A generalized thermal analysis of tubular solar collectors

A generalized thermal analysis of tubular solar collectors such as the CPC, CPC with bare absorber and flat-plate (all having U-tube fluid carriers) is presented. The analysis is applicable for smaller as well as larger size troughs. It has been found that earlier published results of thermal analyses of the CPC collectors are not applicable to all CPC collectors encountered in the literature. The present analysis can, however, be successfully applied to all such collectors.     

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.     

Advanced 3‐D CPC solar collector for thermal electric system

In this paper, the authors propose an innovative non‐tracking three‐dimensional compound parabolic concentrator (3‐D CPC) solar collector, which has excellent thermal efficiency for a high‐temperature range (100–200°C). In the past studies, in order to improve the thermal efficiency of the solar collector in a high‐temperature range, very high concentration ratios and tracking systems have been adopted. However, conventional high concentration solar collectors are not cost‐effective and are inappropriate for small‐rating thermal electric generation systems for residential use. The proposed 3‐D CPC collector has a moderate concentration ratio and does not need tracking. Initially, the tentative 3‐D CPC collector was fabricated and its thermal performance was tested. Next, numerical simulations of the optical characteristics of the 3‐D CPC collector were carried out via the ray‐tracing method. Finally, the specification of the optimal 3‐D CPC collector was clarified. Applications of the thermal electric system will also be mentioned. © 2006 Wiley Periodicals, Inc. Heat Trans Asian Res, 35(5): 323–335, 2006; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20121     

Study of a novel solar adsorption cooling system and a solar absorption cooling system with new CPC collectors

One two-phase thermo-syphon silica gel-water solar adsorption chiller and LiBr-H₂O absorption chiller with new medium CPC (Compound Parabolic Concentrator) solar collectors were investigated. The reliability of adsorption chiller can be improved, because there is only one vacuum valve in this innovative design. Medium temperature evacuated-tube CPC solar collectors were firstly utilized in the LiBr-H₂O air conditioning system. The former system was applied in north of China at Latitude 37.45° (Dezhou city, China), the latter system was applied at Latitude 36.65° (Jinan city, China). Experimental results showed that the adsorption chiller can be powered by 55 °C of hot water. The adsorption chiller can provide 15 °C of chilled water from 9:30 to 17:00, the average solar COP (COPs) of the system is 0.16. In the absorption cooling system, the efficiency of the medium temperature evacuated-tube CPC solar collector can reach 0.5 when the hot water temperature is 125 °C. The absorption chiller can provide 15 °C of chilled water from 11:00 to 15:30, and the average solar COPs of absorption system is 0.19.     

Cost effective asymmetric CPC solar collectors

Low cost CPC solar collectors were designed, constructed and tested. The collectors consist of two separate absorbers, which are horizontally incorporated in a stationary asymmetric CPC mirror. The efficient operation of the proposed collectors is due to the direct absorption of a large part of the incoming solar radiation and to the thermal losses suppression by the inverted surface of both absorbers. Two collector types with the same basic design are presented. The first type has tubular absorbers which are used for direct water heating and the second has flat fin type absorbers with pipe. Test results showed that the proposed collectors operate efficiently and are suitable for hot water applications.     

An evaluation on thermal performance of CPC solar collector

The main objective of this work is the investigation and improvement of thermal performance of evacuated CPC (Compound Parabolic Concentrator) solar collector with a cylindrical absorber. Modified types of this solar collector are always combined with the evacuated glass envelop or tracking system. The conventional stationary CPC solar collector has been compared with the single axis tracking CPC solar collector in outlet temperature, net heat flux onto the absorber and thermal efficiency. Numerical model has been analyzed based on the irradiation determined actually and the results have been calculated to predict the thermal efficiency. Based on the comparison of the measured and calculated results, it is concluded that the numerical model can accurately estimate the performance of solar collectors. The result shows the thermal efficiency of the tracking CPC solar collector is more stable and about 14.9% higher than that of the stationary CPC solar collector.     

A comparison of compound parabolic and simple parabolic concentrating solar collectors

The compound parabolic and simple parabolic solar collectors are analyzed and compared for their ability to accept non-direct radiation and for their respective reflector arc-lengths. The simple parabolic concentrator (SPC) can make use of some non-direct solar radiation if the absorber tube is intentionally enlarged so as to intercept defocussed radiation. A principal advantage of collecting non-direct radiation with a SPC rather than with a compound parabolic concentrator (CPC) is the reduced materials use in the construction of the reflector, but a principal disadvantage is the reduction of acceptance angle to about that of the CPC. However, a SPC with concentration ratio less than 10 can still collect most of the circumsolar non-direct radiation.     

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.     

Comparison of the optics of non-tracking and novel types of tracking solar thermal collectors for process heat applications up to 300 °C

Evacuated CPC (compound parabolic concentrator) collectors with non-tracking reflectors are compared with two novel tracking collectors: a parabolic trough and an evacuated tube collector with integrated tracking reflector. Non-tracking low concentrating CPC collectors are mostly mounted in east–west direction with a latitude dependent slope angle. They are suitable at most for working temperatures up to 200–250 °C. We present a tracking evacuated tube-collector with a trough-like concentrating mirror. Single-axis tracking of the mirror is realized with a magnetic mechanism. The mirror is mounted inside the evacuated tube and hence protected from environmental influences. One axis tracking in combination with a small acceptance angle allows for higher concentration as compared to non-tracking concentrating collectors. Ray-tracing analysis shows a half acceptance angle of about 5.7° at geometrical concentration ratio of 3.2. Losses of well constructed evacuated tube collectors (heat conductivity through the manifolds inside the thermally insulated terminating housing are low) are dominated by radiation losses of the absorber. Hence, reducing the absorber size can lead to higher efficiencies at high operating temperature levels. With the presented collector we aim for operating temperatures up to 350 °C. At temperatures of 300 °C we expect with anti-reflective coating of the glass tube and a selective absorber coating efficiencies of 0.65. This allows for application in industrial process heat generation, high efficient solar cooling and power generation. A first prototype, equipped with a standard glass tube and a black paint absorber coating, was tested at ZAE Bayern. The optical efficiency was measured to be 0.71. This tube-collector is compared by ray-tracing with non-tracking market available tube-collectors with geometrical concentration ratios up to 1.1 and with a low cost parabolic trough collector of Industrial Solar Technology (IST) with an acceptance half angle about 1.5°, a geometrical concentration ratio of 14.4 and a measured optical efficiency of 0.69.     

Impact of different internal convection control strategies in a non-evacuated CPC collector performance

Over the last decade the technological advances observed in solar collector materials, namely better spectrally selective absorber coatings and ultra clear glass covers, contribute to performance improvements and translate into higher operational temperature ranges with higher efficiency values.While the use of Evacuated Tube Collectors (ETCs) is becoming widespread in the thermal conversion of solar energy, non-evacuated solar collectors still hold advantages at manufacturing, reliability and/or cost levels, making them interesting and competitive for a large range of applications, in particularly, in temperature ranges up to 80 °C. However, these advantages have not prevented the major drawback of these collectors when compared to ETCs: thermal losses due to internal convection which prevent their general use in the range of operating temperatures up to 150 °C.Insulation, double glazing or selective coatings can be used in non-evacuated collectors to reduce heat losses. To prevent internal convection losses in these solar collectors, different control strategies have been studied, such as the adoption of different inert gases within the collector cavity, physical barriers reducing air flow velocities over the absorber or cover surfaces or the use of concentration.In the present article, an assessment of adopting such internal convection control strategies in a CPC collector is presented. Each of the presented strategies is assessed in terms of the resulting collector optical and thermal characterization parameters and yearly collector yield. For this purpose, an integrated tool allowing the design, optical and thermal characterization of CPC collectors was developed. The results obtained provide valuable guidelines for anyone wishing to implement any of these strategies in a new collector design.     

Design and analysis of a novel low-temperature solar thermal electric system with two-stage collectors and heat storage units

The proposed low-temperature solar thermal electric generation is based on the compound parabolic concentrator (CPC) of small concentration ratio and Organic Rankine Cycle (ORC). The technologies of CPC and ORC are analyzed, and feasibility of the system is demonstrated. In particular, two-stage collectors and heat storage units are adopted to improve heat collection efficiency. Organic fluid is preheated by flat plate collectors (FPCs) prior to entering a higher temperature heat exchanger connected with the CPC. The two-stage heat storage units are composed of two types of phase change material (PCM) with diverse melting temperatures. The novel configuration is carefully designed to react to different operating conditions. The fundamentals are illustrated for both simultaneous and separate processes of heat collection and power conversion. Mathematic models are built for heat transfer and thermodynamics of the innovative system. Coupling relationship among the proportion of FPC to CPC, the melting temperature of the first-stage PCM and the overall collector efficiency is established. The benefits of the preheating concept and cascaded heat storages are investigated in detail in comparison with the single-stage system. The results indicate that the increase in collector efficiency of the two-stage system is appreciable.     

The effect of variation of angle of inclination on the performance of low-concentration-ratio compound parabolic concentrating solar collectors

Thermal heat transfer in line-axis, symmetric, compound parabolic concentrating solar energy collectors (CPCs) has been investigated and a theoretical numerical model has been developed. The model allows the effect of the angle of axial inclination of an east-west aligned CPC and hence the effect of the latitudinal and tracking configuration of the CPC system on performance to be determined. The internal and external convective heat transfer correlations employed are angular dependent. The variation of convective, radiative, conductive and overall heat transfer coefficients and system efficiency for a range of angular inclinations, concentration ratios, total insolations and beam to diffuse insolation factors are presented graphically. The results demonstrate that there is a 10% variation in convective heat transfer with angle of inclination for low concentration CPCs (i.e. C = 1.5).     

Upper bounds for the yearly energy delivery of stationary solar concentrators and the implications for concentrator optical design

Compound parabolic concentrator (CPC) type collectors have been viewed as the optimal design for totally stationary concentrators. However the CPC is ideal only for uniform incident solar flux averaged over the energy collection period. The actual yearly-averaged incident flux map turns out to be highly non-uniform, as a function of projected incidence angle, which implies that concentration can be increased markedly if optical collection efficiency is compromised. The question then becomes: what concentrator angular acceptance function is best matched to nature's radiation flux input, and how much energy can such a concentrator deliver? The recently-invented tailored edge-ray concentrator (TERC) approach could be used to determine optimal reflector contours, given the optimal acceptance angle function. We demonstrate that totally stationary TERCs can have around three times the geometric concentration of corresponding optimized stationary CPCs, with greater energy delivery per absorber area, in particular for applications that are currently being considered for stationary evacuated concentrators with the latest low-emissivity selective coatings, e.g. solar-driven double-stage absorption chillers (at around 170°C) and solar thermal power generation (at around 250°C).     

Optimization of low temperature solar thermal electric generation with Organic Rankine Cycle in different areas

The presented low temperature solar thermal electric generation system mainly consists of compound parabolic concentrators (CPC) and the Organic Rankine Cycle (ORC) working with HCFC-123. A novel design is proposed to reduce heat transfer irreversibility between conduction oil and HCFC-123 in the heat exchangers while maintaining the stability of electricity output. Mathematical formulations are developed to study the heat transfer and energy conversion processes and the numerical simulation is carried out based on distributed parameters. Annual performances of the proposed system in different areas of Canberra, Singapore, Bombay, Lhasa, Sacramento and Berlin are simulated. The influences of the collector tilt angle adjustment, the connection between the heat exchangers and the CPC collectors, and the ORC evaporation temperature on the system performance are investigated. The results indicate that the three factors have a major impact on the annual electricity output and should be the key points of optimization. And the optimized system shows that: (1) The annual received direct irradiance can be significantly increased by two or three times optimal adjustments even when the CPC concentration ratio is smaller than 3.0. (2) Compared with the traditional single-stage collectors, two-stage collectors connected with the heat exchangers by two thermal oil cycles can improve the collector efficiency by 8.1–20.9% in the simultaneous processes of heat collection and power generation. (3) On the use of the market available collectors the optimal ORC evaporation temperatures in most of the simulated areas are around 120 °C.     

Refrigerants as working fluid in a CPC collector system for electric power generation

Compound parabolic concentrators (CPC) achieve highest possible concentration for any acceptance angle. One of the simplest methods of utilizing the energy of the sun to generate electric power is to use a CPC collector system. A truncated CPC can be used without much loss in concentration. Compared with a full CPC the cost of construction of a truncated CPC is much less. A CPC requires only seasonal adjustments. This further reduces the cost of tracking which is required for other types of concentrators. In this paper it has been shown that the refrigerants R11, R12, R113 and R114 can be satisfactorily used as working fluids in CPC collector systems. By using these working fluids, overall conversion efficiency of 9% can be achieved. Other refrigerants R12, B1, R22, R500, R502, R115 and C318 are found not to be suitable as they produce very low overall conversion efficiencies. In order to produce 20 kW of electricity at 1000 W/m² insolation about 920 truncated CPC collectors (2 m length, 0.15 m aperture) with a concentration ratio of 8 are required. This minimum number of collectors would be required at an overall conversion efficiency of 9%.     

Compound parabolic concentrator technology development to commercial solar detoxification applications

An EC-DGXII BRITE-EURAM-III-financed project called ‘Solar detoxification technology in the treatment of persistent non-biodegradable chlorinated industrial water contaminants’ is described. The objectives are to develop a simple, efficient and commercially competitive solar water treatment technology based on compound parabolic collectors (CPC) enabling design and erection of turnkey installations. A European industrial consortium, SOLARDETOX, representing industry and research in Spain, Portugal, Germany and Italy has been created through this project. Some of the most up-to-date scientific and technological results are given, including the design of the first industrial European solar detoxification treatment plant, the main project deliverable.     

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.     

An innovative,high-efficiency silver/silica nanocomposites for direct absorption concentrating solar thermal power

Using of nanofluids in the concentrating direct absorption solar collectors has the potential of reducing thermal losses because of the excessive temperature of the absorbing surface in the conventional solar collectors. However, increasing the concentration ratio of solar radiation must be followed by increasing the volume fraction of the nanoparticles, which, in turn, has the drawbacks of increasing the settlement and agglomeration rates of the nanoparticles. In this study, we have suggested using the plasmonic nanofluids for volumetric absorption in the concentrated solar power applications because of the less volume fraction of the plasmonic nanoparticles that are required to harvest the concentrated solar radiation. The interaction of concentrated solar radiation with different morphologies of silver nanoparticles coated by silica shell has been computationally studied. Then, the finite element method has been implemented to determine the photo-thermal conversion efficiency for silver nanosphere and nanoplates with a silica shell. Silver nanoparticles coated by silica exhibit a promising potential because of their distinct characteristics. The silica shell is transparent to the visible and near-infrared radiation bands; it also consolidates the intensity of the localized plasmon resonance and so the absorption characteristics, besides its protective role. A high-efficiency low concentration nanofluid has been designed using blended morphologies of Ag nanospheres and nanoprisms with silica-coating–based nanofluid for full-spectrum absorption characteristics. The suggested nanofluid exhibits a promising performance at a volume fraction of 0.0075 wt% where the volumetric solar collector efficiency exceeds 75% under the solar concentration ratio of 50.     

High concentration two-stage optics for parabolic trough solar collectors with tubular absorber and large rim angle

A new two-stage optical design is proposed for parabolic trough solar collectors with tubular absorbers. It can boost the concentration ratio by a factor of 2.5 relative to the conventional design, while maintaining the large rim angles (i.e., low nominal ƒ-numbers) that are desirable for practical and economical reasons. The second stage involves asymmetric nonimaging concentrators of the CPC type, facing segments of the parabolic first stage. The second stage can be accommodated inside an evacuated receiver, allowing the use of first-surface silvered reflectors. The low heat loss of this design opens the possibility of producing steam at temperatures and pressures of conventional power plants, using only one-axis tracking. The improvement in conversion efficiency would be substantial.     

基于TRNSYS的CPC集热器太阳能热水系统的模拟分析

通过TRNSYS软件搭建了复合抛物面聚光器(CPC)集热器太阳能热水系统模型,对广州地区某小型别墅的太阳能热水系统进行了设计,并观察系统在1年(8760 h)中的运行情况。选取了系统在4个典型日的运行情况进行分析,得到了CPC集热器在春分日和冬至日的最高出口温度分别为67.5℃和68.2℃,在夏至日和秋分日的最高出口温度分别为85.7℃和83.3℃。CPC集热器的集热效率随进口流量的增大而增大,随进口温度的下降而升高;经测试,CPC集热器的最佳安装倾角为22°。对CPC集热器和平板集热器的集热性能进行比较后发现,二者的集热功率基本均随太阳辐照度的增加而增加,在冬至日12:00~15:00这个时段,CPC集热器的集热功率是平板集热器的1.5倍。