Available online at www.sciencedirect.com
Procedia Engineering 49 (2012) 316 – 323
Evolving Energy-IEF International Energy Congress (IEF-IEC2012)
Expander modelling in binary cycle utilizing geothermal resources for generating green energy in Victoria, Australia M.Oreijah* , As. Date, A. Date, M. Bryson, A. Akbarzadeh School of Aerospace, Mechanical & Manufacturing Engineering, RMIT University, Bundoora, Vic 3083, Australia Elsevier use only: Revised 24th July 2012; accepted 8th August 2012
Abstract This paper investigates the computational modeling of a binary organic Rankine power cycle. The computer model has been generated to aid the design and manufacture of apparatus that will be utilized during experimentation on the geothermal resources in Victoria, Suitable technology to utilize the low grade heat from the Australia. available geo-fluids has been explained to connect the recent technology with the new approached design. The paper involves the design of the heat engine with the reaction turbine. In addition, theoretical analysis and performance predictions have been presented briefly in the paper to compare between different diameters for the reaction turbine in the organic Rankine cycle engine with power and efficiency. In this paper, realistic recommendations will be discussed to the best of the geothermal energy as sustainable energy resources to be useful for achieving the Australian target. The binary cycle is modeled to evaluate the output power and the efficiency. A computer model for binary cycle is presented in this paper. Thermal-mechanical energy conversion efficiency is predicted using the computer modeling and presented in the analysis.
© 2012 The Authors. Published by Elsevier Ltd. Selection and/or peer-review under responsibility of the International
© 2012 The Authors. Published by Elsevier Ltd. Selection and/or peer-review under responsibility of the International Energy Foundation Energy Foundation
. Keywords: Geothermal Energy; Renewable Energy; Binary Cycle; Rankine Cycle.
Nomenclature D
Rotor Diameter Angular speed
(cm) (radian/s)
h R
enthalpy Radius
Q
Heat transfer
(kW)
U
Tangential velocity (m/s)
W Va Vr T
Output power
(kW)
Absolute velocity
(m/s)
Relative velocity Torque Temperature
(m/s) (N-m) (°C)
T
(kJ/kg) (m)
* Corresponding author. E-mail address:
[email protected]
1877-7058 © 2012 The Authors. Published by Elsevier Ltd. Selection and/or peer-review under responsibility of the International Energy Foundation doi:10.1016/j.proeng.2012.10.143
M.Oreijah et al. / Procedia Engineering 49 (2012) 316 – 323
1. Introduction Victoria, Australia has vast, easily accessible and under-utilized resources of geothermal resources, all in the form of low grade heat at the most of range 1 km to 5 km deep [1]. The solution to utilizing these resources could be applied or is being sought in many countries, developing the power extraction of the ORC technology. It could be the solution to take the advantage of the natural energy resources in Victoria or even worldwide. This paper discusses the modeling technology to utilize the available geothermal resources in Victoria with the ORC technology. Binary cycle power plant is one of the most efficient methods to implement this low temperature resource. By improving this binary system, the possibility to reduce the cost of power production could be increased to support expanding this power generation in Victoria. Moreover, Australia targets a goal to reduce the greenhouse emissions by using more renewable energy resources by 2020. By generating power using the geothermal energy would be useful for achieving this Australian target. The binary cycle will be modified to have a reaction turbine instead of steam turbine or screw turbine. This concept would increase the total efficiency of the binary cycle which would increase the outcome of power generation. This reaction turbine is connected via horizontal axis to the power generator which is important to measure the power outcome. 2. Geothermal Energy in Victoria Geothermal resources are not at temperatures great enough to support hot rock steam generation. Whilst there are ample renewable resources the actual usage of such is very low. According to sustainability Victoria, only 5% of the current energy output is renewable energy[1]. As one can see from the resource maps in Figure 1, Victoria has abundant geothermal resources where the harnessing of such resources can serve to provide a clean source of electricity. Studies have shown that area certain area in Victoria has a temperature of about 80 ºC at a depth of 1.5 km. Whilst this is good resource for geothermal heating, it is not great enough to drive a steam turbine from hot rock technologies. Hence it may not be a viable resource for providing electrical power in Victoria [1].
Fig 1: Potential Geothermal Resources in Victoria for Subsurface Temperature (ºC) at 1500 m (source: SKM 2005)
In Addition, Geothermal energy as it could be a base-load resource for energy when it compared to solar or wind energy. Geothermal energy has the potential to be a sustainable renewable energy resource for the coming generation. Unfortunately, there are not many geothermal power plants in Australia for the purpose of power production. However, Australia has a significant source of safe, secure, competitively priced, emission-free, renewable base-load power for centuries to come [2]. The only geothermal power plant under operation is in Birdsville in Queensland with total production of 150 kW [1]. In 2007: About 27 companies had applied Australia-wide for 197 licenses to explore, do flow tests or demonstration projects and received government grants, some up to $5M [2]. For a low grade heat (85 °C to 175 °C), binary cycle plants are wide utilized and it is an improving technology. Although, the efficiency would be increased by using the wastewater for direct use such as heating.
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3. Geothermal Technologies There are three types of technologies currently used to implement the geothermal energy. First one is single flash steam power plants. It is suitable for high temperature such as 185 ºC and above, it has cylindrical cyclonic pressure to separate the liquid from the vapor which let the vapor rotate the turbine [3]. The second technology is a dry steam power plant. It is the basic power plant to utilize the hydrothermal fluid directly into the turbine. The third technology is binary cycle power plant. It is suitable for low thermal temperature grade below 150 ºC. In this technology the hot water flow through a heat exchanger and exchange the heat with a secondary working flu -pentane or Isobutane has lower boiling temperature at 1 atm. That lead the working fluid to vaporized and drive the turbine. This is closed loop cycle which is no gas emission generated and classified this technology as an organic cycle. The concept of binary cycle is to use the heat transfer from the geothermal fluid to a working fluid that can be boiled at a lower temperature than water can be to be vaporized [4]. The vaporized working fluid will spin the turbine which is connected to a generator to produce electrical power. Then the working fluid will be cooled down in the condenser to start a new cycle in a close loop. Therefore, the binary geothermal power plant has no emissions because the geothermal fluid never exposed to the atmosphere. It will be re-injected back to the geothermal well in closed loop cycle. Binary power plants have virtually no emissions but are relatively less efficient. Figure 2 illustrates a schematic view of a typical binary cycle in basic arrangement.
Hot water
In
Cold water
(90-75) º C
Out
2
2
Expander
3
3
Condenser
Heat Exchanger
Cold water
In
Hot Water
out
1
1
(30-20) º C
Direction of working fluid (Isopentane) Reservoir
Flow meter
Pump
Simplified Schematic of binary system
Fig. 2: Schematic diagram of basic binary cycle power plant
In thermodynamics, the working fluid will have a thermal cycle which is known as Rankine cycle. As shown in Figure 3, the cycle has four reversible processes which is known as T S diagram. It also illustrates the relation of the cycle between 1) From point 1 to point 2: the work W is produced due to working fluid is driven the turbine and where the expansions accurse. In this cycle it is Isentropic expansion because the different in entropy is zero. which work is produced by the cycle working fluid 2) From point 2 to point 3: the heat rejected in form of Isothermal heat from the working fluid to the cooling sink. 3) From point 3 to point 4: the Isentropic compression during which work is performed on the cycle working fluid 4) From point 4 to point 1: the Isothermal heat is added to the working fluid from heat source. In practical cycle, the cycle is more realistic especially between point 1 to 4 and between points 3 to 4. The efficiency increases by minimizing the entropy between points 1 to 2 where the work is produced. The diagram in Figure 3 shows the Rankine cycle in associated with Figure 2.
M.Oreijah et al. / Procedia Engineering 49 (2012) 316 – 323
3.5 3 ui d
Saturated liquid
Va
Li q
3
ur
2
po
2.5
Temprture
2 1.5 1
1
4
4
0.5 0 0
0.5
1
1.5
2
2.5
3
Entropy
Fig.3: Rankine cycle used in binary cycle power plant
4. Experimental Setup in RMIT The experimental setup of the heat engine to test the Rankine cycle is series of projects that were built in RMIT University. The latest rig was used by Bryson is shown in the Figure 4 with the main components [5]. This prototype works as a heat engine to convert the thermal heat into mechanical work which ends up generating electrical power via axis between the expander and the generator. The main components of the experimental rig are expander, generator, condenser, reservoir, feeing pump, flow meter and heat exchanger. The generator is 300 watts, 24 V DC. Around nine thermocouples were used to measure the temperatures of the cycle at different points of the prototype via a data acquisition. The existing expander will be replaced with the new reaction turbine to carry out the Rankine cycle for the working fluid in the heat engine.
Generator
The existing screw expander Heat exchanger
Condenser
Hot water source
Reservoir
Flow meter
Feedin g pump
Fig 4: main: the experimental rig in RMIT lab, top: the screw expander to be replaced with reaction turbine
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The proposed design of the reaction turbine within the binary cycle is shown in Figure 5. As shown in Figure 5, the reaction turbine rotor has two pathways where the working fluid under high pressure enters into the inlet nozzle and expands out from the exit nozzle. The expansion ratio between the inlet nozzle and the outlet nozzle is very high and spins the rotors at very high speed [6].
Inlet
Exit nozzle Top Plate
Fig 5: The proposed design of reaction turbine rotor
5. Turbine Modeling in Binary Cycle The design has been modeled to find out the numerical relationship between the output power and the diameter of the reaction turbine rotor. The concept design of simple reaction turbine has been analyzed by Akbarzadeh and Date [7]. As per their analysis, the computer modeling will help to evaluate the designing of the reaction turbine at the given parameters in the lab. As the power is a dependent on the torque, the torque changes based on the diameter of the reaction turbine rotor. The relationship between the power and the rotor angular speed at different rotor radius is shown in Figure 6. The different diameters have been selected to illustrate the power increment when the rotational speed also increases. Thermal Efficiency Vs RPM at various isentropic Efficiency of nozzle Th=75 C, Tc=25 C, D=0.15m
12.00% 10.00% 8.00%
6.00% 4.00%
n=1.00 n=0.75 n=0.50
2.00% 0.00% 0
5000
10000
15000
20000
25000
30000
Rotational Speed (rpm)
35000
40000
45000
Fig 6: Rotor Rotational Speed Variation with Output Power for nozzle diameter =0.15 and isentropic efficiency varying from 0.5 till 1.00
The parameters of the model are assumed as follow: water temperature inlet to the heat exchanger: Thi=75 ºC, the cool water at the inlet of the condenser temperature: Tci=25 ºC and the working fluid is isopentane. As a result from equations 1 and 2, those are used to calculate the power based on the conservation of energy. The heat transfer 10 kW, and this is used as the modelled geothermal heat input. The output power can be calculated as follow: . W T (1)
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In which Torque is equal:
T
. m Va R
(2)
While:
Va
Vr
U
(3)
and : Vr
2
R2
2( h i
(4)
h e)
As sample of the computer modeling the calculated power at different isentropic efficiencies are: 1- isentropic efficiency =1 P = 0.395 kW 2- isentropic efficiency =0.75 P = 0.343 kW 3- isentropic efficiency = 0.5 P =0.277 kW The range of the rotational speed was between 0 45000 rpm with incensement of 1000 rpm. In both equations 1 and 2, the relationship between the output power and the torque can be summarized that the power output will be relatively increases as the rotational speed increases with different rotor diameters. The turbine at efficiency of 1 is able to generate the highest out power but it is not achievable in reality. At isentropic efficiencies of 0.75 and 0.5, however, the turbine is capable to produce power of 0.343 kW and 0.277 kW respectfully. Power Vs RPM @various rotor diameters Th=75 C, Tc=25 C, Isentropic Efficiency=1.0 1.40
Power (kW)
1.20 1.00 0.80 0.60
D=0.10 m
0.40
D=0.20 m
0.20
D=0.30 m
0.00
0
5000
10000
15000
20000
25000
Rotational Speed (rpm)
30000
35000
40000
45000
Fig 7: Rotor Rotational Speed Variation with Output Power at isentropic efficiency of 1.00 for nozzle diameter varying from 0.1 till 0.3m
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Thermal Efficiency Vs RPM @ various rotor diameters Th=75 C, Tc=25 C, Isentropic Efficiency=1.0
14.00% 12.00% 10.00%
8.00% 6.00%
D=0.10m
4.00%
D=0.20m D=0.30m
2.00% 0.00% 0
10000
20000
30000
Rotational Speed
40000
50000
Fig 8: Thermal efficiency with rotational speed at isentropic efficiency of 1.00 for nozzle diameter varying from 0.1 till 0.3m
In addition, due to the limitation of existing experimental rig, three diameters have been selected to compare the performance for the purpose of designing the new proposed turbine. From the graph in figure 7, these different diameters show the relationship between the net power and the rotational speed (RPM). As the size of the rotor increases from 0.1m to 0.3m, more power produces as the diameter increases with higher rotational speed. That is a result because the diameter is related with the relative velocity (Vr) of the working fluid. By observing the graph in figure 8, the thermal efficiency increases as the power increases at higher rotational speed. 6. Conclusion This study has briefly discussed the expander modelling for binary cycle to utilize the available geothermal resources. The study was about Victoria as the geothermal resources are at low grade heat and this study can be applied at other area which has the same criteria. The conclusion from this paper can be summarized in these following points: 1- As the geothermal energy is available in low grade heat at 1 km to 5 km deep in Victoria and it is green resource energy, it would supply most of the required base-load energy in Victoria in future. 2- Developing the heat engine in order to utilize the thermal heat from geothermal resources would increase the nation-wide renewable energy benefit. 3- Binary Cycle has promising results to be used in the heat engine with low grade heat temperatures from geothermal energy recourses. 4- Using the reaction turbine would decreases the cost and increases the thermal efficiency of the binary cycle as both net power and thermal efficiency increase with larger rotor size. 7. Future work The manufacturing of the new reaction expander will be the next stage of the project to replace the existing expander. Then the experiment will be carried out to analysis the collected data from the experiments and to be compared with the computer modeling analysis. Further technical studies from this project will be published as a result of this study. 8. Acknowledgement The author acknowledges that this project would not be successful without the scholarship provided by the Ministry of Higher Education, Saudi Arabia.
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9. References [1] SKM, The Geothermal Resources of Victoria: The Sustainable Energy Authority of Victoria. 2005, Sinclair Knight Merz constitutes: Melbourne. p. 107. [2] Goldstein, B.A., Hill, A.J., Budd, A.R., and Malavazos, M., Geothermal energy: Australian outlook. MESA Journal, 2007. 45: p. 420. [3] DiPippo, R., Geothermal power plants: principles, applications, case studies and environmental impact. second ed. 2008, Oxford: Butterworth Heinemann. 493. [4] Teguh, B., Suyanto, and M. Trisno, Model of Binary Cycle power Plant using Brine as Thermal Energy Sources and Development Potential in Sibayak IJENS, 2011. 11(02): p. 9. [5] Bryson, M.J., The conversion of low grade heat into electricity using the thermosiphon rankine engine and trilateral cycle, in School of Aerospace, Mechanical and Manufacturing Engineering. 2007, RMIT University: Melbourne. p. 240. [6] Date, A. and Akbarzadeh, A., Design and cost analysis of low head simple reaction hydro turbine for remote area power supply. Renewable Energy, 2009. 34: p. 409-415. [7] Date, A. and Akbarzadeh, A., Design and analysis of a split reaction water turbine. Renewable Energy, 2010, 35: p. 1947-1955.
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