Category Archives: Particle Image Velocimetry (PIV)

Development and Experimental Results of. Thermal Energy Storage Technologies for. Parabolic Trough Power Plants

Doerte Laing, Wolf-Dieter Steinmann, Rainer Tamme, Christoph Richter DLR — German Aerospace Centre — Institute of Technical Thermodynamics


Economic thermal storage is a technological key issue for the future success of solar thermal power plant technologies. Storage technology becomes an urgent issue for the successful implementation of commercial solar thermal plants in the context of an accelerated promotion of such plants in those EU member countries, which are relying on solar power to reach the community goal of doubling the share of renewable energy in the EU energy balance from 6 % today to 12 % in 2010.

Without storage, a solar plant in Southern Europe will operate only 800 hours in real full load and over 2400 hours in part load, yielding about 2000 equivalent full load hours or a solar fraction of about 25% in a base load solar steam plant. With 5-6 hours of storage capacity, a solar plant in Southern Europe will operate about 4000 hours in real full load and reduce its part load operation to 400 hours. Due to improved part load efficiency and doubled utilisation of the power block, solar thermal production costs can be reduced with storage by up to 20%. Larger storage capacities may lead to further reduced costs.

At DLR different storage concepts are developed for application in commercial parabolic trough power plants:

> solid media sensible heat storage for parabolic trough power plants using thermal oil as heat transfer medium in the solar collectors

> phase change energy storage systems for direct steam generation in parabolic troughs

> steam accumulators as buffer storage system for compensation of fast transients in parabolic trough power plants

The temperature range for these storage concepts is between 150°C and 450°C.

Solid media sensible heat storage for thermal oil heat transfer media

Solid media sensible heat storage units have been developed in the project WESPE [1], funded by the German Government from December 2001 until December 2003. The focus of this project lay on the development of an efficient and cheap sensible storage material, on the optimization of the tube register heat exchanger and on the demonstration of this technology with a 350 kWh test unit.

Results and discussions

2.1 Effect of temperature

Fig.3 shows the effect of temperature at 20C, 30C, 45C and 60C when the airflow velocity is 1.35m/s.

From Fig3a, it is seen from four curves that evaporation process is quickened rapidly with the increase of the temperature. Fig3b gives the result that the evaporation velocities fluctuate mainly between 0.40×10 kg sm and 1.27×10 kg sm at 20C, and reaching

0. 80-7.00×10-4kg s-1m-2 at 60C, which is approximately 7 times that at 20C. Since solar collector can heighten the temperature of seawater because of the greenhouse effects, and the results show that temperature is a very important factor to evaporation process, using solar chimney for drying is beneficial to the evaporation process.

2.2 Effect of airflow velocity

Fig.4. shows the effect of airflow velocity when the temperature is 30C. The velocities were 0.52m/s, 1.39m/s,1.60m/s and 1.78m/s.

Clearly, evaporation process increases with the higher wind velocity (From 0.52m/s to 1.60m/s), but it declines when wind velocity reaches 1.78m/s. During the experiments, we find that there is a layer of salt covered the surface of seawater, which hinders the mass transfer from the main body of seawater to the air because of the strong evaporation enhancement. So the design of solar chimney dryer should limit the wind velocity to a proper range. The drive for evaporation is humidity gradients between the body of seawater and the air just above the surface of seawater. So we can enhance the humidity grads by using solar chimney.

Operation and Maintenance issues

Industrial turbines for saturated steam are not designed for daily start-up and shut­down, because this kind of operation would reduce its life time considerably and would require often revisions and costly maintenance. Manufacturers recommend to operate the turbine at low load conditions overnight by means of a fossil-fired back-up boiler or thermal energy storage. So, the use of saturated steam turbines in a solar plant demands the implementation of costly maintenance procedures to assure a good performance and durability. However, the extra maintenance cost required by a saturated steam turbine can not be quantified in general, because the service conditions imposed by every manufacturer are different. This can be done only on a case by case basis. What can be clearly stated is that the O&M cost of a saturated steam turbine is higher.

Concerning operation requirements, saturated steam turbines seems to be less flexible than superheated steam ones, because great changes in the steam parameters can have a more dramatic effect. Nevertheless, the water/steam separator at the interface between the solar field and steam turbine brings some benefits. If the volume of this separator is properly designed, it can also act as thermal energy storage if the turbine is operated with gliding pressure. The amount of saturated water and steam inside this vessel can feed the turbine with saturated steam at gliding pressure for a few minutes, thus overcoming short cloud transients. This is a very important advantage of the saturated steam option.

Summarizing, when O&M issues are considered, DSG solar power plants with saturated steam have advantages and disadvantages when compared to the superheated steam option. However, an economic assessment of these advantages and disadvantages is still unfeasible because of the lack of experience with commercial DSG power plants. Though a theoretical study could be performed, the wide range of uncertainties provoked by the lack of experience very much limit the accuracy of results obtained from such a study.


According to the investigation performed the saturated steam option has a 4 % higher net yearly electricity production of the power plant. On the other hand for the plant size analysed, the initial investment required by the saturated steam option is of about 5% higher than for the superheated steam option. Though its maintenance cost is higher, the saturated steam option requires a less complex solar field and the water/steam separator located at the power block inlet can act as a thermal energy storage, which can feed the steam turbine with saturated steam at gliding pressure for few minutes, thus overcoming short cloudy periods. Further advantages of the saturated steam option are the lower complexity of the solar field and the possibility to use simpler collector options that are able to operate with good efficiency at 260-300°C.

Due to the lack of experience with commercial DSG plants, a complete economic and technical assessment of both options including all the aspects (yearly performance, initial investment, operation and maintenance) is still unfeasible until the first DSG solar power plants are installed and deliver accurate information. Therefore the results presented have to be regarded as preliminary but nevertheless the saturated steam DSG plant seems to be an interesting option for near term application in the lower capacity range.


[1] Eck M., Zarza E., Eickhoff M., Rheinlander J., Valenzuela L.: Applied research concerning the direct steam generation in parabolic troughs’, Solar Energy, Vol. 74, No. 4 , April 2003 , pp. 341-351

Energy savings and Pay Back Time results

In spite of the material and energy requirement needed to produce and install PV and PVT systems (and to treat their components at the end of their technical life), during their operation, they produce clean electricity and heat, thereby displacing conventional energy. Therefore, environmental benefits due to avoided environmental impacts are associated to the system operation phase. We used the data from electricity and heat production by PV and PVT systems (Table 2) to achieve the following, assuming that PV systems electrical output displaces conventional grid electricity considering the European average electricity mix for the calculation. Basing on system energy output and on displaced conventional sources, the "environmental cost” of the systems, in a life cycle perspective, can be matched with the "environmental savings” obtained thanks to their clean operation phase. The values of the energy and CO2 Pay Back Times may be calculated, representing the time period needed for the benefits obtained in the use phase to equal the impacts related to the whole life cycle of the analyzed systems and are summarized in Table 4.

Regarding PBT values, it should be noted that they are, in any case, considerably lower than the expected lifespan of the systems. From these results we observe that the highest PBT values are about 3 years and 3 months, while PV systems lifespan could be assumed to be nearly one order of magnitude higher. As a matter of fact, the most conservative assessments (Kato et al,1998) indicate expected life periods of 15П25 years, while other sources (Travaglini et al, 2000), thanks to aging tests conducted on operating plants, suggest a lifespan of more than 30 years. Besides, LCA results underline that the proposed improved configurations for PV systems (heat recovery by air cooling and TFMS modification) enable the energy output to be significantly increased. The higher energy production from improved PV systems and the consequent energy savings, overcome the increased impacts due to the additional system components (HRU). Thus, the proposed configurations show lower values for the PBTs. Additionally, the heat production compensates the impacts due to the HRU. When the HRU of the PVT system is equipped with a glazed covering, though, the increase in thermal energy production allows a considerable lowering of the PBT values.

Concluding, the use of a glazed covering lowers the electrical output because of the reflection and absorption from the glazing but, on the other side, thanks to the greenhouse effect inside the collector, the amount of heat recovered is widely increased and the result of this two opposite effects is positive, thereby achieving lower PBTs. It is noticeable that the better performance of the studied systems is achieved the more the thermal energy

demand is constant during the year, even though, as underlined in the previous parts of the paper, the "12 months air scenario” is somewhat ideal, since referred to strictly particular industrial cases. The most interesting scenario for domestic applications (in spite of the increased material requirement for the heat exchanger) is the combination of air and water heat recovering systems, that leads to lower the environmental PBT in all the analysed configurations.


Hybrid Photovoltaic/Thermal solar systems with air heat extraction were developed by University of Patras, aiming to the increase of the total efficiency of photovoltaics by providing simultaneously electrical and thermal output. We calculated the energy output for operation and the Energy Pay Back Time (EPBT) and CO2 Pay Back Time (CO2 PBT) of all studied systems, considering the corresponding materials of the horizontal and tilted building roof installation of systems. Estimating all together the extracted results we notice that the system that combines the higher total energy output with the lower values of EPBT and CO2PBT are the PVT/GL and the PVT/TFMS both considered in the configuration with reflectors. These systems can be used on horizontal or tilted building roofs, with better performance for the horizontal roofs. The mounting of the thin flat metallic sheet inside the air channel (TFMS modification) gives higher electrical and thermal output compared to the similar unglazed type of PVT/AIR systems. The addition of the booster diffuse reflectors is positive in all cases although the reduction of EPBT and CO2PBT is small. Concluding, the heat extraction from the PV modules results to cost effective solar devices, that are of positive performance regarding their environmental impact, compared to standard PV modules. The advantages of the hybrid PV/T solar systems makes them attractive for a wider application of photovoltaics, providing heat apart of electricity and increasing therefore the total efficiency of the converted solar radiation into useful energy.


Bazilian M., Leeders F., van der Ree B. G.C. and Prasad D. Photovoltaic cogeneration in the built environment. Solar Energy 71, pp 57-69 (2001)

Bhargava A. K., Garg H. P. and Agarwal R. K. Study of a hybrid solar system — solar air heater combined with solar cells. Energy Convers. Mgmt, 31,5, pp. 471-479 (1991)

Brinkworth B. J., Cross B. M., Marshall R. H. and Hongxing Yang. Thermal regulation of photovoltaic cladding. Solar Energy 61, pp 169-179 (1997)

Brinkworth B. J. Estimation of flow and heat transfer for the design of PV cooling ducts. Solar Energy 69, pp 413-320 (2000)

Chow T. T., Hand J. W., Strachan P. A. Building-integrated photovoltaic and thermal applications in a subtropical hotel building. Applied Thermal Engineering 23, pp 2035-2049 (2003)

Eicker U., Fux V., Infield D. and Mei Li. Heating and cooling of combined PV-solar air collectors facades. In Proc. 16th Europ. PV Conf. 1-5 May Glasgow, UK, pp 1836-1839 (2000)

Frankl P. “Analisi del ciclo di vita di sistemi fotovoltaici” (LCA of Photovoltaic Systems), Ph. D. dissertation thesis, Universita di Roma “La Sapienza”, Roma, May 1996 — available at the Dipartimento di Meccanica e Aeronautica, Universita di Roma “La Sapienza”, Roma, or at the Biblioteca Nazionale, Roma (1996)

Frankl P., Masini A., Gamberale M. and Toccaceli D. Simplified life-cycle analysis of PV systems in buildings: Present situation and future trends. Progress in Photovoltaics: Res. and Appl., pp 137-146 (1998)

Frankl P., Gamberale M., Battisti R., Life Cycle Assessment of a PV Cogenerative System:

Comparison with a Solar Thermal and a PV System. In Proc. 16th European PV Solar Energy Conf., 1-5 May, Glasgow, U. K., pp 1910-1913 (2000).

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Conf. PV in Europe, 7-11 Oct., Rome, Italy, pp 588-592 (2002)

Hegazy A. A. Comparative study of the performances of four photovoltaic/thermal solar air collectors. Energy Convers. Mgmt 41, pp 861-881 (2000)

International Organization for Standardization, ISO 14040:1997, Environmental management -­Life cycle assessment — Principles and framework (1997)

Ito S. and Miura N. Usage of a DC fan together with photovoltaic modules in a solar air heating system. In Proc (CD-ROM) ISES World Congress, 14-19 June, Goteborg, Sweden, (2003)

Kato K., Murata A., and Sakuta K. Energy Pay-back Time and Life-cycle CO2 Emission of

Residential PV Power System with Silicon PV Module, Progr. in. Photovoltaics: Res. and Appl. pp 105-115 (1998)

Lasnier F. and Ang T. G. Photovoltaic Enginnering Handbook, Adam Higler, p 258 (1990)

Lee W., M., Infield D.,G., Gottschalg R. Thermal modeling of building integrated PV systems In Proc. 17th PV Solar Energy Conference, 22-26 Oct, Munich, pp 2754-2757 (2001)

Travaglini G., Cereghetti N., Chianese D., Rezzonico S. Behavior of m-Si plant approaching its 20- year design life. In Proc. 16th Europ. PV Solar Energy Conf.1-5 May, Glasgow, UK, pp 2245­2248 (2000)

Tripanagnostopoulos Y. Nousia Th. and Souliotis M. Low cost improvements to building integrated air cooled hybrid PV-Thermal systems. Proc. 16th Europ. PV Solar Energy Conf.,. 1-5 May, Glasgow, UK, pp 1874-1899 (2000)

Tripanagnostopoulos Y., Nousia Th. and Souliotis M. Test results of air cooled modified PV modules. In Proc. 17th PV Confer. 22-26 Oct, Munich, Germany, pp 2519-2522 (2001a) Tripanagnostopoulos Y., Tzavellas D., Zoulia I. and Chortatou M. Hybrid PV/T systems with dual heat extraction operation. In Proc. 17th PV Solar Energy Conference, 22-26 Oct, Munich, Germany, pp 2515-2518, (2001b)

Tripanagnostopoulos Y., Bazilian M. and Zoulia I., Battisti R. Hybrid PV/T system with improved air heat extraction modification. In Proc. Int Conf. PV in Europe, 7-11 Oct, Rome, Italy, pp 718­721 (2002a)

Tripanagnostopoulos Y., Nousia Th., Souliotis M. and Yianoulis P. Hybrid Photovoltaic/Thermal solar systems. Solar Energy 72, pp 217-234 (2002b)

Tselepis S. and Tripanagnostopoulos Y. Economic analysis of hybrid photovoltaic/thermal solar systems and comparison with standard PV modules. In Proc. Int. Conf. PV in Europe 7-11 Oct. Rome, Italy, pp 856-859 (2002)

Heat loss mechanisms for an evacuated tube collector

Developments in absorber technology have generated selective coatings with high values of absorbance over the solar spectrum in parallel with low levels of emissivity in the infrared region. Using the method described by Bhowmik6 it was possible to show that absorber plate using black chrome coatings exhibit selectivity’s of ~10 whereas the newer so called ‘blue’ coatings exhibit improved selectivities of ~25 due to lower emissivity values. As a result radiation losses have been minimised for evacuated solar tubes in comparison to earlier absorber plate technologies. Also Chow et al7 showed that
conduction and convention loss mechanisms shutdown within evacuated tubes at pressures less than 1×10-3 mbar. The dominant mechanisms of heat loss for the evacuated collector systems are therefore due to convection and conduction in the area of the manifold and corresponding tube connections. The use of Infrared-imaging techniques can reveal this to be the case; Figure 6 shows a thermal map for a vertically mounted direct-flow collector system installed outdoors. The glass temperature of the evacuated tubes was observed to be 12.5 ± 2.5 °C whereas the temperature of the manifold cover indicated by the dashed box was observed to be 32.5 ± 2.5 °C. Modern insulations with low k-values in the order of 2×10-2 Wm-2K-1 can minimise these heat losses from the manifold, which is especially important in low flow systems.

2 Conclusion

It was reported that the optical efficiency of an evacuated tube collector of direct-flow design was stable with incident irradiance power densities. The effect of mass flow-rate on the optical efficiency was found to be significant. The low flow-rate penalty was found to decrease efficiency by up to 25% of the quasi-stable value at higher mass flows. Using a simple empirical technique thermal losses from the collector were calculated under these conditions where Tc was held at 3 K above the ambient temperature. These losses were shown to depend heavily on mass flow and incident irradiance. The influence of increasing collector slope on collector performance was found to be beneficial. Linear losses within the collector were found to decrease by 9% over the range of 0° to 60°, however square dependency losses were found to increase by 750%. However, square dependency losses have a minimal effect on the collector performance and therefore the overall result on increasing the slope was found to be favourable. Losses from the collector system were found to be concentrated around the manifold and the connections to the solar tubes. Losses within the evacuated tube were found to be minimal, radiation losses were dominant for pressures less than 1×10-3 mbar.

3 Further work

Plans to repeat this work for comparison with an evacuated heat-pipe collector system are in the pipeline. Also mathematical modelling of the collector and solar simulator using TRNSYS based set-up may be employed at a later stage for theoretical comparison with experimental results.


I would like to thank my colleagues at Thermomax R&D, David McClenaghan, Gareth McWha and Richard Pelan who helped me with the construction of Thermomax’s first solar simulator, thanks guys I couldn’t have done it without you.






Linear dependency of heat loss coefficient



Square dependency of heat loss coefficient



Collector Absorber Area




Specific Heat Heat Removal Factor



Total incident Irradiance



Mass flow-rate



Useful Gain




Ambient Temperature



Collector Temperature



Collector Inlet Temperature



Coefficient of Heat Loss



Collector slope



Temperature difference across the manifold




Measured Temperature difference Optical efficiency

Transmittance-absorptance product



[1] Solar Energy — The State of the Art: Edited Jeffrey Gordon — Chapter 5 Solar Water Heating by G. L. Morrison: James and James (Science Publishers) Ltd, London, 2001

[2] EN 12975-2:2001 — Thermal solar systems and components — Solar collectors

[3] SRCC STANDARD 100 — Test methods and minimum standards for certifying solar collectors, 1995

[4] C. Muller-Scholl, S. Brunold, U. Frei / Proceedings EUROSUN 2002 Conference in Bologna Italy

[5] K. A.R Ismail, M. M. Abogderah: ASME Journal of Solar Energy Engineer Engineering, 120, 51-58, 1998

[6] N. C. Bhowmik, J. Rahman, M. A. Alam Khan, Z. H. Mazumder: Renewable Energy, 24, 663, 2001

[7] S. P. Chow, D. R. Mills and G. L. Harding: Solar Energy 31 (4), 433, 1983

Project Status

The AndaSol project companies Milenio Solar S. A. and AndaSol-2 S. A. have already successfully initiated all steps for the implementation and permitting as independent renewable power projects in Spain in accordance with Royal Decree 2818/1998 and its subsequent modifications. In March 2003 the first two AndaSol projects have been officially acknowledged as eligible solar thermal electricity production facilities by the Junta de Andalucia and have been registered in the national registry by the Ministry for Economy.

With the progress of permitting, the AndaSol partners Solar Millennium and ACS-Cobra have also been successful in identifying further interested equity investors in the European utility sector and European debt financing institutes, among them the European Investment Bank. At the latest since the publication of RD 436/2004 — in which the Spanish government strongly supports the long-term market introduction of solar thermal power — investors and institutional and commercial lenders demonstrate a strong interest to add commercial solar thermal projects into their investment portfolio.

In parallel to the project development in Spain, Solar Millennium AG together with Flagsol GmbH and SBP further optimized and qualified the SKAL-ET collector design with the financial support of the German Ministry for Environment in the 4350m2 SKAL-ET test loop at the SEGS plants in Kramer Junction (California) in order to provide EPC bidders, financing banks and equity investors with a commercial scale reference. With this experience, Solar Millennium AG with the technical support of Flagsol GmbH developed for the project company Milenio Solar S. A. the detailed EPc specifications.

In spring 2003, Solar Millennium AG with the technical support of Flagsol GmbH started the pre-qualification process of critical key subsystem and component suppliers and the negotiations with interested EPC contractors. A basic project description for discussion with investors and financing institutions was drafted in summer 2003.

A cooperation agreement for the implementation of the AndaSol projects was signed with the Spanish ACS-Cobra group in September 2003. Cobra with its 13.000 employees worldwide is a well-known and experienced turnkey contractor for power plants and industrial installations in Europe, Africa, Asia and Latin America. Cobra is the industrial department of the ACS group, which is after the merger with Dragados, the third largest construction company in Europe with 92.000 employees and annual revenues of € 10,8 billion. After successful investments in the Spanish wind energy market, ACS in
partnership with Solar Millennium wants to establish commercial concentrated solar power in the Spanish, Southern European and Mediterranean renewable power market.

The plant will be established as an Independent Power Producer (IPP) project and will be owned by the project company. The AndaSol projects have been developed and planned by Solar Millennium AG through its Spanish local independent project companies. Shares of the two project companies, Milenio Solar S. A. and AndaSol-2 S. A. will be offered to individual and institutional investors in the Spanish national and European market. Participants may also include the Spanish EPC contractor, interested electric utility companies as well as regional development organizations and regional banks. Various national Spanish industrial companies and regional banks already expressed their high interest to participate in the project companies and to provide financing for the AndaSol projects. A sketch of a potential project implementation structure can be found below.


The total surface area is 7.7 m2. The weight is estimated in 250 kg. The whole structure is assembled with aluminum bars to avoid corrosion. It has 360 flat common mirrors with an area of 100 cm2 (10cmX10cm) each one, the mirrors are directed towards a focal area where a oil storage container is installed, this container has a total volume capacity of about 16 liter. The maximum volume of meal that is possible to be cooked at once is about 8 liter in a commercial pressure cooker.

The structure has two mechanic transmission systems, based upon pulleys and automotive belts, each one moved by a 12 V DC motor.

The concentration system consists of three planes at 0°, 15° and 30°, each mirror has two angle components so that it reflects Sun light over the oil storage container, this proposal is shown in FIG. 4 and FIG. 5

The Holding Point shown in FIG. 6 allows two axis movement so that the oven works 6 hours in Winter and 9 hours in Summer for solar insolation [3], FIG. 7 shows the Sun path at 19° North Latitude where Mexico City is located.

Fig. 7 Useful insolation for Mexico City

Further Development and Market Introduction

At the current status of the development of solar-hybrid gas turbine systems with pressurized volumetric receivers the main issues for further R&D are the verification of O&M assumptions for the receiver and the power conversion subsystem, further increase of the solar share and further cost reduction of the solar components.

Continuation of the 240 kWe solar-hybrid test operation at the PSA is foreseen until summer 2004 with a receiver for air outlet temperatures up to 1100°C. Another future option is the inclusion of high temperature heat storage systems, also leading to an increased solar share.

Although the cost predictions indicate potential competitive applications in the green power market, the introduction of this new technology is hampered by several factors:

Power production cost are still higher than with conventional fossil fuel options.

Up to now, only few possibilities exist for the funding of hybrid systems with fossil contributions above 30% (solar shares <70%).

Exploiting the full potential of high efficiencies of combined cycle plants (>50%) requires power levels above 50 MWe; this means very high investment cost which is not realistic for the introduction of a new technology. Therefore it is clear that market introduction is mainly possible at lower power levels, with the option for future scale-up. At power levels below 10 MWe, gas turbine systems are mainly used for decentralized power generation with cogeneration of heat or cooling power. First cost assessments for such cogeneration units indicated a potential for solar-hybrid gas turbine units [4]. Therefore the first step towards market introduction will be the design and installation of a prototype plant based on a small gas turbine or microturbine in cogeneration mode. Later upscaling will be done to power plants with combined cycle for high efficiency.


The cost-optimized design and performance prediction of solar-hybrid gas turbine plants in the power levels 1.4 MWe, 4.2 MWe and 16.1 MWe for two different locations were shown. An annual mean solar to net electric efficiency of up to 19% was calculated, belonging to the highest conversion efficiencies of solar electric technologies. The cost analysis showed total plant investment costs down to below 1500 €/kW for 2nd generation plants. Solar LEC of 12.8 €cent/kWh at a solar share of 53% are calculated for the largest system in daytime operation (capacity factor 54%). Going to higher power levels will further decrease the specific investment and O&M costs and increase the thermal efficiency. This will lead to a further reduction of solar LEC down to values predicted for other technologies.

The path to enter the market of high power level combined cycles was described. It is foreseen to start after successful long term testing with small scale applications in distributed markets using cogeneration units.


[1] Price H., Lupfert E., Kearney D., Zarza E., Cohen G., Gee R. and Mahoney R. (2002): Advances in Parabolic Trough Solar Power Technology. Journal of Solar Energy Engineering, Vol. 124, pp. 109-125.

[2] Buck R., Lupfert E. and Tellez F. (2000): Receiver for Solar-Hybrid Gas Turbine and CC Systems (Refos). IEA Solar Thermal 2000 Conference. March 8-10, 2000, Sydney, Australia.

[3] Heller P., Pfander M., Denk T., Tellez F. and Ring A. (2004): Test and Evaluation of a Solarized Gas Turbine System. Proc. 12th SolarPACES International Symposium, Mexico 2004 (to be published)

[4] Sugarmen C., Ring A., Buck R., Heller P., Schwarzbozl P., Tellez F., Marcos M. J. and Enrile J. (2003): Solar-Hybrid Gas Turbine Power Plants — Test Results and Market Perspektives. Proc. ISES Solar World Congress 2003, June 14-19, 2003, Goteborg, Sweden.

[5] Buck R., Brauning T., Denk T., Pfander M., Schwarzbozl P., Tellez F., Solar-Hybrid Gas Turbine-Based Power Tower Systems (REFOS). J. Solar Energy Engineering, 124, 2-9 2002

[6] Schwarzbozl P., Schmitz M., Pitz-Paal R. and Buck R. (2002): Analysis of Solar Gas Turbine Systems with Pressurized Air Receviers (Refos). Proc. 1lth SolarPACES International Symposium on Concentrated Solar Power and Chemical Energy Technologies, September 4-6, 2002, Zurich, Switzerland

[7] Pitz-Paal R., Jones S. (1998): A TRNSYS Model Library for Solar Thermal Electric Components (STEC). SolarPACES Technical Report No. 111 -4/98. Koln, Germany, 1998

[8] TRNSYS STEC web page: http://sel. me. wisc. edu/trnsys/trnlib/stec/stec. htm

[9] Schwarzbozl P., Buck R., Sugarmen C., Ring A., Marcos M. J., Altwegg P., Enrile J. (2004): Solar Gas Turbine Systems: Design, Cost and Perspectives. Proc. 12th SolarPACES International Symposium Mexico 2004 (to be published)

Storage Material Development

For the development of solid media storage material the thermo-physical properties of the materials, such as density p, specific heat capacity cp, thermal conductivity Л, coefficient of thermal expansion (CTE) and cycling stability as well as availability, costs and production methods are of great relevance. A high heat capacity (p*cp) reduces the storage volume and a high thermal conductivity Л increases the dynamic in the system. The CTE of the storage material should fit to the CTE of the material of the embedded metallic heat exchanger. A high cycling stability is important for a long lifetime of the storage.

Two different storage materials have been developed in parallel [2], as an innovative storage material a castable ceramic and alternatively, a high temperature concrete. Both developed materials are principally composed of a binder system, aggregates and a small amount of auxiliary materials.

The castable ceramic is based on a binder including Al2O3. The binder is set chemically under ambient conditions and forms a solid, stable matrix, which encloses the aggregates. As main aggregate iron oxides, accumulated as waste material in strip steel production, is used. Auxiliary materials are needed to improve the handling of the ready mixed material, for example as accelerator or for reduction of viscosity.

For the high temperature concrete blast furnace cement is used as binder, again iron oxides are used as main aggregate, as well as flue ash and again a small amount of auxiliary materials.

The material properties have been analyzed at DLR. The results are shown in table 1. Shear stress analysis has proven that the contact between tubes and material is very good at ambient temperature as well as at 350°C.


Castable ceramic

High temperature concrete

Density [kg/m3]



Specific heat capacity at 350°C [J/kgK]



Thermal conductivity at 350°C [W/mK]



Coeff. of thermal expansion at 350°C [10-6/K]



Material strength



Crack initiation

hardly no cracks

several cracks

Table 1 Material properties of storage materials developed at DLR

In an overall view high temperature concrete seems to be the more favorable material. Reasons are the lower costs, higher strength of the material and easier handling of the ready mixed material. However, the further development of cracks in the test modules needs to be investigated, when cycling at operation temperature has been demonstrated. On the other side castable ceramics has a 20% higher storage capacity and 35% higher thermal conductivity and still some potential for cost reduction.

Effect of porous materials

We use porous materials to strengthen the evaporation process by increasing the evaporation area.

2.2.1 Evaporation behaviors with and without porous materials

As we can see from the Fig.5, it is clearly that the technique combined with porous materials can strengthen the evaporation process greatly.

During the experiments, one side of porous materials is dipped into the seawater, and then seawater rises along the materials based on the capillary phenomena. We choose two kind of materials, gauze and non-woven fabric band, which have same area (95×70=6650mm2). And we also learn that non-woven fabric band is better than gauze for evaporation process. Compared with the process with and without porous materials, the efficiency aggrandizes remarkably. So, it is feasible that solar chimney dryer combined with porous materials. And different material has different effect to evaporation process because of different characters.

2.2.2 Evaporation behaviors at different wind velocities with porous materials

As we can see from the Fig6 and Fig7, it is clearly that the evaporation process is strengthened when the wind velocity increases, both for gauze and non-woven fabric band.

The wind velocity is an important factor to the evaporation process because it can remove the air above the surface of seawater quickly, which can enhance the humidity gradients. Compare with the evaporation process, the efficiency for evaporation increases when porous materials are used. And it is obviously effectual when wind velocity changes from 0.55m/s to 1.68m/s (45 C). The encumbrance effect of seawater mass transfer from porous material inside to surface is not very strong because the thickness of porous materials is very thin.

2.2.3 Evaporation behaviors with different pieces of porous material

We investigated the evaporation process with different pieces of porous materials at 0.55m/s to 1.68m/s (45 C). Fig.8-11 shows evaporation behaviors with different pieces of gauze and non-woven fabric band.

The evaporation area increases firstly with the number pieces of porous material increases but still gets a decrease with the further more pieces, except for the tendency in the Figure 11, which revealed that the more pieces of porous materials does not mean the quicker evaporation velocity. Although the increasing of evaporation area makes more water evaporated per minute, but the larger evaporation area will weaken the tendency and finally results in a smaller evaporation velocity. So it is another important factor to use porous materials in solar chimney dryer.

2.2.4 Effect of different location of porous material

We also investigate the effect of the mounted angles between the porous material plane and the wind, which were chosen as 0°,45° and 900. From Figure 12, it is obviously seen
that the effect to evaporation declines when angle increases, because it will generate more resistance to the wind. It is reasonable according to the hydrodynamics.

3. Conclusion:

1. The temperature is one of important influence factor to evaporation. The evaporation process can be strengthened greatly when the temperature increases among the experimental conditions.

2. The airflow is another important factor to the evaporation. The evaporation process can be also strengthened greatly when the airflow velocity increases. But it will be an encumbrance to the evaporation when the airflow is too high. The reason is that the high airflow velocity is a strong force to evaporation of seawater, and then a layer of salt appears to baffle the transfer of water from the main body of seawater to the air. So the airflow velocity should be controlled below 1.68 m/s, in case of affecting the evaporation process.

3. The combination of porous materials makes for the evaporation process greatly. As mentioned above, the gauze is dipped into the seawater and the seawater rises along the materials based on the capillary phenomena, which increases the evaporation surface. The additional function varies with the property of the porous materials. We can see clearly from the Fig.5, the non-woven fabric band is more effective than gauze because of the adsorption ability. The study of porous materials related to this technique will be investigated in the following days.

In conclusion, the solar chimney dryer combined with porous materials was a perfect technique to drying process. In this technique, temperature, airflow velocity, porous materials were important factors to the evaporation process. In this paper, we used wind tunnel investigating the drying process to simulate the situation in the solar chimney dryer and to study the influencing factors. In the following days, we would perform the study of the relationship between the wind tunnel and solar chimney.







drying velocity(kg s-1m-2)


airflow velocity(m/s)


ratio of water to dry materials(Kg/Kg)


averaged ratio of water to dry materials(Kg/Kg)