Dawei Liang and Pedro Bernardes

CEFITEC, Departamento de Fisica, FCT, Universidade Nova de Lisboa,
2825,Campus de Caparica, Portugal

In Sun-pumped lasers, a laser crystal is usually pumped by concentrated solar light through a secondary compound parabolic concentrator (2D-CPC). Maximum concentration can be obtained through this pumping configuration. The output beam quality presents some non-uniformity problems. Aiming at a better laser beam quality, a new light guide pumping scheme is introduced. Nine fused silica light guides of 5x5mm square cross sections were used to form a novel light guide assembly. Except a central straight guide for end pumping, all the other eight guides were curved, four of which were then twisted, so as to pump a laser crystal symmetrically from eight sides through a quartz flow tube. For an efficient side pumping, the output ends of these light guides were slightly polished to a cylindrical lens shape. The output end of the central guide was also spherically polished. A multiplayer HR (for 1.06pm) coating was deposited at one end of a laser crystal. A semi-spherical optical resonator was formed by using an output coupler of 94% reflectivity. A Nd. YAG laser crystal of 5mm in diameter and 20mm in length was mounted. In order to further concentrate the solar light from the straight light guide to the laser crystal, a convex-lens-window was added to one end of the flow tube. In outdoors experiments, 880W of solar power was measured from the output end of nine fused silica guides. About 700W solar power was absorbed by the laser crystal, resulting in 8.6W of laser power with good beam quality.

INTRODUCTION

The idea of directly converting broadband solar radiation into coherent and narrow-band laser radiation is almost as old as the laser itself. The solar laser is much simpler and more reliable due to the complete elimination of the electrical power generation and power conditioning equipments. If lasers are needed in remote locations where sunlight is abundant and other forms of energy are scarce (spaces, for example), a solar laser would seem to be a natural choice. The solar laser power of 1W was firstly produced by Young (1) nearly forty years ago and the solar laser output power was boosted to 18W by Harou Arashi et. al,(2) Weksler et. al. (3) increased the solar laser output to 60W by using a 2-D CPC for side-pumping the laser crystal. End-pumped solar laser was also reported by Cooke (4). Solar laser of different active materials was also produced by A. Yogev et. al. of Weizmann Institute of Science(5).

To maximize the solar radiation that impinges on the laser crystal, it is necessary to use a secondary concentrator. 2-D CPC (Compound Parabolic Concentrator) are usually used in the Sun-pumped lasers because it wraps the solar radiation incident on its aperture around the laser rod and gives an additional concentration. In the past, most efforts in the development of solar pumped lasers were devoted to the achievement of maximum power and high efficiency while the beam quality (6) was neglected. However the beam quality is important for power transmission, satellite communication, frequency doubling and other applications of solar lasers.

Laser beam quality can be improved by pumping uniformly the laser crystal. It is with this aim that a new fused silica light guide assembly-pumping scheme was put forward and tested. The light coupling assembly composed of a straight, curved and a twisted light guides will be introduced firstly. The end-side pumping scheme of the Nd:YAG laser crystal will than be explained. The final result of sun-pumped laser by light guides will finally be discussed.

SURS systems use much shallower curvature than that of ‘involute’ CPCs (CPC’s designed for an acceptance angle similar to that of a flat plate collector). This was sometimes referred to as an ‘ortho-CPC’ during development as it was a straighter design. There are now two variants. Both configurations use the same mathematical formulation for the curvature as conventional CPC’s, but use different values of the variables, notably the acceptance angle and the reference tube from which the reflector curve is drawn.

In the SURS A design, which was the first version discovered, the reflector is designed to direct light to neighbouring tubes rather than the closest tube as in the CPC. Therefore,

there is crossover of radiation from adjacent reflectors, and an array of tubes is presumed rather than one. It is, in essence, a multi-absorber CPC.

Fig. 2. Three tube element of SURS A array showing involved reflectors. Each reflector side is referenced to the more distant tube in the curved V, rather than the nearets as in the CPC. Two dotted tangent lines are shown. The curve criterion is such that light passing along these lines will be reflected straight back along the incident direction. Light incident more normally to the collector aperture will be collected. Between the left and the centre tube, the light ray is collected after one bounce. Between the centre and right tubes, the light ray shown is collected after a double bounce. The double bounce incurs a second reflection loss, but this is less important if mirror reflectance is high as it is with thin glass reflectors.

In SURS A, incoming light tangential to a neighbouring absorber tube is reflected back in the same direction. Light at more normal incidence is reflected either to the neighbouring tube or one of the two reflector segments between adjacent evacuated tubes. Light can be reflected from one reflector to the other and still be collected. This is not done in a CPC. SURS A could also be designed for EW orientation of tubes but this is has not been done.

This design requires two curved reflector strips for each tube, but all reflectors are identical. They may be produced by the conventional thin glass manufacturing technique, except that the reflector backing is pressed into the desired shape before assembly, and then the mirrored glass is pressed together with the curved backing and the glue is allowed to set. It would be important to minimise optical absorption by edge supports as much as possible. There are several possible means of reflector support but this issue is not addressed in this paper.

The configuration appears to offer good water drainage compared to CPCs, because a slight gap can be left at the bottom join, but the whole panel is deeper at about 100mm. In ideal form at low concentration, SURS is as efficient as a CPC if a perfect reflector is modelled, and such SURS designs offer up to 98% in initial light collection before optical losses, similar to a ideal CPC with a gap left between cusp and absorber due to the evacuated space. If the reflectance of the mirror is not high, as with aluminium reflectors, performance may be lower than that of a CPC because of a greater tendency for double ray bouncing. However, because high reflectance thin glass can be used for SURS and not for a tightly curved CPC, effective reflectance can be higher and practical performance greater.

This means that this reflector may have a specific niche market for evacuated tubes provided the glass reflector can be manufactured at reasonable cost.

For spacings of from 16 to 20 tubes in a 1350mm width, the performance of SURS A is excellent with few rays lost in any direction, but for wider tube spacings SURS A starts to lose rays and performance drops off more rapidly than a CPC. A second configuration, SURS B, was developed to improve widely spaced tube module performance.

SURS B

This is visually and practically very similar to SURS A but the curvature criterion is different. A sketch of the collector is shown in Fig. 3.

The B curve is constructed such that light passing tangent to the nearest tube is reflected horizontally across the V. Because of symmetry, it is reflected back out tangentially to the other tube. Light incident inside these lines will be reflected to the tubes, although some rays will not be collected for certain angles. Although SURS A performance is difficult to better for 16 tube spacings or higher, the SURS B configuration appears to have a slightly higher performance than SURS A at wide tube spacings and is the collector of choice for the 14 and 12.5 tube spacings.

The reason for this increasing interest on PV/T hybrid technologies is probably due to the development for the Building Integrated Photovoltaic Application getting always greater in different countries and that many projects require roof installed solar thermal collectors. From that concept the idea to develop both the technologies is straightaway. From an economic point of view there is a reduction of the expected costs and a less competition for the needed space. From an economic point of view the transition to an integrated system results more efficient than the two parted components.

Project criteria that need to be observed are:

1) Vapour and water tightness

2) Aesthetics (Colour, look, shape)

3) Insulation

4) Modularity

5) Simplicity

6) to fit to the energetic required.

The main actors playing an important role on the market should be

1) Technicians (Engineers, architects)

2) The Industry

3) Research centres

DEVELOPMENT

Though at the moment different counties are keen to develop the PV/T technology it is Israel that has reached very interesting levels of applications.

There have been some experienced since 10 years that have proved to be cheap at the climatic and marketing conditions of Israel. They own a technology that seems to match the needs of thermal and electric of a single flat. The cost is about 2000 euro per square meters ( year 2000). The thermal gain is about 1.5 kWh/m2 and the energy production is 0.4-0.8
kWh/m2/d. The pay-back period is less than 3 years. The marginal cost of PV integration is 3 $/Wp

Still exists the need for research and development on the following items:

a) Performances of the component (thermal and electrical aspects and so on)

b) Prediction of the reliability on different climatic situations and seasonal estimation

c) compatibility with existing rules and codes

d) maintenance aspects

e) life time

It is possible to foresee for the near and far future the following deadlines:

Short period: it is necessary for the market preparation

Medium period: development of competitiveness (costs, aesthetics and comfort)

Long period: completion of the integration process.

It is planned to install extra sensors for solar radiation, ambient and system temperatures, mass flow measurement and operating hours of pumps. Figure 4 shows the placement of the sensors in a schematic system. In nearly all solar systems that are chosen, an electronic control exists so that it will be tried for some systems to get additional data from the control. In these systems only at those points where there are no sensors of the control, extra sensors will be installed (e. g. mass flow sensor in the solar circuit).

For detailed measurements of the collector field two systems will be equipped with additional sensors at certain points in the collector field.

Previous experiences show that service and maintenance mainly consist of general revision tasks like checking the concentration of the heat transfer fluid, control of the impermeability of the system, primary pressure of the expansion vessel, control of the system operation pressure etc. In general replacement of certain components is not necessary. These revision tasks mainly cause labor costs. Therefore for the calculation of the cumulative energy demand for maintenance only the driving distance of 30 km (one­way) with a passenger car is considered. Furthermore it is assumed that the inspection is done once a year.

1.3 Primary Energy Saved

The amount of primary energy saved by the thermal solar system PEAsub is determined by the difference between Qconv, tot and the auxiliary primary energy demand required by the thermal solar system Qaux, tot. Here, Qconv, tot represents the total primary energy requirement of a conventional reference system that is necessary to meet the hot water and in case of a combisystem also the space heating demand.

The energy demand of a conventional domestic hot water system as well as the heat losses of the domestic hot water store are based on the European draft standard prEN 12977-2 which specifies a unique European reference system. The yearly heat demand for domestic hot water preparation (including heat losses of the store) is 3589 kWh.

For the determination of the yearly energy demand of the reference system (in the form of oil or gas) Qconv, the efficiency of the boiler (p = 85 %) of the conventional (non-solar) reference system has to be considered. Taking into account the primary energy value of gas of 1,11 kWhprimar/kWh, the yearly primary energy demand of the conventional reference system Qconv, tot amounts to 4687 kWh/a.

The auxiliary primary energy demand required by the thermal solar system can be calculated based on the fractional energy savings. For the calculation of the fractional energy savings the energy saved by the thermal solar system is compared with the energy demand of a conventional (none solar) system. System 1 and system 2 both have fractional energy savings of 55%, which lead to an auxiliary primary energy demand Qaux, tot of 2109 kWh per year.

Simon Furbo, Elsa Andersen, Alexander Thur, Louise Jivan Shah, Karin Dyhr Andersen

Department of Civil Engineering
Technical University of Denmark
Building 118, DK-2800 Kgs. Lyngby
Denmark

Email: sf@byg. dtu. dk,

Fax: +45 45 93 17 55

1 INTRODUCTION

The thermal performance of solar heating systems is strongly influenced by the thermal stratification in the heat storage tank. The thermal performance is increasing for increasing thermal stratification in the heat storage, [1].

Thermal stratification in solar storage tanks is normally established in two ways:

— During charge periods, where heat from the auxiliary energy supply system or from the solar collectors is transferred to the “right” level of the tank. That is, the heat from the auxiliary energy supply system is normally transferred to the top of the tank and the solar heat is transferred to the level in the storage tank, where the tank temperature is close to the temperature of the incoming fluid transferring the solar heat to the tank. For small SDHW systems this is with advantage done by means of a vertical mantle heat exchanger, [2], [3], [4]. For large SDHW systems and for solar combi systems this is with advantage done by means of inlet stratifiers [5],

[6] , [7].

— During discharge periods where heat is discharged from a fixed level of the tank, for instance from the top of the tank for SDHW systems or from a level just above the lower level of the auxiliary volume in a storage tank for solar combi systems. Thermal stratification is best established during discharge if cold water enters the bottom of the tank in SDHW systems during draw-offs, and if the returning water from the heating system enters the tank through inlet stratifiers in solar combi systems [5].

Thermal stratification can be built up to a greater extent than normally if the solar tank is discharged from more than one level, [8], [9]. For instance, a hot water tank for SDHW systems can be equipped with two draw-off pipes, one at the very top of the tank and one at the middle of the tank. In periods where the temperature at the top of the tank is higher than the required hot water temperature, a part of the hot water can with advantage be tapped at a lower temperature through the lower draw­off pipe. In this way the volume of the cold water entering the tank during draw-off is increased, resulting in increased solar collector efficiency, decreased tank heat loss, decreased auxiliary energy supply and consequently increased thermal performance of the SDHW system.

This paper presents the results of theoretical as well as experimental investigations of the thermal advantage of discharge from different levels in solar storage tanks, both for SDHW systems and for solar combi systems.

The hydraulic structures generated in FracTherm can be exported as DXF files. DXF is a common format for CAD (Computer Aided Design) and CAM (Computer Aided Manufac­turing) applications. Thus it was possible to produce an absorber model using a computer — controlled milling machine (plexiglass, scale 1:8, 325.5 mm x 250 mm, Fig. 7).

In order to investigate the flow through a fractal hydraulic structure, some experiments with ink were carried out. At first, pure water flowed through the absorber. Afterwards, ink was injected into the inlet tubing. The results can be seen in Fig. 8. The pictures a) and b) are part of a movie (3 and 4 seconds after injecting, respectively). It can be observed how the ink spreads within the fractal structure. Picture c) shows the ink distribution within the whole absorber model. The pictures a) and b) reveal a laminar profile (parabolic shape). It can also be stated that the profiles are not exactly symmetric, but that the maximum flow speed has moved from the centre of the channels to one of the channel walls (direction in­dicated by arrows). This is probably an effect of secondary flows which occur in curved channels (so called Dean vortices): the fluid moves to the outer wall of a curved channel due to centrifugal forces. Since an asymmetric flow profile can lead to a non-uniform volume flow distribution in the following bifurcation, further investigations on this effect are necessary.

Concerning the thermal behaviour of the absorber, the aim is to obtain a uniform heat transfer from the absorber surface into the fluid. In the experiment shown in Fig. 9 the pro­cess was inverted: after cold water had flowed through the absorber, hot water was injec­ted and the temperature distribution on the surface was observed using thermography. The picture in Fig. 9 was taken 114 seconds after injection. The temperature profile on the right hand side is taken along the white line in the thermography picture on the left hand side (with increasing time from front to back). Apart from the wavy structure — resulting from the periodic changes between fluid channels and solid material — it can be stated that the temperature distribution is rather uniform all over the profile, which also indicates a uniform heat transfer.

It has been found out that the monitoring of the system and the analysis of the operation is necessary, even after several years of operation. Since 1993 many lessons were learned and a kind of standardisation could be reached /3/. But a detailed planning is essential for the special needs of every large-scale solar thermal system.

Two typical failures or not optimal operation will be shown in two examples, which occurred at the solar thermal system of the hospital in Baden-Baden:

After recommendations of /3/ for the sizing of heat exchangers a transmission capacity of 100 W/K/m2 collector area is needed. Following characteristic design values are advised: Temperature input primary: 75 °C Temperature input secondary: 30 °C Temperature output primary: 68 °C Temperature output secondary: 33 °C This means a logarithmic temperature difference of 5 K.

The heat exchanger of the collector loop at the Hospital in Baden-Baden was not designed in the recommended way. The logarithmic temperature difference is 8,5 K instead of 5 K.

100

90- 80-

60

50 40- 30- 20

01 02 03 04 05

The measurement showed that the heat exchanger was sized smaller by approximately 40 %. This can be seen in the diagram in Figure 4. The temperature difference occurs up to 15 K instead of 5 K. Especially at sunny days and therefore high temperatures the available capacity can’t be transferred. The small dimension was due to cost reasons. The consequence is a decreasing of the collector efficiency and thus a smaller solar yield. The installed heat exchanger was not replaced because the delivered solar energy was still in an acceptable range and the warranty could be fulfilled anyway.

After two years of operation it was found that the flow rate of the secondary side of the discharging heat exchanger was decreasing constantly and finally stopped (Fig. 5). After the cleaning of the dirt trap didn’t help the heat exchanger was dismounted. There was a strong calcination on the drinking water side. This was remarkable because there was a
special thermostatic valve which avoided temperatures above 60 °C at this part of the heat exchanger. The measurements proved that the valve worked properly.

After the cleaning the solar thermal system worked like before. One year later the same problem occurred, so now the heat exchanger has to be cleaned at least twice a year. Without the measurement the failure would have been very difficult to detect. It shows that even after 2 or 3 years of trouble-free operation a constant check is necessary.

Fig. 6: Simulated and measured solar yields of the six solar thermal systems from 2000­2003

□ Simulation □ 2000 □ 2001 □ 2002 П2003

Fig. 7: Simulated and measured system efficiency of the six solar thermal systems

The relatively low yield of the solar thermal system at the hospital in Singen is caused by an unfavourable concept in the beginning. As can be seen from the yearly increasing gain the optimisation affords were successfully accomplished. This was for example a better adoption of the control concept and the change of the piping of the hot water storages.

As heating storage tank old hot water storages were used. The place of the connections of these old storages were not optimal which means almost no temperature layers can develop inside the storage.

It is remarkable that the plant at the student residence "Vauban” has reached high solar yields. The reason for this is a low solar fraction. The real hot water consumption especially in the semester brake in summer was not as high as predicted. Therefore the utilisation of solar energy also in times of a high irradiation is guaranteed. The disadvantage is a low solar fraction of app. 17 %. In Figure 8 and 9 the effect of the hot water consumption on the solar yield and the system efficiency can be seen.

Fig. 8 and 9: Influence of the specific load on the annual solar yield of the six plants monitored by the Fachhochschule Offenburg. (Each point in this figure represents a monitoring period of one complete year of operation.)

The specific solar energy price ranges between 0,107 €/kWh and 0,160 €/kWh. The exceeding of the maximum predicted costs of 0,13 €/kWh is due to a tolerance of 10 %, different weather conditions and changes in the consumption. The prices are still twice as much as the conventional energy cost. Including further optimisation potential in costs and solar yield and also the increasing of the costs of fossil energy, the solar technology especially in large-scale system gets competitive.

Tomas Matuska, Czech Technical University in Prague

Borivoj Sourek, Czech Technical University in Prague, ENKI Trebon

Rational use of energy in buildings leads to a concept of active energy facade like transparently insulated massive walls, solar thermal or PV facades, advanced glazings for daylighting purposes or double ventilated facades. The paper is concerned on facade-integrated solar thermal collector concept for water heating in existing building stock in Czech Republic (panel and brick block of flats), which is to come through complex renovation (thermal insulation, windows, heating systems). Thermal behaviour of facade collector compared to standard roof-located collectors has been investigated.

Introduction

Large amount of flats (cca 2.3 millions) is concentrated in housing estates (panel or brick block of flats) in Czech Republic. The housing estates established between 50’s and 70’s should pass through complex renovation. Energy-conscious retrofit takes into account the reduction of building heat losses (thermal insulation, windows, mechanical ventilation with heat recovery), systems of control and measurement, devices for hot water consumption reduction or reconstruction of heating systems (distributed plants) and renewable energy sources exploitation. Solar energy utilization has a large potential for water heating (domestic hot water — DHW) in these buildings. However, there are often problems to locate collector field on the flat building roofs (lift housing, ventilation facility etc) or such roof-located system is rejected by architects (collector field on the flat roof is not an integral part of building). Facade collector concept could help to overcome these technical and aesthetical barriers and to bring another advantages. Today, the concept is tested in first pilot installations in Czech Republic. Technical support in the form of computer simulations is needed to bring the possible problems and risks into light and to suggest efficient solutions for individual applications.

In figure 5 the solar gross heat gain of the different collector fields and the solar irradiation on collector plane (15 °) are shown. The solar heat gain varies in the different years due to the following reasons:

In 2000 the control unit failed and the collectors were operated manually for several months. In addition, the second buffer tank was installed which caused service interruption. Furthermore some components such as a heat exchanger and a pump failed which was not immediately detected.

In 2001 the duct heat store was extended during summer and therefore no heat storage was possible. Since the heat demand in the heat distribution net in summer is less than the heat delivery from the solar collectors some collector fields were manually taken out of operation.

In 2002 only minor operational and technical problems occurred and therefore the solar heat gain was higher than in the previous years.

In 2003 the highest solar heat gains were reached. This was the result of a solar irradiation which was significantly above average. In figure 6 it is seen that the solar gross heat gain for the same solar radiation values tends to be lower in 2003 than in 2002. This is caused by higher preheating temperatures of the collectors due to higher return temperatures from the heat distribution net and the duct heat store than in 2002, see figure 7. Furthermore some minor problems occurred, for example collector damage due to heavy storm. The heat exchanger of the collector field “shopping centre” was cleaned since it was clogged.

The differences in heat gain, in 2003 between 336 and 432 kWh(m2a), of the different collector fields are mainly caused by different return temperatures from the heat distribution net from the buildings to the collector fields.

01 23456789 10

Solar irradiation on collector plane (15 °) in kWh/(m2-day)

Figure 6: Gross heat gain of solar collectors versus solar irradiation on collector plane