basic design and performance evaluation on 5 kwe solar orc under subtropical climate conditions

7/25/2019 Basic design and performance evaluation on 5 kWe Solar ORC under subtropical climate conditions

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Basic design and performance evaluation on 5 kWe Solar ORC1

under subtropical climate conditions2

3

M. Alvesa,* E. Loraa, J. Palacioa, A. Martnezb4

5

aExcellence Group in Thermal and Distributed Generation, Department of6

Mechanical Engineering, Federal University of Itajub, Itajub, Brazil7

e-mail:[email protected]

bCentro de Estudios de Refrigeracin, Universidad de Oriente, Santiago de Cuba,10

Cuba11

Abstract12

The aim of this paper is to present the basic design and an early performance evaluation on a 5 kWe Solar ORC13systems operating under subtropical climate conditions (Cwa) for distributed generation. The system is based on14commercial available equipments, therefore, the model take into account the main physical and mechanical15

phenomena based on experimental data for the main key components. The evaluation is performed using an R-16245fa as working fluid operating at 130C, for the solar irradiation variance throughout a year. The early17

performance analysis shows high amplitude on energy availability, due to the climate conditions, design criteria,18traditional strategy control and no thermal storage. Nevertheless, the system indicates great adeptly, capable of a19minimal average usage factor of 23% with 6.5% system overall efficiency, available for 86% of the year.20

21

Keywords: Renewable Energy, Concentrated Solar Power, Distributed Generation, Organic Rankine Cycle,22

LABS.23

24

INTRODUCTION25

Nowadays, there is already a scientific consensus that climate change is a reality and26

its main causes are human activities [1, 2]. The use of renewable energy sources is one of the27

solutions that can mitigate this problem. Concentrated Solar Power (CSP) technology has28

been expanding significantly in recent years due manufacturing reduction cost; reflect of29

investments on technological development in over the past 25 years. Today, its cost between304.2 up to 8.4 U$/Winstalled; expecting a reduction on the constructions cost of new CSP plants,31

depending on the Direct Normal Irradiation (DNI) on the site, that can reach from 75% up to32

84% of cost reduction by the year 2050 [3]. As showed in the Figure 1, CSP growth is33

expected to continue at large steps; is expected to be delivered over 2.5 GWp to be in34

operation by the end of 2017 [4, 5].35

36


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37Figure 1. Growth of CSP technology [3,5].38

39

However, most of the currently installed CSP plants use a steam Rankine cycle in the40

power block, whichs requires a higher collector temperatures and power density in order to41

be competitive [16]. On the other hand, recent analyses suggest that small-scale systems, such42

as Organic Rankine Cycle (ORC) with parabolic trough collector (PTC) could compete on43

costs of electricity generation with photovoltaic technology and even with Diesel generators44

for isolated areas [6]. Therefore, this paper presents the early start implementation of a solar45

laboratory, LABS, at the Federal University of Itajub, with the goal to start out the46

development and analyze the behavior on distributed solar thermal energy generation in47

Brazil.48

49

The first part of this paper describes the basic design and characteristics of the Solar50

ORC system (CROS). In the second part, is introduced the simulation procedure, which is51

able to perform a performance evaluation during the irradiation change. The last part of the52

paper points out the results of the simulation, evaluating the performance of a CROS for53

subtropical conditions in different seasons.54

55

SYSTEM DESCRIPTION56

The CROS system components are based on commercial available equipments57

(composed according Figure 2), and its basic operational parameters are based on previously58

works [1, 6, 8, 10, 12, 14, 16]; therefore the system is fully developed to maximize its59

availability under subtropical conditions. Characterized by a sub-critical dry-expansion fluid,60

whereas the working fluid adopted is R-245fa, which is heated on the evaporator by hot water61from the solar collector field. The working fluid drives a turbine for power generation, then62


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condensed into liquid state in a water-cooled condenser and its pumped back to the63

evaporator.64

65

66

Figure 2. CROS System.67

68

Solar Collector69

The solar collector is a parabolic through collector (PTC) using a water-base mixture70

as heat transfer fluid (HTF), with an efficiency range of 50-65% for temperatures between 5071

up to 170 C, respecting a maximum pressure of 12 bar. There are 9 collectors, with a total72

net area of 130 m, with single-axis track, aligned east-west, tracking only the sun altitude73

changes. This is the most appropriate configuration, minimizing the reflectance effects of74

solar rays and maximizes absorption during the noonday ensuring the possibility of75

maximum power, at least a period of the day, during the whole year for this latitude [7].76

77

ORC module78

The ORC module has an axial reaction turbine, with an efficiency of 85%, generating79

a nominal net power capacity of 5 kWe, and a peak of 7.5 kWe. The work fluid temperature80

is limited at a maximum of 160 C and maximum pressure of 25 bar. The condenser (4.5 m)81

and the evaporator (5 m) use a counter-flow gasketed-plate heat exchanger, able to operate82

until 180 C @ 25 bar. The pump system has a nominal efficiency around 75%. Also the83

others pumps have the same nominal efficiency.84

85

Operational parameters86

This system is designed to operate during the whole year; therefore the considered87

irradiation level is 500 W/m operating at 130 C. The nominal operational parameters are88


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showed in Table 1. Temperature and operational pressure are showed in a T-s diagram of the89

CROS system (Figure 3).90

91

Table 1. CROS system nominal operational parameters.92

Description: Value Observations:

Pump

consumption

ORC module 310 W Flow rate = 0.167 kg/sSolar collector field 290 W Flow rate = 1.15 kg/sCooling system 170 W Flow rate = 0.910 kg/s

Generation Total gross 6.1 kW Generator efficiency = 95%

Heat

Total irradiation input 65 kWORC module input 41 kWORC module rejection 34.9 kW Max. regeneration capacity = 2.5 kW

Efficiency

Solar collector field 63%CROS system 7.7% ORC module = 12.2%Equivalent Carnot cycle 29.9%Max. cogeneration capacity 60.7% Using all heat rejection

93

94

Figure 3. CROS System T-s Diagram for the working fluid R-245fa.95

96

MODEL97

This section describes the model of the CROS system, which estimated the system98

behavior during an irradiation variation. The modeling approaches were developed under99

MATLAB environment using thermodynamic data from FluidProp, which works with NIST100

references tables [8]. For the simulation was assumed a transient behavior of the solar source101

and the air temperature, whereas beholds a rate flow control strategy. Therefore, kinetic and102

water @1.15 kg/s

water @0.91 kg/s


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potential energy are neglected. Still, were considered pressure drop only on the equipments,103

neglecting on piping.104

105

The calculation routine starts performing on the solar collector field and on the106

evaporator, using a -NTU methodology [9], which is based on the nominal operation107

parameters, estimated a new flow rate on the system according with the amount of solar heat108

input. The exchanger heat capacity (C) is considered constant, given by the equation 1,109

whereas take into account the fluid flow rate (Fm), fluid enthalpy (h) and temperature (T)110

variation.111

112

C =

h. Fm

T (1)113

This new flow rate value affects the efficiency of all other systems, which are114

recalculated throughout an equivalent equipment efficiency equation (based on experimental115

performance test), according for the new heat supply by the solar collector field. For that, the116

efficiency of the solar collector (sc) is affected by the variation of temperature (TmHTF117

mean temperature) and solar irradiance (I), which can be represented by the Figure 4, and118

given by the equation 2[10]:119

120

= B. (A. ) + C. (A. ) + 0,002. + 59,8

= 2. 10 + 1. 10. 1. 10.

B = 2,0458 + 2,8. 10. 1. 10. C =(2,5164 + 5,7. 10. 3. 10. )

(2)

121

122

Figure 4. Solar collector efficiency range versus mean temperature and solar irradiance variance.123

40

50

60

70

20 80 140 200

SolarCollectorEfficienc

y[%]

Heat Transfer Fluid Temperature [C]

300 W/m

1000 W/m


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124On the turbine (t) and pump (p) the efficiency equation take in account the flow rate125

which is transformed into a dimensionless flow rate value (m)calculated by the actual flow126

divided for the nominal flow rate design which 1stand for the nominal flow rate design.127

Both equations 3 and 4 are represented by the Figure 5 and 6 respectively [9]. Also the power128

consumption on the pumps and the power generation at the turbine are modified to match129

these new flow rates, through the equation 5.130

131

= 0.64 () 28.13 () + 52.22 () + 59.7 (3)

= 25 () + 0.83 () + 71.83 () + 27.03 (4)

W =h. Fm

(5)

132

Figure 5. Turbine efficiency versus dimensionlessflow rate.

Figure 6. Pump efficiency versus dimensionless flowrate.

133

134

ANALYSIS METHOD135

The transient behavior of the solar source creates great difficulty to guarantee its best136

performance and functionality in all weather conditions. The climate conditions corresponds137

to a humid subtropical of highlands (Cwa) [11], generally in the form of hot humid summers138

and mild to cool winter, which is the local of the future installation of CROS system on139

LABS at UNIFEI (ItajubMinas Gerais). All of the radiation data (GHI) are retrieved from140

the weather stations on the campus, collect and store all data every 10 minutes. Therefore the141

analysis is based on a quasi-static simulation, following a traditional strategy control,142respecting the following control boundaries [12-14]:143

60

65

70

75

80

85

90

0.0 0.5 1.0 1.5

TurbineEfficiency[%]

Dimensionless flow rate [-]

20

30

40

50

60

70

80

0 0.5 1 1.5

PumpEfficiency[%]

Dimensionless flow rate [-]


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144

Overheating protection (solar collector170 C and ORC module160 C);145

Overcharge protection (turbine/generator7.5 kWe);146

Minimum heat (avoid part-load under 60% of nominal power capacity);147

Minimum starting radiation of 300 W/m (for solar collector primary pump);148

Fixed inlet turbine pressure (23 bar);149

150

Therefore the analysis method consists in two parts: the firsts one, present all the151

possible retrieved data from the model. To be able to better comprehension is used a152

hypothetical day, which consist of an annual mean solar radiation value. Although the second153

part is exactly the same procedure, the difference is the use of the annual solar radiation data,154

which allows to returns fully and daily operational values with a 10 minutes resolution,155

furthermore for enhance comprehension of some of the data is synthetized as a daily average156

value.157

158

Solar radiation chart159Generally solar systems use direct normal irradiation (DNI) as solar data, although the160

use of global horizontal irradiation (GHI) presents only 5% of error if compared with DNI for161

this type of usage and according to the site location [15]. Therefore this change of solar data162

is compensated by the reduce size of the solar system, which implies in losses due to albedo163

on the edges of the solar collector field.164

165

For that, there are two types of solar radiation input chart; the first one is an annual166

mean solar radiation present in the Figure 7, also the annual mean air temperature is included;167

which is considered as environment boundaries. Showed in Figure 8, the second type is a168

three dimensional chart of solar radiation of Itajub, through the graph it is possible to obtain169

information such as: the duration of the solar day (suitable for energy generation),170

environments effects, the amplitude of the radiation, etc., the axes are the respective:171

172

Day [x-axis]364 days, (separated monthly);173

Time [y-axis]From the 6:00 till 19:00;174

Radiation [z-axis]Range from 300 W/m (blue) until 1250 W/m (red).175

176


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177Figure 7. Annual average of solar radiation and air temperature.178

179

180

Figure 8. Radiation variability at Itajub above 300 W/m.181

182

RESULTS AND DISCUSSIONS183

184

Hypothetical day behavior185


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The first test considers a hypothetical 24h day period using solar data from the186

annual average radiation of Itajub (Figure 7). The output, allows the visualization of the187

complete CROS system behavior variation during 24h operation. The Figure 9, show the188

CROS system efficiency and equivalent Carnot cycle efficiency and the net power generated189

by the system. On the Figure 10 is showed the thermal behavior of the system, including solar190

collector, turbine and condenser temperature profile.191

192

193

Figure 9. CROS efficiency and equivalent Carnot cycle efficiency and net power generated.194

195

196

Figure 10. Thermal behavior.197

198

The CROS system first starts the solar collector field primary pump, around 8:30,199

which initiates the heating process of the HTF. Within 20 minutes the system reaches200minimal heat level and starts producing energy with 78% of its nominal capacity. In further201

Nominal Parameters

CROS efficiency 7.7%

Carnot efficiency 29.9%

Net Power 5 kWe

Nominal Parameters

Collector outlet 135 C

Collector inlet 126 C

Turbine inlet 130 C

Turbine outlet 65 C

Condenser outlet 40 C

Condenser inlet 30 C


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20 minutes the system reaches out its nominal producing energy capacity, using only 450202

W/m. Around 9:50, all the efficiency nominal values are reached. Although the solar source203

continues to increase the available power, the system starts to lower down its efficiency. At204

11:00, the protection system starts to operate, and the solar energy supply is cut off, until the205

system comes back to standard patter, this intervention continues for over 3 hours, when the206

irradiation level starts to decrease to sustainable levels. After over 7 hours of availability, the207

system reaches 60% of its nominal capacity and its shutdown.208

209

Annual behavior210

This time using the whole year irradiation data (Figure 8), is possible to create a three211

dimensional chart with the efficiency behavior among a year, Figure 11, with the respective212

axes:213214

Day [x-axis]364 days, (separated monthly);215

Time [y-axis]From the 6:00 till 19:00;216

Efficiency [z-axis]Range from 2% (blue) to 18% (red).217

218

219

Figure 11. Behavior of CROS efficiency during a year of operation.220

221

The Figure 11 indicates that the CROS system could have an instantaneously222

efficiency on the range of 2% to 18%. Also possible to recognize that the system has a greater223


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efficiency and stability during the winter season (Jun. - Aug.); due to its operation close to the224

design point and the low clouds incidence in this period, nevertheless, the available Sun light225

is lower. During the summer (Dec. - Feb.), the incidence of clouds is higher, especially226

cumulus formation in the afternoon, occurring from summer rain to torrential thunderstorm,227

which reduces the system stability and availability during the day. And also due to the228

irradiation amplitude the efficiency decreases. Through the Figures 8 and 11 highlight the229

system latency, even after the passage of a cloud the system keeps its operation up to 20230

minutes before shutting down.231

232

Annual behaviordaily average233

The average information is more valuable for economic analysis. In Figure 12 are234

exposed three different data; the CROS system efficiency, equivalent Carnot cycle efficiency235

and the net power generated, the data represents a daily average during the year236

237

238

Figure12. Characteristic behaviordaily average.239

240

In Figure 12 is possible to visualize the daily average efficiency behaves in the same241

way of showed in Figure 11, reaching globally lower efficiencies during the summer (Dec. -242

Feb.) and higher efficiencies during the winter (Jun. - Aug.), which are respectively in the243

range of 5% to 10%. Even so, due to the traditional control strategy, the energy generation is244

22% higher than the design point.245

246

The system control intervenes on the system flow and it is also able to shut down the247

solar collector field. For this reason is calculated the Overall Equipment Efficiency (OEE). A248

dimensionless value calculated by the difference between the actual time of use of the system249


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(when collect solar energy) by the total time of energy production. When the system switches250

off automatically to self-protection mode, the elapsed time is counted, during this period the251

capture of solar energy is cut off, but continues to produce energy until the reason of the252

overload disappears and system could came back on running.253

254

This operation could be optimized by the use of thermal storage, which the additional255

energy would be directed to the accumulators, allowing the system operate at design point,256

even without the Sun. The presence of accumulator could be a step to make a hybrid system,257

since the thermal storage could be feed by another external source, such as biomass, natural258

gas, etc.259

260

As showed, the time is an important factor for this type of system; in Figure 13 are261

exposed the startup time, operation time, cut off time (when self-protection control is262

activated) and overall equipment efficiency (OEE).263

264

265

Figure13. Time behaviordaily average.266

267

At Figure 13, as presents the startup time has a low variation, keeping less than 25268

minutes. Although the operation time has a high variation due to weather conditions, which is269

possible to remark a sinusoidal behavior. The fact occurs due the seasons, which modifies the270

duration of the day during the yeardays shorter in winter and longer in summer.271

272

The cut off time has the same behavior as the predecessor, reaching highest value273

during the summer period, due to the irradiation amplitude compared with the nominal274radiation project. On the other hand, with inverse behavior; higher is the cut off time, lower is275


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the OEE, whereas reaches a minimum value of 60%. With the data on Figure 12 and 13 is276

possible to synthetize the average data by season on Table 2. Featuring minimum (Min.),277

average (Ave.) and maximum (Max.) values.278

279

Table 2. CROS average operational data by season.280

SeasonSummer

(Dec. - Feb.)Autumn

(Mar. - May.)Winter

(Jun. - Aug.)Spring

(Sep. - Nov.)Annual

Efficiency [%]

Min. 5.67 5.95 7.07 5.69 5.67Ave. 6.57 7.46 8.01 6.88 7.17Max. 9.92 9.65 9.53 10.19 10.19

Net power [kW]Ave. 5.95 5.93 5.93 5.96 5.94Max. 6.09 6.08 6.03 6.08 6.09

Operational time [hours/day]Ave. 7.39 6.62 6.46 7.68 7.00Max. 9.54 8.27 7.50 9.53 9.53

Cut off time [hours/day] Ave. 1.99 1.12 0.56 1.92 1.52Max. 3.21 2.57 1.59 3.34 3.34

Usage factor [%]Ave. 27.01 25.65 23.23 25.78 25.71Max.

1 72.36 82.38 90.86 75.03 78.71

Days available [%] Ave. 89.74 98.72 91.03 85.90 91.35Daily power production

[MW/day]Ave. 158.32 141.22 137.93 164.70 150.54

281

The system efficiency is incontestably higher during the winter, + 11% from the282

annual average. However during the summer, the average efficiency is - 9.7%. On the other283

hand the net power production is stable during the whole year, slightly higher during the284

spring season, + 0.3% above average. The operational time is higher during the spring285

season, + 9.7% above average, which is occasionally due to the climate conditions clouds286

and solar duration day. On the other hand during the winter the operational time is - 7.7%287

lower than average.288

289

The lowest cut off time happens only during the winter season, - 63.2% under average290

due to the design criteria. A better control strategy could mitigate this problem, and also291enhance the system behavior in all weather conditions. Usage factor show interesting values,292

which apparently show higher capacity of the system during the summer, + 16.3% higher293

than the winter season. Nevertheless, the maximum usage factor shows that during the winter294

the usability of the machine is + 25.6% higher than the summer season. This means during295

the winter, almost all of the available sun light is useful, on the other hand during the296

summer, only 72.4% could be retrieved to power generation.297

298

1Maximum usage factortake in account only the available day light.


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The available days only represents the days which the irradiation were under 300299

W/m and/or the system wasnt able to start. Although the best way to evaluate this system is300

to predict the total daily electric power production, which reveals to be on the spring season,301

+ 9.4% higher than the average, and + 19.4% higher than the winter season. That fact302

indicates the possibility of an optimization focused on spring season would be more303

profitable. The enhancement of the system performance for the winter season is limited by304

the available sun light, therefore the others seasons arent; which in this case a 2 hours (10 m305

hot-water tank) thermal storage system could be capable to increase the system performance306

and operational time in + 20.5%, stabilizing the system generation, and also avoiding part-307

load operation.308

309

CONCLUSION310

In this work we present the main characteristics of the basic design of a 5 kWe solar311

ORC plant using a parabolic cylinder technology, based on commercial and available312

equipments, fully developed to maximize its availability under subtropical climate313

conditions. This system will allow other studies like: development of solar thermal energy,314

renewable energy on smart grids, regulation with thermal/mechanical storage, etc. for315

distributed energy generation.316

317

The early studies on the CROS system performance evaluation indicate high variation318

of energy availability among the year. This fact was expected; because it dependency of319

energy source, the Sun light. Nevertheless, the Federal University of Itajub dependencies are320

situated in a valley and also the weather conditions during the seasons arent the most321

adequate for solar thermal energy. Among that fact, the CROS system using a traditional322

control strategy, without a storage system, could reach a minimal average usage factor of323

23.2% during the winter, with a minimal average efficiency of 6.5% (-15.6% from the design324

point) during the summer, and also been available at mostly 86% of the days during a year.325

326

CROS system implementation will allow enhancing simulation tools, creating thermal327

storage system for adverse climate conditions, developing advanced control strategy for solar328

thermal system, etc. This will contribute to advance the knowledge, allowing distributed solar329

thermal generation in any place at Brazil.330

331


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ACKNOWLEDGEMENTS332

The authors want to thank to CAPES, CNPq, FAPEMIG, ANEEL, CEMIG and CPFL333

for their collaboration and financial support in the development of this work.334

335

References336

[1] ALVES, M. S.; LORA, E.; PALACIO, J., (2012). Sizing and parametric study of a33710kWel Solar Organic Rankine Cycle for Brazilian conditions. VII National Congress338of Mechanical Engineering339

[2] RODRGUEZ, C. E. C. ; Palacios ; Venturini ; LORA, Electo Eduardo Silva ; COBAS,340Vladimir Melin ; SANTOS, D. M. ; DOTTO, F. R. L. ; GIALLUCA, V. . Exergetic341and economic comparison of ORC and Kalina cycle for low temperature enhanced342geothermal system in Brazil.Applied Thermal Engineering, v. 52, p. 109-119, 2013.343

[3] POWER, C. S., (2010). Technology Roadmap: Concentrating Solar Power. OECD344

Publishing. Disponvel em: .346

[4] REN21, (2012). Renewables 2012: Global Status Report. Disponvel em:347.348

[5] CSP-WORLD (2013). CSP World Map. Disponvel em: . Acesso em: 1 abril 2013.350

[6] OROSZ, M.; MUELLER, A.; QUOILIN, S.; HEMOND, H. (2009) Small scale solar351ORC system for distributed power. v. 1, n. 3. Disponvel em: . Acesso em: 20/8/2013.353

[7] CIEMAT. (2008). Sistemas Solares Trmicos de Concentracin. Madrid, Espaa: Ciemat,3541-417 p.355

[8]

VANKEIRSBILCK, I., & VANSLAMBROUCK, B. (2011). Organic Rankine Cycle as356efficient alternative to steam cycle for small scale power generation.Proceedings of357the , (July). Retrieved from http://www.orcycle.eu/publicaties_bestanden/HEFAT3582011 - ORC vs steam_final.pdf359

[9] KAKA, S., PRAMUANJAROENKIJ, A., & LIU, H. (2002). Heat exchangers:360selection, rating, and thermal design.361

[10] ALVES, M. S. (2013), Computational Modeling and Optimization of a Solar Organic362Rankine Cycle with Parabolic Trough Collector, Itajub - MG, 195 p. Dissertation363(Master sciences in Energy Conversion)Institute of Mechanical Engineering, Federal364University of Itajub.365

[11] MCKNIGHT, TOM L; HESS, DARREL (2000). "Climate Zones and Types". Physical366Geography: A Landscape Appreciation. Upper Saddle River, NJ: Prentice367Hall.ISBN0-13-020263-0.368

[12] QUOILIN, S., BROEK, M., DECLAYE, S., DEWALLEF, P., & LEMORT, V. (2013).369Techno-economic survey of Organic Rankine Cycle (ORC) systems. Renewable and370Sustainable Energy Reviews, 22, 168186. doi:10.1016/j.rser.2013.01.028371

[13] ESPINOSA, N., TILMAN, L., LEMORT, V., QUOILIN, S., & LOMBARD, B. (2010).372Rankine cycle for waste heat recovery on commercial trucks: approach, constraints and373modelling, 110. Retrieved from http://orbi.ulg.ac.be/handle/2268/62995374

[14] QUOILIN, S. (2011). Sustainable Energy Conversion Through the Use of Organic375Rankine Cycles for Waste Heat Recovery and Solar Applications. University of Lige376
http://www.csp-world.com/cspworldmaphttp://www.csp-world.com/cspworldmaphttp://en.wikipedia.org/wiki/International_Standard_Book_Numberhttp://en.wikipedia.org/wiki/Special:BookSources/0-13-020263-0http://en.wikipedia.org/wiki/Special:BookSources/0-13-020263-0http://en.wikipedia.org/wiki/International_Standard_Book_Numberhttp://www.csp-world.com/cspworldmaphttp://www.csp-world.com/cspworldmap


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[15] VIANA, T. S., (2010). Potencial de Gerao de Energia Eltrica com Sistemas377Fotovoltaicos com Concentrador no Brasil. 127p. Tese (Doutorado)Departamento de378Engenharia Civil, Universidade Federal de Santa Catarina, Florianpolis, SC, Brasil.379

[16] QUOILIN, S., OROSZ, M., HEMOND, H., & LEMORT, V. (2011). Performance and380design optimization of a low-cost solar organic Rankine cycle for remote power381

generation. Solar Energy, 85(5), 955966. doi:10.1016/j.solener.2011.02.010382

basic design and performance evaluation on 5 kwe solar orc under subtropical climate conditions

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