Category Archives: EuroSun2008-2

Wood drying processes

To dry wood the surrounding air must be sufficiently dry so as to absorb its moisture. This can be accomplished either by ventilating or heating the kiln air. During the first stage of the drying cycle, air easily absorbs the moisture and relative humidity inside the chamber may keep close to 100%, with water often beading on the walls in a based-greenhouse structure. As the process evolves wood moisture expelling turns increasingly difficult, mainly due to the low diffusion speed of moisture in wood. At a final stage, when all free water has been lost, only cell bonded moisture is left to be extracted. This final stage is more time and energy consuming, since it requires additional energy supply to break the bonds. Quality regards are present in the intermediate and final drying stages. In this process temperature, relative humidity and wood moisture content are the most relevant quantities [2-3Conventional drying

The main purpose of lumber air drying is to evaporate as much water as possible before end use or transfer to a kiln drier. Air drying can usually proceed until wood moisture content attains 25% to 20%. Another drying methodology must follow if a lower target value is desired. Air drying saves energy costs and reduces required dry kiln capacity, but presents the usual limitations of an uncontrolled process: in winter months drying rates cold be very slow, particularly in raining periods. By other hand under summer hot dry winds wood quality may be degraded as a result of surface shrinking and end splitting, due to severe differential drying (surface vs. interior). Another drawback of this method is the space and long time storage costs of wood stacks, implying large immobilization periods [4]

In kiln drying processes, higher temperatures and faster air circulation are used to considerably increase drying rate. Specific drying schedules/profiles have been developed to control temperature and relative humidity in accordance with the moisture content and stress situation within the wood, in order to minimize shrinkage-caused defects and improving quality. Conventional drying is one of the most expensive processes in wood industry, due to the enormous thermal energy expenditure

[3].

Concerning the advantages of higher temperatures

3.1. General remarks

In general solar energy can be used the better the higher the temperature level is. Even if it can be used exceptionally at low temperatures, e. g. for swimming pool heating, for water heating resp. preheating, for preheating for room heating support or for increase of the back-flow temperature, these processes could be much more efficient at higher collector temperatures. This is to be explained in the following.

3.2. High collector temperatures increase the efficiency factor of the installation:

The conventional backup heating can only be avoided efficiently and permanently if the solar outlet temperature is permanent considerably higher than the relevant desired temperature. As the efficiency factors of boiler and installation are especially bad out of the heating season, every single switching of the boiler avoided by solar heat saves many times the amount of energy that the solar collector system feeds into the tank. If e. g. a boiler working with oil with an efficiency factor of 90 % and an internal water volume of 60 l has to heat this water from 20 °C to 70 °C before it can heat 150 l hot water in the tank by 5 K, only 16 % of the consumed fossil fuel is used for water heating. Has the solar collector system, however, heated the hot water tank sufficiently and the boiler does not need to start, it saves with every solar kilowatt-hour 6.25 kilowatt-hours of fossil energy.

In addition to its annual solar energy harvest the solar collector system helps to save an amount of energy by avoiding boiler and installation losses that can be as large or even larger than the pure solar energy harvest. Low temperature collectors can never have this synergetic effect, not even in summer, because they need delta-T-controllers which usually prevent high temperatures.

Permanently high collector temperatures are coupled with big temperature differences what decreases the volume flow demand. Thus electrical pump energy and pump working time is saved.

New testing facilities are needed

In the development of flat-plate collectors and vacuum tube collectors for domestic hot water and for room heating applications it was sufficient to carry out collector efficiency measurements up to collector inlet temperatures of about 100°C. But the situation is different for the development of collectors which will have their main operating temperature in the range of 80 to 250°C. It is essential to carry out efficiency measurements directly at these high temperatures and not to rely on extrapolations from measured efficiency points at lower temperatures.

Подпись: Figure 6: Efficiency curve measurement for an evacuated tubular collector with a CPC reflector exposed to the solar simulator. The new testing unit, with which accurate measurements up to 200 °C can be carried out, can be seen at the bottom left of the photo.

Therefore, at Fraunhofer ISE a new test facility was developed with which we can determine collector efficiencies at measuring temperatures up to 200 °C. It can be used in indoor measurements (with our solar simulator, see figure 6) and outdoor (with the tracker) [7].

Figure 7 shows examples of measured efficiency curves of three different collectors. All curves were measured in the solar simulator laboratory (indoor measurements) and with the new testing facility.

The measurement points at the highest temperatures were taken at mean fluid temperatures of about 185° — 190°C for all three collectors. The actually measured efficiency points are indicated in the diagram for the evacuated tubular collector 1. It is a collector without a CPC reflector and with relatively narrow gaps between the single evacuated tubes. These measurements were taken at an irradiation of 931.9 W/m2. Therefore also the two other curves are given for this irradiation which is necessary in order to plot all three curves in one diagram. In all measurements the ambient temperature was in the range of about 30°C. The second evacuated tubular collector uses a CPC reflector. The heat losses based on the aperture area are therefore smaller and the efficiency curve is higher than for collector number 1 at higher operating temperatures. The efficiency curves show that both evacuated tubular collectors are suitable for system applications in which the collector operating temperature is in the range above 100°C and may be up to 150°C. The flat-plate collector has a highly selective absorber coating and is glazed with a single, anti-reflectively coated glass.

Подпись: 0 0.05 0.1 0.15 0.2Подпись:Подпись: evacuated tubular collector 2 (with CPC reflector) image278re

£

3

t

Q.

re

£

(Tfiuid-Tambient)/G in (K m2)/W

Figure 7: Measured efficiency curves of three different collectors (indoor measurements with solar simulator, highest mean collector temperatures about 185° — 190°C for all three collectors. The dots show the actually measured efficiency points for collector 1 and the mean collector fluid temperature in the measurement.

3. Conclusions

For a summary on the achievements of the work of IEA-SHC Task 33 SHIP considering the development of process heat collectors I want to make the following statements:

• A good start has been made. Different new collectors for the operating temperature range of 80 to 250°C are under development. The Task had a positive triggering and integrating impact with a high degree of information exchange.

• With regard to realized demonstration plants and built application systems, it has to be stated that almost all of them are working at temperatures below 100°C. Of course, there is also a big potential for solar heat in industrial processes below 100°C, but the full potential for heat up to about 250°C can only be used if new and appropriate collectors are developed. As mentioned, a good start is made, but a lot of development work and research activity is still needed.

• New requirements have to be fulfilled with respect to collector components, materials and system components of the solar loop:

— appropriate heat transfer fluids sufficiently temperature stable, anti-freeze properties, efficient thermodynamic and hydraulic performance

— cost effective reflectors with high performance and long service time

— tracking systems which are reliably operating for the whole service time

— appropriate receivers (with and without selective coating, applicable in vacuum of in atmospheric conditions)

— appropriate and temperature stable pumps, piping and connection systems, heat exchangers and valves and other system components.

• New collector testing standards and testing facilities are required, especially for CPC and concentrating collectors. The aim must be to achieve a full technical and economical comparison for the full range of the collector technologies.

Acknowledgement

I want to thank all colleagues who contributed to Subtask C and to the booklet Process Heat

Collectors. The work contributed by Fraunhofer ISE has been carried out with financial support by the

German Ministry for the Environment, Nature Conservation and Nuclear Safety.

References

[1] Werner Weiss, Irene Bergmann, Gerhard Faninger, (2008), Solar Heat Worldwide — Markets and Contribution to the Energy Supply 2006, Edition 2008, http://www. iea-shc. org/publications/statistics/IEA- SHC_Solar_Heat_Worldwide-2008.pdf

[2] Werner Weiss (2008), Solare Prozesswarme — Potenziale, Einsatzbereiche und Herausforderungen fur die Solarindustrie, 18. Symposium Thermische Solarenergie, Bad Staffelstein 23.-25. April 2008, pp159-164.

[3] Henning, Hans-Martin (Ed.) (2004), Solar-assisted air-conditioning in buildings — A handbook for planners, Springer Verlag Wien New York, ISBN 3-211-00647-8

[4] M. Wieghaus, J. Koschikowski, M. Rommel, (2008) Solar desalination for an autonomous water supply, Desalination and Water Reuse; for more information visit www. solarspring. de

[5] Chr. Thoma, Th. Weick, J. Richter, Th. Siems, M. Rommel (2008) Testing fo solar air collectors, Proceedings of Eurosun 2008, Lisbon

[6] Werner Weiss and Matthias Rommel (Ed.), (2008) Process Heat Collectors, downloadable from http://www. iea-shc. org/publications/downloads/task33-Process_Heat_Collectors. pdf.

[7] M. Rommel, K. Kramer, S. Mehnert, A. Schafer, T. Siems, C. Thoma, W. Striewe, (2007) Testing Unit for the Development of Process Heat Collectors up to 250°C, estec 2007, Proceedings of the 3rd European Solar Thermal Energy Conference, June 19-20, 2007, Freiburg, Germany, pp 414-418

Implementation of the model

Modern computer based simulation models have been developed and adapted to analyze the performance of solar-hybrid gas turbines in commercial system size [4]. Software tools have been developed for combined heliostat field and receiver layout. Some very interesting results have been presented by modeling the power plant with TRNSYS using the STEC model library for annual performance simulation [5]. This model was able to estimate the annual electric output for various design parameters (solar field size, storage size, power block efficiency) and different simple operation strategies. For the whole system of a solar tower power plant consisting of a solar and a conventional part a new model is necessary to be developed for the description of each component of the power block.

The implementation of the solar tower power plant model has been done in the MATLAB/Simulink environment.

MATLAB is a high-performance language for technical computing. It integrates computation, visualization, and programming in an easy-to-use environment where problems and solutions are expressed in familiar mathematical notation.

Simulink is a toolbox in MATLAB that provides an environment for modelling, simulating, and analyzing dynamic systems. It supports linear and nonlinear systems, modelled in continuous time or a sampled time. The implementation of systems can also be multi-rate, i. e. have different parts that are sampled or updated at different rates [6].

Concrete Floor Slab as Heat Store

Simulations were carried out for concrete slabs of 20, 40 and 60 cm thickness. However, the piping of the underfloor heating system was always installed at a depth of 10 cm. The floor thickness shows only a small sensitivity with respect to the solar fraction that can be reached with the system. Doubling or even tripling the floor thickness leads to an increase in solar fraction of only a few percentage points. Therefore, a floor thickness around 30-50 cm is a reasonable value. Floor slabs in this order of magnitude are commonly used in many industrial buildings anyway.

When the results are plotted against the utilization ratio, as it was done for the system concept with storage tank, the points scatter significantly more (see Figure 6). The highest solar fractions are obtained with the standard reference case. In Case 4 (8 kW internal gains), solar fractions are lower because during the day the machine operation leads to high air temperatures and high floor temperatures, therefore the solar thermal system cannot store much heat in the concrete mass. In this case, using the thermal mass as a storage tank is particular unfavorable because the solar energy cannot be stored to be used during the night. In Case 2 (poorly insulated), solar fractions are again lower. In this case, the overall heat demand of the building is much higher and as a result the conventional heating system is turned on more often. The conventional heating system also uses the underfloor heating system, which leads to higher average floor temperatures. If this is the case, then the potential for solar heat to be stored in the floor is decreased, which in turn decreases the solar fraction that can be reached. This effect is even more pronounced in Case 1 (poorly insulated and high air exchange rate). In this case, the overall heat demand of the building is so high that the

Подпись: Figure 6. Nomogram to determine the collector area, the solar fraction and the specific collector yield. The graph is based on a concrete slab thickness of 40 cm. In Figure 6, the light-colored area shows the band of the solar fractions for solar heating systems in factory buildings that use only the concrete slab as heat store. The light-colored area excludes Case 1 because the simulation results have shown that using only the thermal mass as heat store for buildings with very high heat demands does not make sense. The specific collector yields can be significantly higher compared to the cases where a storage tank was used. The reason for this is that the control strategy for thermal mass usage “overheats” the building to a certain extent. This does not necessarily decrease the auxiliary heat demand.

conventional heating system has to be turned on almost all the time. Therefore, solar fractions are decreased dramatically.

Regarding the nomogram for systems using the concrete slab as heat store, it is even more important to remember that the results shown can only give a rough approximation of the solar fractions that can be reached. The results are only strictly valid for the reference buildings described above. Systems without water store are even more sensitive to a change of boundary conditions as could be shown with the different reference cases considered. If for example, a building is much higher than the reference buildings or a higher air temperature is required, this can have a strong influence on the function of the concrete slab as a heat store. It not only increases the heat demand of the building but also decreases the solar fraction that can be reached. Therefore, a more detailed simulation of the building and the heating system may be necessary if the thermal mass is to be used as the only heat store.

As for systems that use a water storage tank, systems should be designed in accordance with the optimum cost-to-benefit ratio (orange area, degrees of solar fraction of the overall heating requirement between 15 and 30%). Degrees of solar fraction of less than 15% are outside the cost- to-benefit optimum since the (slight) rise in the specific yield does not make up for the higher specific system costs of a smaller solar thermal system and would thus lead to higher solar heating costs.

System simulation parameters

Considering the preliminary assessment of a small scale shaft power generation system, yearly simulations where performed, for three different locations, after the following parameters:

• ORC operates at full load (i. e., whenever QinORC is available);

• heat produced in the solar field at ToutSolar = T2 + ATm (i. e. variable fluid flow in the solar field circuit);

• inlet temperature Т1пВЫаг calculated after constant outlet temperature condition and heat exchanger parameters (vide Table 1);

• heat stored in unstratified storage system at T2 + ЬТШ;

• heat delivery priority levels: 1. solar field; 2. storage system; 3. backup system;

• circuit and storage heat losses neglected.

Solar field inlet temperature calculation follows heat exchanger operation, a minimum temperature difference of 5 K and 10 K between heat transfer fluid and working fluid of the ORC was adopted for cycle 2 or cycle 3 respectively, according to regenerator outlet (T), evaporation (Tevap), and

superheating (T2) temperature values [17]. Average heat addition temperature (Ty-2) rounds 113.5 °C, for cycle 2, and 166.6 °C for cycle 3 conditions.

Подпись: QT image237 Подпись: (1)

The use of thermal energy storage (TES) is considered regarding a daily increase of solar based operation. The size of the TES (QTES. max) is given in terms of storage time, traducing, for a general storage material, the amount of energy required to run the system for a given time period (AtTESmax), according to equation 1:

It is important to refer that such simplified system operation conditions are likely to penalize solar field results, considering that no heating regimes are considered in the non stratified storage system and that the solar field is forced to operate permanently under maximum temperature conditions.

Yearly system performance calculations where carried out for three different locations: Almeria (Spain), Cairo (Egypt) and Moura (Portugal) after hourly average data series for global horizontal and diffuse irradiation, as well as ambient temperature, whose average monthly values are presented in figures 2 a) and 2 b).

Regarding a dimensioning assessment of both solar field and storage system, system configurations with and without energy backup where simulated after the following dimensioning parameters:

• total collector area heat є [1500, 2000, 2500, 3000, 3500, 4000, 4500] (m2);

• storage capacity є [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12] (h).

Diversity in brewing

As can be seen from the huge amount of breweries, there is a big variety of beers and every beer has more or less its individual recipe. One can hardly find two identical brewing processes in two different breweries, which is also influenced by the personal preferences of the respective brewer. Besides variations in used raw materials such as grains, hops and yeast, there are differences in time periods and temperature ranges during the wort production. Additionally, there are various technical installations available for mashing, lautering and boiling as well as heating or cooling the wort. This leads to the fact that the detailed knowledge of the brewing process in Brewery “A” might not result in a similar knowledge of any other brewery “B”. Within the brewhouse, there is a great difference in existing mashing and boiling systems.

At the Hutt brewery, the brewing process starts with mashing by mixing crushed malt with 58°C hot water in the mash tun. Within the next two hours, the mash follows a defined time-temperature profile with various temperature rests. Therefore, the shell of the mash tun is heated by steam. The principle of mashing (heating a mixture of water and grains) is more or less the same in all breweries. The diversity is based on the starting temperature, heating rate, way of heating the mash, set temperature and time for mashing as well as the used number and types of the mash tuns.

In the next process step called ‘lautering’, the resulting liquid is separated from the grains. Besides straining the mixture, the resulting draff is washed with hot water (around 80°C), to extract additional sugars. The variety of lautering processes in terms of consumed thermal energy is relatively small compared to mashing or boiling. Main differences can be found by the used lauter tun units. The temperature and proportion of hot water for this process step are more or less in a similar range.

Afterwards, the wort boiling takes place, which is the most energy intensive process step within the brewhouse. Before the wort is boiled, it has to be pre-heated from lautering temperature (around 75°C) to boiling temperature. This can be done by different methods, such as using the boiling copper or an external heat exchanger that is fed by steam, by high pressurised hot water or by recovered heat. After pre-heating, the wort is boiled for a fixed period, while hops are added to the wort. The respective boiling time and temperature is directly linked to the desired amount of evaporated water and the installed boiling system. This leads to different boiling times for different beer recipes or breweries. Another variation during wort boiling is given by the installed heat recovery system. At the Hutt brewery, the occurring vapours are condensed to heat brewing water that is temporarily stored in a hot water tank. Based on the respective installation, the recovered heat can also be used for pre-heating of lauter wort or boiling itself. For pre-heating of lauter wort, the evaporated water is condensed and heats water in a closed heat recovery cycle to a preferably high temperature level. This water is used to heat the lauter wort with a special heat exchanger from 75°C close to boiling temperature. The second possibility, using the recovered heat for wort boiling, is realised by using a special vapour compressor (thermally or mechanically driven). The compressed vapour can be used to heat the wort during boiling with a special heat exchanger.

After boiling, the so called ‘hot trub’ (remaining solid particles) is separated from the wort within a whirlpool. The wort is pumped tangential into the whirlpool, which causes a sedimentation process. After leaving the whirlpool, the wort is cooled by a double-stage heat exchanger. At first, the wort is cooled to approximately 15°C while cold brewing water is heated to 80°C and fed to a hot water storage tank. In a second step, ice water is used to cool the wort to a temperature below 10°C. The separation within the whirlpool is rather similar in all breweries. Solely the temperature level of hot wort can vary, which is based on the respective boiling process. The heat recovery installations for wort cooling are also comparable within different breweries. Usually, the amount of produced wort is similar to the amount of brewing water, heated within wort cooling, and sufficient to cover the demand for mashing and lautering. After wort cooling, the wort production is finished and the wort leaves the brewhouse.

Solar drying

In solar kilns thermal energy comes from solar radiation and can be a reasonable and promising method for almost any wood industry to gain the capacity to dry wood at reduced costs [5-9]. Known past implementations rarely use control embedded in the drying process [11], resulting in poor quality and dry time improvements. However, current instrumentation capabilities allow cost effective control solutions.

In the SECMAD Project, wich means “energy efficient wood drying”, the whole concept of a kiln drier has been reviewed [10,12-14],, namely to reduce its cost and enhance solar energy collection (side walls, double ceiling, etc.). The concept uses natural and mechanical ventilation controlled by an instrumentation and control system, accounting for both internal kiln and external environment conditions.

Figure 1 illustrates the driers prototypes and Figure 2 shows the ventilation and heating concept, with colors indicating the inside conditions of the air: red arrows give indication of direction of hot and dry air while blue ones indicate the direction of cold and humid air. Green arrows indicate the entrance of outside air, being heated by solar air collector at side wall and ceiling. Moisten air is expelled through vents by forced flow, while the fan is turned on. This solar and ventilation dryer is intended to be considerably faster than the traditional open air method and much lesser energy consumption and more cost expensive than conventional kilns.

Two prototypes were installed in two different industries. Prototype I is more appropriate for fresh water charged lumber, which is necessary to remove as fast as possible in order to avoid mould and blue stain when drying softwoods. Prototype II is more appropriate to dry products that need higher temperature and prove easier to remove the water without risk of checks and deformations (poles, agriculture fruits, etc.).

Dimensions of kilns are represented in both figures. Figure 1 prototypes could accept between 40 to 75m3 usable wood volume with equivalent dimensions: 4m inside height; 10 to 20m inside length; 5,60m inside width, but stacking of the wood, ventilation requirements and spaces from the rear wall and from front door gives a actual less wood processing.

Подпись:(Prototype II)

The model of temperature distribution and air circulation is shown in figure 2. The air speed on the ventilators was 13,5 m/s witch provides a volumetric speed of 160m3/min, or in other words, an average of 50 renovations h-1. The average air speed on the wood boards surface was 1,7 m/s on the stacks near the ventilators and 0,7 m/s on the surfaces of stacks more far way.

image252

Fig. 2 — Heat model concept of lumber kiln dryer (cut sights).

High collector temperatures increase the storage capacity

The solar storage tanks display their storage capacity not until the collector temperatures are higher than the desired consumption temperature. The higher the „over-temperature“ in the tank is the higher the storage capacity.

Assumed the heating temperature is adjusted at 60 °C and the back flow has 50 °C. A high temperature collector with low volume flow feeds a buffer tank (start temperature 60 °C) to 90 °C in one step. For every litre at 90 °C 4 litres of water at 60 °C can be fed into the heating network. A low temperature collector needs 3 times the tank volume to yield the same heat energy at 70 °C. If the heat from the buffer tank is used through a heat exchanger, the usable water quantity will be

much smaller and the cooler buffer of less value because the cooler the storage tank is the more usable heat (exergy) is lost by heat exchange.

Optical Design of a High Radiative Flux Solar Furnace for Mexico

C. Estrada1*, D. Riveros-Rosas1, J. Herrera-Vazquez2, Sergio Vazquez-Montiel2, Camilo A. Arancibia-Bulnes1, C. Perez-Rabago1, F. Granados-Agustfn.

1 Centro de Investigation en Energia, Universidad Nacional Autonoma de Mexico. Av. Xochicalco s/n;

Temixco, 62580 Morelos, Mexico.

2 Institute Nacional de Astrofisica, Optica y Electronica. Luis Enrique Erro 1, Tonantzintla, A. P. 216, 72000

Puebla, Mexico.

* Corresponding author, cestrada@cie. unam. mx
Abstract

In the present work, the optical design for a new high radiative flux solar furnace is described. Several optical configurations for the concentrator of the system have been considered. Ray tracing simulations were carried out in order to determine the shape and intensities of the concentrated radiative flux distributions in the focal zone of the system, for comparing the different proposals. The best configuration was chosen in terms of maximum peak concentration, but also in terms of economical and other practical considerations. It consists of an arrangement of 409 first surface spherical facets with hexagonal shape, placed on a spherical supporting structure. The facets have corrected orientations in order to compensate for aberrations. The design considers an intercepted power of 30 kWt and a target peak concentration above 10,000 Suns. For the design, global optical errors from 0 to 6 mrad have been considered, but it is found that the maximum tolerable overall error will be of 4 mrad.

1. Introduction

Modern solar furnace technology starts in the 1950s decade. The first research in these devices was directed towards studying the effects of high temperatures (around 3500°C) on the properties of different materials exposed to highly concentrated solar fluxes [1]. Among their applications is the study of properties like thermal conductivity, expansion coefficients, emissivity, melting points [2], study of ultra refractory materials, and determination of phase diagrams, crystal growth, and purification of materials. At the same time, methods for the measurement of high temperatures in receivers [3], and the flux density of concentrated radiation [4] have been developed. The latter have evolved and image digitalization techniques are used [5], with calorimetric techniques as reference for the images [6].

Among the first built, there were the furnace of Arizona State College in the USA, in 1956 [7], and the furnace of the Government Institute for Industrial Research, in Japan. Solar furnace technology has evolved, and larger furnaces with higher concentration factors have been built, like the one of the CNRS in Odeillo, France, with 1000 kW [8]; the furnace of the Academy of Sciences of Usbekistan, with 1000 kW [9]; the furnace of Paul Scherrer Institute (PSI), of 25-40 kW [10]; the furnace of CIEMAT, in Plataforma Solar de Almeria, Spain, with 45 kW; and the furnace of DLR, in Cologne, Germany, of 20 kW [11].

Mexico has an ideal position for the implementation of solar technologies, due to its favorable geographical location in the Sunbelt of the planet. The estimation is a yearly average of more than 5.5 kWh per square meter of global solar radiation over the country. In particular, in the northwest states this insolation has a very important component of beam solar radiation. This high quality

solar resource makes that area ideal for the implementation of concentrating solar technologies (CST), either for electrical power generation or for the production of solar fuels as Hydrogen. For these reasons, the construction of a high radiative flux solar furnace (HRFSF) was proposed, as a research tool to allow the development of CST in Mexico. Federal funding for the development of this infrastructure has been approved by CONACYT, and the project is now in progress. The HRFSF will be developed in three years, starting from September 2007, by the Centro de Investigacion en Energia of Universidad Nacional Autonoma de Mexico (CIE-UNAM), in collaboration with Instituto Nacional de Astrofisica, Optica y Electronica (INAOE), and other institutions. The applications of this infrastructure are expected to be in the areas of solar chemistry and solar materials processing [12].

The design targets for the HRFSFS are a thermal power of 30 kW, peak concentration above 10000 suns, and a solar spot of 10 cm diameter or smaller. To achieve these targets, the design was carried out by means of ray tracing simulations, to optimize the optical characteristics of the system.

2. Methodology

We started from an initial proposal of a faceted concentrator of around 30 kW, formed by polished first surface glass mirrors with spherical curvature. From the results reported in [13] about the influence of the number of facets in the concentration factors, we sought that the concentrator had the largest feasible number of facets. The size of 40 cm was selected, on the basis of fabrication and mounting considerations. The shape of the facets was chosen to be hexagonal, because this geometry fills adequately the concentrator surface, still being relatively easy to polish. From the point of view of filling the space, square facets would be an interesting option also, but they are difficult to polish adequately.

Also, from the results of [13], it was decided to mount the mirrors on a curved frame, either spherical or parabolic, instead of a flat structure (like from instance in the DLR [11] or in NREL), which would be the easiest option. This has the advantage of reducing the average distance of the facets to the focal point, and therefore reducing the spread of the reflected solar cones.

In Fig. 1, the hexagonal mirrors mounted on a circular frame are projected onto a flat surface. The shape of each mirror must be designed according to this projection to minimize mutual shading.

For this, the vertical dimensions of the mirrors were not altered, changing only the horizontal dimensions in order to fit the mirrors on the arrangement. The difference with respect to regular hexagons is actually small, but it must be taken into account in the fabrication of the facets.

In the present optical system the spherical aberration of the facets is important. Due to this aberration and, in a smaller degree, to comma and astigmatism, the focal region is increased in size (Fig. 2). Facets that are on the most external parts of the concentrator contribute more to these aberration effects, because they are facing the incoming radiation with more inclination.

Aberrations can not be corrected with additional optical surfaces in a system like this. A lot of improvement can be made however, by reorienting the facets to different inclinations. The angle for each facet is chosen such that the ray reflected on the center of the facet is effectively directed towards the focal point of the system, as illustrated in Fig. 3. In this way, even though the facets are mounted on a spherical frame, they are not tangent to the surface of the frame.

image279

C

 

B

 

A

 

Figure 1. Arrangement of hexagons on a spherical frame, projected on a plane (top). Difference between the

largest and the smallest hexagons (bottom).

image280

 

Figure 2. Effects of the spherical aberration.

In addition to the reorientation of the facets, using different focal distances for each one may also help to compensate for the aberrations. Several cases were simulated, which are compared against a continuous paraboloid of equivalent area and focal distance:

• E1: Sphere with reoriented facets. All facets with the same focal distance.

• E2: Sphere with reoriented facets. All facets with a different focal distance, calculated according to their distances to the actual focus.

• E3: Sphere with reoriented facets. Facets in six different groups of equal focal distance.

• P1: Paraboloid with reoriented facets. Similar to case E2, but with paraboloidal frame.

• P2: Paraboloid with reoriented facets. Focal distances in groups, as case E3.

• P3: Continuous paraboloid.

image281 image282

In configurations E3 and P2, only six different focal distances are used, on the assumption that the optical performance could be reasonably good, as compared to the case in which every facet has its own focal distance. This possibility is explored because restricting the number of different focal distances would be very advantageous to reduce fabrication costs. The groups of facets are shown in Fig. 4, and their data reported in Table 1.

Figure 3. Reorientation of facets (b) for correction of the spherical aberration (a).

The modeling of the above configurations was carried out under the assumption of perfect specular reflection; i. e., reflectivity equal to unity and zero optical errors. Based on the results obtained, one of these configurations was selected for further investigation. This configuration was then simulated with different values of the optical error, in order to determine the maximum tolerable value of this parameter for the construction of the system.

Подпись:>

Each simulation run was carried out with two ray trace codes of different nature, to ensure the reproducibility of the results. The first one called Tonalli [13, is a code based on the convolution method [14], and developed in collaboration between CIE-UNAM (Mexico) and CIEMAT, Spain. The second was a routine for reprocessing the results from a commercial optical design software, called Zemax.

Table 1. Groups of focal distances considered for cases E3 and P3

Group

Focal distance (m)

Number of facets

Group

Focal distance (m)

Number of facets

A

3.75

85

D

4.50

56

B

4.00

126

E

4.75

12

C

4.25

130

3. Results and discussion

T he results for the different configurations are presented in Table 1 and Fig. 6. The best result is, as expected, for the control case of a continuous paraboloid (P3). On the other hand, the four configurations E2, E3, P1, and P2, are very similar to each other; there is practically no difference in peak concentration between the paraboloidal and spherical frames, either when all facets have different focal distances (E2 and P1), or when they are grouped in six focal distances (E3 and P2). The case with a single focal distance (E1) is the worst of all, indicating that it is at lest necessary to consider a few different values for the focal distance. Five or six turned out to be a good number in preliminary analyses.

Table 2. Results of the simulations for the proposed configurations.

Configuration

Peak irradiance (kW/m2)

Spot of radius to collect

90% of energy (cm) and amount collected (kW)

Spot of radius to collect

95% of energy (cm) and amount collected (kW)

E1

27 191

4.88

34.81

6.23

36.74

E2

36 323

2.65

34.81

3.16

36.74

E3

36 107

2.67

34.80

3.19

36.74

P1

36 126

2.61

34.49

3.10

36.41

P2

35 396

2.74

34.49

3.27

36.62

P3

38 110

2.02

34.83

2.33

36.76

Once it was determined that a spherical frame is a good option, the next step consisted in studying the effect of the curvature radius of this frame. It was considered that a smaller radius could improve the concentration by reducing the distance the reflected solar cones travel, and therefore limiting the spread of the solar image. In Fig. 6 the variation of peak concentration with curvature radius is presented. It is observed that the best concentration is obtained for curvature radius between 500 and 550 cm. However, the concentration factor for the studied cases has a maximum variation of only 1.3%. on the other hand, the rim angle of the concentrator increases considerably as the radius of curvature is reduced. A large rim angle may result inconvenient for the implementation of receivers in the focal zone, due to the large incidence angles of radiation, as illustrated in Fig. 7. So there is little to gain from the reduction of the radius and there are potential problems with this. Therefore a radius of 7.5 m was fixed for the following simulations, giving a effective focal distance of 3.68 m.

image284

The next study was to analyze the effects of optical errors in the performance of the concentrator. For this purpose, the E3 configuration was considered. Reflectivity was taken as 0.81 and focal distances according to the previously described groups. In Fig. 8 the results are presented for peak concentration, average concentration and spot radius (for 90% collected power). We can see that with values of the optical error between 3 and 4 mrad, the peak concentration is between 15000 and 10500 suns, while spot radius would be between 4 and 5 cm. So, a maximum optical error of 4mrad is required to meet the design targets stated above for the solar furnace.

It is known that optical errors modify not only the concentration factor but also other characteristics of a faceted concentrator, like the optimal focal distance [13]. In Fig. 9 we present

image285 image286

the average irradiance values for different focal distances and optical errors. In this case we consider a configuration of the type E2.

As can be seen from Fig. 8, even tough there is a dependence of the optimal focal distance with optical error, the variation is not sharp. The previously selected focal distance of 3.68 m is well within the optimal range for the errors considered.

It must be pointed out, that the optical error we are considering here is the global error of the system. This includes the alignment and surface errors of the facets, but also the alignment, surface, curvature, and tracking errors of the heliostat. Therefore, the upper bound of 4 mrad must accommodate for all of these effects. Actually, the type of facets considered can be fabricated with very high accuracy, and also an accurate alignment procedure is being developed for the facets of the concentrator. Therefore, the design of the heliostat may prove to be the more critical problem.

A specific heliostat design is in process, where the target would be to minimize all optical errors.

4. Conclusions

The design of the HRFSF aims to create a high quality infrastructure for research in solar concentration systems in Mexico. The design considers a faceted concentrator, formed by 409 hexagonal first surface polished glass mirrors. The mirrors will be attached to a frame of spherical curvature. Ray tracing results indicate that there is very little difference in optical performance between a parabolic and a spherical frame, but the latter may be easier to fabricate. It was found that it is not convenient to use mirrors of equal focal distances, mounted tangent to the frame curvature. Instead, mirror should have different focal distances and must be tilted with respect to the frame. This helps compensating for the optical aberrations. It was found that grouping the mirrors in 6 sets of equal focal distances gives very good results. Also, the effect of optical errors was investigated. It was found that error must be below 4 mrad to reach the design targets.

This work was partially supported by CONACYT (Grant 56918) and UNAM (Grant 372311721).

D. Riveros-Rosas and J. Herrera-Vazquez acknowledge graduate scholarships from CONACYT.

5. References

[1] Glaser, P. E. Solar Energy, 2 (1958) 7-10.

[2] Hisada, T., Mii, H., Noguchi, C., Noguchi, T., Hukuo, N., and Mizuno, M., Solar Energy, 1 (1957) 14-18.

[3] Brenden, B. B. et al. Solar Energy, 2 (1958)13-17.

[4] Loh, Eugene et al., Solar Energy, 1 (1957) 23-26.

[5] Jonhnston, G., Journal of Solar Energy Engineering, 117 (1995) 290-293.

[6] Perez-Rabago, C. A., Marcos, M. J., Romero, M., Estrada, C. A., Solar Energy 80 (2006) 1434-1442.

[7] Kevane, C. J., Solar Energy, 1 (1957) 99-101.

[8] Trombe, F., Le Phat Vinh, A., Solar Energy, 15 (1973) 57-61.

[9] Abdurakhamanov A. A. et al. Applied Solar Energy, 34 (1998).

[10] Schubnell, M., M., Kelle, J., Imhof, A, Journal of Solar Energy Engineering, 113 (1991) 112-116.

[11] Neumann, A., Groer, U., Solar Energy, 58 (1996) 181-190.

[12] Fletcher, E. A., Journal of Solar Energy Engineering, 123 (2001) 63-74.

[13] Riveros-Rosas, D., Sanchez-Gonzalez, M., Arancibia-Bulnes, C., Estrada, C., EUROSUN Congress, Lisbon, Portugal. October 7-10, 2008.

[14] Biggs, F., Vittitoe, C. N.. “Helios model for the optical behavior of reflecting solar concentrators”. Sandia National Laboratories Report, SAND 76-0347, Albuquerque, USA (1979).

[1] Introduction

Solar desalination, an environment-friendly technique, is successfully used for the production of fresh water from saline seawater in many parts of the world. In the last decade, with the rise of fuel cost, an extensive research has been carried out by different institutions worldwide to develop an efficient way of utilizing solar energy for water desalination.

The heat pump is a useful device in transforming low-grade thermal energy into a usable energy source. Heat pumps can use many sources, such as ground [1], to provide useful energy for