The impact of optical and thermal properties on the performance of flat plate solar collectors

The impact of the optical properties on the annual performance of flat plate collectors in a Swedish climate has been estimated with the MINSUN program. The collector parameters were determined with a theoretically based calculation program verified from laboratory measurements. The importance of changes in solar absorptance and thermal emittance of the absorber, the addition of a teflon film or a teflon honeycomb, antireflection treatment of the cover glazing and combinations of these improvements were investigated. The results show that several improvements can be achieved for solar thermal absorbers. A combined increase in absorptance from 0.95 to 0.97 and a decrease in emittance from 0.10 to 0.05 increase the annual performance with 6.7% at 50 °C operating temperature. The increase in performance by installing a teflon film as second glazing was estimated to 5.6% at 50 °C. If instead a teflon honeycomb is installed, a twice as high performance increase is obtained, 12.1%. Antireflection treatment of the cover glazing increases the annual output with 6.5% at 50 °C. A combination of absorber improvements together with a teflon honeycomb and an antireflection treated glazing results in a total increase of 24.6% at 50 °C. Including external booster reflectors increases the expected annual output at 50 °C to 19.9–29.4% depending on reflector material.     

Solar collectors with colored absorbers

The integration of solar collectors in buildings should be compatible with the architectural design, and solar collectors with colored absorbers would be aesthetically preferable. In our laboratory we constructed and tested flat plate solar collectors with colored absorbers for water heating applications. The study includes collectors in their typical form with the protective glazing, and also collectors without glazing. Unglazed solar collectors are not widely used, although they are cost effective solar devices, suitable for low temperature thermal applications. We tested outdoors the constructed models, glazed and unglazed, with black, blue and red brown absorbers. In order to overcome the high thermal losses of the unglazed collectors and the low optical efficiency of the colored absorbers, we used flat booster reflectors. The additional solar radiation input from the reflectors increases the thermal energy output of the collectors, improving their performance. Theoretical steady state efficiency curves are also given for collectors with or without glazing. The presented experimental and theoretical results determine the range of the effective operation of the proposed solar collector types, which can be used in a variety of applications, instead of glazed or unglazed solar collectors with a black absorber.     

Collector performance enhancement with flat reflectors

The use of diffuse and specular flat reflectors to enhance the performance of flat-plate solar collectors has been explored by means of Fortran routines which optimize the size, shape, and placement of reflector and collector. Configuration factors for systems with a diffuse reflector and a collector whose absorptance varies with incidence angle are presented. Specular reflectors are more effective than diffuse reflectors, and, if south-facing, should be used with collectors which are elongated in the east-west direction. Design curves for the specific system of horizontal collector and south-facing reflector are presented. In this system, a moderately sized reflector can increase the midwinter yield per unit collector area by several times.     

Experimental investigation of heat losses from low-concentrating non-imaging concentrators

Heat loss measurements have been performed on a V-trough collector model with concentration ratio 1.56 and with flat absorbers consisting of five parallel reflector troughs aligned east-west. The collector was tilted 45°. Depending on the similarity in geometry between V-troughs and compound parabolic concentrators, the results should in general be valid also for low-concentrating CPCs. The absorbers were heated electrically and the heat losses were calculated from the input power to the absorber surface. Several geometrical and material properties that affect the heat losses from the collector were investigated. It is concluded that the use of transparent insulation, such as Teflon® films, in low-concentrating solar collectors can reduce the heat losses substantially. The reflector emittance in the infrared has an impact on the heat losses. Use of highly emitting reflectors instead of low-emitting reflectors increases the overall heat losses by about 5–8%. The conversion of experimentally measured heat losses into heat losses for real collectors and practical material considerations is discussed.     

Collector,collector-reflector systems—an an analytical and practical study

This paper investigates the use of planar reflectors which substantially improve the performance of flat-plate solar collectors. The experimental setup includes a collector panel and collector-reflector system with maximum useful heat gain. A computer simulation of the analytical and experimental recordings was made to study the comparative features of the systems. The performance study is illustrated as a function of solar altitude, azimuth angles, hour angle and the relative sizes and tilt angles of both the collector and reflector. The use of a booster mirror produces an increase of 10–15°C in water temperature. The shading effect due to the presence of the reflector has also been considered in the present analysis.     

Optimal tilt-angles of all-glass evacuated tube solar collectors

In this paper, a detailed mathematical procedure is developed to estimate daily collectible radiation on single tube of all-glass evacuated tube solar collectors based on solar geometry, knowledge of two-dimensional radiation transfer. Results shows that the annual collectible radiation on a tube is affected by many factors such as collector type, central distance between tubes, size of solar tubes, tilt and azimuth angles, use of diffuse flat reflector (DFR, in short); For collectors with identical parameters, T-type collectors (collectors with solar tubes tilt-arranged) annually collect slightly more radiation than H-type collectors (those with solar tubes horizontally arranged) do. The use of DFR can significantly improve the energy collection of collectors. Unlike the flat-plate collectors, all-glass evacuated tube solar collectors should be generally mounted with a tilt-angle less than the site latitude in order to maximize the annual energy collection. For most areas with the site latitude larger than 30° in China, T-type collectors should be installed with a tilt-angle about 10° less than the site latitude, whereas for H-type collectors without DFR, the reasonable tilt-angle should be about 20° less than the site latitude. Effects of some parameters on the annual collectible radiation on the collectors are also presented.     

Optimal design of orientation of PV/T collector with reflectors

Hybrid conversion of solar radiation implies simultaneous solar radiation conversion into thermal and electrical energy in the PV/Thermal collector. In order to get more thermal and electrical energy, flat solar radiation reflectors have been mounted on PV/T collector. To obtain higher solar radiation intensity on PV/T collector, position of reflectors has been changed and optimal position of reflectors has been determined by both experimental measurements and numerical calculation so as to obtain maximal concentration of solar radiation intensity. The calculated values have been found to be in good agreement with the measured ones, both yielding the optimal position of the flat reflector to be the lowest (5°) in December and the highest (38°) in June. In this paper, the thermal and electrical efficiency of PV/T collector without reflectors and with reflectors in optimal position have been calculated. Using these results, the total efficiency and energy-saving efficiency of PV/T collector have been determined. Energy-saving efficiency for PV/T collector without reflectors is 60.1%, which is above the conventional solar thermal collector, whereas the energy-saving efficiency for PV/T collector with reflectors in optimal position is 46.7%, which is almost equal to the values for conventional solar thermal collector. Though the energy-saving efficiency of PV/T collector decreases slightly with the solar radiation intensity concentration factor, i.e. the thermal and electrical efficiency of PV/T collector with reflectors are lower than those of PV/T collector without reflectors, the total thermal and electrical energy generated by PV/T collector with reflectors in optimal position are significantly higher than total thermal and electrical energy generated by PV/T collector without reflectors.     

Optimized reflectors for non-tracking solar collectors with tubular absorbers

We present an approach to find optimal reflector shapes for non-tracking solar collectors under practical constraints. We focus on cylindrical absorbers and reflectors with translational symmetry. Under idealized circumstances, edge ray reflectors are well known to be optimal. However, it is not clear how optimal reflectors should be shaped in order to obtain maximum utilizable energy for given operating temperatures under practical constraints like reflectivity less than unity, real radiation data, size limits, and gaps between the reflector and the absorber. For a prototype collector with a symmetric edge ray reflector and a tubular absorber, we derive from calorimetric measurements under outdoor conditions the optical efficiency as a function of the incidence angle. Using numerical optimization and raytracing, we compare truncated symmetric edge ray reflectors, truncated asymmetric edge ray reflectors and free forms parametrized by Bezier splines. We find that asymmetric edge ray reflectors are optimal. For reasonable operating conditions, truncated asymmetric edge ray reflectors allow much better land use and easily adapt to a large range of roof tilt angles with marginal changes in collector construction. Except near the equator, they should increase the yearly utilizable energy per absorber tube by several percent as compared to the prototype collector with symmetric reflectors.     

Theoretical variations of the thermal performance of different solar collectors and solar combi systems as function of the varying yearly weather conditions in Denmark

The thermal performances of solar collectors and solar combi systems with different solar fractions are studied under the influence of the Danish design reference year, DRY data file, and measured weather data from a solar radiation measurement station situated at the Technical University of Denmark in Kgs. Lyngby. The data from DRY data file are used for any location in Denmark. The thermal performances of the solar heating systems are calculated by means of validated computer models. The measured yearly solar radiation varies by approximately 23% in the period from 1990 until 2002, and the investigations show that it is not possible to predict the yearly solar radiation on a tilted surface based on the yearly global radiation.The annual thermal performance of solar combi systems cannot with reasonable approximation be fitted to a linear function of the annual total radiation on the solar collector or the annual global radiation. Solar combi systems with high efficient solar collectors are more influenced by weather variations from one year to another than systems with low efficient solar collectors.The annual thermal performance of solar collectors cannot be predicted from the global radiation, but both the annual thermal performance and the annual utilized solar energy can with a reasonable approximation be fitted to a linear function of the yearly solar radiation on the collector for both flat plate and evacuated tubular solar collectors. Also evacuated tubular solar collectors utilize less sunny years with large parts of diffuse radiation relatively better than flat plate collectors.     

Solar thermal collector augmented by flat plate booster reflector: Optimum inclination of collector and reflector

In this report we present a theoretical analysis of a solar thermal collector with a flat plate top reflector. The top reflector extends from the upper edge of the collector, and can be inclined forwards or backwards from vertical according to the seasons. We theoretically predicted the daily solar radiation absorbed on an absorbing plate of the collector throughout the year, which varies considerably with the inclination of both the collector and reflector, and is slightly affected by the ratio of the reflector and collector length. We found the optimum inclination of the collector and reflector for each month at 30°N latitude. An increase in the daily solar radiation absorbed on the absorbing plate over a conventional solar thermal collector would average about 19%, 26% and 33% throughout the year by using the flat plate reflector when the ratio of reflector and collector length is 0.5, 1.0 and 2.0 and both the collector and reflector are adjusted to the proper inclination.     

Experimental analysis of thermal performance of flat plate and evacuated tube solar collectors in stationary standard and daily conditions

New comparative tests on two different types of solar collectors are presented in this paper. A standard glazed flat plate collector and an evacuated tube collector are installed in parallel and tested at the same working conditions; the evacuated collector is a direct flow through type with external compound parabolic concentrator (CPC) reflectors.Efficiency in steady-state and quasi-dynamic conditions is measured following the standard EN 12975-2 and it is compared with the input/output curves measured for the whole day.The first purpose of the present work is the comparison of results in steady-state and quasi-dynamic test methods both for flat plate and evacuated tube collectors. Beside this, the objective is to characterize and to compare the daily energy performance of these two types of collectors. An effective mean for describing and analyzing the daily performance is the so called input/output diagram, in which the collected solar energy is plotted against the daily incident solar radiation. Test runs have been performed in several conditions to reproduce different conventional uses (hot water, space heating, solar cooling).Results are also presented in terms of daily efficiency versus daily average reduced temperature difference: this allows to represent the comparative characteristics of the two collectors when operating under variable conditions, especially with wide range of incidence angles.     

Optimization of stationary nonimaging reflectors for tubular evacuated receivers aligned north-south

Several manufacturers are developing solar collectors with tubular evacuated receivers aligned north-south. Adding low-concentration, wide-acceptance-angle reflectors to such tubes allows greater tube spacings, reducing the number of tubes per area of collector. It also improves collector efficiency, particularly for conditions of high , such as high temperatures or low light levels. This detailed study optimizes the reflector design for maximum daily energy collection and includes the effects of reflection losses, reflector-receiver alignment errors, variation of selective surface absorptance with angle of incidence on the receiver, and losses through the gap between the receiver and the reflector.Three general conclusions have been reached: The use of optimized nonimaging reflectors with tubular evacuated receivers will increase the energy collection efficiency—particularly for high-temperature and harsh environment conditions.Wide-acceptance-angle reflectors are forgiving to receiver-reflector alignment errors. It is neither necessary nor desirable to design reflectors for undersize receivers in order to compensate for misalignments that result from manufacturing tolerances.The daily energy collection of collectors using these reflectors having acceptance half-angles in a range. near 60° is not a sensitive function of the acceptance angle. Manufacturers' final reflector design decisions will probably be based on technical considerations related to fabrication and assembly techniques and possibly on market-related considerations such as collector appearance.     

Intensity distribution in the collector plane from structured booster reflectors with rolling grooves and corrugations

While testing different reflector materials for external reflectors for solar collector arrays, it was found that standard rolled aluminium and corrugated aluminium materials could perform almost as well as mirror-like materials. A ray tracing model was developed to calculate the intensity in the collector plane for solar radiation from reflector materials with grooves or corrugations. Laboratory measurements, for reflector samples, with a specially designed spectral scatterometer were used to determine the angular intensity distribution of the reflected radiation. Calculations with the model using measured intensity distributions show that the scatter from aluminium materials with rolling grooves will be directed close to the specular direction and along an almost circular arc in the collector plane. The intensity in the collector plane will be redistributed slightly upward or downward depending on the season and time of day; therefore, both an increase and decrease in average intensity can occur during the year relative to a mirror-like material with the same total reflectance. For rolled aluminium, a small performance improvement can be achieved compared to a mirror reflector with equal total reflectance. Corrugated surfaces will yield a significant increase in average intensity onto the collector aperture at times when the radiation from a mirror-like reflector would otherwise be lost above the collector.     

The influence of climate and location on collector performance

The influence of annual climate variations on the performance of solar thermal collectors in the northern part of Europe has been investigated. The annual solar collector energy output has been calculated with the MINSUN simulation program using hourly, measured climatic data for the years 1983–98 for three cities situated in the south (Lund), central (Stockholm) and north (Luleå) of Sweden. A synthetic year created with the Meteonorm weather simulation program was also used in the simulations. Two solar thermal collectors were modelled: a flat plate solar collector and a tubular vacuum collector, both of commercial standard.The thermal energy output is strongly correlated to the annual global irradiation at a horizontal surface. The annual average energy delivered from the flat plate collector was 337 kWh/m² for Stockholm (337 for Lund and 298 for Luleå), and from the vacuum tube collector 668 kWh/m² for Stockholm (675 for Lund and 631 for Luleå) at an operating temperature of T=50°C. Maximum deviations from the average value for this 16-year period are around 20% for the flat plate and 15% for the vacuum tube collector, at T=50°C.The relation between global irradiation on a horizontal surface and the annually collected thermal energy at a constant operating temperature could be fitted to a linear equation: q_u=aG(0°)+bT, where q_u is the energy output from the collector, G(0°) the global irradiation at a horizontal surface, T the average temperature of the collector fluid, and a and b fitting parameters in a double linear regression analysis.     

Angular dependence of optical efficiency of evacuated tubular collectors with antireflection coatings and stationary specular reflectors

The efficiencies of η₀ of arrays of evacuated tubular collectors with non-imaging specular reflectors have been determined experimentally using a calorimetric technique and theoretically using a Monte-Carlo ray tracing technique. Results have been obtained for collectors incorporating reflectors of two concentrations, and efficiencies are compared with and without antireflection coatings on the glass envelopes for sunlight incident at angles 0–70°. The reflective properties of all optical components have been modelled in detail for the ray tracing calculations. Experimental and theoretical efficiencies agree within 0.02 for a wide range of angles of incidence. Antireflection coatings which increase the normal transmittance through a glass envelope by 5% result in an increase of 0.025 (about 4%) in collector efficiency. A theoretical study of the dependence of collector efficiency on absorptance of the absorber tube and specular reflectance of the reflectors is also discussed. Experimental and theoretical results have also been obtained for a collector incorporating a specular reflector with an accumulated dust cover. In this system, the reflector exhibits both specular and diffuse components of reflectance.     

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.     

Angular dependent optical properties from outdoor measurements of solar glazings

A method for careful outdoor characterization of the angular dependent solar transmittance of solar glazing is developed and used for investigating the performance of structured and antireflection treated glass. Two identical collectors are series connected in order to get an identical flow. The reference collector is always covered with a thin Teflon foil and the test collector is furnished with the investigated covers. The method is carried out in six steps: (1) The collectors are long term tested in order to get the collector parameters, especially the U-values. (2) The performances of the collector at a temperature close to ambient are monitored during a clear day. (3) The effect of heat losses is eliminated from the knowledge of the U-values, and the optical efficiencies of the collectors are derived. (4) The ratio between optical efficiency of the test — and the reference collector is calculated. (5) Both collectors are tested with a Teflon film, with well-known optical properties, as a cover and the relative optical efficiency is derived. (6) From the relative transmittance between the investigated glazings and Teflon, the absolute value of the angular dependent transmittance is derived. The results show good agreement with conventional optical characterization. The commercial antireflection treatment increases the solar transmittance by 4% and the annual output by 9%. The structured glass shows similar angular performance as a flat glass if the structure is faced outwards, but shows a lower transmittance for incidence angles exceeding 40° if the structure is faced inwards. This means that the annual performance is decreased by 4% by facing the structure inwards instead of outwards.     

Combined collector-reflector systems

The use of flat-plate reflectors can substantially improve the performance of flat-plate collectors. We present the fundamental characteristics of a simulation program to estimate the optimum configuration of reflectors, which should be added to a solar plant, under a variety of design parameters, in order to yield the greatest economic benefit. The performance results for these collector-reflector plants are compared with traditional, single-cover collector plants, as well as with plants made up of collectors with different numbers of cover plates. The flat-plate reflector is incorporated in large-scale solar collector plants; the reflector-enhancement factor and its cost are taken into account. A sensitivity analysis is performed to visualize the behaviour of these novel systems under different cost and operating conditions.     

An improved dynamic solar collector test method for determination of non-linear optical and thermal characteristics with multiple regression

The objective is to characterise the solar collector during a relatively short testing period with no requirement for steady state climatic conditions. This information is then used for predicting annual performance of the collector. A standard collector model that is compatible with the ISO 9806-1 test standard is used with correction terms for beam and diffuse incidence angle modifiers, thermal capacitance, wind speed and sky temperature. This results in a more complete characterisation of the collector. The collector parameters are identified by multiple linear regression, MLR. The method has been tested for characterisation of unglazed collectors, glazed flat plate collectors, evacuated tubular collectors, CPC collectors and concentrating collectors with satisfying results. Typically the correlation coefficient R² is better than 0.99 and the standard deviation of the difference between model and measurement is in the range 3–10 W/m².

In the original method the angular dependence of the optical efficiency and the temperature dependence of the heat losses are supposed to be adjusted to a predetermined function. The most recent development is a routine that makes it possible to accurately identify non-linear optical and thermal performance. This extended MLR method can identify the zero loss efficiency for every angle of incidence interval and the temperature dependent heat losses for every temperature interval. This opens the application of the method to collectors with special incidence angle and heat loss effects that cannot be described easily with a combination of elementary functions. Instead a table of parameter values is determined, which is used directly in standard simulation programmes. This method will further increase the accuracy when comparing different collector designs. It has been used for comparing different glazings and for comparison with spectrophotometric measurements. It has also been used for analysing the heat loss factors for Teflon and honeycomb glazings. Since the total power output of the collector is less dependent on the heat loss coefficient than on the optical efficiency the scattering in this data is larger than for the incidence angle curves.

The reflectance of booster mirrors cannot be derived with the MLR-method with acceptable accuracy. The correlation between direct irradiance and irradiance from the reflector exhibit a very strong correlation. Instead the effective reflectance of the mirror can be estimated by comparison of the measured output with calculation by the complete collector and reflector model. This effective reflectance is not compatible with the specular reflectance obtained from spectrophotometric measurements caused by large differences in acceptance angles.

Standard multiple linear regression available in most spread sheet and statistical programs can be used for the parameter identification in the extended MLR-procedure. The identification takes only a few seconds. At the Älvkarleby Laboratory the test method is now used as a routine tool for the evaluation of new collector materials and designs. The Swedish National testing institute has evaluated the methods with the conclusion that they have a potential for being used in standardised collector testing.     

Augmented solar energy collection using different types of planar reflective surfaces; theoretical calculations and experimental results

The use of planar reflective surfaces can substantially improve the performance of both active and passive solar collectors. In this paper the results of theoretical calculations and experimental tests are presented on the use of different types of flat reflective surfaces to increase the collection of solar energy by flat collectors. Specular, diffuse, and combination specular/diffuse reflective surfaces are discussed. This present work differs from that of other investigators principally in that an attempt has been made to describe the reflective properties of surfaces in more generalized terms than simple specular or simple diffuse. Most real surfaces possess a combination of specular- and diffuse-like reflectivities. The reflectivity properties of a given surface can be measured in the laboratory as a function of incident and reflected angles, and these measured reflective properties can be used in the computer model to predict the increase in collector performance with such a reflector. Thus outdoor tests of a given reflector can be avoided if desired, and yet it is possible to make an estimate of the reflector's contribution to the collector's overall performance. Theoretical calculations of collector energy inputs were done for several distinct types of reflecting surfaces. These calculations based on indoor laboratory measurements of the reflective properties of the surfaces, were compared with experimental results obtained from an outdoor simulation apparatus. Predictions of system performance were made for various collector/reflector configurations, and compared with the performance of an optimally oriented collector without a reflector.