Heat extraction efficiency of a concentric glass tubular evacuated collector

The results of detailed measurements and calculations of the properties of Sydney University/Nitto Kohki evacuated collector tubes have been used to develop a formula for the instantaneous heat extraction efficiency η of a collector panel incorporating the evacuated tubes. The instantaneous efficiency depends on ambient temperature, mean fluid temperature in the collector, solar flux and the design of the manifold used to extract heat from the glass absorber tubes. Manifold design determines the mean temperature difference between absorber tube surface and mean fluid temperature for given operating conditions, and strongly affects the efficiency η of a collector panel. Neither changes in the number of evacuated tubes per unit area of collector, nor variations in solar flux, significantly alter the efficiency decrement Δ η₀ associated with a particular manifold design. Calculated efficiencies agree well with experimental results for collector panels incorporating manifolds of various designs. The formula for efficiency η allows detailed analysis of the relative importance of various energy loss mechanisms in a collector.     

Water-in-glass manifolds for heat extraction from evacuated solar collector tubes

Measurements are reported on three novel manifolds of the water-in-glass type for evacuated all-glasssingle-ended tubular collectors. The manifolds provide for series connection of tubes, but because there is virtually no partitioning of the inner volume of the collector tubes, the manifolds are extremely simple and exhibit low impedance to fluid flow. The efficiency of heat extraction from the tubes has been determined by measuring temperatures at various points on the surface of glass tubes in a panel of area 1.2 m² while heating the tubes electrically to simulate solar energy input. Measurements have been made for a range of tube inclinations (0–80°), water flow rates (0.5–5 lmin⁻¹, water inlet temperatures (13–70°C), and effective solar fluxes (100–1000 W/m²) for two absorber tube diameters. The results show that for a wide range of operating conditions buoyancy effects alone result in efficient heat transfer to the tops of the tubes. The manifold designs described offer a possible low cost solution to the problem of manifolding evacuated collectors for sub-100°C heat extraction for domestic and industrial applications.     

Simulation model of a CPC collector with temperature-dependent heat loss coefficient

We describe a mathematical model for the optical and thermal performance of non-evacuated CPC solar collectors with a cylindrical absorber, when the heat loss coefficient is temperature-dependent. Detailed energy balance at the absorber, reflector and cover of the CPC cavity yields heat losses as a function of absorber temperature and solar radiation level. Using a polynomial approximation of those heat losses, we calculate the thermal efficiency of the CPC collector. Numerical results show that the performance of the solar collector (η vs. ΔT_f(0)/I_coll) is given by a set of curves, one for each radiation level. Based on the solution obtained to express the collector performance, we propose to plot efficiency against the relation of heat transfer coefficients at absorber input and under stagnation conditions. The set of characteristic curves merge, then, into a single curve that is not dependent on the solar radiation level. More conveniently, linearized single plots are obtained by expressing efficiency against the square of the difference between the inlet fluid temperature and the ambient temperature divided by the solar radiation level. The new way of plotting solar thermal collector efficiency, such that measurements for a broad range of solar radiation levels can be unified into a single curve, enables us to represent the performance of a large class of solar collectors, e.g. flat plate, CPC and parabolic troughs, whose heat loss functions are well represented by second degree polynomials.     

Optimisation of evacuated tubular solar collector arrays with diffuse reflectors

The optical efficiencies η_o of arrays of evacuated tubular collectors incorporating plane, triangular and semicircular shaped reflectors coated with flat-white and gloss white paint have been studied experimentally using a calorimetric technique and theoretically using a ray tracing computer program. The results showed that the plane reflector is the optimum design. Detailed studies have been made of the dependence of optical efficiency and incident angle modifier as a function of collector tube separation for collectors incorporating the plane reflector. Two collector panels complete with heat extraction manifold and incorporating the plane reflector, but with different tube spacings were subject to detailed outdoor testing. The results indicated that it is cost-effective to space the collector tubes two or more absorber tube diameters apart.     

Intergration of evacuated tubular solar collectors with lithium bromide absorption cooling systems

By surrounding the absorber-heat exchanger component of a solar collector with a glass-enclosed evacuated space and by providing the absorber with a selective surface, solar collectors can operate at efficiencies exceeding 50 per cent under conditions of ΔT/H_T = 75°C m²/kW (ΔT = collector fluid inlet temperature minus ambient temperature, H_T = incident solar radiation on a tilted surface). The high performance of these evacuated tubular collectors thus provides the required high temperature inputs (70–88°C) of lithium bromide absorption cooling units, while maintaining high collector efficiency. This paper deals with the performance and analysis of two types of evacuated tubular solar collectors intergrated with the two distinct solar heating and cooling systems installed on CSU Solar Houses I and III.     

Investigation of twisted tape inserted solar water heaters—heat transfer, friction factor and thermal performance results

Heat transfer in a solar water heater could be enhanced by means of twisted tapes, inserted inside the fluid flow tubes, which induce swirl flow and act as turbulence promoters. Experimental investigations for a solar water heater with twisted tape inserts having twist pitch to tube diameter ratio ranging from 3–12 have been carried out for varying mass flow rates. The results on heat transfer and friction data have been found to compare well with available results. Within the range of investigated parameters, the heat transfer in the twisted tape insert collectors has been found to increase by 18–70%, whereas the pressure drop increased by 87–132%, as compared to plane collectors. An expression correlating the Nusselt numbers in twisted tape and plane collectors, the twist pitch ratio has been developed in the form of Nu_s/Nu=1.3+2.88/y, which predicts the heat transfer within the range of the present investigation. Results conclude that such collectors would be preferable for higher grade energy collection as it is also at higher rate.Solar water heaters having twisted tape inserts inside the flow tubes perform better than the plane ones. It has been observed that heat losses are reduced (due to the lower value of the plate temperature) consequently increasing the thermal performance by about 30% over the plane solar water heaters under the same operating conditions. The effect of twisted-tape geometry, flow Reynolds number and intensity of solar radiation on the thermal performance of the solar water heater has been presented. It has been found that the twisted-tape collectors perform remarkably better in the lower range of flow Reynolds number (Re≈12,000), beyond which the increase in thermal performance is monotonous. It has also been found that such collectors might perform even better at higher values of intensity of solar radiation.     

Design considerations for solar collectors with cylindrical glass honeycombs

A properly designed cell structure placed between the solar absorber and outer cover glass can substantially reduce natural convection and infrared reradiation heat losses. Glass has merit for such a cellular structure or honeycomb because it is an inexpensive, abundant and stable material with low thermal conductivity and outstanding optical characteristics. To optimize the design of a honeycomb structure, i.e. to minimize the cost of the solar energy collected Z, requires the determination of the honeycomb solar transmission as a function of incidence angles of the sun, infrared effective emittance, cell Nusselt number, and cell wall conductance as well as an estimate of appropriate costs. For an array of circular tubes, the design parameters are wall thickness b, cell diameter d_i, and cell length L. It is difficult to make b less than about 0.2 mm. Typically, d_i must be no larger than 1.6 cm. Increasing decreases reradiation and conduction losses, but also decreases solar energy transmission. For d_i = 9.53 mm and b = 0.198 mm, optimum values ranged from 3 to 12 for collector temperatures (above ambient air temperature) between 22°C and 100°C. Since the Z vs curves have fairly broad minimums, values less than 9 can be used with less than a 3 per cent penalty in cost at the higher temperatures.A comparison of collector effiency characteristics indicated that cylindrical glass honeycomb collectors with nonselective-black absorbers were markedly superior to single-glazed selective-black and double-glazed nonselective-black collectors, especially at higher collector temperatures. Cost effectiveness studies also indicated honeycomb collector superiority at temperature differences between the working fluid and ambient air greater than about 35°C.     

Buoyancy effects and the manifolding of single ended absorber tubes

Measurements are reported on some manifolds of the fluid in glass type for evacuated all-glass tubular collectors. Temperatures at the closed end of glass tubes were measured while heating the tubes electrically to simulate solar energy input. Both parallel and series connection of the tubes are considered. The results show that thermosiphon effects can be used to achieve balanced flows in the parallel configuration, and for both parallel and series configurations efficient heat extraction can be achieved from the tubes with no partitioning of the inner volume of the tubes.     

Theoretical flow investigations of an all glass evacuated tubular collector

Heat transfer and flow structures inside all glass evacuated tubular collectors for different operating conditions are investigated by means of computational fluid dynamics. The investigations are based on a collector design with horizontal tubes connected to a vertical manifold channel.Three different tube lengths varying from 0.59 m to 1.47 m have been modelled with five different inlet mass flow rates varying from 0.05 kg/min to 10 kg/min with a constant inlet temperature of 333 K. Under these operating conditions the results showed that:

• the collector with the shortest tube length achieved the highest efficiency,

• the optimal inlet flow rate was around 0.4–1 kg/min,

• the flow structures in the glass tubes were relatively uninfluenced by the inlet flow rate,

Generally, the results showed only small variations in the efficiencies. This indicates that the collector design is well working for most operating conditions.     

Design and test of non-evacuated solar collectors with compound parabolic concentrators

The intermediate range of concentration ratios (1.5X–10X) which can be achieved with CPCs without diurnal tracking provides both economic and thermal advantages for solar collector design even when used with non-evacuated absorbers. The present paper summarizes more than 3 yr of research on non-evacuated CPCs and reviews measured performance data and critical design considerations. Concentrations in the upper portions of the practical range (e.g. 6X) can provide good efficiency (40–50 per cent) in the 100–160°C temperature range with relatively frequent tilt adjustments (12–20 times per year). At lower concentrations (e.g. 3X) performance will still be substantially better than that for a double glazed flat plate collector above about 70°C and competitive below, while requiring only semi-annual adjustments for year round operation. In both cases the cost savings associated with inexpensive reflectors, and the optimal coupling to smaller, simple inexpensive absorbers (e.g. tubes, fins, etc.) can be as important an advantage as the improved thermal performance.The design problems for non-evacuated CPC collectors are entirely different from those for CPC collectors with evacuated receivers. For example, heat loss through the reflector can become critical, since ideal CPC optics demands that the reflector extend all the way to the absorber. Recent improvements in reflector surfaces and low cost antireflection coatings have made practical a double-glazed non-evacuated CPC design. It is calculated that a 1.5X version of such a collector would have an optical efficiency η_o = 0.71, a heat loss coefficient U = 2.2 W/m²°C and a heat extraction effciency factor F′ ≥ 0.98, while requiring no tilt adjustments.     

Heat losses from parabolic trough solar collectors

Parabolic trough solar collector usually consists of a parabolic solar energy concentrator, which reflects solar energy into an absorber. The absorber is a tube, painted with solar radiation absorbing material, located at the focal length of the concentrator, usually covered with a totally or partially vacuumed glass tube to minimize the heat losses. Typically, the concentration ratio ranges from 30 to 80, depending on the radius of the parabolic solar energy concentrator. The working fluid can reach a temperature up to 400°C, depending on the concentration ratio, solar intensity, working fluid flow rate and other parameters. Hence, such collectors are an ideal device for power generation and/or water desalination applications. However, as the length of the collector increases and/or the fluid flow rate decreases, the rate of heat losses increases. The length of the collector may reach a point that heat gain becomes equal to the heat losses; therefore, additional length will be passive. The current work introduces an analysis for the mentioned collector for single and double glass tubes. The main objectives of this work are to understand the thermal performance of the collector and identify the heat losses from the collector. The working fluid, tube and glass temperature's variation along the collector is calculated, and variations of the heat losses along the heated tube are estimated. It should be mentioned that the working fluid may experience a phase change as it flows through the tube. Hence, the heat transfer correlation for each phase is different and depends on the void fraction and flow characteristics. However, as a first approximation, the effect of phase change is neglected. Copyright © 2013 John Wiley & Sons, Ltd.     

A graphical method of measuring the performance characteristics of solar collectors

A graphical method to measure average and instantaneous efficiencies of a solar concentrator used for heating and boiling liquids and a flat plate collector is presented. The overall heat loss coefficient for the collectors and the optical loss factors: γ(τa)_b—the product for a concentrator and (τa)—the product for a flat plate collector, are also obtained. The method involves measuring the temperature of stagnated liquid in the absorber/collector as a function of time at noon. The efficiencies obtained are correct to within 5% of the efficiencies obtained from accurate measurements involving solar radiation data, the design parameters of collectors and the physical characteristics of the materials used in the fabrication of collectors.     

Simulation of an enhanced flat-plate solar liquid collector with wire-coil insert devices

Solar liquid collectors are potential candidates for enhanced heat transfer, but there are just a few studies focused on this topic. However, enhancement techniques can be applied to thermal solar collectors to produce more compact and efficient designs. This work presents the study of heat transfer enhancement in a tube-on-sheet solar panel with wire-coil inserts, using TRNSYS as the simulating tool. The numerical simulation methodology predicts the thermohydraulic flow behaviour of enhanced and standard tube-on-sheet solar collectors, evaluating the local losses, friction coefficients and Nusselt numbers as functions of the operating parameters. The standard and the enhanced collectors have been simulated under the same ambient, radiant and operating conditions. The standardized efficiency curves according to the standard UNE-EN 12975-2 are provided. The enhanced collector increases the thermal efficiency values by 4.5%. A parametric study was performed to relate the fluid and flow characteristics with the heat transfer enhancement by wire-coil inserts. The simulations were performed for different working fluids (water and propylene glycol/water mixtures) in a mass flow rate range from 15 to 120 l/h m².     

Parameters of solar collectors tested with a KMITT solar simulator

This paper reports on the experimental performances of flat plate solar collectors tested with a solar simulator under steady-state conditions, in terms of collector efficiency, η, and ratio of temperature difference and solar radiation (T_fi-T_e)/I_T. T_e was the effective heat-sink temperature of the tested collector and could be evaluated from temperatures of the collector's cover, ambient and light source panel (or infrared filter). Techniques for converting values of the collector's parameters, F_RU_Le and F_R(τα)_e, obtained from the indoor tests to match outdoor results were demonstrated. The adjusted results agreed well with those of the outdoor data in the case of a collector having a flat glass cover. For a collector having a convex plastic cover, the estimated optical efficiency was lower than that of the outdoor result.     

Mathematical modeling and thermal performance analysis of unglazed transpired solar collectors

Unglazed transpired collectors or UTC (also known as perforated collectors) are a relatively new development in solar collector technology, introduced in the early nineties for ventilation air heating. These collectors are used in several large buildings in Canada, USA and Europe, effecting considerable savings in energy and heating costs. Transpired collectors are a potential replacement for glazed flat plate collectors. This paper presents the details of a mathematical model for UTC using heat transfer expressions for the collector components, and empirical relations for estimating the various heat transfer coefficients. It predicts the thermal performance of unglazed transpired solar collectors over a wide range of design and operating conditions. Results of the model were analysed to predict the effects of key parameters on the performance of a UTC for a delivery air temperature of 45–55 °C for drying applications. The parametric studies were carried out by varying the porosity, airflow rate, solar radiation, and solar absorptivity/thermal emissivity, and finding their influence on collector efficiency, heat exchange effectiveness, air temperature rise and useful heat delivered. Results indicate promising thermal performance of UTC in this temperature band, offering itself as an attractive alternate to glazed solar collectors for drying of food products.The results of the model have been used to develop nomograms, which can be a valuable tool for a collector designer in optimising the design and thermal performance of UTC. It also enables the prediction of the absolute thermal performance of a UTC under a given set of conditions.     

Collector efficiency factor F' for absorbers with rectangular fluid ducts contacting the entire surface

The most commonly used absorbers in flat-plate collectors are manufactured as finned tubes. In this article, an alternative design is investigated: the absorber consists of a rectangular, narrow duct, in which the fluid contacts the entire surface. Under the conditions of fully developed laminar flow and negligible heat resistance of the absorber plate material, relations are developed to calculate the temperature distribution within the fluid. These results are used to derive a formula for the collector efficiency factor F' of narrow-duct absorbers, which is then evaluated for water and a commonly used glycol antifreeze liquid as the collector fluid. Finally, the optimisation of narrow-duct absorbers is investigated with consideration of the thermal heat gained by the collector and primary energy consumed by the pump of the solar system due to the pressure drop in the absorber. It is concluded that duct heights in the range of 3–6 mm should be chosen. Collector efficiency factors F' around 0.98 may be expected.     

Correlations for natural convective heat exchange in CPC solar collector cavities determined from experimental measurements

A detailed experimental study was undertaken to analyse the natural convective heat transfer in CPC cavities, a complex function of collector orientation, geometrical aspect ratios and thermal boundary conditions at the enclosure walls. Results are reported for CPC solar collectors with full-, three quarter- and half-height reflectors, C_R = 2 and a 100 mm wide flat plate absorber. Experiments were conducted using a purpose built solar simulator under controlled lab environment employing realistic boundary and thermal conditions. The effects of simultaneous tilting of the solar collectors about both transverse and longitudinal axes, truncation of the reflector walls and inlet water (collector heat removal fluid) temperature on the natural convective heat flow characteristics inside the CPC cavity have been determined. It is concluded that the correlations developed for prediction of natural convection characteristics in rectangular, annuli and V-trough enclosures are not appropriate for application to CPC solar collectors with divergence ranging from 150% to 300%. Based on the experimental data a correlation is presented to predict the natural convection heat loss from the absorber plate of solar collectors for a range of water inlet temperatures.     

Performance evaluation of solar photovoltaic/thermal systems

The major purpose of the present study is to understand the performance of an integrated photovoltaic and thermal solar system (IPVTS) as compared to a conventional solar water heater and to demonstrate the idea of an IPVTS design. A commercial polycrystalline PV module is used for making a PV/T collector. The PV/T collector is used to build an IPVTS. The test results show that the solar PV/T collector made from a corrugated polycarbonate panel can obtain a good thermal efficiency. The present study introduces the concept of primary-energy saving efficiency for the evaluation of a PV/T system. The primary-energy saving efficiency of the present IPVTS exceeds 0.60. This is higher than for a pure solar hot water heater or a pure PV system. The characteristic daily efficiency η_s* reaches 0.38 which is about 76% of the value for a conventional solar hot water heater using glazed collectors (η_s*=0.50). The performance of a PV/T collector can be improved if the heat-collecting plate, the PV cells and the glass cover are directly packed together to form a glazed collector. The manufacturing cost of the PV/T collector and the system cost of the IPVTS can also be reduced. The present study shows that the idea of IPVTS is economically feasible too.     

Efficiency and exergy analysis of a new solar air heater

It would be misleading to consider only the cost aspect of the design of a solar collector. High service costs increase total costs during the service life of solar collector. The most effective way to save energy is by increasing the efficiency in a solar collector by the heat transfer coefficient.In our study, five solar collectors with dimensions of 0.9×0.4 m were used and the flow line increased where it had narrowed and expanded geometrically in shape. These collectors were set to four different cases with dimensions of 1×2 m. Therefore, heating fluids exit the solar collector after at least 4.5 m displacement. According to the collector geometry, turbulence occurs in fluid flow and in this way heat transfer is increased. The results of the experiments were evaluated on the days with the same radiation. The efficiencies of these four collectors were compared to conventional flat-plate collectors. It was seen that heat transfer and pressure loss increased depending on shape and numbers of the absorbers.     

The potential applications and advantages of powering solar air-conditioning systems using concentrator augmented solar collectors

Non-concentrated evacuated tube heat pipe solar collectors have been reported to show higher fluid temperatures with improved thermal performance in the low to medium temperature range (?60 °C) due to low heat losses but suffer higher heat losses at the medium to higher temperature range (?80 °C) which reduces their efficiency compared to concentrated evacuated tube heat pipe solar collectors. To operate as stand-alone systems capable of attaining temperatures in the range of 70-120 °C, an innovative concentrator augmented solar collector can be an attractive option. The performance of a combined low-concentrator augmented solar collector in an array of evacuated tube heat pipe solar collectors defined as concentrator augmented evacuated tube heat pipe array (CAETHPA) and an array of evacuated tube heat pipe collectors (ETHPC) were tested and compared and results presented in this paper. The analysis of the experimental data allows concluding that the use of a CAETHPA is a more efficient alternative for integrating renewable energy into buildings with higher fluid temperature response, energy collection and lower heat loss coefficient compared to the use of evacuated tube heat pipe collector array (ETHPA).