Thermal Energy Storage for Solar Energy Utilization: Fundamentals and Applications

Solar energy increases its popularity in many fields, from buildings, food productions to power plants and other industries, due to the clean and renewable properties. To eliminate its intermittence feature, thermal energy storage is vital for efficient and stable operation of solar energy utilization systems. It is an effective way of decoupling the energy demand and generation, while plays an important role on smoothing their fluctuations. In this chapter, various types of thermal energy storage technologies are summarized and compared, including the latest studies on the thermal energy storage materials and heat transfer enhancements. Then, the most up-to-date developments and applications of various thermal energy storage options in solar energy systems are summarized, with an emphasis on the material selections, system integrations, operational characteristics, performance assessments and technological comparisons. The emerging and future trends are finally outlined. This chapter will be a useful resource for relevant researchers, engineers, policy-makers, technology users, and engineering students in the field

Lattice Boltzmann simulation of sound absorption of an in-duct orifice

Two-dimensional time-domain numerical investigation of sound-induced flow through an orifice with a diameter 6mm is conducted by using lattice Boltzmann method. Emphasis is placed on characterizing its acoustic damping behaviors. The main damping mechanism is identified as incident waves interact with the shear layers formed at the orifices rims and the acoustic oscillations destabilize the shear layers to form vortex rings. And acoustic energy is converted into vortical energy. To quantify the orifice damping effect, power absorption coefficient is used. It is related to Rayleigh conductivity and describes the fraction of incident acoustical energy being absorbed. Numerical simulations are conducted in time domain by forcing a fluctuating flow with multiple tones through the orifice. This is different from frequency-domain simulations, of which the damping is characterized one frequency at a time. Comparing our results with those from Howe theoretical model, good agreement is observed. In addition, orifice thickness effect on its damping is discussed.Published versio

Lattice Boltzmann investigation of acoustic damping mechanism and performance of an in-duct circular orifice

In this work, three-dimensional numerical simulations of acoustically excited flow through a millimeter-size circular orifice are conducted to assess its noise damping performance, with particular emphasis on applying the lattice Boltzmann method (LBM) as an alternative computational aeroacoustics tool. The model is intended to solve the discrete lattice Boltzmann equation (LBE) by using the pseudo-particle based technique. The LBE controls the particles associated with collision and propagation over a discrete lattice mesh. Flow variables such as pressure, density, momentum, and internal energy are determined by performing a local integration of the particle distribution at each time step. This is different from the conventional numerical investigation attempting to solve Navier-Stokes (NS) equations by using high order finite-difference or finite-volume methods. Compared with the conventional NS solvers, one of the main advantages of LBM may be a reduced computational cost. Unlike frequency domain simulations, the present investigation is conducted in time domain, and the orifice damping behavior is quantified over a broad frequency range at a time by forcing an oscillating flow with multiple tones. Comparing the numerical results with those obtained from the theoretical models, large eddy simulation, and experimental measurements, good agreement is observed.Published versio

Geometric shapes effect of in-duct perforated orifices on aeroacoustics damping performances at low Helmholtz and Strouhal number

In this work, experimental studies are conducted to measure the aeroacoustics damping performances of 11 in-duct perforated plates in a cold-flow pipe with a variable Mach number. These in-duct plates have the same porosities but different number N and geometric shaped orifices. Here six shapes are considered, i.e., (1) triangle, (2) square, (3) pentagon, (4) hexagon, (5) star, and (6) circle. It is shown that the orifice shape has little influence on power absorption Δ and reflection coefficient R at a lower Helmholtz number He ≤ 0.0903. However, as He is increased, the in-duct plate with a star-shaped orifice is shown to be with much lower Δ in comparison with that of other plates with different shape orifices. In addition, the perforated orifice with the same shape and porosity but a larger N is shown to be associated with 20% more power absorption at approximately He = 0.1244. Δmax is observed to be approximately 85% at about He = 0.0244, as Ma≈0.029. To gain more insights, the quasi-steady model is applied, depending on the Strouhal number Sr. The transition from quasi-steady flow behaviors to unsteady behaviors occurs at approximately Sr = 0.45. The measured minimum reflection coefficient Rmin occurs at Ma ≈ 0.024. This experimental finding is consistent with the quasi-steady prediction.NRF (Natl Research Foundation, S’pore)Published versio

Experimental studies of mitigating premixed flame-excited thermoacoustic oscillations in T-shaped Combustor using an electrical heater

In this work, attenuating unsteady heat-driven thermoacoustic oscillations in a T-shaped standing-wave Rijke-type combustor is numerically and experimentally studied. For this, 2D numerical studies are conducted first on a T-shaped standing-wave combustor. In such combustor, a heater with constant surface temperature of 1100 K is confined in the bottom branch. However, a secondary heater with a controllable surface temperature is enclosed in the horizontal bifurcating branch. When the secondary heater is not actuated, large-amplitude limit cycle oscillations are successfully generated. However, as the secondary heater surface temperature is increased to 1600 K, the limit cycle oscillations are completely mitigated. To validate these findings, experimental study is then conducted on a T-shaped combustor. A premixed flame is enclosed in the bottom branch and an electrical heater is implemented to attenuate unstable combustion oscillations generated by the flame. When the electrical heater is not actuated, premixed flame-excited thermoacoustic oscillations are generated at approximately 210 Hz. However, with the heater being actuated, sound pressure level is successfully reduced from 130 dB to 85 dB. The present work opens up an alternative control approach to enable combustors being operated stably.This work is financially supported by National Natural Science Foundation Key Project of China with Grant No. 91541121, Scientific Research Fund of Hunan Provincial Education Department with Grant No. 6B235 and National Natural Science Foundation of China with Grant Nos. 51206148, 51476145, 51476146 and 51506079. It is gratefully acknowledged

Efficacy of angled metallic fins for enhancing phase change material melting

Research on enhancing phase change material (PCM) heat transfer is concentrated in latent heat thermal energy storage (LHTES) field, especially with utilization of metallic fins. One interesting fin parameter that was less explored for a rectangular PCM system, is the metal fin's inclined angle. This research aims to experimentally validate the hypothesis and evaluate the efficacy of angled metallic fins enhancing PCM melting in a sidewall heated cuboid LHTES system. Experiments indicate that three-dimensional PCM melting in the LHTES unit can be characterized as two dimensional. The angled fins considerably influence temperature evolution of local solid PCM around fins. Compared to the horizontal fin, positive inclined fins with angles of +30° and +15° prolong the PCM melting time by 4.0% and 3.8% respectively, while the downward tilted inclined fins at −15° and −30° promote PCM melting by up to 5.2% melting fraction difference. Extending simulations with seven fin angles and three fin lengths explicate the substantial effect on PCM heat storage, melting time, and temperature uniformity. Particularly, the longest fin at the downward angle of −15° reduces the PCM melting time most. The study shows feasibility of utilizing downward angled fins to enhance PCM transient melting in the LHTES unit.C. Ji thanks the support from Fundamental Research Funds for Central Universities (Grant No. 22120200417 and 22120210158) and Shanghai Committee of Science and Technology (Grant No. 21ZR1466000) in China

Experimental Study on Transient Ignition Characteristics of Acoustic Excited Methane Jet Diffusion Flames

The ignition process of fuel plays an important role in the flame development and emission characteristics, which has attracted intensive attention in the combustion field. However, the transient ignition process for jet flames under acoustic excitation is rarely reported. In the current study, the effect of external acoustic excitation with different frequencies on the ignition process of methane jet diffusion flames has been studied experimentally using high-speed color and schlieren imaging systems. The fuel nozzle used in the experiment features a concentric ring structure, with fuel in the middle and air around it. The acoustic excitation was added to the air side through the loudspeaker, and the frequency of the acoustic excitation was set as 10 Hz, 30 Hz, 50 Hz and 100 Hz, respectively, while a case without external excitation was used as the control group. It is found that the periodic vortex structure propagates downstream in the flow field after acoustic excitation is added, which leads to an uneven velocity distribution in the flow field and the appearance of a local high-speed zone. The acoustic excitation of 30 Hz and 50 Hz can reduce the probability of successful ignition, which is mainly because the acoustic wave propagates in the flow field and causes drastic velocity changes near the ignition position. For the case of 100 Hz, the acoustic perturbation is confined in a small region near the nozzle exit, while the flow field velocity is slightly higher than the case without acoustic excitation

Investigation on the stability of the Rijke-type thermoacoustic system with an axially distributed heat source

Due to the incurred damages to the combustors, large-amplitude self-sustained thermoacoustic oscillations are unwanted in many propulsion systems, such as liquid/solid rocket motors and aero-engines. To suppress these thermoacoustic oscillations efficiently, the mechanism of thermoacoustic instability needs to be clarified. Following the previous experimental work, the transitions to instability in a Rijke-type thermoacoustic system with an axially distributed heat source are studied numerically in this paper. The URANS numerical method is utilized and verified by means of a mesh sensitivity analysis. The influences of the axially distributed heater length, the heater location, and the mean flow velocity on the nonlinear dynamic behaviors of thermoacoustic oscillations are evaluated. To explore the corresponding mechanism behind these influences, the principle of acoustic energy conservation has been applied. The acoustic energy gains from the thermal-acoustic coupling are quantified via Rayleigh’s integral, and their phase differences are calculated by the cross-correlation function. The acoustic damping induced by the vortex dissipation is qualitatively analyzed by the characteristics of the flow fields in the Rijke tube. Finally, as the heater length, the heater location, or the mean flow velocity is varied, three mechanisms of the transitions to instability in a Rijke-type thermoacoustic system are identified

Experimental and theoretical studies of aeroacoustics damping performance of a bias-flow perforated orifice

Aeroacoustics damping performance of an in-duct perforated orifice with a bias flow in terms of acoustic power absorption Δ and reflection χ coefficients are evaluated in this work. For this, experimental measurements of a cold-flow pipe system with a diameter of 2b with an in-duct perforated plate implemented are conducted over the frequency range of 100 to 1000 Hz first. The effects of (1) the downstream pipe length Ld, (2) porosity η, (3) bias flow Mach number Ma and (4) the orifice thickness lw are experimentally evaluated on affecting the noise damping performance of the in-duct perforated orifice. It is found that decreasing Ld leads to increased Δmax (maximum power absorption). However, the orifice thickness plays a negligible effect at lower frequency, and a non-negligible role at higher frequency range. The maximum power absorption Δmax and reflection coefficients χmax are found to be approximately 80% and 90% respectively. There is an optimum porosity or Mach number corresponding to Δmax. In addition, Δ and χ are periodically changed with the forcing frequency. To simulate the experiments and gain insights on the damping performance of the orifice with a diameter of 2a, an 1D theoretical model that embodies vorticity-involved noise absorption mechanism is developed. It is based on the modified form of the Cummings equation describing unsteady flow through an orifice and the Cargill equation describing acoustically open boundary condition at the end of the downstream duct. It is shown that Δ and χ are strongly related to (1) the bias flow Mach number Ma, (2) forcing frequency ω, (3) porosity η, (4) and the downstream pipe length Ld. Comparing with the experimental measurements reveals that good agreement is obtained. This confirms that the present experimental and theoretical study shed lights on the optimum design of in-duct orifices.National Research Foundation (NRF)This work is supported by the University of Canterbury, New Zealand with Grant No. 452STUPDZ, and National Research Foundation, Prime Minister’s Office, Singapore, with Grant No. NRF2016NRF-NSFC001-102 and National Natural Science Foundation of China with Grant No. 11661141020. This financial support is gratefully acknowledged

Investigation of Harmonic Response in Non-Premixed Swirling Combustion to Low-Frequency Acoustic Excitations

The propagation mechanism of flow disturbance under acoustic excitations plays a crucial role in thermoacoustic instability, especially when considering the effect of non-premixed combustion on heat release due to reactant mixing and diffusion. This relationship leads to a complex coupling between the spatial distribution of the equivalence ratio and the propagation mechanism of flow disturbance. In the present study, the response of a methane-air non-premixed swirling flame to low-frequency acoustic excitations was investigated experimentally. By applying Proper Orthogonal Decomposition (POD) analysis to CH* chemiluminescence images, the harmonic flame response was revealed. Large Eddy Simulation (LES) was utilized to analyze the correlation between the vortex motion within the shear layers and the harmonic response under non-reacting conditions at excitation frequencies of 20 Hz, 50 Hz, and 150 Hz. The results showed that the harmonic flame response was mainly due to the harmonic velocity pulsations within the shear layers. The acoustically induced vortices within the shear layer exhibited motion patterns susceptible to harmonic interference, with spatial distribution characteristics closely related to the oscillation modes of the non-premixed combustion