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.

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:

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.

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.

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.

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

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.

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

Technical advantages of the solar activated glass facade are:

• 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 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.

An early production SURS collector using University of Sydney licenced double cermet coating evacuated tubes was tested by ITW (ITW, 2001) in Stuttgart, and measured optical efficiency is shown in Fig. 5. The reflector used was an aluminium reflector with reflectance of approximately 0.91, and the similarity to the earlier SURS B calculated plot in Fig. 4 is evident when the lower mirror reflectance is taken into account. However, the peak efficiency curve is flattened to some extent, possibly due to inaccuracies in reflector formation. In spite of this, the collector performed at the top of the range for evacuated tube modules tested by ITW, with very high diffuse acceptance because of the projecting tubes. Fig. 6 shows the measured thermal performance under standard conditions.

Conclusions

A practical evacuated tube system using using a multi-absorber CPC reflector design has been developed to a commercial product for domestic solar water heating, and performs very well with high diffuse acceptance and will not be as sensitive to snow build up as a conventional CPC system. The SURS concentrator can also be developed to use thin glass reflectors because of the low reflector curvatures used.

References

Winston, R. (1974). Principles of solar concentrators of a novel design. Solar Energy 16, (2), 89.

Mills D. R. 1995. Two-stage collectors approaching maximal concentration. Solar Energy 54, no.1, pp. 41-47.

Institut fur Thermodynamik and Warmetechnik (ITW), 2001. Forschungs- und Testzentrum fur Solaranlagen (TZS), Pfaffenwaldring 6, D-70550 Stuttgart

і

0.9 0.8 0.7 0.6 Ц 0.5 0.4 0.3 0.2 0.1 0

Fig.6. Measured efficiency at different temperatures from a production collector module. A sketch of the reflector system is also shown. Obtained courtesy of ITW.

E. M. D. Pereira, L. C.S. Mesquita[8], J. M.G. Rocha, M. J. Silva, D. P. Dias, R. Schirm, A. S.C. Diniz[9].

GREEN SOLAR, Pontificia Universidade Catolica de Minas Gerais.

Rua Dom Jose Gaspar 500, Predio 50, Belo Horizonte, MG, BRAZIL. 30535- 610 Email — green@pucminas. br

INTRODUCTION

Since the early 1990’s, the market for solar water heaters (SWH) in Brazil has experienced strong growth. From 36.000 m2of solar collectors commercialized in 1990, the market grew to 335.000 m2 in 2003, and reached a peak of 480.000 m2 during Brazil’s electricity supply crisis in 2001. In response, the manufacturers of SWH, worried about the development of a sustainable market, decided to implement a testing program for their products. The idea was to create a voluntary program that would test collectors, tanks and systems, as a way to establish minimum quality criteria and to help consumers compare different products. It was hoped that this would increase confidence, while at the same time permitting governments and utilities to launch tender processes for the purchase of solar water heaters.

Consequently, in 1997 an agreement was reached between the manufacturers’ association (ABRAVA) and the Brazilian government agency responsible for metrology, standardization and industrial quality, INMETRO. Since 1984, INMETRO had been managing a successful labeling program created for electrical appliances. After the initial agreement, the solar energy laboratory of the Catholic Pontificate University of Minas Gerais (GREEN SOLAR) was chosen to execute the tests, and to be a partner in the development of the program’s standards and procedures.

GREEN SOLAR, INMETRO and all participating manufacturers jointly manage the voluntary program. Although it is not mandatory to have the tests performed to sell in the Brazilian market, manufacturers that do not participate face restricted access to government financing and tender processes. Since its onset, ABRAVA established the participation in the program as a prerequisite to membership. It is estimated that ABRAVA member companies account for over 80% of the sales of SWH in Brazil.

Collectors were the first type of equipment tested, starting in 1998. Then, in 1999, tanks were added to the program. As of January 2004, 171 models of solar collectors and 136 models of tanks have been sent to the laboratory for tests. Of these, 96 collectors and 117 tanks have already been fully tested. In 2003, a verification mechanism was implemented that allows the collection of samples from manufacturers’ stocks, through unannounced inspections.

Because of suitable ground conditions an ATES is used for seasonal heat storage. A water-bearing ground layer in a depth of 15 to 30 m below ground surface was made accessible by two wells with a distance of 55 m. During charging-periods ground water is produced from the cold well, heated up by a heat exchanger and injected into the warm well. For discharging the flow direction is reversed: warm water is produced from the warm well, cooled down by a heat exchanger or the heat pump and injected into the cold well. The maximum design flow rate is 15 m3/h. Due to the changing flow directions both wells are equipped with pumps, production and injection pipes. To make sure that no oxygen can enter the ground water circle a pressurized nitrogen system fills the part of the wells above the groundwater level. An entry of oxygen into the aquifer system must be avoided because of a high risk of precipitation and well clogging. As a result of the low charging temperatures no water treatment is necessary.

The suitability of the ground layers has been investigated in advance by an extensive hydro-geological test programme. The hydraulic conductivity kF of the aquifer was found to be between 6И0-5 m/s and 9И0-5 m/s in different parts of the layer, the mean volumetric heat capacity is 2.7 MJ/m3K, the mean thermal conductivity 3.2 W/mK.

The general methodology for the determination of the energy payback time of solar combisystems is the same as explained above. The particularities that arise from various different system concepts for the realization of solar space heating will be explained in the following. This will be demonstrated by calculating the energy payback time for four solar combisystems — two combisystems without integrated burner and two combisystems with integrated burner.

4.1 System Boundaries

In order to be able to compare the energy payback time of solar combisystems with different system concepts, system boundaries have to be defined.

The system boundary for solar combisystems without integrated burner, i. e. where the boiler is a separate component, is directly at the store. The auxiliary heating loop with boiler and hydraulic station is not taken into consideration as these components do not represent specific solar components. They are also necessary for a “conventional” heating system without using solar energy. The same applies to the hot water loop. Components that are situated beyond the system boundary are not considered.

Solar combisystems with integrated burner are thermal solar systems where a gas or oil burner is integrated in the store. As for combisystems without integrated burner, the system boundary is directly at the store. But in this case the balance comprises the integrated burner and the hydraulic station. As flue pipes are situated outside of the system boundary, they are not taken into account.

1.4 Credits

In order to be able to compare solar combisystems with integrated burner with solar combisystems without integrated burner, standardized components are defined that can later be credited to solar combisystems with integrated burner. This will be done by special credits, as already described above with the store credit for the hot water preparation. Depending on the system concept these credits comprise burner, hydraulic station and controller of the heating loop. The single components are balanced with the average values indicated below.

The reference hydraulic station consists of a 3-way valve, a mixer, a pump, 3 m copper tubing with insulation and an expansion vessel of 35 l for the heating loop. The cumulative energy demand for the production of this reference hydraulic station amounts to 247 kWh.

For the reference controller of the heating loop a controller with a weight of 1.25 kg (including temperature sensors) is defined. The cumulative energy demand for the production of this reference controller amounts to 89 kWh. The nominal power of the reference controller is specified with 3 W. This results in an annual energy consumption of 26 kWh. With the primary energy equivalent for electric power (3.8 kWhprim/kWh) the cumulative energy demand for the operation of this reference controller amounts to 100 kWh per year.

As reference burner a standard burner for oil or gas with a weight of 45 kg is specified. It is composed of different materials like mild steel, stainless steel, aluminium, copper and polypropylene. The cumulative energy demand for the production of this burner amounts to 972 kWh.

The application of the credits will be demonstrated with the help of the calculation of the cumulative energy demand for the production of four different solar combisystems.

As shown in Table 8, a solar combi-system with integrated burner can be credited with a maximum cumulative energy demand of 1308 kWh (system 5). In the case that the controller of the combisystem also includes the control algorithm for the heating loop, credits for the controller are also applicable for solar combisystems without integrated burner, as can be seen in system 3.

As the operation of the controller consumes electrical power, the amount of power consumption has to be credited too. Therefore the cumulative energy demand of the operation will be reduced by a credit of 100 kWh that represents the cumulative energy demand of the reference controller of the heating loop.

1.5 Example

Table 9 contains the calculated results for the four solar combisystems. The determination of the primary energy that will be saved by the thermal solar system during its lifetime will be explained below.

The yearly primary energy demand of the conventional system includes the primary energy demand for hot water preparation as well as the primary energy demand for space heating. According to the European draft standard prEN 12977-2 an amount of 2945 kWh is assumed for the hot water consumption and an amount of 644 kWh is considered for heat losses of the store. The space heating demand of a single family house with low energy consumption standard and approximately 130 m2 heated area is 9090 kWh. This results in a total energy consumption of 12679 kWh per year. Taking into account the efficiency of the boiler of p = 85% and the primary energy equivalent of oil or gas with 1.11 kWhprimar/kWh yields to the primary energy demand of the reference system Qconv, tot. Finally the primary energy saved is calculated by subtraction of the auxiliary heat demand from the primary energy demand of the reference system.

5. Conclusion

The energy payback time is a suitable method for the integral assessment of thermal solar systems. Solar domestic hot water systems have energy payback times between 1.3 to 2.3 years. For solar combisystems typical energy payback times are slightly higher, from 2.0 to

4.3 years. Taking into consideration that thermal solar systems have a minimum lifetime of 20 years or more, the substantial potential for saving of primary energy has hence been demonstrated.