7th International Scientific Conference on “Energy and Climate

Change”

Kostis Palamas Building, Athens, GreeceOctober 8-10 , 2014

Proudly Kenyan

WR

Energy and Exergy Analysis Concepts: Modeling of Olkaria II Geothermal Power Plant in Kenya.

Authors Mr. Nyambane NDr. Gatari M. JDr. Githiri J. G

Dr. Mariita N. O

Outline Introduction/Background Description of Olkaria II geothermal power plant Thermodynamic Analysis Methodology Results and Discussions Conclusions Recommendations Questions and Comments

Introduction Energy is an essential component for economic

development of any country.• Improves the economy

For the case of Kenya• Demand is higher than supply

To bridge this gap• Invest in energy generating projects• Improve the efficiency of existing ones

Introduction

Kenya is endowed with huge potentials of geothermal energy

• Over 7000 MW• Only 5 % is being exploited for electricity generation

The efficiency of conversion of the exploited geothermal energy is demanding due to

• Huge exergy losses in plant subsystems• Turbine(s) and Condenser(s)

Introduction Studies have shown huge exergy losses occur in the

turbine(s) and condenser(s).• Aligan (2001)• Kwambai (2005)

Minimal efforts have been made to curb this menace.• For turbine, improve the steam condensation process• Translates to higher vacuum pressure in condenser• Higher exergy drop across turbine• Increase exergy efficiency and power output

Introduction

Steam condensation efficiency is influenced by cooling water temperature.

• The lower the cooling water temperature the efficient the steam condensation process

Kenya being in tropical region experiences high ambient temperature which leads to

• High cooling water temperature• High condenser pressure

The big question is, “how do we minimize the effect of ambient temperature on cooling efficiency of geothermal power plant?”

Absorption Chiller

In this study an absorption chiller system has been adopted for Olkaria II geothermal Power plant in Kenya.

• Incorporated into the cooling system.• Cool water out of the cooling tower.

Study by Tesha (2009), have shown a 131 kW power increase for a geothermal power plant that adopts an absorption chiller as an integrated condenser unit.

Description of Olkaria II Geothermal Power Plant, Kenya

Cooling TowersTurbines, Condensers and Generators

Separator

Transmission line

Power Distribution Station

Thermodynamic Analysis

Thermodynamic Analysis Exergy flow diagram

Absorption Chiller Cycle

QdQd

1122

QabsQabs

2626

1515

cold

1414

QshxQshxhot

SHX

2525

2323

2424

cold

QevQevhot

cold

1212 1313

QrhxQrhxhot

RHXcoldcold

3232

3131

3030

2929

2828

QcondQcondhot

Condcold

2727

1717

1616

2222

2121

Absorption Chiller Model

Methodology

Parameters considered during data collection were • Pressure(s)• Mass flow rate(s)• Temperature(s)

The measured parameters served as inputs to the modelled system for simulation on Engineering Equation Solver (EES).

Methodology

Absorption chiller Model• Based on simple steady state• Uses mass and energy balance principles• Formulated on the basis of UA and LMTD

EES codes are developed and simulated using EES software to compute the output parameters.

Iterations for the evaporator temperature were run.

Results and DiscussionsTable 1: Comparison of efficiencies between the current geothermal power plant (GPP) and the geothermal power plant with hybridized cooling system (HCS)

Subsystem Desired Exergy for each

subsystem (MW)

Exergy wasted in each subsystem

(MW)

Exergy destroyed in each subsystem

(MW)

Exergetic efficiency of each

subsystem (%)

  Current GPP

GPP with HCS

Current GPP

GPP with HCS

Current GPP

GPP with HCS

Current GPP

GPP with HCS

Turbine 37.5 39.13 8.04 6.8 7.7 7.3 83.0 84.3Condenser 5.0 4.9 0.15 0.14 2.89

 2.3 63.4 65.9

Energy Efficiency of turbine 19.4 20.2Overall Exergetic efficiency of Plant as a function of steam into transmission lines

52.4 65

Overall Exergetic efficiency of Plant as a function of geo-fluid 46.6 55.6

Results and Discussions

Adoption of the hybrid cooling system showed some positive results.

• Increase in power output from 35.6 MW to 37.2 MW• Decrease in exergy loss in the turbine(s) and condenser(s)• Net annual gain of 819,542 US $

Results and Discussions

Figure 1:Relationship between condenser pressure and changes in cooling water temperature.

Figure 2: Relationship between turbine power

output and changes in condenser pressure

10 11 12 13 14 15 16 17 18 19 20 21 224

4.254.5

4.755

5.255.5

5.756

6.256.5

6.757

7.257.5

7.758

8.258.5

Cooling water temperature [oC]

Con

den

ser

pre

ssu

re [

kP

a]

4 4.5 5 5.5 6 6.5 7 7.5 8 8.536

36.5

37

37.5

38

38.5

39

39.5

40

40.5

41

41.5

42

Condenser pressure [kPa]

Tu

rbin

e ou

tpu

t p

ower

[M

W]

Results and Discussions

Figure 3: Condenser heat transfer rate due to cooling water temperature and flow rate changes at constant condenser pressure of 5 kPa.

10 12 14 16 18 20 22140000

160000

180000

200000

220000

240000

260000

Cooling water temperature [oC]

Cond

enser

heat

trans

fer ra

te [K

W]

Cooling water flow rate [kgs-1]

2194

2000

1800

1500

Conclusions

By lowering cooling water temperature from 25 0C to 16 0C• Condenser pressure reduced from 6.9 kPa to 5.9 kPa• Output power increased by 1.5 MW• Exergy loss in condenser reduced by 0.59 MW• Exergy loss in turbine reduced by 0.4 MW• Overall exergy efficiency improved by 12.6 %

Recommendations

There is need to carry out an economic analysis to ascertain the cost of investing in the absorption chiller

Evaluate the payback period which will help the investors to evaluate if the investment is economically feasible.

Thank You

Questions & Comments