Category Archives: Particle Image Velocimetry (PIV)

SIMULATION MODEL

Calculations were carried out with the simulation model Mantlsim developed at the Technical University of Denmark, [10], [3], [4] and [11]. The two tested low flow systems with the data given in Table 1 are taken into calculation. Weather data of the Danish Test Reference Year [12] is used in the calculations.

The daily hot water consumption is 4.6 kWh, corresponding to 100 l water heated from 10°C to 50°C. Hot water is tapped with an energy quantity of 1.53 kWh three times each day: 7 am, 12 am and 7 pm. The required hot water draw-off temperature is 50°C.

The thermal performance of the system with the two draw-off levels was calculated with one draw-off pipe at the very top of the tank and with different positions of the second draw-off level.

2.2.1 SIMULATION RESULTS

Fig. 3 shows the calculated yearly net utilized solar energy of the system as a function of the relative position of the second draw-off level and as a function of the auxiliary set point temperature.

Fig. 3. Calculated net utilized solar energy of the SDHW system as functions of the position of the second draw-off level and of the auxiliary set point temperature.

Extra thermal performance

top

Relative position of second draw-off level

Auxiliary volume temperature

50.5 "C

-M— 55 "C — A-60 "C -•—65 "C

Fig. 4. Calculated extra percentage net utilized solar energy of the system by using two draw-off levels as functions of the position of the second draw-off level and of the auxiliary set point temperature.

Fig. 4 shows the calculated yearly percentage increase of the net utilized solar energy by using a second draw-off pipe as functions of the relative position of the second draw-off level and the set point temperature for the auxiliary energy system. If the relative position of the second draw-off level is at the top of the tank, the system is identical to the standard system with only one draw-off level at the very top of the tank. The thermal advantage of using a second draw-off pipe is strongly influenced by the auxiliary set point temperature. The higher the set point temperature the larger the advantage. If the auxiliary set point temperature is only 0.5 K higher than the required draw-off hot water temperature, the extra thermal performance by using a second draw-off level is about 1%. If the auxiliary set point temperature is 15 K higher than the required hot water temperature, the extra thermal performance by using a second draw-off level is about 13%. The second draw-off level is best placed in the middle of the tank.

Figs. 5 and 6 show the calculated yearly net utilized solar energy of the system and the calculated extra percentage net utilized solar energy of the system by using two draw-off levels as a function of the position of the second draw-off level for four different daily hot water consumptions: 50, 100, 160 and 180 l. Hot water is tapped by means of three daily draw-offs with the same draw-off volume: At 7 am, 12 am and 7 pm. Hot water is tapped at 45°C at 7 am and at 7 pm, while hot water is tapped at 40°C at 12 am. During all hours the top auxiliary volume is heated to 50.5°C. The draw-off temperatures are realistic, since hot water is not used at the same temperature level in practice. Further, in practice the set point of the auxiliary energy supply system is often 5-10 K higher than the required hot water draw-off temperature. The figures show that the net utilized solar energy is increased by about 6% by using two draw-off levels. Again, the second draw-off level is best placed in the middle of the tank. It should be mentioned, that there is a need for development of an advanced control system before solar tanks in practice can supply the consumers with different draw-off temperatures.

Fig. 5. Calculated yearly net utilized solar energy of the SDHW system as a function of the position of the second draw-off level for different daily hot water consumptions.

Fig. 6. Calculated extra percentage net utilized solar energy of the SDHW system by using two draw-off levels as a function of the position of the second draw-off level.

Selectively solar-absorbing coatings on a copper plate 1-1. Synthesis of TiOxNy precursor

1.2 mol of 1, 2, 4-Triazole (Kanto Chemicals Co. Ltd., 98.0 %) was dissolved in 2 mol of piperidine (Kanto Chemicals Co. Ltd., 98.0 %). 0.4 mol of titanium tetraisopropoxide (Kanto Chemicals Co. Ltd., 97.0 %) and 0.4 mol of 1-amino-2-propanol (Kanto Chemicals Co. Ltd., 99.0 %) were mixed directly in a four necked round bottom flask equipped with thermometer, condenser and drying tube. 0.4 mol of Urea (Kanto Chemicals Co. Ltd., 99.0 %) was dissolved in 2 mol of N, N-Dimethylformamide (DMF, Kanto Chemicals Co. Ltd., 99.5 %). The solution was poured into the flask. Then, the mixture was heated by oil bath at 85 °C for 2 h. The solution was evaporated, so that corresponding TiOxNy precursor content became about 8 %.

Measurement of the ventilation rate

Most collectors are equipped with special openings for enabling a controlled ventilation. Low — cost collectors or collectors with wooden frames are not really tight and allow considerable uncontrolled ventilation. Both contribute to changes of the micro-climate.

A procedure for measurement of the ventilation rate must take into account both. The working group Materials for Solar Thermal Collectors (MSTC) of the Solar Heating an Cooling Programme (SHCP) of the International Energy Agency (IEA) developed a method for Ventilation rate measurements and tested it by performing a round robin test of the same collector in several laboratories: IBE (DK), TNO (NL), SPF (CH), ISE (D). The method was to measure the air-flow-rate through an additional opening in the back-plane of the collectors while applying a positive or negative pressure difference in the order of some Pascal between collector and ambient. The results of the different labs are shown in figure 3. The good

agreement proved the applicability of the procedure. The variation of the ventilation rate of different commercial collectors is shown in figure 4. The ventilation rate was normalised by relating it to the volume of the collector in order to enable a comparison.

The function of pressure and flow rate follow the expected parabolic shape (figure 4a). The ventilation rate was defined as the flow-rate in collector-volumes per hour at a pressure — difference of 1 Pa. The collectors with the high flow-rates were untight wooden constructions. The values are usually not reached in operation. The flow-velocity measured by a micro­anemometer in front of a ventilation hole of a controlled ventilated collector during stagnation conditions (displayed in figure 5) follows the air temperature measured in the gap between absorber and glazing. The total air exchange during this sunny winter day was about 7 volumes per day. The air-exchange during night-time was nearly 50% of the day-time, with reversed direction, of course, because of the relative cooling in the clear nights. This might cause accumulation of moisture in the collector, when the collector temperature is below the dew-point and/or the insulation material was dried before.

Figure 4: Measurement of the ventilation rate of different collectors

Collector-building simulation

Energetic behaviour of solar system based on facade-integrated collector has been investigated through a computer simulation. Simulation was aimed to characterise performance of facade solar system for hot water preparation in block of flats building and to obtain information on effect of facade collector on building performance.

Computer simulation was performed using Transient System Simulation Program — TRNSYS [6]. TRNSYS model for integrated facade collector was composed from a multizone model and the solar system model with thermal interconnection between them. Only a one zone in the central part of the considered building facade was modelled to investigate the block of flats building performance with facade collector. TRNSYS model used for simulations is shown in Figure 7. Facade construction was divided to two surfaces, one of them has been coupled to collector absorber (absorber temperature is identical with the last layer surface temperature).

Solar systems (facade, roof) were modelled as conventional ones — collector connected to storage tank with stratification. Facade solar collector with slope 90° was modelled as thermally coupled to building facade as described above. Roof solar collector was modelled separately with slope 45°. Thermal characteristics of the collectors were obtained from detailed simulation through KOLEKTOR model. Standard parameters of hot water were used (heating from 12 °C to 55 °C, max. temperature 85 °C). Solar systems have been modelled with high flow forced circulation (100 l/h/m2).

Two facade types were investigated for application of facade collector. First, middle-weight facade is common to panel block of flats buildings based on 27 cm ceramzit-concrete panels. These types of construction represent a wide range of buildings in large housing estates. Second, heavy-weight facade based on 45 cm brick represents the old buildings antecedent to the panel housing. Both types should be renovated with respect to construction problems, indoor comfort and energy consumption. In the model, applications with overall thermal resistance R =1,3 and 6 m2K/W for building envelope were considered. Windows with heat insulating glazing were used (U =1.7W/m2K). Overall surface area of zone facade is 9.0 m2, the window area is 3.2 m2, the wall is 6 m2. Splitting of the wall into two surfaces allows changing the collector / facade area ratio for parametric analysis.

Simulations were performed for different cases which can be considered in decision­
making for building renovation. Simulated cases were: panel wall — base case (envelope insulation with/without roof collector)

— integrated case (envelope insulation with facade collector) brick wall — base case (envelope insulation with/without roof collector)

— integrated case (envelope insulation with facade collector)

Parametric analysis for different facade construction resistances R, collector field surfaces Ac, required solar fractions and orientations were performed. Test reference year for Prague (TRY_Prague) was used as a climate database in the system and building simulations. Principal observed parameters for the building behaviour were energy consumption in winter season, overheating characteristics (inside temperatures, PPD values) in summer season and possible temperature-induced risk in construction. For the solar system, specific annual solar gains qs, u, solar fraction f system efficiency n and unutilizable energy qs, nu due to collector stagnation were the required outputs.

Results

Results of parametric simulations of solar fraction achieved by investigated solar systems (roof, facade) are shown in Figure 8. The solar fraction is plotted against the specific area of collector field Ac/Vaku. Interesting solar fraction values for facade collectors result from higher insulation levels (R = 3, 6 m2K/W). While the facade collector area should be increased by 30% compared to roof collector (45°) area to achieve 60 % solar fraction, for solar fraction above 70 %, the required facade collector area is comparable or lower.

However, roof collector gets to much more higher levels of stagnation conditions, which lead to possible operation problems and material degradation. Vertical position of the facade collector results in a well-balanced useful solar gains profile and very low level of unutilizable energy gains in comparison with the roof collector case.

In the Figure 8, the unutilizability factor bnu is introduced and plotted. Unutilizability factor is defined as a ratio of solar energy gains available from solar collector, but not used due to upper temperature limits (Tmax) in the storage tank, to total available solar energy gains Qs from collector field (solar system gains Qs, u utilized for water heating + unutilized Qs, nu).

b

Comparison of solar fraction f, specific solar system gains qs, u and solar system efficiency П, annual profiles for roof and facade solar system (R = 6 m2K/W) at annual solar fraction 60 and 70% is shown in Figure 9. Facade solar collector orientation impact on system gains and achievable solar fraction is shown in Figure 10 (compared with roof collector with south orientation).

Interaction of facade solar system with building has been investigated for winter (from October to April) and summer (from June to August) season. Performance analysis through collector-zone coupled modelling has been done for two usual building types, middle-weight (panel) and heavy-weight (brick). Results for winter season were put in the Table 1. With increasing heat insulation level, the heat gains caused by facade collector tends to be negligible.

Type

R = 1 m2K/W

R =3 m2K/W

R =6 m2K/W

middle-weight (panel)

9.9

4.0

2.1

heavy (brick)

10.3

4.1

2.2

Tab. 1 Facade heat gains caused by collector integration (kWh/m2.a)

South-oriented buildings often suffer with overheating problems in summer season. Temperature of the building envelope raises and considerable heat gains through the facade and windows could contribute to space overheating. Facade collector, due to good level of thermal insulation and absorber temperatures kept under 70 °C (low level of collector stagnation as resulted from system simulation), doesn’t cause notable temperature increase inside the building. Average temperature Tinside and PPD value (predicted percentage of dissatisfied) inside the zone adjacent to facade with collector were derived from frequency histograms for summer season and these are shown in Figure 11 in dependence on collector/facade area ratio (solar fraction 60 % and 70 %). Panel wall case, due to lower storage capability (middle-weight wall), results in higher average temperature values than brick wall case (heavy-weight wall). Application of facade collector thermally coupled to the wall moves the average temperatures no more than 1 K higher.

23.0

Fig. 11 Average temperature Tinside and PPD in the zone with facade collector in summer

season (June to August)

Figure 12 shows the temperature profiles in the facade collector-building construction (middle-weight ceramzit panel) during the typical summer day from 8 am to 8 pm. There are individual modelled layers outlined in the figure. The solar system heats the storage tank and during the day the absorber temperature raises up to 70 °C. Thermal insulation layers are affected by absorber, the first layer extremely. It should be made of materials capable to withstand high temperatures (up to 150 °C), e. g. mineral wool. Next layers (ceramzit panel) are kept at moderate temperatures with minimal variation during the day. These layers are mainly affected by the temperature variation inside the building. Brick wall case behaves in similar way with lower variations in indoor temperatures with respect to higher inertia.

System modelling

The solar ventilation system described above consists of four main components: the slates, the PV module, the DC motor/fan combination and the duct. The flow rate in the system is governed by the interaction of the three latter components. Furthermore, since maximising the daily volume of air necessitates using a system with minimal resistance (i. e. smallest length of duct), the optimisation methodology depends primarily on the coupling between
the PV module and DC motor/fan combination. For a fixed length of duct, the maximum flow rate of air is obtained at the motor/fan maximum possible speed.

1.1 Photovoltaic module model

The I-V characteristic of the PV module can be described by the following equation, which is derived from the equivalent circuit described by Applebaum[3]

V + I ■ R S

I = I G — I о ( Є " — 1) (1)

where I is the PV module output current (A), IG is the light-generated current (A), I0 is the diode reverse saturation current (A), V is the PV module’s output voltage (V), RS is the series resistance of the solar cell (Q) and A is a curve fitting parameter (V).

This equation and the parameters included in it (i. e. IG, I0 and A) are valid at a given irradiance and temperature. Many methods are available so that the I-V characteristic of the PV module can be adapted to different levels of irradiance and module temperature. This work makes use of a new method which has been previously investigated by our research group at Napier. For a given irradiance (G, W/m2) and PV module temperature (Tmod, °C), the module short circuit current, its maximum power, its voltage and current at maximum power and its open circuit voltage are calculated. The parameters I0, A and IG are then calculated and substituted in Eq. 1 for an I-V characteristic which is valid at these conditions of G and Tmod.

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]