Tetra Tech, Inc.

10306 Eaton Place, Suite 340

Fairfax, VA 22030

phone 703-385-6000

fax 703-385-6007

TO: Paul Shriner and Jan Matuszko, EPA

FROM: John Sunda, SAIC and Kelly Meadows, Tetra Tech

DATE: April 17, 2009

SUBJECT: Modular Cooling Tower Technology and Costs

Closed-cycle cooling (in the form of 

However, in estimating compliance costs associated with closed

typically only considered permanent

basins. Closed-cycle cooling may also be 

towers. Modular cooling towers are portable cooling units that can supplement or even 

replace once-through cooling water flows at el

users of cooling water. While this

over permanent cooling towers

does not presently have enough

technology.

Typical Modular Tower System

Much of the technical information below was obtained through correspondence with Blue 

Stream Services, which has partnered with the modular tower manufacturer Varitech 

Equipment Company. The services provided by the vendors primarily involve short

and long-term rentals, especially for systems that may require use during only a portion 

of the year.1

 

1 Although modular units may be purchased, purchase costs were not available. One reason that the vendor 

may be focusing on temporary and seasonal

energy requirements are higher for modular towers c

towers may be more cost-effective for systems that would be required for a very long period of time and 

that are used continuously or for a large portion of the year.

MEMORANDUM

10306 Eaton Place, Suite 340

Paul Shriner and Jan Matuszko, EPA

John Sunda, SAIC and Kelly Meadows, Tetra Tech

, 2009

ooling Tower Technology and Costs

cycle cooling (in the form of cooling towers) is one form of flow reduction

However, in estimating compliance costs associated with closed-cycle cooling, 

permanently-built onsite structures with concrete cold water 

cycle cooling may also be achieved through the use of modular cooling 

Modular cooling towers are portable cooling units that can supplement or even 

through cooling water flows at electric generators and other significant 

While this alternative technology offers a number of benefits 

over permanent cooling towers and could significantly reduce facility downtime

enough information to assess the widespread applicability of this 

System

Much of the technical information below was obtained through correspondence with Blue 

Stream Services, which has partnered with the modular tower manufacturer Varitech 

Equipment Company. The services provided by the vendors primarily involve short

term rentals, especially for systems that may require use during only a portion 

 

Although modular units may be purchased, purchase costs were not available. One reason that the vendor 

may be focusing on temporary and seasonal-use systems may be that, as described below, the operating 

energy requirements are higher for modular towers compared to permanent towers. As a result, permanent 

effective for systems that would be required for a very long period of time and 

that are used continuously or for a large portion of the year.

1

flow reduction. 

ng, EPA has 

structures with concrete cold water 

modular cooling 

Modular cooling towers are portable cooling units that can supplement or even 

ectric generators and other significant 

offers a number of benefits 

and could significantly reduce facility downtime, EPA 

assess the widespread applicability of this 

Much of the technical information below was obtained through correspondence with Blue 

Stream Services, which has partnered with the modular tower manufacturer Varitech 

Equipment Company. The services provided by the vendors primarily involve short-term 

term rentals, especially for systems that may require use during only a portion 

Although modular units may be purchased, purchase costs were not available. One reason that the vendor 

use systems may be that, as described below, the operating 

ompared to permanent towers. As a result, permanent 

effective for systems that would be required for a very long period of time and 
2

System Operation

Modular cooling towers are typically used in either a helper mode2or a recirculating 

mode. In the recirculating mode, hot water can be withdrawn directly from the hot water 

discharge channel, cooled and then returned to the intake channel upstream of the intake 

pumps or in the intake pump well. The modular tower system can recirculate all or only

a portion of the total cooling system flow requirements. To recirculate all of the flow, the 

modular cooling tower pumping rate is matched to the associated condenser cooling 

water pumping rate through the use of float controls in the intake channel.

System Set-Up

The simplest installation is to withdraw water from the facility’s effluent channel and 

pump the water through the towers, where it then flows by gravity to the existing intake 

channels or cooling water pump wells. The upstream portion of the intake can then be 

blocked off so that the existing recirculation pumps remain functional while intake 

construction occurs. Other engineering solutions for cooling water pump and piping 

configurations are possible. In some cases, there may be challenges in finding a suitable 

path for the placement of the temporary cooling water recirculation pipes, but 

engineering solutions should be available in most circumstances.

Modular towers can be installed while the plant is in operation and can be operated on an 

as-needed basis. The installation can be done quickly; once the units are trucked inplace, it takes approximately one day per unit to complete the installation. This allows 

modular towers to be used as a temporary source of cooling water during the period when 

an intake must be shut down, such as allowing for the retrofit of 316(b) compliance 

technology.

The Varitech units are transported to the site on a truck trailer bed. In earlier designs 

prior to this one, the modular units either remained on the truck trailer or were removed 

using a crane. One advantage of the newer Varitech units is that they can be trucked 

onsite, jacked up so that the truck trailer can move out, and then the tower height can be 

adjusted using built-in legs that are easily leveled. No crane is necessary, thus reducing 

mobilization and demobilization costs. These towers require very little site preparation 

as long as the site is moderately level.

System Design

The basic modular cooling tower configuration uses PVC pipes running along the ground 

to and from the towers, skid-mounted pumping units that pump water through the towers, 

and the tower units themselves. The vendor typically provides all the necessary

equipment, including pumps, piping, control system, electrical cable and transformers. 

The vendor can also provide all necessary services, including engineering design, 

mobilization/installation, contract operation and maintenance, and 

 

2 The helper mode provides help in compliance with 316(a) requirements only, so this application will not 

be discussed further here.
3

removal/demobilization. In short, the vendor can provide all necessary equipment and 

services with the exception of permitting activities. In many respects, operation and 

maintenance activities for modular towers will be similar to those for permanent towers, 

except that these tasks would be performed by the vendor. One benefit of modular 

towers is that it will be easier to perform maintenance on the modular units while 

maintaining generation at near full capacity, since the temporary loss of the flow from 

one unit at a time represents only a small component of total cooling water flow.

In the past, large portable towers generally ranged in size from 500 ton to 2,000 ton 

capacities.3 Recently, however, Varitech has begun manufacturing portable 3,000 ton 

units capable of handling up to 10,000 gpm of cooling water. A typical design flow for 

these units is 10 MGD or 6,945 gpm (Blue Stream 2008b). These units are the largest 

portable units available and provide the most efficient use of available space. 

With the use of multiple side-by-side tower units, a cooling system of any size capacity 

can be assembled. For example, Blue Stream recently worked on a system for the 2,600 

MW Cumberland Plant in Cumberland City, Tennessee, operated by the TVA (see 

below). This plant requires cooling tower flow capacity of 500,000 gpm. In this case, 

the towers have been installed in a helper tower configuration to eliminate the need to 

derate (reduce plant generation) during summer source water low-flow conditions that 

have been exacerbated by recent drought.

The specifications for the 3,000 ton units are:

Flow Range: 5,000 – 10,000 gpm

Design Flow: 6,945 gpm (10 MGD)

Tower Width: 12 ft

Tower Length: 50 ft

Tower Height: 15 ft excluding ground clearance

Fans: 20 direct drive at 15 Hp each

Fan Energy Req.: 300 Hp

Shipping Weight: 56,000 lbs

Operating Weight: 88,000 lbs

Source: Varitech 2008.

The towers are constructed of fiberglass-reinforced plastic, stainless steel, and PVC 

components to prevent corrosion. The towers contain high efficiency cellular sheet PVC 

drift eliminators.

System Requirements

The system requirements are available space for placement of the towers and a route for 

the piping from the warm water discharge channel to the towers and then from the tower 

cold water outlet to the intake channel. Each unit is 12 ft wide by 50 ft long and requires 

a side clearance of 12 ft between units placed side-by side and 24 to 36 ft between rows 

 

3 Note that in this context a “ton” is a measure of cooling capacity equivalent to 12,000 BTUs/hr.
4

to allow for proper air circulation (Blue Stream 2008b). The result is that, for each 10 

MGD of design flow volume, an area of approximately 2,000 sq ft including the side 

clearance is required. Looking at the tower footprint alone, these 3000 ton units can cool 

11.6 gpm/sq ft. The footprint size of the permanent cooling towers derived using the 

EPRI cost methodology (as described in Chapter 8 of the TDD) indicates they cool only

3.4 gpm/sq ft. Thus, the modular towers are more compact and should require less space. 

The reason for this is the higher fan energy requirement (see discussion below) and high 

efficiency design of the units and fill material. 

Assuming that the total side clearance for both sides for permanent cooling towers is 

equal to the tower width, a similarly sized permanent cooling tower would require about 

4,000 sq ft. The difference in space requirements may be somewhat less than the factor 

of 2 that is implied by the above estimates, since the smaller modular units will have 

plumes that may be more prone to recirculation, especially if they are placed in arrays of 

multiple rows and close together using minimum clearance. A 10,000 gpm skid-mounted 

pump requires a space of 6 ft by 6 ft (Blue Stream 2008a).

Another system requirement is a power source. The fan energy requirements for modular 

towers are about 50% higher than the fan energy requirements estimated using EPRI’s 

methodology for a similar design flow. The higher fan energy is part of the modular 

tower design that allows for the towers to be more compact, taking up less space than 

permanent towers.

A comparison of the total pump and fan energy requirements reported by the vendor to 

estimates derived from EPRI’s methodology indicates that the modular towers may

require about 70% more electrical energy than equivalent permanent towers to operate. 

That means that pumping energy requirements may also be higher for modular towers. 

This may be due to the fact that modular towers use multiple pumps and pipes that are 

smaller in size than those used in permanent towers. The use of smaller pipes and pumps 

tends to result in greater friction losses and less efficient pump operation.

Non-Water Quality Impacts

In general, non-water quality impacts (e.g., noise, drift, water consumption, plume) 

should be similar for both modular and permanent cooling towers.

Example Installations

The largest example facility provided by the vendor is the 2,600 MW Cumberland Plant 

in Cumberland City Tennessee (operated by the TVA). The facility’s design intake flow 

(DIF) is over 2 billion gallons per day (BGD). This plant operates the towers in a helper 

mode due to problems with low river flow, which have led to problems in meeting the 

facility’s 316(a) discharge limits.

Another facility is the 370 MW WS Lee Steam Station in Anderson County, South 

Carolina (operated by Duke Energy). The facility recently purchased modular towers and
5

is using them in a recirculating mode to reduce intake flow volume. Existing permanent 

cooling towers already located onsite are operated in as helper towers; the new modular 

towers are operated in a recirculating mode during the summer because the river level 

drops too far. The modular tower recirculating flow volume is approximately 100,000 

gpm, which is close to the facility’s actually intake flow (AIF). So during the summer,

the facility routes the helper tower effluent to the modular towers, recirculating the water 

back to the intake and thus running water through two sets of towers. 

System Costs

Table 1 below presents the rental costs and total energy requirements for modular towers 

(as supplied by the vendor). These costs require a minimum rental period of four months 

per year but facilities are only charged for the period the towers remain in use in excess 

of 4 months each year after that (i.e., beyond the minimum 4 month rental, a facility is 

only charged for time the towers are in use; if they are idle, there is no additional fee). 

The rental costs include the tower units, pumps, piping, controls, and maintenance. 

These costs do not include items such as site preparation, freight, installation labor, 

electrical supply equipment, electricity to operate the pumps and fans, or system removal 

labor and freight (Blue Stream 2008b).4The vendor noted that since the clients are 

power companies, they usually provide the electrical supply components using their own 

resources.

Table 1. 

 

4

Such costs (especially freight) are very site-specific. Freight costs could be $3,000 - $4,000, but can 

change with fuel costs and distance traveled.

GPD GPM Dollars/Month

Number of 

Modular 

Units

Power 

Requirement 

(MW)

EPRI Average 

Capital Cost

EPRI O&M 

Costs

EPRI Total 

Power 

(MW)

20,000,000 13,889 $120,400 2 1.0 $5,300,000 $25,600 0.58

40,000,000 27,778 $240,800 4 2.0 $10,600,000 $51,200 1.13

60,000,000 41,667 $361,200 6 3.0 $15,900,000 $76,800 1.67

80,000,000 55,556 $481,600 8 4.0 $21,200,000 $102,400 2.22

100,000,000 69,445 $602,000 10 5.0 $26,500,000 $128,000 2.75

120,000,000 83,334 $722,400 12 6.0 $31,800,000 $153,600 3.29

140,000,000 97,223 $842,800 14 7.0 $37,100,000 $179,200 3.83

160,000,000 111,112 $963,200 16 8.0 $42,400,000 $204,800 4.36

180,000,000 125,000 $1,083,600 18 9.0 $47,700,000 $230,400 4.9

200,000,000 138,889 $1,204,000 20 10.0 $53,000,000 $256,000 5.45

220,000,000 152,778 $1,324,400 22 11.0 $58,300,000 $281,600 5.98

240,000,000 166,667 $1,444,800 24 12.0 $63,600,000 $307,200 6.51

260,000,000 180,556 $1,565,200 26 13.0 $68,900,000 $332,800 7.05

280,000,000 194,445 $1,685,600 28 14.0 $74,200,000 $358,400 7.58

300,000,000 208,334 $1,806,000 30 15.0 $79,500,000 $384,000 8.11

320,000,000 222,223 $1,926,400 32 16.0 $84,800,000 $409,600 8.65

340,000,000 236,112 $2,046,800 34 17.0 $90,100,000 $435,200 9.19

360,000,000 250,000 $2,167,200 36 18.0 $95,400,000 $460,800 9.72

380,000,000 263,889 $2,287,600 38 19.0 $100,700,000 $486,400 10.26

400,000,000 277,778 $2,408,000 40 20.0 $106,000,000 $512,000 10.79

1

Each unit consists of a 3,000 ton tower at 6,945 gpm with minimum 4 months rental period

2Calculated using the EPRI Cooling Tower Calculation Worksheet (EPRI 2007)

Source: Blue Stream 2008B

System Design Flow Modular Tower Rental1Permanent Constructed Tower2

Comparison of Modular and Permanent Cooling Tower Costs and Energy Requirements
6

Cost of Modular Towers Compared to Permanent Towers

For comparison, Table 1 also includes the capital and O&M costs and the energy

requirements for a permanent cooling tower system, as derived from the EPRI

methodology. A present-value calculation using an interest rate of 5% and the 

assumption that the towers will operate for four months per year indicates that the breakeven time period is about 14 years (see below for a detailed discussion). This means that 

if the modular towers are used for 4 months or less each year, they will be more 

economical than permanent towers if they are needed for a period of less than 14 years. 

If the duration of tower operation is extended to 12 months/year, the break-even time 

period is less than four years. The break-even calculation takes into consideration the 

costs presented in Tables 1 though 4 below (including estimated cost of energy

requirements), but assumes that the non-power O&M costs for permanent towers (Table 

1, Column 7 — EPRI O&M) are the same for both 4 and 12 months of operation. The 

energy costs are a large component of O&M costs. For permanent towers operated 

continuously throughout the year, energy requirements can represent up to 90% of total 

O&M costs. The results of the break-even analysis demonstrate that modular towers may

be more financially advantageous for facilities that require flow reduction for only a 

portion of the year and for facilities in which the cooling tower technology will not be 

needed for the full service-life of permanent towers constructed onsite.

Estimation of Costs for Temporary Use During Construction Downtime 

The temporary installation of modular cooling towers is an available technology option 

for minimizing construction downtime costs. This approach also has the added benefit of 

maintaining generating capacity at baseload plants which may be important in regions or

time periods where extended facility downtime would be detrimental to maintaining 

reliable generation. Below is an estimate of the costs for a typical temporary rental tower 

installation. For each facility with an estimated net downtime period (due to installing a 

compliance technology as a result of the proposed rule), the equations below can be used 

to estimate the cost of this downtime technology option. These costs can then be 

compared to the lost generation costs of shutting down the generating units. EPA could 

then identify the facilities that could benefit economically from employing this option 

and adjust the downtime costs accordingly.

Each project will have a minimum cost associated with installation and demobilization 

costs plus rental costs (minimum four months for Blue Stream). The decision to utilize 

modular towers to reduce downtime costs will be somewhat dependent on whether the 

estimated lost generation costs for the net downtime period exceeds the total cost for 

tower rental by a large enough margin to make it economically worthwhile. Lost 

generation costs will also be dependent on both the downtime duration and facility’s 

capacity utilization rate (CUR) during the downtime period. Due to the installation and 

minimum rental period costs, this technology will likely be more attractive to facilities 

that expect longer downtime requirements and a high CUR during the outage. The 

decision to use temporary cooling towers may not be limited to just economic 
7

considerations for the facility, as the need to maintain minimum regional generating 

capacity and the cost of replacement power generation may also factor into the decision. 

Table 2 presents the estimated one-time installation and demobilization costs that are not 

covered by the vendor rental costs. The total costs apply to all applications regardless of 

duration. The shipping, installation, power supply, demobilization, and other costs not 

covered by the vendor fees are derived using the assumptions below:

 Freight – transportation costs per tower (each way) is $6,000 including shipping 

of towers and ancillary equipment.

 Installation labor – installation time of one day per tower is assumed to require six 

persons for two 8-hr shifts per tower.

 Demobilization labor - same as for installation.

 Labor Rate - $50/hr ($38/hr in 2002 adjusted for inflation to 2009).

 Site Preparation - assumed as $4,000 per unit (estimate based on BPJ).

 Electrical hookup – material and labor cost is assumed to be $20,000 per unit 

(estimate based on BPJ).

 Allowance – additional costs equal to 50% of total for above items (freight, labor, 

site preparation, and electrical) to account for all other costs.

 Power – cost estimate for power to operate pumps and fans are based on a CUR of 

85% and an average wholesale power cost of $65/Mwh.

Table 2

GPD GPM

Freight Two 

Way

Mob/Demob. 

Labor Site Preparation Electrical

Total Fixed Costs 

Excluding 

Rental*

20,000,000 13,889 $24,000 $9,600 $8,000 $40,000 $122,400

40,000,000 27,778 $48,000 $19,200 $16,000 $80,000 $244,800

60,000,000 41,667 $72,000 $28,800 $24,000 $120,000 $367,200

80,000,000 55,556 $96,000 $38,400 $32,000 $160,000 $489,600

100,000,000 69,445 $120,000 $48,000 $40,000 $200,000 $612,000

120,000,000 83,334 $144,000 $57,600 $48,000 $240,000 $734,400

140,000,000 97,223 $168,000 $67,200 $56,000 $280,000 $856,800

160,000,000 111,112 $192,000 $76,800 $64,000 $320,000 $979,200

180,000,000 125,000 $216,000 $86,400 $72,000 $360,000 $1,101,600

200,000,000 138,889 $240,000 $96,000 $80,000 $400,000 $1,224,000

220,000,000 152,778 $264,000 $105,600 $88,000 $440,000 $1,346,400

240,000,000 166,667 $288,000 $115,200 $96,000 $480,000 $1,468,800

260,000,000 180,556 $312,000 $124,800 $104,000 $520,000 $1,591,200

280,000,000 194,445 $336,000 $134,400 $112,000 $560,000 $1,713,600

300,000,000 208,334 $360,000 $144,000 $120,000 $600,000 $1,836,000

320,000,000 222,223 $384,000 $153,600 $128,000 $640,000 $1,958,400

340,000,000 236,112 $408,000 $163,200 $136,000 $680,000 $2,080,800

360,000,000 250,000 $432,000 $172,800 $144,000 $720,000 $2,203,200

380,000,000 263,889 $456,000 $182,400 $152,000 $760,000 $2,325,600

400,000,000 276,000 $480,000 $192,000 $160,000 $800,000 $2,448,000

*Includes 50% allowance to account for all other costs.

Costs are in 2008 Dollars

System Design Flow Installation and Demobilization Costs

Calculated Installation and Demobilization Costs
8

Table 3 presents the tower rental costs based on the data in Tables 1 and 2 and includes 

the total costs for a rental duration of up to 16 weeks, the weekly rental costs for periods 

exceeding 16 weeks, and power costs which are shown separately. Note that the tower 

rental costs already include tower O&M for labor and materials, with the exception of the 

cost of power to operate pumps and fans. 

Table 3

The total downtime for all cost modules ranges from <4 weeks to 15 weeks, with the 

exception of installing cooling towers at nuclear facilities which has a relatively high 

downtime value of 32 weeks. In the 2004 Phase II rule, the total downtime values were

converted to net downtime by subtracting 4 weeks of routine maintenance downtime.5 A

second possible approach to consider downtime is to use facility-specific data to calculate

facility-specific estimated routine maintenance durations ranging from 1 to 8 weeks. 

Using this approach, the calculated net downtime periods for any facility could range 

from 0 to 14 weeks. Since both of these approaches would yield outages that are less 

than the minimum four-month (16 weeks) rental period, the minimum cost will apply to 

most technology modules at non-nuclear facilities. The only variable cost is the power to 

operate the pumps and fans. 

 

5 Based on EPA’s site visits, 4 weeks is a typical outage period for many facilities.

GPD GPM

Total Minimum 

Costs for Tower 

Rental Up to 16 

Weeks including 

Installation/

Demobilization

Tower Rental 

Additional 

Weeks 

(Dollars/Week)

Number of 

Modular Units

Power 

Requirement 

(MW)

Estimated Weekly 

Power Costs 

(Dollars/week)2

20,000,000 13,889 $604,000 $30,100 2 1.0 $3,285

40,000,000 27,778 $1,208,000 $60,200 4 2.0 $6,570

60,000,000 41,667 $1,812,000 $90,300 6 3.0 $9,855

80,000,000 55,556 $2,416,000 $120,400 8 4.0 $13,140

100,000,000 69,445 $3,020,000 $150,500 10 5.0 $16,425

120,000,000 83,334 $3,624,000 $180,600 12 6.0 $19,710

140,000,000 97,223 $4,228,000 $210,700 14 7.0 $22,995

160,000,000 111,112 $4,832,000 $240,800 16 8.0 $26,280

180,000,000 125,000 $5,436,000 $270,900 18 9.0 $29,565

200,000,000 138,889 $6,040,000 $301,000 20 10.0 $32,850

220,000,000 152,778 $6,644,000 $331,100 22 11.0 $36,135

240,000,000 166,667 $7,248,000 $361,200 24 12.0 $39,420

260,000,000 180,556 $7,852,000 $391,300 26 13.0 $42,705

280,000,000 194,445 $8,456,000 $421,400 28 14.0 $45,990

300,000,000 208,334 $9,060,000 $451,500 30 15.0 $49,275

320,000,000 222,223 $9,664,000 $481,600 32 16.0 $52,560

340,000,000 236,112 $10,268,000 $511,700 34 17.0 $55,845

360,000,000 250,000 $10,872,000 $541,800 36 18.0 $59,130

380,000,000 263,889 $11,476,000 $571,900 38 19.0 $62,415

400,000,000 276,000 $12,080,000 $602,000 40 20.0 $65,700

1

Each unit consists of a 3,000 ton tower at 6,945 gpm with minimum 4 months rental period

2 Based on wholesale power cost of $65/Mwh and fan/pump power utilization of 90%.

Costs are in 2008 Dollars

System Design Flow Modular Tower Rental Fan and Pump Power 1

Modular Tower Costs 
9

The costs shown in Table 3 were plotted against the recirculating flow rate and the data 

were fitted to straight lines. Table 4 presents cost estimation equations derived from this 

data. Equations for O&M costs for pump and fan energy are presented in both megawatts 

at full capacity and in dollars per week assuming that the energy usage will be 85% of 

full capacity over the operating period and the average wholesale cost of electricity is 

$65/Mwh.6

Table 4

Additional Cost Information

The rental fee is charged only for the months the towers are used; the equipment can stay

onsite the remainder of the year at no extra charge. Thus, installation/demobilization 

costs can be amortized over the entire rental contract period. As a result, modular 

cooling towers are often rented for periods of several years or more. Another benefit is 

that rental towers can be financed through the facility’s operating budget, thus avoiding 

the time-consuming planning and approval requirements for such a major capital 

expenditure as a cooling tower.

Modular towers may be capable of producing colder water than permanent cooling 

towers, which in turn could offset a portion of the extra power requirements through a 

reduction in the turbine efficiency energy penalty. The vendor stated that the modular 

towers had an approach of 7 to 10 F, while the EPRI documentation specified an 

approach of 10 F for permanent towers (Blue Stream 2009). The approach is the 

difference between the temperature of the cold water exiting the tower and the wet bulb 

temperature of the air and thus, towers with lower approach values will produce colder 

water. The modular tower cited here is designed to be efficient and compact and 

accomplishes this, in part, by using a higher fan power and thus may produce colder 

water.

Cooling towers can be designed with different approach values, generally ranging from 5 

to 15 F with 10 F being a typical design value. In general, a tower with a lower 

approach will cost more and in the case of permanent towers will be larger in size. The 

316(b) Phase I Rule based cooling tower costs on a vendor-supplied tower sizing factor 

 

6 Either approach could be used in the economic analysis, but using megawatts would be the most accurate. 

Generally speaking, the relative magnitude of the O&M cost component is equal to about 1/3 of the 

monthly tower rental costs.

Modular Cooling Tower Costs Equation Constant Units

Total Costs ?16 Weeks (TC16) TC16 = DIF(gpm) x Constant 43.527 Dollars

Added Weekly Costs for >16 Weeks TCwk = DIF(gpm) x Constant 2.169 Dollars/week

Variable O&M - Pump & Fan Power (OMMW) OMMW= DIF(gpm) x Constant 0.0000700 MW 

Variable O&M - Pump & Fan Power (OM$$)* OM$$= DIF(gpm) x Constant 0.6707 Dollars/week*

Energy Penalty -Heat Rate (EP) EP=MWS x Constant 0.02 MW 

MWS = Total facility steam generating capacity

* Based on assumed power cost of $65/Mwh and 85% power utilization

Costs are in 2008 Dollars

Cost Equations for Modular Cooling Towers
10

of $30/gpm for a cooling tower with an approach of 10 F and a sizing factor of $50/gpm 

for a tower with an approach of 5 F.

7

(USEPA 2000) Thus, based on these sizing 

factors, it is estimated that a reduction of the approach from 10 F to 5 F would result in 

an increase in capital costs for a permanent tower of approximately 67%. 

It is estimated that a 1.0 C reduction in the cooling water temperature can decrease the 

heat rate of a thermal power plant by 5 Kcal/KWh.

8

(National Productivity Council of 

India (NPCI) 2009) Using this factor, a coal-fired plant with a heat rate of 9,000 

BTU/KWh (thermal efficiency = 37.9%) would increase its output by 0.12% for each 1.0

F reduction in cooling water temperature during warmer periods of the years. Another 

analysis of a theoretical pressurized water nuclear reactor predicted that an increase in 

cooling water temperature by 1.0 C would decrease output by 0.45%. (Durmayaz and 

Sogut 2006) This is equivalent to a change in output of 0.25% for a 1.0 F change in 

cooling water temperature, or roughly double the NPCI-based estimate. Note that an 

approach of 5 F is generally considered the lower limit for tower design, due to the fact 

that the tower size will increase exponentially as the approach decreases, resulting in 

greatly diminishing returns in terms of performance versus costs.

The quality of the water being circulated through the towers (e.g., concentration of 

suspended of solids) may affect the type of tower fill that can be used, which can in turn 

affect the efficiency of the tower and the approach value. The modular towers use a film 

fill that is more susceptible to plugging and fouling than splash fill. Film fills come in 

different designs that can be tailored to the water quality. If the water contains solids or 

is susceptible to biofouling that could plug the smaller holes/channels, then a fill with 

larger holes/channels must be used, which will reduce the efficiency of the tower.

Applicability for 316(b)

Modular cooling tower systems would be most useful for projects requiring a temporary

source of cooling water during major repairs or construction at existing once-through 

intakes and cooling tower systems, such as during the downtime associated with 

retrofitting a new intake technology or closed-cycle cooling system (although there are 

limitations on the latter, as discussed below). The modular towers would be available to 

those facilities with sufficient space for the towers and where the existing cooling water 

pumps and pump wells or intake channels can be blocked off (see Figure 1).9 Modular 

towers require somewhat less space than permanent towers and, since the modular 

cooling towers consist of many individual units, they can also be placed in configurations 

 

7 These factors were then scaled up using factors for additional cost components such as installation, etc., 

but the relative ratio of total capital costs should be similar.

8 These estimates of the effect of cooling water temperature on plant output are presented as specific 

examples found in literature. Most technical sources are reluctant to provide a quantitative estimate of this 

sort because the actual effect will vary depending on the turbine system design and the specific operating 

conditions.

9

In Figure 1, a sheet pile wall was used to block off the intake canal to ensure that returned water was 

recirculated, essentially eliminating the withdrawal of intake water with the exception of make-up water. 

Make-up water was withdrawn from the discharge canal.
11

that allow for efficient use of available space, including space that might not otherwise be 

useful for conventional multi-cell cooling towers.10

Potential Sites for Modular Towers

Modular towers may provide a significant reduction in downtime and the associated costs 

at some facilities. In some cases, especially those where the interruption period is fairly 

long or if the facility is a baseload facility, the downtime costs (due to lost generation) 

can be much higher than the tower rental costs, resulting in substantial overall savings for 

the project when modular cooling towers are used. As described in Chapter 8 of the 

Technical Development Document (TDD), EPA estimated that 100% of normal 

generating revenue will be lost during net construction downtime;11 however, in cases 

where modular towers are compatible with site conditions, this is likely an overestimate

for facilities with long downtime periods, since it does not consider modular towers as an 

available cost-saving option. The actual downtime costs would simply be the modular 

cooling tower rental costs for the period, including mobilization/demobilization costs.

Modular cooling towers are also useful at facilities that are subject to periodic 

interruptions or reductions in the quantity of water withdrawn for cooling purposes, and 

can be a cost-effective option for facilities that require supplemental cooling or flow 

reduction for only a portion of the year.12

Limitations on Applicability

This technology is most appropriate for compliance technologies that involve major 

modifications to the intake structure or relocation of the intake inlet. There are 

limitations regarding the use of modular cooling towers at nuclear facilities and in 

conjunction with the installation of new cooling towers.

At nuclear plants, the service water may be pumped from a combined service water and 

condenser cooling water intake, or it may be from a separate intake. Nuclear facilities 

have more stringent safety requirements for the service water system than for the 

condenser cooling water system. The service water system is considered part of the 

emergency and safety system, which has very rigorous certification requirements 

regarding the manufacturing, installation, and maintenance of each component. 

 

10 One option for facilities that have tight space constraints is to place all or a portion of the tower units or 

pumps on floating barges. Of course, use of barges may be limited by factors such as conflict with 

waterbody navigation or poor suitability of the adjacent waterbody for barge use. Figure 2 shows how 

modular towers can be laid out in a variety of configurations that may provide for more efficient use of 

space and use of space that might not be suitable for typical multi-cell permanent towers.

11 For many facilities, EPA assumed that construction would be performed during a facility’s regularly 

scheduled maintenance outage (normally four weeks).

12 As described below, rental costs for multi-year tower rental are assessed only for the period they are used 

each year. Eventually, after many years, the total cost of tower rental will exceed the cost of permanent 

towers. The time that it takes to reach this point is much longer if towers are used for only a portion of the 

year than if they are used year-round. 
12

Obtaining certification for a new technology can be a rather involved process with no 

guarantee that the technology will become certified. 

During intake construction downtime, the intake no longer functions as a cooling water 

source and thus, when modular towers are used during construction downtime to replace 

this cooling water, they become the sole source. For this reason, it is unlikely that 

modular towers would be considered for situations where they would become at any time 

during plant operation the sole source of cooling water for the service water system at a 

nuclear facility. 

At nuclear facilities where the condenser cooling water has a separate intake, the 

installation of modular towers would still be required to meet rigorous seismic and fire 

control requirements. Even if these requirements were met, there may be a perception of 

modular towers as a “temporary technology” that could influence the Nuclear Regulatory

Agency’s decision to permit such a technology application. 

For the reasons discussed above, modular towers are not likely to be used as an alternate 

cooling water source to keep a nuclear plant operating during construction activities. It 

may, however, be possible to use them in situations where they function as an alternate 

source of cooling water where once-though operation can be maintained as a backup, 

provided the technology can meet the technical requirements.

For facilities installing new (permanent) cooling towers, modular towers are also unlikely 

to prove useful. In order for modular towers to be used as an alternate source of cooling 

water during the construction downtime, space must be available for the towers and 

piping for both the permanent and modular towers. This essentially doubles the space 

required; most facilities would be unlikely to have this much available space.

Implications for EPA’s Engineering Costs

One difficulty of incorporating this potential construction cost-saving option into the 

compliance technology economic analysis is that there is insufficient information to 

determine for which facilities and construction projects this technology would be viable. 

The primary obstacle that may limit the technology viability is the availability of 

sufficient space to place the modular cooling towers. However, this technology should 

be viable for more facilities than conventional permanent cooling towers because 

modular towers require somewhat less space and also allow for more flexibility in the 

placement of individual units, including the availability of alternatives such as placing 

some or all of the units on barges. Conventional cooling towers are generally too large to 

be installed on floating barges. 

In general, limitations on the availability of sufficient space for modular towers will be 

greatest at facilities that are located in higher density urban areas, where there is 

insufficient available space within the plant property boundaries as well as there being no 

undeveloped land adjacent to the plant that could be purchased and used for siting 

cooling towers. 
13

Also, the elevation of the available land is an important consideration. Optimal locations 

for cooling towers should be at elevations that are not much higher than those of the 

generating unit condensers to ensure that pumping costs are economically acceptable.

Summary of Advantages and Disadvantages

The advantages and disadvantages of modular cooling towers (as compared to permanent 

cooling towers) are summarized below.

Advantages include:

 Can be more economical than permanent towers where requirements for use 

are temporary or seasonal

 Can be installed quickly

 Can be installed with minimal site preparation

 Can be installed without use of explosives that might be necessary for 

permanent tower concrete basins

 May be available to more facilities because they require less space than 

conventional cooling towers

 Can be installed in almost any configuration

 Can be installed on barges

 Can be used to provide temporary supply of cooling when existing intake is 

being modified

 Rental cost includes equipment maintenance and repair

 Can be paid for using facility operating budget

Disadvantages include:

 Economic benefit over permanent towers diminishes or disappears as period 

of use per year or number of years in use increase

 Energy requirements (and thus operating costs) are higher than for permanent 

towers

Analysis

The CUR at which the plant would have otherwise been operating during the downtime is 

an important factor in determining whether the lost generation cost would be high enough 

to justify using this technology option during construction downtime. To analyze how 

capacity utilization could affect this decision, the lost generation costs and total tower 

rental costs were calculated and then compared for different CUR values and different net 

downtime durations in weeks. The typical generating capacity associated with each 

recirculating flow rate was calculated using the average cooling water requirement of 707 

gpm per MW. Lost revenue was assumed to be $45/Mwh, based on an assumed 

wholesale value of $65/Mwh minus an assumed fuel cost of $20/Mwh. The tower costs 

and lost generation costs were then calculated for different downtime durations in weeks 
14

and CUR values in increments of 10%. The data were then reviewed to determine the 

minimum number of weeks for assumed CUR values at which the lost generation cost 

exceeded the tower rental costs.13

Using an assumed value for cooling water requirements of 707 gpm/MW, a facility 

requiring 400 MGD of recirculating cooling water (last row in Tables 1-3) would 

generate approximately 400 MW of electricity at full power. Lost generation costs for 

the loss of 400 MW of electric power generation are estimated to be approximate 

$18,000/hr using the assumed value for lost revenue of $45/ MWh. This is equivalent to 

a loss of $2.4 million dollars per week if the capacity utilization rate (CUR) is 80%. 

Using this example, at $65/MWh, a reduction in the approach of 1.0 F (increase in 

efficiency of 0.12%) will increase revenue by $3,120/hr or $419,000 per week at 80% 

CUR. 

Table 5 presents the results of the analysis and indicates that the use of temporary cooling 

towers to reduce downtime costs will not likely be cost-effective for construction projects 

with net downtime durations of less than 6 weeks for facilities with a high CUR during 

the downtime period. Nor would temporary towers be economical for facilities with 

CUR values of <40% for the range of estimated construction downtime durations for 

most of the technology cost modules considered by EPA. Since most facilities would 

likely schedule the construction downtime to coincide with a period of time where power 

demand is lower, the CUR during the downtime period will likely be lower than the 

annual average CUR for the plant.

Table 5

 

13 The lost revenue cost estimates derived here are intended only for obtaining a rough estimate of which 

types of facilities would be likely to use such towers and for what minimum downtime periods would the 

use of rental towers be expected to be economical. Actual lost revenue costs are derived in the economic 

analysis using a much more detailed plant-specific approach. Note that the relative costs of the towers 

compared to lost generation were generally the same regardless of the size of the system (i.e., the DIF).

CUR During 

Rental Period

Breakpoint 

(Weeks)

20% See Note*

30% 14

40% 11

50% 9

60% 7

70% 6

80% 6

90% 5

* Uneconomical - monthly tower costs 

almost equal to lost generation 

BreakPoint - When Lost 

Generation Cost Exceeds 

Rental Cost
15

Table 6 presents a comparison of estimated downtime costs for a 400 MW and a 1000 

MW power plant, assuming two different capacity utilization rates: one for a baseload 

plant (80%) and one for an intermediate load plant (60%). The net downtimes are based 

on EPA estimates for cost modules 4, 7, 12, and 14, and are associated with relocation of

intakes to a submerged offshore location. These net downtimes are the net values after 

subtracting out a four-week concurrent scheduled maintenance downtime event. Table 6 

shows that downtime cost for longer durations can be reduced substantially by use of 

modular towers, especially at baseload facilities. The last column is the break-even point 

for these examples, similar to that shown in Table 5. 

Table 6

Comparison of Estimated Downtime Costs for

Use of Modular Tower Versus Plant Shut Down

As an example of the potential benefits of a lower tower design approach value, if the 

example 400 MW facility in row one of Table 5 is used along with the 0.12% (output 

increase /1.0 F) factor described above, the effect of reducing the cold water temperature 

by 1.0 F would be to increase plant revenue by up to $3,800,000 for the nine-week 

period if this were to occur during a period of continuously elevated air temperatures. 

Note that the benefit of increased generating efficiency is only available during periods 

when the wet bulb temperature is high enough to result in a turbine back-pressure that is 

well above the design back-pressure of the turbine. 

Conclusions

In summary, modular towers are a technology that offers a potentially significant 

reduction in facility downtime related to compliance with the proposed rule. Facilities 

that are able to install the modular towers may actually face no downtime at all, which 

would eliminate a significant component of the costs of the proposed rule and replace it 

with the smaller, temporary cost of modular tower rentals. (See the Environmental and 

Economics Benefit Analysis for a discussion of the role of downtime costs in EPA’s 

estimation of national economic impacts.) However, EPA is not able to estimate how 

many facilities would be able to employ these modular towers, nor has it attempted to 

estimate the overall cost savings of using them. As a result, EPA has not adjusted its 

national cost estimates to include the use of modular cooling towers.

Plant 

Size

Design 

Intake 

Flow1CUR2

Net 

Downtime3

Downtime 

Costs - Lost 

Revenue for 

Plant Shut 

Down4

Modular 

Tower Rental

Modular 

Tower Power 

Costs5

Downtime 

Total Cost 

Using 

Modular 

Towers

Downtime Cost 

savings for 

Using Modular 

Towers

Percent 

Reduction 

in 

Downtime 

Costs

Break 

Even 

Net 

Downtime

MW MGD % Weeks Dollars Dollars Dollars Dollars Dollars % Weeks

400 407 80% 9 $21,800,000 $12,200,000 $1,600,000 $13,800,000 $8,000,000 37% 5.7

400 407 60% 9 $16,300,000 $12,200,000 $1,200,000 $13,400,000 $2,900,000 18% 7.4

1000 1018 80% 10 $60,500,000 $30,500,000 $4,300,000 $34,800,000 $25,700,000 42% 5.8

1000 1018 60% 10 $45,400,000 $30,500,000 $3,200,000 $33,700,000 $11,700,000 26% 7.4

1

 Estimate based on average of 707 gpm/MW

2

 Capacity Utilization Rate During Downtime

3

 9-10 Weeks is the net downtime for cost modules 4, 7, 12, and 14

4

 Based on CUR and net revenue loss of $45/MWh

5

 Based on CUR and power cost of $65/MWh
16

References

Blue Stream Inc., Richard Brister. Telephone contact record by John Sunda, SAIC. 

Regarding technical requirements and costs of modular cooling towers. May 30, 2008a.

Blue Stream Inc. Email from Richard Brister to John Sunda, SAIC. Re: Request for 

Modular Cooling Tower Cost Information In Support of EPA 316(b) Rule. May 30, 

2008b.

Blue Stream Inc. “Estimated Cooling Cost Chart” (attachment to May 30, 2008 email 

from Richard Brister to John Sunda, SAIC).

Blue Stream Inc., Richard Brister. Telephone contact record by John Sunda, SAIC. 

Regarding technical requirements and costs of modular cooling towers. June 10, 2008c.

Blue Stream Inc., Richard Brister. Telephone contact record by John Sunda, SAIC. 

Regarding modular tower energy requirements and approach. March 25, 2009.

Durmayaz, Ahmet and Sogut, Oguz. Influence of Cooling Water Temperature on the 

Efficiency of Pressurized-Water Reactor Nuclear-Power Plant. International Journal of 

Energy Research. Vol. 30 No. 10. pp 799-810. 2006.

National Productivity Council of India (NPCI). Guide Book 3 – Energy Efficiency in 

Electric Utilities. Chapter 7 – Cooling Towers. No Date. Accessed on March 25, 2009 

at website: http://www.em-ea.org/Guide%20Books/book3/Chapter%203.7%20Cooling%20Tower.pdf. 

EPA. 316(b) Phase I – Proposed Rule. Economic and Engineering Analyses of the 

Proposed Rule. Appendix A. August 2000.

EPRI. Tower Calculation Worksheet. September 17, 2007.

Varitech Equipment Company. Fact Sheet: VTECT-ID3000 Portable Cooling Tower 

(equipment specifications). Accessed on May 30, 2008 at website: 

http://www.rentalcoolingtowers.com/product-specifications.htm
17

Figure 1. Intake Canal with Sheet Pile Wall, Forming a Closed Loop System
18

Figure 2. Example of Flexible Configuration