Improving the energy efficiency of a 110 MW thermal power plant by low-cost modification of the cooling system

Abstract

The electric power system of the Republic of Serbia relies mostly on lignite-fired thermal power plants, with 70% of the total electricity generation. Most of these plants are over 30 years old, and investment in their modernization is necessary. The energy efficiency of the 110 MW coal-fired power plant in which the condenser is cooled by the mechanical draught wet cooling towers system is analyzed in this paper. Attention is primarily devoted to operating conditions of the cold end of the plant, i.e. to the interrelationship of the condenser and cooling towers. Most important parameters that affect the operation of the cooling towers system are ambient air temperature and relative humidity, specific mass flow rate, and temperature of cooled water. With the existing cooling system, the overall energy efficiency of the plant is low, especially in the summer months, even less than 30%, due to adverse weather conditions. By upgrading existing cooling tower system by adaptation of two additional cooling tower cells, overall energy efficiency can be increased by 1.5%. The cooling tower system rehabilitation investments payback period is estimated to be less than one year. Static method for economic and financial assessment is used.

Keywords

Cooling tower specific mass flow rate energy efficiency investments power plant

Introduction

As a primary component of the cold-end system in thermal power plants, the cooling towers play an important role in cooling the circulating water from the condenser, hence its efficiency has a great impact on the total cycle efficiency of power plants. The achievable temperature in the cooling tower, which is the main objective of this paper, is considered as one important economic factor while designing power plants. According to these issues, studying cooling tower performance and its effective factors on getting power production are highly important. Increasing efficiency in all production processes has become an imperative, with special emphasis on improvement in energy efficiency and resource conservation. The continuously increasing complexity of the industrial systems has led to their construction and operation having a detrimental effect on natural resources and environment.¹ As main electricity producers in many countries, coal-fired power plants are considered to be one of the largest greenhouse gasses emitters; effective diagnosis of the problems in the specific plant and appropriate energy management can reduce overall energy consumption and costs of electricity production, but also can reduce carbon emissions.²

There are several ways to improve the performance of a power plant. One of them is to improve the performance of the cooling system. Some ways of improving the cooling system of a power plant performance are the following³:

Estimation of the technical performance of present equipment and an analysis of the opportunities to improve their efficiency

Improving the maintenance procedures

Implementing new proven technology in the current energy system

Increase capacity of the existing cooling tower

Install a new tower

The effect of climatic changes is shown to be important in the design of more effective cooling technique and to device methods to compensate for the loss in plant efficiency.⁴ Lakovie et al.⁵ showed that dimensioning a cooling tower according to ambient air parameters that are higher than current standard recommendations ensures required cooling water temperature for efficient plant operation and gives experimental confirmation for this statement.

In the near future, research work will be based on finding adequate solutions to the problem of the lack of sufficient cooling water. In fact, today in the world, between 60 and 80% of the water is used for cooling, primarily in thermal power plants. For the purposes of providing drinking water, it is used only 2–6% of the total water sources.⁶ The amount of cooling water needed for coal-fired and nuclear power plants is certainly one of the most important factors in planning of electricity capacities enlargement and rehabilitation of the existing plants. In the future, the focus should be in avoiding once-through cooling systems and replacing them with closed-cycle cooling systems with cooling towers, as it is noted in Fleischli and Hayat.⁷ This is also important concerning environmental protection of riverine ecosystems, which in the recent years becomes one of the top priorities.⁸

A cooling tower is a heat removal device used to transfer waste heat absorbed in the circulating cooling water systems to the atmosphere. Cooling towers are relatively simple devices with an extremely complex process. The complexity is in the input parameters that are largely beyond the control of change: either directly as a change in the parameters of atmospheric air or through feedback to the plant from which the heat is removed. In the cooling tower, the temperature of cooling water depends on the parameters of the atmospheric air and specific mass flow rate.⁹

In a cooling tower system, the most important moist air parameter for cooling tower design and operation is the wet bulb temperature, which strongly influences the enthalpy of the air. The influence of other parameters such as dry bulb temperature and atmospheric air pressure is within negligible ranges which occur in the normal operation of cooling towers. The variation of ambient air parameters can be great during a day or month; they will cause the variation of cooling tower heat capacity and supply water temperature.¹⁰

Many factors determine the type, size, and shape of a cooling tower. The final choice is determined by many factors such as individual user requirements, economic considerations, local weather patterns, and esthetics. The two basic cooling tower designs in use today are the natural draft tower and the mechanical draft tower.¹¹ Natural draft cooling towers rely on natural forces to move air through the cooling section of the tower. The most recognizable example of the natural draft tower is the large hyperbolic tower used by many nuclear power generation stations. Hyperbolic towers work much like a chimney, where the air flow is induced by convection.

Mechanical draft towers have air forced through the structure by a fan. The air flow can be pushed through by fans located at the base of the tower (referred to as forced draft) or it can be pulled through by fans located at the top of the tower (referred to as induced draft). Induced draft towers tend to be larger than forced draft units. Due to their technical and economical characteristics, wet, mechanical draught cooling towers are suitable for elastic operation for large and low capacities. Investments in the cooling tower with the natural draft is 6–9 $/kW while the cooling towers with forced draft investments are smaller, 5–8 $/kW. Cooling towers with forced draft are used for smaller installations while in the case of power plants 150–200 MW the use of cooling towers with a natural draft is justified.

In this paper, the energy efficiency of the 110 MW coal-fired power plant “Kolubara A” located in western Serbia was analyzed from its cold-end efficiency point of view. This plant was built in 1979 and still makes important part of electricity generation system in the country.¹² Power plant performance and efficiency erode after about 25–30 years of operation,¹³ so proper maintenance and investments in this particular case are necessary. In this paper, the possibility of low investment recovering of the cooling tower system has been analyzed, in accordance with specific geographical and climatic data. After preliminary energy audit of the plant supported by government of the Republic of Serbia, the authors came to a conclusion that operation of the cooling system of this plant should be improved. Possibility and effects of putting into operation two more cooling tower cells in order to increase the energy efficiency of the power plant is considered in this paper.

Thermodynamic and mathematical model of reference plant

“Kolubara A” thermal power plant is the oldest active power plant in the Republic of Serbia. It is located on the edge of the Kolubara coal basin in Veliki Crljeni, nearby Lazarevac. It has started working in 1956 with two power units of 32 MW. Today this power plant has five units. 110 MW Kolubara A5 Unit of the power plant was chosen as the reference plant for the consideration of the influence of parameters of atmospheric air to the energy efficiency of the plant. It has a closed-cycle type cooling system with mechanical draught cooling towers.

A cooled system of 10 cooling towers for the condenser block A5 110 MW has been designed, but only eight cooling towers are in function. In the general plan of the plant there is a possibility for constructing two additional cells. A concrete construction for the two cells is already placed. Required financial investments for these two additional cells are related to the filling system, the system for spilling water and fans.

In this paper, the reasonableness and economic feasibility of repairing and putting into operation of existing cooling cells No. 9 and No. 10 are analyzed. The results of a calculation when the plant is in operation with eight cooling cells and when in function are 10 cooling cells are compared, together with the necessary investment and payback period, as well as the period of amortization invested assets calculation.

The economic analysis of building additional four (instead two recommended) cooling tower cells was not done. The reasons for this decision are plant is old enough that in the near future it would be necessary to reconsider if it will stay in basic load of the country power system or it will be transferred to a peak load. For the cells No. 9 and No. 10 concrete construction is already finished and requires less initial investment. The gain in overall efficiency of the plant by putting in operation 12 cooling tower cells would be visible only in the few hottest months. Therefore, further investments in the cells No. 11 and No. 12 are considered unnecessary by the authors; because of that thermodynamic calculation for 12 cooling towers is presented in the paper but is not discussed.

The boiler produces a steam pressure of 127.5 bar at a temperature of 535°C. This vapor expands in the high-pressure turbine to 36.2 bar and continuous to overheating process in the reheater at a temperature of 535°C, after which steam is expanded in low-pressure turbine to the final pressure. The pressure in the condenser is 0.075 bar in the design conditions. The plant has two regulated steam subtraction, at pressures of 30 and 2 bar. The cooling water flow rate through the condenser is 4000 kg/s. The condenser of this power plant is cooled with water which is previously cooled in a mechanical draft cooling tower. The condenser of this plant is a surface heat exchanger with a surface area of 6900 m² made of 14,635 tubes, the inner diameter of a tube is 18 mm and the outer 20 mm. Figure 1 gives a simplified schematic view of the plant.

Figure 1.

Schematic view of the plant.¹⁴

A mathematical model of the condenser is based on the equations of heat transfer and equations of mass and energy balance¹⁵

Q = K F Δ t_{m}

(1)

Q = D_{k} (i_{p} - i_{k})

(2)

Q = G_{w} c_{w} (t_{w 2} - t_{w 1})

(3)

Δ t_{m} = \frac{(t_{p} - t_{w 2}) - (t_{k} - t_{w 1})}{l n \frac{t_{p} - t_{w 2}}{t_{k} - t_{w 1}}}

(4)

Evaporative water cooling decreases its temperature during simultaneous execution of two physical processes^16–18: heat convection due to the temperature difference between water and air, and the evaporation of water in the atmospheric air due to differences in concentrations

q = q_{α} + q_{β} = α (t_{w} - t_{a}) + r β_{p} (p^{″} - p)

(5)

The partial pressure of water vapor in the saturated and no saturated air is represented by the following expression

p^{″} = \frac{X^{″}}{X^{″} + 0.622} p_{b}, p = \frac{X}{X + 0.622} p_{b}

(6)

where

X^{″}

and

X

are air humidity ratio in the saturated air on border surface and in the core of air stream, 0.622 represents a constant ratio of water vapor and air.

After a certain amount of settling the terms of the evaporated liquid we get

q_{β} = r β_{x} (X^{″} - X)

(7)

Whereby $β_{x} = 1.61 p_{b} β_{p}$ represents mass transfer defined in relation to the difference of moisture in the air.

The volumetric mass transfer coefficient is determined from the following form

β_{v x} = \frac{β_{x} F}{V}

(8)

In practice, it is not possible to reach an exact solution of this equation and is used as the empirical equation for its calculation. In Berman,¹⁹ an empirical equation is represented as

β_{x v} = B {(w_{v} ρ_{v})}^{m} q_{w}^{n}, [kg / m^{3} h (kg / kg)]

(9)

where:

$q_{w} = \frac{q}{3.6}$ −specific mass flow rate (kg/m²s), w_v—air velocity

The constants in this expression are B = 1050, m = 0.53, and n = 0.39.

Merkel developed the theory for the thermal evaluation of cooling towers in 1925. Merkel equation has the following form¹⁹

\frac{β_{x v} V}{G} = \int_{t_{1}}^{t_{2}} \frac{c_{w} d t}{h^{″} - h}

(10)

Merkel integral requires certain assumptions. The first assumption is about neglecting the change of water flow rate in energy balance. The second assumption states that air exiting the cooling tower fill is saturated and this state can be characterized only by its enthalpy. The last assumption of the Merkel’s model states that Lewis factor L_ef = 1.²⁰

Merkel’s integral analytical solution is not possible. Therefore it solves graphic. Graphics solution of Merkel integral requires quite a lot of work. This integral cannot be solved analytically due to the nonlinear dependence of the enthalpy of saturated air and temperature. However, when the working conditions for the cooling tower are not extreme (when the towers working in the field of customary temperature) above integral can be solved approximately analytically. Then it is assumed parabolic dependence of the enthalpy of saturated air and temperature using the following expression

i^{″} = a t^{2} + b t + c (kJ/kg)

(11)

For the calculation of heat exchange in cooling towers it is assumed that the coefficients amounts a = 0.119, b= − 1.575, and c = 40.¹⁰ With such constants adopted, on the field of water temperature for referent plant, thus calculate the enthalpy of saturated air shows very little deviation from the values obtained from the diagrams.

Solving the Merkel integral and including constants that are adopted for the parabolic dependence of the enthalpy of saturated air, the following equation can be written

V = \frac{G_{w}}{β_{x v}} \cdot \frac{c_{w} Δ t}{Δ i_{m}} [m^{3}]

(12)

Δ t = t_{w 1} - t_{w 2}

(13)

Δ i_{m} = \frac{(i_{2}^{″} - i_{2}) - (i_{1}^{″} - i_{1})}{l n \frac{i_{2}^{″} - δ i^{″} - i_{2}}{i_{1}^{″} - δ i^{″} - i_{1}}}

(14)

\begin{matrix} i_{1}^{″} = 0.119 a t_{w 2}^{2} - 1.575 t_{w 2} + 40 \\ i_{2}^{″} = 0.119 t_{w 1}^{2} - 1.575 t_{w 1} + 40 \end{matrix}

(15)

δ i^{″} = \frac{i_{1}^{″} - i_{2}^{″} - 2 i_{m}^{″}}{4}

(16)

i_{2} = i_{1} + \frac{c_{w}}{λ} (t_{w 1} - t_{w 2}) \to Δ i_{m} = f (t_{w 2})

(17)

A solution of equation (12) is water temperature on the outlet of the cooling tower as a function of atmospheric air parameters, inlet water temperature, flow rate of the water, and air flow rate. In this way, it can be seen the overall impact of various variables on the water temperature at the outlet of the cooling tower, and thus the overall efficiency of the plant.

Overall energy efficiency of the power plant is

η = \frac{P}{B H_{i}}

(18)

The results of analysis

In the thermal power plant with a closed system of cooling water, the temperature of a cooled water depends on the parameters of the atmospheric air (dry temperature and relative humidity) and the specific mass flow rate of the cooling tower. The parameters of atmospheric air are obtained from Republic Hydrometeorological Service of Serbia.²¹

In this paper mean value of relative humidity was used. For winter period average value of relative humidity of 78% was taken, while for the summer period it is 50%. These values of relative humidity are typical for the area where the thermal power plant is located.

The temperature of cooling water affects on operating conditions of the condenser which causes a change in its capacity and condensing pressure. Condensers are typically heat exchangers which have various designs and come in many sizes ranging from rather small (hand-held) to very large industrial-scale units used in plant processes.²² Change of the condensation pressure leads to changes in the energy efficiency of the plant.²³ The calculation results include the atmospheric air temperature change from 10 to 38°C and two specific mass flow rates, corresponding to eight and 10 operating cooling cells.

The specific mass flow rate of the fill pack q is defined as the quantity of water that is distributed over a 1 m² cross-section of the fill pack (gross cross-section of the tower in the area where the fill is placed) per unit of time.¹⁹ Depending on the number of cooling tower cells in operation, there are different specific mass flow rates of the fill pack. Technical characteristics of fill type are given in Table 1 and results are presented in Table 2.

Table 1.

The characteristics of the fill used in block A5.

The fill material	PVC
The dimensions of film	1000 × 600 × 30 mm
Film thickness	0.5, 0.6, 0.77 mm
Height of the waves	30 mm
The specific mass flow rate	6–20 t/m²/h
The surface of the heat exchange	109.3 m²/m³
Maximum operating temperature	−50°C to +60°C

Table 2.

The value of the specific mass flow rate in relation to the number of cooling towers in operation.

Number of operating cells	Specific mass flow rate (kg/m² s)
8	3.85
10	3.077
12	2.56

As it can be seen from Table 2, increasing the number of cooling towers reduces the specific mass flow rate of each cell. Spilling the same amount of water through the greater cross-section of the fill pack increases the contact surface of the water that needs to be cooled and the air to which cooling is done. That leads to better conditions for heat and mass transfer, and the end result is lower water temperature at the outlet of the tower.

In Table 3 and Figure 2, the dependence of the cooling water temperature leaving cooling tower on the temperature of the atmospheric air for a different number of given cells. It is obvious that reducing the number of cells decreases the temperature of the cooling water, at any temperature of the atmospheric air. In Figure 3 the results are given in the form of diagrams.

Table 3.

Values of temperature of cooled water depending on the temperature of the atmospheric air and the number of cells.

Number of cells	$t_{s} = 10$ (°C)	$t_{s} = 20$ (°C)	$t_{s} = 28$ (°C)	$t_{s} = 34$ (°C)	$t_{s} = 38$ (°C)
8	29.4	30.75	32.12	37.2	38.01
10	28.2	28.65	30.22	36.6	37.1
12	27.3	27.84	29.6	36	36.4

Figure 2.

Change the temperature of cooled water depending on the number of towers.

Figure 3.

Temperature of the cooled water during one average summer day.

In order to validate applied model and analysis, the results of the measurements on site are shown herein. During one year, the parameters of the atmospheric air were measured every day at 00:00, 3:00, 6:00, 12:00, 15:00, 18:00, and 21:00. Cooling water temperature leaving cooling tower system was also measured in those intervals, and mean values of every measured data was used in the power plant cold-end simulation.¹⁰ For the present state, with specific mass flow rate of 3.846 kg/m²s, the results of the measurements for one summer day are given in Figure 3.

With known size of the cooling surface of the condenser, steam flow rate through the condenser, the flow rate and temperature of the cooling water, for the constant load of turbine unit, the dependence of the condensation pressure as a function of temperature of cooling water for the condenser of this reference plant and hydraulic load is obtained. The results are shown in Table 4 and in Figure 4.

Table 4.

Condensing pressure.

The temperature of the atmospheric air
Number of cooling towers	10 (°C)	20 (°C)	28 (°C)	34 (°C)	38 (°C)
Number of cooling towers	Condensing pressure $p_{k}$ (bar)
8	0.083	0.091	0.1	0.125	0.131
10	0.075	0.08	0.09	0.12	0.125
12	0.07	0.073	0.084	0.115	0.118

Figure 4.

Characteristics of condensator.

Table 5 shows the results of this analysis. The results show that there are deviations from the designed condenser pressure which is a p_k=0.075 bar. The change of the condensation pressure is lower when there is a larger number of cooling towers in operation. Also, with the increase of atmospheric air temperature this difference increases. For lower values of atmospheric air temperature there are lower pressure changes.

Table 5.

Change of condenser pressure.

The temperature of the atmospheric air
Number of cooling towers	10 (°C)	20 (°C)	28 (°C)	34 (°C)	38 (°C)
Number of cooling towers	$Δ p_{k}$ (bar)
8	0.008	0.016	0.025	0.05	0.056
10	0	0.005	0.015	0.045	0.05
12	−0.005	−0.002	0.009	0.04	0.043

Figure 5 shows that the condensation pressure changes depending on the temperature of the atmospheric air and the number of cooling towers in operation.

Figure 5.

Change of the condensing pressure depending on the temperature of atmospheric air and number of cooling towers.

As noted in Truhnii and Losev²⁴ with the increase of pressure in the condenser for 1 kPa, the total energy efficiency of the power plant is reduced by 1.0–1.5%. In this particular case, it was assumed that the increase in pressure in the condenser of 1 kPa, the efficiency decreases by 1.5%. The results are shown in Table 6. The designed value of the degree of utilization (efficiency), at a pressure of 0.075 bar is η = 35%. Lower efficiency is adopted because of low caloric coal used in this power plant. The Kolubara basin is one of the most important coal basins in Serbia but the quality of this coal is not remarkable. This coal contains a small amount of carbon and a significant proportion of moisture.

Table 6.

Change of the total efficiency.

The temperature of the atmospheric air
Number of cooling towers	10 (°C)	20 (°C)	28 (°C)	34 (°C)	38 (°C)
Number of cooling towers	η (%)
8	33.8	32.6	31.25	27.5	26.6
10	35	34.25	32.75	28.25	27.5
12	35.75	35.3	33.65	29	28.55

Table 6 shows that there are deviations from the projected designed value of the total efficiency of this plant. Increasing of the atmospheric air temperature leads to total efficiency decreasing of the power plant. This decrease is less pronounced in the case of 10 cells in operation. This is especially noticeable at high temperatures of atmospheric air. It means that the plant with 10 cells in operation works 1.5% more efficient than a plant with eight cells under the same operating conditions.

Economic analysis of implementation additional cooling towers

The economic justification for putting in operation two more cooling towers (the installation would function with 10 instead of the current eight cells) was done. This is particularly important because of increasing the energy efficiency up to 1.5% in the case of 10 operating cells.

There are two principal methods for the economic and financial assessment of different investment projects, static and dynamic.³

Static methods are useful for a comparison with the results of dynamic procedures, for approximate and quick assessments or when the time differences between various payments/revenues are short enough or can be even neglected (less than three years).

In the dynamic method, the value of monetary flows depends on the time at which the transaction takes place. In energy performance improvement projects, there are a present, one-time payment of investment costs and future revenues from energy cost savings.

In this paper static method was used. The static payback period is calculated as a ratio of the capital investment to annual returns

n = \frac{I_{o}}{N_{p}}

Where:

$I_{o}$ —initial investment costs

$N_{p}$ —average annual net returns.

The economic justification of the investments was done by using the following elements²⁵:

Nominal power, $N$ =110 MW

The annual number of hours of operation, $h_{o}$ = 6000 h

The degree of time utilization for nominal power, $t_{u}$ =0.65

Specific investments in cooling tower 6.4 euro/kW

Interest on funds invested 8%

The price of the energy produced at power plant $n_{p} = 4 .11$ euro cent/kW h

The price of electricity for own consumption 37 euro cent/kW

In these conditions, it is

Annual energy production

w = t_{u} \cdot h_{o} \cdot N = 0.65 \cdot 6000 \cdot 110 = 429, 000 MW h

Additional production due to increased total efficiency by 1.5%

Δ w = 0.015 \cdot w = 6 435 MW h = 6, 435, 000 kW h

Average annual net returns

N_{p} = n_{p} \cdot Δ w = 0.0411 \cdot 6, 435, 000 = 264, 478.5 EUR

The fans of two additional towers will consume electricity which must be included in the own consumption of electricity in the block 110 MW. These fans consume electricity of 180 kW, each 90 kW.

Own consumption of electricity in the block 110 MW is 10450 kW

S_{p} = (10, 450 + 180) \cdot 0.37 = 3933 EUR

There are a large number of consumers of electricity in the thermal power plant. These are usually asynchronous and synchronous motors that drive a large number of fans, pumps, coal mills, etc. The own consumption of electricity is $S_{p} = (0.05 \div 0.1) P_{n o m}$

Investment in two additional cooling towers, the power of 13 MW each

T = 2 \cdot 13, 000 \cdot 6.4 \cdot (1 + 0.08) = 179, 712 EUR

Initial investment costs

I_{o} = S_{p} + T = 183, 645 EUR

On the basis of these elements, the payback period of investment is

n = \frac{I_{o}}{N_{p}} = \frac{183, 645}{264, 478.5} = 0.69 years

The analyses lead to a conclusion that the payback period of the invested funds is quite short, less than a year. Therefore putting in function the two already constructed, existing cooling towers, in place in the surrounding of the plant will lead to significant gains in terms of the efficiency of the entire system.

Economic analysis indicates that for a short period of time and with certain investments more energy efficient power plants can be gained. Energy efficient capital investments in the construction of facilities are gained for the investors and they have social effects too. If some country has power plants which are energy efficient that leads to economy efficiency.

Conclusion

In this paper, it was considered the power plant “Kolubara A,” located in Serbia, with a given designed data. The cooling system of this power plant is composed of eight operational cooling tower cells. In winter, this plant can achieve energy efficiency close to designed, but a problem occurs in the summer period. The increase of ambient air temperature directly has a negative effect on the efficiency of the plant. With increasing the number of cooling towers, from the current eight to 10, there is an increase in energy efficiency, as it is shown in this paper.

The energy and economic analysis of justifiability for putting into operation two additional cooling tower cells were done. With 10 instead eight cells, the specific mass flow rate of the cooled water decreases. Heat and mass transfer in the cooling tower system intensifies and temperature of the cooled water decreases. Decreasing the temperature of the cooling water leads to decreasing condenser back pressure and thus energy efficiency of the power plant increases, as it is shown in this paper. The payback period of the investments necessary for this solution is less than one year, according to economic analysis.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

Milica Jović is assistant at Faculty of Mechanical Engineering Nis. She is also a PhD student on the same faculty. Her research area are thermal engineering, thermal power plants, energy efficiency, cooling towers, energy management.

Mirjana Laković is Associate Professor at Faculty of Mechanical Engineering Nis. She has a PhD, Energy efficiency of the steam block with once-through and closed-cycle cooling system depending on the atmospheric air parameters. Her research area are thermal engineering, process engineering, industrial management, energy management.

Miloš Banjac is Full Professor at Faculty of Mechanical Engineering Belgrade. He is also Assistant Minister at the Ministry of Mining and Energy of the Republic of Serbia. He has PhD, Heat transfer in the case of a turbulent axial-symmetrical jet in a flat hot plate. His research area covers renewable energy, thermodynamics, energy management, energy efficiency.

Appendix

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