Modelling and Simulation of Parabolic Trough Power Plant

Transcription

1 Modelling and Simulation of arabolic Trough ower lant João. Silva Instituto Superior Técnico / Technical University of Lisbon (IST/TUL), ortugal Abstract: In ortugal the increase in the production of electricity with origin in renewable sources has been achieved based on hydric and wind resources, but the Sun has been under-exploited. The portuguese territory has high levels of solar radiation and it's necessary to find solutions that take advantage of this fact. In this context, it's important to consider the concentrated solar power systems as an option. The most mature of these technologies is the parabolic trough. In this paper it is presented a model that describes, from the point of view of the electricity production, the behavior of a parabolic trough power plant. Based on this model it was built a simulator to obtain the performance of the plant for some locations in ortugal. Through the results some conclusions are drawn. Keywords: Renewable Energies - Sun - Concentrated Solar ower - arabolic Trough 1. INTRODUCTION In the last years there was in ortugal a large increase in the production of electricity with origin in renewable sources. This is due to the 's price and also the international guidelines to reduce the carbon emissions. In 1 the share of renewable energies exceed 5% of the total production. This production is mainly related with the hydric and wind resources. Besides these two natural resources ortugal has another one: the Sun. However, even though the portuguese territory presents high levels of solar radiation, this resource has been little used. This fact is related to a strategic decision that initially privileged the hydric resources, by exploring, and the wind, but also with the cost of photovoltaic. Now it is necessary to focus on the development of solar energy that can complement the hydric and wind, that present a reduction in summer. In this context, it's important to consider the concentrated solar power (CS) as an option, because it has huge potential and represents a less expensive alternative to photovoltaic. This new way to generate energy is in large development, especially, in the United States, Spain and some African countries. ortugal begins now with the first demonstration projects. Instead to what occurs on photovoltaic, the CS systems initially convert solar energy into al energy and then into electric energy. There are four systems that perform the conversion of solar energy into al energy: parabolic troughs, linear Fresnel reflectors, central receivers and parabolic dishes. Although they have different characteristics and formats, the operating principle is the same. Reflector elements concentrate the solar rays into a receiver, where a heat transfer medium, usually a fluid, is heated. The conversion of al energy into electrical energy is made of conventional mode, with the drive of a turbine. In the case of using a al storage system, which is optional, part of the heat generated during the day can still be stored by a liquid or solid medium, with the possibility to be used at night to keep the turbine in operation. This paper is a extended abstract of a thesis, Silva, J.. (11). The purpose of this work is, essentially, study a parabolic trough power plant, with al storage system, because this technology is the most mature of CS systems. The main objectives are: modulation of a parabolic trough power plant, validation of the model and build of a simulator to assess the performance of the plant for several locations of the portuguese territory. Based on the obtained results some conclusions, regarding the behavior of the system, are drawn.. STATE OF ART The four CS technologies can be categorized by the way they focus the Sun's rays and by the receiver type as show in Table 1. Table 1 - CS technologies Line Focus oint Focus Fixed Receiver Linear Fresnel Reflectors Central Receivers Mobile Receiver arabolic Troughs arabolic Dishes.1arabolic Troughs In this systems, parabolic trough shaped mirror reflectors are used to concentrate sunlight on to ally efficient receiver tubes placed in the troughs focal line, as show in Fig.1. The troughs are usually designed to be installed along one axis north-south to perform the tracking of the Sun from east to west. A al transfer fluid, such as a synthetic al, circulates in these tubes. The fluid is heat by the Sun's concentrated rays and then pumped through a series of heat exchangers to produce superheated steam. The steam is converted to electrical energy in a conventional steam turbine generator, which can either be part of a conventional steam cycle or integrated into a combined steam and gas turbine cycle.

2 Fig. 1 - arabolic Trough. Linear Fresnel Reflectors An array of nearly-flat reflectors, that track the sun along a single axis, concentrates solar radiation onto elevated inverted linear receivers, as show in Fig.. In these receptors flows water that is converted into steam. This system is similar to a parabolic trough with the advantages of low costs, for structural support and reflectors, fixed fluid joints, a receiver separated from the reflector, and long focal lengths that allow the use of flat mirrors. Fig. 3 - Central Receiver.4 arabolic Dishes A parabolic dish-shaped reflector tracks the sun along two axes and concentrates sunlight on to a receiver located at the focal point of the dish, as show in Fig.4. The concentrated beam radiation is absorbed into a receiver to heat a fluid or gas. This fluid or gas is then used to generate electricity in a Stirling engine or a micro turbine, attached to the receiver. Fig. - Linear Fresnel Reflectors.3 Central Receivers The central receiver systems use a circular field of large mirrors with sun tracking along two axes, called heliostats, to concentrate the solar radiation onto a central receiver mounted at the top of a tower, as illustrated in Fig.3. A heattransfer medium in this central receiver absorbs the highly concentrated radiation reflected by the heliostats and converts it into al energy, which is used to generate superheated steam for the turbine. To date, the heat transfer medium demonstrated include water/steam, molten salts and air. Fig. 4 - arabolic Dish 3. ARABOLIC TROUGH OWER LANT MODEL The parabolic trough power plant Andasol-1, in Spain, served as the basis to the various considerations made in this work. This choice is due to the fact that the Andasol power plants are a world reference and have been designed based on climatic conditions similar to that occurring in ortugal. 3.1 General Description and Operating Modes The Andasol-1 has an installed capacity of 5 MW and a al storage system with capacity to allow de operation of power plant at full load for about 7.5 hours. The structure of this power plant can be decomposed in three blocks: solar field, al storage and power bock. This structure is shown in Fig.5. The solar field is constituted by parabolic trough collectors, where the solar energy is converted into al energy. The al storage system consists of two tanks, a cold one and a hot one, where the storage medium is saved. The conversion of al energy into electric energy occurs in

3 the power block that has as main constitutive elements: steam generator system, turbine, generator and cooling system. Fig.5 - arabolic trough power plant structure This type of power plant can present in a summer day, four different operation modes, depending on the daytime: in the morning, during the day, in the evening and at night Operation in the morning After sunrise, the collectors begin to follow the Sun. arabolic mirrors concentrate the solar radiation to absorber tubes, in which a heat transfer fluid (HTF) flows. This fluid then transmits its al energy to heat exchangers. The steam, which is generated there, drives a turbine and electricity is produced by the connected generator Operation during the day If Sun radiation is strong enough, the solar field supplies sufficient energy to generate electricity and fill up the storage system simultaneously. When the storage system is being filled up, the storage medium is heated by the surplus HTF flow rate, through a heat exchanger, and pumped from the cold tank into the hot tank Operation in the evening In the evenings or when the sky is cloudy, the solar field can supply the energy, which is required to drive the turbine, together with the storage system. For this purpose, the storage medium will heat the HTF flow rate missing. Now, the storage medium is pumped into the cold tank from the hot tank Operation at night After sunset, al energy is exclusively supplied by the storage system. Thus, the HTF necessary for generate steam to drive the turbine will be entirely heated by the storage medium until the cold tank is empty. 3. Solar Field 3..1 Constitution and principle of operation In Andasol-1 power plant the solar field is divided into four sections, with power block in the center of these, as show in Fig.6. Fig.6 - Solar field structure In each section, the collectors are arranged in rows, according to north-south direction, and grouped into loops. Each loop consists of four collectors arranged in two parallel rows of collectors each, as illustrated in Fig.7. Fig.7 - Loop of collectors The solar field is constituted by 64 collectors grouped in 156 loops. The sections north and south are crossed, each, by two pipes, according east-west direction, which carry the HTF from the power block to each loops, also making his return. The tube that carries the fluid to the loops is called header supply and the tube that makes the return is called the return header. The HTF is pumped from the steam heat exchangers in the power cycle to the east and west solar field, through the east and west supply headers. The supply headers distribute the HTF through parallel loops of solar collectors. The HTF travels away from the supply (cold) header through one row of the collector loop and back toward the return (hot) header through the other row. The hot HTF from the collector loops then merges in return headers and is pumped back to the central power plant. Since entering the solar field, the temperature of the HTF increases of around 1 C, from 93 C to 393 C. The HTF used is a synthetic, namely Therminol V-1, which is a mixture of biphenyl and diphenyl oxide, which is stable at temperatures below 4 C. The collectors track the Sun with great precision through a hydraulic drive and concentrate the solar radiation, with the reflector mirrors, in the absorber tube. The absorber tube is a steel tube and is surrounded by a glass envelope. The space between the steel tube and the glass is evacuated to limit heat

4 losses from the absorber tube to the surrounding environment. In Andasol-1 power plant, there are used Eurotrough-15 collectors and absorber tubes Solel and Schott. 3.. Model This paragraph follows atnode, A. (6) and Montes, M. J. (8). i. Absorbed ower The absorbed power from solar radiation (in W/m ) is given by: abs DNI cos IAM R E SF (1) opt Each parameter is described below. a. Direct Normal Insolation (DNI) S L Avail The direct normal insolation represents that portion of solar radiation reaching surface of the Earth that hasn't been scattered or absorbed by the atmosphere. The adjective "normal" refers to the direct radiation as measured on a plane normal to its direction. b. Angle of incidence () The angle of incidence is the angle between the beam radiation on a surface and the plane normal to that surface, as show in Fig.8. Fig.8 - Angle of incidence on a parabolic trough collector This angle varies over the course of the day and results from the relationship between the Sun's position in the sky and the orientation of the collectors for a given location. To determine the position of the Sun is necessary to know some angles. One of them is the declination angle. This angle is the angular position of the sun at solar noon, with respect to the plane of the equator. As the Earth rotates around the Sun through the course of a year, the declination angle will change, within a range of -3.45º 3.45º. The Fig.9 illustrates this description. Fig.9 - Declination angle The declination angle is given by: 84 n 3.45sin 36 () 365 where n is the day number of the year (since January 1st). The position of the Sun also depends on the hour angle, that represents the angle between the local meridian and the plane containing the center of the Sun. The hour angle is zero when the Sun is in the line with the local meridian. This angle comes as a result of the rotation of the Earth, which spins on its axis at a rate of 15º per hour, and is given by (in degrees): 15( t 1) (3) s where t s is the solar time in hours. To obtain the solar time is necessary to adjust the local time (LCT). The relationship between solar time and local time (in hours) is: t s EOT LCT LC D 6 where LCT is in hours, EOT is the equation of time in minutes, LC is the longitude correction in hours and D is the Daylight Savings Time adjustment. The equation of time is the difference between the mean solar time and real solar time. The equation of time used is: EOT E1 E cos B E3 sin B E4 cos B E5 sin B E ; E. ; (5) E ; E ; E where: 36 B ( n 1) (6) 365 The longitude correction is the component that reflects the difference between time of the local meridian and the time of standard meridian. This factor is given by: L loc L LC st (7) 15 where L loc is the local longitude and L st is the standard meridian longitude, both in degrees. The D is 1 during Daylight Savings Time and during standard time. (4)

5 The last angle required to determine the Sun's position is the zenith angle. The zenith angle is the angle between the line of sight to the Sun and the vertical, it is the complementary angle of solar altitude angle. It can be related to both the declination angle and the hour angle by the following relationship: cos Z cos cos cos sin sin (8) where is the latitude of the place. Once the declination angle, hour angle and zenith angle are known, the angle of incidence can be calculated. The incidence angle for a plane rotated about a horizontal northsouth axis with continuous east-west tracking to minimize the angle of incidence is given by: cos cos Z cos sin (9) c. Optical efficiency ( opt ) Since the solar radiation passes through the aperture plane of the collector, until to be absorbed by the absorber tube, various losses occur. The optical efficiency counts these losses as is given by: opt (1) where is the mirror reflectivity, is the interception factor, is the transmissivity of the glass envelope and is the absorbtivity of the absorber tube. d. Incidence Angle Modifier (IAM) The optical parameters are factors that are reduced when the angle of incidence increases. To account this effect its introduced a new parameter called incidence angle modifier, that is given by: K IAM (11) cos In this equation, K is: K cos k1 k (1) where k 1 and k are characteristic coefficients of each type of collector, obtained experimentally. e. Row Shadow (R s ) Due to the low solar altitude angle of the Sun in the morning, the most eastern row of the collectors will receive full Sun, but this row will shade all subsequent rows in the west. As the Sun rises and the collectors track the Sun, this mutual row shading effect decreases, however it re-appears in the later afternoon and evening. The Fig.1 shows this phenomenon during the morning. Fig.1 - Row shadow effect in the morning Row shading decreases the collector performance by decreasing the amount of radiation incident on the collectors. The width of the mirror aperture which receives incident radiation is defined as the effective mirror width. The row shadow factor is the ratio of the effective mirror width to the mirror wide and can be related with zenite angle, incidence angle and length of spacing between rows of troughs by: R S Wef Lspacing cos Z (13) W W cos where W ef, W and L spacing are in meters. This equation is bounded with a minimum value of (rows are fully shaded) and a maximum value of 1 (rows are not shaded). f. End Losses (E L ) End losses occur at the ends of the absorber tubes, where, for a nonzero incident angle, some length of the absorber tube is not illuminated by solar radiation, reflected from the mirrors, as show in Fig.11. Fig.11 - End loss at the end of the absorber tube The end losses are a function of the focal length of the collector (f), the length of the collector (L col ) and the incident angle: E L tan 1 f (14) L col where f and L col are in meters. g. Solar field available (SFAvail) This factor is the fraction of the solar field that is operable and tracking the Sun. ii. Thermal Losses The al losses occur on collectors and also on solar field piping, like the pipes linking the collectors and loops. a. Collector al Losses As the in the receiver tubes absorbs energy, its temperature will increase. This temperature increase creates a difference between the temperature of the fluid and the temperature of the surrounding ambient air. These losses occur by mechanisms of conduction, convection and radiation. An expression to quantify these losses (in W/m) is:

6 losscol L L c1 c L c 1 at1 a1t 1 a L c (15) ( bt 1 b1t 1 b)( DNI /9)cos where a i and b i are characteristic coefficients of each type of absorber tubes and T 1 represents the difference between the temperature (T ) and the ambient temperature (T amb ). A temperature (in ºC) is given, approximately, by: T Tfield outlet Tfield inlet (16) where Tfield outlet and T field inlet are, respectively, the outlet temperature and the inlet temperature of the in solar field. Division by the mirror aperture width is performed to express the receiver heat loss as per unit of mirror aperture area (W/m ). b. Solar Field iping Thermal Losses Thermal losses from the piping leading to and from the loops in the solar field are accounted (in W/m ) by following equation: loss pip L 3 p1t1 LpT1 Lp3T1 L.1693 p1 ; L. p 1683; L p iii. Collected ower 7 (17) The collected power, per unit of aperture area, is the difference between the absorbed power and al losses and is given by: collected abs losscol loss pip (18) Thus, the mass flow (in kg/s) available from the solar field is: collected Aaperture m (19) c ( T T ) p fieldoutlet fieldinlet where A aperture represent the total area of mirror surface of the solar field (m ) and c (J/kg.ºC) the specific heat of the. p For the Therminol V-1, the specific heat is given by: c 4 p T T () 3.3 Thermal Storage System Constitution and principle of operation The al storage system uses as storage medium a mixture of 6% sodium nitrate (NaNO3) and 4% potassium nitrate (KNO3). Therefore, it can be considered as a salt. The system is composed by: two al tanks, a cold one and a hot one, -to-salt heat exchangers and circulation pumps, as show in Fig.1. The salt is storage in the cold tank at 91ºC and at 384ºC in the hot tank. Fig.1 - Thermal storage system elements The basic operation strategy is to charge al storage when the flow rate exceeds the necessary flow rate for steam generation. Surplus flow travels through the -to-salt heat exchangers and the salt leaves the cold tank and extracts heat from the, and then enters the hot tank. When the flow rate provided by the solar field is insufficient, the al storage system is discharged. The discharge process occurs in the same heat exchangers but the flow is reversed and the salt is pumped from the hot tank, to heat the, into the cold tank. The first law of odynamics requires a temperature drop across heat exchanger, which implies that the flow rate, heated by the salt in discharge process, will be at a lower temperature than the temperature of the flow rate directly from solar field. This decrease in temperature will result in a decrease in power generation Model The model that describes the behavior of the al storage system can be obtained considering the heat exchangers as one and getting the power balance equation for that exchanger. Thus, the charge process is described by the following equation: m c ( T T ) m c T (1) salt p salt hot tank cold tank ex sto And the discharge process is given by: m c T m c ( T T ) () sto p p ex salt p salt hot tank coldtank where m represents the mass flow rate that travels sto through the heat exchanger (kg/s) and T is the difference between the inlet and outlet temperatures of the in the exchanger. The salt flow rate that travels between tanks is represented by m (kg/s) and the specific heat by c (J/kg.ºC). Finally, salt T hot t and ank coldtank p salt T are, respectively, the temperatures of the hot tank and cold tank, and efficiency of the exchanger. The specific heat of the salt is given by: c 17 ex is the p T salt salt (3) where T salt represents the temperature of the salt (ºC). 3.4 ower Block Constitution and principle of operation The power block of the Andasol-1 power plant is, essentially, a Rankine power cycle of 5 MW with reheat. The Rankine power cycle, which is reproduced in Fig.13, has as main

7 characteristics elements: two turbines, in series, one of high pressure and the other of low pressure, a reheater, five closed feedwater heaters, a deaerator, a steam generation system, two pumps and a condenser. outlet of the deaerator, to a pressure slightly higher than the input in the system of the steam generator, as it still has to cross two heaters until it reaches that point. With the return of feedwater to the pre-heater, part of the steam generation system, the cycle is complete Model The al power delivered by the (in W) is given by: m c T Tpb ) (4) pb ( p pbinlet outlet where (kg/s), m is the mass flow rate sent to power block pb T pb and inlet pboutlet temperatures of the power block. T are the inlet and outlet The total al power delivered to the cycle is given by: steam. gen (5) Fig.13 - Flow diagram for power cycle It should be noted the inclusion of a reheater, in order to avoid an excessive humidity fraction in steam, upon leaving the turbine, since the presence of water droplets reduces the useful life of the turbine, due to the wear caused by the shock with the turbine blades. To reduce the content on dissolved gases, such as oxygen and carbon dioxide, its introduced a deaerator, preventing this way, corrosion problems associated with the presence of those in hot water. It s also important to notice that a cooling tower is associated to the condenser. The cycle begins when the passes through the steam generation system, constituted by three sections. In the first section, called pre-heater, the feedwater temperature, in liquid state, is increased to the saturation point. Then it passes into an evaporator, being transformed in high quality steam. In the final section, a superheater will produce superheated steam, through high temperature at that point, at 371ºC and 1 bar. The superheated steam travels through the high pressure turbine, where it expands and, consequently, propels the turbine blades. At this point of the cycle, two extractions are taken from the high pressure turbine, which will be used to preheat feedwater in two closed feedwater heaters (#5 and #6 in Fig.13). Upon exiting the high pressure turbine, the steam is directed to a reheater, where it is superheated to approximately the same temperature reached outside of the superheater and a pressure of about 17.5 bar. This superheated steam then passes to the low pressure turbine, where again the steam expands and propels the turbine blades. In this turbine, four steam extractions are taken: one is directed to the deaerator (#4) and the remaining three to the feedwater heaters (#1 - #3). The steam leaving the low pressure turbine is condensed in a surface condenser by heat exchange with circulating water. This water, which circulates in a closed circuit, is cooled using an induced draft cooling tower. The feedwater, resulting from the condensed steam, is pumped to a sufficiently high pressure, about 15 bar, to allow it to pass through the three low pressure feedwater heaters and into the deaerator. The feedwater is pumped again at the where steam.gen is the efficiency of the steam generator and the reheater. In this context the mechanical power available from the turbine is: (6) mec where is the al efficiency of the cycle. In the cycle under consideration, this can be given in terms of power delivered to the al cycle, based in Montes, M. J. et all. (9), by: (7) with in MW. Finally, the electric power that can be delivered to the net is given by: net. (8) mec loss el aux where loss.el are the electrical losses on the generator and on the transformer and aux are the losses associated with the auxiliary services of the power plant. 4. SIMULATION Based on the models presented in the previous section a parabolic trough power plant simulator was conceived. The main parameters of the simulator are presented in Appendix A. The simulator was used, initially, to make a model validation and also to obtain, for some regions of the portuguese territory, the daily performance, the annual performance, the influence of al storage system and operation at a guaranteed power of the power plant. For this task there were required the hourly values of DNI, obtained at SODA site, and ambient temperature, obtained at VGIS- Europe site. 4.1 Validation To make the validation test there were used the simulator input values corresponding to the location of the Andasol-1 power plant, in Guadix, Spain. For the 876 hours of the year there were obtained the results of the Table.

8 Insolation (W/m ) Insolation (W/m ) Insolation (W/m ) Insolation (W/m ) Table - Obtained results for Andasol-1 location Annual DNI (kwh/m ) 4.4 Electrical power produced (GWh) Annual efficiency 16.6% Data of Solar Millenium (8) indicate that for 136 KWh/m of annual DNI, the power plant produces approximately 18 GWh of electricity. The result is an annual efficiency of 16.5%. As can be seen, this value is similar to the annual efficiency obtained in the simulation, which may represent an indicator of the validity of the simulator developed. 4. Daily erformance To illustrate the daily performance, of the parabolic trough power plant, it was made the hourly average of the DNI's and the ambient temperatures during the summer days as well as the winter days. This procedure aims to obtain a typical day that characterizes each of the seasons. To calculate the position of the Sun, necessary for the simulation, it was considered February 3rd, for the winter, and August 5th, for summer. The results obtained for Beja and Bragança are presented in Figs.14, 15, 16 and Fig.14 - erformance in a typical summer day, in Beja Beja Hour of day DNI ower Collected ower to Storage ower from Storage ower to ower Block Electrical ower Bragança Hour of day DNI ower Collected ower to Storage ower from Storage ower to ower Block Electrical ower Fig.15 - erformance in a typical summer day, in Bragança ower (MW) ower (MW) Fig.16 - erformance in a typical winter day, in Beja Fig.17 - erformance in a typical winter day, in Bragança In a typical summer day, the operation of the plant begins, approximately, at 8 am and the nominal operation is reached at 9 am for each location. The charge of the al storage starts at this moment until 6 pm when the discharge begins, due to reduced solar radiation. In Beja the stored energy is higher, for this fact the operation ends later than in Bragança. The operation during a typical day of winter, for each location, starts at 1 am and the possibility to store energy is, practically, nonexistent. The power plant operates for about 7 hours at a variable power and the nominal power its only achieved at 3 pm in Beja. 4.3 Annual erformance To obtain a more global notion of the performance of the power plant, for these locations, their behavior was simulated for 365 days of the year and there were obtained values of the energy produced, annual operation hours at nominal power and annual efficiency. These results are presented in Table 3. Table 3 - Obtained results for annual performance Location Annual DNI (kwh/m ) Beja Hour of day DNI ower Collected ower to Storage ower from Storage ower to ower Block Electrical ower Bragança Hour of day DNI ower Collected ower to Storage ower from Storage ower to ower Block Electrical ower Energy produced (GWh) Annual operation (hours) Annual eficiency Beja % Bragança % ower (MW) ower (MW)

9 Insolation (W/m ) Operation (hours) Insolation (W/m ) Operation (hours) The energy produced in Beja is substantially higher than Bragança, due to the difference between the levels of solar radiation verified for each location. In Bragança the annual efficiency is also lower due to the low temperatures that causes an increase of al losses. 4.4 Influence of Thermal Storage System To evaluate the influence of al storage system it was performed the same simulation mentioned above but without al storage. The results obtained are in Table 4. Table 4 - Obtained results for influence of al storage Location Energy produced (GWh) Annual operation (hours) Annual efficiency Beja % Bragança % Is shown in Figs.18 and 19 the influence of al storage system, for each month, in the operation hours at nominal power Beja lower levels of radiation and for this is no such need to have a al storage system. 4.5 Operation at a Guaranteed ower In previous simulations the parabolic trough power plant had a mode of operation to obtain the maximum power at all times, charging the al storage just when the nominal operation were guaranteed. However, it is interesting to simulate the performance of the plant when it wants to get an operation at a guaranteed power for long periods of time. When that power is achieved, the storage is charged. In the process of discharge the aim is also ensuring only this power. The operation of the plant will only be made at a higher power, if the al storage is already full. It was considered the operation of the plant, in Beja, to some values of guaranteed power that allow the operation during most of the time, throughout the summer and the winter. The results obtained are presented in Tables 5 and 6, where it can also be observe the hourly average deviation. Is shown in Figs., 1 and the operation at guaranteed power of 5 MW during the summer. Table 5 - Operation at guaranteed power during the summer ower (MW) Time of guaranteed power Hourly Average Deviation (MW/h) % % % DNI Electrical ower 6 Jan Fev Mar Abr Mai Jun Jul Ago Set Out Nov Dez With Thermal Storage Without Thermal Storage 7 4 Fig.18 - Comparative of operation hours, with and without storage, for each month, in Beja 6 Bragança ower (MW) Jan Fev Mar Abr Mai Jun Jul Ago Set Out Nov Dez With Thermal Storage Without Thermal Storage Fig.19 - Comparative of operation hours, with and without storage, for each month, in Bragança In the plant operation without al storage there is a significant reduction in the energy produced, about 5 GWh in Beja and 35 GWh in Bragança. This reduction occurs especially in the summer months. In this situation, Bragança presents a higher annual efficiency than Beja, because has Hour Fig. - Operation at guaranteed power of 5 MW between June 1th and July th DNI Hour Electrical ower Fig.1 - Operation at guaranteed power of 5 MW between July 1th and August th ower (MW)

10 Insolation (W/m ) DNI Hour Electrical ower Fig. - Operation at guaranteed power of 5 MW between August 1th and September th Table 6 - Operation at guaranteed power during the winter ower (MW) Time of guaranteed power Hourly Average Deviation (MW/h) % % % 4. Through the results, its observed that throughout the summer is possible to ensure a significant power during much of the time. For the winter, the power values are reduced drastically. 5. CONCLUSIONS This paper showed that the CS systems, specifically the parabolic troughs, should be an option to take in account in the development of the portuguese energetic park. The inclusion of a al storage system in the power plant, enable this technology to produce electricity after the sunset and the production can be adapted to the consumption. One of the conclusions drawn in this study is the annual efficiency of the plant, about 16%. In a typical summer day the power plant operates during 16 hours, covering the period of peak of the load diagram. However in a typical winter day the operation occurs only during 7 hours of very irregular way. The annual energy produced can exceeds the 173 GWh. The studied power plant can be operate at a significant power guaranteed, during the most of the time, in the summer. This situation does not arise in the winter. For this facts its possible to conclude that the parabolic trough power plant presents a good performance in the summer, contrary to what occurs in the winter. The reverse happens, normally, in wind and hydric power plants. For this, the system studied can represents a complement of the electrical production of these plants. A solution to improve the performance of the power plant, especially in the winter, is the hybrid operation using nonrenewable resources. In this context it can be used fuel burners to heat the HTF or directly in the steam generation. Another solution is to add the solar field to fossil fuel plants such as coal plants or combined-cycle natural gas plants ower (MW) REFERENCES Montes, M. J. (8). Análisis y propuestas de sistemas solares de alta exergía que emplean agua como fluido calorífero, Tesis, Escuela Técnica Superior de Ingenieros Industriales. Montes, M. J., Abánades, A., Martinez-Val, J. M. (9). Solar multiple optimization for a solar-only al power plant, using as heat transfer fluid in the parabolic trough collectors, Escuela Técnica Superior de Ingenieros Industriales. atnode, A. (6). Simulation and erformance Evaluation of arabolic Trough Solar ower lants, Thesis, University of Wisconsin-Madison. VGIS-Europe. Available at Silva, J.. (11). Central de Canal arabólico: Modelação e Simulação do Sistema, MSc Thesis, Instituto Superior Técnico. SODA - Solar Radiation Data. Available at Solar Millenium (8). The parabolic trough power plants Andasol 1 to 3 - The largest solar power plants in the world, available at Appendix A. SIMULATOR ARAMETERS Variable Description Value k Coefficients of IAM k W Collector aperture width 5.76 m L spacing Length of spacing between rows of troughs 17. m f Focal length of the collectors 7.1 m L col Collector length 15 m Fraction of the solar field that is SF Avail operable 1 a.154 a 1.1 a Coefficients for al losses b.36 b 1.9 b A aperture Total aperture area 51 1 m max Maximum mass flow rate m provided by solar field 117 kg/s c Oil specific heat J/kg.ºC p c p salt Salt specific heat J/kg.ºC m salt Mass of salt available 8 5 kg ex Heat exchanger efficiency.97 nom m pb Oil mass flow rate sent to power block in nominal conditions 59.5 kg/s steam.gen Steam generator efficiency.98 nom Thermal efficiency in nominal conditions.381 nom lant nominal power 5 MW net