Flat Micro-heat Pipe Arrays Research Articles

Thermal performance and optimization of an integrated collector–storage solar air heater based on lap joint-type flat micro-heat pipe arrays

This study seeks to optimize the performance of an integrated collector–storage solar air heater (ICSSAH) based on lap joint-type (LJT) flat micro-heat pipe arrays (FMHPAs) and latent thermal storage through numerical simulation. A novel dynamic heat transfer model is developed by incorporating the heat transfer correlation describing the thermal characteristics of phase change material (PCM) in narrow and elongated confined spaces within plate fins and validated using experimental data to address the issue of deviation caused by the fixed thermal resistance of the PCM in conventional numerical simulation methods. The validated model is used to simulate and predict the dynamic performance of LJT-FMHPA-ICSSAH under different heat transfer structures. Several optimization parameters, including the air gap thickness, the optical properties of the collector and glass cover plates, and fins structure, are considered. Simulation results indicate that the heat lost to the external environment through the glass cover plate accounted for over 70 % of the total heat dissipation. The optimal combination included a 50 mm gas layer; copper-based blue titanium coating; ultra-white glass; and fin spacing, thickness, height, and length of 5, 0.2, 60, and 195 mm, respectively. Under autumn/winter operating conditions, the optimized thermal performance is 1.13–1.35 times higher than the performance prior to optimization.

Thermal performance of two evacuated tube solar collectors with flat heat pipes

To promote the transition to clean heating methods in rural areas and improve solar energy utilization efficiency, two new types of solar water collectors with evacuated tubes, namely finned and finless collectors, are constructed with multi-channel flat tubes and flat micro-heat pipe arrays, and comparison experiments are conducted. By integrating with multi-channel flat tubes, the micro-heat pipe arrays, which function as the central heat transfer component, are capable of attaining effective thermal transmission. The multi-channel flat tubes have a substantial surface area available for heat transfer with the thermal fluid, while the evacuated tubes exhibit superior thermal insulation performance. The structural discrepancy between the two collectors lies in the presence or absence of louvered fins attached to the flat heat pipes inside the evacuated tubes. The comparative analysis of the two collectors includes time constant, thermal resistance analysis, distribution of temperature, normalized temperature difference, pressure drop, and actual operation effect under different conditions. The average thermal efficiency of the finless collector (59.4 %) is lower than that of the finned collector (78.2 %) on a typical experimental day. Then, further investigations are conducted to explore the effects of heat transfer fluid volume flow rate and inlet temperature on the thermal performance of finned solar water collector. With the heat transfer fluid volume flow rate of 120 L/h and the inlet temperature at 15 °C, the finned collector can achieve its maximum thermal efficiency of 86.4 %.

Experimental investigation of a novel thermal storage solar air heater (TSSAH) based on flat micro-heat pipe arrays

A novel thermal storage solar air heater (TSSAH) is proposed in this study. This TSSAH is composed of a vacuum glass tube, flat micro-heat pipe arrays (FMHPA), and a thermal storage material (i.e., paraffin) with a phase change temperature of 58 °C. The FMHPA acts as the core heat transfer component. Considering the low heat conductivity coefficient of paraffin, louver fins are utilized to enhance heat transfer. Thermal performance is analyzed and discussed. During thermal collection, natural convection evidently contributes to enhancing heat transfer. Increasing solar radiation intensity can improve collection efficiency. Thermal collection efficiency increases from 70.22% to 77.28% when solar radiation is increased from 675 W/m2 to 835 W/m2 under an ambient temperature range of 20.8 °C–23.3 °C. A high ambient temperature results in high collection efficiency that can reach 80.59%. During thermal discharge, enhancing the air volumetric flow rate can evidently increase the amount of useful energy and shorten discharge time. From 80 m3/h to 170 m3/h, the amount of useful energy is increased from 335 W to 550 W, and discharge time is shortened from 309 min to 195 min under inlet temperatures ranging from 24.1 °C to 24.2 °C. A high inlet temperature generates a high outlet temperature but decreases the amount of useful energy. From 19.7 °C to 32.0 °C, outlet temperature is increased from 30.4 °C to 36.6 °C, and the amount of useful energy is reduced from 692 W to 287 W at an air volumetric flow rate of 200 m3/h. During thermal discharge, the average amount of useful energy can reach 692 W at an inlet temperature of 19.7 °C and an air volumetric flow rate of 200 m3/h, indicating that the proposed TSSAH can release heat quickly. In the experiment scale, the discharged heat is 6150–6450 kJ at a phase change material temperature scale of 85 °C–35 °C.

Numerical and experimental investigations of latent thermal energy storage device based on a flat micro-heat pipe array–metal foam composite structure

Latent heat thermal energy storage (LHTES) is crucial in the application of renewable energy and waste heat recovery. A novel LHTES device with a flat micro-heat pipe array (FMHPA)–metal foam composite structure is designed in this study to obtain excellent heat transfer performance. An evaluation standard called integrated power is proposed to assess and compare the structural advantages of the LHTES device with others. Performances of FMHPA, temperature distribution, effort of inlet temperature and velocity of heat transfer fluid (HTF) are also studied in the experiments. A three-dimensional numerical model is developed to investigate the effort of porosity and pore density of metal foam on the charging process. Results show that the FMHPA–copper foam composite structure improves the performance of the LHTES device. This structure exhibits stronger heat transfer performance than that of other devices. The increasing inlet temperature of HTF has a better promotion effect on power than raising HTF velocity. High porosity is conducive to natural convection but detrimental to heat conduction. High pore density is disadvantageous to natural convection and does not affect heat conduction. • Strengthen of flat micro-heat pipe arrays and metal foam are studied. • The temperature distribution of PCM are experimentally studied. • The effect of temperature and velocity of HTF are experimentally studied. • The porosity and pore density of copper foam are numerically studied. • An evaluation standard is proposed to evaluate the structural advantages.

A comparative experimental investigation on thermal performance for two types of vacuum tube solar air collectors based on flat micro-heat pipe arrays (FMHPA)

Two types of solar air collectors (SACs), namely, a conventional-tube collector (C-TC) and a transparent-tube collector (T-TC), are designed on the basis of flat micro-heat pipe arrays (FMHPA) and are compared. FMHPA is used as the core heat transfer element for both collectors. Absorption coatings are located at different places. It is located at, namely, on the inner wall of vacuum tube and on the FMHPA evaporation section in the C-TC and the T-TC. A large volumetric flow rate of air contributes to high efficiency. Results of the comparative experiment show that even if the thermal efficiency of the C-TC (77.6%) is smaller than that of the T-TC (85.0%), the useful energy of the C-TC (641 W) is larger than that of the T-TC (497 W). Moreover, the COP values of the C-TC and T-TC are 11.4 and 8.9, respectively. The unit initial investments are 0.15 and 0.21 $/W. Hence, considering the heating capability and price advantage, the C-TC is a better collector form than the T-TC in the same collector gross area. The collector pressure drop does not exceed 19.1 Pa. These results can serve as a reference for the selection and design of SACs for heating applications and crop drying.

Thermal performance of integrated collector storage solar air heater with evacuated tube and lap joint-type flat micro-heat pipe arrays

To alleviate the problem of global warming and the energy crisis, this study proposed an integrated collector storage solar air heater that uses evacuated tubes as solar absorbers and paraffin as thermal storage material. In the proposed unit, lap joint-type flat micro-heat pipe arrays serve as heat conductors that transmit the solar energy absorbed by the evacuated tubes to the thermal storage tank or transfer the heat stored in the tank to the air flow channel. Outdoor experiments were carried out to obtain the thermal performance of the proposed device during charging and discharging. The effects of weather conditions, supply air flow rates, and inlet temperature on the thermal response of the integrated collector storage solar air heater were reported. The thermal storage efficiency during the experiment period ranged from 56.1% to 67.5% when the mean outdoor temperature ranged from −5.7 °C to 36.2 °C. The mean thermal extraction power and thermal released efficiency reached 1268.8 W and 98.5%, respectively. The results of the energy conversion process analysis and mathematical fitting showed that the mean charging efficiency was linear with the normalized temperature difference. The mean outlet temperature and mean extraction power were linear with the inlet temperature and exponential with the air flow rate. A conceptual design of an air heating system using the integrated collector storage solar air heater for a 9.9 m2 building was presented. The benefit pre-evaluation revealed that the system can reduce carbon emissions by approximately 5.8 tons over its life cycle.

Thermal performance investigation of an integrated collector–storage solar air heater on the basis of lap joint-type flat micro-heat pipe arrays: Simultaneous charging and discharging mode

In this part of the investigation, the thermal performance of an integrated collector–storage solar air heater (ICSSAH) on the basis of a lap joint-type flat micro-heat pipe array during simultaneous charging and discharging mode is experimentally studied. Paraffin is used as a phase change material, and air acts as a heat transfer fluid in a built-out structure of ICSSAH. The thermal performance of ICSSAH is assessed under an outdoor experiment to explore the effect of seasonality, volume flow rate, and daily accumulative solar radiation. Thermal energy analysis is described and calculated in detail according to the experimental measurements. Results showed that the daily mean thermal efficiency in summer is significantly higher than the transition season. The smaller the volume flow rate is, the lower the thermal efficiency will be. However, the effect of solar radiation on thermal efficiency is small during the test conditions. The highest daily thermal efficiency at the volume flow rate of 141 m3/h is approximately 78.1% in the present study when the solar irradiance ranges from 0 W/m2 to 874 W/m2 and the ambient temperature varies from 27.8 °C to 35.4 °C.

Numerical investigation of the thermal performance enhancement of latent heat thermal energy storage using longitudinal rectangular fins and flat micro-heat pipe arrays

The performance of a new type of latent heat thermal energy storage (LHTS) device based on flat micro-heat pipe arrays (FMHPAs) with longitudinal rectangular fins is numerically studied by enthalpy–porosity technique based on finite volume method (FVM) in this research. The numerical model is verified correct. The temperature distribution and phase transition process in different directions of the interior of a thermal storage tank and the effects of fin height, spacing, and thickness on charging power and thermal storage capacity are also analyzed numerically. Results show that phase interface is presented in U type in the horizontal direction. In the vertical direction, the phase change material (PCM) among fins melts from up to down when the fin spacing is larger than 6 mm, and the opposite occurs when the fin spacing is less than 6 mm. The thermal storage capacity of the LHTS device is reduced drastically when the fin spacing is less than 4.14 mm. For fin height, the structure of multiple rows of FMHPAs with dwarf fins is recommended because it exhibits large power and exerts a small negative effect on thermal storage capacity. Fin thickness has a minimal effect on charging power and thermal storage capacity. The study results provide optimization design guidance for LHTS devices based on FMHPAs under various application backgrounds, such as solving the contradiction between energy supply and demand in solar thermal systems and the peak load shifting of electricity in heat pump systems.

Experimental investigation of an integrated collector–storage solar air heater based on the lap joint-type flat micro-heat pipe arrays

This article presents an integrated collector–storage solar air heater (ICSSAH) based on lap joint-type (LJT) flat micro-heat pipe arrays (FMHPA) and latent thermal storage (LTS). FMHPA for thermal collection and for thermal storage/release were separated and overlapped. The discharging process is not affected by the length of the collection segment. Approximately 21.03 kg of 52# industry paraffin wax was utilized as thermal storage material (TSM) to store solar energy, and the collection area was set to 1.03 m2. The influence of environmental factors on the charging process and the effects of heat transfer fluid's parameters on the discharging process were studied. An average efficiency curve was proposed according to the analysis of experimental data to evaluate thermal storage performance during charging, and a correlation between Re and Nu was fitted to predict the convective heat transfer coefficient during the discharging of the device. LJT-FMHPA-ICSSAH exhibits good thermal characteristics during experimental conditions. The thermal storage/extraction efficiency reached 73.8%/97.1%, and the average thermal storage/extraction power reached 623.7 W/815.9 W.

Experimental study on the new type of electrical storage heater based on flat micro-heat pipe arrays

A new type of electrical storage heater that utilizes latent heat storage and flat micro-heat pipe arrays (FMHPAs) was developed. The thermal characteristics of the heater were tested through experimentation. The structure and operating principle of the storage heater were expounded. Three rows of FMHPAs were applied (three rows with five assemblies each) with a mass of 28 kg of phase change material (PCM) in the heat storage tank. Electric power was supplied to the PCM in the range of 0.2‒2.04 kW, and air was used as heat transfer fluid, with the volume flow rate ranging from 40‒120 m 3 /h. The inlet temperature was in the range of 15‒24°C. The effects of heating power, air volume flow rate, and inlet temperature were investigated. The electrical storage heater exhibited efficiencies of 97% and 87% with 1.98 and 1.30 kW of power during charging and discharging, respectively. Application of the proposed storage heater can transfer electricity from peak periods to off-peak periods, and the excess energy generated by wind farms can be stored as heat and released when needed. Good economic and environmental benefits can be obtained.

Experimental study on an integrated collector storage solar air heater based on flat micro-heat pipe arrays

This study investigated the performance of an integrated collector storage solar air heater based on latent heat storage and flat micro-heat pipe arrays. The structure and working principle of the device were explained in detail. The effective area of the collector is 0.93m2, and the mass of the phase change material (PCM) is 45.8kg. An integrated collector storage experimental system was set up, and outdoor experiments were conducted to demonstrate the feasibility of the device for energy storage and air heating. The temperature distribution of the PCM was recorded during charging and discharging to monitor the propagation of the melting and freezing front. Efficiency and power during charging and discharging were also calculated. The integrative device exhibited 59% and 91.6% efficiencies for solar air charging and discharging at 393 and 344W power, respectively.

Heat Transfer Properties of a Latent Thermal Storage Unit with Flat Microheat Pipe Arrays

AbstractA thermal storage unit (TSU) with flat microheat pipe arrays (FMHPAs) as heat transfer core is experimentally investigated in this study. A comprehensive evaluation parameter of the TSU def...

Thermal performance of solar air collection-storage system with phase change material based on flat micro-heat pipe arrays

In this study, a new type of solar air collection-storage thermal system (ACSTS) with phase change material (PCM) is designed using flat micro-heat pipe arrays (FMHPA) as the heat transfer core element. The solar air collector comprises FMHPA and vacuum tubes. The latent thermal storage device (LTSD) utilizes lauric acid, which is a type of fatty acid, as PCM. The experiments test the performance of collector efficiency and charging and discharging time of thermal storage device through different air volume flow rates. After a range of tests, high air volume flow rate is concluded to contribute to high collector efficiency and short charging and discharging time and enhance instantaneous heat transfer, whereas an air volume flow rate of 60m3/h during discharging provides a steady outlet temperature. The cumulative heat transfer during discharging is between 4210 and 4300kJ.

Thermal performance of a new CPC solar air collector with flat micro-heat pipe arrays

A new type of compound parabolic concentrator (CPC) solar air collector (SAC) with flat micro-heat pipe arrays (FMHPA) was investigated in this study. A cylindrical absorber was constructed by inserting the FMHPA into an evacuated glass tube to transport heat during the working process. The new FMHPA-CPC SAC is an edge ray collector with a concentration ratio of 1.8. The core concept is the integration of the FMHPA, an absorber tube, and a CPC reflector as a heat-collecting unit. Thermal performance investigation was conducted theoretically and experimentally. This study mainly analyzed the effect of different factors on the thermal performance of the collector. The efficiencies and heat loss coefficients of the new collector were determined. Experimental curves with different testing conditions are presented. The optimal value of efficiency was determined for the SAC. The aperture area of each CPC collecting unit is approximately 0.4 m2. The new FMHPA-CPC SAC was tested in Beijing, China. The average efficiency was approximately 61% at a volume flow rate of 320 m3/h, with a radiation of 799 W/m2 (ambient temperature of 28.8 °C). The instantaneous efficiency reached 68%, depending on the solar radiation, air volume flow rate, and ambient temperature. In addition, the time constant of the collector was approximately 14.8 min, with an air volume flow rate of 260 m3/h.

Experimental Investigation on the Performance of a Novel Solar air Heater Based on Flat Micro-heat Pipe Arrays (FMHPA)

Abstract In order to improve the efficiency of solar air heater, a novel solar air heater with flat micro-heat pipe arrays (FMHPA) is presented in this study. The structure of FMHPA-solar air heater, the working principle and experimental test facility have also been described in detail. The efficiency, the pressure drop and the time constant are determined for the FMHPA-solar air heater. The results showed that the average thermal efficiency of the solar air heater is up to 70% of stable operation and time constant is about 14 minutes. Experimental results indicate better thermal efficiency for FMHPA-solar air heater compared to a traditional flat plate heater.

Experimental study on the thermal performance and pressure drop of a solar air collector based on flat micro-heat pipe arrays

A new solar air collector that combines the use of flat micro-heat pipe arrays (FMHPA) and evacuated tube is developed and investigated. Using FMHPA as the central transporting component in a solar air collector is an effective approach to improve the collector’s thermal properties and reduce its pressure drop. The thermal properties and pressure drop of the FMHPA solar air collector are analyzed in detail. The main objective of this paper is to report test results of the FMHPA solar air collectors which includes the effects of different seasons and airflow rates on the thermal efficiency and the pressure drop of the air collector. In summer, the thermal efficiency of the collector reaches 73% under stable operation. In the testing airflow range condition, the pressure drop is less than 25Pa when the flow rate is below 201.6m3/h. The relative uncertainty of thermal efficiency is approximately 7.73%.