非正南方向太阳集热器面积补偿的理论研究

该文以昆明和北京为例,对我国南北地区分别选用平板型管翼式集热器和圆柱吸热体真空管集热器进行了模拟计算。结果显示,水平面上日均太阳辐射、环境温度、屋顶方位及倾角对集热面积补偿有不同程度的影响;其中,屋顶方位角和倾角是影响集热器处于非正南和正南方向且最佳倾角时采光面积之比A/A0的主要因素。本文的研究成果可为太阳能建筑一体化设计提供部分有价值的参考数据。     

Optical and thermal optimization of stationary non-evacuated CPC solar concentrator with fully illuminated wedge receivers

Stationary low concentrator collectors (C < 2), of the CPC type, are of great interest for thermal energy supply of industrial processes, at temperatures below or equal to 100 °C. In particular, concentrators with fully illuminated V inverted absorbers have attractive properties for thermal energy conversion.Numerical analysis of the geometric and optical characteristics of different low concentration CPC’s (C between 1 and 2) with fully inverted wedge absorbers, shows that the cavities with the minimal relationship between the length and height of the reflector surface and the aperture, (L/A) and (H/A), and the lower average number of reflections 〈n〉 correspond to the lowest angular acceptance concentrator. If a concentration of 1.2 is desired, the smallest ratios of (L/A) and (H/A) and mean number of reflections 〈n〉 occur for C = 2 (θ_a = 30°). However, when the annual generated thermal energy is also considered (for example, for Recife, tilt equals latitude, fluid temperature equals 50 °C, East–West orientation), a very large maximum value in the concentration region between 1.4 and 1.6 (acceptance angles of 38.68° e 45.58°) occurs. The simulation results indicate, that while the operational temperature rises, the ratio between the annual generated thermal energy by the CPC and a good quality flat-plate collector becomes greater than 1: for CPC with 1.2 concentration these ratios become 1.0 at 50 °C and 1.35 at 80 °C. The improvement in the reflectivity of the reflector surface of the CPC rises significantly this relation, i.e., if the reflectivity exceeds from 0.86 to 0.96 the CPC of the concentration relation 1.2, operating at 80 °C may generate 55% more thermal energy than flat-plate collector.     

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.     

A thermo-economical optimization of a domestic solar heating plant with seasonal storage

In this study, the thermo-economic optimization analysis to determinate economically optimal dimensions of collector area and storage volume in domestic solar heating systems with seasonal storage is presented. For this purpose, a formulation based on the simplified P₁ and P₂ method is developed and solved by using MATLAB optimization Toolbox for five climatically different locations of Turkey. The results showed that the required optimum collector area in Adana (37 °N) for reaching maximum savings is 36 m²/house and 65 m²/house in Erzurum (39 °N) for same storage volume (1000 m³). The effects of collector efficiency on solar fraction and savings are investigated. The simulation results showed that the solar fraction and savings of the selective flat plate collector systems are higher than the other black paint flat plate collector systems.     

An experimental investigation on the effect of pH variation of MWCNT–H2O nanofluid on the efficiency of a flat-plate solar collector

The effect of pH variation of MWCNT–H₂O nanofluid on the efficiency of a flat-plate solar collector was investigated experimentally. The experiments were carried out using 0.2 wt.% MWCNT with various pH values, 3.5, 6.5, and 9.5, and Triton X-100 as additive. The procedure of ASHRAE standard was used for testing the thermal performance of flat-plate solar collector. Results show that by increasing or decreasing the pH values with respect to the pH of isoelectric point, the positive effect of nanofluid on the efficiency of solar collector is increased.     

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.     

Design and test results of a low-capacity solar cooling system in Alicante (Spain)

Despite its attractiveness, solar cooling technology is still in an early stage of development. Most installations currently in operation show differences in the collector area per kilowatt of cooling capacity that cannot be explained only by project-specific circumstances. The purpose of this paper was twofold. First, to answer some questions that came up during the design process of the plant by using a TRNSYS system model and statistical tools. Second, to gain knowledge about the plant operation and validate the TRNSYS model through measured data. The system was equipped with a flat-plate collector field of 38.4 m². A lithium bromide-water single-effect absorption chiller (17.6 kW) was selected in order to provide chilled water to fan-coils. Performance data were registered at the solar plant working with a 1000-l heat storage tank and a required temperature of 80 °C to drive the absorption machine. An average of 29% of the solar energy incident on the solar collectors’ surface was transferred to the hot water storage. The registered average COP of the absorption chiller was 0.691. The performance data were compared with the values predicted by the TRNSYS plant model and a high level of agreement was obtained.     

Effect of solid volume fraction and tilt angle in a quarter circular solar thermal collectors filled with CNT–water nanofluid

Solar thermal collectors have significant importance due to its wide use in solar thermal technology. Augmentation of heat transfer is a key challenge for solar thermal technology. A quarter circular solar thermal collectors is investigated throughout the paper introducing carbon nanotube (CNT)–water nanofluid in the cavity. Tilt angle of this type of collector plays a vital role and heat transfer can be maximized for a particular tilt angle and solid volume fraction of the nanofluid. Galerkin weighted residual of FEM has been applied for the numerical solution of the problem. Grid independency test and code validation have been assessed for the accuracy of numerical solution. In this paper a wide range of solid volume fraction (δ = 0 to δ = 0.12) and tilt angle (ϕ = 0 to ϕ = 60°) has been investigated for Rayleigh number (Ra = 10⁵–10⁸) with varying dimensionless times. It has been found that both solid volume fraction and tilt angle play vital roles for the augmentation of heat transfer and a good heat transfer characteristic can be obtained by compromising between these two parameters. The results are shown using streamline, isotherm contour and related graph and chart.     

Performance assessment of an integrated free cooling and solar powered single-effect lithium bromide-water absorption chiller

In this study, performance assessment of an integrated cooling plant having both free cooling system and solar powered single-effect lithium bromide–water absorption chiller in operation since August 2002 in Oberhausen, Germany, was performed. A floor space of 270 m² is air-conditioned by the plant. The plant includes 35.17 kW cooling (10-RT) absorption chiller, vacuum tube collectors’ aperture area of 108 m², hot water storage capacity of 6.8 m³, cold water storage capacity of 1.5 m³ and a 134 kW cooling tower. The results show that free cooling in some cooling months can be up to 70% while it is about 25% during the 5 years period of the plant operation. For sunny clear sky days with equal incident solar radiation, the daily solar heat fraction ranged from 0.33 to 0.41, collectors’ field efficiency ranged from 0.352 to 0.492 and chiller COP varies from 0.37 to 0.81, respectively. The monthly average value of solar heat fraction varies from 31.1% up to 100% and the five years average value of about 60%. The monthly average collectors’ field efficiency value varies from 34.1% up 41.8% and the five-year average value amounts about 28.3%. Based on the obtained results, the specific collector area is 4.23 (m²/kW_cold) and the solar energy system support of the institute heating system for the duration from August 2002 to November 2007 is 8124 kWh.     

Jet impingement interaction with cross flow in horizontal porous layer under thermal non-equilibrium conditions

In the present article the jet impingement cooling of heated portion of a horizontal surface immersed in a thermally non-equilibrium porous layer is considered for investigation numerically with the presence of a cross flow. The mathematical model is derived for steady, two-dimensional laminar flow based on Darcy model and two-energy equation for fluid and solid phases. A parametric study is carried out by varying the following parameters: cross flow to jet flow velocity ratio parameter (0 ⩽ M ⩽ 1); porosity scaled thermal conductivity ratio parameter (0.1 ⩽ K_r ⩽ 1000); heat transfer coefficient parameter (0.1 ⩽ H ⩽ 1000); Péclet number (1 ⩽ Pe ⩽ 1000) and Rayleigh number (10 ⩽ Ra ⩽ 100). The total average Nusselt number is defined based on the overall thermal conductivity, which is assumed to be the arithmetic mean of the porosity scaled thermal conductivity of the fluid and solid phases. The total average Nusselt number as well as the average Nusselt number for both fluid and solid phases is presented for different governing parameters. It is found that the presence of a weak cross flow in a jet impinging jet may degrade the heat transfer. The results show that the average Nusselt number calculated from the thermal equilibrium model are the maximum possible values and these values can be reproduced by large values of H × K_r.     

Thermosyphon heat-pipe evacuated tube solar water heaters for northern maritime climates

Under transient climatic conditions previous research has reported that evacuated tube solar water heaters (ETSWHs) with heat-pipe absorbers are the most effective solution for collection of solar energy. The cost of such systems is greater than the mass produced “water in glass” evacuated tube solar water heater mainly manufactured in China. Previous studies have reported that the costs of solar water heating can be reduced through the adoption of thermosyphon fluid circulation. Well designed thermosyphon systems are as effective as pumped systems but with lower capital and running costs. To investigate if costs could be reduced and performance levels maintained, outdoor testing of three thermosyphon heat-pipe ETSWHs primarily designed for pumped fluid circulation was carried out under a northern maritime climate. Experimental data from a year’s side by side monitoring of two thermosyphon ETSWHs (both with the same area of 2 m²) was collected and used to validate a correlation based on a modified version of the f-chart design tool between the observed and expected performance for both systems. The R² value between measured and predicted monthly solar fractions was greater than 0.99 for both systems. The R² value between measured and predicted diurnal solar fractions was calculated as greater than 0.95 for both systems. The only difference between the two was that one utilised internal heat-pipe condensers whilst the other used external ones. The system with internal condensers was found to be 17% more efficient. A simplifying assumption of a constant temperature rise across the collectors reduced the calculations required to predict the performance of thermosyphon heat-pipe ETSWHs and was also statistically significant. To determine if the assumption was valid for other thermosyphon heat-pipe ETSWHs with different collector parameters a third system with internal condensers an area of 3 m², a heat removal factor (F_R) of 0.816 based on the absorber area and a collector loss coefficient (F_RU_L) of 2.25 W m^?2 K^?1 was assembled and its performance monitored, when the same assumption was applied the R² value between the measured and predicted daily solar fractions was calculated as 0.96 experimentally demonstrating that this relationship was still statistically significant for another heat-pipe thermosyphon ETSWH with different collector parameters.     

An improved dynamic test method for solar collectors

A comprehensive improvement of the mathematical model for the so called transfer function method is presented in this study. This improved transfer function method can estimate the traditional solar collector parameters such as zero loss coefficient and heat loss coefficient. Two new collector parameters t and m_fC_f are obtained. t is a time scale parameter which can indicate the heat transfer ability of the solar collector. m_fC_f can be used to calculate the fluid volume content in the solar collector or to validate the regression process by comparing it to the physical fluid volume content if known. Experiments were carried out under dynamic test conditions and then test data were processed using multi-linear regression method to get collector parameters with statistic analysis. A comparison of the collector parameters obtained from the improved transfer function (ITF) method and the quasi-dynamic test (QDT) method is carried out. The results show that the improved transfer function method can accurately obtain reasonable collector parameters. The influence of different averaging time intervals is investigated. Based on the investigation it is recommended to use on line calculation if applicable for the second-order differential term with 6–9 min as the best averaging time interval. The measured and predicted collector power output of the solar collector are compared during a test of 13 days continuously both for the ITF method and the QDT method. The maximum and averaging error is 53.87 W/m² and 5.22 W/m² respectively of the ITF method while 64.13 W/m² and 6.22 W/m² of the QDT method. Scatter and relative error distribution of the measured power output versus the predicted power output is also plotted for the two methods. No matter in either error analysis or scatter distribution, the ITF method is more accurate than the QDT method in predicting the power output of a solar collector.In conclusion, all the results show that the improved transfer function method can accurately and robustly estimate solar collector parameters and predict solar collector thermal performance under dynamic test conditions.     

Experimental study of natural convection in PV roof solar collector

The aim of this paper is to propose the PV roof solar collector (PV-RSC) to investigate the natural convection heat transfer and estimated the convective heat transfer coefficient in the channel. The experimental set-up was composed of a PV panel on the upper layer and the lower layer is aluminum plate of the channel. The inclination angle and air gap of channel were fixed at 30° and 15 cm, respectively. The channel width is 0.7 m, and length is 1.2 m. The data analysis were confirmed the effect of radiative exchange influent to natural convection within the channel. On the basis of the experimental results, an empirical formula is found; the Nu as a function of Ra_s sin30, that is Nu_s = 0.3282 (Ra_s sin30)^0.2249. The correlation obtained to range 3 × 10⁸ < Ra_s sin30 < 7 × 10⁸. A comparison between PV-RSC and normal PV panel, it was confirmed that the PV-RSC could be generated electric power than that normal PV panel by about 30 W; and also the percentage of power generation increase was rising about 25% throughout the day.     

Heat transfer of impinging jet-array over convex-dimpled surface

A detailed heat transfer measurement over a convex-dimpled surface of impinging jet-array with three eccentricities (E/H) between jet-centre and dimple-centre is performed. These surface dimples considerably modify heat transfers from smooth-walled scenarios due to different impinging topologies for jet array with modified inter-jet reactions. Heat transfer variations caused by adjusting jet Reynolds number (Re) and separation distance (S/D_j) over the ranges of 5000 ⩽ Re ⩽ 15,000 and 0.5 ⩽ S/D_j ⩽ 11 with three eccentricities of E/H = 0, 1/4 and 1/2 are examined. A selection of experimental data illustrates the isolated and interactive influences of Re, S/D_j and E/H on local and spatially averaged heat transfers. In conformity with the experimentally revealed heat transfer physics, a regression-type analysis is performed to generate a set of heat transfer correlations, which permit the evaluations of spatially averaged Nusselt numbers over central jet region of dimpled impinging surface.     

Effect of flow geometry parameters on transient heat transfer for turbulent flow in a circular tube with baffle inserts

The effect of the flow geometry parameters on transient forced convection heat transfer for turbulent flow in a circular tube with baffle inserts has been investigated. The characteristic parameters of the tubes are pitch to tube inlet diameter ratio H/D = 1, 2 and 3, baffle orientation angle β = 45°, 90° and 180°. Air, Prandtl number of which is 0.71, was used as working fluid, while stainless steel was considered as pipe and baffle material. During the experiments, different geometrical parameters such as the baffle spacing H and the baffle orientation angle β were varied. Totally, nine types of baffle inserted tube were used. The general empirical equations of time averaged Nusselt number and time averaged pressure drop were derived as a function of Reynolds number corresponding to the baffle geometry parameters of pitch to diameter ratio H/D, baffle orientation angle β, ratio of smooth to baffled cross-section area S_o/S_a and ratio of tube length to baffle spacing L/H were derived for transient flow conditions. The proposed empirical correlations were considered to be applicable within the range of Reynolds number 3000 ⩽ Re ⩽ 20,000 for the case of constant heat flux.     

A MCRT and FVM coupled simulation method for energy conversion process in parabolic trough solar collector

A coupled simulation method based on Monte Carlo Ray Trace (MCRT) and Finite Volume Method (FVM) is established to solve the complex coupled heat transfer problem of radiation, heat conduction and convection in parabolic trough solar collector system. A coupled grid checking method is established to guarantee the consistency between the two methods and the validations to the coupled simulation model were performed. Firstly, the heat flux distribution on the collector tube surface was investigated to validate the MCRT method. The heat flux distribution curve could be divided into 4 parts: shadow effect area, heat flux increasing area, heat flux reducing area and direct radiation area. The heat flux distribution on the outer surface of absorber tube was heterogeneous in circle direction but uniform in axial direction. Then, the heat transfer and fluid flow performance in the LS-2 Solar Collector tube was investigated to validate the coupled simulation model. The outlet temperatures of the absorber tube predicted by the coupled simulation model were compared with the experimental data. The absolute errors are in the range of 1.5–3.7 °C, and the average relative error is less than 2%, which demonstrates the reliability of the coupled method established in this paper. At last, the concentrating characteristics of the parabolic trough collectors (PTCs) were analyzed by the coupled method, the effects of different geometric concentration ratios (GCs) and different rim angles were examined. The results show the two variables affect the heat flux distribution. With GC increasing, the heat flux distributions become gentler, the angle span of reducing area become larger and the shadow effect of absorber tube become weaker. And with the rim angle rising, the maximum value of heat flux become lower, and the curve moves towards the direction φ = 90°. But the temperature rising only augments with GC increasing and the effect of rim angle on heat transfer process could be neglected, when it is larger than 15°. If the rim angle is small, such as θ_rim = 15°, lots of rays are reflected by glass cover, and the temperature rising is much lower.     

Electrohydrodynamic enhancement of in-tube convective condensation heat transfer

An experimental investigation of electrohydrodynamic (EHD) augmentation of heat transfer for in-tube condensation of flowing refrigerant HFC-134a has been performed in a horizontal, single-pass, counter-current heat exchanger with a rod electrode placed in the centre of the tube. The effects of varying the mass flux (55 kg/m² s ⩽ G ⩽ 263 kg/m² s), inlet quality (0.2 ⩽ x_in ⩽ 0.83) and the level of applied voltage (0 kV ⩽ V ⩽ 8 kV) are examined. The heat transfer coefficient was enhanced by a factor up to 3.2 times for applied voltage of 8 kV. The pressure drop was increased by a factor 1.5 at the same conditions of the maximum heat transfer enhancement. The improved heat transfer performance and pressure drop penalty are due to flow regime transition from stratified flow to annular flow as has been deduced from the surface temperature profiles along the top and bottom surfaces of the tube.     

Maximum-power-point tracking control of solar heating system

The present study developed a maximum-power point tracking control (MPPT) technology for solar heating system to minimize the pumping power consumption at an optimal heat collection. The net solar energy gain Q_net (=Q_s ? W_p/η_e) was experimentally found to be the cost function for MPPT with maximum point. The feedback tracking control system was developed to track the optimal Q_net (denoted Q_max). A tracking filter which was derived from the thermal analytical model of the solar heating system was used to determine the instantaneous tracking target Q_max(t). The system transfer-function model of solar heating system was also derived experimentally using a step response test and used in the design of tracking feedback control system. The PI controller was designed for a tracking target Q_max(t) with a quadratic time function. The MPPT control system was implemented using a microprocessor-based controller and the test results show good tracking performance with small tracking errors. It is seen that the average mass flow rate for the specific test periods in five different days is between 18.1 and 22.9 kg/min with average pumping power between 77 and 140 W, which is greatly reduced as compared to the standard flow rate at 31 kg/min and pumping power 450 W which is based on the flow rate 0.02 kg/s m² defined in the ANSI/ASHRAE 93-1986 Standard and the total collector area 25.9 m². The average net solar heat collected Q_net is between 8.62 and 14.1 kW depending on weather condition. The MPPT control of solar heating system has been verified to be able to minimize the pumping energy consumption with optimal solar heat collection.     

The daily performance of a solar pond integrated with solar collectors

The present study deals with heat storage performance investigation of integrated solar pond and collector system. In the experimental work, a cylindrical solar pond system (CSPS) with a radius of 0.80 m and a depth of 2.0 m and four flat plate collectors dimensions of 1.90 m × 0.90 m was built in Cukurova University in Adana, Turkey. The CSPS was filled with salty water of various densities to form three salty water zones (Upper Convective Zone, Non-Convective Zone and Heat Storage Zone). Heat energy collected by collectors was transferred to the solar pond storage zone by using a heat exchanger system which is connected to the solar collectors. Several temperature sensors connected to a data acquisition system were placed vertically inside the CSPS and at the inlet and outlet of the heat exchanger. Experimental studies were performed using 1, 2, 3 and 4 collectors integrated with the CSPS under approximately the same condition. The integrated solar pond efficiencies were calculated experimentally and theoretically according to the number of collectors. As a result, the experimental efficiencies are found to be 21.30%, 23.60%, 24.28% and 26.52%; the theoretical efficiencies to be 23.42%, 25.48%, 26.55% and 27.70% for 1, 2, 3 and 4 collectors, respectively. Theoretical efficiencies were compared with the experimental results and hence a good agreement is found between experimental and theoretical efficiency profiles.     

Estimating effective thermal conductivity of two-phase materials

In this paper, correlations are proposed to estimate the effective thermal conductivity of two-phase materials. For any α, Maxwell equation for 0.0 < c ⩽ 0.10 and phase inverted Maxwell for 0.9 ⩽ c ⩽ 1 are considered. For concentrations between 10% and 90%, and low α (<20), an equation based on the unit-cell approach (constant isotherms) is proposed. For α > 20, three correlations are proposed based on field solution approach which includes three α ranges viz. medium (20 ⩽ α ⩽ 100), high (100 ⩽ α ⩽ 1000) and very high (1000 < α). The predicted effective thermal conductivity of two-phase system is compared with well-established models. Comparison of the predicted values of the correlations with experimental results is also made. The predictions of effective thermal conductivity of two-phase materials match well with the experimental values.