Heat Transfer Fluid Mass Flow Research Articles

Advanced Controllers for Heat Transfer Fluid Mass Flow Rate Control in Solar Tower Receivers

Efficient control strategies for managing the mass flow rate (MFR) of heat transfer fluids (HTF) during cloud transients in Solar Tower receivers play a pivotal role in optimizing plant profitability and receiver durability. This study focuses on the performance and durability of Solar Tower receivers during cloud transients. It evaluates adaptive feedback and feedforward control methods, which adjust the flow rate of heat transfer fluids based on real-time measurements of direct normal irradiance (DNI), receiver outlet and receiver panel outlet temperatures. The effectiveness of an aggressive all-sky and conservative clear-sky control strategy is explored against a conventional PI controller, emphasizing energy efficiency and receiver longevity. Simulations using a thermal Modelica model resembling a 100 MWel Crescent Dunes-like solar tower plant reveal that both advanced controllers provide precise setpoint tracking, while the PI controller struggles. The conservative controller which has a cloud standby mode prevents overheating during cloud transients by using a clear sky mass flow rate, while the aggressive controller uses the receiver panel outlet temperatures to correct for upstream tube temperature variations allowing for fast tracking correction and disturbance rejection, albeit with slight overshoots. Furthermore, the controllers significantly decrease the creep-fatigue damage accumulated in the receiver panels during cloudy days, due to limiting the increase in wall temperature spikes when cloud events end. Overall, this study underscores the pivotal role of HTF mass flow rate control systems in influencing receiver system failure modes and longevity and offers a new tool in controller design and operation assessment.

Enhanced performance of photovoltaic thermal module and solar thermal flat plate collector connected in series through integration with phase change materials: A comparative study

The decline in photovoltaic cell efficiency due to temperature elevation, coupled with solar energy's inherent intermittency, presents challenges for solar systems, including photovoltaic panels and solar thermal (ST) collectors. The integration of phase change materials (PCMs) emerges as a promising solution to enhance thermal energy storage and regulation, thereby improving system performance and sustainability. This study investigates the performance of series-connected photovoltaic thermal (PVT) and ST systems with integrated PCMs from energy and exergy viewpoints, utilizing a 3D transient numerical simulation. The proposed system's performance is compared with that lacking a PCM across various heat transfer fluid mass flow rates. Suitable PCM selection based on melting temperature is explored, along with PCM layer thickness and nanoparticle augmentation. The research highlights that PCM integration effectively regulates PV cell temperature, enhancing electrical power and exergy rate. While increasing the mass flow rate improves thermal and electrical power, it reduces the thermal exergy rate. Comparing different PCMs (RT28 HC, RT35 HC, and RT44 HC), the RT28 HC PCM provides the lowest temperature of PV cells and the best electrical performance during the day. Moreover, increasing PCM layer thickness results in lowering PV cell and outlet temperatures and storing more heat. Nanoparticles added to PCMs lead to a marginal improvement in energy and exergy, especially for the system with RT28 HC. This analysis emphasizes PCM integration's role in enhancing solar thermal system performance, highlighting the importance of considering the PCM's melting temperature for effective thermal energy management.

Design of a Parabolic Trough Collector Solar Field for a Pilot CO2 Capture Plant from Natural Gas Combined Cycle Power Plant Flue Gas.

A parabolic trough collector solar field was designed to supply the heat needed to regenerate the CO2-rich monoethanolamine in a solar-assisted carbon capture system. Process design modeling was performed for 90% of the CO2 removal from 1% of the flue gas produced by a 255 MWe natural gas combined cycle power plant. Calculations with and without 24 h of thermal energy storage by increasing the solar collector size needed and providing a buffer vessel to store hot heat transfer fluid were performed. A dynamic analysis of the solar field using the hourly solar forecast was performed to investigate how heat transfer fluid mass flow changes during January and June to maintain the desired parabolic solar field outlet temperature needed for CO2 reboiler operation. The calculations provided here present an explicit method to calculate relevant solar field design parameters that can be scaled up and used in solar energy-assisted gas capture processes.

Numerical investigation and extensive parametric analysis of cryogenic latent heat shell and tube thermal energy storage system

• Cryogenic latent heat thermal energy storage for boil-off gas reliquefaction. • Phase change numerical modelling using the enthalpy method application on Dymola. • Parametric study on the system geometric design and the fluid mass flow rate. • Study of radial and longitudinal fins as heat transfer enhancement techniques. • Performance evaluation using charging/discharging rates and energy storage data. Latent heat thermal energy storage systems have shown promising energy management solutions in different domains due to their high efficiency and energy density. Nowadays, most studies focus on moderate to high operating temperature applications such as solar power plants, while few studies exist for cryogenic applications such as the liquefied natural gas (LNG) domain. This article aims to assess the thermal performance of a shell and tube latent heat thermal energy storage system. The purpose of this system is to reliquefy the excess boil-off gas (BOG) generated on LNG carriers, using n-Butane as a suitable phase change material (PCM). The system is numerically investigated using Dymola software, where the solidification and melting evolution of the PCM is accounted for by using the enthalpy method and applying the finite volume method discretization technique. This system works on two operating modes: charging and discharging. During charging, the PCM solidifies and stores the cold energy during the LNG evaporation to be discharged at later times to condense the BOG while the PCM melts. An extensive parametric study is carried out to assess the impact of different factors on the system’s thermal performance. Results showed that the existence of natural convection would significantly enhance the PCM performance. Moreover, the selection of the tube diameter, PCM thickness, and the heat transfer fluid mass flow rate is crucial and is a compromise of several factors. On the other hand, radial and longitudinal fins were assessed as heat transfer enhancement techniques, where a parametric study on the fins’ volume, thickness, and number is carried out. The excessive addition of fins is expected to obstruct the natural convection forces; however, a considerable enhancement in the thermal performance, especially for the radial ones, is observed.

Thermal Performance Analysis of a Linear Fresnel Concentrating Solar Collector Absorber Tube

Linear Fresnel solar collectors are suitable for a wide range of applications, which include steam generation for power generation, industrial process heat, solar cooling, institutional and domestic hot water production etc. The thermal performance of a linear Fresnel solar collector absorber tube was investigated in this study for different fluid mass flow rates, heat flux intensities, heat transfer fluid inlet temperatures, ambient air temperatures and external loss heat transfer rate. Turbulent flow liquid water heat transfer fluid and uniform circumferential heat flux distribution boundary were considered under steady-state conditions. The fluid flow was considered incompressible. This study was limited to a first order model approximation for thermal performance of a linear Fresnel solar collector absorber tube. The heat transfer fluid and absorber tube material properties were considered constant and independent of temperature. The results indicated that the absorber tube outlet heat transfer fluid temperature decreased with an increase in the Reynolds number and increased with the heat flux intensities. Thermal efficiency increased with an increase in the heat transfer fluid mass flow rate and however, decreased with increase in pressure drop and the consequent pumping power requirement. It was also found that the thermal efficiency of the collector decreased with an increase in the fluid bulk inlet temperature and decreased with an increase in the external loss heat transfer rate.

Thermal performance of an accumulator unit using phase change material with a fixed volume of fins

Melting rate enhancement inside an accumulator unit for a phase change material (PCM) has been numerically investigated in a shell-and-tube (S-T) arrangement that incorporates the circular type of fins, considering a fixed total volume of fins. The number of fins was chosen as the design parameter. The heat transfer fluid (HTF) was chosen as water that flows through the selected accumulator unit during the process of melting. The computational fluid dynamics (CFD) technique has been utilized for the simulation of the accumulator using ANSYS FLUENT software by considering a two-dimensional axisymmetric cylindrical accumulator in a vertical direction. During the melting process, two heat transfer mechanisms were considered, namely, conduction and convection. The HTF flow was 5 L/min with a flow temperature of 358 K for charging. The predicted complete melting time of the PCM decreased by 53.125%, 65.625%, 71.875%, and 71.875% for fins numbering 6, 18, 30, and 36, respectively, compared with the S-T without fins where it took approximately 480 minutes to melt completely. The optimal fin parameters are recommended in this study as follows: number of fins N = 30, thickness t/d = 0.01858, interval d/L = 0.03227, and aspect ratio t/h = 0.015. These recommended values maximize the thermal performance of the selected accumulator unit. The effects of changing the flow rate of the heat transfer fluid and its inlet temperature have been found to be significant on the PCM melting rate. The total melting time of the PCM is found to be reduced by about 57% when the inlet flow temperature is increased from 343 to 358 K. Moreover, the total melting time reduces by about 70.5% with an increase in the heat transfer fluid mass flow rate from 0.15 to 5 L/min. Furthermore, increasing the HTF flow rate from 0.15 to 5 L/min leads to a reduction of about 61.1% in the predicted total time that is required for solidification. The temperature differences for low flow rates are greater than those for high flow rates. The novelty of this study is in investigating the performance at a fixed total volume of fins.

Performance optimization of a nanofluid-based photovoltaic thermal system integrated with nano-enhanced phase change material

In this study, a nanofluid-based photovoltaic thermal system integrated with nano-enhanced phase change material is numerically simulated using a transient three-dimensional model. Aluminum oxide nanoparticles are dispersed into both the heat transfer fluid, which is water, and the Rubitherm series of organic phase change material. Response surface methodology is applied to gain a predictive model for estimating the behavior of the system in terms of four different design parameters of phase change material layer thickness, heat transfer fluid mass flow rate, and the mass fraction of nanoparticles through the phase change material and working fluid. The relationships between the mentioned parameters and responses, including electrical and thermal power, exergy, and entropy generation along with their interaction impacts on system performance are obtained. Finally, employing single and multi-objective optimization, different scenarios are defined to optimize the system based on the designer’s goals. The results reveal that the dispersion of nanoparticles within the heat transfer fluid leads to a better improvement in photovoltaic thermal system performance compared to its addition to the phase change material. Based on the multi-objective optimization method to maximize electrical and thermal exergies, simultaneously, results show the best electrical and thermal exergies of the system can be achieved to 135.92 and 2.44 W/m2, respectively.

A systematic assessment on a solar collector integrated packed-bed single/multi-layered latent heat thermal energy storage system

Thermal energy storage (TES) methods stand out as a key technology that eliminates the supply-demand imbalance in cooling or heating systems. This paper focuses on investigating the charging performance of the solar-aided packed-bed latent heat TES system. An in-house 1D transient mathematical model is developed to simulate the latent heat TES system's heat transfer phenomena. The TES tank is integrated with a flat plate solar collector to include the variable boundary condition effects of solar energy and ambient air. The enhanced thermal conductivity approach is implemented into the mathematical model to consider the natural convection effects of the PCM's liquid phase, and the method is validated. The mathematical model of the packed bed latent heat TES tank is also validated with the experimental and numerical results from the literature. As design and working parameters, the mass flow rate of heat transfer fluid (HTF) and spherical storage material diameter are varied. Single-, double- and triple-phase change material (PCM) arrangements are also studied by varying the melting temperature of PCM along the flow direction. The time-wise variation of energetic and exergetic efficiencies of the integrated system, PCM mean temperature inside the storage tank, and the liquid fraction in flow direction are investigated. Further extensive parametric analyses are conducted for heating season months, i.e., November, December, January, and February. As a result, the best design and working conditions for the integrated latent heat TES system within the range investigated are proposed. Also, it is concluded that decreasing the capsule diameter and increasing the HTF's mass flow rate has a positive effect on system indicators.

A numerical simulation of a linear Fresnel solar reflector directed to produce steam for the power plant

The concentrating solar systems of linear Fresnel reflectors offer the ability to generate electricity from solar energy. The main objective of this paper is to perform a transient numerical simulation on a linear Fresnel solar reflector directed to produce superheated water steam for the power plants, in order to determine its efficiencies (optical and thermal), where the heat transfer fluid mass flow inside the copper absorber tube is 0.045 kg/s. The dimensions of the studied linear Fresnel solar collector are the same dimension as the modular system Nova-1 of NOVATEC SOLAR Company. The dimension of the studied model is 1D. For this purpose, El-Oued region has been designated as a study field. The twelve recommended average days for months have been selected for the study. This paper was based on a numerical solution using the finite differences method. MATLAB has been used as a software language. The optical efficiency of the LFR solar receiver has reached 53.7% where the thermal efficiency has reached 37.5%, while the pressure drop within the absorber pipe has reached 02 bars. As for superheated steam temperatures variations, it ranged between 233.35 °C and 263.75 °C, knowing that the water steam inlet temperature when entering the absorber tube is equal to 140 °C. As for the overall coefficient of heat loss, its mean value is 5.84 W/m2.°C. The results obtained are very encouraging to invest in this steam power plant, especially in the desert areas, which are rich in solar energy.

Selection of metal hydrides-based thermal energy storage: Energy storage efficiency and density targets

Thermo-chemical energy storage based on metal hydrides has gained tremendous interest in solar heat storage applications such as concentrated solar power systems (CSP) and parabolic troughs. In such systems, two metal hydride beds are connected and operating in an alternative way as energy storage or hydrogen storage. However, the selection of metal hydrides is essential for a smooth operation of these CSP systems in terms of energy storage efficiency and density. In this study, thermal energy storage systems using metal hydrides are modeled and analyzed in detail using first law of thermodynamics. For these purpose, four conventional metal hydrides are selected namely LaNi5, Mg, Mg2Ni and Mg2FeH6. The comparison of performance is made in terms of volumetric energy storage and energy storage efficiency. The effects of operating conditions (temperature, hydrogen pressure and heat transfer fluid mass flow rates) and reactor design on the aforementioned performance metrics are studied and discussed in detail. The preliminary results showed that Mg-based hydrides store energy ranging from 1.3 to 2.4 GJ m−3 while the energy storage can be as low as 30% due to their slow intrinsic kinetics. On the other hand, coupling Mg-based hydrides with LaNi5 allow us to recover heat at a useful temperature above 330 K with low energy density ca.500 MJ m−3 provided suitable operating conditions are selected. The results of this study will be helpful to screen out all potentially viable hydrides materials for heat storage applications.

Experimental Evaluation of a Paraffin as Phase Change Material for Thermal Energy Storage in Laboratory Equipment and in a Shell-and-Tube Heat Exchanger

The thermal behavior of a commercial paraffin with a melting temperature of 58 °C is analyzed as a phase change material (PCM) candidate for industrial waste heat recovery and domestic hot water applications. A full and complete characterization of this PCM is performed based on two different approaches: a laboratory characterization (mass range of milligrams) and an analysis in a pilot plant (mass range of kilograms). In the laboratory characterization, its thermal and cycling stability, its health hazard as well as its phase change thermal range, enthalpy and specific heat are analyzed using a differential scanning calorimeter, thermogravimetric analysis, thermocycling and infrared spectroscopy. Laboratory analyses showed its suitability up to 80 °C and for 1200 cycles. In the pilot plant analysis, its thermal behavior was analyzed in a shell-and-tube heat exchanger under different heat transfer fluid mass flow rates in terms of temperature, power and energy rates. Results from the pilot plant analysis allowed understanding the different methods of heat transfer in real charging and discharging processes as well as the influence of the geometry of the tank on the energy transferred and required time for charging and discharging processes.

Estimation of heat loss from a cylindrical cavity receiver based on simultaneous energy and exergy analyses

Abstract This study undertakes the experimental and theoretical investigation of heat losses from a cylindrical cavity receiver employed in a solar parabolic dish collector. Simultaneous energy and exergy equations are used for a thermal performance analysis of the system. The effects of wind speed and its direction on convection loss has also been investigated. The effects of operational parameters, such as heat transfer fluid mass flow rate and wind speed, and structural parameters, such as receiver geometry and inclination, are investigated. The portion of radiative heat loss is less than 10%. An empirical and simplified correlation for estimating the dimensionless convective heat transfer coefficient in terms of the Re $\mathrm {Re}$ number and the average receiver wall temperature is proposed. This correlation is applicable for a wind speed range of 0 . 1 $0.1$ to 10 m/s. Moreover, the proposed correlation for Nu $\mathrm {Nu}$ number is validated using experimental data obtained through the experiments carried out with a conical receiver with two aperture diameters. The coefficient of determination R 2 and the normalized root mean square error (NRMSE) parameters were calculated, and the results show that there is a good agreement between predicted results and experimental data. R 2 is greater than 0 . 95 $0.95$ and the NRMSE parameters is less than 0 . 06 $0.06$ in this analysis.

Experimental transient performance of a heat pump equipped with a distillation column

A series of experiments were conducted on a heat pump equipped with a distillation column. The system was operated with R32 and with a 30/70% by mass mixture of R32/134a to examine the difference between the transient performance trends with a pure fluid (R32), and those with a zeotropic mixture (R32/134a). Additionally, the effects of varying heat transfer fluid mass flow, compressor speed, and accumulator sump heat input were examined. Each test was 1 h in duration. The heat pump capacities did not generally achieve steady state during the R32/134a tests. Steady state was generally achieved during the R32 tests. As a percentage of the final (end-of-test) capacity, the rate of capacity increase was greater during the R32/134a tests than during those conducted with the pure fluid. The R32/134a tests exhibited capacity oscillations early in each transient that were not present during the R32 tests. The results show that circulating refrigerant mass and composition are the primary controlling factors with regard to transient capacity.