Category Archives: Particle Image Velocimetry (PIV)

Sun-tracking principle

The point of interest in sun tracking is to follow the path of the sun on local visible sky. This is because in this area, the receiver part of the conversion system has to intercept the sunshine during the day.

The orientation principle is based on the input data referring to the position of the sun on the sky dome. For the highest conversion efficiency, the sun rays have to fall normal on the receiver so the system must periodically modify its position in order to maintain this relation between the sun rays and the panel. (Fig .2)

The positions of the Sun on its path along the year represent an input data in designing the tracking system, so the geometrical relationship between the Earth and the Sun has to be considered.

The Earth describes along the year a rotational motion following an elliptical path around the sun. During one day (24 hours) the Earth also spins around its own axis describing a complete rotation (360°).

Comparative with the first mentioned motion which doesn’t affect the behaviour of the sun on the sky dome, the second one generates the sunrises and the sunsets.

The variation of the altitude of the sun on the celestial sphere during one year is determined by another rotational motion called precession responsible for a declination of the Earth axis in consideration with the plane of the elliptic yearly path. This motion generates the seasons because of the alternative exposure of the northern and southern hemisphere to the sun rays trajectory. So the combined motions of the Earth are similar with behaviour of a spindle.

Considering as a reference point the ground plain of the loco, the relative movement of the Sun along the year covers a sector in the celestial sphere.

This "belt” of trajectories results from the combination between the daily rotation of the Earth around its axis and the precession motion of the Earth axis (Fig.3) [1, 4, 6].

The fact that the orientation can be obtained by two independent rotational motions has to be considered in the mechanism design process.

There are particular conditions imposed by the principle of orientation:

— the motions have to be independent in respect to the relative motions of the Sun;

— the motion that tracks the daily trajectory of the Sun is recommended to be a rotation around an axis parallel with the polar one.

Saturated Steam Process with Direct Steam Generating Parabolic Troughs

M. Eck8, E. Zarzab

aDLR, Institute of Technical Thermodynamics, Pfaffenwaldring 38-40, 70569 Stuttgart bCIEMAT, Plataforma Solar de Almeria (PSA), P. O. Box 22, 04200 Tabernas (Almeria),

Spain

Abstract — The direct steam generation (DSG) in parabolic trough collectors is an attractive option regarding the economic improvement of parabolic trough technology for solar thermal electricity generation in the multi Megawatt range. The European DISS project has proven the feasibility of the direct steam generation under real solar conditions in more than 4000 operation hours [1]. Within the European R&D project INDITEP the detailed engineering for a pre-commercial DSG solar thermal power plant with an electrical power of 5 MW is being performed. This small capacity is chosen to minimise the risk for potential investors.

Regarding DSG solar thermal power plants only steam cycles using superheated steam have been investigated so far. In this paper a steam cycle operated with saturated steam is investigated for the first time. For near term applications this might be an interesting alternative in the chosen small capacity range. This choice would offer some specific advantages:

• Lower complexity of power block and thus lower investment but also lower efficiency of the power block

• Simple set up of the collector field

• Proven safe operation and higher thermal efficiency of the collector field

Introduction

During operation of the DISS facility, the life size DSG test facility at the Plataforma Solar de Almena (PSA), it turns out that the so called recirculation mode is advantageous for near term application mainly because of its safe operation of the collector field [1]. A recirculation mode driven collector row is subdivided into an evaporating and a superheating section by a separator. At the end of the evaporating section the excess water is separated from the steam flow and recirculated to the collector inlet. The saturated steam thus produced is fed into the superheating section and is superheated up to 400°C or more. In the end the superheated steam is expanded in a turbine that runs a generator.

the detailed engineering for a first DSG solar thermal power plant is performed. A size of 5 MW was chosen for this plant. This size might be too small to guarantee a cost — effective operation of the plant but it minimises the financial risk of potential investors. For the detailed design a site close to the Spanish city of Seville was selected (latitude: 37°24’ N; longitude: 5°58’ W). A dry cooling condenser is considered. Due to the high ambient temperature at the site in summertime a condensation pressure of 0.1 bar has been considered for the design point (June 21st), which is equivalent to a condensation

temperature of 45.8°C. A minimum steam quality at the turbine outlet of 0.85 is allowed. The basic boundary conditions are summarised in Table 1.

Due to the small size of the turbine, the number of steam extractions at the turbine is limited. In case of a 5 MWe turbine there is only a single extraction line. The according bleed stream is used for dearation in the feed water tank. The dearator pressure is set to

5.6 bar. The schematic diagram of such a simple steam cycle operated with superheated steam is shown in figure 1. The collector field is operated in recirculation mode. In this paper this basic operation mode is compared to a DSG plant operated with saturated steam. The according schematic diagram of this option is displayed in figure 2.

Figure 2: Schematic diagram of the saturated steam cycle

In figure 2 the collector field is also operated in recirculation mode. The saturated steam leaving the field separator is fed directly to the saturated steam turbine. Again there is a single extraction line for dearation. In this case the steam re-entering the second stage of the turbine has to be dried in a second separator to guarantee the minimum steam quality at the outlet of the second stage. The saturated water at the outlet of the second separator is fed to the feed-water-tank.

Study of variations in optical efficiency for a direct-flow evacuated tube collector with relation to incident irradiance, mass flow and slope

T. P. Williamson, B. Bauer, P. T. McEntee, P. M. McKernan, Thermomax Ltd, Balloo Crescent, Bangor, BT19 7UP, Northern Ireland, tel: +44 (0) 2891 270411, tommy_williamson@thermomax. co. uk

Current test methods for solar collectors require that the irradiance distribution over the collector aperture should be >800 Wm-2 during data acquisition. However, evacuated tube collectors have been designed to operate at northerly latitudes where irradiance distributions are commonly lower than this value during a large part of the year. Reported here are the results of an investigation into how the optical efficiency of a direct-flow evacuated tube collector varies with respect to incident irradiance in the range 200 — 1400 Wm-2 using a simulated solar irradiance source.

A number of domestic solar thermal control systems have utilized a variable flow­rate method to manage the contribution of useful energy gained by the system. However to date little experimental data relating to how varies with flow-rate for evacuated collectors was available in the literature1. Stated here are the results for optical efficiency variation of an evacuated tube collector with mass flow from 0.02 — 0.15 kgs-1 and the corresponding variation in AT for two irradiance levels.

Many evacuated solar collectors are installed with slopes p which do not conform to those recommended by standard test methods (i. e. 45°), the consequences are that the relative efficiencies of these installations are unknown. Evaluation of for slopes in the range 0° to 60° with respect to the horizontal plane were investigated.

1 Introduction

Standardised testing2,3 for thermal solar collectors has proven to be an effective method for direct system comparison. By applying a strictly controlled input parameter technique, different collector performances can be quantitatively evaluated. Valuable additional information can be gained in relation to the collector characteristics by shifting these input parameters away from their recommended magnitudes. The variation of optical efficiency По for a direct-flow evacuated collector with relation to the incident irradiance G, mass flow m and slope p with respect to the horizontal plane are reported. Global heat losses UL from the collector were calculated under these experimental conditions where the collector temperature Tcwas held at ~3 K above the ambient temperature; which is the upper limit for optical efficiency measurements according to EN12975-2. All optical efficiencies stated are in relation to the collector absorber area.

Development of a new support structure for Parabolic Trough collectors VC1 (Full surface collector)

Fa. Carpe Diem Solar 72348 Brittheim, SchwarzwaldstraRe 11

Solar heat Power stations using Parabolic Trough collectors have been in commercial use in the U. S.A. for over 15 years. With an output of 354 MW they have, to date, delivered 11.400 GWh of electrical power to the net and represent the most successful technique for converting solar radiation into electrical power.

However, because of sinking prices for fossil fuels since 1991, no new solar heat Power stations have been built.

Photo 4

Detail view of the Eurotrough structure

Because of changes in the Spanish energy provision policy in 2002, important conditions have been created for the building of 2 new Parabolic trough power stations in Guadix, each with 50 MW output.

In Guadix a part of the days output will be stored in salt heat stores so that the turbines will be able to run during the night.

Demand profile for Parabolic trough collector structures

• High torsional rigidity against wind last

• Optical precision for the focussing mirror

• Compact transportability

• Low manufacturing costs in materials and assembly

• Flexibility in size and shape

• Suitability for erection in deserts and areas of high salinity e. g. coastal areas

• High long term reflectivity

Concept

We are developing a structure for Parabolic trough power stations that offers clear advantages compared with the current versions shown above.

• The support frame comprises the fewest possible components.

• The closed profile has very high rigidity, which is indispensable for this application.

• The closed profile structures make corrosion protection easier to achieve.

• The completed structures should be simple to construct and easy to assemble.

• As much as possible of the functional geometry i. e. the parabolic form should be contained within the construction process.

• Reduction of handlings technique

o The simplest possible on site assembly, which can be performed by a semi­skilled workforce, without expensive building cradles. o Simple positioning of the mirrors is made possible

• Use of more cost effective thin profile reflector surface allows flexible size and shape.

• Circa 25% cost saving compared with present structures.

• The completely enclosed skeleton structure offers high rigidity with a low use of materials.

• Through the low number of components (about 12 stabilising ribs and the covers) a short, problem free, on site assembly process is possible.

SHAPE * MERGEFORMAT

As the structured components can be transported separately, the necessary transport volume per structure is very low. The top cover of the parabolic surface consists of 3x12m long and 2.20m wide trapezoidal profile metal sheets. For transport these can be stacked on top of each other so that containers of several hundred can be carried up to the maximum load capacity of the vehicle. Similarly the lower cover sheets and the sickle shaped forming ribs can be stacked for transportation.

The metal pressing and shaping techniques used allow cost effective manufacturing of the stabilising ribs and increased accuracy.

The parabolic shaped hull structure combines the two essential demands of a parabolic trough (Mirror geometry and support structure). Because of this mirror panels need not be used. Thin surface reflectors offer the same quality, are considerably cheaper and can be glued directly onto the given form. This results in significant cost reductions. Alternatively, reflector foil could be used. The ALANOD company offer appropriate surface treated aluminium sheets for solar applications.

• Through the integrated construction of Reflector and support structure, storm damage and glass breakage can be considerably reduced.

• The very high glass insurance costs of the mirror panels used up to now are well known. The previous point shows that these costs could also be reduced.

Environmental Relevance

The planned production process promises a noticeably lower electricity and overall energy use in the manufacture of the structure.

• An important savings factor is the use of thin sheet reflectors (e. g. 1mm thick glass mirror replaces the 4mm thick mirror panels used at the moment).

• Energy consuming welding technology is replaces by simple glue and rivelting techniques.

Protection of Fossil resources

Through a more cost effective and simpler manufacturing process, the competitiveness of environmentally friendly, solar heat power stations enhanced. Because of this the adoption of renewable energy technology in the general market would be increased. This would proportionally reduce the operation of conventional power stations. Fossil fuel recourses would be conserved and CO2 emissions would be avoided.

SHAPE * MERGEFORMAT

Solar heating system

The annual results for this system are shown in Table 6. As the characteristics of all systems are the same, the inlet temperature depends on the heating load which affects the temperature of the stored water and thus the temperature of the water entering the solar collector (from the bottom of the storage tank).

This behaviour is illustrated in the comparative graphs of the monthly useful energy (Qu) and auxiliary energy (Qaux) for the four types of collectors considered shown in Figs 4 and 5 for Nicosia and Athens respectively. As can be seen in both cases the performance (Qu) of the collectors during summertime is lower than that of other mid-season months (March and October). No auxiliary is required during the period April to November.

Table 6 Annual results for solar heating ^ system

Collector

Type

Collector

characteristics

Nicosia

Athens

Qu (GJ)

Qaux (GJ)

Qu (GJ)

Qaux (GJ)

A

a=0.95, є=0.1

112.0

1.625

112.8

6.308

B

a=0.85, £=0.1

94.33 (15.8)

3.026

94.47 (16.3)

8.228

C

a=0.95, £=0.9

67.57

4.996

64.64

11.06

D

a=0.85, £=0.9

57.58 (14.8)

7.106

53.29 (17.6)

13.44

Heating load (GJ)

52.05

51.58

Note: Number in brackets represent percentage difference with respect to black

absorber

—♦—Qu-A —о—Qaux-A —■—Qu-B —в—Qaux-B —A—Qu-C —A—Qaux-C —•—Qu-D —e—Qaux-D

Months

Fig. 4 Monthly useful and auxiliary energy of the solar heating system for the different

collectors considered for Nicosia

2.1 Industrial Process Heat System

The annual results for this system are shown in Table 7. Typical monthly results for type A collectors for both Nicosia and Athens are shown in Table 8.

Table 7. Annual results for the industrial process heat system

Collector

Type

Collector

characteristics

Nicosia

Athens

Qu (GJ)

Qaux (GJ)

Qu (GJ)

Qaux (GJ)

A

a=0.95, є=0.1

729.3

55.04

722.6

86.56

B

a=0.85, £=0.1

671.9 (7.9)

79.36

653.6 (9.5)

123.0

C

a=0.95, £=0.9

587.2

114.9

543.4

176.4

D

a=0.85, £=0.9

537.3 (8.5)

143.5

488.6 (10)

208.8

Note: Number in brackets represent percentage difference with respect to black

absorber

Table 8 Monthly performance of the industrial process heat system for Nicosia and Athens _______________ for type A collector in (GJ).______________________

Results for Nicosia

Results for Athens

Month

Qins

Qu

Qaux

Qenv

Qins

Qu

Qaux

Qenv

JAN

131.7

52.9

11.2

0.653

136.6

52.7

13.4

0.659

FEB

129.8

49.1

8.48

0.601

146.1

52.0

8.33

0.709

MAR

164.3

59.7

8.05

0.737

177.5

60.3

8.79

0.811

APR

180.1

60.8

1.86

0.827

178.5

59.0

5.79

0.835

MAY

171.8

61.7

5.30

0.697

196.7

63.8

6.62

0.829

JUN

185.4

64.6

1.52

0.735

206.8

64.7

4.80

0.837

JUL

201.5

68.5

0.48

0.827

222.7

67.7

3.23

0.926

AUG

205.8

71.0

0.48

0.874

222.5

73.2

2.84

0.927

SEP

184.7

65.7

1.03

0.828

216.4

68.8

2.18

0.952

OCT

181.2

65.7

2.68

0.816

180.0

61.3

7.64

0.832

NOV

152.2

56.8

5.17

0.750

131.6

48.4

11.9

0.686

DEC

125.2

52.9

8.80

0.653

127.8

50.8

11.1

0.655

SUM

2013.7

729.3

55.04

8.998

2143.0

722.6

86.56

9.657

Comparative graphs of the monthly useful energy (Qu) and auxiliary energy (Qaux) for the four types of collectors considered are shown in Figs 6 and 7 for Nicosia and Athens respectively. As can be seen again the performance of the color collectors is somewhat lower than that of the black colored collectors. The difference in performance between the respective cases is almost constant in all months of the year. The maximum useful energy collected occurs in both locations during the month of August and is about 71 GJ and 73 GJ (black selective absorber) for Nicosia and Athens respectively.

It can be generally concluded from the results presented in this section that color collectors give in most of the cases about 10% lower performance than collectors painted with black paint either for the normal paint or selective. This means that 10% more collector area would be required to obtain the same performance as the black colored collectors, which is acceptable.

3. CONCLUSIONS

In this paper applications of solar collectors with colored absorbers in a house heating, multi-flat residential or office buildings, and industrial process heat applications are
presented. These systems are simulated on an annual basis at two different locations at different latitudes, Nicosia, Cyprus (35°) and Athens, Greece (38°).

The results show that although the colored collectors present lower efficiency than the typical black type collectors, the difference in energy output depends on the absorber darkness. For a medium value of the coefficient of absorptance (a=0.85), the colored collectors give satisfactory results regarding the drop of the amount of collected energy for the two locations (about 10%), compared to collectors with black absorbers (a=0.95).

This implies the use of proportionate larger collector aperture area to have the same energy output as that of typical black colored collectors.

REFERENCES

[1] Tripanagnostopoulos Y., Souliotis M. and Nousia Th., Solar Collectors with Colored Absorbers, Solar Energy, 68: 343-356, 2000.

[2] Medved S., Arcar C., Cerne B. A large-panel unglazed roof-integrated liquid solar collector-energy and economic evaluation. Solar Energy 75, 455-467, 2003.

[3] Crnjak Orel Z., Gunde Klanjsek M. and Hutchins M. G. Spectrally selective solar asorbers in different non-black colours. In Proc. Int Conf. WREC VII (CD-ROM), Cologne, Germany, 29 June-5 July, 2002.

[4] TRNSYS program Manual, Solar Energy Laboratory, University of Wisconsin, Madison,

USA, 1996.

[5] Petrakis M. Kambezides H. D, Lykoudis S, Adamopoulos A. D, Kassomenos P. Michaelides I. M, Kalogirou S. A., Roditis G., Chrysis I. and Hadjigianni A., Generation of a "Typical Meteorological Year” for Nicosia, Cyprus. Renewable Energy, 13: 381­388, 1998.

[6] Pissimanis D., Karras G., Notaridou V. and Gavra K., The generation of a "Typical Meteorological year” for the city of Athens. Solar Energy 40: 405-411, 1988.

Solar-Hybrid Gas Turbine Plants: Status and Perspective

Reiner Buck, Peter Heller, Peter Schwarzbozl, Deutsches Zentrum fur Luft — und Raumfahrt (DLR), Institute of Technical Thermodynamics, Pfaffenwaldring 38-40, D-70569 Stuttgart, Germany

Chemi Sugarmen, Arik Ring, Research & Development Department, ORMAT Industries Ltd., Industrial Area, POB 68 Yavne 81100, Israel

Felix Tellez, Renewable Energy Department, CIEMAT, Avda. Complutense 22, E-28040 Madrid, Spain

Juan Enrile, SOLUCAR, Avda. de la Buhaira, 2, E-41018 Sevilla, Spain

The integration of solar energy into gas turbine power systems can lead to lower cost for solar power production. The paper describes the status and perspective of solar-hybrid gas turbine systems. A solar-hybrid test system was set-up consisting of a pressurized solar receiver cluster and a modified helicopter gas turbine with a generator. The test system was operated up to receiver temperatures of 960°C, delivering up to 230 kWe to the grid. Controlability of the hybrid system was very good.

Design and performance assessment of prototype plant configurations in the power levels from 4 MWe to 16 MWe are presented. Expected cost figures for the solarized gas turbine, the solar tower plant and further equipment as well as for operation and maintenance are discussed. Levelized electricity costs of about 13 €cent/kWh with an annual solar share of 53% are calculated for a 16 MWe combined cycle. The perspective for market introduction is outlined.

Introduction

The reduction of fossil-fuel based power production by using solar power technology is one important step in the international commitment of CO2 reduction. The direct way of producing electric power from solar energy, the photovoltaic technology (PV), is gradually extending its focus from purely decentralized small scale systems towards large-area bulk power production. Generating costs of solar electricity below 10 €cent/kWh are predicted for 2010. In contrast, solar thermal power plants produce high temperature heat that is converted to electricity by conventional power cycles. The nine commercial parabolic trough plants in the Californian dessert (SEGS) produce electricity from solar energy with an overall efficiency of 10-14% and at levelized electricity costs of 16-19 €cent/kWh.

Future large systems of 200 MW with 12h storage are forecasted with system costs of 2500 €/kW and generating costs below 5 €cent/kWh [1]. Similar projections are made for other solar-only technologies. In any case, the key to cost reduction lies in mass production after successful market penetration.

One major option for an accelerated market introduction of solar thermal power technology are solar-fossil hybrid power plants. Their advantage compared to solar-only systems lies in low investments due to an adaptable solar share, reduced technical and economical risks due to a fully dispatchable power, and a higher system efficiency because of reduced part load operation and less start-up and shut-down losses. Another important aspect is that for renewable power generation technologies like wind turbines and PV a conventional power capacity has to be kept in stand-by to compensate the fluctuating power supply of renewable energies. This is a kind of renewable-conventional hybrid power system but with completely separated system technologies. Real hybrid plants share great parts of their system, hence leading to economical advantages.

Using the high conversion efficiencies of Combined Cycle systems (now above 50%) or recuperated gas turbines leads to a decrease in the required heliostat area. As the heliostat field is the major cost contribution in solar tower plants, this results in a cost reduction.

In this paper the current status of the technology for solar-hybrid gas turbine systems is discussed, including their market perspective.

A NEW RECEIVER FOR PARABOLIC TROUGH COLLECTORS

N. Benz, F.-D. Doenitz, Th. Kuckelkorn

SCHOTT, Business Unit Solar Thermal Schott Rohrglas GmbH, D-95660 Mitterteich, Germany Phone: +49 9633 80 401, Fax: +49 9633 80 757 e-mail: nikolaus. benz@schott. com

ABSTRACT

SCHOTT developed a new receiver for parabolic trough collectors used in solar thermal power plants. In particular we developed a new absorber coating which is temperature stable up to 500 °C with excellent optical properties. We found an anti reflective coating for the special glass tubing with high solar transmittance, durability and improved abrasion resistance. To reduce breakage of the glass-to — metal sealing, which is the main cause for damages in existing power plants, we developed a new sealing with adapted CTE of metal and glass. To meet the requirements we had to introduce a new glass type suitable for large tubes. A new design approach minimizes the undesirable shading of the absorber by the bellows. Finally we achieved an active length of the receiver of more than 96%.

INTRODUCTION

Solar technology is one of the key business issues of SCHOTT in the future. The activities are bundled in a strategic business unit: the joint venture with RWE for PV together with the solar thermal division producing evacuated tube collectors for residential applications.

End of 2001 SCHOTT started to develop a receiver for parabolic trough collectors to enter the upcoming market of solar thermal power generation.

The development is part of the joint project PARASOL, supported by the German Federal Ministry for the Environment. Project partners are the Flagsol GmbH and the German Aerospace Center (DLR).

Parabolic trough collectors

More than a decade after the last construction of a solar power plant with parabolic troughs, the interest in this technology is rising again. Projects are scheduled or even in ogress in Europe, USA and some other countries in the sun belt [1]. The upcoming new market for solar power is not only driven by the global search for clean energy. Apart from the four countries which anticipate funding from the GEF, the most promising opportunities are opening up in the US and in Europe: In the US projects benefit from the renewable portfolio standard in several states of the south-west which require a certain percentage of electricity supply to come from renewable sources. In Spain a “feed-in-tariff” was introduced to guarantee a premium payment for electricity generated in solar thermal power plants.

The receiver requires the most challenging technology and has a decisive influence on the overall efficiency. It consists of a cylindrical absorber placed along the focal line to capture the energy reflected by parabolic mirrors. To achieve high efficiency the absorber must show high solar absorptance as well as low thermal emittance. To suppress heat conduction losses, the absorber is usually insulated with vacuum enclosed by a
transparent cover. Therefore a glass tube is used which is coated with anti reflective films for high solar transmittance. Metal bellows are used to accommodate for thermal expansion difference between the steel tubing and the glass envelope. To provide high vacuum where gas heat conduction is totally suppressed, a durable and tight glass-to — metal seal (GMS) is required.

Structural synthesis method

The synthesis is based on Multibody Systems Method (MBS) according to which a mechanical system is defined as a collection of bodies with large translational and rotational motions, linked by simple or composite joints [5]. The functional design process at structural level consists in the following stages:

♦ Identification of all possible graphs on basis of the following input data:

— spatiality of the multibody system;

— type of the geometrical constraints gc (simple or/and compound);

— number of bodies nb;

— the mobility of the multibody system M.

♦ Selection, from multitude of the identified graphs, of the graphs that are admitting supplementary conditions imposed by the specific field of utilisation.

♦ Successive transformation of the selected graphs into mechanisms by:

— mentioning the fixed body and the role of the other bodies (ex.1-fixed body, 2-input body, 3-output body etc.);

— identification of distinct graphs versions based on the preceding particularisation;

— transformation of these graphs versions into mechanisms by mentioning the types of constraints gc (rotation, translation etc.) [2], [7].

The graphs of the multibody system are defined as a features based on the modules introduced in the next figure and are considering the number of bodies and the relationships between them.

The identification of all possible graphs starts with definition of the types of the geometrical restrictions between the bodies considering the chosen space S (gc, min= 1, gc, max = S-1) [9]. In the Fig.4 the notations “R” and “T” represent restriction rotation type and respective translation restriction type. All the other notations represent composite joints as combinations of the ones mentioned before.

Fig.4 Restriction types

For example, in the planar space (S = 3), all the possible graphs can be designed using the restrictions types from Fig.4, where gc= 1 (Fig.4.a), gc = 1+1 (Fig.4.b), gc = 2 (Fig.4.c), considering the correlations between the number of bodies nb, the mobility M and the sum of the geometrical constraints Igc.

The relation between M, S, nb, Zgc is [8.]:

M=S(nb-1)-Zgc (1)

In respect with the relative motions of the sun on the sky dome the mobility of the mechanisms that orients the receiver of the conversion system deals with a degree of freedom equal with 2.

The Site

The performance of the system strongly depends on the site chosen. The performance analysis in this study is performed for a site close to Seville (Spain). Time series for the ambient temperature and the direct normal irradiation (DNI) have been derived from satellite data. The yearly sum of the DNI for the site is 2012 kWh/m2. The distribution of the mean monthly DNI in W/m2is displayed in figure 3.

N

/

S

_

800,00-1000,00 □ 600,00-800,00

□ 400,00-600,00

□ 200,00-400,00 0,00-2 00,00

j

t

Hour [-]

Figure 4 displays the distribution of the mean monthly ambient temperature. Since a dry cooling condenser is chosen for the power plant the ambient temperature affects the condenser temperature of the power plant and thus its efficiency. Accordingly the power block efficiency will decrease in summer due to the higher ambient temperature.

The parabolic troughs used for the power plant cannot use the complete DNI but only

the DNI multiplied by the cosine of the incident angle. Figure 5 displays the corrected DNI sorted according to the number of hours they occur. For the simulation it is assumed that the power plant is only operating at a corrected DNI higher than 250 W/m2. According to figure 5 this limit is exceeded for 2770 hours per year. Thus the useful solar energy is reduced to 1726 kWh/m2a compared to 2012 kWh/m2a. Plant operation for values lower than 250

W/m2 is not profitable due to thermal losses in the solar field and high parasitic losses in the whole plant.

Experimental set-up

The thermal solar collector was of a direct-flow design; which consisted of tubes with a co­axial heat transfer conduit connected to a manifold with a parallel inlet/outlet pipe configuration. Twenty evacuated tubes were installed equating to an absorber area of 2.046 m2, with vacuums in the order of 10-5 mbar. Filtered water was the heat transfer fluid used throughout this study, with no anti-freeze component being added to the system. All data was recorded using Thermomax’s in-house solar simulator as shown in Figure 1. Thirty-six Tungsten Halogen lamps (3500 K) were used to produce 18 kW of power to irradiate the collector surface. This system was capable of generating average irradiances in the range 200 to 1500 Wm-2. A correction factor for the spectral discrepancy between the solar simulator and natural sunlight was applied using the effective transmittance — absorptance product method4 and the cool sky was used to minimise the influence of
thermal irradiance on the collector. The solar simulator rig (i. e. solar cradle and cool sky) was capable of a 90° rotation between the vertical and horizontal planes. Acquisition of the data was recorded on in-house software program designed using National Instruments LabVIEW 7, where the deviation of the measured parameters was consistent with EN12975-2.