Annual Simulation of the Thermal Performance of Solar Power Plant ...

Proceedings of the ASME 2016 10th International Conference on Energy Sustainability ES2016 June 26-30, 2016, Charlotte, North Carolina

ES2016-59516

ANNUAL SIMULATION OF THE THERMAL PERFORMANCE OF SOLAR POWER PLANT FOR ELECTRICITY PRODUCTION USING TRNSYS Mohamed H. Ahmed National Research Centre Dokki, Giza, Egypt

Alberto Giaconia ENEA Casaccia research center Casaccia, Rome, Italy

Amr M. A. Amin Academy of Scientific Research and Technology Kaser Elainy, Cairo Egypt ABSTRACT The process of generating electricity using solar energy took a great interest in the recent period for its contribution to the reduction of the fossil fuel consumption and the harmful emissions to the environment. The main task of this article is to simulate the thermal performance of a solar power plant for electricity production using a parabolic trough concentrator for accumulating the solar heat. The plant includes a stratified storage tank, steam generator, steam turbine and an electric generator. The simulation studies the effect of the design parameters of the solar field and the storage tank on the annual performance of a 1 MWe solar electric power plant. The simulation platform TRNSYS was used to model the solar power plant including the solar concentrator field, the storage tank, and the steam generator. The simulation predicts the instantaneous and annual heat energy collected by the solar concentrator and the heat energy rate supplied, extracted, and stored in the storage tank. It predicts also the rate and the quality of the steam produced. This analysis was applied to four sites in Egypt to study the effect of the solar radiation on the energy produced in those sites.

by extraordinary pioneers such as Eriksson [2], Eneas [3], Shuman [4], and Francia [5, 6]. In the 1980s, the first large trough, dish and tower arrays were installed in response to the challenges of the1970s oil crises. The establishment of the solar power plants using solar concentrators increased significantly in many countries in recent periods. Electricity production using the solar power plant is affected by different designs, operations, and meteorological parameters. Modeling and simulation activities play a key role in the design phase and the performance optimization of complex energy processes. It is also expected that they will play a significant role in the future of power plant maintenance and operation. Numerous efforts have been made by scientists for reaching the optimal variables that can lead to higher efficiency and the highest possible electricity production. Modeling the behavior of the solar power plant was carried out by different scientists. Some studies have been conducted to simulate separate components of the solar power plant as there are other studies performed on all components of the plant. Hottel and Whillier developed a model to calculate the conversion of solar radiation into useful thermal energy [7]. The collector heat loss coefficient is considered constant along the absorber. Cooper and Dunkle derived a differential equation to take into account the variation of the heat loss coefficient [8]. Fraidenraich et al. obtained a closed form solution that enables the calculation of the profiles of the absorber temperature, fluid temperature, and the power gained along a parabolic linear focus collector [9]. A number of proprietary computer performance simulations have been developed for modeling the performance of parabolic trough plants. Luz international limited developed an hourly simulation model that was used to help in designing the solar electric

INTRODUCTION Solar thermal electricity may be defined as the result of a process by which directly collected solar energy is converted into electricity through the use of concentrator. Solar thermal electricity on grid was not achieved until the 1980s. However, the basic technology for the production of mechanical energy (which could be converted to electricity using a conventional generator) had been under development for about 140 years, beginning with Mouchot and Pifre in France [1], and continued

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generating systems (SEGS) plants [10]. Flabeg solar international developed a performance simulation model to market parabolic trough plants and conduct design studies for clients [11]. The behavior of a typical 30 MWe plant was studied by Lippke [12]. Part of this analysis was a new solar field model derived based on extensive experimental results of an Ls-2 collector. Rolim et al. developed an analytical model for a solar thermal electric generating system with parabolic trough collectors [13]. The energy conversion of solar radiation into thermal power along the absorber tube of the parabolic collector is studied taking into consideration the nonlinearity of the heat losses and its dependence on the local temperature. The conventional Rankine cycle is treated as an Endoreversible Carnot cycle, whereby the mechanical and electric power is calculated. Good agreement is obtained when comparing the results of this model with an experimental data belonging to SEGS installed in the Mojave Desert. Mokheimer et al. developed code to evaluate the optical and thermal efficiencies of a parabolic trough collector (PTC) solar field; in addition to detailed cost analysis of the solar field [14]. In this regard, a computer simulation code was developed using engineering equations solver (EES). This simulation code was validated against ThermoFlex code and data previously published in the public literature. Excellent agreements between results of the two models were observed. Baligh used an open source Modelica library called ‘ThermoSysPro’ [15]. This library has been mainly designed for the static and dynamic modeling of power plants. It can be used also for other energy systems such solar system. Baligh focused on solar electricity generation for which two test cases were developed: a dynamic model of a linear parabolic trough solar power plant and a model of a solar hybrid combined-cycle power plant with a linear Fresnel field. The used model gives an accurate figures and checks precisely the performances and the design given by the manufacturers beside other advantages. Popel et al. developed a model based on TRNSYS software for mathematical simulation of solar thermal power plants [16]. The model has been devoted to simulation and thermodynamic analysis of the Hybrid Solar-Fuel Thermal Power Plants (HSFTPP) with gas turbine installations. Three schemes of HSFTPP have been assembled and tested under TRNSYS. For this purpose, 18 new models of the schemes components such as gas and steam turbines; compressor, heat exchangers, steam generator, solar receiver, condenser, and controllers have been elaborated and incorporated into the TRNSYS library components. DLR and Sandia national laboratories (SNL) have developed a special library for use with the TRNSYS thermal simulation software to model parabolic trough solar power plants [17]. The TRNSYS simulation environment was selected for use in modeling solar thermal power systems for a number of reasons including modularity, flexibility, and ease of use. Commercially available power cycle modeling codes have many standard components; but frequently limit the user’s ability to create new components, tend to be quite expensive, and are not capable of modeling annual performance using weather file data

as input. The latest update of TRNSYS, version 17, was used for this work. It has a number of improvements to the graphical user interface that were found to be very useful. The objective of this paper is to study theoretically the effect of the operating parameter on the performance of the plant components and on the overall plant performance. It also investigates the effect of different plant locations on the plant performance. A model of the solar field, storage tank, and the power cycle was developed using the TRNSYS program. This model includes the steam generator, steam turbine, and the electric generator. The actual version of the TRNSYS which is distributed among users includes standard TRNSYS component library and provides customers with the opportunity to simulate operation and to carry out transient analysis of different types of solar installations [18]. In addition, the Thermal Energy Systems Specialists TESS library incorporated into the TRNSYS library components presented opportunity to use new components for the solar power plant such as the cooling tower, steam turbine, steam generator…etc. A detailed model of a real solar thermal power plant is built using steady state power plant simulation software. The plant includes numerous parabolic trough collectors tracking the sun on a single axis. A heat transfer fluid flows in the focal line of the troughs collecting solar heat. This heat is transferred to high pressure water from the power block. This process generates steam at high pressure and temperature where it is pumped to a steam turbine. The steam turbine is connected to an electric generator. DESCRIPTION OF THE SYSTEM As shown in Fig. 1, the system consists of a numerous number of parabolic trough collector PTC forming the solar field. The solar field contains three loops. Each loop contains six collectors. The length of the parabolic trough collector is 98.4 m and the parabola width is 5.9 m. The solar power plant contains also one stratified thermal storage tank, steam generator, steam boiler for the insufficient solar energy case, steam turbine, electric generator, and pumps. The parabolic trough collector concentrates the solar radiation on a receiver tube located on the focus line. The receiver tube absorbs the concentrated radiation and transforms it into heat. This heat energy is transferred to the Heat Transfer Fluid (HTF) which is molten salt mixture (60% NaNO3+40% KNO3). The HTF flows from the receiver tube to a stratified storage tank. A constant speed pump delivers the HTF from the top of the storage tank to a steam generator where the HTF transfers its heat to pressurized water. Consequently, this water is transformed into superheated steam. The steam boiler is inserted between the steam generator and the steam turbine to compensate the shortage of the thermal energy supplied from the storage tank. The super saturated steam flows to the steam turbine which is connected directly to an electric generator. The outlet steam from the turbine flows to the condenser to be converted into liquid. Then it can be pumped again through the pump to the steam generator. Cooling tower was connected to the condenser to reject the heat from the cooling water.

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Pump Linear parabolic concentrator type536

Steam generator type636a

Steam turbine type628

Electric generator type599

Stratified Storage tank type4a

Pump

Condenser type598

Pump Pump

Cooling tower type51a

Figure 1. Description of the Solar Power Plant

high-temperature molten salt to heat the steam. This model will attempt to meet the user-specified steam outlet condition. However, it may be limited by the inlet molten salt temperatures and flow rate. This device may operate in either a counter-flow or a parallel-flow configuration. The device is assumed to be off if either of the inlet flow rate is zero, or if the inlet steam enthalpy is already at or above the desired outlet steam enthalpy.  Steam boiler (type 638): This component model will attempt to meet the specified steam outlet condition (outlet steam enthalpy) in case of insufficient heat energy from molten salt.  Steam turbine model (type 592d) simulates a steam turbine relying on an isentropic efficiency approach to calculate the performance of the steam turbine given the steam inlet conditions and the turbine backpressure.  Condenser model simulate the condensation process for the steam exiting the turbine so that it can be pumped through the steam generation system. The condenser model assumes a constant temperature difference between the condensate and the cooling water as well as a constant rise in cooling water temperature. Therefore, the condensing pressure depends only on the condensate inlet temperature.  Cooling tower: In the cooling tower a hot water stream is in direct contact with an air stream and cooled as a result of sensible heat transfer due to temperature differences with the air and mass transfer resulting from evaporation to the air. The air and water streams can be configured in either counter flow or cross flow arrangements. Ambient air is drawn upward through the falling water. Most towers contain a filler material which increases the water surface area in contact with the air. A cooling tower is usually composed of several tower cells they are in parallel and share a common sump. Water loss from the tower cells is replaced with make-up water to the sump. This component models the performance of a multiple-cell counter flow or cross flow cooling tower and sump.

MODELING OF THE SOLAR POWER PLANT. The proposed model of the plant was built using the TRNSYS simulation platform in addition to the Thermal Energy Systems Specialists TESS library. The TRNSYS model for the solar power plant consists of four loops as shown in Fig. 2. The first one is the thermal energy collecting loop that contains the solar concentrator field (parabolic trough concentrator), stratified storage tank, and a circulating pump. The second loop is feeding thermal energy to the steam generator which contains the stratified storage tank, the steam generator and the circulation pump. The third loop is the steam Rankine cycle which contains the steam generator, the power block (steam turbine and the electric generator), the pump and the condenser. The fourth loop is the cooling loop for the steam condenser. The component models are linked together to form the desired system. That system would permit flexibility in modeling different design and operating parameters for different configurations. The description of the function of each component model is presented next.  Linear Parabolic Concentrating Solar Collector (Type 536) models a type of solar collector called a linear parabolic concentrator that is commonly used in high-temperature applications. In this type, the fluid passes through a long evacuated tube that runs along the north-south axis. The Type 536 parabolic concentrator is modeled based on theoretical equations developed in Solar Engineering of Thermal Processes [19].  Stratified thermal energy storage tank with uniform losses (type 4a) models the thermal performance of a fluidfilled sensible energy storage tank subjected to thermal stratification. It is modeled by assuming that the tank consists of 20 fully-mixed equal volume segments.  Steam generator (type 636a): This component models the heat recovery steam generator (HSRG); a device which uses

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Figure 2. The Solar Power Plant TRNSYS Model

All design and operating data for all components such as the solar collector dimensions, the storage tank dimensions, and the capacity of the power block (the steam turbine and the electric generator) were provided for the simulation program. Table 1 presents some samples of these data.

THE RESULTS The results focus on the plant components’ performance at different meteorological parameters of the four selected sites in Egypt which are at different latitudes. The four selected sites are Mersa-Matruh (latitude 31.35, longitude 27.24), Cairo (latitude 30.04, longitude 31.23), Asyut (latitude 27.17, longitude 31.18), and Aswan (latitude 23.57, longitude 32.49). Results of 7 days were selected to present the performance of the plant’s components in summer (2-9/ Jun.) and in winter (1320/ Jan.). Figure 3 shows the beam radiation on a horizontal surface of the plant in summer. The beam radiation falling on the mirror area of about 10500 m2 recorded high values for Aswan and Asyut sites. At those sites, the radiation reaches a

TABLE 1. DESIGN PARAMETERS FOR THE SOLAR POWER PLANT.

Property Collector field

Quantity

unit

Collector type

PTC

-

Parabolic width

5.9

m

Collector length

98.5

m

No. of collector

6

-

No, of loop

3

Total mirror area

10500

m2

Mirror reflectivivty

0.96

-

Concentration ratio

0.85

-

Storage tank Type

Cylindrical

Tank volume

40

m3

Tank height

5

m

Tank loss coefficient

3

kJ/h m2 K

Steam boiler capacity

4

MW

Electric generator output

1

Mwe

Turbine capacity

1

Mw

Power block Figure 3. The Instantaneous Beam Radiation Incident on A Horizontal Surface in Four Selected Sites of Egypt From 2- 9/Jun.

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value of about 35000 kJ/hr. The lowest value for the beam radiation falling on the mirror appears in Cairo where it records a value of 28000 kJ/hr. This low value of the beam radiation in Cairo can be attributed to the pollution and humidity which has a big role in transforming the direct beam radiation into diffuse radiation. The results of the simulation program for three days in June illustrate the thermal energy collected from the parabolic trough collector field for the different selected sites as shown in Fig. 4. From the figure we can observe that Aswan and Asyut have nearly the same profile and higher values of the collected energy from the parabolic trough collector compare to Cairo and Mersa-Matruh. The maximum thermal energy rate gained from the collected in Aswan and Asyut is 8600 and 7950 MJ/hr, respectively. The maximum thermal energy rate gained in Cairo and Marsa-Matruh is 6760 and 6580 MJ/hr, respectively. The reduction ratio of the thermal energy collected from solar concentrator due to installing the plant in Cairo and MersaMatruh ranges from 14 to 19 %. The molten salt outlet temperature of the parabolic trough collector was presented for the select four sites though 7 days in summer. As observed in the previous figure for the beam radiation and the thermal collected energy, the outlet temperature of the molten salt has a higher value in Aswan and Asyut compare to Cairo and Mersa-Matruh as shown in Fig. 5. The maximum outlet temperature of the molten salt is 563, 502 and 478 °C for Aswan, Mersa-Matruh and Cairo, respectively. The previous results can be attributed to the high value of the beam radiation and the ambient temperature in the south area of Egypt. The inlet steam conditions to the steam turbine were adjusted to be as follows: the inlet enthalpy is 3260 kJ/kg ᵒC and the inlet pressure is 50 bars. In addition to the steam generator, a fuel steam boiler was added to the plant to ensure supplying the previous steam conditions to the steam turbine.

Mersa-Matruh

Cairo

Asyut

Aswan

Cairo Aswan

Temperature [º C]

Mersa-Matruh Asyut

2

Time [hr]

Figure 5. The HTF Outlet Temperature From the PTC Collector Through Seven Day for the Selected Sites of Egypt.

Figure 6 presents the thermal heat energy required for the steam boiler to ensure the supply of constant steam conditions at different solar collector areas for the four selected sites of Egypt. The collector surface area varied in the simulation from 8720 to 13930 m2. The annual energy required by the steam boiler decreases with increasing the collector mirror area for all sites under investigations. Increasing the collector area from 8720 to 13930 m2 leads to a decrease in the energy required from the boiler by about 105, 85, 54.7 and 76.4 GJ for Mersa-Matruh, Cairo, Asyut and Aswan, respectively. Comparing the energy required from the boiler at a collector area of 10500 m2, Aswan presents the lowest value, where the plant needs thermal energy of about 148 GJ from the boiler. While the required thermal heat energies from the steam boiler are 173, 240 and 168 GJ for Mersa-Matruh, Cairo, and Asyut, respectively. 2 m2 m2 2

m2

Energy [ GJ]

Rate of energy gained [MJ/hr]

m2

2

Mersa-Matruh

Time [hr]

Cairo

Asyut

Aswan

Selected sites

Figure 4. The Collector Thermal Power Gained for the Selected Sites of Egypt

Figure 6. The Total Energy Required by the Boiler at Different Collector Areas for the Selected Sites

5


18000

Figure7 illustrates the annual thermal energy gained from the solar parabolic trough collector, the annual electric energy generated, and the annual input energy required from the steam boiler for the selected sites. From the figure, we can observe that the highest annual energy gained by the solar concentrator is achieved in Aswan where it records 8300 GJ while the electric energy generated is 8130 GJ and the annual input fuel energy required by the steam boiler is 148 GJ. The lowest annual energy gained by the solar concentrator is achieved in Cairo, where it records a value of 3710 GJ while the electric energy generated is 3090 GJ and the annual fuel energy required by the steam boiler is 240 GJ. The rates of energy supplied from the solar concentrator to the tank and delivered from the tank to the steam generator were presented for three days in summer and winter in Fig. 8 and 9, respectively. From Fig. 8, we can observe a sharp increase and decrease in the quantity of heat energy supplied to the stratified storage tank from the solar concentrator before and after the solar noon. The maximum value of the energy rate that is supplied to the tank is about 16600 MJ. The energy supplied to the load (steam generator) from the storage tank is nearly stable around 8710 MJ. The period of supplying energy to the steam generator ranges from 6 to 8 hours. In winter, Fig. 9 presents the heat energy supplied from the solar concentrator and delivered to the steam generator. It has an average value of about 6650 MJ/hr which is small relative to the summer. The rate of heat energy delivered to the steam generator ranges from 6450 to 6880 MJ/hr. The stability of energy supplied to the load is very low due to the insufficient thermal energy gained from the solar concentrator. In addition, the period of receiving energy from the solar concentrator does not exceed 3 hours, while the rate of energy supplied to the steam generator from the storage tank ranges from 6880 to 7580 MJ/hr for a period ranging from 0.5 to 2 hours only.

Energy to load Energy from source

16000

Rate of energy [MJ/hr]

14000 12000 10000 8000 6000 4000 2000 0 3000

3010

3020

3030

3040

3050

3060

3070

Time [h]

Figure 8. The Rate of Energy Supplied from the Collector to the Storage Tank and Delivered from the Tank to the Steam Generator During Three Days in Summer.

Figure 10 presents the instantaneous efficiency of the plant which is defined as the rate of electric energy gained from the electric generator divided by the total incident beam radiation falling concentrator area for Aswan, Cairo and Mersa-Matruh. From the figure, we can compare the plant efficiency among the previous cities. From the comparison we observe that, a significant variety of the efficiency among the three cities. We can also see that, the lowest plant efficacy was observed in Aswan which is 9.5 %, while the highest efficiency was observed in Cairo which reaches around 31.4 %. The highest value of the efficient appear in the figure at the early mooring and late evening for all cities can be attribute to the low input solar energy while the output electric energy still constant, where the fuel boiler compensates the shortage of solar energy. 10000 Energy to load Energy from source

9000 Solar energy gained Electric_energy Boiler energy

8000

Energy [GJ]

Rate of energy [ MJ/hr]

7000 6000 5000 4000 3000 2000 2

1000 0 269 Mersa-Matruh

Cairo

Asyut

Aswan

279

289

299 Time [hr]

309

319

329

Selected Sites

Figure 9. The Rate of Energy Supplied from the Collector to the Tank and Delivered from the Storage Tank to the Steam Generator During Three Days in Winter

Figure 7. The Annual Collector Energy Gained, Electric and Boiler Energy for the Selected Sites

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Casaccia research center (Rome, Italy) for providing the required input data.

0.35 Mersa-Matruh Aswan Cairo

0.30

REFERENCES

0.20

[1] [2] [3] [4]

0.15

[5]

Efficincy

0.25

0.10

[6] 0.05

[7] 0.00 3000

3020

3040

3060

3080 Time [hr]

3100

3120

3140

3160

Figure 10. The Overall Efficiency of the Solar Power Plant for the Aswan, Cairo, and Mersa-Matruh During One Week in June.

[8] [9]

CONCULSIONS This article presents a dynamic simulation model of a complete solar power plant for electricity generation using TRNSYS platform along with the TESS library. The paper focuses on the dynamic simulation of the solar power plant at four different locations in Egypt. From the results, the locations of Aswan and Asyut have the highest rates of solar thermal energy gain. Those sites also have the highest outlet fluid temperatures from the concentrator. The beam radiations for these two Cities have higher values compared to the other cities. In summer, the outlet temperature range of the molten salt is from to 2 ᵒC for Aswan and Asyut, respectively. The total annual solar thermal energy gained from the selected sites are 8300, 6570, 3710 and 5530 GJ for Aswan, Asyut, Cairo and Mersa-Matruh, respectively. The annual input fuel energy required by the steam boiler for Aswan is 173 GJ which is the lowest value compared to other cities. The fuel energy required by the steam boiler for Cairo has the highest value which is 207 GJ. The previous conclusions proved that Aswan and Asyut are the best sites in Egypt for installation of the solar plant. The proposed model successfully presents the dynamic simulation that can estimate the performance of all components of the plant at different conditions.

[10]

[11]

[12]

[13]

[14]

[15]

[16]

[17]

ACKNOWLEDGMENTS This work was funded and supported by a grant from the Academy of Scientific Research and Technology. (ASRT) and from the European Union’s Seventh Framework Program (FP7/2007-2013) through MATs project under grant agreement no. 268219. The authors are grateful to the ASRT and European Union’s Seventh Framework Program and also to the staff of the Energy Technologies Department (DTE) unit of ENEA-

[18] [19]

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