PERFORMANCE ASSESSMENT OF A SOLAR DRIVEN COMBINED STEAM AND ORGANIC RANKINE CYCLES

Transcription

1 PERFORMANCE ASSESSMENT OF A SOLAR DRIVEN COMBINED STEAM AND ORGANIC RANKINE CYCLES Fahad A Al-Sulaiman Mechanical Engineering Department King Fahd University of Petroleum and Minerals Dhahran, Saudi Arabia, fahadas@kfupmedusa Ahmet Z Sahin Mechanical Engineering Department King Fahd University of Petroleum and Minerals Dhahran, Saudi Arabia ABSTRACT In this paper, the performance improvement of a new solar thermal system which consists of combined steam and organic Rankine cycles as compared to another system that considers only steam Rankine cycle (SRC) is presented This study shows that there is a considerable efficiency improvement when combined system is used as compared to only SRC The thermal efficiency increases considerably from 25% for only SRC to 32% for the combined system The net electrical power increases from 61 MW for SRC to 78 MW when the combined system is used In addition, this study considers sizing different electrical power outputs from the combined system and the effect of that on the solar collector aperture area KEYWORDS solar energy, Rankine cycle, steam turbine, organic turbine, performance, efficiency 1 INTRODUCTION The depletion of fossil fuels and increment of CO 2 emissions requires finding alternative efficient energy sources Hence, there is a need to improve the existing energy technologies based on renewable energy sources Parabolic trough solar collectors (PTSC) are considered the most established solar thermal technology for power production Hence, this technology has been selected in this study as a heat source for steam Rankine cycle Several studies examined the performance of PTSC integrated with steam Rankine cycles, e g [1-4] However, there is no study that examined the performance of PTSC integrated with combined SRC and organic Rankine cycle (ORC) Therefore, the results of this study are original and will be useful to both industries and researchers Solar energy can be used to provide heat to steam Rankine cycles (SRC) An organic Rankine cycle (ORC) is a type of a Rankine cycle that uses an organic fluid and works at a medium or low temperature Thus, the ORC can be used as a bottoming cycle for SRC In this study, the performance of a combined system using SRC and ORC and integrated with PTSC is examined and compared with a SRC integrated with PTSC In this paper, key performance operating variables are examined These variables are electrical power output, thermal efficiency, thermal energy, and aperture area of the solar collectors 1

2 2 SYSTEMS DESCRIPTION In this study, two systems are examined The first one is PTSC combined with SRC, as shown in Figure 1 The second system is the same as the first one; however, an ORC is integrated with SRC as a bottoming cycle to SRC, as shown in Figure 2 The details of the geometric data of the solar collectors are given in Table 1 The type of the collector selected is LS-3 collector The temperature at the exit of the solar collectors is 663K (390 o C) [5] This temperature is considered the maximum practical operating temperature of the selected oil in the PTSC The oil selected is Thermonil-VP1 This oil has good heat transfer properties and a good temperature control [6], and, therefore, it is being used in many different power plants driven by PTSC, [3] and [7], =, / (4) where the subscript solp indicates the of the solar field The net power for the combined system, both SRC and ORC is defined as =, +, / (5) The net electrical efficiency for the non-combined system, considering only SRC, is defined as, =, The net electrical efficiency of the combined system is defined as = (6) (7) 3 ENERGY MODELING Energy modeling is presented in this section The main input data used in the code are given in Table 1 The energy analysis of the PTSC in this section is based on the equations presented in [8-10] The performance equations of the overall system are presented next The power produced by the steam turbine is defined as = (h h ) (1) where h is the enthalpy and the subscript st indicates steam The net power of the steam Rankine cycle is defined as, = / (2) where the subscript g indicates generator and the subscript mo indicates motor Similarly, the net power of the organic Rankine cycle is defined as, = / (3) where the subscript ot indicates organic turbine and the subscript op indicates the of the ORC The net work produced from the steam Rankine cycle is defined as Figure 4 illustrates the effect of pressure variation on the efficiency It demonstrates that the electrical efficiency, 4 RESULTS AND DISCUSSION Variation of steam turbine inlet pressure and steam turbine power on key performance parameters are studied These parameters include thermal efficiency and electrical power production of the system, area of solar collectors and the useful energy from the collectors The system is examined under solar irradiation density G b = 08 and G b = 07 kw/m 2 Figure 3 shows the effect of turbine inlet pressure variation on the electrical power It illustrates the power variation for two solar radiation densities = 08 and 07 kw/m 2 ) This figure quantifies the electrical power improvement gained when combined SRC and ORC are used as compared to only SRC The power increment is significant, which is approximately 28% when combined SRC and ORC are used It is shown that the net electrical power from the SRC cycle is around 61 MW for G b = 08 kw/m 2 On the other hand, when combined SRC and ORC are used, the power increases to around 78 MW In contrast, for the lower solar radiation density, G b = 07 kw/m 2, the net electrical power from the SRC is around 54 MW and when ORC is integrated, the electrical power rises to around 69 MW This figure illustrates that the maximum power for G b = 08 kw/m 2 is at a pressure of 10 MPa and for the G b = 07 kw/m 2 is at 115 MPa when only steam turbine is used, is approximately 25% Alternatively, when combined SRC and ORC are used, the 2

3 efficiency improves considerably to around 32 % Moreover, this figure demonstrates that for the lower solar radiation density case, G b = 07 kw/m 2, the pressure has more effect on the efficiency as compared to the higher solar radiation density case, G b = 08 kw/m 2 Figure 5 demonstrates the effect of the turbine inlet pressure on the useful energy from the solar collectors It can be noticed that as the pressure increases, the useful energy decreases noticeably This behavior is explained as follows As the pressure increases, the temperature at evaporator inlet, state 4, increases while the temperature at the evaporator exit, state 5, is kept constant Hence, the SRC absorbs less heat and, consequently, the temperature at states 3 and 1 are higher under high pressure at state 5, as compared to low pressure at state 5 As a result, the useful energy from the collectors decreases as the pressure increases where the temperature at state 1 is assumed constant, 390 o C, which is the maximum practical temperature for the selected synthetic oil (Therminol-VP1) Figure 6 presents the variation of the aperture area of the solar collectors and the electrical power produced from the steam turbine for G b =08 kw/m 2 case It is shown that as the electrical power from steam turbine increases, the total aperture area of the solar collectors increases linearly The aperture area increases from 320,000 m 2 at 50MW to 640,000 m 2 at 100MW On the other hand, the number of solar collector rows increases from 460 at 50 MW to 925 at 100 MW 5 CONCLUSION In this paper, the performance improvement of integrated ORC with SRC is quantified as compared to SRC; both operated by solar thermal energy The study reveals that considerable electrical power increase can be gained when combined SRC and ORC are used as compared to only SRC This study shows that for the G b = 08 kw/m 2 case, the net electrical power production is 61 MW for the SRC while, when combined SRC and ORC are used, the electrical power increases considerably to 78 MW Similarly, the electrical efficiency increases from 25% to 32% ACKNOWLEDGEMENT The authors acknowledge the support of King Fahd University of Petroleum & Minerals (KFUPM), Dhahran, Saudi Arabia, for this work through project # JF REFERENCES [1] JL Wolpert, and SB Riffat, Solar-powered Rankine system for domestic applications, Applied Thermal Engineering, Vol 16 (4) (1996), pp [2] Y You, and E J Hu, A medium-temperature solar thermal power system and its efficiency optimization, Applied Thermal Engineering, Vol 22 (4) (2002), pp [3] MJ Montes, A Abánades, and JM Martínez-Val, Performance of a direct steam generation solar thermal power plant for electricity production as a function of the solar multiple, Solar Energy, Vol 83 (5) (2009), pp [4] E Zarza, M E Rojas, L González, J M Caballero, and F Rueda, INDITEP: The first pre-commercial DSG solar power plant, Solar Energy, Vol 80 (10) (2006), pp , (2006) [5] LUZ International Limited, Solar electric generating system IX technical description, LUZ International Limited, (1990) [6] H Price, ; E Lüpfert, D Kearney, E Zarza, G Cohen, R Gee, and R Mahoney, Advances in parabolic trough solar collectors technology ASME, Journal of Solar Engineering, Vol 124, (2002), pp [7] Therminol, Heat Transfer Fluids by Solutia Inc, Therminol VP-1, wwwtherminol com/ pages/products/ vp-1asp, 2011 [8] F A Al-Sulaiman, I Dincer, F Hamdullahpur, Exergy modeling of a new solar driven trigeneration system, Solar Energy, Vol 85 (9) (2011), pp [9] S Kalogirou, Solar energy engineering: processes and systems Elsevier, 2009 [10] J Duffie, and W Beckman, Solar Engineering of Thermal Processes John Wiley & Sons, Inc 2006 [11] V E Dudley, G J koib, A R Mahoney, T R Mancini, C W Matthews, M Sloan, and D Keamey, Segs ls-2 solar collector test results Report of Sandia National Laboratories, SANDIA , (1994) [12] NREL, Heat Transfer Analysis and Modeling of a Parabolic Trough Solar Receiver Implemented in EES, NREL,

4 NOMENCLATURE A ap : Aperture area, m 2 G b : Solar radiation density, W/m 2 h : Enthalpy, kj/kg : Mass flow rate in the steam Rankine cycle ORC : Organic Rankine cycle SRC : Rankine cycle T : Temperature, K : Power, kw, : Power of the organic Rankine cycle, kw, : Power of the steam Rankine cycle, kw : Power of the combined steam and organic Rankine cycles, kw : Power of the steam turbine, kw Greek letters η : Efficiency Subscripts 0 : Atmospheric conditions el : Electrical power ev : Evaporator g : Generator i : Inlet j : Property value at state j o : Organic op : ORC ot : ORC turbine r : receiver st : u : Useful 4

5 Parabolic trough solar collectors field 2 5 Evaporator turbine 4 6 Electrical generator Solar 8 9 Condenser Fig 1: Schematic of parabolic trough solar collectors integrated with steam Rankine cycle Parabolic trough solar collectors field 2 5 Evaporator turbine 4 6 Electrical generator Solar ORC 11 8 Heat Exchagner 9 ORC turbine Electrical generator 10 Condenser Fig 2: Schematic of parabolic trough solar collectors integrated with the steam and organic Rankine cycles 5

6 W st, net W st, net W net W net W (kw) Q u, o (kw) Q u, o Q u, o P 5 (kpa) Fig 3: Power versus turbine inlet pressure P 5 (kpa) Fig 5: Useful solar energy versus turbine inlet pressure Electrical efficiency (%) η el, st η el, st η el, st + ORC η el, st + ORC Number of collector rows A ap (m 2 ) P 5 (kpa) W st (kw) Fig 4: Electrical efficiency versus turbine inlet pressure Fig 6: Number of solar collector rows and total aperture area versus steam turbine power 6

7 TABLE 1 MAIN INPUT VALUES TO THE SYSTEM and Rankine cycles and ORC turbine efficiency 85% and ORC efficiency 80% Baseline mass flow rate of the steam Rankine Cycle 94 kg/s Baseline steam turbine inlet pressure 115 MPa Baseline organic turbine inlet pressure 12 MPa Baseline organic inlet pressure 035 MPa Baseline steam turbine inlet temperature 663K Pinch point temperature of the evaporator 10 K Electrical generator efficiency 96% Electrical motor efficiency 96% Solar collectors [4] and [12] Single collector width 576 m Single collector length 1227 m Ambient conditions Ambient temperature K Ambient pressure 1013 kpa 7