Category Archives: The Experimental Analyze Of The Solar Energy Collector

Energy conservation

The heater saves about 17 MJ of energy per day. There are 300 clear days in a year at Jodhpur and, therefore, solar water heater can be used for 300 days per year, accordingly, calculations have been made of energy savings per year in relation to different fuels, and payback periods have been computed. The economic evaluation and payback periods have been computed by considering interest rate = 10%; maintenance cost = 5% of cost of the solar heater, and inflation rate in fuel prices and maintenance cost = 5 % per year. Cost of the heater is Rs. 8,000.00 (Euro 1.0 = Rs

60.0) . The cash flow of the heater with respect to different fuels has been carried out and it has been found that the cash flow is maximum with respect to fuel coal and minimum with respect to kerosene. The payback period is least, i. e. 1.42 yr, with respect to coal and maximum, i. e. 3.72 yr, with respect to kerosene (The cost of kerosene is highly subsidised). The payback periods are in increasing order with respect to fuel: coal, electricity, firewood, LPG, and kerosene. The estimated life of this solar water heater is more than 15 years. The shorter payback periods suggests that the use of ICS solar water heater is economical. The use of integrated collector storage solar water heater will conserve substantial amounts of commercial and non-commercial fuels, which are consumed for obtaining hot water.

4. Conclusion

Integrated collector storage (ICS) type solar water heater can provide 100 litres of hot water at an average temperature of 57.3o C that can be retained to 43.0o C till next day morning when cold water temperature was 17.0o C. The efficiency of the heater has been found to be 61.3%. The heater saves about 17 MJ of energy per day. Cost of the heater is Rs. 8,000.00 (Euro 1.0 = Rs

60.0) . The cash flow of the heater with respect to different fuels has been carried out and it has been found that the cash flow is maximum with respect to fuel coal and minimum with respect to kerosene. The payback period is least, i. e. 1.42 yr, with respect to coal and maximum, i. e. 3.72 yr, with respect to kerosene (The cost of kerosene is highly subsidised). The payback periods are in increasing order with respect to fuel: coal, electricity, firewood, LPG, and kerosene. The estimated life of this solar water heater is more than 15 years. The shorter payback periods suggests that the use of ICS solar water heater is economical. The use of integrated collector storage solar water heater will conserve substantial amounts of commercial and non-commercial fuels, which are consumed for obtaining hot water.

References

[1] IMD, Solar Radiation Atlas of India, India Meteo. Department, New Delhi. 1985.

[2] D. J. Close, Solar Energy, 6 (1962) 33-40 .

[3] J. I. Yellot, R. Sobotka, Trans. ASHRAE, 70 (1964) 425.

[4] C. L. Gupta, H. P. Garg, Solar Energy, 12 (1968) 163-82.

[5] K. S. Ong, Solar Energy, 16 (1974) 137-48.

[6] N. M. Nahar, Energy 5 (1984) 461-464.

[7] G. L. Morrison, N. H. Tran., Solar Energy, 33 (1984) 515-26.

[8] G. L. Morrison, J. E. Braun, Solar Energy, 34 (1985) 389-405. ‘

[9] M. Vaxman, M. Sokolov, Solar Energy, 37 (1986) 323-31.

[10] N. M. Nahar, and J. P. Gupta, Energy Conversion and Management, 27 (1987), 29-32.

[11] Norton, B., Probert, S. D. and Gidney, J. T., Diurnal performance of thermosyphonic solar water heaters — an empirical prediction method’, Solar Energy, 39 (1987) 251-65.

[12] N. M. Nahar, Int. J. Ambient Energy, 9 (1988) 149-54.

[13] N. M. Nahar, Int. J. of Energy Res. 16 (1992) 445-452.

[14] N. M. Nahar, Energy Convers. & Mgmt. 32 (1991)371-374.

[15] N. M. Nahar, Int. J. of Energy Research 16 (1992) 445-452.

[16] N. M. Nahar, International Journal of Renewable Energy, 26 (2002) 623-635.

[17] N. M. Nahar, International Journal of Energy and Buildings, 35 (2003) 239-247.

[18] B. J. Brinkworth, Solar Energy, 71 (2001)389-401.

[19] I. Tanishita, Present status of commercial solar water heating in Japan. ISES Conf., Melbourne, Paper 2/73,1970.

[20] S. J. Richards, D. N. W. Chinnery, A solar water heater for low cost housing. NBRI Bull., 41, CSIR Res. Rept. 237, South Africa, 1967.

[21] H. P. Garg, Solar Energy, 17 (1975) 167-172.

[22] N. M. Nahar, Energy Conversion and Management, 23(1983) 91-95.

[23] N. M. Nahar, J. P. Gupta, J. P., Int. J. of Energy, 12(1988), 147-153.

[24] N. M. Nahar, J. P. Gupta, Applied Energy 34 (1989)155-162.

[25] D. Fairman, H. Hazan, I. Laufer, Solar Energy, 71(2001) 87-93

[26] Y. Tripanagnostopoulos, M. Souliotis, Th. Nousia, Solar Energy, 72(2002)327-350.

[27] M. Smyth, P. C. Eames, B. Norton, Solar Energy, 75 (2003) 27-34.

[28] M. Souliotis, Y. Tripanagnostopoulos, Solar Energy, 76 (2004) 389-408.

[29] A. Madhlopa, R. Magawi, J. Taulo, Solar Energy, 80 (2006) 989-1002.

Dynamic heat loss coefficient with operative flap (set-up B)

The collector heat loss coefficient UL was determined with the ventilation flap in operation for the experimental set-up B and data of Sept. 9 and Sept. 14. The dimensions of the slit aperture, the flap’s sensitivity setting and the collector tilt angle are given in Fig. 7. Fig. 7 shows that UL

increases for larger AT from the interval 5.4 — 6.8 W/(m2 K) to the interval 8.2 — 9.3 W/(m2 K) due to the self-triggered opening of the ventilation flap. Here, a constant collector heat capacity of Ce=5 kJ/(m2 K) was chosen. The experimental set-ups A and B have different dimensions for the thermal insulation and the distance between collector cover and absorber, hence the values for UL and Ce from set-up A an B cannot be directly compared.

The trend of UL for September 9 and 14 in Fig. 7 shows that the heat loss coefficient increases steeply when the flap is opened and converges then to a final value. This is due to the dimensions of the flap opening and slit aperture. At first the dimensions are sufficient to provide sufficient cooling. With increasing AT an asymptotic limit is reached where the geometric and aerodynamic limit is so that the maximum heat loss of the collector through passive ventilation is reached.

For Sept. 14 the initial and the final value for UL are larger than for Sept. 9. This is caused by the enlargement of the slit aperture at the bottom of the collector from 15 cm to 20 cm and the changed flap’s sensitivity settings. Due to the larger slit aperture (bottom) the heat loss is already larger from the beginning (Sept. 14). Further, due to the higher sensitivity of the flap setting the flat opens wider for certain AT, which results in a higher heat loss coefficient.

9

Sept. 9, 2006

9

Sept. 14, 2006

8

,»• *

8

• ♦ ♦♦

7

t

7

. /

6

* . « . *

6

. *

5

5

4

4

3

3

Tilt angle: 60%

2

Tilt angle: 42%

2

Flap setting: 6 mm

Flap setting: 8 mm

1

Slit aperture (bottom): 20 mm

Slit aperture (bottom): 15 mm

0

———— 1———— 1———— 1———— 1———— 1———— 1———— 1————

= (Tbs

— Ta )

^T = (Tbs

— Ta )

0

Fig. 7. Heat loss coefficient UL as a function of the temperature difference AT = (Tabs, avg — Ta) with ventilation
flap in operation; measurements for different tilt angles, flap settings and bottom slit apertures [set-up B];

2. Summary

Passive ventilation has been investigated as a method to limit overheating in polymeric collectors during thermal stagnation. Measurements were performed outdoors at two experimental set-ups, which consisted respectively of a ventilated and a non-ventilated (reference) collector.

For set-up A, a maximum temperature reduction up to 30 K between ventilated — and reference collector was measured. Corresponding results were obtained in [1,2] by modelling and in [3] by measurements; for the latter the ventilation was performed on the backside of the absorber.

In the present studies the temperature in the solar collector was reduced from above 145 °C to less than 120 °C. The temperature reduction was obtained with a distance of 28 mm between the absorber surface and the collector cover. The slit aperture openings were varied between 10­20 mm. This changed the temperature reduction with 5-10 K, with largest cooling effect for the large slit aperture. The temperature difference between the reference and the ventilated collector increased with increasing tilt angle.

In set-up B, the slit aperture in the bottom of the collector frame was constant while an adjustable, temperature-triggered flap determined the slit aperture in the top. It was shown that the longitudinal thermal expansion of the polymeric absorber is sufficient to trigger the ventilation flap in the top and initiate the ventilation. The ventilation flap concept seems to work effectively.

A simple method to determine the heat capacity of glazed polymeric solar collectors by means of stagnation temperature measurements has been investigated. The advantage of the method is that the collector efficiency can be obtained from the collector’s stagnation temperature and solar irradiation measurements only. The disadvantage is that additional information about optical and heat removal properties is required. This method is a valuable tool to determine the heat loss coefficient and the heat capacity (and hence the collector efficiency) of building integrated, polymeric collectors.

References

[1] M. Kearney, J. Davidson, S. Mantell, Polymeric absorbers for flat plate collectors: Can venting provide adequate overheat protection?, Journal of Solar Energy Engineering, ASME, August 2005.

[2] J. P. Roberts, M. J. Brandemuehl, J. D. Burch, K. M. Gawlik, Overheat protection for passive solar water heating systems using natural convection loops. Madison: Proc. of ASES Annual Conference Solar 2000, Wisconsin, 2000.

[3] S. J. Harrison, Q. Lin, L. C.S. Mesquita, Integral stagnation temperature control for solar collectors. SESCI 2004 Conference University of Waterloo, Ontario, Canada, 2004.

[4] Wavin B. V. Solar collector with plastic tubes for transfer medium — and heat-exchangeable overheat protector to open housing wall for convection cooling. Patent, Publication number: NL8105671, Publication date: 1983-07-18.

[5] K. Resch, G. M. Wallner, Thermotropic materials for overheating protection of solar collectors. In Proceedings of the EUROSUN 2008 Conference, October 7-10, 2008, Lisbon, Portugal.

[6] K. Resch, G. M. Wallner, Overheating protection with thermotropic resin systems: Effect of material structure and morphology on light-shielding efficiency. In Proc. of the EUROSUN 2008, Oct. 7-10, 2008, Lisbon, Portugal.

[7] K. Resch, R. Hausner, G. M. Wallner, Modeling of an all-polymeric flat-plate collector with thermotropic overheating protection. In Proceedings of the EUROSUN 2008 Conference, October 7-10, 2008, Lisbon, Portugal.

[8] R. Griessen, M. Slaman. Solar collector overheat protection. Patent number: P6017675NL; Patent/IP status: filed.

[9] R. Hausner, C. Fink, R. Riva, Quantifizierung des Stagnationsverhaltens von thermischen Solarsystemen.

Staffelstein: Tagungsbeitrag 13. Symposium Thermische Solarenergie, 2003.

[10] J. Gjessing, Ventilering som metode for a redusere stagnasjonstemperatur i solfangere. Master thesis at the University of Oslo, November 2006.

[11] N. Rumler. Untersuchung von Kunststoffkollektoren hinsichtlich Uberhitzung im Stillstandsfall. Master thesis at HTWK Leipzig with practical project at University of Oslo, January 2007.

[12] General Electric Company, GE Advanced Materials Plastics, NORYLs Resin EN150SP Data sheet, 2005.

[13] A. Olivares, J. Rekstad, M. Meir, S. Kahlen, G. Wallner. A test procedure for extruded polymeric solar thermal absorbers, Solar Energy Materials & Solar Cells 92(4), 2008, p. 445-452.

[14] S. Kahlen, G. M. Wallner. Degradation behavior of polymeric materials for solar thermal applications, In Proc. 27th PDDG Meeting, Aston University, Birmingham, England, September 2007.

[15] J. A. Duffie, W. A. Beckman (1991). Solar Engineering of Thermal Processes, 2nd ed. Wiley Interscience, New York.

[16] EN 12975:2. Thermal solar systems and components, Solar collectors-Part 2. European Standard EN 12975-2:2003.

[17] M. Kohl. Messung des solaren Absorptionsgrades und des thermischen Emissionsgrades, MeBprotokoll. Fraunhofer Institute for Solar Energy Systems, 27.04.2000.

[18] H. Visser, P. van Staalduinen, B. G.C. van der Ree, H. P. Oversloot, A. J. Koelemij, S. Bijma. Assessment and recommendations for application of the SolarNOR energy roof/facade, Second draft. Report number 95-BBI-R1217, Project no: 526.6.3582. TNO, The Netherlands, 1996.

[19] J. Gjessing, J. Rekstad, M. Meir. A method to determine the u-value and the heat capacity of glazed polymeric solar collectors. Manuscript prepared for submission for Buildings and Energy; status: September

Trough angular misalignment

Beside the collection efficiency, other optical characteristics have been monitored to evidence how much they are affected by geometrical deformations of the solar trough profile. The second study combines the consequences of mirror deformations with misalignment and tracking errors.

The crucial optical features to be considered in examining alignment and sun tracking are angular misalignment and acceptance angle of the solar trough collector.

The angular misalignment is simulated tilting the solar trough, with a rigid rotation of parabolic mirror and absorber around the axis of parabolic vertexes. Analogously to the previous study, the parameters defining the solar trough layout are: f=780mm, D=50mm, G=70mm, T=2mm. Figures 4-5 show parabolic mirror profile and circular absorber section, whose centre is located in the parabola focus. Figure 5 illustrates the rigid rotation of parabolic mirror and absorber, for a tilt angle of 1.1°. For rotation in the right direction, the collected light impinges on the left portion of the metal pipe, instead of being symmetrically distributed as shown in Fig. 4, corresponding to tilt angle 0°.

image037

Fig. 4. Parabolic trough without tilt. Fig. 5. Parabolic trough with tilt of 1.1°.

image038

Fig. 6. Effect of angular misalignment.

The effect of collector angular misalignment is assessed considering the collection efficiency E (ratio between focused and entering light). The behaviour of collection efficiency is reported in Fig. 6, for misalignment angles in the range (-1.5°; 1.5°). The absorber centre is located in the focal point of parabolic mirror. The curve evidences that the collection efficiency almost maintains its maximum value between -1.1° and 1.1° for the solar trough under test. This limit angle represents the acceptance angle of the solar trough collector: significant energy losses will appear for angular misalignments exceeding the acceptance angle 1.1° (in Fig. 5).

Since the consequences of angular misalignment depend on the geometrical parameters of solar collector, this second study proceeds combining the angular misalignment effects with the mirror deformations effects.

Thermotropic polymer blends

In general polymers are incompatible with one another as a result of low entropy of mixing and the positive energy of mixing between polymers. Exceptions to this rule are for example metastable, partly miscible systems which exhibit a Lower Critical Solution Temperature (LCST). At low temperatures the polymers interact via salt formation, hydrogen bonding, complex formation, п-electron interaction or dipolar interaction. The miscibility decreases with increasing temperature associated with the formation of domains. As a result the layer turns opaque. Thermotropic polymer blends are poured mostly as films from an organic solvent on a glass or a polymer substrate [2,3,20].

Typical polymer blends developed for overheating protection purposes are based on acrylate polymers mixed with either chlorinated rubber or polystyrene [23]. Other systems are styrene- hydroxyethylmethacrylate based with polypropyleneoxide as a second polymer [24,25].

In general thermotropic polymer blends are environmental-friendly and can be produced in a large area at low costs. As to their switching range thermotropic polymer blends are well suited for solar thermal applications. Thermotropic polymer blends undergo a transition from a highly transmitting state to a highly reflecting state (change in solar transmittance by 52%) at temperatures variable between 30 and 130°C [20,23,24]. However, these material types show a switching within a broad temperature range along with high reversibility within a broad time-frame (up to 15 hours) [26]. Furthermore the materials are susceptible to humidity and UV radiation and exhibit problems with long-term stability [27]. To apply thermotropic polymer blends as overheating protection devices of solar collectors further developments should focus on the adjustment of switching temperatures between 55°C and 80°C and on the improvement of the long term stability and switching performance.

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.

image131

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.

Sustainability

• 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.

Reliability

• 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.

image064

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.

image144

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.

image145

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.

image015

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

ETC1a/b

image088 Подпись: FPC3c

FPC2c

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

with

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.

2

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.