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.     

The experimental study of energy features for solar air heaters with different turbulator configurations

To assess the thermal performance in the climate conditions of western and central Iraq, the advantages of using a solar air collector with various turbulator absorber plates are experimentally explored. Four distinct kinds of absorber plates are provided flat plate (F), triangular (T), rectangular (R), and circular (C) turbulators at different air mass flow rates. The collector's economic properties and overall thermal performance are compared to the conventional flat plate turbulator heating systems. The main findings suggest that delta turbulators improve collector economics and overall thermal performance by generating vortex and dampening the formation of the thermal boundary layer in the direction of airflow. Furthermore, when the mass flow rate increases, the thermal performance improves, and the efficiency increases for all mass flow rates, resulting in good thermal performance for the rectangular plate collector when compared to other collectors. When compared to other types of configurations, the daily average efficiency of solar air collectors for flat plate (F), triangular (T), rectangular (R), and circular (C) turbulators are 28%, 67%, 39%, and 48%, respectively, at 50° tilt angle while at 90° tilt angle they are 44%, 76%, 54%, and 63%, respectively, as ṁ = 0.0377 kg/s. The maximum daily average efficiency fitted with rectangular turbulators have about 86% at the largest ṁ = 0.1 kg/s. This study will also give a unique direction to the work trend in the western and central parts of Iraq throughout the winter months.     

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.     

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.     

A hybrid high efficiency single-basin solar still

In this work, an attempt has been made to enhance the distillate output of a single-basin solar still by coupling it with a flat plate solar collector and by coating a thin layer of SnO₂ on one side of the transparent cover plate. The heat transfer fluid was circulated between the still and the collector through a heat exchanger and storage tank by thermosyphonically induced flow. It was observed that good insulation around the storage tank considerably increased the yield at night due to the decrease of ambient temperature. Thermosyphonically induced flow eliminated the need for pumps and control units. A layer of SnO₂ on the transparent cover lowered the thermal radiation loss, one of the major sources of heat energy loss in a solar still. Values for solar radiation, ambient air temperature, salt-water temperature in the basin, temperature of air-vapour mixture within the still, input and output temperatures of the heat exchanger heat transfer fluid, and the distillate yield were collected by a data acquisition system. After making the above modifications, the distillate yield was measured to be 6·745 litres per square metre per day for a September solar radiation of 17820 KJ at Istanbul-Gebze. To compare the distillate yield, a conventional solar still with similar dimensions to those of the improved still was constructed. The overall efficiency of the improved still was found to be 3·26 times the efficiency of the conventional still.     

The real utility ranges of the solar selective coatings

The efficiency of the solar selective coatings (η_sel) with various combinations of the optical properties (solar absorptance and thermal emittance), and their impact on the performance of solar thermal systems of different concentration ratios (CRs) have been analyzed. The stagnation temperatures of the selective coatings have been measured using stagnation temperature measurement (STM) chamber. From the results of the simulation study, it is recommended that selective solar absorber coatings should be used only in systems with CR=1 (solar flat plate collectors) and its use in systems with high CR values (parabolic collector) is an additional expenditure with reduced efficiency. The dependence of the performance of the system with CR=1 on the low emittance of the absorber coating has also been experimentally confirmed by the stagnation temperatures recorded for different selective coatings measured in an STM chamber, a system with unit CR. For systems with CR=1, a simple and new parameter, specific area ratio (SAR) has been proposed which further pinpoints the exact solar system wherein the selective coatings are to be used.     

Comparative analysis of a solar‐driven novel salt‐based absorption chiller with the implementation of nanoparticles

The present study exemplifies the comprehensive thermal analysis to compare and contrast ammonia‐lithium nitrate (NH₃‐LiNO₃) and ammonia‐sodiumthiocynate (NH₃‐NaSCN) absorption systems with and without incorporation of nanoparticles. A well‐mixed solution of copper oxide/water (CuO/H₂O) nanofluid is considered inside a flat‐plate collector linked to an absorption chiller to produce 15‐kW refrigeration at ?5°C evaporator temperature. Enhancements in heat transfer coefficient, thermal efficiency, and useful heat gain of the collector are evaluated, and the effect of these achievements on the performance of both absorption chillers have been determined for different source temperatures. A maximum 121.7% enhancement is found in the heat transfer coefficient with the application of the nanofluid at 2% nanoparticle concentration. The maximum coefficient of performance observed for the NH₃‐NaSCN chiller is 0.12% higher than that for the NH₃‐LiNO₃ chiller at 0°C evaporator temperature. Contradictory to this, the average system coefficient of performance of the NH₃‐LiNO₃ absorption system has been found 5.51% higher than that of the NH₃‐NaSCN system at the same evaporator temperature. Moreover, the application of the nanofluid enhanced the performance of the NH₃‐NaSCN and NH₃‐LiNO₃ systems by 2.70% and 1.50%, respectively, for lower generator temperature and becomes almost the same at higher temperatures, which altogether recommends the flat‐plate collector–coupled NH₃‐LiNO₃ absorption system be integrated with a nanofluid.     

Some theoretical and experimental aspects of built-in solar storage

The possibility of using a built-in storage solar water heater in Jordan has been investigated. Experimental and theoretical results were obtained using a 90 × 90 × 10 cm solar heater which was tilted at an angle of 30° to the horizontal.It was found that the efficiency of the built-in storage solar heater may reach as high as 78% with a maximum increase in the water temperature of 70°C and at low cost compared to the conventional flat plate collector. It is therefore expected that such solar system may be used to provide reasonably sufficient hot water in Jordan. Analytical solution, based on the assumption that the water temperature equals that of the plate, was used to solve the governing equations. While a numerical solution was used to solve the equations under the condition that the plate temperature is not equal to that of the water.It was found that the assumption of the temperature of the plate (T_p) is equal to the temperature of water (T_w) is only justifiable at early hours of operation in the morning.     

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.     

Development of advanced solar assisted drying systems

The Solar Energy Research Group in the Universiti Kebangsaan Malaysia has been set-up more than two decades ago. One of the activities is in the field of solar thermal process, particularly in development of solar assisted drying systems. Solar drying systems technical development can proceed in two directions. Firstly simple, low power, short life, and comparatively low efficiency-drying system. Secondly, the development of high efficiency, high power, long life expensive solar drying system. The group has developed four solar assisted drying systems namely (a) the V-groove solar collector, (b) the double-pass solar collector with integrated storage system, (c) the solar assisted dehumidification system for medicinal herbs and (d) the photovoltaic thermal (PVT) collector system. The common problems associated with the intermittent nature of solar radiation and the low intensities of solar radiation in solar thermal systems can be remedied using these types of solar drying systems. These drying systems have the advantages of heat storage, auxiliary energy source, integrated structure control system and can be use for a wide range of agricultural produce.     

Theoretical thermal performance analysis of two solar‐assisted heat‐pump systems

The thermal performance of two different schemes of solar‐assisted heat‐pump systems has been theoretically studied. In first scheme, the evaporator of the heat pump is taken directly as the solar collecting plate and always maintained at the ambient temperature. As there is no heat loss from the collecting plate, the thermal efficiency of the collector is high and equals the solar absorptivity of the collecting plate. As suggested, the heat‐pump evaporator of the second scheme is placed in a novel fresh water solar pond/tank with high efficiency. Since the evaporator operates at a relatively high temperature, the COP of the heat pump can be increased. The calculated results show that the COP of a solar‐assisted heat pump using the second scheme is considerably higher than that of the first scheme. Copyright © 1999 John Wiley & Sons, Ltd.     

Experimental investigation on the thermal efficiency and performance characteristics of a flat plate solar collector using SiO2/EG–water nanofluids

Application of nanofluids in thermal energy devices such as solar collectors is developing day by day. This paper reports the results of experiments on a flat plate solar collector where the working fluid is SiO₂/ethylene glycol (EG)–water nanofluid with volume fractions up to 1%. The thermal efficiency and performance characteristics of solar collector are obtained for mass flow rates between 0.018 and 0.045 kg/s. The curve characteristics of solar collector indicate that the effects of particle loading on the thermal efficiency enhancement are more pronounced at higher values of heat loss parameter. The results of this work elucidate the potential of SiO₂ nanoparticles to improve the efficiency of solar collectors despite its low thermal conductivity compared to other usual nanoparticles.     

The Influence of Collector Fluid Inlet Temperature on the Performance of a Solar-Assisted Absorption System Using Step-Finned Flat-Plate Collector

In the present work, an extensive analysis is developed for an evaluation of the thermal performance of a solar-powered H₂O/LiBr absorption cooling system using a step-fin flat-plate collector (SFC). The performance parameters, namely, collector efficiency factor, heat removal factor, and collector efficiency, for the SFC is derived. A system simulation model has been developed to analyze the system performance—that is, to identify an operating criterion as a function of the collector fluid inlet temperature (T _FI). It has been observed from the results that the performance of the system depends strongly on T _FI. Simulation results show that the system operates optimally (maximum coefficient of performance) at an optimal T _FI. When the system runs at this optimal value of T _FI, minimum collector material is required. Thus, when using SFC in place of a rectangular-fin flat-collector, thirty-five percent or more collector material can be saved. However, it has been observed that the effect of thermal conductivity on the plate volume of SFC has a marginal effect.     

Efficient single glazed flat plate photovoltaic-thermal hybrid collector for domestic hot water system

This paper deals with the design of a single glazed flat plate Photovoltaic-Thermal (PV-T) solar collector. First, the thermal and electrical performances of several single glazed flat plate PV-T concepts based on water circulation are investigated, using a simple 2D thermal model, then different ways of improvement are presented. It mainly consists in focusing on the heat transfer between PV cells and fluid, and also on the optical properties of materials. Thus, the most appropriate concept configuration has been identified and suitable material properties have been selected. A prototype collector has been designed, built and tested. A high thermal efficiency was reached at zero reduced temperature. For this level of thermal efficiency, the corresponding electrical efficiency has is lower than efficiency of a standard PV panel using the same technology. However, this solar PV-T collector is reaching, in these standard conditions, the highest efficiency level reported in the literature.     

Exergy analysis of domestic-scale solar water heaters

Solar water heater is the most popular means of solar energy utilization because of technological feasibility and economic attraction compared with other kinds of solar energy utilization. Earlier assessments of domestic-scale solar water heaters were based on the first thermodynamic law. However, this kind of assessment cannot perfectly describe the performance of solar water heaters, since the essence of energy utilization is to extract available energy as much as possible. So, it is necessary to evaluate domestic-scale solar water heaters based on the second thermodynamic law.No matter the technology process, from the property of energy utilization perspective, we can separate the technology process into three intimately related sub-procedures, namely conversion procedure, utilization procedure, and recycling procedure. An energy analysis entitled ‘Three Procedure Theory’ can be conveniently conducted as presented by Professor Hua Ben. Compared with other theories of energy analysis, three procedure theory exhibits great advantages. The utilization procedure puts forth requirement for the design of parameters in conversion procedure and sets up limits in the consideration of recycling procedure. Of course, under specific conditions, the utilization procedure also receives feedback from other procedures. Three procedure theory furnishes us a good platform to perform energy analysis.The study in this paper is based on three procedure theory. Exergy analysis is conducted with the aim of providing some methods to save cost and keep the efficiency of domestic-scale solar water heater to desired extent and at the same time figuring out related exergy losses. From this survey, it is shown that for an ordinary thermally insulated domestic-scale solar water heater, D_ju (exergy losses due to imperfectly thermal insulation in collector) and D_jR (exergy losses due to imperfectly thermal insulation in storage barrel) cannot be avoided. D_ku (exergy losses due to irreversibility in collector) is mainly caused by irreversibility of heat transfer and D_kR (exergy losses due to irreversibility in storage barrel) is dominated by the mixing of water at different temperature. D_ku acts as the driving force for the system while D_kR is of little contribution. A good design of storage barrel with little D_kR will go a long way in improving exergy efficiency. An equation for computing D_kR is presented. For the collector, which is the core of the domestic-scale solar water heater, a judicious choice of width of plate W and layer number of cover is necessary. We define collector exergy efficiency η_xc to be η_xc=E_xo/E_xu. The relation between collector exergy efficiency and width of plate together with layer number of cover is also analysed.     

Experimental and theoretical investigation of a solar heating system with heat pump

In this study, the performance of a solar heating system with a heat pump was investigated both experimentally and theoretically. The experimental results were obtained from November to April during the heating season. The experimentally obtained results are used to calculate the heat pump coefficient of performance (COP), seasonal heating performance, the fraction of annual load met by free energy, storage and collector efficiencies and total energy consumption of the systems during the heating season. The average seasonal heating performance values are 4.0 and 3.0 for series and parallel heat pump systems, respectively. A mathematical model was also developed for the analysis of the solar heating system. The model consists of dynamic and heat transfer relations concerning the fundamental components in the system such as solar collector, latent heat thermal energy storage tank, compressor, condenser, evaporator and meteorological data. Some model parameters of the system such as COP, theoretical collector numbers (N_c), collector efficiency, heating capacity, compressor power, and temperatures (T₁, T₂, T₃, T_T) in the storage tank were calculated by using the experimental results. It is concluded that the theoretical model agreed well with the experimental results.     

Comparative thermal performance evaluation of an active solar distillation system

In this paper, thermal models of all types of solar collector‐integrated active solar stills are developed based on basic energy balance equations in terms of inner and outer glass temperatures. In this paper, hourly yield, hourly exergy efficiency, and hourly overall thermal efficiency of active solar stills are evaluated for 0.05 m water depth. All numerical computations had been performed for a typical day in the month of 07 December 2005 for the climatic conditions of New Delhi (28°35′N, 77°12′E, 216 m above MSL). The thermal model of flat‐plate collector integrated with active solar still was validated using the experimental test set‐up results. Total daily yield from active solar still integrated with evacuated tube collector with heat pipe is 4.24 kg m^?2 day^?1, maximum among all other types of active solar stills. Copyright © 2007 John Wiley & Sons, Ltd.     

A solar ejector air‐conditioning system using environment‐friendly working fluids

In this paper, the performance of the solar‐driven ejector air conditioning with several environment‐friendly working fluids is studied. The effect of the fluid nature and operating conditions on the ejector performance is examined. This performance is calculated using an empirical correlation. Thermodynamic properties of functioning fluids are obtained with a package REFPROP7. It appears that the refrigerant R717 offers the highest coefficient of performance (COP). For generator temperature T_B = 90°C, condenser temperature T_C = 35°C and evaporator temperature T_E = 15°C and with R717, the COP of ejector air‐conditioning system is 0.408. Using a meteorological data for the city of Tunis, the system performance is computed for three collector types. The air‐conditioning season and period were taken for six months from April to September. The daily period is between 8 and 17 h. For the solar air‐conditioning application, the COP of the overall system varied from 0.21 to 0.28 and the exergy efficiency varied from 0.14 to 0.19 with the same working conditions and total solar radiation (351–875 Wm^?2) in July. Copyright © 2008 John Wiley & Sons, Ltd.     

非金属管式平板太阳能集热器研究

运用平板太阳能集热器热迁移因子F_R的表达式,研究集热器管间距W和管外径D_o对F_R的影响规律。研究发现,随着W的减小,F_R逐渐增大,当W趋近于D_o时,F_R趋近最大,此时集热管导热系数对F_R影响甚微,即可采用非金属材料替代金属材料制作集热管;随着D_o增大,F_R先增加而后逐渐下降。研制了非金属管式平板太阳能集热器,建立了其性能试验测量系统。试验结果表明,当工质流量为0.165 kg/s时,F_R为0.738,平均集热效率为54%。运用BP神经网络算法,建立了非金属管式平板太阳能集热器出口热流体温度的预测模型,并将预测结果与试验结果进行了比较。