Transcript
Page 1: Hydro energy potential of cooling water at the thermal power plant

Applied Energy 88 (2011) 4005–4013

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Applied Energy

journal homepage: www.elsevier .com/locate /apenergy

Hydro energy potential of cooling water at the thermal power plant

Vladimir D. Stevanovic a,⇑, Aleksandar Gajic a, Ljubodrag Savic b, Vladan Kuzmanovic b, Dusan Arnautovic c,Tina Dasic b, Blazenka Maslovaric a, Sanja Prica a, Bojan Milovanovic b

a Faculty of Mechanical Engineering, University of Belgrade, Kraljice Marije 16, 11120 Belgrade, Serbiab Faculty of Civil Engineering, University of Belgrade, Bulevar Kralja Aleksandra 73, 11000 Belgrade, Serbiac Electrotechnical Institute ‘‘Nikola Tesla’’, Koste Glavinjica 8a, 11000 Belgrade, Serbia

a r t i c l e i n f o a b s t r a c t

Article history:Received 25 May 2010Received in revised form 11 February 2011Accepted 2 April 2011Available online 23 April 2011

Keywords:HydropowerCooling waterThermal power plant

0306-2619/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.apenergy.2011.04.003

⇑ Corresponding author. Tel.: +381 11 3370561; faxE-mail addresses: [email protected] (V.D.

ac.rs (A. Gajic), [email protected] (L. Savic), vladak@[email protected] (D. Arnautovic), [email protected] (B. Maslovaric), [email protected] (S. P

The hydro energy of the gravity water flow from the coal-fired thermal power plant units to the river inan open cooling system of turbine condensers is determined. On the basis of statistical data for a longtime period, the water net head duration curve due to the river annual level change, as well as the reduc-tion of the hydro energy potential due to the thermal power plant overhauls periods, are evaluated in thecase study of the Thermal Power Plant ‘‘Nikola Tesla B’’ in Serbia. A small hydro power plant is designedfor the utilization of this hydro energy, and the economic benefits of the project are calculated. The inter-nal rate of returns and pay back periods are calculated in dependence of the electricity price and totalinvestment costs. The increase of profitability is assessed, bearing in mind that the plant might be real-ized as the Clean Development Mechanism project according to the Kyoto protocol. The obtained resultsshow that the project is economically attractive, and it can be carried out with standard matured solu-tions of hydro turbines available at the market. Even for the relatively low electricity price from smallhydro power plants in Serbia of 0.08 €/kW h the internal rate of return and the pay back period are17.5% and 5.5 years.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Natural hydro energy sources of rivers are widely used for theelectricity production [1]. However, the exploitation of large hydroenergy sources is mainly saturated, especially in developed coun-tries. Further construction of large hydro power plants (HPP) is of-ten burdened with the unacceptable high investments, and/orundesirable environmental consequences. Hence, in recent years,a wide interest and activity is directed towards the utilization of en-ergy potentials of small streams, and building of small HPP [2–4], aswell as towards other natural hydro power sources, such as seawaves [5]. Besides these natural sources, there is also potential toutilize hydro energy in technical systems. Generally, such solutionsmust a priori satisfy several conditions: they should not violate thetechnical system safety, they must be environmentally acceptable,and they must be energetically and economically beneficial.

Recently, a few projects considered the utilization of the hydroenergy of the turbine condenser cooling water at thermal and nu-clear power plants. Wherever possible the water for the turbine

ll rights reserved.

: +381 11 3370364.Stevanovic), [email protected] (V. Kuzmanovic),

rs (T. Dasic), bmaslovaric@rica).

condenser cooling is supplied from and returned to a naturalsource, such as a river, lake or sea. The cooling-water flows fromthe thermal or nuclear plant back to the natural water sourcedue to gravity since the discharge of the cooling water at the plantis at a higher elevation. The hydro energy of this return cooling-water flow can be utilized for the electricity production in a smallHPP. Since the cooling-water flow is the necessary prerequisite forthe thermal or nuclear power plant operation, its residual energy(that is always available during plant operation) can be regardedas renewable. For example, a HPP with a generation capacity of7.5 MW uses the available hydro energy of the sea water, whichserves as a coolant for eight units of a thermal power plant (TPP)in South Korea [6]. Also, a HPP with two hydro turbines with capac-ity of 5 MW each, has been built at the Bulgarian Kozloduy nuclearpower plant (with two units, the capacity of 1000 MW each) [7].The HPP is operated by the cooling water from the Danube river.

The energy utilization of water stream that is returned from thetechnical system to the environment is also applied at the seawaterreverse osmosis plants [8,9]. The energy of the brine stream at ahigh pressure is recovered by the hydraulic turbine at the plantoutlet, where the turbine is directly coupled with the centrifugalpump that supplies the seawater to the desalination plant. Beforethe 1980s, Francis turbines were applied, but later on they were re-placed by Pelton turbines, since they provide higher system effi-ciency. Recently, the so-called isobaric-chamber devices, or the

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pressure exchange devices have been introduced, in order to fur-ther improve the energy-recovery efficiency [9]. For the existingTPP, the main difficulty for the application of such devices is a largedistance (tens of meters, or more) between the location of the cool-ing water intake, and the location of the cooling water dischargeback to the river. The other major drawback for the applicationof the pressure exchange devices is a need for a large number ofunites, since the water flow through one pressure exchange uniteis below one cubic meters per second, while the cooling-water flowrate at the TPP is of the order of tens of m3/s (depending on theplant power). Obviously these issues should be the topics for fur-ther engineering investigation and development.

The purpose of this paper is to demonstrate and evaluate the hy-dro energy potential of the river water that is used for cooling of theturbine condensers at the TPP. The water is pumped from the riverto the TPP condenser, and returned back to the river by gravity flow.The water-flow energy potential is determined by its net head andflow rate. While the flow rate of the cooling water is more or lessconstant, its net head significantly changes in time, depending onthe river water level variations. The hydro energy of the cooling-water flow is available only during the TPP operation, while duringoverhauls and plant trips it is not available. The influence of the nethead temporal variation and the annual TPP operational period onthe energy potential of the cooling-water flow are evaluated inthe case study of the thermal power plant ‘‘Nikola Tesla B’’ in Serbia.The calculation is based on statistical data for a period of two dec-ades. A small HPP is designed in order to use the cooling water en-ergy, and the economic benefit of the project is estimated.

2. Approach to the feasibility study of the cooling water energyutilization at a TPP

The utilization of energy of the cooling water gravity flow fromTPP back to the river differs from the conventional utilization of theriver stream energy in several issues. The cooling-water flow ispractically constant during the TPP operation, since a large plantis designed for the base load production. It means that TPP oper-ates at the design power and with constant operational parametersthroughout the year, leading to a constant cooling-water flow rate.On the contrary, an available flow rate considerably varies for aconventional run-of-the-river HPP. Also, the cooling-water flowcompletely stops during overhauls or plant trips.

For a cooling water HPP, a net head is obtained due to the factthat the water level in the pool downstream of the TPP condensershas to be at a higher elevation than the maximum water-surfacelevel of the recipient river. During the most time of the year, theriver elevation is much lower than the above mentioned maxi-mum, providing the head for HPP. On the other side, for a conven-tional HPP, the head must be provided by the dam.

Having in mind these differences the feasibility study of thecooling water energy utilization is performed through the follow-ing main steps:

– The duration curve for the gross head between the thermal powerplant and the river water-level should be calculated, using thehydrological statistical data for the available time period.

– The head loss of cooling-water return-flow from the thermalpower plant towards the recipient river should be calculatedby taking into account the friction losses along the channeland local losses. Knowing the head loss, the available net headis determined.

– In general, the hydro turbine should be selected for the low nethead and a high water flow rate. The minimal and maximal val-ues of the net head, acceptable for the turbine operation, shouldbe defined based on the turbine design characteristics.

– The electricity production should be calculated based on theannual net head duration curve, the maximal and minimalheads acceptable for the hydraulic turbine operation, thewater flow rate, the total HPP efficiency (taking into accountthe efficiency of the hydraulic turbine, the mechanical trans-mission system and the electric generator), as well as reduc-tions due to the estimated overhaul periods and plant trips.

– The civil work design should provide the site-location anddimensioning of the power house and appurtenant structureswith the hydro-mechanical equipment, resulting with the cap-ital costs, as well as the efficient construction management.

– The present value of the total costs should be determined basedon the total cost of equipment, and civil work, and the presentvalue of operational and maintenance costs. Also, the value ofthe annual electricity production should be calculated. The pro-ject profitability should be determined considering the pay backperiod and the internal rate of return.

– Since the project contributes to the reduction of the carbondioxide emission and it will be performed in the non-Annex Icountry to the United Nations Framework Convention on Cli-mate Change, the benefits of its execution within the frame-work of the Clean Development Mechanisms of the Kyotoprotocol has to be considered.

The procedure described in this paper is applied and evaluated tothe real case of the open cooling water system at the coal-fired ther-mal power plant, as an example of the project implementation.

3. Cooling water system at the thermal power plant

Utilization of hydro energy of the water flow for the cooling of aturbine condenser is studied for the case of the coal-fired TPP ‘‘Nik-ola Tesla B’’ in Serbia. The plant has two identical units with thepower of 620 MW each. The plant thermal unit has two condensers,the main one for the condensation of steam that exits from the mainturbine, and the auxiliary one for the small turbine which is a primemover for the steam boiler feedwater pump. These condensers arecooled with the water from the Sava river (Fig. 1). The cooling-waterflow is provided by two parallelly connected pumps. After passingthrough the main and auxiliary condensers, the cooling water is col-lected in a water pool from which it spills, and by gravity flows backto the river. The head, resulting from the difference between the poolelevation and the river level, and the considerable water flow rate of20 m3/s (per each unit), provide an energy potential that can be uti-lized by a small HPP. The cooling-water flow rate is practically con-stant, while the water head depends on the river water level, rangingbetween 69 m and 78 m above the see level, as indicated in Fig. 1.The water levels of the Sava river at the TPP site-location are re-corded during the 20 years period, from 1986 till 2006.

Concrete buried channels, conveying the cooling water from theTPP pools towards the existing outlet structure at the river bank, areshown in Fig. 2. There are four such channels, one per each of thetwo existing units, while the other two channels were built for an-other two planned units. At present, the construction of one addi-tional (third unit) is in preparation, and the hydro energypotential of the cooling water for this unit is also taken into accountin this study. All channels have the same quadratic 3 m � 3 mcross-section, while their lengths vary, due to the different dis-tances between the TPP units and the river bank. A planned locationof the small HPP site at the river bank is also presented in Fig. 2.

4. Estimation of net head

The gross water head Hg is represented by the difference of theupstream Huwl and downstream Hdwl water levels. The upstream

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Fig. 1. The cooling water system at one unit of the TPP ‘‘Nikola Tesla B’’. Cooling water pumps are denoted with CWPs, top levels of the main and auxiliary condensers’ aredenoted with Hmc and Hac respectively, Huwl and Hdwl are respectively the upstream water level in the water pool and the downstream river water level, while the level of thewater pool wall is Hwpw. Indicated are inner diameters of the pipelines in millimetres.

Fig. 2. Discharge cooling-water channels from the pools at the plant units towards the river.

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water level is the free surface of the jet that spills from the TPPpool, and the downstream water level is the river water surface.Daily water levels of the Sava river at the plant location for the20 years period are averaged, and presented in Fig. 3. The highestriver water level is in the spring (in April), while the lowest is atthe end of August and at the beginning of September. These dataare used for the calculation of the annual change of the gross head,and the results are also presented in Fig. 3. As shown, the highestgross head is at the end of the summer, due to the lowest riverwater level in this period.

The flow head losses, from the TPP pool unit, towards the HPPintake structure chamber, are calculated taking into account thefriction losses along the concrete channel, and the local hydrauliclosses. The local head losses take place at the water spill over theTPP pool weir, at the channel bends, and the inflow into the intakechamber just in front of the hydro turbine units. For the channelwall roughness of 1.0 mm, the channel lengths of 470 m, 545 mand 620 m (from the three units towards the HPP intake chamber),the water flow rate of 20 m3/s per channel, and the total local loss

coefficient of 1.39 (the same for all three channels), the calculatedhead losses DHl are 1.2 m, 1.3 m and 1.4 m for the Units 1, 2 and 3respectively. By subtracting the head loss from the gross waterhead Hg the net head Hn is obtained. The duration curves of thegross and net heads are averaged for the annual period, based onthe available data of the Sava river level, and presented in Fig. 4.The maximum available net head is up to 6 m, while the mean va-lue is 3.2 m (calculated as the difference of the mean gross head of4.6 m, as presented in Fig. 3, and the maximum head loss of 1.4 m,that corresponds to the Unit 3). About 56% of the maximum headloss of 1.4 m is due to the friction along the channel, while the restis caused by the local resistances. Such a high losses might be re-duced by increasing of the cooling-water channel cross-section,which will reduce the velocity from the present value of 2.22 m/s. However, the civil works on the reconstruction of the existingchannels are not feasible, due to the existing infrastructure at theplant, and the need for the plant shut down during these works.This is one of the restrictions that could arise when the small hydropower plant is built at the existing technical system.

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69.0

70.0

71.0

72.0

73.0

74.0

75.0

1 2 3 4 5 6 7 8 9 10 11 12

Wat

er le

vel (

m)

Months

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

6.5

Gro

ss h

ead

(m)

Monthly mean valuesAnnual mean value

Fig. 3. The averaged water level of the Sava river and the averaged gross head between the surface of the water pool at the TPP and the river level during the year based on20 years daily records (from 1986 till 2006).

0 0.2 0.4 0.6 0.8 1

Annual period (-)

Hea

d (m

)

Gross headNet head

8760 h

ΔHl

0

1

2

3

4

5

6

7

8

Fig. 4. Duration curves for the gross and net heads between the water pools in TPPunits and the HPP at the river Sava bank (averaged for the period from the year1986 till 2006).

4008 V.D. Stevanovic et al. / Applied Energy 88 (2011) 4005–4013

5. Hydro turbine selection and electricity production

For the low net head of 3.2 m, and the total water flow rate of2 � 20 m3/s = 40 m3/s, for the present state of two TPP units, or3 � 20 m3/s = 60 m3/s with planned additional unit, the applicationof the axial type hydro turbines, such as the Kaplan, propeller orbulb turbines, is appropriate [3]. The application of the double reg-ulated Kaplan turbine, or the bulb turbine, is considered as themost suitable solution. The propeller turbine is not proposed, dueto its substantial decrease of efficiency, when the operationalparameters diverge from the optimal design values. The turbineswill be equipped with adequate type of synchronous generator,with static excitation system and other necessary electrical equip-ment. The synchronous generators will be electrically connected tothe internal consumption of the thermal power plant. Also, it isconsidered that the best solution for the reliability of the coolingwater energy utilization in the coupled thermal and hydro powerplants operation is to put up one hydro power unit per one thermalpower unit. In this way, the failure of one hydro unit or thermalpower unit, would have the least consequences on the HPP output.Introduction of more than one hydro unit per thermal unit willraise the investment costs. The Kaplan and bulb turbines offeredby several vendors have been surveyed. Although the operationalcharacteristics of turbines offered by different vendors are remark-ably different, the conservative judgment leads to the conclusionthat under the water flow rate of 20 m3/s, the hydro turbineoperates under the net head in the range from 2.5 m up to 5.0 m,

with the averaged plant efficiency of 0.82 (this value of the plantefficiency is also suggested in [10]). The adopted minimum headof 2.5 m is the lower value for the application of the conventionalhydro turbines [11], while for even lower head values, the applica-tion of the hydrokinetic turbines is analyzed [12]. The plant effi-ciency is determined as the product of the hydro turbineefficiency, the speed increaser efficiency (which multiplies thelower turbine rotation speed to the higher rotation speed requiredby the electric generator), and the synchronous generator effi-ciency. The overall plant design is presented in the next section.

The electricity production in kW h, during the year, is deter-mined as

Eel ¼ 8:76 � q � g � _V � gHE �Z 1

0HðxÞdx� DEr ð1Þ

where 8.76 is the annual number of hours divided by thousand, q isthe water density, g is the gravity, _V is the water volumetric flow,gHE is the mean value of the hydro power plant efficiency, and thenet head that can be utilized in the hydro turbine, H(x), is boundedby its maximum (Hn,max) and minimum (Hn,min) values, and it isdetermined according to

HðxÞ ¼Hn;max; HnðxÞ > Hn;max

HnðxÞ; Hn;min 6 HnðxÞ 6 Hn;max

0; HnðxÞ < Hn;min

8><>: ð2Þ

The net head Hn depends on the time duration denoted as x, andit is determined according to Fig. 4. The last term on the r.h.s. of Eq.(1) counts for the electricity that cannot be produced due to over-haul periods at the thermal power plant, when there is no flow ofthe cooling water. These overhaul periods at both units at the TPP‘‘Nikola Tesla B’’ are averaged for the period from the first connec-tion to the electrical grid untill recent time, and the result is pre-sented in Fig. 5. Taking into account the number of hours inoverhauls per each month during the year, as presented in Fig. 5,and the available net head during the year (the net head for eachmonth is obtained by subtracting the head loss from the gross headpresented in Fig. 3), the reduction of the electricity production iscalculated using the first term on the r.h.s. of Eq. (1). Although thelongest overhaul period is in May (the fifth month in Fig. 5), thereduction of the electricity production is zero because the averagednet head in May is very low, i.e. below 2.5 m, and anyway the HPPcannot produce electricity during this period of the year. The re-verse holds for June. The overhaul period in this month is also highand the water level of the river descends, and there is a maximum

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1 2 3 4 5 6 7 8 9 10 11 12

Months

Tim

e (h

)

0

50

100

150

200

250

300

Fig. 5. Average monthly overhaul periods during the year per one unit of the TPP‘‘Nikola Tesla B’’ (averaged from the first connection to the grid in 1984 (Unit 1) and1986 (Unit 2) till June 2007).

V.D. Stevanovic et al. / Applied Energy 88 (2011) 4005–4013 4009

reduction in electricity production. The total annual reduction ofelectricity production due to overhaul periods for one unit of thethermal power plant is 0.3052 GW h.

The possible electricity production in the HPP that utilizes theenergy of the cooling water at the TPP ‘‘Nikola Tesla B’’ is calculatedwith Eq. (1), and the results are presented in Table 1, depending onthe number of units in operation. For the operation of two or threeunits, the available net head is determined according to the lowestnet head for the units in operation. For instance, when Units 1 and2 are in operation, the net head for the Unit 2 is taken into account.The adjustment of the operation, according to the available nethead of the Unit 1 (which is higher than in Unit 2), would meanan increase of the water level in the cooling water pool in the Unit2, but that is not acceptable since it would lead to the flooding ofthe plant in Unit 2.

The impact of the cooling water temperature change through-out the year on the electricity production is not higher than 0.6%.Namely, the cooling water temperature at the exit of the thermalplant cooling system changes between the minimum value of8 �C in the winter, and the maximum value of 35 �C degrees inthe summer. This maximum variation of the cooling water temper-ature leads to the water relative density variation that is not higherthan 0.6% (from 999.85 kg/m3 at 8 �C to 994.04 kg/m3 at 35 �C).Since the electricity production depends linearly on the water den-sity, according to Eq. (1), the maximum water temperature varia-tion influences the electricity production also by a value nothigher than 0.6%. The temperature change has a significant influ-ence on the cooling water viscosity, but the viscosity influenceon the water head drop is negligible since the friction coefficientin flow channels with rough walls does not depend on the viscosity(i.e. on the Reynolds number) for high values of the Reynolds num-bers, as it is presented in the Moody’s chart for friction factors [3].In this study, the minimal value of the Reynolds number is4.8 � 106 at 8 �C and the relative roughness is 0.0003 (calculatedas the ratio of the wall roughness of 0.001 m and the hydraulicdiameter of 3 m). According to the Moody’s chart the friction coef-ficient does not depend on the Reynolds number for its valueshigher than 4 � 106, and the calculated value of the relative rough-ness of 0.0003.

Table 1Annual electricity production in the HPP depending on the number of units inthe water flow rate per unit is 20 m3/s, duration of the net head is presentedis 0.3052 GW h and the overall plant efficiency is 0.82).

Units of thermal power plant in operation Unit 1

Annual production of electricity Eel, (GW h/god) 3.88

The water temperature variation influences the necessary suc-tion head of the turbine, which is calculated as [3]

Hs ¼patm � pv

qgþ v2

2g� rHn ð3Þ

where patm is the atmospheric pressure, pv is the water vapour pres-sure, v is the hydro turbine outlet average velocity, r is the cavita-tion coefficient and Hn is the net head. The change of the watervapour pressure from 0.0107 bar at 8 �C to 0.0563 bar at 35 �C,and the corresponding water density change, lead to the reductionof the first term on the r.h.s. of Eq. (3) for 0.47 m, i.e. the suctionhead of the turbine is reduced accordingly. Hence, the hydro turbinesetting should be done according to the Hs value, determined for thehighest possible cooling water temperature.

6. Civil works

Within this section, the function, location, and dimensioning ofthe civil works for the small HPP are considered. The power houseand the appurtenant structures are presented in Fig. 6.

The basic principals for selection and location of the powerhouse and the appurtenant structures were not to jeopardize theoperation of the existing TPP ‘‘Nikola Tesla B’’, and to comply withthe environmental conditions, prescribed for the reach of Sava riv-er downstream of the plant. The other criteria and limitationswere: the location of the existing structures of TPP, the suitablemechanical and electrical equipment to be implemented, minimalscope and expenses of the civil works, with the appropriate staticand functional stability.

In order to keep the permanent availability of the cooling-waterchannels for the TPP, the hydro power plant power house could notbe located directly within the existing buried channels. Hence, theflow is by-passed through a short chamber-like diversion, by a sys-tem of gates (Fig. 6). The diversion chamber enables the capacityfor the flow-regulation (including the start-up). A side-channelemergency spillway, with a chute and an energy dissipater, shouldpass the superfluent flow from the chamber into the Sava river, incase of the rapid shut-down, (Fig. 7).

In the selection of the appurtenant structures location, the mainprincipal was to minimize the reconstruction of the existing struc-tures. The redirection of the cooling-water flow through the exist-ing channels will be enabled by the appropriate coordination of thegates to be implanted. In the channel walls, the openings will becut, where the automatically governed gates will be embedded.

The intake structure and the chamber dimensions are selectedso that the most of the available head is utilized (Fig. 7). Also,the chamber size enables favourable hydraulic conditions for theHPP exploitation, especially during the start-up and the shut-down. The wall and slab thickness enables the structure stabilityduring the construction and operation, especially having in mindan extremely unfavourable influence of the uplift, during the highstages of Sava river.

The gates enable the efficient flow redirection from one cooling-water channel to another, and to the chamber, depending on thesystem operation requirements. Hence, the existing outlet struc-ture will be reconstructed, with the new flow-regulation gates.

operation (the operational net head is in the range from 2.5 m to 5.0 m,in Fig. 4, the reduction in electricity production due to overhaul periods

Unit 2 Units 1 + 2 Units 1 + 2 + 3

3.78 7.45 10.83

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Legend:

1. Cooling water flow

2. The existing outlet structure

3. The existing earth channel

4. The intake structure with the chamber

5. Power house

6. Tail water

7. Emergency spillway

8. Chute

9. Gate plateau

10. Access to powerhouse

11. The existing road

12. A pipeline for the heating of equipment in the cooling water intake station during extremely cold winter days.

Fig. 6. Hydro power plant layout.

Fig. 7. Hydro power plant cross-section (see the legend of Fig. 6).

4010 V.D. Stevanovic et al. / Applied Energy 88 (2011) 4005–4013

For the case when the HPP is out of service, the flow will be passedthrough the outlet structure, directly into the Sava river.

The power house is designed in compliance with the selectedequipment. The tailwater connects the diffuser directly to the Savariver.

The redirection of the cooling flow during the small HPP con-struction is foreseen through the existing channels, having in mindthat the two of the four channels are not in operation, yet. Thereconstruction works will be performed entirely during the mainte-nance of the TPP units. At the first stage, the chamber, the power-house, and the emergency spillway will be built. The structureswithin the river channel have to be erected during the low riverstages, under the sheet pile protection. The second stage will takeplace during the maintenance of the TPP second unit. In this stage,the channels II, III, and IV will be reconstructed, and all the gates inthe channel II will be implanted (Fig. 6). The second phase could beperformed parallel to the first one. The third stage will be done dur-ing the maintenance of the TPP first unit, and after the second stageworks have been completed. At this time, the flow will be diverted

by the previously installed gates, from channel II into channel III,enabling the work at channel I and the wall between channels Iand II. At this stage, channel I will be reconstructed, with the gatesembedded in this channel. Also, the gates between channels I and II,and between channel I and the intake structure will be implanted.After the third stage, the refilling, construction of the plateau andthe service road will be built. The required civil works are presentedin the Gantt chart in Fig. 8.

7. Environmental aspects

Natural water temperatures of the Sava river vary considerably,with minimal values in January and February, and maximal in Julyand August. The average year temperature is around 12.7 �C, themaximal average monthly temperature is 23 �C, and the maximaldaily temperature 29 �C.

The most significant impact of the TPP ‘‘Nikola Tesla B’’ on thetemperature regime of the Sava river occurs in the periods of low

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Fig. 8. Flow-chart diagram for civil works.

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flow and high water temperatures. The basic requirements for theHPP operation are:

– The quantity of water taken for cooling of the turbines has to belower than 25% of the instant river flow.

– The temperature of the Sava river after the mixing with thewater from the cooling system must not be higher than 28 �C.

On the base of the available data, it can be concluded that thewater temperature in the cooling-water canals is approximately9 �C higher than the water in the Sava river (at the TPP ‘‘Nikola Te-sla B’’ intake structure area). The intake structure of the TPP ‘‘Nik-ola Tesla A’’ is located 17 km downstream of the TPP ‘‘Nikola TeslaB’’. In the critical period of a year, the temperature difference at thetwo intake structures (of the Power Plants ‘‘Nikola Tesla B’’ and‘‘A’’) is maximum 2.5 �C, with the average difference of around 1 �C.

The small HPP is designed to utilize the entire cooling-waterflow from the thermal power plant, which should not affect theaverage temperature in the cross-sections of the Sava river closeto the tail water outlet. However, the flow velocities of the hotwater discharging into the Sava river will be lower comparing tothe present conditions (due to the lower energy), reducing the like-lihood of mixing the hot water with the water in the river. Conse-quently, it can be expected that the flow of hot water will bedeveloped close to the right river bank.

The flow of the Sava river enters a sharp right bend, with theoutside angle of around 140�, some 10 km downstream from theTPP ‘‘Nikola Tesla B’’ outlet structure. Such a sharp winding causesa helicoidal circulation, with the strong mixing of the water acrossthe entire cross-section; hence, the temperature regime of thedownstream flow of the Sava river should not be considerablychanged comparing to the present state.

On the base of the available data, it can be concluded that thesmall HPP will have minor influence on the Sava river, comparingto the present conditions. No cross-section temperature measure-ments were available, so the influence of the hot-water flow cannotbe accurately estimated. Having in mind the previously stated (nochanges of the hot-water flow discharged due to operation of the

small hydro power plant, and significant influence of the sharp rightbend 10 km downstream of the outlet) it can be predicted that thesmall HPP will have insignificant influence on the Sava river, com-paring to present conditions. That influence is limited on the area ofaround 10 km downstream from discharge of hot water into theriver.

8. Economic evaluation

Specific costs of electric and mechanical equipment, which rep-resents investment costs divided by the installed capacity, de-crease with the increase of the installed capacity and net head,according to the following relation [4]

I0EM ¼bo

Pb1n Hb2

n

ð4Þ

where the specific costs I0EM are calculated in Euros/kW, Pn is thenominal power in kW, Hn is the net head in m, and the constantsare bo = 3300 €, b1 = 0.122, b2 = 0.107. The total investment costsfor the HPP are calculated as

I ¼ ðI0EM � z � Pn þ ICVÞð1þ f Þ ð5Þ

where z denotes the number of units, ICV is the cost of civil works,while f takes into account the costs of equipment installation, theelectrical connecting to the grid, the design work, etc. The parame-ter f ranges between 0.05 and 0.10 [4], and the value of 0.075 isadopted. The present value of the annual operation and mainte-nance (O&M) costs is calculated according to

COM ¼ ðm1 � I0EM � z � Pn þm2 � ICVÞXj¼n

j¼1

ð1þ g1þ i

Þj ð6Þ

where m1 is the fraction of O&M costs in the total investment in themechanical and electrical equipment, m2 is the fraction of O&M inthe investment costs in civil works, g is the annual increase of thesecosts, and i is the standard interest rate. The number of years of theplant operation is denoted with n. The following values are adopted:

Page 8: Hydro energy potential of cooling water at the thermal power plant

0.06 0.07 0.08 0.09 0.1 0.11 0.12

Electricity price (€/kWh)

Valu

e (€

/yea

r)

2 units 3 units

0.00200,000.00

400,000.00600,000.00

800,000.001,000,000.00

1,200,000.001,400,000.00

Fig. 9. Value of annual electricity production in the HPP that utilizes energy of thecooling-water flow at the coal-fired TPP ‘‘Nikola Tesla B’’ versus electricity price.

0.06 0.08 0.1 0.12

Electricity price (€/kWh)

Inte

rnal

rate

of r

etur

n (%

) / P

ay

back

per

iod

(yea

rs)

2 Units 3 Units

Pay back period

Internal rate of return

0

5

10

15

20

25

30

Fig. 10. Internal rate of return and pay back period for the HPP project that utilizesenergy of the cooling-water flow at the coal-fired TPP ‘‘Nikola Tesla B’’.

4012 V.D. Stevanovic et al. / Applied Energy 88 (2011) 4005–4013

m1 = 0.025, m2 = 0.015, g = 0.03, i = 0.08, and the planned period ofoperation is 20 years. The variable maintenance and operation costs(which could be caused by the need to replace some parts of equip-ment, during the planed lifetime of the plant), are neglected. Thepresent value of the total costs is calculated as

CT ¼ I þ COM ð7Þ

and presented in Table 2. The two solutions are considered. The firstone is with two hydro turbines for the existing TPP units, and thesecond is with three hydro turbines, having in mind the planned thirdTPP unit. The adopted rated power per hydro turbine unit, used in theinvestment costs prediction is 800 kW. The same cost for the civilworks, for the both solutions (with two and three units), is assumed.The power of three hydro turbines is 2.4 MW. That presents 36% of thetotal nominal power of the main cooling water pumps (6.6 MW), re-quired to provide the cooling water for the three TPP units.

The present value of the electricity produced during the HPPoperational lifetime of n years is predicted according to

VEel ¼ Eel � cel

Xn

j¼1

ð1þ e1þ i

Þj ð8Þ

where cel (€/kW h) is the electricity price, i is the standard interestrate, and e is the rate of annual increase of electricity price(e = 0.03). The value of the annual electricity production Eelcel,depending on the electricity price, is shown in Fig. 9.

The economic evaluation of the HPP project is performed withthe calculation of the internal rate of return and the simple payback period. The internal rate of return iIRR is calculated assumingthat the present value of electricity production during the HPP life-time is equal to the present value of all costs

Eel � cel

Xn

j¼1

1þ e1þ iIRR

� �j

¼ Iþ m1 � I0EM � z �Pnþm2 � ICV� �

�Xj¼n

j¼1

1þ g1þ iIRR

� �j

ð9Þ

where the plant operation for n = 20 years is adopted. The simplepay back period SPBP is calculated as the ratio of the total invest-ment costs and the value of annual electricity production

SPBP ¼ IEel � cel

ð10Þ

The results are presented in Fig. 10. For the present price ofelectricity for the small HPPs in Serbia of 0.08 (€/kW h), the inter-nal rate of return is 17.5%, and the pay back period is 5.5 years.These parameters indicate that the presented project is economi-cally attractive. The sensitivity of the internal rate of return andthe pay back period of the total investment cost for the smallHPP with three units, is shown in Fig. 11. The results are presentedfor the 20% increased, and 20% reduced total investment costs.

If the project of the given small HPP would be performed as theClean Development Mechanism (CDM) project [13], than its contri-bution to the reduction of carbon dioxide (CO2) emission would bequalified with certain amount of Certified Emission Reductions(CERs). One CER corresponds to the reduction of one tone of CO2,

Table 2Present value of costs according to Eqs. (4)–(7).

Power perunit, Pn

(kW)

Totalpower, zPn

(kW)

Specific cost ofequipment, I0EM (€/kW), Eq. (4)

Total cost ofequipment,IEM ¼ zI0EM Pn (€)

Twoturbines,z = 2

800 1600 1245 1,992,000

Threeturbines,z = 3

800 2400 1245 2,988,000

and it has a certain market value. It is determined that in Serbiaone kWh of electric energy is produced from fossil fuels with theemission of 1.04 tones of CO2. Hence, the annual production of10.83 GW h with three hydro turbine units (Table 2), will lead tothe reduction of CO2 emission in the amount of 11149 tones, whichresults in 11149 CER. For the assumed CER values of cCER = 6 €/tCO2,10 €/t CO2 and 15 €/t CO2, the possible income is 66894 €, 111490 €

and 167235 €, respectively. According to the CDM project rules,this income can be achieved in 10 years. The internal rate of returnin this case is calculated from the following equation

Eel � cel

Xn

j¼1

ð 1þ e1þ iIRR

Þj þ CER � cCER

X10

j¼1

1

ð1þ iIRRÞj

¼ I þ ðm1 � I0EM � z � Pn þm2 � ICVÞ �Xj¼n

j¼1

ð 1þ g1þ iIRR

Þj ð11Þ

The calculated internal rate of return for the small HPP project,realized as the CDM project is presented in Fig. 12 for different val-ues of the electricity cel and CER values. As presented in Fig. 12, theinternal rate of return can be increased in the range from 1% in caseof 6 €/CER to 3% in case of 15 €/CER.

Cost of civilworks, ICV

(€)

Totalinvestment, I(€), Eq. (5)

Present value of O&Mcosts, COM (€), Eq. (6)

Present value oftotal costs CT (€),Eq. (7)

1,670,000 3,936,650 944,425 4,881,075

1,670,000 5,007,350 1,258,600 6,265,950

Page 9: Hydro energy potential of cooling water at the thermal power plant

0.06 0.07 0.08 0.09 0.1 0.11 0.12

Electricity price (€/kWh)

Inte

rnal

rate

of r

etur

n (%

) / P

ay

back

per

iod

(yea

rs)

0.8*I 1.0*I 1.2*I

Pay back period

Internal rate of return

0

5

10

15

20

25

30

35

40

Fig. 11. Internal rate of return and pay back period dependence on the totalinvestment costs I.

0.06 0.07 0.08 0.09 0.1 0.11 0.12Electricity price (€/kWh)

Inte

rnal

rate

of r

etur

n (%

) CCER=15€CCER=10€

CCER=6€

without CDM

10

15

20

25

30

Fig. 12. Internal rate of return dependence on the value of the certified emissionreduction.

V.D. Stevanovic et al. / Applied Energy 88 (2011) 4005–4013 4013

9. Conclusions

Recently, a wide interest and activity is directed towards theutilization of energy potential of small streams, and other naturalhydro power sources. In this paper, considered is the hydro energypotential of the gravity water flow from the coal-fired TPP to therecipient river. Since the discharge of the cooling water from theTPP has to be at a higher elevation than the maximum water-sur-face level of the recipient river, a significant head residual is pro-duced for the lower river-levels. The issuing energy potential, sofar unexploited, shall be utilized for the electricity production bya small hydro power plant (HPP).

The existing structures of the TPP cooling system (intake struc-ture, cooling-water pool, discharge outlets, etc.), will be used forthe proposed HPP. Generally, such a project has to satisfy severalconditions: the hydro energy of the cooling-water flow is availableonly during the TPP operation; it must be environmentally accept-able and energetically and economically beneficial; it must not vio-late the technical TPP system safety.

Bearing this in mind, for the permanent availability of the cool-ing-water outlets, the HPP power house should not be located withinthe existing structures. Hence, the flow must be directed to thepower house by a system of gates. The design must also providefor an efficient phased construction, entirely during the TPP mainte-nance. An emergency spillway should pass the superfluent flow intothe recipient river, in case of the rapid HPP shut-down. The turbine

type should be selected regarding the available cooling-water flowrate and net head.

The economic benefits of the project should include the internalrate of returns and pay back periods, depending of the electricityprice and total investment costs. Bearing in mind that an HPP couldbe realized as the Clean Development Mechanism project (accord-ing to the Kyoto protocol), the expected increase of profitabilityshould be evaluated.

The case study of utilization of cooling water energy potential atthe coal-fired TPP ‘‘Nikola Tesla B’’ in Serbia is presented, to sup-port the previous considerations. The nominal power rate of thedesigned HPP with three turbines is 2.4 MW, while the internalrate of return and the pay back period are 17.5% and 5.5 years,for the present electricity price from small HPPs in Serbia. The ob-tained results show that the project is economically attractive, andit can be realized with standard matured solutions of hydro tur-bines available at the market. The influence of the total investmentcosts uncertainty for ±20% on the project economy is predicted, aswell as the increase of the project profitability if it would be real-ized as the Clean Development Mechanism project. The presentedresults show that the utilization of the water stream energy withinthe cooling system of the TPP is technically feasible and economi-cally attractive and it is an additional source of clean energy.

Acknowledgments

This work was supported by the electric company ThermalPower Plants ‘‘Nikola Tesla’’ and by the Ministry of Science andTechnological Development of the Republic of Serbia (Grant174014). The project was conceived by Milorad Jovanovic andMilos Milic from the Thermal Power Plants ‘‘Nikola Tesla’’, andthe authors are thankful for their generous help.

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[11] US Department of Energy, Energy Efficiency and Renewable Energy; Wind andHydropower Technologies, Feasibility Assessment of the Water EnergyResources of the United States for New Low Power and Small Hydro Classesof Hydroelectric Plants, Tech. Rep. DOE-ID-11263; January 2006.

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