Average Thermal Efficiency Research Articles

EXERGETIC AND ENERGETIC EVALUATION OF AN INNOVATIVE SOLAR AIR HEATING SYSTEM COATED WITH GRAPHENE AND COPPER OXIDE NANO-PARTICLES

In the 21th century, renewable energy has to play very important role in socio-economic and industrial development. This paper evaluates the exergy- energy analysis, which is based on the second law of thermodynamics. The triangular solar heater is developed to determine the heat transfer rate, thermal efficiency, exergy efficiency and Bejan number. In addition, we have examined the effects of entropy generation with respect to solar radiation and ambient temperature of air. Absorber plates coated with graphene and copper oxide nano-particles by the different percentages (0.1%, 0.2%, 0.3% & 0.4%) doped into black paint which increases the absorption of heat. The Reynolds number (4500≤R_e≤22700) varies for the fixed selective coating on absorber plate and mass flow rate. The experimental observations were performed for constant mass flow rate of air ranging from 0.0035kg/s to 0.018 kg/s. The experimental result gives the average thermal efficiency enhancement of 3.58% for 0.3% graphene/CuO-black paint. Entropy generation is more for 0.1% and minimum for 0.3% graphene/CuO-black paint coating. The entropy generation analysis concludes that the entropy generation increases with increasing the mass flow rate. Exergy efficiency enhancement can be found 0.169%for 0.3% with respect to 0.1% graphene/CuO-black paint.

Performance study of a combined low-concentration bifacial photovoltaic/thermal system with glass channels

The combination of photovoltaic and photothermal, often abbreviated as PV/T, is a promising approach that has been increasingly studied to improve solar energy conversion efficiencies. In this paper, a low-concentration bifacial PV/T system (LC-BiPV/T) which uses double-axis tracking module and direct-contact glass-channel was built. The electric and thermal performances of this system are studied experimentally. When the series and parallel waterflow configuration is compared, the result shows that the average electric efficiency of the direct contact glass-channel LC-BiPV/T system is 13% and the average thermal efficiency is 48.13% when running at series configuration. When the average electric efficiency is 14.90% and the average thermal efficiency is 58.69% at parallel configuration. In general, the thermal and electric performance of this system are excellent, and the series configuration can further improve both efficiencies of the system. This experimental result can aid on the improvement of future energy-using.

Performance study on photovoltaic thermal building façade component in multi-energy generation during winter

Photovoltaic thermal systems have gained tremendous popularity in the production of electric and thermal energy. In this paper, the photovoltaic thermal modules for the building façade assisted by heat pump system is proposed which combines the photovoltaic modules with an evaporator part of the heat pump system to produce hot water and electrical energy. Also, the photovoltaic thermal panels are used to preheat the cold ambient fresh air without heat pump operation. The proposed system was constructed at the Institute of Building Energy, Dalian University of Technology, China to study the ambient fresh air heating characteristic, electrical power generation, and hot water generation through performance evaluation indices under natural weather conditions. It was found that the average electrical, thermal, and overall efficiencies are 8.8%, 26%, and 50%, respectively during the pre-heating of fresh air. While the average air temperature is 15.2°C inside an air gap. The average COP for water heating is 3.91 during the water heating mode. This study could be used as a guide for photovoltaic thermal solar-assisted heat pump systems on building envelopes in a multi-energy generation under different weather conditions. Practical application: The study considers the photovoltaic thermal modules for building façade not only to generate the electrical energy and pre-heated fresh air but also to generate the hot water when assisted with the heat pump system. This research could assist researchers and engineers in the field of photovoltaic thermal façade systems in multi-energy generation such as for the production of electricity, heated/cooled fresh air, and hot water generation.

Experimental Investigation of the Thermal Performance of a Solar Air Heater

Solar air heaters are widely used in many low temperature applications such as space heating, crops drying, desalination..etc. It collects solar radiant energy and transforms it into heat through a fluid (air) flowing inside the system. The outside cold air is heated through the system and delivered to the required application. It is simple, economic and clean. In this study, an experimental investigation is carried out using a test-rig installed at the laboratories facility for Center of Solar Energy Research and Studies (CSERS) in Tajoura-Libya equipped with all necessary measuring instruments and devices. The aim of this study is to investigate the effect of process air mass flow rate on the thermal performance of a solar air heater working at different operating conditions under the prevailing conditions of Tajoura-Libya. Experiments were conducted on specified days in August 2019, October 2019 and January, 2020.Results show that there is a noticeable increase in the air temperatures of the solar air heater as incident solar radiation values increase during the day time, especially at afternoon. The maximum average outlet air temperature measured reaches 60 oC which is suitable for space heating and crops drying applications. Useful heat energy collected is directly proportional to the incident solar radiation. Increasing air mass flow rates leads to a corresponding decrease in the temperature at different locations in the solar air heater. Furthermore, the average thermal efficiency values of the solar air heater range from 35% to 65%. Average overall heat loss coefficient values tend to decrease with the day time. Finally, the present study results coincide with literature and show a good agreement.

Experiment study of a hybrid concentrated photovoltaic/thermal(CPV/T) system

In order to improve the efficiency of solar energy utilization, a hybrid concentrated photovoltaic/thermal(CPV/T) system equipped with 750× Fresnel lens and roll-bond heat collector of water-bifurcated channel was proposed. The system produces both electricity and heat performance of the system was tested based on the outdoor measurement in a clear day. The experimental results show that the maximum generating power of the system can reach 3.656KW, with an average generating power of 3.31KW. The total generating energy of the system reached 25.21KWh from 9:00 am to 5:00 pm. The maximum and average electrical efficiency are 25.36% and 24.53%, respectively. The maximum and average thermal efficiency are 63.92% and 58.72%, respectively. The total efficiency can be more than 80%.

Thermo-economic design, optimization, and evaluation of a novel zeotropic ORC with mixture composition adjustment during operation

Organic Rankine cycle (ORC) is a proposing technology that converting low temperature (usually lower than 200 °C) thermal energy into power. Mostly, the ORC is operated under off-design conditions and the operation performance is deteriorated remarkably from that achieved under rated condition. In the present study, a novel composition-adjustable zeotropic ORC is proposed. A liquid separation condenser-based unit, coupling the heat transfer intensification and mixture composition tuning, is conceptually designed, and integrated into the zeotropic ORC. A regulation strategy for working fluid composition is developed. A thermo-economic evaluation and optimization model and a mixture composition adjustment model are formulated to investigate the superiority of the proposed ORC. A sequential method and genetic algorithm (GA) are applied to conduct the thermodynamic optimization, component design, and thermo-economic optimization of the proposed ORC. A case study is conducted to validate the thermo-economic superiority of the proposed composition adjustable ORC driven by geothermal energy. Results show that the proposed ORC features 0.52% higher annual average net power output, 2.20% higher annual average thermal efficiency, and 21.43% lower average electricity production cost than conventional ORC. The fluid composition-regulating system can achieve the target composition within acceptable time.

Energy and exergy analyses of a mixed-mode solar dryer of pear slices (Pyrus communis L)

A mixed-type solar dryer operating with three different modes of operation (mixed-mode natural convection (MM-NC); mixed-mode forced convection (MM-FC) and indirect-mode natural convection (IM-NC)) were evaluated to reduce the moisture content of pear of 82.15 ± 0.13% up to 8.86 ± 1.3% (wet basis). The drying times were 5.25, 4.5, and 6.6 h; the drying efficiencies were 13.6, 11.2, and 26.6%, and the average instantaneous thermal efficiencies of the solar air heater were 33.0%, 56.3%, and 30.9% for the MM-NC, MM-FC, and IM-NC modes, respectively. The drying chamber’s exergy efficiency values in the MM-NC mode was 65.2%, while the lowest values were in the MM-FC mode (54.4%). The MM-FC mode had a more significant improvement potential (308.76 kJ) than the other modes. The highest average value of the energy utilization ratio was 29.1% in the IM-NC mode. The experimental data shows that the MM-NC mode had a higher exergy efficiency than the other two operation modes. The IM-NC mode shows a lower improvement potential, taking better advantage of the supplied energy. Having these results would be expected that by increasing the load, the solar dryer’s thermal and exergy efficiency in the MM-NC mode and MM-FC mode could be improved.

A Comparison Study on Energy Performance between Different PV/T Systems and PV System in Highland Cold Winter

This paper aims to investigate the enhancement performance of two different PV/T systems on power generating and space heating in highland cold area in winter, compared to a standard PV system. A comparison system with a same size of PV system, liquid-cooled PV/T system and heat pipe PV/T system has been designed and constructed in Ganzi city, China. The testing on the energy performance of these systems are carried out under real weather condition for several days. The experimental results indicated that the daily average electrical efficiency of the PV system, heat pipe PV/T system and liquid-cooled PV/T system ranges from 11.8% to 13.0%, 11.9% to 12.7%, and 11.7% to 12.7%, respectively, with a similar average value of 12.5%, benefiting to the low ambient temperature of local weather. The average daily heating power, thermal efficiency, and total efficiency of the heat pipe PV/T system range from 780.9 W to 954.6 W, 42.9% to 56.9%, and 52.0% to 66.3%, respectively. While for the liquid-cooled PV/T system, they range from 757.1 W to 846.1 W, 37.4% to 44.2%, and 46.4% to 53.5%, respectively. According to the comparison result, the heat pipe PV/T system has a better energy performance than liquid-cooled type.

The performance analysis on a novel purified pv-trombe wall for electricity, space heating, formaldehyde degradation and bacteria in activation

On the one hand, PV-Trombe wall is a combined solar system that meets dual functions of space heating and electricity output, but the utilization on the recovered solar heat by PV-Trombe wall is very limited. On the other hand, thermal catalytic oxidation and thermal sterilization are two advanced air purification technologies and both they have huge application potential with solar heat. Therefore, a novel purified PV-Trombe wall for electricity, space heating, formaldehyde degradation and bacteria inactivation was proposed. Firstly, the system thermal and mass transfer model was established and verified by experimental data. Secondly, the comprehensive performance with the PV coverage ratio of 0 was analyzed. Finally, the effect of PV coverage ratio on the system performance was investigated. Accordingly, the main results were: ① Under PV coverage ratio of 0, the average daily air thermal efficiency and formaldehyde single-pass ratio were 0. 46 and 0. 35, respectively. Meanwhile, the total generated volume of clean air by formaldehyde degradation was 93. 4 m3 . Five kinds of bacteria were fully thermal inactivated for several hours. Accordingly, the total generated volume of clean air were 188. 3, 173. 0, 201. 4, 189. 9 and 200. 2 m3 for E. coli, L. monocytogenes, L. plantarum, S. Senftenberg and S. cerevisiae, respectively. ② The PV coverage ratio played a different role on the system comprehensive performances. For specific performances, the PV coverage ratio only had a positive influence on electrical performance and had negative effect on other performances such as thermal, formaldehyde degradation and bacteria inactivation performances. However, considering the system comprehensive performances, the electrical energy was the additional product and the low PV coverage ratio was suggested. ③ The works could provide possible support to the prevention and control of COVID-19. © 2021, Editorial Department of Journal of University of Science and Technology of China. All rights reserved.

Modeling of an Organic Rankine Cycle Waste Heat Recovery system for automotive engine applications

The remarkable investments made by manufacturers over the last few decades have contributed to improving the performance of internal combustion engines in every aspect: lower polluting emissions, greater specific power and thermal efficiency. Despite this, on an average, about 40% of the thermal power theoretically available from the combustion of the fuel is still stored in the exhaust gases and therefore dispersed in the environment. In this work the modeling and validation of a waste heat recovery (WHR) plant will be described, combining the engine with a low temperature Organic Rankine Cycle (ORC) system, in order to investigate the feasibility of this system on board of a vehicle, analyzing the quantity of thermal power recovered and made available in the form of electrical power. The ORC plant is modeled using a 0D/1D thermo-fluid dynamic approach. Starting from experimental tests, a map-based model for the piston pump and the scroll expander has been developed. The model has been validated through the use of a vector optimization technique, exploiting a genetic algorithm (MOGA). Subsequently, this system has been coupled to a spark ignition engine for automotive applications, adapting its speed range to comply with the ORC experimental tests. To have an accurate control over the expander inlet temperature, a bypass circuit and two throttles actuated by a PI controller have been implemented. The simulations were performed by considering 18 engine points at maximum load and different rpm. An average thermal efficiency increase of the system of 2.6% was obtained by introducing the recovery plant, and wide improvement chance can be foreseen in the case of ORC full-power use.

Evaluation and Improvement of Thermal Energy of Heat Exchangers with SWCNT, GQD Nanoparticles and PCM (RT82)

Today, due to the reduction of energy resources in the world and its pollutants, energy storage methods and increase the thermal efficiency of various systems are very important. In this research, the thermal efficiency and energy storage of two heat exchangers have been investigated in series using phase change materials (RT82) and single wall carbon nanotubes (SWCNT) and graphene quantum dot nanoparticles (GQD) In this research, two heat exchangers have been used in combination. The first heat exchanger was in charge of storing thermal energy and the second heat exchanger was in charge of heat exchange. The reason for this is to improve the heat exchange of the main exchanger (shell and tube) by using heat storage in the secondary exchanger, which has not been addressed in previous research. The results of this study showed that using two heat exchangers in series, the thermal efficiency of the system has increased. Also, the heat energy storage of the double tube heat exchanger was obtained using phase change materials in the single-walled carbon nanotube composition of about 3000 W. The average thermal efficiency of the two heat exchangers as the series has increased by 52%. In general, the effect of the two heat exchangers on each other was investigated in series with two approaches (energy storage and energy conversion) using fin and nanoparticles, which obtained convincing results.

Experimental investigation of nanomaterial doped in black paint coating on absorber for energy conversion applications

In the present study, the absorber plate of the triangular solar air heater system is coated with graphene nanomaterial doped in black paint. This is done to analyze the performance based on various mass flow rate of air and solar intensity. The different percentages of graphene nanomaterial (0.1%, 0.2%, & 0.3%) are doped in black paint which increases the absorption of heat. The Reynolds number is varied for the fixed selective coating on absorber plate and mass flow rate. Also the heat transfer of air and absorption rate on the absorber plate is increased . The thermal efficiency, exergy efficiency and improvement potential are also enhanced. In addition, the effects of entropy generation are examined with respect to solar radiation and ambient temperature. The results are presented for constant mass flow rate ranging from 0.0036 to 0.0181 kg/s. The experimental result gives the average thermal efficiency enhancement of 4.91% for BP-II. Entropy generation varies 0.1571 to 0.7189 W/K. Entropy generation is maximum for BP-I and minimum for BP-II. It concludes that the entropy generation increases with increase in mass flow rate. Exergy efficiency for BP-II is found to be 4.27% higher as compared to BP-I.

Helically coiled solar cavity receiver for micro-scale direct steam generation

Concentrated solar systems have a large potential for power generation and water pumping in rural off-grid settlements with a high solar irradiance resource. With the use of a solar dish concentrator, superheated steam can be directly generated in a solar cavity receiver. Volumetric expanders show promise for micro-scale power production (below 1 kW). However, naturalistic experimental investigations of micro-scale direct steam generation using a solar dish is limited. In this work, a low-cost helically coiled cavity receiver and a novel solar concentrator manufactured from off-the-shelf components are experimentally investigated for steam generation at water pressures of above 3 bar. Working fluid temperatures were measured throughout the coil length and pressures were measured at the inlet and outlet of the receiver coil. Using a concentrator with a total incident area of 2.70 m2 and a water flow rate of 0.294 g/s, the working fluid captured 861 W of heat with an outlet temperature of 343 °C at an average solar irradiance of 757 W/m2. The average thermal efficiencies of the collector and receiver were determined to be 42% and 50%, respectively, for the testing period. Furthermore, the average second-law efficiency was 12%. Flow patterns within the coil were investigated by considering the temperature differences between the coil surface and the working fluid. A heat loss model was developed that could predict the steady-state heat loss rate with an accuracy of 97%. The observations made during the study and the results obtained highlight important design aspects that need to be considered in future work.

Investigation of the effects of solar tracking system on performance of the solar air dryer

Design of the collector in solar air collector dryer (SACD) system is very important factor for high drying performance and the effective use of solar radiation. The main purpose of this study is to increase the thermal efficiency of the solar air collector (SAC) by adding a two-direction solar tracking feature supported by programmable logic controller (PLC) to the SAC. A movable SAC and fixed SAC with same dimensions were designed and integrating both collectors into the drying cabinet, generating two different SACDs. Semielliptical sliced products were dried at similar air flow rate and ambient conditions in both drying systems. The results indicated the highest SAC thermal efficiency was 91% in the SACD with solar tracking feature, while the lowest SAC thermal efficiency was 34% in the other SACD. In addition, the drying process in the solar tracking feature SACD was performed with an average drying efficiency of 66%, while the fixed SACD was performed with an average drying efficiency of 32%. Based on the results, the main role of PLC solar tracking feature in SACD system is to increase the average thermal efficiency by 42% and to double the average drying efficiency.

Enhancement of Solar Desalination Still Productivity Using Flash Evaporation (Dept.M)

A solar desalination system with flashing chamber is experimentally investigated. The main objective of the present study was lo improve the performance of a traditional single slope solar still through a design modification using step-wise water basin instead of flat basin and coupling the solar still with a spray water system. These modifications can increase the solar still capability to capture more solar energy. The effect of using the spray system for seawater is investigated experimentally at different spray water velocities, and flow rates on the productivity and efficiency of the solar still. It was found that the productivity and performance of the system were significantly positive dependent on both the mass flow rate of the impure water and the linear speed of the nozzles holder. They increase slightly with the using of a porous medium in basin of the still as a heat storage sink. There is an optimum mass flow corresponding to the holder linear speed with respect to both thermal efficiency and distilled water production. The unit daily productivity reaches 6.7 L/m2.day and the average daily thermal efficiency reaches 77.35%. The maximum accumulated productivity is obtained at 250 rpm.

Assessment of double-pass pin finned solar air heater at different air mass ratios via energy, exergy, economic, and environmental (4E) approaches.

In this study, an assessment based on energy, exergy, economic, and environmental approaches on a double-pass (DP) solar air heater (SAH) having pin finned absorber at different air mass ratios up and down the absorber is investigated experimentally. Four air mass ratios are considered: (i) all the air mass flow passes up the absorber and returns to pass down the absorber (DP), (ii) 2/3 of the airflow passes up the absorber and returns to mix with the remainder of air to pass down the absorber (2/3 DP), (iii) the same as (ii) but 1/3 of the air passes up the absorber (1/3 DP), and (iv) all the air mass passes only down the absorber (single pass, SP). For all mass ratios, the performance of pin finned SAH (P_SAH) is compared with that of flat SAH (F_SAH). The results indicated that the air temperature rise and energy and exergy efficiencies of P_SAH are highly greater than those of F_SAH. The highest average thermal efficiency of F_SAH is 56.7% obtained at DP flow condition, whereas the highest value of P_SAH is 65.21% obtained at 2/3 DP with an increase of 17.6% compared with F_SAH. Also, P_SAH has higher average exergy efficiency of about 34.7% compared to F_SAH. Furthermore, P_SAH achieves energy payback time (EPBT) lower than that of F_SAH, while P_SAH has higher embodied energy. The findings indicated that F_SAH at SP airflow pattern has the maximum energy cost (0.0427$/kWh), whereas P_SAH at 2/3 DP airflow pattern achieves the minimum energy cost (0.037$/kWh). Finally, the proposed P_SAH system appears to be more viable from exergoeconomic and enviroeconomic approaches compared to F_SAH.

Drying kinetics of tomato (Solanum lycopersicum) and Brinjal (Solanum melongena) using an indirect type solar dryer and performance parameters of dryer

Tomato (Solanum lycopersicum) and brinjal (Solanum melongena) are food products that are frequently wasted in Indian markets as their shelf lives are only a few days. An indirect type solar dryer (ITSD) has been developed. It’s performance and drying kinetics of brinjal and tomato slices have been analyzed. The moisture content of tomato decreased from 15.667 to 0.803 kg/kg of dry basis (db) and that of brinjal reduced from 10.111 to 0.498 kg/kg of db. The drying curve was fitted with the different models of existing studies. The average effective moisture diffusivity is estimated and it is 3.60 × 10−9 and 4.00× 10−9 m2/s for tomato and brinjal, respectively. Mass transfer coefficient was in the range of 0.82 × 10−4 to 2.85 × 10−3 m/s for tomato and 1.11 × 10−4 to 3.32 × 10−3 m/s for brinjal. The heat transfer coefficient was in the range of 0.089 to 2.888 W/m2 K and 0.1066 to 3.3564 W/m2 K for tomato and brinjal, respectively. The activation energy for tomato and brinjal was 21.19 and 19.46 kJ/mol, respectively. The average thermal efficiency of the collector and dryer was 59.05% and 31.4% during tomato drying and 58.42% and 25.16% for brinjal drying, respectively.

Experimental and theoretical analyses of a double-cylindrical trough solar concentrator

Parabolic trough, Fresnel, heliostat mirror, or dish-type collectors are used to obtain high temperature in solar thermal applications. The operation of this system is similar to that of a Fresnel collector, except for its reflective mirror, which is cylindrical and not flat. The experiments were carried out in the solar energy system manufactured for producing hot water, superheated water, and steam in the cylindrical trough solar concentrator. According to the test results, the thermal power of the system and the average thermal efficiency of the collector were a maximum of 15 kW and approximately 35 %, respectively.

Energy and exergy analysis of the drying of corn grains

A mixed cabin solar dryer was designed and tested in Brazil in the spring equinox, for the drying of corn grains. The key advantage of the current design is its ability to work even in locations with no electricity because a photovoltaic module is used to power an electrical heater and fans. The analysis of the drying process was performed based on the first and second laws of thermodynamics. For average solar radiation and ambient temperature of 710 W/m2 and 30 °C, respectively, the average thermal efficiency obtained was 21% and the exergy efficiency ranged between 10% and 66%, with an average value of 23%. It was observed that the corn grains reached the desired moisture content of 13% in 8.5 h, while a similar sample subjected to natural sun drying failed to reach this moisture content in 24 h. The PV module was also used to preheat the drying air, allowing an average increase in the airflow temperature of 14 °C, reaching a maximum of 27 °C.

Wall-suspended trays inside stepped distiller with Al2O3/paraffin wax mixture and vapor suction: Experimental implementation

In the present work, we conducted some modifications in the design and operation of stepped distiller to improve its performance. The stepped distiller was modified by installing fixed suspended trays over the vertical walls of the steps to increase the evaporative and exposure surface areas. Also, we created a parallel cavity under the steps’ liner to work as a heat storing space. This cavity was filled by a paraffin wax mixed by Al2O3 nanoparticles as a phase change material (PCM). Besides, the stepped solar distiller performance was studied under integrating the distiller with an external condenser. Furthermore, the performance of the stepped solar still was investigated under different fan speeds of zero, 400, 800, 1200, 1600, 2000, 2400, and 2800 rpm. The performance of the solar still was evaluated under the different conditions individually and combined. Results revealed that the stepped solar distiller, either with or without condenser, had better performance than the reference solar still. At the case of using suspended trays only, the stepped distiller recorded a productivity of 4175 mL/m2 a day versus 3800 mL/m2 a day for the reference distiller with an improvement of 9.8%. Moreover, using the suspended trays and Al2O3/paraffin wax mixture in the parallel cavity increased the productivity of the solar still by around 40%, where the productivity was 5740 and 4100 mL/m2 a day for the stepped and reference stills, respectively. In addition, the optimum fan speed obtaining the maximum productivity improvement for the stepped distiller over the conventional still was 2000 rpm, where the productivity of the stepped still was improved by 55% over that of the reference still. At this fan speed of 2000 rpm, the daily distillate of the conventional and stepped distillers was 4000 and 6200 mL/m², respectively. Finally, the average daily thermal efficiency of the stepped solar still was 46%, 48.7%, and 52.4% when using the suspended trays, suspended trays with PCM, suspended trays with PCM and fan at 2000 rpm, respectively. While the average thermal efficiency of the reference basin distiller was ranged between ~ 38 – 41%. Finally, the cost of the distilled water of stepped still and reference still is 0.014 and 0.015 $/L, respectively.