## Thermodynamics of Regenerative Distillation

Desalination of seawater can be accomplished in principle by many different processes. The same fundamental thermodynamic laws govern every such process. By means of a thermodynamic analysis for the so-called "differential process”, as it proceeds in series of infinitesimal steps with complete equilibrium being maintained at all times, the reversible, isothermal work for any steady state process regardless of mechanism may be found to be approximately —

Wmin = 2.98 kw-hr/1000 gallons or 2.83 kJ/kg

This figure (or a similar one depending on operating temperature and data chosen) is the one usually quoted as the minimum theoretical work. Practically, it is a very unrealistic figure, not only because it assumes complete reversibility of all operations, but also because it would involve pumping an infinite amount of feed water and the pumping work would then be infinite [2].

Distillation is a vaporization process driven by heat. Essentially, it is a heat pumping process driven by a heat engine. Here the work requirement depends not only on the quantity of heat to be pumped but also on the temperature difference over which it is pumped. When heat is used to vaporize water from a salt solution, the vapor evolved contains the full amount of heat of evaporation and the only net result is a slight degradation in temperature due to the Boiling Point Elevation (BPE) of a salt solution. This heat can all be reutilized merely by restoring it to its former temperature — i. e. by heat pumping. By using the first law of thermodynamics and the Carnot relationships, the minimum heat (Qk) necessary to drive an ideal distillation process over a given temperature range AK can be shown to be —

This parameter represents the amount of "effects” that the device is able to perform. "Effects” stand for the amount of kilograms of distillate obtained from a heat input equal to the latent heat of evaporation of 1 kilogram of seawater. A GOR of 1 thus means that there was no gained effect and the unit mass of distilled water corresponds with the amount of energy needed to evaporate that unit mass. Obviously, GOR above 1 indicates a system that is regenerating its heat for multiple effects. gOr serves as a factor by which distillation devices of different design may be compared with respect to their heat efficiencies.

We see therefore that in order to maximize the efficiency of a distillation process from a thermodynamic approach (and minimize irreversibilities) the whole process of heat insertion and removal should take place at just a fraction of a degree difference from each other (small AT). This however would require infinitely large surface areas and is thus impractical. Likewise, ideally, one would desire that the salinity of the incoming water remain constant and not increase as part of it is evaporated. This however, would require an infinite flow of water to be processed through the system and is also impractical. As

the thermal energy inserted into the system must be ultimately removed it would make sense to benefit from the removed heat in such a way that it could be reinserted into the system. Finally, in order to operate at a higher thermodynamic efficiency the temperature range of operation must also be large. Following is a basic diagram describing a process that incorporates these issues.

Figure 1- Parameters of a thermally optimized distiller

The described optimized enclosure operating over a large temperature range, with a small temperature drop, while regenerating its latent heat of condensation, will still have a limited ability to produce distillate. The amount of heat needed to "pre-heat" an amount of cold incoming seawater is considerably less than the latent heat released while the same equivalent amount of distillate condenses. Thus it is apparent that there is a practical limit to which a realistic regeneration device is able to reuse its heat. This is a function of the high and low temperature heat reservoirs, and the temperature drop necessary for heat transfer between the evaporator and condenser. Thus, the highest possible GOR any distillation device is able to feasibly operate at, may be expressed as

(3)11 MdT^T^.AT)-

Where Mr is the mass flow ratio between the cold incoming water (Ms) and the distillate (Md) and is defined using the first law of thermodynamics and humid air relationships.

____________________________________________ С pm *(Тк» ~ AT ~ Г„и )_______________________________________

I Cpa ■ {Thli — )- Сr ■ [g7to ■ (Тш )~ ПГрри ‘(Tpau )]+ hfgO^tJbq, 4J)

Ц 07 ha ~ .

Here Cpa and Cpw are the specific heats of the air and the water respectively and represents the specific humidity.

It is important to remember that this relationship represents the maximum efficiency attainable in a realistic regenerative distillation process and gives an indication as to how close a given device is operating with respect to the maximum possible. Thot and Tcoid are defined by the environment, and temperature drop — AT is defined by the size and quality
of the heat transfer surfaces. In effect therefore, one may “pre-define” the theoretical efficiency limitations of any given regenerative distillation device simply by defining these three factors. The upper and lower temperature limits are generally defined by the environment and AT depends on how much investment is made in the size and quality of the heat transfer surfaces. The goal of this research project has been to determine, by using CFD tools, how to promote the most effective natural convection so as to allow the device to approach this optimal efficiency at the given operating parameters.

## Intermediate model: flow distribution in a fin-and-tube absorber

In this subsection, the flow distribution through an absorber is analysed. The absorber has 327 riser tubs with a diameter of 6 mm and a length of 1m which are connected to two manifolds with a diameter of фм. The riser tubs are mounted with no separation (no fins) between them. Therefore, the width of the absorber is in the range of 2 m. The risers will be numbered from 1 upwards, where the riser 1 is the one next to the inlet. The absorber has a z-configuration: the water enters the absorber through one ending of the inlet-manifold and is distributed through the different risers; after passing through the risers, water is collected in the outlet-manifold, and exists the absorber from the ending closer to the riser number 327. Investigated is the flow distribution in the risers for different diameters of the manifold, фм, and for two different total mass flow rate through the whole absorber: 25 l/hrn2 and 50 , using water as thermal fluid. Results are shown in figure 2. Given are the mass flow rates through each riser і, йц. These values are normalised by the ideal flow rate m0 which will result from a uniform distribution of the fluid flow through the riser, i. e fn0 = m/n, where n is the number of riser tubs.

iii= 25 1/hm2 m= 50 1/hm2

Figure 2: Numerical results with the intermediate model: flow distribution through a absorber with a z-conflguration, with two manifolds of a diameter of and 327 riser tubs (riser 1 = riser next to the inlet). Given is the mass flow rate through each riser i, , normalised by tha mass flow rate for a uniform distribution of the flow, . Two different total mass

flow rates, m, are analysed: a) m = 25 l/hrn2; b) .

Studies like this are of major importance in order to assure an appropriate flow distribution through the absorbers in order to obtain best thermal performance of the collector, [20].

## Validation

The best approach in validating an estimated service life from accelerated testing is to make use the results from the accelerated life tests to predict expected change in material properties or component performance versus service time and then by long-term service tests check whether the predicted change in performance with time is actually observed or not.

The results of validation tests therefore can be used to revise a predicted service life and form the starting point also for improving the component tested with respect to environ­mental resistance, if so required. It should be remembered that the main objective of ac­celerated life testing is to try to identify those failures which may lead to an unacceptable short service life of a component. In terms of service life, the main question is most often, whether it is likely or not, that the service life is above a certain critical value.

In the case studies of Task 27 outdoor tests at different test sites are performed for meas­urement of microclimatic variables and for validating predicted loss in outdoor performance from accelerated test results. Tests are performed by CSTB in Grenoble (France), ENEA in Rome (Italy), INETI in Lisbon (Portugal), ISE in Freiburg (Germany), NRELin Colo — rado/Florida/Arizona (USA), SP in Boras (Sweden), SPF-hSr in Rapperswil (Switzerland) and Vattenfall in Alvkarleby (Sweden). In Figure 7 a view of the test site at INETI in Lisbon is shown.

 Figure 7View of the outdoor exposure site with facilities for monitoring of climatic data at INETI in Lisbon

Conclusions

The work in IEA Task 27 on durability assessment of static solar energy materials has shown that it is possible to employ a systematic approach in the evaluation of the expected

service life of the materials studied. Based on the work performed recommended test pro­cedures will be worked out for qualification of new materials with respect to durability.

Figure 7 Results from outdoor exposure of antireflective glazing materials performed at SPF-HSR Rapperswil, Switzerland. The decrease in the solar transmittance with time is due to soiling effects, which vary very much with exposure site.

For recommended durability test procedures to be accepted as international standards, it is of utmost importance to demonstrate their relevance for predicting real in-service long­term performance. We think that the work of Task 27 will meet this requirement.

Acknowledgement

The authors sincerely want to thank the colleagues and participants in the work of Task 27 on the static solar materials for contributions to this paper: Michael Kohl and Volker Kubler (Fraunhofer ISE Freiburg), Ole Holk (DTU Copenhagen), Gary Jorgensen (NREL, Golden Colorado), Bjorn Karlsson (Vattenfall Utvecklings AB Alvkarleby), Manuel Lopes Prates (INETI Lisbon), Kenneth Moller (SP Boras), Marie Brogren, Arne Roos, Anna Werner (Uni­versity Uppsala), Michele Zinzi (ENEA Rome), and Michele Ghaleb (CSTB Genoble)

[1]

## Numerical Simulation (FDTD)

As a first estimation, the behavior of the micro-structured low-e coatings was investigated by way of numerical simulation using the finite difference method in the time domain (FDTD) [3]. The FDTD method uses the difference form of Maxwell’s curl equations

— = Vx H

dt

and

— = -—Vx E,

dt J

where D=rE, є is the relative permittivity and /jo is the vacuum permeability [4].

These difference equations are used to calculate the time dependent development of the electric and magnetic field, iterating the calculation of Maxwell’s equations over many time steps. The frequency dependence is calculated by Fourier transformation into the frequency domain. As the Fourier transform of a small pulse

contains all frequencies, the response of a system to excitation by plane waves of several frequencies can be calculated in one numerical simulation, where the system to be examined is excited by a narrow Gaussian pulse [5].

For the simulations, only the wavelength selectivity of a metal mesh was examined, the influence of the glass substrate on the transmittance or reflectance spectrum was not considered. The mesh consisted of metal cylinders with infinite conductivity, thus being perfectly reflecting (see Figure 5).

The simulation was implemented using periodic boundary conditions, so that dependent scattering of an infinite array of cylinders was calculated.

## New pumping configuration for Nd:YAG solar laserby optical fibers

Pedro Bernardes and Dawei Liang
CEFITEC, Dept. of Physics, F. C.T, The New University of Lisbon,

Quinta da Torre, 2825 Campus de Caparica, Portugal

A new pumping configuration by optical fibers was used to produce a Nd :YAG solar laser power. The sunlight was concentrated by a primary parabolic mirror. The solar energy from the focus was transmitted to a pump cavity by means of a two- stage transmission system. The solar power of 355W was transmitted by a fused silica angle transformer with circular input and hexagonal output cross-sections. Both angle reduction and beam uniformity were achieved, suitable for its light coupling to a compact 37 optical fiber bundle with NA=0.4. In order to cover all the output area of the angle transformer, each fiber with 1.5mm diameter was hexagonally polished at its input end. The total output solar power of 184W was measured from the 37 optical fiber bundle. The optical fibers were mounted in a semi-cylindrical pattern around the flow-tube, by means of an aluminium part that provided 4×9 matrix fiber distributions. To concentrate efficiently the light energy from the optical fibers to the laser crystal, 2-D curve polishing at the output ends of the optical fibers were done. The diameters of the Nd:YAG laser rod (4mm) and of the flow tube (8mm) were dimensioned to achieve the maximum energy flux inside the active medium. To ensure maximum absorption, a double-pass pumping scheme was accomplished by applying a gold reflector onto half of the internal wall of the flow tube. Using an output coupler of 94% reflectivity, Nd:YAG laser operation was achieved, resulting in a maximum output power of 2.46W. The proposed configuration is scalable trough application of more optical fibers along the flow — tube of a longer laser rod. Further improvements in homogeneity of the absorbed pumping power can be obtained by using large numerical aperture optical fibers mounted around laser rod.

1. Introduction

By converting directly incoherent and broadband sunlight into monochromatic laser radiation, sun-pumped solar lasers find many applications in space ranging from power transmission, propulsion to earth and atmospheric sensing. Due to the complete elimination of the electrical power supply unit and the simple optical to optical pumping scheme, the solar laser is more reliable in some specific applications. The first Nd:YAG solar laser was reported by Young1, who obtained a laser output of 1W for an efficiency of 0.57%. Improvement in higher output power was achieved by Arashi2, Weksler3 and more recently Cooke4. The actual research in solar pumped laser is still mainly devoted to achieving higher output power and higher laser efficiency while other aspects like the beam quality, uniformity of the pumping and flexibility (the flexible separation of a primary and a secondary concentrator) were not much stressed.

A solar-pumped laser utilizes a two-stage system that incorporates a focusing first-stage primary parabolic mirror that tracks the sun and a second-stage, usually a non-imaging concentrator. The laser head and its associated optics are placed near or directly at the focus of the collector. The impossibility to take apart primary and secondary concentrator penalizes the flexibility, and turns the two-stage system unsuitable for certain applications. The efficient transport of concentrated sunlight to a remote target by solar fiber-optic mini­dishes scheme with high efficiencies was reported recently by Gordon5, which may
constitute an important advance for solar-pumped laser research due to utilization of large numerical (NA=0.66) optical fibers. Non-imaging optics plays an important role in solar lasers by providing means for concentrating sunlight to intensities approaching the theoretical limit. Based on the edge-ray principle, the compound parabolic concentrator (CPC) that gives the maximum concentration for a two dimensional cavity, is the most commonly used for side-pumping solar laser. Although the non-imaging pump cavity provides a large amount of pump power, it does not give a Gaussian absorption pumping profile, affecting the laser output beam quality6. In side-diode pumped laser, the close — coupled fiber optic or glass plate pumping geometry approaches the ideal TEM00 mode — matched absorption distribution7,8, which, may be beneficial to designing sun-pumped lasers.

The experimental results of a fiber optic solar-pumped laser are presented. The new side­pumping scheme allows the separation for many meters of the radiant source at the focus and the active medium by means of a bundle of 36 optical fibers. The output ends of the 36 fibers are displayed in a close-coupled geometry half way around the medium in order to achieve pumping homogeneity. A double pass pumping scheme allows an efficient absorption of the pumping energy.

The fiber optic solar laser system is given in Fig.1. A primary parabolic mirror with 150cm diameter, 67 cm of focal length, 85% reflectivity and a small centred hole of 8cm diameter was used. A plane mirror of 14cm diameter was used to invert the incoming concentrated solar light to the input end of a 2 meter light guide-optical fiber bundle assembles, by which concentrated solar light was transmitted to a convenient place for flexible pumping of the small solar laser rod. Due to the flexibility of the fiber bundle, constant solar power was obtained while the primary mirror was working in the direct tracking mode. Flexibility in solar energy transmission, allows the location of the laser head outside the focal area of the primary parabolic mirror.

Some descriptions of the technical parameters for the solar laser system by optical fibers ranging from the flux distribution at the focus to the light-coupling scheme to the laser crystal will be given in the following sections. Angle-dependent light interception and transmission efficiencies will also be discussed.

## Mathematical formulation

The fluid flow and heat transfer phenomena involved in processes of water storage tanks are described by the Navier-Stokes and energy equations. Assuming a Newtonian fluid behaviour, with constant physical properties with the exception of density variations which are treated assuming Boussinesq approximation (relevant in buoyancy terms of momentum equations), viscous dissipation and the influence of pressure in temperature negligible and non-participant radiation medium, the governing equations can be written as follows:

 V • V = 0 (i) ^ + pv ■ = — Vp + v • T — рЯіЗ (T — To) (2) dT -► f к (3) PW + Pv. m=v-(-vi)

where і is time; p mass density; v velocity; f stress tensor that is evaluated considering Stokes’ law; p pressure; <f gravity; temperature; T0 reference temperature; ер specific heat at constant pressure; & thermal conductivity; and /3 thermal expansion coefficient. Thermo­physical properties considered in this work are listed in table 1.

Table 1: Thermo-physical properties. Units in SI.

 Property Material Water Plexiglass p 1000 262 Cp 4169 1050 к 0.5552 0.17 P 9.32 10“4 — /3 2.76 10-‘1 —

Reproducing the test sequence proposed above, at the beginning the tank temperature is set at 20oC, which corresponds to preconditioning phase (P1).The inlet mass flow rate has been imposed according to those recommended by the test sequence. Thermal losses of the tank have been modelled considering a heat transfer coefficient of 3 W/m2K at lateral walls, and at the top and bottom of the tank. Ambient temperature has been fixed at 20oC. At the outlet, the injected flow rate has also been imposed, and temperature derivative has been assumed null.

## Economical analysis

The aim of investigations carried out in this work is to optimise the use of small solar heating systems for domestic sector. Demonstration project has been realized to

determine the investment cost and expenditure for construction and mounting. In a dialogue with Bulgarian solar collector manufacturers and importers, a price for small solar heating systems was analysed.

For the installation investigated in this work full price of investment is 750 Euro. This price corresponds to the Bulgarian economical standards and includes solar equipment available on the Bulgarian market in its lower price level.

Solar heating economy has to only been analysed by comparing the investment costs to the value of the calculated solar production. Two-year exploitation of solar installation shows that it can be used both in summer and in winter periods, which improves solar heating economy. In table 1 are shown results for overall solar production of installation. Calculations are made by using the theoretical model, but most of results are approved by experiments.

Calculated yearly solar energy production for a typical climatic conditions in south regions in Bulgaria is 1220 kWh. If substituted energy is electricity, which price in Bulgaria is about 0.07 Euro, the cost of solar production can be assessed to the 85.26 Euro per year. This gives payback time 8.8 years.

 Month Solar radiation, kWh/day Utilized radiation, kWh/day % solar fraction 1 3.36 0.26 3.26 2 6.20 1.61 20.09 3 7.72 2.49 31.05 4 10.02 3.72 46.40 5 11.22 4.47 55.00 6 12.24 5.16 64.42 7 13.47 5.90 73.67 8 12.94 5.71 71.29 9 11.76 5.13 64.03 10 9.00 3.74 46.60 11 4.60 1.46 10.22 12 3.10 0.31 3.02
 Table 1. Yearly Solar Production

1. Conclusions

The thermal stratification in domestic solar hot water systems has been investigated both experimentally and numerically. Special test module with monitoring system registers all needed parameters to analyse efficiency and physical behaviour of the system. Mathematical model for thermal accumulator was validated to wide investigation scope. The main purposes of experiments relate to investigate the influence of serpentine location in the tank on thermal performance of the system. Three different configuration of serpentine location have been investigated.

Serpentine location in bottom zone of the tank realizes unstratified thermal accumulation in solar installation. Thermal stratification can be arrived with serpentine location in the top zone of the tank. Results show that the stratification in tank improves thermal efficiency up to 15-20%. This can results in using smaller collector area to prepare hot water.

Thermal efficiency in solar installations is highest when thermal stratification is stable and it is formed with heat exchange in hot and cold zone. This ensures high thermal efficiency of solar collectors and delivers useful energy on demand.

References

1. J. A.Duffie and W. A. Beckman, Solar engineering of thermal processes, Wiley Interscience, New York, 1980.

2. G. F.Csordas, A. P. Brunger, K. G.T. Hollands and M. F. Lightstone, Plume entraintment effects in solar domestic hot water systems employing wariable-flow — rate control strategies, Solar Energy 49 (6), 497-505 (1992).

3. A. Shahab, An experimental and numerical study of thermal stratification in a horizontal cylindrical solar storage tank, Solar Energy 66 (6),409-421 (1999).

4. Y. Hoseon, C. J. Kim, C. W. Kim, Approximate analytical solutions for stratified thermal storage under variable inlet temperature, Solar Energy 66 (1) 47-56 (1999).

5. Zurigat et al., A comparision study of one-dimensional models for stratified thermal storage tanks. ASME J. Solar Energy Eng. 111, 205-210 (1989)

6. Shtrakov St.,A. Stoilov, Solar hot water installation with stratified accumulation, 8th

Arab International Solar Energy Conference and Regional World Renewable Energy

Congress, Bahrein 2004

## Knowledge Formalisation

There are several types of objects to formalise and supervise the knowledge. The most important are: diagnosis objects, question-class objects and rule objects.

The different diagnostic objects are defined and organised in the diagnostic hierarchy. They are represented in a tree structure which enables to distinguish them either in roughly or in finer diagnosis in arbitrary depth. They represent all the given diagnosis for possible error sources in solar installations. Concerning diagnostic objects many attributes can be set, e. g. the „a priori
frequency" or a proposal for the resolution of an error.

We have eventually found about 60 different diagnosis of possible errors. The hierarchy of question classes includes all question classes and the corresponding question objects. We tried to enter all possible and relevant error symptoms and we reached about 60 symptoms in 25 question classes.

Rule objects link up answers to the questions with their possible solutions, e. g. in very simple form such as “IF A AND B THEN C”, or also such bounds as “N from M” in arbitrary depth. Each kind of classification has its own rule objects.

Out of a number of approximately 60 different diagnoses, about 150 rules in heuristic and safe classification are necessary.

Application

After the start of D3 a dialog interface appears. On this interface the user interacts with the system, which answers questions to indicate the characteristics. First the user must indicate a rough symptomatology of the problem.

Depending on the evaluation given by the system, further question classes appears.

When desired, the user can find help for each question, or the cause of the current question class. If all questions are answered, the system evaluates the characteristics and indicates to the user the most plausible solution(s).

The user can consult the different propositions of solution given by the computer thanks to graphs or other special figures so that the diagnosis is easier to understand.

3. Conclusion

In its present structure the solar expert system enables users to easily find error sources thanks to the knowledge of solar installations which are put on disposal.

In the future the collection of the characteristics will be completed with explanations, pictures and propositions of solutions so that even a user with a few or without knowledge in solar installations can use the system.

We have also planed to integrate the solar expert in a WWW-browser which offers the advantage that changes and actualizations in the knowledge basis are more available. Thanks to this, the use of the system will become much easier. Presently we are just testing the second version of the software. After its probation it should be free-of-charge distributed.

## Investigation of a Solar active glass facade

H. Kerskes, W. Heidemann, H. Muller-Steinhagen

Universitat Stuttgart, Institut fur Thermodynamik und Warmetechnik (ITW)
Pfaffenwaldring 6, D-70550 Stuttgart
Tel.: 0711/685-3536, Fax: 0711/685-3503
Email: kerskes@itw. uni-stuttgart. de, Internet: http://www. itw. uni-stuttgart. de

1. Introduction

The use of solar thermal systems for hot water preparation and space heating in single family houses is the state of art. For further dissemination of solar thermal energy multi family houses and industrial — as well as business-buildings promise great potential. For such buildings solar cooling can also assume importance. In contrast to single-family houses the ratio of roof area to heated space is much smaller for these buildings. For bridging this gap solar active facades are suitable. It is expected that these components will take their part in the future solar market.

In this article the investigation of a solar active glass facade is described. This facade consists of a solar collector integrated into a conventional double-glassed window. To improve the collector efficiency reflector stripes are properly arranged as shown in the figure1. One half of the window area is covered by absorber and therefore diffuse and direct irradiation can still enter a room behind the facade.

Fig. 1 : Front view and cross section of the window collector

• the use of solar thermal energy,

• controlled room illumination,

• prevention of overheating.

In this project theoretical and practical investigations of the glass facade will be carried out to analyse the thermal behaviour under realistic outdoor conditions.

From an architectural point of view these technical advantages will have to be combined with the aesthetic appearance. This new device fulfils both.

To establish this new technology outdoor measurements under realistic conditions are necessary.

## THE FABRICATION OF FUSED SILICA LIGHT GUIDES

The fused silica light guides of cross sections (5X5mm) were provided by Beijing King Quartz Cooperation. The light guides are of good optical quality. Four light guides were curved to a designed curvature under high temperature environment (hydrogen flames). A pure graphite mould was also used to help the correct bending of these light guides. The other four light guides in diagonal positions were firstly curved to the desired shape. The input ends were then twisted 900. It was discovered that a much better light concentration to the crystal was achieved by twisting the light guides at their input ends. In order to fit compactly these light guides into free spaces in diagonal positions in Fig.5, the input end of the twisted light guides should be carefully polished. For focusing tightly the solar power to the central core region of the laser crystal, the input ends of all these light guides were slightly and spherically polished. The output ends were also polished in cylindrical lens shape.

Fig.7 The output ends of five principal Fig.8. End and side pumping scheme

light guides for end-side pumping. light guide assembly.

The four light guides in diagonal positions were removed to demonstrate clearly the free space where the flow tube is to be mounted. The output end of the central light guide was used to realize end pumping and the others were used for side pumping.

The side-pumping scheme from eight light guides is shown in Fig.8, where the central focus region was clearly seen. Light guides from eight directions could pump the laser crystal uniformly.