An investigation into the differing efficiencies of working fluids used in binary cycle power plants.

Alec Perry-Cain 1*

1 University of Technology Sydney, P.O Box 123, MaPS, Broadway NSW 2007. E-Mail: alec.perry-cain@student.uts.edu.au

* Author to whom correspondence should be addressed; E-Mail: alec.perry-cain@student.uts.edu.au

DOI: http://dx.doi.org/10.5130/pamr.v4i0.1449

Abstract: In this paper the efficiency of three common working fluids in binary cycle geothermal power plants were compared by modeling the properties of the fluid within the working parameters of the Dora-II geothermal power plant. It was concluded that ammonia has the highest efficiency of the fluids and R-134a was the least efficient.

Keywords: geothermal power efficiency; R-134a; n-pentane; ammonia; working fluid; Dora-II; geothermal; binary cycle; Rankine; geothermal power plant

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Copyright 2017 by the authors. This is an Open Access article distributed under the terms of the Creative Commons Attribution 4.0 Unported (CC BY 4.0) License (https://creativecommons.org/licenses/by/4.0/), allowing third parties to copy and redistribute the material in any medium or format and to remix, transform, and build upon the material for any purpose, even commercially, provided the original work is properly cited and states its license.

 

Nomenclature
R-134a 1,1,1,2-tetrafluroethane
ηI First Law efficiency
ϵ Cycle efficiency
ηII Second law efficiency
W Mass flow rate of working fluid
G Mass flow rate of geothermal fluid
hn Specific enthalpy at point n in simple Rankine cycle
Tn Temperature at point n in simple Rankine cycle
sn Specific entropy at point n in simple Rankine cycle

1. Introduction

Non-renewable power, such as coal fired power plants, are by nature a limited energy resource. They are limited by both the amount of resources available to be used to fuel power generation and the detrimental effects that the by-products of these plants have on the earth. The major renewable sources are wind, solar, hydro-electric, and tidal power. These sources have restrictive properties that limit where and how often they can produce energy. Wind, solar and tidal power are inconsistent in times that they can generate power as they are reliant on favourable conditions to produce power. Renewable power sources such as wind turbine plants require wind that is strong enough to turn the blades but not so fast as to burn out or cause excessive stress on the turbine itself, severely limiting the conditions that they are effective in. (1,2) Solar is in a similar position in that it requires light from the sun to be incident on the panels to be able to produce power. (1) This limits the operation times to during the day and efficiency can be reduced by cloud cover. The energy in the light from the sun, i.e., areas further away from the equator go through periods of large reductions in incident light as well as decreased energy due to the thicker atmosphere. Tidal power is limited to the number of times the tide changes a day to produce the power. Whilst this is both regular and predictable if the flow of tidal water increases above the operational speed of the generator then the system can be damaged if it is not shut down pre-emptively. Whilst these systems can have greater efficiency for power production than geothermal plants, they have lower consistent power generation when compared to fossil fuel, nuclear and geothermal power. Geothermal power stations can maintain a consistent level of power output as they are not reliant on atmospheric conditions, the location of the sun, or the position of the moon to generate power. As long as the location chosen can produce enough hot water then power generation will be consistent day or night all year round.

The performance of a geothermal power plant is limited by the temperature of the geothermal water. A standard geothermal power plant that uses geothermal water as the working fluid requires water temperatures with a minimum reservoir temperature of 180˚C. (2) The minimum temperature for geothermal power is set so that when the water reaches the turbines it has been converted to super-heated steam and expands fast enough to drive the turbine. To increase the spread of geothermal powerplants, binary cycle systems have been developed. These plants use a closed loop with a working fluid that is not water to generate the power. The working fluids used in these systems are chosen because they have lower boiling points and lower specific heat capacity compared to water (5,6). These fluids are used in binary systems as their low boiling points allow the fluids to be in a gaseous state at lower temperatures than water, thus being able to drive a turbine at lower temperatures. This allows binary powerplants to be built in areas with lower reservoir temperatures that currently range from 70˚C to 250˚C. (2) The fluids that are currently being used in current production plants include 1,1,1,2 Tetrafluroethane (R-134a), ammonia and n-pentane. (5,6,7) One property that is present in ammonia that is not in R-134a or n-pentane is that when in significant concentration, as is present in this simulation, is that ammonia is very corrosive. Ammonia is know to corrode copper- and zinc- containing alloys as well as attacking rubber and plastics. (3)

2. Methods

The purpose of this research is to determine to what degree the working fluid impacts the efficiency of a binary cycle geothermal power plant.

Plant efficiency was modelled using the equations used by Subbiah and Natarajan (4).

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Where ηI is the first law efficiency, ϵ is the cycle effectiveness and ηII is the second law efficiency.

These equations are matched to a simple binary cycle geothermal power plant (see figure 1). Using these equations and data researched from papers. The effects of the three fluids was compared by using the current working fluid in the production plant then using the same brine and condenser data, and changing the enthalpy and entropy data for each fluid. The enthalpy and entropy was calculated using the Fluid Property Calculator from the International Refrigerant Consortium. (5) The fluid data in table 1 was sourced from Aylward & Findlay’s SI Chemical Data 7th edition (6) and from the GESTIS substance database (3) The data in table 2 was taken from the paper by Ganjehsarabi et al. (7) That data was then used as the basis for calculating the efficiencies of R-134a and ammonia. The mass flow rate, temperature and pressure were kept as constants for the plant. The calculated data is shown in table 3 and 4.

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Figure 1. Simple Rankine cycle process

The efficiency was also graphed against the price per kg of the fluids. The prices were obtained from Alibaba.com, as it supplies in commercial amounts.

The fluids chosen for this paper were restricted to ones that are currently being used in power plants that are currently supplying power for consumers.

3. Results and Discussion

The fluid comparison was based on a combination between the operational specifications of the Dora-II geothermal plant in Turkey and the simplistic Rankine cycle. We can see from the results that ammonia is more efficient than n-pentane and R-134a. The efficiency is noted to be a result of the amount of change in enthalpy between the stages in the Rankine cycle. When the efficiency was graphed against the price per kg of the fluid it was seen that ammonia is again the best option for a working fluid in this plant.

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Figure 2. Comparison of working fluid efficiency.

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Figure 3. Graph of price against efficiency of the working fluids

4. Conclusions

A comparison of the fluids show ammonia as the most efficient of the three fluids investigated. This indicates that to achieve a higher level of efficiency at the Dora-II power plant the working fluid of n-pentane should be replaced by ammonia. However, using ammonia will be more expensive in up keep as it is more corrosive then n-pentane and R-134a. This puts the plant at a higher risk of leaks and reduction of efficiency. Leaks will be of great concern when running both ammonia and n-pentane as they are both flammable. From that perspective, R-134a would be a more suitable fluid as it is not corrosive and is not readily flammable. It is not recommended to replace the working fluid with R-134a, as it will greatly reduce the efficiency of the plant. Therefore, n-pentane is the best choice for a long term working fluid. It will reduce the cost of maintenance compared to ammonia and has a higher efficiency than R-134a. It was also concluded that the working fluid can have a theoretical impact on efficiency of 13%.

Acknowledgments

Dr Jurgen Schulte – Subject coordinator – Provided guidance and mentorship.

References and Notes

1. Lyon, Katrina and Rayner, Mark. Fact sheet 10: tidal energy. [Fact sheet]. Perth : s.n., 1999.

2. Efficiency of geothermal power plants: A worldwide review. Moon, Hyungsul and Zarrouk, Sadiq J. Auckland : New Zealand Geothermal Workshop 2012 Proceedings, 2012.

3. Institue for Occupational Safety and Health of the German Social Accident Insurance. GESTIS Substance Database. Institut fur Arbeitsschutz der. [Online] [Cited: 05 10, 2017.] http://gestis-en.itrust.de/nxt/gateway.dll/gestis_en/000000.xml?f=templates$fn=default.htm$vid=gestiseng:sdbeng$3.0.

5. Industrial Refrigeration Consortium. Fluid Property Calculator. Industrial Refrigeration Consortium Website. [Online] [Cited: 05 10, 17.] https://www.irc.wisc.edu/properties/.

6. Blackman, Allan and Gahan, Lawrie. Aylward & Findlay's SI Chemical Data, 7th Edition. s.l. : John Wiley & Sons Australia, 2013.

7. Exergetic performance analysis of Dora II geothermal power plant in Turkey. Ganjehsarabi, Hadi, Gungor, Ali and Dincer, Ibrahim. 1, Bornova : Energy, 2012, Vol. 46.

8. National Wind Watch. National Wind Watch FAQ - Technology. [Online] [Cited: May 5, 2017.] https://www.wind-watch.org/faq-technology.php.

9. Direct,drive permanent magnet generators for high-power wind turbines: benefits and limiting factors. Semken, R Scott, et al. 1, 2012, IET Renewable Power Generation, Vol. 6, pp. 1-8.

10. Reducing CO2 emission and improving exergy based performance of natural gas fired combined cycle power plants by coupling Kalina cycle. Singh, Omenda K. and Kaushik, Subhash C. 1, New Dehli : Energy, 2013, Vol. 55.

11. Australian Government, Department of health. Environment Tier II assessment for Ethane, 1,1,1,2-Tetrafluro-. National Industrial Chemicals Notification and Assessment Scheme. [Online] 04 26, 2017. [Cited: 05 16, 2017.] https://www.nicnas.gov.au/chemical-information/imap-assessments/imap-assessments/tier-ii-environment-assessments/1,1,1,2-tetrafluoroethane#References.

12. The Chena hot springs 400kW geothermal power plant: Experience gained during the first year of operation. Holdmann, Gwen. Fairbanks : s.n.

Appendix

Table 1. Table of working fluid properties
Properties Fluid
R-134a N-pentane Ammonia
Mol Mass (g/mol) 102.03 72.1 17
Specific heat capacity (gas) (J/(K*mol) 87 120 35
Boiling point (Celsius) -26.3 36.1 -78
melting point (Celsius) -103.3 -129.7 -33
flash point (Celsius) none -49 630

 

Table 2. Data for n-pentane at each stage of the simplistic Rankine cycle
    Fluid Phase Temperature (°C) Pressure (kPa) Specific enthalpy (kJ/(kg*K)) Specific entropy (kJ/kg) Mass flow rate (kg/s)
Point in simple rankine cycle 0 Air Gas 17.1 1013 290.25 1.67 4350
1 Air Gas 29.1 1013 302.3 1.71 4350
               
2 n-pentane Liquid 41.87 141.85 20.66 0.065 118.61
3 n-pentane Liquid 89.3 1185.5 133.88 0.39 118.61
4 n-pentane Vapour 133.08 1185.5 510.23 1.33 118.61
5 n-pentane Vapour 83.7 141.85 444.46 1.38 118.61
               
6 Brine Liquid 169 1296.96 715.11 2.03 231.94
7 Brine Liquid 124.4 1296.96 523.15 1.57 231.94

 

Table 3. Data for R-134a at each stage of the simplistic Rankine cycle.
    Fluid Phase Temperature (°C) Pressure (kPa) Specific enthalpy (kJ/(kg*K)) Specific entropy (kJ/kg) Mass flow rate (kg/s)
Point in simple rankine cycle 0 Air Gas 17.1 1013 290.25 1.67 4350
1 Air Gas 29.1 1013 302.3 1.71 4350
               
2 R-134a Liquid 41.87 141.85 290 1.12 118.61
3 R-134a Liquid 89.3 1185.5 322 1.05 118.61
4 R-134a Vapour 133.08 1185.5 368 1.18 118.61
5 R-134a Vapour 83.7 141.85 329 1.24 118.61
               
6 Brine Liquid 169 1296.96 715.11 2.03 231.94
7 Brine Liquid 124.4 1296.96 523.15 1.57 231.94

 

Table 4. Data for ammonia at each stage of the simplistic Rankine cycle.
    Fluid Phase Temperature (°C) Pressure (kPa) Specific enthalpy (kJ/(kg*K)) Specific entropy (kJ/kg) Mass flow rate (kg/s)
Point in simple rankine cycle 0 Air Gas 17.1 1013 290.25 1.67 101.83
1 Air Gas 29.1 1013 302.3 1.71 101.83
               
2 Ammonia Liquid 41.87 141.85 1560 6.27 118.61
3 Ammonia Liquid 89.3 1185.5 1630 5.47 118.61
4 Ammonia Vapour 133.08 1599.58 1740 5.75 118.61
5 Ammonia Vapour 83.7 438.5 1650 6.54 118.61
               
6 Brine Liquid 169 1296.96 715.11 2.03 231.94
7 Brine Liquid 124.4 1296.96 523.15 1.57 231.94

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