Category Archives: The Experimental Analyze Of The Solar Energy Collector

The solar air collector of Kollektorfabrik

Kollektorfabrik has laid the foundations to produce a solar air collector which meets the technical requirements of efficient solar energy usage. This collector complies also with the needs of installers and craftsmen, customers and investors.

The field test in private households with heat exchangers for domestic hot water production and direct use of air for heating was started in September 2008. Larger collector fields for solar heat for industrial processes are currently under negotiations.


Fig 2. A demonstration field of an early prototype stage with five modules.

Characteristics of the air collector of Kollektorfabrik

• In comparison with unglazed or flat-plate collectors a higher temperature level can be supplied for processes by means of vacuum tubes.

• For space heating applications in private households a further system with heat exchanger and fan is available.

• Optimized area ratio between absorber and header surface.

• Different geometries and sizes are possible.

• Different designs are possible, for e. g. header with different colors and different sizes.

• Safe and fast installation without the need for long instructions.

• Lightweight construction (ca. 20 kg/m2) for loadsensitive sub-structures.

• The solar thermal air collector of the Kollektorfabrik has a total area of ca. 9.2 m2. A typical household would use about two or three air collector modules for domestic hot water production and space heating.

• A heating system perfectly fitted around roof windows and a smooth adaptation of the dormers of a roof can be realized by means of vacuum tube of different lengths.


• The usage of decentralized renewable energy represents a major contribution to secure environment, supply independency and to deal with depleting resources. The use of solar thermal heat is a cooperatively easy and effective way to do so. With the introduced collector, high solar fractions are easy to realize thus achieving an important impact on the energy supply chain.

• From an economic point of view, a reasonable investment is strictly connected to its life time, its total costs and total benefits of ownership. By integrating the solar air collector into a suitable application not only the energy supply is secured in an ecologically way. The economic investment also achieves sustained success.


• High solar fraction of typical solar thermal systems comes often along with a partial energy overrun in summer. Ideal, easy to run applications need the most heat when the radiation is at the maximum (e. g. solar cooling). If this is not the case, additional components are — depending on the size of the collector field — absolutely necessary to deal with stagnation problematics in summer. These would be e. g. space consuming big storages, advanced intelligent controller with nightcooling (heat rejection), redundant pumps, electricity backup unit, rating rules for expansion vessels, special connecting schemes and advanced solar fluids.

Whereas the collector developed by Kollektorfabrik is intrinsically safe. If the system, a sensor or a actuator fails, even if the system was not installed properly, the collector does not deteriorate during stagnation condition neither does it damage any other part of the system.

• Therefor, no particular measures are necessary to guarantee safety during weekend, lunchtime, process interception or vacations of companies, schools, public buildings etc. The solar air system can resume after a break and even start from full stagnation and inner absorber temperatures up to 250°C without the risk of thermal shocks.

• The collector of Kollektorfabrik meets the requirements based on the test conditions of the European Norm for collector testing (EN 12975-2).

Cost effectiveness

• Kollektorfabrik has developed a long-lasting intrinsically safe collector with the focus on maximum energy output at high temperatures (30°C — 130°C) in the cold and hot seasons.

• Some details were implemented that enable an easy and fast mounting of the collector, thus reducing costs connected to installation.

• Special attention was given to make the collector lightweight so it can be moved without a crane. A Team of two persons can easily transport the collector to a roof e. g. through a roof light and install it. This lowers the costs of installation.

• Kollektorfabrik initiated the development of a fan with extremely low power consumption which could even be driven as standalone system in combination with pv-cells.

• With the scientific assistance of the Fraunhofer Institute for Solar Energy Systems the aerodynamics were optimized through CFD-Simulations and proofed on the test facilities of the Fraunhofer ISE. This way, a maximum benefit can be achieved at a minimum auxiliary power.

• There are several possibilities to store the heat from hot air. It could be transferred into water and stored in a water tank or it could be stored directly and cost effective in the thermal mass of walls and floors or lossless in sorption materials.

Different possibilities to influence the technical and optical appearance like color shadings, different length of the tubes, different sizes and different angles of the tubes make the solar air collector field of Kollektorfabrik unique. It becomes a part of the house, the building or the application the collector is made for. This grades up the application itself, modernizes the building and makes the object of the heat supply more valuable.

Experimental Apparatus

Tests shall be performed with system components installed in accordance with the manufacturer’s installation instructions. The collector shall be mounted in a fixed position facing the equator within a range of ±10 and located in such a manner that a shadow should not be cast onto the collector at any time during the test period.


The schematic representation of experimental apparatus for test procedure system is shown in Fig. 1. It is an open circle loop that contains solar collector, storage tank, valves, and measurement sensors, such as flow meter, radiometer, temperature sensors etc. There are three temperature sensors used in storage tank for two reasons: (a) to obtain the stratification profile in the storage tank along the test and (b) to determine when the homogenized temperature in the storage tank is reached (see Fig. 1.).

Fig.1. Experimental apparatus for the performance test

The responsibility of the loop is also to recirculate the fluid, using a small pump to allow the quick circulation of the water from the storage tank to the collector. The loop has also an air vent, whose operation can drain off the air to make the flow rate at a stable level. The whole loop shall be insulated to ensure a heat loss rate of less than 0.2 W/K and protected with reflective weatherproof coating, so that calculated temperature loss or gain along the homogenize procedure does not exceed 0.2 K under test conditions.

Additionally the ambient temperature is measured using a shaded thermal resistance 1 m above the ground approximately and not closer than 1.5 m to the collector and system, the inlet and outlet water storage tank temperatures are measured with thermal resistance, global solar irradiance sensors are also integrated on the collector plane and an anemometer is also installed in order to measure the wind direction and speed.

Air collector systems

Air collectors can be found in systems for heating or pre-heating of the ventilated air in buildings (Fig. 3). All-polymeric solar air collectors or

Подпись: Fig. 3. Principle of a solar air collector system; collectors with polymeric collector components are found in the market as small stand-alone units for dehumidification of week-end houses, cabins, garages, storerooms, etc. and for heating of large industrial buildings and residences. An example, which obviously includes the advan­tages of using polymers for solar collectors is shown in Fig. 3 and Fig. 8 (d): A modular roof­ing system of building integrated air collectors, which replaces conventional roof cladding and contributes to space heating.

’’Commercial” installations in Spain and Tunisia

In late 2007 a Fresnel process heat collector with 352 m2 aperture area (176 kWpth) was installed on the roof of the Escuela Superior de Ingenieros (ESI), a university building of the Faculty of Engineering in Seville, Spain. The collector has a total length of 64 m (16 modules, 4 m length each) and otherwise a similar design as the ones in Freiburg and Bergamo. The collector powers a double effect H2O/LiBr absorption chiller (Broad), with maximum cooling capacity of 174 kWth, for air-conditioning of the building. At this site the wet-cooling tower for heat rejection, which is usually necessary for H2O/LiBr absorption chillers, will be substituted by a water heat exchanger

fed by water out of the nearby river Guadalquivir. The double effect absorption chiller offers a high COP of up to 1.3, which makes this system a further attractive application of solar process heat for solar thermal cooling. First operation experience of the system is positive and measurement results are expected soon.


Figure 5. The PSE Fresnel collector in Seville, Spain, with an aperture area of 352 m2 (176 kWpth).

The latest installation of our collector is at a winery in Tunisia, where the collector powers a 5TR NH3/H2O chiller from Robur. The installation was realized in the frame of a European funded project (MEDISCO), which will cover monitoring and performance evaluation of the system.


Figure 6. The PSE Fresnel collector in Tunisia (MEDISCO project)

Both, the installation in Spain as well as the one in Tunisia are research / demonstration projects for solar cooling. However, from the viewpoint of the collector manufacturer both are commercial projects, which indicate the start of commercialization of the PSE linear Fresnel process heat collector.

Actuation and tracking system

As shown in Fig. 6, an angle plate, bended as a sector of a circle, is screwed at the front side of the collector prototype. Over this guidance a chain with a tensioning mechanism is led, which can be driven by a step motor over a gearwheel. The step motor is placed together with the control chip in a cabinet at the front side of the carrier. The control chip receives the sensors’ signal, which is placed on the top side of the collector. If the irradiation is not vertical to the aperture section, the control chip receives a differing voltage of the two photo cells insight the sensor which activates the tracking. The motor stops when the sensors’ signal is again equal zero.


Fig. 6. Tracking system with sensor, control chip and step motor

The mentioned concept drives only one collector module, which was sufficient within the proto­type stage. But since every trough needs its own actuation, the concept is not capable for an ar­rangement of a number of collectors in a field. In addition the actuation, especially the combination of chain, gearwheel and step motor, is not capable for a serial production, since the concept con­tains too many single parts. Furthermore the chain has a slip which makes the tracking rather insuf­ficient if it is not clamped properly.

Planned optimization: As an optimized actuation, the new trough will be driven by a suspension link. These kinds of actuators, which are available as standardized components for tracking satel­lite antennas or big photovoltaic modules, have an adequate accuracy and enough power to drive several troughs. Each collector row is given an own suspension link and an own sensor. Thus, the rows operate autonomously and remain in operation, if one row is damaged or not in operation for attendance reason. A standardized component such as a PLC (programmable logical controller) will be used in the new concept.

Prediction of the steam-producing power

This model is based on a total of 210 outdoor stagnation experiments, which were carried out between 2003 and 2007 on three different collector types with a total of eight different connection variations (Fig. 1).

Подпись: ETC1c

Подпись: FPC2a Подпись: FPC2b


image088 Подпись: FPC3c


The SPP of a collector array depends on numerous parameters such as collector efficiency, system pressure and the piping of the collectors. During the stagnation process, we assume that the two — phase mixture in the collector array has the temperature of saturated steam $s. The theoretical collector performance during stagnation Pstag at the moment of maximum steam spread is calculated as follows:

Подпись:Pstag = GT, stag -Л0 “ a1 (S. )“ a2 (®. “ )2


Pstag Theoretical collector performance during stagnation W/m2

GT, stag Effective irradiance during stagnation W/m2

Ss Boiling point of the heat transfer medium °C

Sa Ambient air temperature °C

p0 Conversion factor of the collector —

a1 Temperature-independent heat loss coefficient W/m2K

a2 Temperature-dependent heat loss coefficient W/m2K2

The boiling point 0S of the common heat transfer medium, which consists of a mixture of 60% water and 40% propylene glycol (40%), can be calculated with the help of the system pressure psys at the moment of maximum steam spread:

3S = 100°C + 35.1K • ln (pSyS) (2)

Подпись: Fig. 2. Correlation of the steam-producing-power (SPP) of the investigated collector arrays versus the theoretical stagnation power Pstag.

This calculation takes into account the influence of the system pressure psys, which has an impact on the stagnation behaviour through a changed boiling point. High-performance evacuated-tube collectors are more efficient during the stagnation process and therefore tend to a higher SPP. Hence, an interdependency of SPP and theoretical collector performance during stagnation Pstag is to be expected. Furthermore, the developed model, which describes the correlation of Pstag and SPP, is only influenced by the draining behaviour of the collector array. Fig. 2 shows the dependency of the measured SPP-levels on the theoretical collector performance during stagnation Pstag for the different collector types and array connections.


Theoretical stagnation power Pstag in W/m

Although there sometimes have been measured considerable differences in the SPP-values, Fig. 2 shows a clear trend. As expected, SPP rises with theoretical collector performance during stagnation Pstag. Furthermore, almost all lines of best fit have a positive axis intercept, i. e. a great amount of steam is produced by the collector arrays, although the theoretical collector performance during stagnation is zero. This is particularly clear to be seen with the measurement results from variant FK2a, where SPP levels of 60 W/m2 are recorded although the theoretical collector performance during stagnation is zero. The main reason for this discrepancy is the false model assumption, that the collector temperature during stagnation equals the boiling point 0S of the collector field. In fact, this assumption is often not valid for collectors with unfavourable draining

behaviour, because the relatively large amount of liquid remaining in the collector can significantly reduce the average collector temperature.

From the measurements at the outdoor test arrays at ISFH we can derive three classes of collector arrays with good (A), moderate (B) and bad draining behaviour (C). For these three classes the following correlations with rounded coefficients can be produced:

Class A: SPP = 15% Pstag + 10 W/m2

Class B: SPP = 20% Pstag + 40 W/m2 (3)

Class C: SPP = 25% Pstag + 80 W/m2

The designation of the collector draining behaviour during stagnation process is the basis for the following design process. If we calculate SPP using the model equations (3), a standard deviation between the measurement results and the prediction of 25% may be expected.

Development of solar collectors with FracTherm® aluminium roll-bond absorber

M. Hermann

Fraunhofer Institute for Solar Energy Systems, Heidenhofstr. 2, 79110 Freiburg, Germany
Tel.: +49 7 61 / 45 88 — 54 09, Fax: +49 7 61 / 45 88 — 94 09
michael. hermann@,ise. fraunhofer. de


Aluminium becomes more and more interesting as a solar absorber material. One possibility to produce an absorber entirely made of aluminium is given by roll-bond technology which is well established for large-scale series production of e. g. evaporators for refrigerators. This technology offers the possibility to build solar absorbers with high efficiency, since the channel design can be varied without additional costs. At Fraunhofer ISE a computer algorithm called FracTherm® was developed which is capable of generating a fractal-like, multiply branched channel design on a given area (also non-rectangular), similar to natural hydraulic networks. The aim of this bionic approach was to obtain a high efficiency by means of a uniform flow distribution as well as a low pressure drop. A small solar collector with FracTherm® roll-bond absorber (0.59 m x 1.0 m) had already been built and compared with absorbers featuring serial (meander absorber) and parallel (harp absorber) channel arrangements, respectively, within the research work of a doctoral thesis. Since the test absorber was relatively small, the focus of current research and development is on investigations of larger absorbers with sizes typical for solar collectors. This work, which is still at the beginning, will be done together with industry partners within the European project BIONICOL. This paper describes the challenges coming up with the development of large aluminium roll-bond absorbers with FracTherm® channel design including issues such as inside and outside corrosion, application of a selective coating, the connection of several roll-bond absorbers, fluid flow investigations and stagnation behaviour.

Keywords: FracTherm®, roll-bond, bionic, aluminium absorber

1. Introduction

State-of-the-art solar collectors with fin-and-tube absorbers mostly feature serial (serpentine or meander absorber) or parallel (harp absorber) channel arrangements. However, serial arrangements lead to a high pressure drop due to the channel length, whereas the parallel arrangements can feature non-uniform flow distributions (depending on diameters and lengths of riser and header tubes) [1]. The bionic so-called FracTherm® approach developed at Fraunhofer ISE (Fig. 1) allows creating fractal­like, multiply branched channel structures on a given area which lead to a low pressure drop as well as a uniform flow distribution. The resulting complex structures are not suitable for being built as a fin- and-tube construction, but they can easily be realized as aluminium roll-bond absorbers. Small

FracTherm® test absorbers (0.59 m x 1.0 m) had already been designed, built and compared with meander and harp absorbers within the frame of a doctoral thesis. These investigations had already been published in detail in [2], [3] and [4]. The thermal efficiency had been very high for all test absorbers. It could have been shown that the flow distribution of the FracTherm® absorber was much more uniform and the pressure drop for high volume flows was up to 8 % less than in the roll-bond harp absorber. However, both the results of the experiments carried out so far and the construction of the collector are not directly scalable to a standard size collector (1.0 m x 2.0 m). Fig. 2 shows the small test absorber investigated earlier and a standard size collector with a possible FracTherm® channel structure. The questions arising with the development of a large aluminium absorber with FracTherm® channel design are to be answered within the European research project BIONICOL.


Fig. 1. The FracTherm® algorithm


Fig. 2. Test absorber (left) and standard size absorber to be developed (right)

MEGASOL: a new technology for building big, cheap solar water heating collectors on site

Kerr MacGregor*

MacGregor Solar, 31 Temple Village, Edinburgh, Scotland EH23 4SQ
tel +1875 830 271

* Corresponding Author: kerr @macgregorsolar. com


This paper describes a novel method of building large, cheap solar water heating collectors on site. It is based on using synthetic rubber pipes with enhanced thermal conductivity which are squeezed against the underside of an aluminium solar absorber sheet. The collector can easily be built on site using unskilled labour at a relatively low cost. In addition the collectors are freeze-tolerant and can handle corrosive fluids.

1. Introduction

Several attempts have been made (Bartelsen et al, 1999) to use polymers pipes for solar heat collectors. However, these require specially formed absorber plates to encase the pipes. This method uses plain, flat absorber sheets which are cheaper and more readily available.

2. Construction of absorber

The starting point is a foundation board which can be of timber or chipboard. A layer of thin insulation e. g. reflective bubble polythene is then laid on the foundation board. A grid of the polymer pipes is then laid on the insulation, spaced at about 150mmcentres. The pipes can be temporarily held in place by adhesive tape. Then a sheet of highly conductive material, e. g. aluminium, is laid on top of the pipes and is fastened using screws to the underlying foundation board. The thickness of aluminium and pipe spacing are chosen so that the polymer pipes are squeezed against the underside of the aluminium, but are not closed to the passage of fluid. The aluminium sheet is then painted with a solar absorbing paint. Alternatively, aluminium which has a selective surface can be used. Finally, glazing such as twin wall polycarbonate is located over the absorber and held by glazing bars. A diagram showing a cross-section of the collector is shown in Fig 1 below.


Fig 1: A cross-section of the absorber 3. Suitable piping material

Obviously the pipe material should have good thermal conductivity and long life. EPDM has been previously used. Its thermal conductivity can be enhanced by including materials such as carbon black or graphite in its composition. That can also increase the mechanical strength of the polymer. For example, the addition of 40phr of carbon black to EPDM can increase thermal conductivity from 0.2 to 0.5 W/mK and tensile strength from 3Mpa to 17Mpa. It should also increase resistance to UV degradation, though in this application the tubing is shielded from solar radiation.

Acceptance angle and mirror deformations

Our second study considers the interactions between three different quantities: acceptance angle, vertical edge displacement Delta Z and collection efficiency maximum Emax. The results of these energetic and angular analyses are summarised in Fig. 7. The configuration examined in this study includes a linear parabolic mirror of focal length f=780mm. The absorber is a metal pipe of diameter D=50mm, enclosed into a glass tube of diameter G=70mm and thickness T=2mm.


The acceptance angle is estimated examining plots of collection efficiency versus angular misalignment (tilt angle), like the curve in Fig. 6. The solar trough collector is tilted for assessing the acceptance angle and, at the same time, the mirror is deformed, as described in Sections 2-3. The parameter chosen to indicate the deformation of mirror surface is again the vertical edge displacement Delta Z. Starting from the results of the first study, the range considered for Delta Z is (-2.5 mm; +2.5 mm).

The critical optical characteristic in this second study is the acceptance angle obtained for every solar trough with deformed mirror. It represents the maximum tilt angle for which the collection efficiency approximately maintains its maximum value Emax, presenting only very little energy losses (<1% of Emax). The acceptance angle basically depends on the respective positions of mirror surface and absorber, as well as, on other characteristics not examined in this paper.

Figure 7 reports acceptance angle and collection efficiency maximum Emax as a function of the vertical edge displacement Delta Z. The collection efficiency has been calculated for all examined cases and its maximum value is reported in the labels of Fig. 7; these Emax values are obtained without angular misalignment.

As it could be supposed, Figure 7 evidences that the acceptance angle reaches its maximum value for Delta Z = 0, corresponding to the parabolic profile (see Table 1). Then the acceptance angle decreases when the absolute value of Delta Z improves, but the curve is asymmetrical. In analogy to the different results obtained in the first study for the two cases of elliptic or hyperbolic deformations, the behaviour of the curve in Fig. 7 depends on the deformation type.

The previous study individuated two different limiting values for Delta Z: 2.5 mm for the elliptic case and -3.0 mm for the hyperbolic case. For edge deformations included in this range of Delta Z values (-3 mm; +2.5 mm), the collection efficiency maintains its maximum value, so these mirror deformations do not introduce energetic losses. The first study does not consider any angular misalignment of absorber and deformed mirror.

Whereas the second study simulates a more realistic situation, where misalignment errors interact with mirror deformations, and the consequences are assessed analysing the variations of acceptance angle and collection efficiency.

The maxima of collection efficiency present only minor variations, but their behaviour is not symmetrical with respect to Delta Z = 0. The optimum position corresponds to vertical edge displacement Delta Z = 1, for elliptic deformation of the mirror.

1. Conclusions

A solar trough collector has been analysed using ray tracing simulations. The main optical components of the system are linear parabolic mirror and absorber, composed of a metal pipe surrounded by a glass tube. The reference layout for the solar trough has focal length f=780mm and absorber dimensions D=50mm, G=70mm, T=2mm.

The extensive research has investigated several possibilities for the optical configuration, varying most of trough geometrical features. For the parabolic mirror, it has considered the dependence on mirror width, length and focal length; the effect of mirror deformations and errors in surface finishing. For the absorber, it has examined metallic pipe diameter and shape, glass tube diameter and thickness. Beside these geometrical features the research has studied angular misalignment (of solar trough) and absorber displacement with respect to parabolic mirror. All mentioned effects have been analysed taking into account the sun tracking. The main aspects considered in this latter analysis are solar trough positioning, with respect to Earth rotation axis, and errors in daily and monthly tracking.

This paper summarises the results concerning two studies developed in the framework of the extensive research on solar trough collectors. The first study introduces an original methodology to reproduce rigid deformations of the linear parabolic mirror. Then it examines the consequences of mirror deformations on the collection efficiency of solar trough. The second study analyses the interactions between mirror deformations, misalignment and tracking errors.

In both studies the light concentrated by the parabolic mirror and received by the absorber is expressed as collection efficiency, corresponding to the ratio between focused light and entering light. The other fundamental parameter introduced is the vertical displacement of deformed mirror extreme Delta Z, chosen to indicate the amount of mirror deformation.

The methodology to simulate the deformations is based on the introduction of conic constant K and conic equation to represent the mirror profiles. The deformation can be of two types: elliptic, for -1 < K < 0 or hyperbolic for K < -1. While for K = -1 the conic equation represents a parabolic profile. The vertical edge displacement Delta Z in our convention is positive in the elliptic case and negative in the hyperbolic case, while Delta Z = 0 for the parabola.

This procedure to replicate the deformations of a parabolic mirror is simple and efficient. But the most interesting result is that it seems to reproduce the imperfect rigidity and the flexibility of a real solar collector.

The result of the first study is the identification of two different limits of Delta Z for the elliptic case (2.5 mm) and for the hyperbolic case (-3.0 mm). For mirror deformations with Delta Zin the interval (-3 mm; +2.5 mm), the collection efficiency keeps its maximum value, indicating that the corresponding mirror deformations do not cause losses in the collected energy.

In the second study the deformation effects are combined with the angular misalignment, obtained by a rigid tilt of mirror and absorber. Here the fundamental optical characteristic is the acceptance angle, representing the maximum misalignment angle for which the collection efficiency maintains its maximum value.

The results of the second study is a plot summarising the interaction between collection efficiency, acceptance angle and mirror deformations. The collection efficiency is the most important quantity in the application to solar light exploitation, because it indicates the level of performance in energy collection of the solar system. While for the aspects of alignment and sun tracking the crucial parameter to be taken into account is the acceptance angle of solar collector.


The research has been developed in the framework of the S. A.L. T.O. project. S. A.L. T.O. (Solar assisted cooling Toscana) is a research integrated project POR Ob. 3 Toscana 2000/2006 Misura D4, partially financed by the REGIONE TOSCANA-settore Promozione e sostegno della Ricerca (Tuscany Region). Thanks are due to the industrial partners FAIT group and CEVIT for their support to our activities in developing Solar Cooling Systems.


[1] C. Ciamberlini, F. Francini, G. Longobardi, M. Piattelli, P. Sansoni Solar system for the exploitation of the whole collected energy Optics and Laser in Engineering 39/2, 233-246 (2003).

[2] F. Francini, D. Fontani, D. Jafrancesco, L. Mercatelli, P. Sansoni Solar internal lighting using optical collectors and fibres, 6338-22, Proceedings of SPIE Vol. #6338, Optics & Photonics SPIE Conference San Diego — USA 13-17 Aug. 2006.

[3] F. Francini, D. Fontani, D. Jafrancesco, L. Mercatelli, P. Sansoni Optical control of sunlight concentrators, 6339-08, Proceeding of SPIE Vol. #6339, Optics & Photonics SPIE Conference San Diego — USA 13-17 Aug. 2006.

[4] F. Francini, D. Fontani, D. Jafrancesco, L. Mercatelli, P. Sansoni Designing solar collectors and optical fibers for daylighting. A novel system exploits solar energy by collecting and channeling sunlight to illuminate interior spaces — Illumination & Displays — Science and Technology: SPIE Newsroom DOI: 10.1117/2.1200612.0487 (2006).

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Thermotropic systems with fixed domains

In thermotropic systems with fixed domains scattering particles, which exhibit a sudden change of refractive index with temperature, are statically embedded in a matrix material. At low temperatures the layer is translucent, as the refractive indices of matrix and domain are almost equal. The differing temperature dependence of the refractive index for the components above the switching threshold causes the thermotropic film to turn opaque [2].

One way to manufacture thermotropic systems with fixed domains is the incorporation of thermotropic core-shell polymer particles in a thermoplastic matrix. Such a material is commercially available from EMS Chemie AG (Switzerland) [28]. However, the gradual transition from clear to cloudy of this material is not optimal for solar collector applications.

Other thermotropic systems with fixed domains are based on the dispersion of paraffin in the matrix of a curable resin [6,29,30]. Such thermotropic systems possess a high potential for active solar thermal systems. The materials show a steep and rapid switching process within a small temperature range and an extraordinary high reversibility at low hysteresis [6,30]. The switching threshold is easily adjustable between 25 and 100°C [6,29,30]. A solar transmittance below 85% in clear state is reported. At present a moderate change in transmittance by 25% is achieved [31]. The material production is environmental friendly and cost-efficient. For thermotropic systems with fixed domains no comprehensive research on long-term stability and ageing was reported yet. However, as the additive is statically embedded in the matrix, the materials may posses a high durability. Further research should focus on the adjustment of appropriate switching temperatures, an improvement of the switching range and on the guarantee of sufficient long-term stability even under demanding conditions.