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292 Industrial Cooling Systems

XII.6 Cost comparison between the various types of coolingtowers

The cost elements of cooling systems are mainly of three kinds:•  investment costs,•  costs related to energy consumption (i.e. efficiency),•  and maintenance costs.

For power plants, operating costs related to energy have to take into account the financial gain

that is linked to the difference in efficiency between different options. Generally for power  plants, the comparison of different options is executed through a socio-economic method basedon an ‘actualised’ balance with an ‘actualisation’ ratio that varies from country to country (e.g.8% for France, 5% for Germany and Italy, 10% for Portugal). This method is described inreference L. Caudron, "Les réfrigérants atmosphériques industriels", Collection de la Directiondes Études et Rechèrches d´Électricité de France, 1991.

The ‘actualised’ balance is composed of:•  investment added with indirect charges to technical solutions considered I;•  algebraic ‘actualised’ expenses (maintenance of equipment) and receipts in operation

(production during the estimated life « tf  »);•  « Pi » is the balance of expenses and receipts of the year « i », supposed in the middle of the

year.

The balance is represented by the following enumeration with « α »as the actualisation ratio:

I + Σi =1i= tf  Pi /((1 + α)I-½)

With the expenses counted positively, the criterion of choice between different solutions is thelowest actualised balance.

 21 Flue gas desulphurisation.

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Industrial Cooling Systems 293

In the case of mechanical draught cooling towers, one may suppose that the maintenance costsare very similar because they are mainly linked to the maintenance of the fans. By taking intoaccount the first two criteria and by selecting the least expensive solution as the reference, TableXII.3 shows that the wet system is much more economical than the dry system, natural draughtmore than mechanical draught. From an economic point of view dry systems would be lessrecommendable as they are more expensive and have a higher influence on the cost of kWh. Sodry systems may be recommended only in the case of lack of water.

Table XII.3: Comparison of different types of recirculating cooling systems with a lifetime of 25

years and an actualisation ratio of 8% (study on EDF units of 1300 MWe)

[L. Caudron, "Les réfrigérants atmosphériques industriels", editions Eyrolles]

Wet cooling tower

Wet/dry

cooling

tower

Dry cooling towerType of refrigeration

systemNatural

draught

Induced

draught

Induced

draught

Natural

draught

Induced

draught

Approach K (dry air 11°C/wet air 9°C)

12.5 12.5 13.5 16 17

Nominal condensationpressure (mbar)

63 63 66 82 80

Thermal power (MWth) 2458Electrical power Delivered (MWe)

1285 1275 1275 1260 1240

Fan power MW 0 10 12 0 26Pump power MW 13 13 8 14 13Cost of refrigerant 1 1.25 2.3 5.7 4.8Cost of cold end 1 1.1 1.6 3.6 3.1Difference of cost of kWh/cost of kWh (%)

0 1.0 2.4 8.4 8.9

Table XII.4: Comparison of wet cooling towers and aircooled condenser with a life-time of 20 years

and an actualisation ratio of 8% for a combined cycle unit 290 MWth

Wet cooling towerType of refrigeration

systemOnce-through Natural

draught

Induced

draught

Air-cooled

condenser

Approach K (dry air 11°C/wet air 9°C)

/   ≈8   ≈8   ≈29

Nominal condensation

pressure (mbar)34 44 44 74

Thermal power (MWth) 290 290 290 290

Difference of electricalpower delivered (MWe)

+ 0.6 0 0 - 1.8

Fan and pump power (MW)

1.9 1.95 3 5.8

Global difference onelectrical power in(MEuro)

-4.7 -2.9 0 12.6

Difference in cost onwater consumption(MEuro)

-8.9 -8.9 0 0

Difference in cost of cooling system in(MEuro)

-3.0 1.9 0 8.9

Cost of cooling system 0.82 1.11 1 1.54Global balance of costs

(MEuro)-16.5 -1.0 0 12.6

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294 Industrial Cooling Systems

The same comparison can be done for combined cycle plants.  Table XII.4  shows that drysystems are more expensive again than wet systems, but the difference is smaller than in case of conventional power plants. The difference between mechanical and natural draught is small andis more or less comparable. Wet systems are preferred to dry systems. Maintenance costs,eventual taxes for make-up or blowdown flows of water and costs of chemical productsnecessary to the treatment of water are not taken into account in this table, which mayunderestimate the cost of wet systems or overestimate the cost for dry cooling. Thus, dry

systems may be recommended depending on the price of water and water treatment for wetsystems or taking into account the lifetime of the plant, where a shorter lifetime reduces thedifferences between dry and wet systems.

An important factor in cost comparisons is the efficiency or rather the loss of efficiency due tocooling with less efficient cooling systems. This loss is measured in the dimensionless energy-temperature factor kWth/MWth per degree temperature difference of the cooling water (in K). Inthe following theoretical example this factor is derived [Paping, pers. comm.].

From the definition that 100 bar steam of 530 °C is equal to 3451 kJ/kg it follows (using Mollier diagram) that:

50 [mbar] 32.7 [°

C] 2110 [kJ/kg]60 [mbar] 35.6 [°C] 2130 [kJ/kg]70 [mbar] 38.8 [°C] 2150 [kJ/kg]

Above mentioned vacuum pressures and their related condensation temperatures are also relatedto an average cooling water temperature in Europe of 15 °C together with a increase of 10 °C of the cooling water in the condenser itself.Including the heat transfer coefficient of the condenser, the condensate will leave it with a totaltemperature of 30 °C and an inseparable vacuum pressure of about 43 mbar (see Table XII.3and Table XII.4). Thus, to calculate the energy-temperature factor for increasing coolinginfluent temperatures the calculation for is started with 50 mbar.

The efficiency is calculated following the Carnot cycle resulting in an efficiency, which is inline with the commonly used 40% for conventional power plants:

at 50 mbar = (3451 - 2110) / (3451 - 4.18 * 32.7) x 100= 40.4609%at 60 mbar = (3451 - 2130) / (3451 - 4.18 * 35.6) x 100= 40.0037%at 70 mbar = (3451 - 2150) / (3451 - 4.18 * 38.8) x 100= 39.5583%

The minimum efficiency loss expressed per degree temperature difference under ideal(thermodynamic) circumstances:

efficiency difference between 50 mbar and 60 mbar = 4.572%o per 2.9 K differenceefficiency difference between 60 mbar and 70 mbar = 4.454%o per 3.2 K difference

efficiency difference between 50 mbar and 70 mbar = 9.026%o per 6.1 K difference

This efficiency loss can be further expressed with respect to the total efficiency and per K:

4.572%o / (2.9 K * 0.4) = 3.9 kWth/MWth per K difference4.45%o / (3.2 K * 0.4) = 3.5 kWth/MWth per K difference

9.026 %o / (6.1 K * 0.4) = 3.7 kW th/MWth per K difference

From this simplified calculation it appears that for an efficiency of about 40% the loss or gain per degree of temperature difference of the cooling water can be estimated using the factor of 3.5 kWth/MWth per K.