Category Archives: Particle Image Velocimetry (PIV)

A SIMPLE MULTI-COMPOUND SOLAR. CONCENTRATOR INTENDED TO IMPROVE THE. PERFORMANCE OF FLAT-PLATE SOLAR ARRAYS

Eduardo Rincon Mejia, Fidel Osorio Jaramillo and Fernando Vera Noguez

Facultad de Ingenierfa de la UAEMex
Cerro de Coatepec, C. U., 50130 Toluca, Mdxico
Tel.: + (52) (722) 214 08 55; Fax: + (52) (722) 215 45 12;

INTRODUCTION

This paper describes a new and simple multi-compound solar concentrator (MCC), which can be installed with easy in solar arrays or in single flat-plate solar collector if the absorbers can resist temperatures up to 140° C without damage, increasing very significantly its performance with a very modest investment. For a pair of flat collectors, the MCC consists essentially of two parabolic mirrors of the CPC type at the extreme sides of the collectors, and other pair of smaller elliptic fitted between them. At the bottom side of the array a flat mirror is placed.

This way, the following benefits are achieved: the gathering of solar energy increases due to the augmentation of the total area of aperture of the array, the stagnant and operation temperatures are both increased due to the concentration of solar radiation, the mean thermal efficiency is increased also, and the driving force for thermo-siphon convection is almost doubled. All the forgoing effects yield a very much high performance of the system, increasing its economical efficiency too. A variation of this MCC consists of a pair of flat mirrors (placed instead the parabolic ones of the first option) and a pair of parabolic mirrors (instead of the elliptic pair of first option) between the flat collectors. This option is simpler, cheaper and easy to implement, but the increase in performance is smaller. Nevertheless, it could be a very good election for inexpensive flat collectors.

In Mexico more than 100.000 m2 of solar flat collectors with metallic absorbers of copper or aluminum for water heating for domestic, industrial or services applications has been installed. These solar collectors are reliable, safe and very economically efficient; most of them costs less than 200 dollars for m2 installed, they do not need pumps due to the thermo-siphon effect and the storage and labor of installation is so cheap that the total investment is recovered in about three years or less, while their time of life are more than 10 years. However, their performance can be boosted if a simple and cheap multi-compound concentrator, tailored to the size of the available commercial flat collectors is implemented to the arrays. This can be made if the absorbers resist temperatures up to 140° C without damage.

It is expected that the present development would contribute to the massive the use of solar collectors in Mexico and other developing countries. The main limitation of these solar concentrators is that they cannot be implemented for many flat collectors with plastic absorbers, because they are degraded due to the high temperatures and UV deteriorative effects.

NOMENCLATURE

a Width of a solar flat collector

b = 2a + e Width of an array of two flat collectors

c1, c2, cn Adjusting coefficients for the function Ta (t)

Ac Aperture area of each solar collector

B Linear coefficient of thermal losses (W/m2 ° C)

C Quadratic coefficient of thermal losses (W/m2 ° C)2

Cg Geometric solar concentration (non-dimensional)

CPC Compound Parabolic Concentrator, placed at the extremes of the array

cec Compound Elliptic Concentrator, placed between two solar flat collectors

e Gap between two flat collectors

exc Eccentricity of the elliptic mirrors

F (t) Acceptation function

G Total irradiance (W/m2)

Gm Maximum irradiance in a day (in Mexico, about 1000 W/m2)

MCC Multi-compound solar concentrator

N Day length (yearly mean value: 12 hours)

Pu Thermal useful power (W)

Qu Gathered energy (useful heat) per m2 of flat collector (W/m2)

Qu Useful power (W)

t Instantaneous time (seconds)

ta Dawn instant

Ta Ambient temperature (° C)

Ti Fluid inlet temperature (° C)

Ta (t) Ambient temperature as a function of time

Greek symbols

Angle modifier function Efficiency variable (m2 ° C/W)

Thermal efficiency for flat collectors without MCC Thermal efficiency of flat collectors with MCC

Mean thermal efficiency in a given period Maximum thermal efficiency of flat collectors Angle of incidence of beam radiation Acceptance half-angle of the MCC Specular reflectance of the mirrors Angular parameter or coordinate Truncation angle (maximum value of t)

Comparison of Systems

Together with an evaluation of the outcome of this project, a special aspect is the comparison of centralized and decentralized solar domestic hot water systems for terraced houses. Alongside the classical, self-sufficient ‘one-family house’ type of hot water central heating system, exists the possibility to connect the solar units of terraced houses together and to collect the heat in a central tank. The hot water is then directed to each individual house via a simple network of pipes. The production and distribution of heat for the heating system is also done via the central network.

The objective of the project, i. e. whether centralized systems for the combined supply of several houses with a solar collector area of 20-60 m2 can be an appropriate alternative to standard solar systems for each individual house was thoroughly investigated. The project offered optimal conditions for this investigation, as both system types were put directly

alongside each other so that both would be used under the same conditions (weather, orientation, demands made on the system).

2 Method

In addition to the technical and primary energy assessment of the systems with the help of a comprehensive measurement program and parallel computer simulation tests, a further important criterion of a hot water system, i. e. the economic implications involved, were also investigated. Not simply the initial investment, but the annual total costs with regard to the investment made, as well as the running costs incurred, were taken into account. As an alternative to the centralized freshwater storage system as used in Gelsenkirchen, the performance of other centralized systems on the market were also simulated, not just from the point of view of energy efficiency but in terms of economy. The simulation program used was MATLAB-Simulink®-Toolbox”CARNOT”/1/, a program specially developed by the Solar Institute Julich for researching conventional heating units and likewise innovative thermal solar systems. The results were generalised in order to be applicable to other solar settlements. However, the presence of influences which are not directly quantifiable should also be taken into account.

3 Results

As a result of the project, a research paper entitled „Advice on the Planning of Solar Hot Water Systems in Housing Settlements" was produced /2/. Following on from this, the most important findings both from the real-life comparisons made in Gelsenkirchen as well as the continuing generalized research done on the basis of computer simulations and economic analyses, were documented as a guideline for general use.

STATEMENT OF PROBLEM

A funnel-shaped solar cooker has been developed to concentrate the energy brought by sunlight into a cooking pot or jar, and then trap the heat using the greenhouse effect. A simple system allows pressure-cooking to increase the cooking rate while releasing steam. In this way, solar fusion energy can be used for cooking and pasteurizing water. The Line Concentrated Solar Cooker will be particularly useful in sunshine-rich countries where the vast resource of the sun’s fusion energy is appreciated.

Generally in Parabolic Concentrator collectors concentrate sunlight to a point. This parabolic concentrator was first tried in the 1860’s but is difficult and expensive to build and can only cook small quantities of food. There is also a significant safety problem associated with concentrating sunlight to a point, where damage to eyesight can occur quickly. The Solar Box Cooker uses a glass plate to admit sunlight while keeping infrared radiation ("heat") effectively trapped inside the box. A pot or jar is placed inside the box, preferably a black color to absorb the sunlight. The cooking pot radiates heat mostly as infrared radiation while the glass or plastic plate on top of the box inhibits cooling by the outside air. A weakness of the solar box cooker is that sunlight is admitted only through the top window, while heat escapes from all sides of the box. Good insulation is therefore required along the sides and bottom of the box, and typically two or three nested boxes are used to improve the insulation. Cooking is also slow, typically requiring rotation of the box to follow the sun over a 1 — 3 hour period.

In order to overcome all the above mentioned problems a special device is developed called Line Concentrated Solar Cooker also known as Solar Funnel, which allows sunlight to enter from all sides except from the bottom which would substitute for the box altogether. A black jar or can inside the plastic bag would serve as the cooking vessel. By using a cone-shaped reflecting solar funnel, sunlight would be concentrated along the axis of the funnel at the bottom, where the cooking can or jar would be placed. Safety comes from the fact that the sunlight is concentrated along a line deep inside the funnel where the eyes cannot go without blocking the sun.

Novel Concept Of Nonimaging Single Reflection Solar Energy Concentrator

Dionyz Gasparovsky, PhD.

Ilkovicova 3, 812 19 Bratislava, Slovak Republic
Tel.: +421 903 455 035, Fax: +421 2 654 25 826
e-mail: hung@elf. stuba. sk

Introduction

Many solar applications require temperatures higher than those that can be achieved by common flat-plate collectors. Temperatures over 100 °C are necessary e. g. for industrial process heat. Such temperatures can be obtained by means of solar energy concentrators.

Advantages of concentrating the solar radiation can bring in addition to higher temperatures also decrease in heat losses and material savings due to smaller size of absorber, if taking into account that costs for material absorber per square meter can be possibly higher than costs for e. g. concentrating mirrors. On the other hand, using the concentration, two other kinds of losses will raise: losses of diffuse radiation and optical losses.

There exist a variety of solar energy concentrators for different purposes. For low — temperature applications, inexpensive concentrators of diffuse radiation can be used. For these concentrators, acceptance angle 0A defines the ability to concentrate the diffuse radiation and also its concentration factor C. To this class of concentrators belongs e. g. nonimaging types like CPC (Compound Parabolic Concentrator), V-trough types, cylindrical concentrators etc.

This paper deals with development of a new type of concentrator, novel concept of which is based on functionality of CPC by means of flat mirrors, primarily designed for needs of SME’s (Small and Medium Enterprises). The CLON project is being ellaborated under the 5th Framework Programme of the EU.

Background

In many publications one can meet the solutions of solar energy concentrators, which seem to be similar to the CLON concentrator by shape, however, conceptually they are different. Analyses of those similar designes showed that their properties do not reach many of advantages of the CLON. In first order they are not single-reflecting, they have a lack of satisfactory concentration factor or they are not able to concentrate diffuse radiation.

Concepts similar to CLON are usually named as „V-trough" concentrators (with multiple mirrors), mainly in english written publications. Russian authors, who developed a variety of such designs, name the device like „Flat Focline" [1], [2], [3] (symmetrical single-mirror concentrator), „Single Focline" [2], [3] (non-symmetrical concentrator with one only flat
mirror). Device with multiple mirrors is described e. g. in [4] and [3] but only the direct solar radiation is focusable.

A common property of all the mentioned concepts is flat shape of absorber at the output area. There exist also a concept of concentrator for frontal placement of absorber [5], [6],

[7] and [8], however, this is not a concept following the principle of CPC rather a parabolic trough concentrator.

As an example, concept proposed first by Grilikhes and Zaitsev, furtherly improved by Vartanian [3] is most similar to the concept of CLON. Difference is, in fact, that this concentrator focuses only direct radiation incoming exactly at 0°. Vartanian found out the optimal angle for first zone of the concentrator to be equal to 01 = 67° 30′. Inclination angles and sizes of the rest zones can be found using the following algorithm with results presented also in table 1.

і

Zcos 0dk

k=l__________

Я-© dl 2

(1)

©

C

II

О9

ьр

(2)

COS © dl

cos ©i

(3)

C = l + np^ cos © dl

l=1

(4)

Goals

The project CLON is built upon idea of novel concept of solar energy concentration, not approximating the curvature of CPC, rather simulating the functionality of the CPC. CLON aims, in general, to the development of concentrating collector of practical output for SME’s, including three chosen installations: 1. Industrial plant (process heat), 2. Swimming pool, 3. Agriculture (drying of products).

The current stage of the project is oriented to ellaboration of the novel concept in details with creating the mathematical model of the concentrator and analyses of its properties, as a preparatory work for construction of the device and manufacturing of the prototypes. Goals of the first stage can be briefly listed as follows:

• To formulate the functioning principle of the concentrating device and its general optical scheme.

• Based on the optical scheme of well-known and well-studied type of solar energy concentrator — the CPC, to achieve the same effect of single reflecting concentration in the framework of defined angular range by the means of inexpensive flat mirrors.

• To define the basic parameters of the concentrator.

• Based on the optical scheme, to derive the graphical method of calculation of the concentrator.

• Optimalisation of graphical method and derivation of numerical method for calculation.

• Analyses of geometrical properties of the concentrator.

Further goals of the first stage include selection of materials for each of the components, detailed analyses of optical properties and mechanical model of the collector. These tasks are actually under ellaboration.

CFD simulation of a laboratory thermosyphon system

In order to investigate in detail the thermosyphon effects, the authors are studing a sim­plified thermosyphon system with a geometry that will permit both the construction of a controlled experimental set-up, and the modelling of the whole system with CFD techniques,

First studies on a bidimensional configuration have already been carried out. The two dimensional system considered in this first stage is shown in figure 6. The computational domain used, including details of the mesh, is also given in figure 6. The mesh has been concentrated through the collector and in the tank walls principally (see the solid triangles in

figure 6.b, where n represents the mesh characteristic parameter). The mesh used for the simulations here presented corresponds to n=2 (i. e. 50×90 control volumes).

The system has been simulated during 24 hours exposed to ideal outdoor conditions following a procedure similar to the one-day test proposed in the standards (ISO-9459). The initial time is 8 a. m. In figure 7 three different maps along the studied time period are shown. It can be seen how the velocity increases in the first hours of the simulation due to the received heat flow and, approximately at 6 p. m., when the heat flux stops, the inversion flow appears. Also, the benefits of the insulating material can be observed due to the time the tank remains at a high temperature.

The first point to consider is the degree of detail this method provides. From each time step all maps can be known and comparing the simulation results from experimental results becomes much easier, direct and efficient.

There are two main weak points: the time increment and the CPU time. The time increment that has been used is 0.1 s, value too low for long-term simulations [15] but needed to quickly converge the system at each time step, and to properly evaluate the transient phenomena. If greater time step is used, the number of inner iterations increases worsen the CPU costs, and errors due to temporal discretization increase. The one-day test on mesh n=2 has spent more than 8 days of CPU time. Obviously, this is not acceptable if a three-dimensional study has to be carried out so, in spite of the possibilities this method provides, it becomes necessary to study the way to shorten this CPU time. An alternative is the Multiblock method (or domain decomposition method, [1]) because it would avoid the simulation of the internal solid and improve the mesh distribution along each compound (block). Moreover, Multiblock methods would give the possibility to parallelise and use different CPUs at the same time. The critical point of the Multiblock method and which is currently focusing the attention of the authors is the transfer of information between subdomains in multiconnected grids (elliptical situations), as it occurs with thermosyphon systems, were solutions without physic sense can be obtained.

Conclusions

Computational fluid dynamics, CFD, offers a valuable tool to obtain local and extensive information of the fluid dynamic and thermal behaviour of thermosyphon systems.

Detailed CFD simulations of whole thermosyphon systems have not are not yet possible because the large computational resources (time and memory) required. However, CFD can be used to model components or parts of the components of the system with a rea­sonable CPU time. Furthermore, CFD simulations can also be used to obtain information which is required by other more simplified models like local Nusselt numbers or skin friction coefficients, with no need to construct expensive experimental units.

Due to the constant and fast improvement of the CFD techniques and increase of compu­tational resources, the authors expect to be able to perform 3-dimensional CFD simulations of complete termosyphon systems very soon. Main work currently carried out by the authors in this line focus on the development of efficient multiblock techniques.

Acknowledgments

This work has been funded in part by The European Commission under the “Energy, Environment and Sustainable Development” Programme, Framework Programme V, 1998­2002, project contract number CRAFT-1999-72476.

[1] etical and computational approaches. CRC Press, 1998. [6]

Performance tests

For system simulation calculations an empirical simulation model of the MD-module must be developed which is based on its measured performance data. Exact performance measurements must be feasible to determine improvements concerning new module constructions. The dynamical behaviour of the MD-module is a very important property for the system design with respect to an intermittent operation of a solar thermal collector field as heat source.

A test facility allows the determination of the specific energy consumption of the modules for different inlet temperatures and different feed volume flows. Also the dynamic start up and cool down behaviour can be investigated.

The specific energy consumption and the GOR value were determined depending on the feed volume flow and the evaporator inlet temperature. The measured parameters are the distillate volume flow, the feed volume flow, the condenser in — and outlet temperature and the evaporator in — and outlet temperature. The additional heat supplied into the system from outside can be calculated from the temperature difference between the condenser outlet and the evaporator inlet, the feed volume flow and the specific heat capacity cp of the feed. The heat demands for different feed flow rates between 200 and 400 l/h depending on the evaporator inlet temperature are shown in the right hand diagram of figure 6. The diagram to the left shows the corresponding distillate volume flow. The GOR value can be calculated by dividing the product of distillate output and the specific enthalpy of evaporation (m-distiiiate * r) by the heat input (Q^n). For example the calculated GOR for a volume flow of 350 l/h at an evaporator inlet temperature of 75°C (r70°C=2321.5 kJ/kg) is 5.5. The specific energy consumption per cubic meter distillate for these operation conditions is in the range of 117kWh / m3.

Figure 7: Investigations on the dynamic performance of the MD-module.

For larger systems a pressurised heat storage is used if the investment costs for the MD — modules are much higher than the additional system costs for the storage system. In that case the MD-modules can be operated 24 hours a day.

The advantage of a storage system concerning the energy efficiency is that the system can be operated at the optimal working point (temperature and volume flow) for a long part of the daily operation period. On the other hand storage heat losses and lower collector efficiencies caused by higher collector temperatures which are necessary for a reasonable storage, decrease the energy efficiency.

Micro-structuring and real low-e coatings

The simulations described above were carried out for a mesh of cylinders with infinite conductivity. Real low-e coatings normally use layers of silver or gold to establish the low emissivity in the IR spectral range. These metals have a high but not an infinite conductivity. Nevertheless, to get a first impression of the properties of meshes made out of real metals, it is possible to transfer the results of the numerical simulations to conventional low-e coatings.

A layer of metal with infinite conductivity would be opaque regardless of the thickness of the layer. The transmittance of a mesh of cylinders made of the metal calculated in the numerical simulations derives from radiation passing through the gaps between the metal bridges of the mesh. With the increase in transmittance due to this radiation it is possible to calculate the increase in transmittance due to micro-structuring of a real low-e coating. This means that the transmittance Treal of a mesh made out of a real low-e coating on a glass substrate will be between the transmittance Tcoat of the conventional low-e coated pane and the transmittance Tfloat of an uncoated pane of float glass, depending on the transmittance TmeShof the infinite conductive mesh:

Teal(2) = Tco„(2) + Tmesh(2) ■ (^,(2) — ^(2))

The same applies to the low-e properties: as there is no additional absorption, the emissivity Sreai of the mesh made of a conventional low-e coating will be between the emissivity £,coat of the conventional coated pane and the emissivity £float of an uncoated pane of float glass, depending on the transmittance Tmeshof the infinite conductive mesh:

^real ^"coat (2) + ^"coat (2))

The solar transmittance and low-e properties are then calculated by averaging these spectral values over the solar spectrum and the spectrum of heat radiation respectively. Through micro-structuring, the solar transmittance of the Pilkington Optitherm SN coating seen in Figure 2 could be increased by nearly 10 percent points from 0.62 to 0.72. The emissivity would increase from 0.048 to 0.060. The transmittance of this micro-structured coating is shown in Figure 9.

Conclusion

Numerical simulations using the FDTD method have shown that the solar transmittance of low-e coatings can be considerably increased. The penalty is a slightly increased IR emissivity.

Micro-structured low-e coatings are currently being processed. Measurements will follow accordingly.

Literature

[1] R. E. Bird, R. L. Hustrom, L. J. Lewis, “Terrestrial Solar Spectral Data Sets”. Solar Energy Vol. 30 (6), pp. 563-573 (1983)

[2] “International Glazing Database”, National Fenestration Rating Council (2003) http://www. nfrc. org

[3] K. S. Yee, “Numerical solution of initial boundary value problems involving Maxwell’s equations in isotropic media”. IEEE Transactions on Antennas and Propagation Vol. 14(3), pp. 302-307 (1966)

[4] J. C. Maxwell, “ATreatise on Electricity and Magnetism” (1873)

[5] D. M. Sullivan, “Electromagnetic simulation using the FDTD method”. IEEE Press, Piscataway (2000)

[6] Advanced Systems Analysis Program (ASAP), Breault Research Organization, Tucson, Arizona

http://www. breault. com/html/soft_asap. html

The light guide and optical fiber assemble

A fused silica light guide of 12cm length, 07mm circular input and 010,5mm hexagonal output cross sections was placed in the primary focus. Due to the existing difference in the input/output diameter dimensions, this light guide was used to reduce input angles smaller then 37° into output angles minor then 23° which matched well with the numerical aperture of the optical fibers. The light guide has also the function to homogenise the output light power at its output end, resulting in equal input light power distribution for the optical fibers. The hexagonal form of the output end of the light guide permits also the best light coupling with the fiber bundle. At the output end of the light guide, a bundle of 37 optical fibers was coupled, each of them with 1.5mm diameter. Mounting individually each of the 37 optical fibers into an aluminium part, a compact hexagonal bundle is formed.

SHAPE * MERGEFORMAT

0, 8

— 0,3 0,2

Overview on new developments of solar thermal collectors for the temperature range of 80° to 250°C

Matthias Rommel

Fraunhofer Institute for Solar Energy Systems ISE
Heidenhofstr. 2, D-79110 Freiburg
email: matthias. rommel@ise. fraunhofer. de

Since the beginning of December 2003 a new IEA Task on "Solar Heat for Industrial Processes" is working. The Task is organised as Task 33 of the Solar Heating and Cooling Programme and as Task 4 of the SolarPACES Programme. It is referred to as Task 33/4 in the following. This new Task will contribute to somehow bridge the gap in information exchange and in technology development between solar thermal collectors for domestic hot water and room heating applications on the one side and high and medium temperature parabolic trough collectors on the other side. The objective of the whole Task is to make use of the huge potential for solar heat in the industry and to open new market sectors for the solar thermal industry. The aim is to integrate solar thermal systems into industrial processes in the best and most suitable way.

One of the objectives of the IEA Task 33/4 is to develop, improve and optimise collectors with a potential for integration in industrial processes with a temperature level up to 250 °C. This is the field of work of Subtask C of Task 33/4.

In the paper an overview on the new collector developments in the temperature range of 80°C to 250°C is given. These collectors are investigated in the Task. The collector technologies cover new collectors from improved flat plate collectors, anti-reflectively double-glazed flat-plate collectors, low concentration CPC collectors, vacuum tube collectors and new developments of low cost parabolic trough collectors.

The input to the paper and to the presentation comes from all experts of the Subtask C of the IEA Task 33/4.

Comparison of Sensors and CM21

To evaluate the quality of the instantaneous measurements, for two representative days (one with clear and one with clouded sky) the instantaneous values of the sensors when G(device) >10 W/m2 were statistically analysed for their deviation to the CM21. The guaranteed accuracy of the instantaneous measurements of the reference pyranometer CM21 is ± 0,5 %. For the analysis, mean value, standard error, the maximum upper and the maximum lower deviation were calculated for the absolute and relative deviation between sensor and pyranometer. The relative deviation was calculated by dividing the difference between sensor value and pyranometer value through the pyranometer value. This approach allows the comparison of the sensors independent from the level of irradiance.

For estimating the accuracy of long-term measurements, the sums of total irradiance measured by the sensors over a period of 8 days with changing weather were compared to the energy measured by the CM21. As a potential source of error for long-term measurements, the overnight off-set of the sensors was observed for its effect.

The measurements were carried out in Vienna in October 2003. The outdoor test stand was installed with a declination of 45 degrees, an azimuth of 0 degrees and an inclined surface with a length of 116 cm and a horizontal width of 160 cm, which is dyed black for simulating the roofage. The 10 tested irradiance sensors were placed approximately in middle height of the stand. Simultaneously to the irradiance, the ambient air temperature was measured by 2 calibrated Pt100 sensors, one mounted directly on the test stand and irradiation sheltered by an isolated and ventilated pipe-in-pipe system, the other one mounted closely beside the test stand sheltered by a standard irradiation protection casing. Over a period of 8 days the measured values were recorded partly in one minute and two minute intervals. Figure 1 and 2 exemplify the recorded trends of all measured devices during a day with clear sky and one with clouded sky. It can be seen, that with one exception the irradiance sensors tend to measure lower instantaneous irradiances than the reference-pyranometer.

Figure 2: Recorded Trends during a sector of October 23rd 2003

SHAPE * MERGEFORMAT