles

Simplified pumped thermal energy storage using a two-way Stirling cycle

The present work introduces a concept of pumped thermal energy storage based on the Stirling cycle. It provides a smaller sized energy storage and power conversion unit than what is typically proposed using pumped thermal energy storage systems, while offering high round-trip efficiencies despite a simple concept. Conventional systems are typically Brayton cycle based with two-tank liquid thermal storages on both the hot and the cold side. The proposed concept utilizes a single Stirling machine for both energy discharge and charge, and this is the first study where this approach is investigated. It is combined with a single-tank liquid thermal storage on the hot side and a fan and radiator combination on the cold side. Round-trip efficiencies for several storage fluids are analyzed using high-fidelity validated models, with relevant components and power losses. It is shown that round-trip efficiencies of up to 49 % at the generator are possible when using high-temperature fluids and with low-temperature synthetic fluids round-trip efficiencies of 32 % are obtained.

DSC and TGA Measurements of Room Temperature Ionic Liquids (RTILs) Containing Ammonium Alum or Aluminum Nitrate with Amide Compounds

Characterization of some room temperature ionic liquids (RTILs) as thermal storage media and heat transfer fluids in thermal applications were investigated. Five ionic liquids prepared from ammonium alum NH4Al(SO4)2.12H2O as inorganic salt with urea NH2CONH2 or acetamide CH3CONH2 as organic compounds, and aluminum nitrate salt Al(NO3)3.9H2O either with urea or acetamide compounds in two mole ratios were investigated using Thermo-Gravimetric Analysis (TGA), Differential Scanning Calorimetry (DSC), X-ray diffraction (XRD), Fourier Transform Infrared (FTIR) spectroscopy and conductivity measurements. Thermo-physical properties such as enthalpy, heat capacity and thermal energy storage capacity were determined. It was found that ammonium alum: urea (USIL) in the mole ratio (1:5) and aluminum nitrate: acetamide (AN2IL) in the mole ratio (1:22) ionic liquids to be characterized with high density, wide liquid temperature range, high chemical stability, high heat capacity and high thermal energy storage capacity. Based on experimental results, it is concluded that ionic liquids could be considered as a candidate for liquid thermal storage media and heat transfer fluids.

Evaluation of Annual Efficiencies of High Temperature Central Receiver Concentrated Solar Power Plants with Thermal Energy Storage

The current study has examined four cases of a central receiver concentrated solar power plant with thermal energy storage using the DELSOL3 and SOLERGY computer codes. The current state-of-the-art base case was compared with a theoretical high temperature case, which was based on the scaling of some input parameters and the estimation of other parameters based on performance targets from the Department of Energy SunShot Initiative. This comparison was done for both current and high temperature cases in two configurations: a surround field with an external cylindrical receiver and a north field with a single cavity receiver. The optical designs for all four cases were done using the DELSOL3 computer code; the results were then passed to the SOLERGY computer code, which uses historical typical meteorological year (TMY) data to estimate the plant performance over the course of one year of operation. Each of the four cases was sized to produce 100 MWe of gross electric power, have sensible liquid thermal storage capacity to generate electric power at full rated production level for 6hours, and have a solar multiple of 1.8.There is a fairly dramatic difference between the design point and annual average performance. The largest differences are in the solar field and receiver subsystems, and also in energy losses due to the thermal energy storage being full to capacity. Another notable finding in the current study is the relatively small difference in annual average efficiencies between the Base and High Temperature cases. For both the Surround Field and North Field cases, the increase in annual solar to electric efficiency is <2%, despite an increase in thermal to electric conversion efficiency of over 8%. The reasons for this include the increased thermal losses due to higher temperature operation and operational losses due to start-up and shut-down of plant sub-systems. Thermal energy storage can mitigate some of these losses by utilizing larger thermal energy storage to ensure that the electric power production system does not need to stop and re-start as often, but solar energy is inherently transient. Economic and cost considerations were not considered here, but will have a significant impact on solar thermal electric power production strategy and sizing.

Application of [C4min][Tf2N] Ionic Liquid as Thermal Storage and Heat Transfer Fluids

Imidazolium ionic liquids with [Tf2N]- anions were synthesized and characterized using nuclear magnetic resonance (NMR) spectrometer, differential thermal analysis (DTA) and differential scanning calorimeter (DSC). Properties of these ILs such as thermal stabilities, heat capacities and thermodynamic properties were measured. Feasibility of these ILs as liquid thermal storage and heat transfer fluids was investigated. It is found that [C4min][Tf2N] has several advantages over other ILs such as wider liquidus temperature range, lower viscosity, higher chemical stability, moderate heat capacity and storage density. The calculated storage densities for [C4min][Tf2N] was more than 180 MJ/m3 when the inlet and outlet field temperature are 210{degree sign}C and 310oC. Based on our experimental results, it can be concluded that ionic liquids are suitable as liquid thermal storage and heat transfer fluids in solar power generation applications.

Transient thermal analysis of a spherical liquid storage tank during charging

A spherical liquid thermal storage tank made up of two concentric spheres is modeled using the transient one-dimensional energy equation in the spherical coordinates. A numerical solution is obtained for the modeling partial differential equation using a fully implicit finite difference method. The solution is used to study the transient thermal behavior of the spherical liquid tank during the charging mode, which is presented in terms of the spatial and temporal temperature distributions inside the liquid tank, as well as the variation of energy stored with time. The effects of various design and operating parameters such as the tank size, inlet and ambient temperatures, and inlet mass flux on the thermal storage characteristics of the tank are fully investigated. Further, the thermal storage performance of the spherical tank is compared with that of an equal volume cylindrical packed bed under the same operating conditions.

Numerical Study of Flow and Temperature Stratifications in a Liquid Thermal Storage Tank

A two-dimensional, time-dependent mathematical model has been developed to investigate the flow and temperature stratification phenomena in a liquid thermal storage tank for a solar energy system. This model is based on the principles of both natural and forced convective heat transfer. To obtain the numerical solution for this set of nonlinear partial differential equations, an improved Alternative Direction Implicit (ADI) scheme was developed, which enables us to obtain solutions for very large Grashof and Reynolds numbers for the first time. In this analysis, we present the numerical results on the flow patterns and temperature distributions in a liquid storage tank for various Reynolds and Grashof numbers together with the numerical scheme.