EuroSun2008-2

Energy Performance and Optimisation

Tmopt program was used to couple the Tmsys simulations software with GenOpt, a generic optimisation program. Trnopt acts as an interface programme between the two software programs and streamlines the optimization process.

The optimisation process has as objective function the maximisation of the production of desalted water. The result is the optimal operating temperature of the fluid at the outlet of the solar field system coupled to the Rankine Cycle. This temperature has been optimised considering two different time periods: hourly or daily. The optimal daily and hourly temperature conditions are determined for both cycles considering two typical days corresponding to a typical situation in winter and in summer. The results for both cases are shown in Table 2 and Table 3.

As an example of the hourly results, Fig. 4 shows the results of the water system and working at a set point using the hourly and daily optimal temperatures in the selected summer day.

Table 2. Performance of the system operating at the optimum daily temperature

Cycle

Day

Solar

irradiation

[kWh/day]

Turbine

capacity

[kWh/day]

Optimal daily Temperature

[°С]

Desalted Water Production [m3]

Steam Rankine cycle - Reverse Osmosis plant

24 January

2585

150

235

44

26 June

6960

986

325

294

Organic Rankine cycle - Reverse Osmosis plant

24 January

2585

129

262

38

26 June

6960

768

354

228

Table 3. Performance of the system operating at the optimum hourly temperature

Cycle

Day

Solar

irradiation

[kWh/day]

Turbine

capacity

[kWh/day]

Optimal daily Temperature [°C]

Desalted Water production [m3]

Steam Rankine cycle - Reverse Osmosis plant

24 January

2589

151

variable

37

26 June

6960

988

variable

295

Organic Rankine cycle - Reverse Osmosis plant

24 January

2589

129

variable

39

26 June

6960

768

variable

229

Подпись: (a)

image051

(b)

Fig. 4. Results for the optimal operating temperature of the fluid of the solar system. (a) daily (b)

hourly

2. Conclusion

A model for the optimal integration of Rankine cycles and solar thermal plants to drive Reverse Osmosis desalination plants has been presented. This model combines rigorous models for the simulation of the Rankine and solar field subsystems. The objective is to calculate the optimal operation temperature to produce the highest amount of desalted water. An example using trough solar collectors and water and pentane as working fluids was presented.

Using the trough solar collector and the two selected fluids the results in terms of energy efficiency and production of desalted water are very similar operating the system at an optimal hourly temperature at the collector’s outlet or at an optimal daily temperature. However, significant differences where found between the optimal temperatures in winter and summer days. So, this would mean that the set point for the solar field outlet temperature should be changed throughout the year to obtain the best global system performance but this change will have little effect throughout the same day. The performance using water is similar to that of n-pentane due to the high efficiency of the selected trough collector at high temperatures and the high solar radiation available in the selected geographical location. In the future the developed model will be extended to study also other types of solar collectors and working fluids.

Acknowledgements

This work is financially supported by the Ministerio de Educacion y Ciencia of Spain, OSMOSOL project, ref. ENE2005-08381-C03-03.

References

[1] IDA Desalination, Yearbook 2007-2008, Water Desalination Report, Global Water Intelligence, UK.

[2] L. Garcia-Rodriguez, Seawater desalination driven by renewable energies: a review, Desalination, 143

(2002) , 103-113.

[3] L. Garcia-Rodriguez, Renewable energy applications in desalination: state of the art, Solar Energy, 75

(2003) , 381-393.

[4] S. A. Kalogirou, Seawater desalination using renewable energy sources, Progress in Energy and Combustion Science, 31 (2005), 242-281.

[5] E. Mathioulakis, V. Belessiotis, E. Delyannis, Desalination by using alternative energy: Review and state - of-the-art, Desalination, 203 (2007), 346-365.

[6] J. McHarg, R. Truby West Coast researchers seek to demonstrate SWRO affordability. Desalination & Water Reuse, 14 (2004) 10-18.

[7] Osmosol - Desalacion por osmosis inversa mediante energia solar termica, Memoria de proyecto, Proyectos de investigacion, Ministerio de Educacion y Ciencia, 2006. https://www. psa. es/webeng/projects/joomla/osmosol/

[8] S. Canada, G. Cohen, R. Cable, D. Brosseau, H. Price, Parabolic trough organic Rankine cycle solar power plant, NREL/CP-550-37077, Presented at the 2004 DOE Solar Energy Technologies, Denver (USA), 2004.

[9] G. Burgess, K. Lovegrove, Solar thermal powered desalination: membrane versus distillation technologies. Proceedings of the 43rd Conference of the Australia and New Zealand Solar Energy Society, Dunedin, November 2005.

[10] D. Manolakos, G. Papadakis, E. Sh. Mohamed, S. Kyritsis, K. Bouzianas, Design of an autonomous low - temperature solar Rankine cycle system for reverse osmosis desalination, Desalination 183 (2005), 73-80.

[11] D. Manolakos, G. Papadakis, S. Kyritsis, K. Bouzianas, Experimental evaluation of an autonomous low - temperature solar Rankine cycle system for reverse osmosis desalination, Desalination 203 (2007), 366-374.

[12] Delgado-Torres, A. M., Diseno Preliminar de un Sistema de Desalacion por Osmosis Inversa mediante Energia Solar Termica, PhD thesis, Universidad de La Laguna (Tenerife, Spain), 2006.

[13] J. C. Bruno, J. Lopez-Villada, E. Letelier, S. Romera, A. Coronas, Modelling and Optimisation of Solar Organic Rankine Cycle Engines for Reverse Osmosis Desalination, Applied Thermal Engineering (2008), doi:10.1016/j. applthermaleng.2007.12.022.

[14] TRNSYS 16 - A transient system simulation program, version 16, 2004.

[15] S. A. Klein, Engineering Equation Solver (EES), F-Chart Software, http://www. fchart. com

[16] ROSA - Reverse osmosis system analysis software, ver. 6.1.3, Dow Water Solutions, 2006.

[17] A. C. McMahan, (2006). Design and Optimization of Organic Rankine Cycle Solar-thermal Power Plants, Master of Science, Solar Energy Laboratory, University of Wisconsin-Madison (USA).

[18] A. M. Patnode, (2006). Simulation and Performance Evaluation of Parabolic Trough Solar Power Plants, Master of Science, Solar Energy Laboratory, University of Wisconsin-Madison (USA).

[19] J. Lopez-Villada, J. C. Bruno, E. Letelier, S. Romera, A. Coronas, Simulacion con Trnsys de Sistemas Solares Termicos para Desalinizacion mediante Osmosis Inversa, XIV Congreso Iberico y IX Congreso Iberoamericano de Energia Solar, Libro de actas 647-652, Vigo, 2008.

EuroSun2008-2

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