The positioning system can be viewed as a chain of referencing elements that starts at the sun position and ends at the collector receiver. In fixed mirror concentrators this chain is different from moving reflector systems such as parabolic dishes or parabolic troughs. In those systems the receiver is connected directly to the reflector through a fixed structure. Therefore, using closed loop control systems, the chain can be reduced to:

The precision of the relative position of the sun to the reflector is mainly determined by the precision of the sensor used. Thus, in the manufacturing process it is only necessary to reference accurately the reflector to the receiver, two elements that have no relative motion. In fixed mirror concentrators the former reference chain is slightly different:

In this case, the reflector and the receiver have relative motion and therefore should be linked through the tracking mechanism. Then, it is necessary to precisely reference the tracking mechanism to the reflector and the receiver to the tracking mechanism.

Another particular characteristic of the fixed mirror concentrators is that the average focus distance for a given reflector width is higher than in moving reflector systems. This fact increases the precision requirements of both the tracking mechanism and the mechanical referencing system. Furthermore, this large focus distance could be a handicap to building integration in those cases where the visibility of the collector system is not desired.

Given those special characteristics two main design choices were made:

1. The width of each reflector was reduced as much as possible (546 mm in the current prototype)

2. In order to reduce the cost of the tracking mechanism the reflectors and the receivers were arranged in arrays of eight lines of evacuated tubes sharing the same tracking and positioning structure.

The figure 5 shows a drawing of the final design of the current prototype. The tracking mechanism consists of four rotating arms that support and reference eight receiver rows. The receivers are standard “Sydney” evacuated tubes of 1.5 m in length and an absorber 47 mm in diameter.

A. Vidal*1, T. Denk2, A. Valverde2, A. Steinfeld3, L. Zacarfas4, J. C. de Jesus4 and

M. Romero1

1 CIEMAT, 28040 Madrid, Spain

2 PSA-CIEMAT, 04200 Tabernas (Almeria), Spain

3 PDVSA INTEVEP, 1070-A Caracas, Venezuela

4 ETH, 8092 Zurich, Switzerland Corresponding Author, alfonso. vidal@ciemat. es

Abstract

Hybrid solar/fossil endothermic processes, in which fossil fuels are used exclusively

chemical source for H2 production and concentrated solar power is used exclusively

energy source of process heat, offer a viable route for fossil fuel decarbonization and a transition path towards solar hydrogen. Research in recent years has demonstrated the efficient use of solar thermal energy for driving endothermic chemical reforming reactions in which hydrocarbons are reacted to form syngas. This process produces not only a highly useful and transportable end product, but also results in the storage of a significant fraction of solar energy in the chemical bonds of the fuel molecules.

The steam-gasification of petroleum derivatives and residues using concentrated solar radiation is proposed as a viable alternative to solar hydrogen production. Therefore, PDVSA, CIEMAT and ETH started a joint project with the goal to develop and test a 500 kW plant for steam gasification of petcoke. This report summari zes the major accomplishments and challenges of upscaling the installation at the SSPS — tower of the Plataforma Solar de Almeria.

Keywords: Solar Chemistry, Gasification, Central Receiver, Petroleum Coke, Slurry

1. Introduction

Gasification, which is a means to convert fossil fuels, biomass and wastes into either a combustible gas or a synthesis gas for subsequent utilization. The feedstocks include coal, natural gas (for reforming applications), refinery residues and biomass/wastes in combination with coal.

The use of high temperature solar heat to drive the endothermic reaction associated with coal gasification has been suggested and investigated in the last 20 years. The advantages of supplying solar energy for process heat are three-fold:

• Calorific value of the feedstock is upgraded

• Gaseous products are not contaminated by the by products of combustion; and

• Discharge of pollutants to the environment is avoided.

An important example of such hybridization is the endothermic steam-gasification of petroleum derivatives and residues (petcoke) to synthesis gas (syngas), represented by the simplified net reaction:

x

CHxOy + (1 — y)H2O ^ (2 +1 — y)H2 + CO

where x andy are the elemental molar ratios of H/C and O/C in petroleum tar, respectively. In a previous paper, the chemical thermodynamics and reaction kinetics of reaction were examined [1].

With regard to refinery residues (bottoms), these can take several forms depending on the design on the refineries and their products. In particular, our study comprises one type of these refinery residues: solid materials such as coke which is a by-product from the processing of heavy and extra-heavy oils using delay-coking technology. Application of these technologies is resulting in increased yields of low value refinery residues such as residual fuel oil, coke and petroleum tar, furthermore stringent environmental regulations appears to be reducing the markets for these residues — particularly petroleum coke [2].

The project of solar petroleum coke gasification is a joint cooperation between the company Petroleos de Venezuela (PDVSA), the Eidgenossische Technische Hochschule (ETH) in Zurich / Switzerland, and the Centro de Investigaciones Energeticas, MedioAmbientales y Tecnologicas (Ciemat) in Spain. The primary goal is to develop a clean technology for the solar gasification of petroleum coke and other heavy hydrocarbons.

The project is divided into three phases. In a first step, after performing in-depth studies of the thermodynamic and kinetic behaviour, a small 5 kW prototype was tested in the Solar Furnace of PSI / Switzerland [1]. Goal was to demonstrate the feasibility of the solar gasification, to determine critical process parameters, to identify possible difficulties, and finally to get a solid data base for the scale up step in phase 2. One important result was the decision to use slurry for the feeding of the reactor [3].

In phase 2, the design, construction, and operation of a 500 kW reactor are foreseen. The design of the reactor itself was done by ETH and upstream and downstream system by Ciemat. Construction is managed by Ciemat, and operation will be done at the SSPS-tower at the Plataforma Solar de Almeria during 2008. In phase 3 finally, a 50 MW solar gasification plant located in Venezuela will be designed.

Length and orientation of the collector rows had to be adapted to meet the constraints given by existing infrastructure at the site. Performance calculations with different sets of meteorological data for the region confirmed that the highest annual energy yield could be achieved with the collector tracking axis oriented to NNW, perpendicular to the wall of the adjacent production hall [4]. Due to inconsistencies in the radiation data available for that region from different sources, there is still a great uncertainty about the expected annual thermal energy provided by the system, ranging from some 9.300 kWh/a to 19.200 kWh/a. Some 1250 productive operating hours may be expected per year, with major proportion in part load (Figure 4).

Fig, 4: Expected annual load distribution of the solar steam generator

The methodology to integrate solar process heat in industry follows the pathway that was developed within the IEA Task 33 SHIP, subtask B “Investigation of industrial processes”. The approach highlights the importance of energy efficiency as a first step, before an efficient solar process heat plant is designed. The energy efficiency measures include energy demand reduction by technology optimization, but as well heat recovery possibilities within the whole production process (system optimization — heat integration). Based on the reduced heat demand, an optimized energy-consumption profile of the company can be developed, which is then used to plan the practical utilization of renewable energies, especially solar process-heat, for the production. Only with this approach a real economic and efficient solar process heat plant can be designed that fits well into the energy supply system of a company. The following reasons can support this statement

[5]:

• The optimisation of the production process reduces the overall energy demand and prevents an over-dimensioning of the energy supply systems (e. g. the solar plant).

• The technological optimisation of unit operations can result in different energy demand and different temperature levels of the processes. Both parameters have a large impact on the considerations of solar process heat.

• The investigation of processes leads to detailed know-how of unit operations and their operational data (temperatures, operation schedule etc.) and gives the necessary overview to identify the ideal integration for solar process heat into the production system.

Consideration of all possibilities of heat recovery and the use of waste heat (heat integration) makes sure that no supplementary heat is introduced to the production where recovered or waste heat is available as an already existing energy source.

The following steps are taken for the considerations on the integration of a solar plant in industrial processes:

• Data acquisition of all energy data within the company (energy demand and energy availability)

• Calculation of the overall energy balance of the company

• Reviewing of possible measures for enhancing energy efficiency (technological improvements, best available technologies, reduction of heat losses etc.)

• Calculation of the (theoretical) minimal heating and cooling demand with external energy sources

• Design of a heat exchanger network (heat integration)

• Definition of the heat demand that can be sensibly covered by solar thermal applications or other renewable energy sources

• Design of the renewable heat supply system

• Economic analyses

Tools for solar integration in industry

The Matrix of Indicators: Industrial sectors vary in structure and heat demand. Therefore a systematic approach is needed to describe the processes in energetic terms. Also, the minimization of the heat demand of an industry can be achieved by several approaches: (a) applying changes in the process (application of competitive energy technologies), (b) applying changes in the energy distribution system (application of heat integration systems) and (c) applying changes in the energy supply system (application of heat pumps/co-generation systems and/or application of solar thermal systems).

Solar thermal systems, on the other hand, vary in layout and design. Therefore a classification of the different hydraulic schemes is needed to point out their suitability to be applied for the energy supply in production processes.

In order to fulfil the above issues on a theoretical level, a tool that systematically includes process engineering and energetic information of industrial sectors with a potential for application of solar thermal systems has been developed. The aim was to design a decision support system that gives the user a large information database for all crucial steps that have to be taken when designing a solar heating system for industrial processes. These steps include the overview of the processes, important parameters of the energy supply of unit operations, benchmark data on energy consumption, competitive technologies, hydraulic schemes for solar integration and successful case studies.

Heat integration: A correct way to integrate (waste) heat into a process is described by the pinch theory [Ferner, Schnitzer, 1990] that was developed by Linhoff et. al. in the 1970s. With the pinch

analysis the heat and cold demand of the whole production is plotted in one diagram that shows the energy (heating or cooling) demand of the processes and at which temperatures this energy is needed. Some very important statements can be drawn from this analysis:

• How much energy can theoretically be saved by heat recovery?

• How much external heating demand does the production process have? Which temperature level is necessary?

• How much external cooling demand does the production process have? Which temperature level is necessary?

The pinch analysis is a strong tool for a first estimation of the energy saving potential by heat recovery (which later has to be adapted due to practical and/or economic reasons). Secondly, the analysis shows at which temperature levels the demanded heat/cold is necessary — important information for a possible solar process heat plant.

In the framework of the IEA Task 33 SHIP the development of a Heat Integration Tool started that is especially suitable for low temperature applications and batch processes. This tool could already be applied and tested for the Austrian case studies.

The basis for the application of the pinch analysis is the profound knowledge on the energy demand and energy availability streams of the production system. For the Austrian case studies, this data could either be acquired directly from the company’s data information system or measurements (ultrasonic measurements of fluid flows) had to be conducted.

Solar Simulation

The implementation of solar energy can either be done into the energy supply system, but as well directly at the process. An important aspect is to integrate the solar energy in no competition to possible available waste heat available within the process. Here the storage design plays a crucial role and the load management of waste heat, solar heat, energy supply and the demand schedules. In many industrial applications storages have to be implemented to achieve an increase in the efficiency of solar heat (and waste heat).

For the design of solar plants the Computer Software T-Sol was used. This tool allows the calculation of solar yields, collector areas, storage dimension and investment cost assessments of the solar plants. Based on the energy data achieved after the energy optimisation, the decision of the integration of the solar plant was done by the process engineers and the solar experts to achieve the highest yields of solar energy and the best overall economic system.

2 Results

In Styria, 10 case studies were conducted in 2007 in order to develop concepts for energy conservation and for the implementation of solar heat. In four of these ten case studies, detailed concepts, based on extensive measurements and calculations, were developed. The suggested measures in terms of heat-integration, technological innovations and the use of solar process-heat result in savings that amount to more than 28 Mio. kWh/a for all 10 companies, implying an annual reduction of 5.830 t CO2. The economically recommended collector-area for those companies, which the solar plant was thoroughly examined, was 2.790 m2 in total for 5 of 10 companies.

The following table gives an overview of the companies, their sectors, the solution that were drawn and the savings that could be achieved.

It is important to point out that most of the solar plants designed for the companies focus on the supply of hot process water that is stored in a central hot water storage tank. Partly this hot water is directly applied for production processes (company Nr.3 and Nr. 10), partly hot water is used for the general hot water household mainly for washing and cleaning activities (company Nr.2 and 6).

In both cases hot water is first transferred to the storage and if necessary temperature is elevated by the back up system. Again it is shown in the work with the case studies that storage plays a crucial role and that simulation is needed for complex storage tanks which are in the centre of several heat demand and heat availability streams with different schedules.

Additionally the case studies highlight the importance of demand reduction first. This includes energy efficiency measures and heat integration, but as well the optimisation of industrial processes to lower temperature processes. The economics of the overall project can be strongly

improved if the solar plant can operate with higher efficiency at lower temperature levels (company Nr. 10).

Economical estimation showed that for the case studies pay back times between 1-5 years could be reached.

3 Conclusions

Several potential studies, including the in this paper presented study within the project Styrian Promise for the region of Styria, show the high potential for the integration of solar heat in industrial processes.

For the realisation of this potential it is necessary to follow a specific methodology to ensure the application of solar thermal energy in an economic and sustainable way in industrial companies. Tools for this methodology have been elaborated in the framework of Austrian national projects and within the Subtask B of the IEA Task 33 SHIP. These tools have been applied and tested during the performance of the case studies.

The case studies showed the economic feasibility of the integration of solar thermal heat in combination with a full energy efficiency concept. The methodology applied leads to clear concept based on a detailed know-how of the energy demand structures of the company.

The work with the companies showed that crucial technical questions of future research will a. o. mainly focus on

the reduction of temperature requirements of industrial processes and

complex storage simulations for heat integration and solar applications in batch processes.

References:

[1] ECOHEATCOOL (IEE ALTENER Project), www. ecoheatcool. org: The European Heat Market, Work Package 1, Final Report published by Euroheat & Power.

[2] H. Schweiger et al.: POSHIP (Project No. NNES-1999-0308) The Potential of Solar Heat for Industrial Processes, Final Report. Barcelona, 2001

[3] C. Vannoni, R. Battisti. S. Drigo: Potential for Solar Heat in Industrial Processes. Study performed within Task 33 “Solar Heat for Industrial Processes” of the IEA Solar Heating and Cooling Programme and Task IV of the IEA SolarPACES Programme. Madrid, 2008.

[4] Sachs L. (1984), Angewandte Statistik.- 6. Aufl., Berlin-Heidelberg-New York-Tokyo

[5] C. Brunner, B. Slawitsch, K. Giannakopoulou, H. Schnitzer: Industrial Process Indicators and Heat Integration in Industry. Report performed within Task 33 “Solar Heat for Industrial Processes” of the IEA Solar Heating and Cooling Programme and Task IV of the IEA SolarPACES Programme. Graz, 2008.

The performance analysis was carried out for a heliostat located 100m north of the tower. The orienta­tion of the MMA is adjusted to obtain optimum annual reflection efficiency (270° azimuth angle, 37° elevation angle).

Three configurations of the MMA were evaluated:

• ideal MMA with antireflective coating (“ideal, ARC”)

• ideal MMA without antireflective coating (“ideal, noARC”)

• realistic MMA with antireflective coating (“real, ARC”).

The reference heliostat (“reference”) is included for comparison.

Figure 8 shows the results for the described configurations for noon at equinox (21.3. 12:00).

Details of the results are given in This characteristic is much more obvious when looking at the per­formance over a day.

Figure 10 shows the daily performance for the selected heliostat at equinox (21.3.) compared to the most optimistic case: the ideal MMA with antireflective coating. Although the MMA performance comes close to that of the reference heliostat at noon, it drops off much more at lower sun positions. As mentioned before, the main reason for this behaviour is the cosine loss.

Table 1. The MMA in the ideal configuration with antireflective coating shows a slightly lower performance
than the reference heliostat (about -5%). The greatest loss of reflection efficiency is due to the light twice pass-
ing through the cover glazing resulting in reflection and absorption losses. For the ideal MMA without antire-
flective coating, these effects are even more pronounced and make the MMA efficiency drop by about 13%.
Further, in the realistic case the reduced area of the mirror facets allows a significant amount of radiation to
pass between and outside of the facets, thus reducing the overall efficiency by about 18%, compared to the ref-
erence heliostat.

Figure 8: Performance of all configurations (21.3. 12:00)

Figure 9: Annual performance of all configurations

If the performance is evaluated on an annual basis, the MMA losses are significantly higher than that of the reference heliostat.

Figure 9 shows the annual performance of all cases. The difference to the previous situation is mainly the increase in cosine losses. The projected area of the MMA is affected by the fact that the position of the box is fixed, and the incidence angles of the solar radiation cover a wide range. In contrast, a con­ventional heliostat is always tracked in such a way that the mirror normally points midway between the

sun vector and the vector towards the receiver, i. e. the projected area is greater. The detailed results are given in table 2.

This characteristic is much more obvious when looking at the performance over a day.

Figure 10 shows the daily performance for the selected heliostat at equinox (21.3.) compared to the most optimistic case: the ideal MMA with antireflective coating. Although the MMA performance comes close to that of the reference heliostat at noon, it drops off much more at lower sun positions. As mentioned before, the main reason for this behaviour is the cosine loss.

Since the MMA’s can be installed beside one another without shading their neighbours, the MMA sys­tem allows a denser packing of the heliostat field. Due to their movement, conventional heliostats need a minimum spacing between them to achieve low shading. However, usual heliostat field layouts are optimized taking this effect into account, and the resulting shading losses are normally quite low. Nev­ertheless, the land use factor is better with a MMA type heliostat. Further analysis is required to find out how complete MMA heliostat fields perform, compared to conventional heliostat fields.

4. Conclusion and Prospects

The discussed mini-mirror array promises a low-cost heliostat design. A first demonstrator unit with a reduced size has been developed and built with high-quality materials. The present unit uses conven­tional step-motor drives and ball and socket joints. However, the low-cost potential is counterbalanced by approximately 30% less optical performance compared to conventional heliostat designs, mainly due to increased cosine losses. In the next step of the current project the performance of the test unit will be analysed in detail with the artificial sun of the SIJ and an ecobalance will be worked out. In further studies the heliostat field performance and the cost reduction options shall be carried out.

5. Acknowledgements

The project is funded by the German Ministry for the Environment, Nature Conservation and Nuclear Safety under contracts 03UM0074 and 03UM0075.

References

[1] F. Krawiec, J. Thornton, M. Edesess, 1980, An Investigation of Learning and Experience Curves, SERI, Golden, Colorado, Contract No. EG 77 C 01 4042

[2] G. J. Kolb, S. A. Jones, M. W. Donnelly, D. Gorman, R. Thomas, R. Davenport, and R. Lumia, 2007, Heliostat Cost Reduction Study, Sandia Report SAND-2007 3293, Sandia National Laboratories, Albuquer­que

[3] C. L. Mavis, 1989, A Description and Assessment of Heliostat Technology, Sandia Report SAND87- 8025, Sandia National Laboratories, Albuquerque[4] R. Buck, E. Teufel, 2008, A Comparison and Op­timization of Heliostat Canting Methods, J. Solar Energy Engineering 2008 (accepted for publication)

[5] F. Ansorge, J. Wolter, H. Hanisch, B. Hoffschmidt, M. Reindl, Device for Concentrating Light, Particu­larly Sunlight, International Patent WO 2006/005303 A1, 19.1.2006

As mentioned above, the experience gathered during the various central receiver technology projects 7 8 9 and materials treatment activities carried out in the PSA’s Solar Furnace1,2 have been combined in the design of the test assembly. Figure 2# below is a diagram of the main system components.

Air is blown through the ceramic absorber of the solar receiver where it is heated, supplying the energy source for an industrial, high-temperature process in the ‘treatment chamber’. The ‘treatment chamber’, which is right behind the absorber, is the main innovation with respect to a conventional volumetric receiver system.

A blower forces the air into the system and regulates the air temperature through speed variation. The air passes through the whole system, from the receiver to the blower. There is an auxiliary fresh air inlet just at the blower entrance, regulated with a by-pass valve, so the temperature of the air passing through is below 200°C which is the maximum allowed for the blower.

Fig. 2. Schematic view of the device and the air circuit

The receiver temperature can be controlled with the opening percentage of the Solar Furnace’s flux shutter.

The operator of the system is able to control both the blower speed and the aperture of the shutter on-line from the control room. The next step would be to implement an automatic control loop, able to handle both variables in order to make the reference temperature to follow a desired T-t profile.

The receiver has a 30 cm diameter to fit in the solar furnace focus. The main focus characteristics are shown in Figure 3 below.

Power information on Solar Furnace Target

Total Power…………………………… = 68 kV

Peak oflrradiance………………….. = 3029 kW/nf

Statistic analysis ol Irradiance distribution:

Slant Ranee…………………………… = 7.445 m

Centroid location………………….. = (0.013,-0.012) m

Peak location………………………… = (0.016,-0.017) m

90-Percent Energy Radius…….. = 0.132 m

Maximum rms-radtus…………… = 0.063 m

Minimum rms-radius…………….. = 0.058 m

……… = 1.08

Elhpticity direction………………… = 183.6 deg

rim.

Fig. 3. Flux map in the PSA’s Solar Furnace

For any solar thermal system involving solar collectors, the efficiency of solar collectors will always be needed to observe its performance. Instantaneous efficiency of the collectors is defined as the ratio of useful energy absorbed by the working fluid to the amount of solar radiation falling on the collector.

The Coefficient of Performance (COP) is used to describe the performance of a heat pump system. It is defined as the ratio of energy released in the condenser coil and water tank to energy used to run the compressor.

where Xpp is defined as:

3. Results and Discussion

The meteorological condition for the day during the experiment conducted in December is shown in Figure 2. It can be seen that the ambient temperature is relatively constant with a value close to 30 °C. The maximum solar radiation value reaches around 1000 W/m2, and it varies during the period of measurements with the lowest value around 400 W/m2

The efficiency of the liquid solar collector is shown in Figure 4. The collector, at 2 m2, has an efficiency in the range of 50 to 60 percent. As shown in Figure 5, the coefficient of performance (COP) tends to have a relatively stable value. It varies between 8 and 9.

From Figure 6, it can be seen that the maximum water production rate is close to 0.9 kg/hr. The performance ratio is a function of water production rate, thus the change of production rate will affect the performance ratio. From Figure 6, the performance ratio is shown to be close to 1.3.

О

<1)

О

c

TO

E

о

t

<1)

CL

Once the models of each component of the circuit are validated, these models are interconnected to simulate the whole system, as shown in Figure 8.

The following “causalities” are observed:

• the pump imposes the refrigerant flow rate,

• the expander imposes its supply pressure and exhaust temperature,

• the evaporator imposes the refrigerant superheating and the pump exhaust pressure,

• the condenser imposes the expander exhaust pressure and the pump supply pressure.

This simulation model is not fully predictive, because the subcooling at the exhaust of the condenser is defined as a model input. In order to predict this subcooling, a refrigerant charge model would have to be included.

Figure 9 shows the prediction of the output shaft power for the global cycle model. All measurements are predicted within a 10 % accuracy. Errors of each model are indeed cumulated, which leads to a lower accuracy for the global model than for the individual components.

4. Conclusion

This paper proposes a semi-empirical model of an ORC involving a relatively limited number of parameters. The comparison between predicted values and experimental results show a fairly good agreement (for the cycle model as well as for the different sub-models).

The experimental study carried out shows a good expander isentropic effectiveness and demonstrates the viability of utilizing a mass-produced compressor as an expander in a small scale ORC. This represents an important step towards realizing the cost reductions that would make a kilowatts-sized Solar ORC economical for developing countries.

That the overall cycle efficiency remains limited is partly explained by the low temperature of the heat source and by a low pump efficiency. The former can be rectified by using higher temperature heat sources, while the latter can be addressed by selecting of a pump optimized for the pressure ratio and flow rate of the ORC.

Future work will focus on the integration of the ORC model into a global model including the solar collector in order to size the system, define a control strategy, and optimize the working conditions and the components.

References

[1] Hung, T.-C., Waste heat recovery of organic Rankine cycle using dry fluids, Energy Conversion and Management (2001), Vol. 42, pp. 539-553..

[2] Incropera F. P., D. P. DeWitt, Fundamentals of Heat and Mass Transfer (2002), John Wiley & Sons

[3] International Energy Agency (2006), World Energy Outlook 2006, Paris

[4] E Kane, M., Larrain, D., Favrat, D., Allani, Y., 2003, Small hybrid solar power system, Energy (2003), Vol. 28, pp. 1427-1443

[5] Klein S., Beckman W. (2008), Engineering Equation Solver (EES), University of Wisconsin, Madison

[6] Liu, B.-T., Chien, K.-H., Wang, C.-C., Effect of working fluids on organic Rankine cycle for waste heat recovery, Energy (2004), Vol. 29, pp. 1207-1217.

[7] Lemort, V., Teodorese, I. V., Lebrun, J., Experimental study of the integration of a scroll expander into a heat recovery Rankine cylce, Proc. 18th Int. Compressor Engineering Conference at Purdue (2006), Purdue University, West Lafayette, C105.

[8] Maizza, V., Maizza, A., Unconventional working fluids in organic Rankine cycles for waste energy recovery systems, Applied Thermal Engineering (2001), Vol. 21, pp. 381-390.

[9] Price, et. al. Advances in Parabolic Trough Solar Power Technology, Journal of Solar Energy Engineering (2002), Volume 124, Issue 2, pg. 123

[10] Wei D., Lu X., Lu Z., Gu, J., Dynamic modeling and simulation of an Organic Rankine Cycle (ORC) system for waste heat recovery, Applied Thermal Engineering (2008), Volume 28, pp. 1216-1224

[11] Wenqiang, Liu, et. al. Techno-economic assessment for off-grid hybrid generation systems and the application prospects in China, World Energy Council (2007)

[12] Yamamoto, T., Furuhata, T., Arai, N., Mori, K., 2001, Design and testing of the Organic Rankine Cycle, Energy (2001), Vol. 26, pp. 239-251.

Another critical point for the outcome of the project was the reflector manufacturing. Despite the low geometrical concentration ratio of the device (about 11), due to the large focus distance, the optical precision required it is similar to parabolic troughs that work with concentration ratios several times higher. On the other hand, in order to integrate the collector in building roofs, weight must be kept as low as possible.

For the reflecting surface, thick glass mirrors with permanent curvature were discarded almost from the beginning due to their high weight, high production costs for small manufacturing series and the low curvatures required. Therefore the two main alternatives considered were:

1. Glass on Metal Laminates (GOML)

2. Solar grade specular aluminium sheets

Eventually the second option was chosen mainly because of weight considerations. Aluminium sheets can be used in sandwich structures filled with polyurethane foam. In this process polyurethane is injected between two metal sheets inside a mould and the pressure due to expansion of the foam forces the metal sheets to take the shape of the mould (Fig. 6).

With this method small reflector pieces (1 x 1.5 m approx.) can be easily produced (Fig 7.a) and then this pieces are assembled to the reflector frame (see Fig. 5. and 7.b) to form the complete reflector of 4.5 x 6 m.

7.a)

An accurate control of the reaction conditions of the foam is of critical importance for the optical quality of the obtained surface because incomplete reactions lead to surface distortions. Nevertheless, under controlled conditions the optical quality obtained is excellent. The figure 8 shows a comparison between a theoretical radiation distribution on the focus, estimated with ray tracing, for a prefect geometry and assuming a material dispersion of о = 7.5 mrad, and the experimental curve obtained from a photograph of the focus for normal incidence.

Despite the importance of this component for the performance of the collector, at the beginning of the project it was decided to try to use market available solutions whenever possible, instead of developing a new component.

Nevertheless, as can be seen in figure 2, at different times of the day the solar radiation reaches the receiver from a different direction. Therefore either the absorbent surface should be cylindrical or it should be oriented to the centre of the reflector for each incident angle.

The orientation of the receiver surface could be an interesting option, particularly if the concentrator were to be used in combined thermal/PV applications, but it was decided not to implement it in the first prototypes in order to reduce the amount of mechanisms.

Therefore a receiver with a cylindrical absorber with a diameter between 45 and 55 mm was required. The only type of absorber available with this geometry were the Sydney evacuated collectors. In this kind of tubes, the heat is primarily absorbed by the inner glass layer and then transferred to a U shaped copper tube or to a “heatpipe” through an aluminium fin. In the figure 9.a a section of the tube with a U-pipe is shown.

The main advantages of this kind of collector are its low cost and a very good sealing of the evacuated volume. On the other hand, the low conductivity of the glass absorber requires high temperature increments in order to evacuate the incident radiation. This problem is increased by the fact that it is very difficult to ensure a perfect contact between the glass and the aluminium fin, and between the aluminium fin and the copper tube. In the figure 9.b the simulated temperature distribution is shown for the case of the concentrated radiation reaching the tube laterally. In this model, only conduction between the different elements has been taken into account, and an air filled gap of 0.1 mm is assumed at both the glass-aluminium and the aluminium-copper interfaces. Although a more complex model would be required in order to accurately predict the absolute temperature values, the results obtained are representative of the relative temperature gradients between the glass zone and the metal parts.

Those relatively large temperature gradients can produce a low efficiency of the collector and breaking of the glass tube due to thermal stresses. Nevertheless it is difficult to theoretically predict those problems because they are highly sensitive to factors, such as the perfect contact between the different parts, that are very difficult to control. Therefore it was decided to experimentally evaluate the collectors for normal incidence (Fig. 10).

For an input fluid at a ambient temperature (21°) and a normal direct radiation of 750 W/m2, the efficiency obtained for the reflector-receiver system was about 70 %, and the stagnation

temperature of the tubes was 297 °С. Due to time issues no other efficiency measurements were carried out, and the obtained values were considered to be sufficient for the first prototype.

Fig. 10. Experimental setting

Regarding to the breaking of the tubes, during the stagnation tests no breaking of the tubes was observed. During the efficiency tests, only two failures were observed, and both of them were produced during manipulations or sudden changes in the fluid regime that could be easily avoided during normal operation. Therefore it was decided to use standard Sydney evacuated tubes for the receiver of the collector.

Nevertheless, during the first tests of the first collector prototype several tube failures have been observed. Most of them have been originated at either one of the tube tips (Fig. 11). Although a complete study of the causes of the breaking is still not completed, it is likely that most of the failures were related to the thermal stresses induced at the tube tips. Whether it is possible to reduce those thermal stresses with an improved fin design is still under analysis.

The SSPS-CRS plant was inaugurated as part of the International Energy Agency’s SSPS project (Small Solar Power Systems) en September 1981. At the present time, as with the CESA-I plant, it is a test facility devoted mainly to testing small solar receivers in the 200 to 350kW thermal capacity range. The heliostat field is made up of 92 39.6 m2 first generation units. Under typical conditions of 950 W/m2, total thermal capacity of the field is 2.7 MW and peak flux obtained is above 2.0 MW/m2. 99% of the power is collected in a 2.5m dia. circumference and 90% in a 1.8m circumference.

For these experiments, two height levels at the tower were taken into consideration to find the optimum location. For this experiment, the receiver reactor will be placed at the 40m-level of the SSPS-tower. The performance that can be expected from the receiver was presented in a previous paper [4].