Performance optimization of evacuated tube collector for solar cooling of a house in hot climate

Evacuating the space connecting cover and absorber significantly improves evacuated tube collector (ETC) performance. So, ETCs are progressively utilised all over the world. The main goal of current study is to explore ETC thermal efficiency in hot and severe climate like Kuwait weather conditions. A collector test facility was installed to record ETC thermal performance for one-year period. An extensively developed model for ETCs is presented, employing complete optical and thermal assessment. This study analyses separately optics and heat transfer in the evacuated tubes, allowing the analysis to be extended to different configurations. The predictions obtained are in agreement with experimental. The optimum collector parameters (collector tube length and diameter, mass flow rate and collector tilt angle) are determined. The present results indicate that the optimum tube length is 1.5 m, as at this length a significant improvement is achieved in efficiency for different tube diameters studied. Finally, the heat generated from ETCs is used for solar cooling of a house. Results of the simulation of cooling system indicate that an ETC of area 54 m², tilt angle of 25° and storage tank volume of 2.1 m³ provides 80% of air-conditioning demand in a house located in Kuwait.     

Thermal conversion with fluorescent concentrators

Fluorescent planar collector-concentrators are a new possibility for the conversion of solar energy into thermal energy. The collection and concentration of direct and diffuse radiation is feasible, using a transparent sheet of material doped with a fluorescent dye. The collector offers the advantage of separating the global irradiation into different spectral regions. This geometrical and spectral concentrated light can be converted with adapted highly selective absorbers into high temperature heat. Intensity and spectral region of the sunlight and the selectivity of the absorber determine the thermodynamically possible maximum absorber temperature. A test collector with a fluorescent concentrator area of 0.8 m² with an absorber pipe of 3 mm diameter in an evacuated glass tube was built. At a total irradiation of 850 W/m² on the fluorescent collector surface, a maximum stagnation temperature of 555°C (828 K) was reached. Under diffuse light conditions (150 W/m²), stagnation temperatures above 250°C (523 K) were measured. Thermodynamic calculation, experimental setup and results are given.     

Thermal performances comparisons of the glass evacuated tube solar collectors with shapes of absorber tube

The thermal performance of a glass evacuated tube solar collector is numerically and experimentally investigated. The solar collector considered in this paper consists of a two-layered glass tube and an absorber tube. Air is used as the working fluid. The length and diameter of this glass tube are 1200 and 37 mm, respectively. Four different shapes of absorber tubes are considered, and the performances of the solar collectors are studied to find the best shape of the absorber tube for the solar collector. Beam irradiation, diffuse irradiation, and shade due to adjacent tubes are taken into account for a collector model to obtain a realistic estimation. In addition, a single collector tube with only beam irradiation is studied as a simplified model, and the results of the simplified model are compared to those of the collector model to identify the difference between these two models. The performance of a solar collector is affected by the shape of the absorber, incidence angle of solar irradiation, and arrangement of collector tubes. The results obtained from the simplified model are very different from those from the collector model, which considered not only beam and diffuse irradiation but also shade due to adjacent tubes.     

Performance modelling of a parallel tube polymer absorber

Making use of radiation and convective heat transfer theory the efficiency of a collector fitted with an absorber consisting of spaced parallel tubes is modelled on a personal computer. The absorptance of the tubes for sunlight and the convection coefficient between the back insulation and the tubes are adjusted until agreement between experimentally determined values and calculated values are obtained. The effects of variables not tested experimentally are then investigated.Covering the top of the back insulation with a sufficiently thick metal film to make it isothermal, reduced the dependence of the efficiency on tube spacing from 0.834%/mm to 0.615%/mm for inter tube spacings between 4 mm and 12 mm. A collector with an absorber with an inter tube spacing of one tube diameter and with an isothermal back is only 5.2 percentage points less efficient than one with (twice as many) tubes touching one another at the sides.Efficiency is slightly dependent on wall thickness, decreasing from 64.5% for a 0.5 mm wall to 61.5% for a 3 mm wall. This is because the resistance of the hydrodynamic boundary layer forms a significant part of the total resistance to heat transfer. For outside diameters between 5 and 11 mm, efficiency varies by only 0.5 percentage points as long as the diameter/pitch ratio stays the same.For small scale unsophisticated manufacture, the absorber with spaced polymer tubes presents an attractive alternative to more conventional designs.     

Optical evaluation and analysis of an internal low-concentrated evacuated tube heat pipe solar collector for powering solar air-conditioning systems

An optical evaluation and analysis of an internal low-concentrating evacuated tube heat pipe solar collector designed to enhance the collection of solar radiation for medium temperature applications is presented in this paper. The internal low-concentrating evacuated tube heat pipe solar collector was designed with an acceptance angle of 20° given a geometrical concentration ratio of 2.92. The truncation of the upper part of the reflector giving a geometrical concentration ratio of 1.95 was carried and enabled the internal low-concentrating evacuated tube heat pipe collector to be enclosed by a borosilicate glass tube with 100 mm and 93 mm outer and inner diameters, respectively. Ray trace analysis at different transverse angles determines optical efficiencies, related optical losses and flux distribution on the absorber of the internal low-concentrating evacuated tube heat pipe solar collector. A detailed two dimensional ray trace techniques considering only the direct insolation component predicated overall ray’s acceptance of 93.72% and optical efficiency of 79.13% from transverse angles of 0° to 20°.     

Technical note: Performance study of a linear Fresnel concentrating solar device

A linear Fresnel concentrating solar device was developed and its performance was studied. Various combinations of reflecting mirrors were tried to achieve the temperature at focus. A tubular absorber made from aluminium pipe (0.075 m diameter and 0.53 m long) filled with Hytherm-500 oil was used. The absorber tube was placed at focus of the collector. The quantum of heat absorbed by oil in absorber tube with 10, 15, and 20 numbers of mirrors was evaluated. Rise in oil temperature in absorber tube with different set of reflecting mirrors was compared. The efficiency of this solar device with different set of mirrors was also compared. It was observed that increase in peak oil temperature in absorber tube was not directly related with number of mirrors. The overall efficiency of the concentrator during initial 30 min of operation with 10, 15 and 20 number of mirrors was found to be 20.5, 17.6 and 16.8% respectively.     

Pool boiling heat transfer in a vertical annulus with a stepped outside tube

Effects of a stepped outside tube on nucleate pool boiling heat transfer in a vertical annulus with closed bottoms have been investigated experimentally. For the study saturated water at atmospheric pressure and a heated tube of 200 mm length and 19 mm diameter were used. The shape of an outside tube of the annulus has step geometry and the ratio between the upper and lower regions are varied from 0.23 to 4.43. The existence of the stepped outside tube changes heat transfer characteristics and its effect becomes evident as the gap ratio is smaller than 1. For the tested gap ratios heat transfer deterioration is observed as the heat flux is higher than 80 kW/m² comparing to the single unrestricted tube. The major cause for the tendency is attributed to the loss of liquid agitation intensity around the stepped regions of the annulus.     

Heat transfer analysis of receiver for large aperture parabolic trough solar collector

Parabolic trough solar collector (PTSC) is one of the most proven technologies for large‐scale solar thermal power generation. Currently, the cost of power generation from PTSC is expensive as compared with conventional power generation. The capital/power generation cost can be reduced by increasing aperture sizes of the collector. However, increase in aperture of the collector leads to higher heat flux on the absorber surface and results in higher thermal gradient. Hence, the analysis of heat distribution from the absorber to heat transfer fluid (HTF) and within the absorber is essential to identify the possibilities of failure of the receiver. In this article, extensive heat transfer analysis (HTA) of the receiver is performed for various aperture diameter of a PTSC using commercially available computational fluid dynamics (CFD) software ANSYS Fluent 19.0. The numerical simulations of the receiver are performed to analyze the temperature distribution around the circumference of the absorber tube as well as along the length of tube, the rate of heat transfer from the absorber tube to the HTF, and heat losses from the receiver for various geometric and operating conditions such as collector aperture diameter, mass flow rate, heat loss coefficient (HLC), HTF, and its inlet temperature. It is observed that temperature gradient around the circumference of the absorber and heat losses from the receiver increases with collector aperture. The temperature gradient around the circumference of the absorber tube wall at 2 m length from the inlet are observed as 11, 37, 48, 74, and 129 K, respectively, for 2.5‐, 5‐, 5.77‐, 7.5‐, and 10‐m aperture diameter of PTSC at mass flow rate of 1.25 kg/s and inlet temperature of 300 K for therminol oil as HTF. To minimize the thermal gradient around the absorber circumference, HTFs with better heat transfer characteristics are explored such as molten salt, liquid sodium, and NaK78. Liquid sodium offers a significant reduction in temperature gradient as compared of other HTFs for all the aperture sizes of the collector. It is found that the temperature gradient around the circumference of the absorber tube wall at a length of 2 m is reduced to 4, 8, 10, 13, and 18 K, respectively, for the above‐mentioned mass flow rate with liquid sodium as HTF. The analyses are also performed for different HTF inlet temperature in order to study the behavior of the receiver. Based on the HTA, it is desired to have larger aperture parabolic trough collector to generate higher temperature from the solar field and reduce the capital cost. To achieve higher temperature and better performance of the receiver, HTF with good thermophysical properties may be preferable to minimize the heat losses and thermal gradient around the circumference of the absorber tube.     

On the correlations between collector efficiency factor and material content of parallel flow flat-plate solar collectors

The collector efficiency factor F^′, besides the collector heat loss coefficient U_L, characterizes the thermal quality of a solar collector. As F^′ is strongly influenced by the tube distance w and the absorber plate thickness δ, F^′ is also correlated with the material content of absorber plus tubing. Due to the future mass production of collectors and to the restricted copper resources (in the literature, a range until 2026 is given), the role of material savings can be expected to become more and more important. This paper focuses on the correlations between F^′ and the material content of absorber and tubing for flat-plate collectors with the fin-and-tube geometry. The correlations between w, δ, F^′ and material content are presented in a new type of nomograph. This nomograph indicates the values of w and δ that minimize the material content (for a given F^′). For a typical absorber with F^′=0.90, material savings of 25% can theoretically be achieved without any deterioration of F^′, by reducing the absorber plate thickness and the tube distance. The resulting plate thickness is below 0.1 mm; the respective tube distance will be about 7 cm. Practical restrictions are discussed. In a sensitivity analysis, the influence of different parameters on F^′ is investigated. The most important parameters are w, U_L,δ and the Reynolds number. The technique chosen for contacting tube and absorber has only a minor influence on F^′.     

Parabolic trough collector with rhombus tube absorber for higher concentration ratio

In the present study, the concentration ratio of the parabolic trough collector using rhombus tube absorber has been estimated. An analytical technique has been developed to determine the optimum size of the rhombus tube absorber for given trough dimensions. The optimum size of the rhombus tube absorber is 13.8% smaller than the circular tube absorber for the LS3 trough with no change in intercept factor. The maximum improvement in the concentration ratio is found to be 31.5% for the troughs with rim angle 90° in comparison to circular tube absorber. Results also indicate that rhombus tube absorber can be employed for a range of rim angle 75 to 90 degree.

Abbreviations: CR: concentration ratio; CSP: concentrated solar power; MCRT: Monte Carlo ray tracing; PTC: parabolic trough collector     

Design construction and test run of a solar powered solid absorption refrigerator

The intermittent system used CaCl₂ and NH₃ as absorbent and refrigerant, respectively. The absorbent was mixed with 20% by weight of CaSO₄, as cement, and prepared as hard porous granules of 5–10 mm sizes.The double glazed collector/absorber/generator unit used clear PVC and plane glass sheets, with the former as the outer cover. Overall collector plate exposed area was 1.41 m². A stagnant water evaporative condenser was designed and constructed of re-inforced sandcrete, with steel condenser tubes, and was coupled to the system. The evaporator was a spirally coiled steel tube immersed in a pool of stagnant water. Absorbent cooling during absorption was by natural convection of air over the collector plate and tubes, coupled with sky radiation. All construction, except the collector covers and porous condenser walls, were of steel sheets and tubes.Ambient temperatures during absorption and generation ranged over (25°–35°C). Tests indicated that cooling capacities of the NH₃ condensed were around 714 KJ m⁻², while effective cooling obtained was equivalent to an ice production of 1 kg m⁻², per day.     

Optimization of the photovoltaic thermal (PV/T) collector absorber

In an effort to reduce the cost of conventional fin and tube photovoltaic thermal (PV/T) collectors a novel mathematical analysis was developed which determines the optimum absorber plate configuration having the least material content and thus cost, whilst maintaining high collection efficiency.The analysis was based on the “low-flow” concept whose advantages include: improved system performance, smaller pump (less expensive with lower power consumption), smaller diameter tubes requiring lower thickness and thus cost of insulation, less construction power and time for the optimum absorber configuration.From the optimization methodology developed it was found that very thin fins (typically 50 μm) and small tubes (of 1.65 mm inside diameter for the risers, in the header and riser arrangement and 4.83 mm for the serpentine arrangement), with a tube spacing of 62 mm and 64 mm (both corresponding to 97% fin efficiency) and a mass of 1.185 kg/m² and 2.140 kg/m², respectively, can be used. This optimum serpentine absorber plate contains 40.50% less material content and mass, as compared to the serpentine prototype proposed by others. In one such design a mass of 3.596 kg/m² was used (with 10 mm diameter tubes, 95 mm tube spacing and 200 μm thick absorber).To predict the performance of the determined optimum configurations, a steady-state model (using the EES code) was developed. To validate the steady-state model two prototypes, one in Header and Riser and the other in Serpentine configuration, were built and tested. It was found from the experiments that there is a good agreement between the computational and the experimental results. Moreover, it was found that optimum PV/T configurations do indeed have thermal and electrical performance comparable to non-optimum ones of greater mass and cost.     

CFD analysis of the thermal losses on the upper part of a flat solar collector

To investigate the reduction of heat losses on the upper part of a flat solar collector, a two‐dimensional study was carried out by CFD analysis using Fluent. For this purpose, the heat transfer behavior in the air gap over a wide range of thicknesses of the latter (1‐20 mm) and the addition of a second glass cover fixed at midheight of the air gap spacing have been investigated. For small thicknesses of the air gap (1‐8 mm), the heat transfer is essentially conductive. An increase in the thickness leads to the intensification of the natural convection which induces high thermal losses. The simulation results have shown that the addition of a second cover glazing leads to the weakening of the natural convection and thus to an average enhancement of the solar collector temperature over the range of thicknesses studied of approximately 17%. The overall thermal losses coefficient is then reduced by an average of 26% compared with the single‐glazed solar collector. They have also shown that the thickness of the air gaps resulting in the minimum overall heat losses is 8 mm and that the thickness of the second glass cover has no significant effect on these results. In addition, this study has highlighted the importance of taking into account the radiation heat transfer in establishing the thermal balance of a flat solar collector. Indeed, this consideration leads to an average decrease of the absorber temperature of about 30%.     

Optimization of a fixed solar thermal collector

Criteria are presented for optimizing solar thermal energy collection. These criteria are then used in setting the design of a fixed solar thermal energy collector. This design is obtained by proceeding carefully through a series of optimization steps. While seeking near optimum performance, features have been retained which should lead to low cost. Initial optimization steps lead to an all glass vacuum collector tube whose side and lower walls are internally silvered to provide optimal Winston concentration on an interior glass tube coated with a selective absorber. Heat transfer calculations, performed for an array module of these collector tubes, produce values for the radiation, heat conduction and pumping losses and indicate operating conditions which minimize these losses. Near this minimum, heat conduction and pumping losses are small and can usually be neglected. Liquids provide much better heat transfer than gases. For liquid heat transfer fluids, the minimum loss collector tube window width (setting the transverse scale) is ～3 cm and tube length ～4 m, depending somewhat upon array area and the weighting used for the various losses. A window width of～5 cm and tube length～2 m should provide lower cost fabrication, while still allowing operation near minimum loss. Skills now used in the glass and lighting industry are expected to lead to low cost production of these tubes.     

Measurement of radiation distribution on the absorber in an asymmetric CPC collector

A method to estimate the annual collected energy and the annual average optical efficiency factor is suggested. The radiation distribution on the absorber of an asymmetric CPC collector with a flat bi-facial absorber is measured for three different absorber mounting angles using a photo diode. The annual optical efficiency factors and a relative measure of the annual collected energy are determined for collectors with the absorber fin thickness 0.5 and 1 mm, and for a collector with a teflon convection suppression film mounted around the absorber. With the local optical efficiency factors and the annual incident solar energy distribution considered, the analysis indicates that the energy gain for a mounting angle of 20° is higher than for a collector with 65° absorber mounting angle. The annual collected energy is increased with 6–8% if the absorber fin thickness is increased from 0.5 to 1 mm. The annual average optical efficiency factor is relatively independent of the absorber mounting angle. It was found to be 0.87–0.88 for a collector with a 0.5 mm thick absorber fin and 0.92 for a collector with a 1 mm thick absorber fin or for a collector with 0.5 mm thick absorber fin with a teflon convection suppression film added. The low annual average optical efficiency factor is not caused by the uneven irradiance distribution but by the relatively high U_L-values.     

Analysis of wind flow around a parabolic collector (2) heat transfer from receiver tube

Parabolic collectors of commercial solar thermal power plants are subject to variable convection heat transfer from the receiver tube. In the present study heat transfer from a receiver tube of the parabolic trough collector of the 250 kW solar power plants in Shiraz, Iran, is studied taking into account the effects of variation of collector angel of attack, wind velocity and its distribution with respect to height from the ground.The governing equations for the two-dimensional steady state wind flow include continuity, momentum and energy equations and RNG-based k–ε model for turbulence scheme. Finite volume discretization method is used to solve the governing equations with wall function boundary condition and the SIMPLE approach is employed to iterate for the pressure correction and convergence of the velocity field. The momentum equation contains buoyancy force when the buoyancy effect is high and force convection effect is low.Computation is carried out for various wind velocities and different collector orientations with respect to wind direction. For solution of the energy equation, temperature of the receiver tube is taken as 350 K and ambient temperature is assumed to be 300 K. Various recirculation and temperature fields were observed around the receiver tube for different flow conditions. Effect of collector orientation on the average Nu number for the receiver tube was found negligible when the wind speed is low (Re⩽4.5×10⁵ based on the collector aperture). But when the wind velocity is high (Re>4.5×10⁵), the collector effect on the variation of Nu around the glass cover of the absorber tube is considerable.     

The performance of a cylindrical solar water heater

A cylindrical solar water heater is designed and manufactured in the Department of Mechanical Engineering, University of Bahrain. It consists of a cylindrical tube made from high quality glass having a length of 0.8, 0.14 m outer diameter and a thickness of 6 mm. A copper coil tube in the shape of spiral rings, with the tube inner diameter of 2 mm and outer diameter of 3.175 mm, painted black, serves as a collector to the incident solar energy on the cylinder wall. The thermal performance was evaluated extensively throughout the months of March and April 2002; a maximum temperature difference of 27.8 °C between inlet and outlet of the solar water heater at a mass flow rate of 9 kg/h was achieved. The efficiency of the cylindrical solar water heater was calculated. The maximum value during the experimental period was found to be 41.8%. This reveals a good capability of the system to convert solar energy to heat which can be used for heating water. An economic analysis has reveals that the cylindrical solar water heater compared with the flat plate collector is cost effective.     

Experimental study of the performance of a circular tube solar collector with closed-loop oscillating heat-pipe with check valve (CLOHP/CV)

This paper describes the performance of a circular glass tube solar collector with a set of closed-loop oscillating heat-pipes with check valves (CLOHP/CV). The assembly was divided into three sections, i.e. circular glass tube, adiabatic gap and condenser water tank. A circular 10-set glass tube solar collector of 0.058 m diameter and 1.50 m length was housed on a collecting plate. The inside circular glass tube consisted of a CLOHP/CV and collecting plate. The adiabatic gap was 0.05 m. The condenser water tank was made from a 0.03 × 0.05 m² zinc sheet. The CLOHP/CV consisted of the collecting plate cover with inner diameter of 0.003 m and 26.40 m total length per set, and it contained two check valves with evaporator 1.50 m long, adiabatic gap 0.05 m and condenser 0.30 m long. R-134a was used as the working fluid with filling ratio of 50%. The CLOHP/CV arrangement was aligned at an inclination angle of 18 degrees from the horizontal plane, with 6 turns per set. A 0.001 m thick aluminum sheet was used to make a collecting plate with 0.10 × 1.50 m² test area. Efficiency evaluations were conducted during daylight hours over a 2-month period and included extensive monitoring and recording of temperatures with type-K thermocouples placed at key locations throughout the system. The results confirmed the anticipated fluctuation in collector efficiency dependent on the time of day, solar energy irradiation, ambient temperature and circular tube surface mean temperature. An efficiency of approximately 76% was achieved, which correlates with the efficiency of the more expensive heat-pipe system. The CLOHP/CV system offers the additional benefits of corrosion-free operation and absence of freezing during winter months.     

Experimental Investigation on Flow Boiling Heat Transfer and Pressure Drop of HFC-134a inside a Vertical Helically Coiled Tube

An investigation on flow boiling heat transfer and pressure drop of HFC-134a inside a vertical helically coiled concentric tube-in-tube heat exchanger has been experimentally carried out. The test section is a six-turn helically coiled tube with 5.786-m length, in which refrigerant HFC-134a flowing inside the inner tube is heated by the water flowing in the annulus. The diameter and the pitch of the coil are 305 mm and 45 mm, respectively. The outer diameter of the inner tube and its thickness are respectively 9.52 and 0.62 mm. The inner diameter of the outer tube is 29 mm. The average vapor qualities in test section were varied from 0.1 to 0.8. The tests were conducted with three different mass velocities of 112, 132, and 152 kg/m²-s. Analysis of obtained data showed that increasing of both the vapor qualities and the mass fluxes leads to higher heat transfer coefficients and pressure drops. Also, it was observed that the heat transfer coefficient is enhanced and also the pressure drop is increased when a helically coiled tube is used instead of a straight tube. Based on the present experimental results, a correlation was developed to predict the flow boiling heat transfer coefficient in vertical helically coiled tubes.     

Theoretical and Experimental Studies of the Ultra-Thin-Channel Solar Water Collector

Ultra-thin-channel solar water collector efficiency (UCSWC) was investigated theoretically and experimentally. An ultra-thin-channel solar water collector was constructed using several flat plates with an ultra-thin fluid channel formed using an adjustable flexible silicon frame inserted between the absorber plate and bottom plate. The advantages of the ultra-thin-channel solar water collector are low absorber plate temperature and low total water mass flow rate, resulting in considerable collector efficiency improvement with high outlet fluid temperature and low pump power requirement. A simple and general modeling method was developed to predict the collector efficiencies and mean temperatures of the glass cover, absorber plate and fluid. Good agreement was achieved between the calculated and experimental values. The superior collector efficiencies of the UCSWC are obtained as 82.2% and 75.5% for the inlet temperatures 30°C and 70°C, respectively, operating at a total fluid mass flow 8.3 × 10^?3 kg/s and solar radiation incident of 1100 W/m².