Evacuated Tube Research Articles

Integrated novel solar distillation and solar single-effect absorption systems

Novel hybrid single-slope solar distillation systems integrated with single-effect solar absorption cooling system are designed, analyzed and optimized for production of cooling effect and freshwater, simultaneously. The innovative integrated systems are capable of recovering significant portion of the released heat of their own components that is otherwise wasted; with superior performance and lower cost than available systems. The integration of the two solar systems is proposed in three strategic heat recovery configurations: (S1) recovers waste heat from the water vapor leaving the generator, (S2) recovers waste heat from the strong lithium bromide solution leaving the generator, and (S3) recovers waste heat from the hot liquid water leaving the generator. The results show that the productivity of the still is boosted by three-folds compared to the conventional system. S2 negatively affects the COP of the absorption system, however, it improves still productivity to higher values than S1.The COP of S3 is comparable to S1, while the still productivity of S3 is the highest with only slight increase in the area of the evacuated tube collector. S1 has the advantage of enhancing the still productivity and reducing the condenser cooling load without affecting the COP of the basic absorption system. The integrated system achieved an impressive cooling capacity of 20 kW, and still productivity of 10 kg/m2-day with COP of 0.85 at conservative solar radiation of 500 W/m2. The cost of the distilled water in S3 (0.047 $/kg) is lower than in S2 (0.054 $/kg) and S1 (0.061 $/kg).

Thermo-Economic Comparisons of Environmentally Friendly Solar Assisted Absorption Air Conditioning Systems

Absorption refrigeration cycle is considered a vital option for thermal cooling processes. Designing new systems is needed to meet the increasing communities’ demands of space cooling. This should be given more attention especially with the increasing conventional fossil fuel energy costs and CO2 emission. This work presents the thermo-economic analysis to compare between different solar absorption cooling system configurations. The proposed system combines a solar field, flashing tank and absorption chiller: two types of absorption cycle H2O-LiBr and NH3-H2O have been compared to each other by parabolic trough collectors and evacuated tube collectors under the same operating conditions. A case study of 200 TR total cooling load is also presented. Results reveal that parabolic trough collector combined with H2O-LiBr (PTC/H2O-LiBr) gives lower design aspects and minimum rates of hourly costs (5.2 $/h) followed by ETC/H2O-LiBr configuration (5.6 $/h). H2O-LiBr gives lower thermo-economic product cost (0.14 $/GJ) compared to the NH3-H2O (0.16 $/GJ). The absorption refrigeration cycle coefficient of performance ranged between 0.5 and 0.9.

Thermo-Economic Comparisons of Environmentally Friendly Solar Assisted Absorption Air Conditioning Systems

Absorption refrigeration cycle is considered a vital option for thermal cooling processes. Designing new systems is needed to meet the increasing communities’ demands of space cooling. This should be given more attention especially with the increasing conventional fossil fuel energy costs and CO₂ emission. This work presents the thermo-economic analysis to compare between different solar absorption cooling system configurations. The proposed system combines a solar field, flashing tank and absorption chiller: two types of absorption cycle H₂O-LiBr and NH₃-H₂O have been compared to each other by parabolic trough collectors and evacuated tube collectors under the same operating conditions. A case study of 200 TR total cooling load is also presented. Results reveal that parabolic trough collector combined with H₂O-LiBr (PTC/H₂O-LiBr) gives lower design aspects and minimum rates of hourly costs (5.2 $/h) followed by ETC/H₂O-LiBr configuration (5.6 $/h). H2O-LiBr gives lower thermo-economic product cost (0.14 $/GJ) compared to the NH₃-H₂O (0.16 $/GJ). The absorption refrigeration cycle coefficient of performance ranged between 0.5 and 0.9.

Performance Investigation of Flat Plate and Evacuated Tube Collectors under Jordan Climate Conditions Using TRNSYS Software

The performance of the Flat Plate Collector (FPC) and Evacuated Tube Collector (ETC) for domestic hot water applications under Jordan climate conditions was theoretically investigated using TRNSYS software. The presented simulation model can provide a long-term evaluation of system performance at different weather conditions. This simulation was carried out during 24 hours in 21 June, is considered as a hot climate and 21 January is considered as a cold climate; under the meteorological conditions of both Irbid and Aqaba cities in Jordan. The inlet water temperature of the first collector, collector area, water flow rate and tilt angle were considered 15℃, 2.04 m2, 0.0139 kg/s and 45 degrees respectively. The outlet temperature of each collector was considered as the inlet of the previous collector. Three collectors in series were applied to increase the output temperature up to 90-100℃. The results showed that TE increased with the decrease in the inlet temperature, which means the useful energy gain (Qu) of the first collector is greater than the 2nd and the 3rdcollector. The useful energy gain (Qu) of ETC is greater than of FPC, and the useful energy gain (Qu) of cold climate is less than hot climate. The output temperature of ETCs is greater than that of FPCs for Irbid city in cold and hot climate. Furthermore, for Aqaba in cold climate and in hot climate the outlet temperature of ETCs is higher than FPCs. Additionally, the daily thermal performance of the ETCs is significantly better than the FPCs in cold climate. Meanwhile, thermal performance of both collectors are close in a hot climate.

Mathematical Modelling and Analysis of Hybrid Solar Desalination System Using Evacuated Tube Collector (ETC) and Compound Parabolic Concentrator (CPC)

Mathematical Modelling and Analysis of Hybrid Solar Desalination System Using Evacuated Tube Collector (ETC) and Compound Parabolic Concentrator (CPC)

Thermal performance of heat pipe evacuated tube solar collector integrated with different types of phase change materials at various location

This paper presents an experimental investigation of a new method of phase change materials (PCMs) integration in one or both the evacuated tube (ET) and two separated tanks adjacent to the water tank of heat pipe evacuated tube solar water heater collector (HP/ETC). In this method, the evacuated tube was filled with medical paraffin wax as thermal heat storage, while the two separated tanks were filled with paraffin wax (grade-A). Therefore, owing to the thermal isolation of the evacuated tube and PCM storage tanks, heat is efficiently accumulated and stored for a longer time. The advantage of this new method is that improved overall solar water heater performance by delayed heat release, thereby supplying hot water for a long time at high demand or when low solar intensity. Four HP/ETSCs with gravity assist heat pipe (GAHP) rigs were used for a comparative study. The first HP/ETC was equipped with PCM in its ET and two PCM storage tanks. The second and third HP/ETCs were integrated with PCM in an ET and two PCM storage tanks, respectively. While the fourth HP/ETC was considered a reference as it was left without PCM. Each heat pipe was filled with a 0.7 filling ratio of pure acetone. The tests are carried out with two different rates of water flow (i.e. 1 and 2L/h). The results demonstrate that the integration of PCM in both the ET and the separated tanks gives an efficiency improvement of 55.7%, While the integration of PCM in the ET led to an increase in the efficiency of 49.9%. Whereas the efficiency has been improved with the integration of PCM in the separated tanks by about 36.5%, compared with PCM-free reference collector.

Submicrometer-Gap Thermionic Power Generation Based on Comprehensive Modeling of Charge and Thermal Transport

Over the past years, thermionic energy conversion (TEC) with a reduced inter-electrode vacuum gap has been studied as an effective way to mitigate a large potential barrier due to space charge accumulation. However, existing theoretical models do not fully consider the fundamental aspects of thermionic emission when the inter-electrode gap shrinks to the nanoscale, which results in underestimation of thermionic power generation for such small gaps. The present work addresses this challenge by comprehensively modeling charge and thermal transport processes with specific consideration of nanoscale gap effects, such as image charge perturbation, electron tunneling, and near-field thermal radiation. Carefully conducted energy balance analysis reveals that if optimized, submicron-gap TEC can excel the micron-gap counterpart with $\sim$4 times the power output and ~5-10 % higher energy conversion efficiency. Moreover, the high-temperature collector of the submicron-gap TEC, which is due to thermionic and near-field radiative heat transfer, allows the addition of a bottom-cycle heat engine to further enhance the power and efficiency when combined. Electric field concentration due to engineered surface roughness is also examined as a potential approach to produce an additional increase in power generation. We believe that the present work provides a theoretical framework for submicron-gap thermionic power generation as a promising energy recycling scheme for high-quality heat sources.

Comparative study of modelling the thermal efficiency of a novel straight through evacuated tube collector with MLR, SVR, BP and RBF methods

Data-based methods are useful for accurate modelling of solar thermal systems. In this work, several artificial neural network (ANN) techniques are proposed to predict the thermal performance of an all-glass straight through evacuated tube solar collector. These are compared to support vector regression analysis. Extensive experimental data sets were collected for training the ANN models. Solar radiation intensity, ambient temperature, wind speed, mass flow rate and collector inlet temperature were selected as the input layer to predict the thermal efficiency of the solar collector. The prediction precision of the ANN models was compared to the multiple linear regression and support vector regression model using different criteria. The Radial Basis Function (RBF) neural network method gave the best prediction accuracy followed by the Back Propagation (BP) model. The sensitivity of the model to changes in the input variables (solar radiation intensity, collector inlet temperature, fluid flow rate and wind speed) was also investigated showing the largest dependency on solar radiation.

An integrated absorption cooling technology with thermoelectric generator powered by solar energy

A novel integrated solar absorption refrigeration system with a thermoelectric generator and thermoelectric cooler is presented. The proposed system is of a 20-kW single-stage lithium bromide absorption cycle driven by solar evacuated tube collectors or by the heat rejected by the thermoelectric cooler module. The governing equations of the thermodynamic model are solved, and the results are validated. It was found that the coefficient of performance of the system approaches a constant value at a generator temperature of 100 °C. The best performance of the system was found in the case of placing the thermoelectric cooler between the generator and the condenser. The coefficient of performance and the overall thermal efficiency were found to be 1.12 and 73.4%, respectively. Furthermore, a 20% rise in the generator temperature would increase the power produced by thermoelectric generator modules by 60%. An increase of 20% in the thermoelectric generator modules would result in a reduction of 11.5% in its efficiency, while doubling the temperature difference would increase the generated power by a factor of three.

A novel active-mode indirect solar dryer for agricultural products: Experimental evaluation and economic feasibility

This article presents a novel active-mode indirect solar dryer consisting of highly efficiency through-flow evacuated tube collector for air heating. A DC fan powered with solar photovoltaic (PV) module is provided for forced air flow through this dryer. The dehydration chamber is of tray type with loading capacity of 6 kg–45 kg depending on the drying product. The dryer was evaluated to dry fenugreek leaves (Trigonella foenum-graecum) and turmeric (Curcuma longa) in semi-continuous mode; and respective overall thermal efficiency was found to be 34.1% and 23.6%. The respective thermal efficiency in open sun drying (OSD) was 5.7% and 5.4%. The quality of dried fenugreek leaves and turmeric was found better than OSD. Economics of the solar dryer indicate the capital cost is negligible in comparison to cumulative present worth of life cycle savings. The values are 211,262 INR (1 US$ = 74.57 INR) and 229,152 INR respectively for fenugreek leaves and turmeric; and capital cost of the dryer is only 50,000 INR. The low value of payback period also confirms economic viability of this dryer. The generalized economic curves have also been plotted.

Energy, exergy, enviroeconomic & exergoeconomic (4E) assessment of thermal energy storage assisted solar water heating system: Experimental & theoretical approach

In this study, exergy, energy, enviroeconomic & exergoeconomic analysis of evacuated tube collector with/without latent heat storage material has been done for different flow rates i.e. 0.13, 0.26, 0.40 & 0.46 LPM. In this manuscript, palmitic acid has been used as a latent heat storage material. The storage material was filled in the annuls between the absorber pipe and heat pipe termed as palmitic acid integrated evacuated tube collector system while another heat pipe based evacuated tube collector system was tested without latent heat storage material termed as evacuated tube collector without storage system. The results of the experimental investigation revealed that daily thermal & exergy efficiencies of palmitic acid integrated evacuated tube collector increased in the range of 36-44% and 28-35% respectively in comparison to evacuated tube collector without storage.Furthermore, storage based evacuated tube collector system mitigated 21-28 tCO2/lifetime and 6-8 tCO2/lifetime based on environmental and exergoenvironmental approaches respectively. Whereas, evacuated tube collector without storage mitigated 14-19 tCO2/lifetime and 4-6 tCO2/lifetime based on environmental and exergoenvironmental approach respectively. In comparison to collector without storage, the revenue generated by collector with storage was found to be 95-112 $/lifetime and 22-25 $/lifetime higher based on energy and exergy approach respectively.Overall, palmitic acid integrated evacuated tube collector based continuous flow solar water heating system found to be the most promising system comparison to the collector without storage for different selected flow rates of water.

Investigation on The Thermal Performance of Evacuated Glass-Thermal Absorber Tube Collector (EGATC) for Air Heating Application

Existing design of Heat-Pipe Evacuated Tube Collector (HP ETC) for solar water heating require storage tank and additional heat exchanger required for air heating application which leads to the extra spacing and costing requirement. HP ETC have better thermal performance to produce high outlet temperature than flat plate collector (FPC), especially during diffuse solar radiation. But HP ETC normally focusing on water heating system. Furthermore, HP ETC and FPC installation need to be positioned either to south or north facing to ensure the solar thermal collector absorbs more solar radiation. Meanwhile, HP ETC need to be tilt at the correct angle to maximize the performance of the system. These could lead to design limitation. The aim of this research is to develop the new design of Evacuated Glass-Thermal Absorber Tube Collector namely EGATC for drying application. It was developed from conventional HP ETC evacuated glass tube. In this study comparison result of EGATC and HP ETC performance were evaluated. The three days outdoor experiment proves that the performance of EGATC was better than HP ETC in air heating application which is provide higher outlet temperature. Based on the result, EGATC (Day 1: 50.9 oC, Day 2: 53.9 oC, Day 3: 49.2 oC) performed better with slightly higher temperature at outlet temperature compare with HP ETC (Day 1: 46.7 oC, Day 2: 50.3 oC, Day 3: 46.9 oC). It is concluded that EGATC have better performance in term of temperature different and outlet temperature as compared to HP ETC. EGATC (Day 1: 53.6%, Day 2: 50.6%, Day 3: 49.8%) also have greater efficiency in term of heat storage capability as compared to HP ETC (Day 1: 42.7%, Day 2: 41.6%, Day 3: 41.1%). Regarding energy buffer storage, EGATC have better energy storage compared to HP ETC at sudden weather change such as diffuse solar radiation during clouds. The outlet temperature of EGATC (42.3 oC) was remained slightly higher compared to HP ETC (39.9 oC) at the beginning. The outlet temperature gradually drops slower during discharging period until the end of the experiment for 15 minutes towards outlet temperature 41.1ºC and 37.2ºC for both EGATC and HP ETC with temperature difference 1.2ºC and 2.7ºC respectively.

Performance evaluation of hybrid photovoltaic thermal (PVT) solar dryer for drying of cassava

A prototype hybrid Photovoltaic Thermal (PV – T) solar dryer aided with Evacuated Tube Collector (ETC) is presented for drying of cassava slices under the meteorological conditions of Thanjavur, Tamilnadu, India. Further the morphological, structural and physical properties of thin layer drying kinetics are studied and compared with natural sun drying. The designed dryer reduces the moisture content of cassava from 91.5% (wb) to 10.67% (wb) which is considered to be a safe level for storage in 8 h. There is a wide scope to save the conventional fuel by adopting the designed solar dryer as the air temperature inside the chamber rise is 30–40 °C than the ambient temperature. Non – linear regression analysis is performed to find out the drying kinetics of cassava using IBM SPSS 23 statistical package. Two - term model is proved to be the most suitable model for predicting thin-layer drying behaviour of cassava slices with R2 = 0.982, χ2 = 0.017553 for hybrid dryer and R2 = 0.998, χ2 = 0.001247 for open sun drying. XRD results exhibit ‘A’ type crystalline pattern that indicates semi–crystalline nature of cassava for both hybrid and sun dried cassava. Structural, morphological and proximate composition analysis reveals that the quality of hybrid dried cassava is better in terms of physical and chemical compositions than sun dried cassava. The proposed hybrid dryer is capable of producing high standard dried products for exporting and can make a good profit.

Investigation on solar water heater with different types of turbulators

Solar water heaters have become unavoidable renewable energy systems for the most of the residential, commercial and industrial hot water requirements. Evacuated tube collector (ETC) based solar water heaters are widely used for the aforementioned purposes in recent days due to their relative advantages over other systems. Exclusively, the ETC systems are very economical and efficient for the passive operation. Hence, they are keenly installed in the commercial and residential buildings. A major hiccup with ETC systems in passive mode of operation is the poor water circulation at the lower end of the ETCs, which make them inactive in most of the time and reduces the effective absorber area. This work is planned to curtail the aforementioned issue of the ETCs by inducing turbulence using two different types of turbulators, namely coiled turbulator and matrix turbulator inside the ETC. The experiments were accomplished in three modes namely, ETC without turbulator (Simple-ETC), ETC with coiled turbulator (CT-ETC) and ETC with matrix turbulator (MT-ETC). The results were compared and analyzed. The major results revealed that the presence of the turbulators significantly enhanced the ETC tube mean temperature by stimulating turbulence. They simultaneously improved the solar gain and thereby, mean tank water temperature was boosted-up. Quantitatively, the tank water temperature was escalated by 3.5 °C and 7.5 °C, with the aid of coiled turbulator and matrix turbulator, respectively. Further, the matrix turbulator seems ahead in improving heat transfer inside the ETC compared to coiled turbulator.

Performance analysis of a PCM integrated domestic solar water heater by numerical simulations

The present work attempts to analyse the performance of a non-pressurised domestic solar water heater (DSWH) integrated with phase change material (PCM) during the two modes of operation, namely the charging and discharging modes. PCM's melting and the solidifying rate has been identified as the primary criterion influencing the heater's performance under standard test conditions. A co-axially placed cylindrical PCM container is modelled along with the evacuated tube collector (ETC) type water heater. A transient, three-dimensional numerical solver assuming constant solar insolation is used to study the phase change characteristics assisted by natural convective effects within the DSWH. The solver discretises the conservative equations using the finite volume method (FVM). Average solar insolation of 800 W/m2 has been fixed on the upper semi-cylindrical surface of ETC tubes and 35% of it as the reflected radiation falling on the ETC tubes' bottom surface. During charging, the melt fraction plot indicates that seven hours of charging is insufficient for the PCM to completely melt primarily due to the reduced heat conductivity of the PCM. Therefore, the extended period of discharge at the specified temperature expected by incorporating PCM is limited by the fraction of material remaining as solid.

Investigation on ETC solar water heater using twisted tape inserts

The solar water heaters are finding their application in a number of sectors such as, residential buildings, processing industries, commercial establishments. They can be scaled based on the requirements. Among the different kind of solar water heaters, all-glass evacuated tube type (ETC) solar water heaters are considered to be the better choice for the most applications due to their efficient operations with simple design constraints compared to other type of water heaters. However, there is a large scope to boost-up the performance of the ETC type solar water heaters to the next level with the appropriate modifications in the evacuated tubes, which will help to advance the overall efficiency. The present work proposes the amalgamation of twisted tapes within the evacuated tubes (ET) of the ETC solar water heaters to attain the enhanced performance of the system. The twisted tapes with two different twist ratios of 2 and 3 were used for the investigations. The twisted tapes were made-up of copper tapes of width 15 mm. The experimental trials were conducted on a passive flow ETC solar water heater on the days having alike solar insolation and ambient conditions in three different modes namely, ETC water heater without any twisted tapes (ETC-Plain), ETC water heater with twisted tapes comprising twist ratio of 2 (ETC-Twist 2) and the ETC water heater with twisted tapes comprising twist ratio of 3 (ETC-Twist 3). The results evidenced that the twisted tapes helped to improve the evacuated tube temperature and the tank water temperature of the ETC type solar water heater. The efficiency of the ETC type solar water heater was enhanced by 6.8% and 4.6%, respectively using twisted tapes comprising twist ratio of 2 and 3. Furthermore, the twisted tape with smaller twist ratio was determined to be efficient in improving the performance of the ETC type solar water heater.

Performance of Solar Adsorption Cooling System with Different Solar Collectors Technologies

The use of different technologies for collecting solar energy in the operation of the adsorption chiller unit is the topic of this article. A theoretical study was done through the simulation program TRNSYS. In this research, the effect of parabolic trough concentrator, compound parabolic concentrator, and the evacuated tube collector on the thermal performance of the absorption refrigeration unit was investigated in terms of cooling capacity, coefficient of performance a and the operating period of the adsorption chiller through the summer climatic condition of Cairo. The results proved that the parabolic trough concentrator (PTC) has the best effect on the performance of the 7 kW adsorption unit, where the maximum outlet collector temperature was 102 °C compare to 95.8 and 87.3 °C for the Compound Parabolic Concentrator (CPC) and the Evacuated Tube Collector (ETC) respectively. The collector type does not affect the COP of the adsorption unit but affects the operation time. For solar collectors of 20 m2, the weekly operation periods were 47, 32.24, and 25.75 hours for the PTC, CPC, and ETC respectively. These values increase with increasing the collector area. Also, for different storage tank capacities, the PTC has the best effect on the operation period of the adsorption unit compare to the other solar collectors.

Energetic and exergetic study of dual slope solar distiller coupled with evacuated tube collector under force mode

In this article, energetic and exergetic analysis of symmetric dual-slope solar still, oriented East-West at a latitude of 28°35′ N and integrated with ETC under forced operation is carried out. The performance is evaluated at an optimal flow rate within the vacuum tubes with a viewpoint of maximum heat extraction. The energetic and exergetic efficiencies are increased by ~ 8.0% and ~ 6.0%, respectively, with an increase in flow rate from 0.01 to 0.24 kg/s. With increased water temperature, evaporative fraction exergy increases, ranging 0.2–0.9. The liner, water mass, glass cover, and collector have daily exergy efficiencies of ~ 8.12, ~30.1, ~41, ~18.0%, while overall exergy efficiency and global exergy efficiency are estimated as 4.9 and ~ 7.1%, respectively.

Performance Enhancement and Improvement of Energy Efficiency in Evacuated Tube Heat Pipe Collector with Nanocomposite: A Review

Solar collector tube is used to combine incident energy and convert it into heat energy. Recently several studies have been done on improving the performance of the solar collector. Solar energy is clean energy and does not create pollution. The sun gives more power than we need. Solar collector consists of an absorber unit which transfers the heat energy of the sun to a working fluid. It is easily seen that solar intensity has more effect on the evacuated tube solar collector. In solar applications like distillation of salty water, heating, drying, air conditioning, and air heating, evacuated tube collector gives higher efficiency than flat plate collector. The evacuated tube solar collector technology gives good performance in bad weather conditions than the conventional system because vacuum inside the glass tube allows it to store higher percentage of heat. The challenge in the production of future generation evacuated tube collector technology is the control and management of operating temperature and heat generation. Heat pipe of solar collector controls operating temperature and prevent overheating that common problem in solar heating application. After chemical treatment, the heat pipe will control the freezing and prevent the evacuated tube. Heat transfer equipment is used to transfer the heat from one fluid to another fluid in many industries. The performance of solar collector with heat pipe depends on various factors such as solar radiation, ambient temperature, and flow conditions. With increasing mass flow rate, the heat transfer coefficient of the nanofluids increases. The use of nanofluids with evacuated tube heat pipe solar collector gives more efficiency and increase the output.

Design, Construction and Performance of a Solar Cloths Drying System

This paper presents the design, construction and performance evaluation of an efficient stand-alone PV- integrated solar clothes drying system for drying applications. The solar clothes drying system was constructed to reduce the reliance on unsustainable electric dryers, which are high electricity-consuming appliances resulting in the emission of large CO2 amounts to the environment. All these developments were geared towards improving the performance and reliability of the alternative sustainable energy systems and help to reduce environmental hazards. After testing different materials for the construction and the evacuated collector tube, the result shows that copper is the best for high performance and efficiency followed by aluminum. But aluminum was used for the casing and a copper tube for the evacuated collector tube. From the performance evaluation test of the hybrid solar clothes drying system, it was observed that The trend of the variation in temperature was a result of the weather condition the solar drying system was able to dry a pair of clothes within 3 hours during the outdoor test and about 3 hours and 45 minutes during the indoor test with a relative humidity of 30%. The economic analysis of the solar drying system was also carried out and the payback period was calculated to be 2.87 years, which was small compared to the life of the hybrid solar drying system.