Joachim Koschikowski, Matthias Rommel, Marcel Wieghaus

Fraunhofer-Institut fur Solare Energiesysteme ISE
Heidenhofstr.2, D-79110 Freiburg, Germany
Tel +49-761-4588-5294
Fax +49-761-4588-9000
email ioako@jse. fhg. de

INTRODUCTION

In many places world wide drinkable water is already a scarce good and its lack will rise dramatically in the future.

Today, sea and brackish water desalination plants are well developed in industrial scales. Each day about 25 Mio. m3 of the world water demand is produced in desalination plants. These “water factories” are in the capacity range up to 230.000 m3/d and can provide big cities with drinkable water. Small villages or settlements in rural remote areas without infrastructure do not profit from these techniques. The technical complexity of the large plants is very high and can not easily be scaled down to very small systems and water demands.

Furthermore, the lack of energy sources as well as a missing connection to the grid complicates the use of standard desalination techniques in these places. The use of renewable energy sources as wind or solar radiation can compensate this lacks.

The absence of drinkable water in arid and semi-arid regions often corresponds with a high solar insolation, this speaks for the use of solar energy as the driving force for water treatment systems. These systems must be adapted to the special conditions required by the alternating operation conditions caused by solar energy powering and to low water demands, challenging ambient conditions and the lacks of well trained technicians for setup and maintenance. So the main focus of the development work is on the construction of robust systems which operate maintenance free in a stand-alone mode. The systems have to be modular in order to resize them to a wide range of user profiles and they must be able to withstand different raw water compositions without chemical pre-treatment in order to develop standardised stand-alone systems for all current types of sea and brackish water. Mainly two different options are given for using solar energy as the driving force for desalination: PV (photo voltaic) coupled RO (revere osmosis) systems and solar thermally driven distillation systems. Also PV and thermally driven VC -(vapour compression) systems are possible.

This paper reports on an ongoing development of solar thermally driven stand­alone operating desalination systems for a capacity ranges of 0.1 to 20 m3 per day. To achieve these aims separated desalination units based on the membrane distillation technique (MD) with internal or external heat recovery function are coupled with high effective solar thermal collectors. The implemented heat source for very small capacities is a corrosion free, sea water resistant thermal collector developed by Fraunhofer ISE in 1999. For larger systems a design is used which is based on a separated collector loop coupled to the brine loop by a seawater resistant heat exchanger. In that case standard flat-plate collectors or vacuum tube collectors are used.

Two options for small-scale stand alone operating systems

For seawater desalination two principal different techniques are existing, thermal driven systems and pressure driven systems. Also for small-scale stand alone operating systems this two options are given.

The most common type of thermally driven stand-alone operating desalination Figure 1: Example for a simple solar still. system is the solar still type (figure 1). It

consists of a basin with a dark bottom in

order to absorb solar radiation. The top is covered by a solar radiation transparent plate made from glass or polymer. The saltwater is evaporated by the heat transformed from the solar insulation. The vapour condenses on the cold surface of the top plate and the formed distillate is collected in a trough. This construction is quite simple and many different designs which are all based on the same principle exist. Due to the fact that its thermal efficiency is very low, the specific collector area per cubic meter desalted water is very high. The experience with simple solar stills were negative, especially with respect to the low system efficiency (V. Janisch, 1995). In advanced solar thermally driven desalination systems the desalination unit must be separated from the solar collector in order to achieve high efficiencies on the heat generator site and to integrate a heat recovery into the desalination unit.

An other option for decentralised, stand alone operating desalination is the use of reverse osmosis (RO) modules and photo voltaic (PV) coupled high pressure pumps.

Osmotic pressure results from a concentration gradient between two salt solutions. If there are two solutions of different concentration separated by a permeable membrane with a diameter of pores smaller than salt ions, then the liquid from the lower concentration site permeates through the membrane to the higher concentration site until the hydrostatic pressure of the water column (see figure 2) is equal to the osmotic pressure. The osmotic pressure mainly depends on the height of the concentration gradient.

To produce water with a low salt concentration from a higher concentrated solution, an external pressure on the concentrate site is necessary which is higher than the osmotic pressure. This principle is called reverse osmosis (RO). For technical applications RO is used to produce fresh water from sea or brackish water. For sea water desalination pressure pumps are operated between 60 and 80 bar. Today circa 42% of the world wide installed plant capacity for sea and brackish water desalination is based on RO.

While these grid coupled RO systems are very well developed, it is known that difficulties exist to operate small scale stand — alone systems which are supplied by PV or wind energy. The comparison between solar thermally driven evaporation systems and PV driven RO systems with respect to the long-term system efficiency, reliability and appropriateness can not finally be assessed.

Membrane distillation (MD)

Membrane distillation is another technique which is operated with thermal energy but also uses a membrane for the separation of pure water from the concentrated solution. Apart from some experimental systems the MD-technique is not used for desalination up to now, but with regard to the implementation in solar driven stand alone desalination systems it holds important advantages. The most important advantages are:

• The operating temperature of the MD-process is in the range of 60 to 80 °C. This is a temperature level at which thermal solar collectors show a good performance.

• The membranes used in MD are proved against fouling and scaling.

• Chemical feed water pre-treatment is not necessary.

• Intermittent operation of the module is possible without heat storage.

• The system efficiency and the high product water quality is almost independent from the salinity of the feed water.

1992, Findley 1967, Schofield 1987) is

Contrary to membranes for RO, which have a pore diameter in the range of 0,1 to

3,5 nm, membranes for membrane distillation have a pore diameter of about 0,2 pm. The separation effect of these membranes is based on the fact that the polymer material it is made from, is hydrophobic. This means that up to a certain limiting pressure liquid water can not enter the pores. Molecular water in the form of steam can pass the membrane. In figure3 the principle functioning of membrane distillation is discribed.

On the one side of the membrane there is salt water, for example at a temperature of 80°C. If there is a lower temperature at the other side of the membrane, for example by cooling the condenser foil to 75°C, then there exists a water vapour partial pressure difference across the membrane. This is the driving force that makes the water passes through the membrane. The water vapour condenses on the low temperature side and distillate is formed.

For the design of a solar powered desalination system the question of energy efficiency is very important since the investment costs mainly depend on the area of solar collectors to be installed. Also the power consumption of the auxiliary equipment (for example the pump) which will be supplied by PV has an important influence on the total system costs. Therefore, the system design has to be focused on a very good heat recovery function to minimise the need of thermal energy. Heat recovery can be carried out by an external heat exchanger or by an internal heat recovery function were the feed water is directly used as coolant for the condenser channel.

Heat-

exchanger1

The principle of the internal set-up of the MD-module with internal heat recovery function is shown in figure 4. All together, there are three different channels: the condenser channel, the evaporator channel, and the distillate channel. The condenser and the distillate channel are separated by a impermeable condenser foil, while the evaporator and the distillate channel are separated by a hydrophobic, steam permeable membrane. The hot water (e. g. 80°C inlet temperature) is directed along this membrane, passing the evaporator channel from its inlet to its outlet while cooling down (e. g. 30°C evaporator outlet temperature). The feed water (e. g. 25°C inlet temperature) passes the condenser channel in counter flow from its inlet to its outlet while warming up (e. g. 75°C outlet temperature). The partial pressure difference caused by the temperature difference on both sides of the membrane is the driving force for the steam passing the membrane. The heat of evaporation is transferred to the feed water by condensation along the condenser foil. Thus the heat of evaporation is (partly) recovered for the process. Because the energy for evaporation is removed from the brine, the brine temperature decreases. The liquid distillate is gained from the distillate outlet on a temperature level between feed in — and brine — outlet. The heat input which is necessary for the required temperature gradient between the two channels (e. g. 5°C) is introduced into the system between the condenser outlet and the evaporator inlet. Thus the thermal energy consumption of the system is given by the volume flow rate and the temperature lift of the feed water between these two points. The heat recovery function has an important influence on the energy consumption of the system. In thermal desalination processes the "Gained Output Ratio” (GOR) is an important parameter for the evaluation and assessment of the heat recovery function. The GOR can be calculated as the quotient of the latent heat needed for evaporation of the produced water and the energy input supplied to the system from out site. A possible design for a MD module is a spiral-wound module construction. A sketch of the channel assembly is shown in the cross section in figure 5. The picture on the right hand shows a test module in the performance test facility.

The technical specifications of the MD module are:

• hydrophobic PTFE membrane, mean pore size 0.2 pm

• height 650 mm

• diameter 300 mm

• membrane area 7 m2

• feed temperature at evaporator inlet 60-85 °C

• specific thermal energy consumption 100-150 kWh/m3distinate (GOR about 4 to 6)

• distillate output 20-30 l/h

• all parts are made of polymer materials (PP, PTFE, synthetic resin)

Figure 6 shows the transmittance of the metal mesh as a function of the wavelength for different cylinder diameters according to the FDTD simulations. The distance between the cylinders is 1875 nm.

The mean transmittance for wavelengths smaller than the distance between the cylinders decreases when the diameter of the cylinders increases. This is in accordance with the expectations from geometrical optics, which applies for wavelengths which are small compared to the distance between the cylinders. Figure 7 shows the transmittance of a grid of reflecting cylinders according to the laws of geometrical optics, calculated with the ray-tracing program ASAP [6].

It is apparent that the mean transmittance for wavelengths below the distance between the cylinders is higher according to the FDTD simulations than it is according to geometrical optics. This is due to diffraction by the cylinders, whose dimension is of the magnitude of the wavelength. This phenomenon is not accounted for by geometrical optics.

To derive the transmittance of the mesh for solar or heat radiation, the transmittance spectrum in Figure 6 is averaged over the corresponding spectral distributions. Figure 8 shows these properties calculated for cylinders with diameters of 250 nm, 500 nm and 1000 nm. These values of solar and heat transmittance can be used as guidelines to estimate the increase in solar transmittance and low-e properties of micro-structured coatings made from metals with finite conductivity.

The light flux distribution at the focus of the primary parabolic mirror was mainly determined by its focal distance and by its rim angle. The accurate alignment of the primary parabolic mirror was considered essential for achieving the minimum focus spot. Since the focused on-axis and the defocused performances of a concentrator gave different flux distributions9, the primary mirror was accurately aligned in order to obtain minimum light spot at the focus. Under these conditions, a detailed map of the light distribution was generated by a small aperture (1.0mm) radiometer scanned across the focal plane. Detailed results on and off the focus were obtained. A typical flux distribution at the focal plane of the primary concentrator is given in Fig.2.

The average solar insolation at the scan time was 800W/m2. A near-Gaussian type distribution was obtained, with the maximum solar flux being 25.6W/mm2. Based on the flux distribution at the focal plane, the total solar power of 1258W was calculated which matched well with experimental measurement.

Matthias Rommel, Thorsten Siems, Rainer Becker, Kurt Schule
Fraunhofer Institute for Solar Energy Systems ISE
Heidenhofstr. 2, D-79110 Freiburg
email: matthias. rommel@ise. fraunhofer. de

The operation of collector fields under stagnation conditions has to be regarded as an important operation mode. A failure-free behaviour of a thermal system under stagnation conditions is important in order to achieve a long system life time. The stagnation behaviour has to be well understood and dealt with for the successful further development of small and large solar thermal systems.

The basic processes that take place when solar collector fields with pressurised closed solar loops are subjected to stagnation conditions were investigated in the past years in different projects. Fraunhofer ISE has worked in this field and elaborated important contributions to it /1, 2/. For information on the basic investigations see for example the contributions from ISE and AEE to Eurosun 2000 and from AEE to Eurosun 2002 /3/.

The proper control and containment of stagnation conditions is especially important for the further development of large solar thermal systems for multi-family houses that are designed to contribute not only to the domestic hot water demand but also to the room heating demand. On the one hand, these applications are extremely important to increase the share of solar energy in the European thermal energy market and to reduce CO2 emission. On the other hand, in these applications large collector fields will regularly be under stagnation conditions during summer time so that it is extremely important to achieve a non-problematic stagnation behaviour.

Fraunhofer ISE now carries out new experimental and simulation investigations in the frame of a larger joint project of several German partners and companies. The aims of this joint project are described in the paper by A. Schenke et al. ‘Outline of a joint research project of SWT, ZfS, ISFH and Fraunhofer-ISE: Analysis and evaluation of large scale thermal solar „combi-plants“ ‘ to this Eurosun 2004 conference. The joint project is supported by the German Ministry of Environment, Environmental Protection and Reactor Safety (BMU).

Method and approach

One of the main aims of the investigations carried out by Fraunhofer ISE is to study in detail the stagnation behaviour of single collector modules. For these experimental investigations the solar simulator and indoor test facility of Fraunhofer ISE is used /4, 5/. A complete pressurised closed solar loop was set up under the solar simulator. The emptying behaviour and process that occurs due to stagnation conditions will be analysed for different collectors
and different hydraulic absorber designs. One of the aims is to determine the maximum steam production rate from the measurements. The further aim is to conclude from the observations and measurements of a single collector module on the behaviour of a complete collector field in which a certain number of modules are connected in series and some of these rows are connected in parallel.

Experimental set-up

Figure 1: Indoor-collector test facility of Fraunhofer ISE with solar simulator.

The pipes of the solar loop connected to the collector are equipped with temperature sensors. The distance between two adjacent sensors is one meter. The sensors are attached to the outer wall of the copper pipe. The pipe is insulated with temperature resistant insulation material (EPDM foam, 165°C). The total length of the pipes prepared like that is 15 m on the left connection of the collector field and 15 m on the right.

In the experimental set-up it is taken care that the pipes are installed such that they continuously lead downwards to the pump, membrane expansion vessel and filling valves of the solar loop, see Figure 2. Under these conditions, the steam reaches into the pipes and consecutively passes the temperature sensors when stagnation conditions start and the collector fluid evaporates further in the course of the stagnation situation.

Figure 3 and Figure 4 show first measurement results which were measured with one collector module of an evacuated tubular collector. The fluid is a water/glycol mixture in this measurement. Figure 3 shows the course of the temperatures. Tin and Tout are measured at the collector inlet and outlet. These two sensors are immersed in the fluid. All other temperature sensors are attached to the outer wall of the pipes of the solar loop. Figure 4 shows the pressure (above ambient pressure) in the solar loop measured near to the membrane expansion vessel.

1 Collector, pe = 2 bar, pMAG = 1,5 bar, Fluid = Glycol, VMAG = 33I, 03/2004

Figure 3: Measured temperatures at collector inlet and outlet and along one half of the pipe of the solar loop.

The lamps of the solar simulator were switched on at 18:15h. The different phases of the

stagnation process can be seen in the measurement:

1. phase 18:30h to 19:15h: Expansion of the fluid. Temperature at the collector inlet and outlet is increasing rapidly. The temperature in the pipes increases very little and only due to the fact that the ambient temperature in the room of the test facility with the solar simulator increases from 20 to 25°C.

2. phase 19:15 to 19:30h: The first evaporation occurs and the steam presses the fluid out of the collector. The over pressure at the expansion vessel is 2.15 bar, Tin=130°C.

3. phase 19:30h to 20:00h: phase with saturated steam — emptying of the collector by boiling. The temperatures at the different sensors on the connecting tube increase very rapidly whenever the steam reaches them. The maximum pressure of 2.7 bar is reached. The steam reaches somewhere between the temperature sensors that are 9 and 10 m away from the collector inlet.

4. phase 20:00h to 9:15h: phase with overheated steam. The steam producing power of the collector is reduced because an increasing part of the absorber is not filled with fluid any more. The depth to which the fluid is penetrating the connecting pipe is decreasing. Whenever the fluid/steam front passes a temperature sensor the temperature falls rapidly. Finally at 22:30h the fluid is only 1 m apart from the collector inlet. Later in the course it reaches almost back to the collector inlet. Then there is no motion of the fluid in the connecting pipe any more and the fluid in the loop decreases all along the tube due to
heat losses of the pipe.

5. phase 9:15h to end of measurement: At 9:15h the simulator is switched out and the irradiation drops suddenly from 1070 W/m2 to zero. The collector is refilled by the fluid from the connecting tubes. When the fluid enters the absorber the increase again shortly due to steam produced from heat that was stored in the absorber material.

2 bar, Pmag = 1,5 bar, Fluid = Glycol, VMAG = 33I, 03/2004

Figure 4: Pressure (above ambient pressure) measured in the solar loop at the height of the expansion vessel. At the point in time when the pressure reaches its maximum the collector reaches its maximum steam producing power.

Conclusions

As mentioned before, these are just first measurements. The experiments and their evaluation are ongoing. More results and conclusions to be drawn with respect to the stagnation behaviour of large collector fields will be presented at the conference.

Information on the status and the results of the joint project can be found on the website http://www. Solarkombianlagen-XL. info.

Literature

/1/ Konrad Lustig, Experimentelle Untersuchungen zum Stillstandsverhalten thermischer Solaranlagen, Dissertation, University Karlsruhe, elaborated at Fraunhofer ISE, 2002.

/2/ Konrad Lustig, Matthias Rommel and Dirk Stankowski, EXPERIMENTAL RESEARCH OF STAGNATION IN SOLAR THERMAL SYSTEMS, Eurosun 2000 Copenhagen.

/3/ Hausner, Fink, Stagnation Behaviour of thermal solar systems, Eurosun 2002 Bologna

/4/ Joachim Koschikowski, Neuer Solarsimulator zur Indoor-Vermessung thermischer Solarkollektoren am Fraunhofer ISE, OTTI 2002 Bad Staffelstein

/5/ Joachim Koschikowski, Charakterisierung des neuen Solarsimulators am Fraunhofer ISE, OTTI 2003 Bad Staffelstein

The scientific work is financed by the German Federal Ministry for Environment, Environmental Protection and Reactor Safety (BMU). The authors gratefully acknowledge this support. The authors are responsible for the content of this publication.

Alexander Storch, Jan Schindl
Business Unit Renewable Energy
Osterreichisches Forschungs — und Prufzentrum Arsenal GesmbH
Faradaygasse 3, A-1030 Vienna
Phone +43(0)50550-6381; Fax -6390
alexander. storch@arsenal. ac. at

The presented project surveyed the quality of irradiance sensors used for applications in solar systems. By analysing an outdoor measurement, the accuracies of ten commercially available irradiance sensors were evaluated, comparing their results to those of a calibrated Kipp&Zonen pyranometer CM21. Furthermore, as a simple method for improving the quality of the results, for each sensor an irradiance-calibration was carried out and examined for its effectiveness.

Irradiance sensors in thermal solar systems

Irradiance sensors have two major fields of application in thermal solar systems. First, they can provide input parameters for an automated control and supervision of thermal solar systems. For this application, the accuracy of the instantaneous measurements is crucial. The second application is the measurement of sums of irradiation for a monitoring. To meet the rising demand for quality assurance, the monitoring of solar systems is an increasingly requested method for observing the performance and controlling the function of thermal solar facilities. The recorded sums are needed for the calculation of certain characteristic values which are used for the comprehensive rating of the solar systems. To support an increased implementation of monitoring devices, the costs of the measurement equipment has to be kept as low as possible while maintaining a necessary accuracy. Due to their low costs of about a tenth of those of a pyranometer, irradiance sensors are now commonly used in monitoring devices for irradiance measurements. Some of the suppliers guarantee an accuracy of less than 5 % for the annual sum.

Abdoussalam Ba* ,Amadou Hamadou* ,Hamidou Arouna Saley**

*Centre National d’Energie Solaire BP 621 Niamey, Niger **Msc UDUS Sokoto Nigeria

ABSTRACT: Traditional ovens working with large amount of wood are used by butchers to roast mutton in Niger. As we know, this country is mostly occupied by Sahara desert. It is quite important to preserve its forest and all initiative to reduce wood consumption is welcome. That is one of the reasons that a solar oven is conceived. It is a hot box type solar cooker that has parallelepiped form with 1200 mm length, 975 mm width, and 755 mm height, the all with four rollers feet. The absorber is half cylinder, constituted with a black-painted sheet and with 1100 mm length and 965 mm diameter. The oven has a double glass cover and two reflectors permitting the increase of solar radiation in the box. The external wall is constituted of wood board on which a layer of varnish has been putted. Between the board and the absorber there is a glass wool insulation of 25 mm thickness. Tests have been run to characterise the oven: -temperature profile in the box (from the bottom to the glass cover) — efficiency of the cooker calculated — economic aspects

1. Introduction

Niger Republic is a land locked West African country covered on the % of its territory by Sahara desert. The major part of the population (10 900 000 inhabitants) lives on the remaining 25%, in the south part of the country.

Annual forestry production was 910 759 tons of wood while the consumption was 2 293 398 tons in the year 1997. Fuel wood is the main energy resource of the population, which 94% of the needs are satisfied by that resource. In the large cities, mainly in the capital city Niamey, hundreds of cubic meters are burned daily for cooking.

Solar energy is abundant: 6kwh/day; 9 to 10 sunshine hours per day. Could this resource be used in the aim of reducing wood consumption? That is the aim of the present study

The concept of the window collector was setup by J. M. Robin et. Al. In /1/ an overview of the state of art is given.

To evaluate the thermal performance of the window collector, a numerical investigation is made. A simple model describing the thermal behaviour of the system is implemented under the TRNSYS environment. The model takes both passive and active solar gains of the window collector into consideration. A dynamic building simulation was carried out with a solar combi system for space heating and domestic hot water preparation. The goals of the simulation are the estimation of the possible yearly energy saving and its influence on the room temperature behind the collector.

The necessary collector parameters for this simulation are obtained from a collector test in accordance to EN 12975-2. The collector testing was performed with a first prototype of the window collector. For this testing the collector was treated like a conventional collector. The thermal efficiency of the window collector is of course the efficiency of a typical flat plate collector due to the fact that only half of the aperture area is covered by the absorber. The k-value obtained from this test method is over predicted and therefore not suitable for the window collector.

Under realistic conditions with the window collector integrated into a wall the heat losses will be much smaller than the calculated one taking the above mentioned k-value into account.

A one-family house at WQrzburg with a yearly energy demand of 12674 kWh served as a reference. A schematic description of the solar combi system with 750 litres of tank-in-tank storage is given below (Fig 2).

Comparison is done for the energy savings due to a wall-mounted conventional flat plate collector of 15 m2 and the window collector of the same area. The yearly energy saving was calculated as 22.2% for the flat plate collector and 19.2% for the window collector.

This is a remarkable result given that the absorber area of the window collector is only half that of the flat plate collector. The primary reason for this better performance of the window collector is its optically optimised design (distance of absorber tubes, reflector arrangement) under the prevailing irradiance condition on a vertical wall and the secondary reason is an additional passiv solar gain. On account of the special optical property of the window collector this passive solar gain is dependent on the incident angle

(IAM). Corresponding the IAM the collector shades the room located adjacent behind the window collector depending on the position of the sun. Ray-tracing studies are undertaken to evaluate this. The results of this study are depicted in Fig. 3.

The TRNSYS simulation described is not detailed enough to capture the interaction of the window collector and the room behind to calculate the influence on the indoor climate and comfort. The simulation results showed that even in summertime the room temperature may not rise above 25°C due to the shading effect of the window collector where as an ordinary window of the same size yields a temperature of 35°C.

A more detailed description of the simulation results is given in /1/

Despite the availability of the room temperature through TRNSYS simulation the comfort level inside the room is not fully discovered. The comfort level might be influenced by the possible high temperatures at the window collector. First measurements using infra-red camera showed temperatures of the inside glass surface up to 55° under stagnation condition corresponding to maximum a temperature at the absorber of 102°C. The effect of the window collector on the room climate still has to be investigated.

To test the high power solar energy transmission capacity of light guide assembly, an experimental set-up was built. Owing to the difficulty in measuring directly the total solar power from the light guide assembly, each individual light guide was mounted on its correct light coupling position. A Moletron power meter was used to measure the output power from each straight, curved or curved and twisted light guide. A total solar power of about 880W was then deduced. The output light divergence from each light guides was less than 300, which ensured the future light coupling to the laser crystals.

Nd:YAG End-Side Laser Pumping

With the curve polishing at the output end of the light guides, a spot-size in the focal region of 4mm was achieved. Theoretical analysis confirmed that 80% of the sunlight transmitted by the light guide matched the rod cross section. To achieve maximum flux energy in the active medium, a 5mm diameter Nd:YAG rod was chosen. The crystal with a nominal neodymium concentration of 1.1.% and 20mm in length, was coated at one end to be
highly reflection for A=1.064pm. The other end was plane and antireflection coated. The Nd:YAG rod was inserted in the flow-tube (see Fig. 9) of 12mm external diameter and 30mm in length.

The resonant cavity was formed by a 94% reflectivity output coupler of -1.0m radius positioned at 300mm from the Nd:YAG rod. To prevent laser rod damage from the UV radiation and to reduce unwanted temperature rise, a doped flow tube was used. Demineralised water was used as a coolant and the flow rate was 4l/min. The equilibrium temperature of the coolant for cw-Sun pumped laser operation was 28°C.

The continuous laser output power was measured as a function of the power transmitted by the light guide assembly (Fig.10). To achieve variation of the output power, the primary mirror was masked with ring pieces of non-reflecting material, affecting the collecting radius of the primary concentrator. Besides controlling the incident sunlight power in the active medium, this practice had an effect on the flux distribution within the Nd:YAG rod.

For a solar irradiance of 800W/m2, the maximum measured cw-laser output power was 8.6W, corresponding to an overall efficiency of 0.97% and 1.8% slope efficiency. The threshold was determined to be approximately 300W of the power collected by the light guide. Considering the total solar power at the focus, the total efficiency was 0.68%.

CONCLUSIONS

Due to the symmetric end-side pumping of a Nd:YAG laser crystal by a fused silica light guide assembly, satisfactory solar laser efficiency was obtained. The new light guide assembly permitted tailoring the pumping flux distribution within the active medium at the cost of slightly lower solar laser efficiency, due to the transmission loss of the light guides. Much experimental work was done in finding the optimum light guide shape and in achieving the minimum focused end-side pumping zone with the flow tube. The ZEMAX ray-tracing program confirmed some of the experimental research.

T. Muller*. W. Wagner, R. Hausner, AEE INTEC, Gleisdorf, Austria,

M. Kohl, S. Herkel, Fraunhofer ISE, Freiburg, Germany,

B. Orel, National Institute of Chemistry, Ljubljana, Slovenia,

K. Hofler, TB fur Bauphysik, Graz, Austria

* Phone: +43 3112 5886 16, Fax: +43 3112 5886 16, email: t. mueller@aee. at

Coloured absorbers are a major demand of architects for the design of fagade integrated solar thermal collectors. But coloured absorbers have shown an inferior thermal performance compared to selective coatings of state-of-the-art collectors so far. Within the project Colourface selective colour coatings have been developed and ageing tests of the coatings have been performed. Four colours have been chosen as absorber coatings for test collectors. The colours were blue, green, auburn and grey. The efficiency of these test collectors was measured using the dynamic collector test method according to EN 12975-2. The blue and green coloured absorbers have shown thermal performances comparable to black solar varnish coated absorbers. The auburn coating showed less absorptivity but also less emissivity than blue and green resulting in a slightly lower efficiency curve. The efficiency of the grey coated absorber was the lowest of all tested collectors as it was expected. Seven wall constructions that are commonly used in Austria and Germany have been investigated to find out whether the direct integration of collectors into the wall is possible without harming the building materials or. Finally, two pilot systems — a newly constructed two-family house and a retrofit building — have been realised with fagade collectors. Temperatures and relative humidities in the collector and inside the wall construction have been monitored and analysed. The results of the project are presented in this paper.

Development of Paints

Thickness insensitive spectrally selective (TISS) paints have been developed by KI, the National Institute of Chemistry in Ljubljana, Slovenia. Several compositions of low-emitting pigments, coloured low-emitting pigments and black absorbing pigments have been screened with respect to their spectral selective properties in combination with a good colour effect. Finally three colours (blue, green, auburn) with absorptivity values of a > 0.8 and emissivity values of є < 0.5 have been selected to be used for test collectors and two pilot systems. Additionally, the colour grey was chosen as a fourth colour since architects see a high application potential for this colour.

Testing of Paints

The testing of the selectivity of the colours has been performed by Fraunhofer ISE in Freiburg, Germany. Absorption and emission values before and after ageing tests have been measured for all colour samples. Temperature tests were conducted at 178°C. Condensation tests were performed with a sample temperature of 40°C. Both tests were repeated with exposure of the samples to UV radiation. Only the samples which have shown no significant changes after the tests have been selected for the coating of the test collectors and the collectors for the pilot systems.

Tests of solar collectors are performed according to several international standards. Table 1 presents the procedures and standards currently in use. The collectors are divided in two categories related to their application: sanitary or swimming pool. The tests are also divided in two groups. The first group deals with minimum durability tests and the second with thermal behavior. The thermal efficiency test is performed after all group 1 tests have been finished.

Thermal efficiency tests are performed using an open-loop test configuration, according to ASHRAE 93-1986 specifications, with a constant head tank to maintain constant pressure through out the test. Figure 1 illustrates the collector testing facility, with one collector undergoing thermal efficiency tests, while two other collectors are subjected to the non­operational exposure test.

Once the tests are performed, the data is processed to generate the information for the label. The label for the collectors is similar to other labels used for appliances in Europe and Brazil. Figure 2 shows the version that has been in use since 2004. At the top, the label presents basic information including manufacturer, brand, model, maximum working pressure and application (sanitary or swimming pool). Then there is a classification in 7 different categories, from A to G. Below the graphic classification, the label presents the specific average monthly energy production (kWh/month. m2) and the average monthly energy production (kWh/month). Finally, the label sets out the gross collector area (m2) and the collector’s efficiency (%).

The efficiency is calculated using a standard linear efficiency equation derived from the test data and assuming the following operating point:

T — T

— a = 0,02 (1)

Gt

There was no attempt to create a number that would be closely related to the actual average energy production in different regions of the country, since the high diversity of climates makes it impossible to establish one single number that would be representative. A single number for the whole country was used in an attempt to keep the information presented to the consumer as simple as possible, while still providing a tool that would allow straightforward comparisons between different collectors. The use of a single number representing energy performance is used also for other products tested under the labeling program, e. g., refrigerators and air-conditioning units. These products are tested under a certain set of conditions, and their estimated energy consumption is stated for such conditions. Their actual energy consumption will vary depending on the operating conditions, as will the production of a solar collector.

There have been several efforts to educate consumers about the information on the label, and to make clear that average energy production values should not be used to size SWH systems.