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Steam Power Plant Condenser Cooling - Part 2: Comparison of Alternatives

Written by:  • Edited by: Lamar Stonecypher
Published Feb 24, 2010 • Related Guides: Department Of Energy

Steam power plant water is used primarily for the condensing unit. The cooling alternatives, once through cooling, wet cooling tower, air cooled condenser, hybrid wet/dry cooling, and cooling ponds each have advantages and disadvantages. Water use, economics and % use in U.S are compared here.

Introduction

Steam power plant water use has been coming under increasing scrutiny because of the large quantities of water involved and the many competing uses for water. The cooling options for the condensing unit (once through cooling, wet cooling tower, air cooled condenser, hybrid wet/dry cooling, and cooling pond) were introduced in the article: Steam Power Plant Cooling - Part 1: Introduction to Alternatives. Here the condenser/cooling options will be compared in terms of water withdrawal, water consumption, economics, and percentage of U.S. generating capacity using each option.

The Five Condenser/Cooling Alternatives

Rankine Power Cycle with Once Through Cooling
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Rankine Cycle with Cooling Tower
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Rankine Cycle with Air Cooled Condenser
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Three of the major options for steam power plant water cooling and the condenser are summarized in the diagrams in this section. There is a diagram for i) once through cooling, ii) wet cooling tower, and iii) air cooled condenser. When a cooling pond is used, it would simply replace the cooling tower shown in the second diagram. The fifth alternative, hybrid wet/dry cooling, would have an air cooled condenser and some features of a wet cooling tower system.

Water Withdrawal and Consumption Comparison

Water Use Comparison
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A comparison of typical water withdrawal rates and water consumption rates is given in the table at the left, for once-through cooling, cooling tower, and cooling pond systems, with fossil fueled and nuclear fueled steam power plants, based on information from a 2002 report of the Electric Power Research Institute (EPRI, 2002). (Click on the table to view a larger version.)

Note that the water withdrawal rate for once through cooling is two orders of magnitude larger than that for pond or cooling tower systems. The withdrawal rate for pond and cooling tower systems is simply replacement for the water evaporation into the atmosphere. There are only small differences in the rate of water consumption for the three systems. Some sources show the rate of water consumption for once through cooling being much lower than in this table. EPRI (2002) estimates this rate as being only slightly lower than for a cooling tower or pond, based on increased evaporation from the source water body due to the heated water being discharged to it.

No figures are shown for an air cooled condenser, because the power plant water use for an air cooled condenser is much less than for the three options shown in the table.

Percentage of Use in the U.S.

The percentage of U.S. steam power plants of several types, for use

Percentage Use of Cooling Types
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of four types of condenser/cooling systems, is shown in the table at the right. The information in the table came from a 2008 report of the Department of Energy, National Energy Technology Laboratory report (DOE/NETL, 2008). Most of the steam power plants built in the U.S. before 1970 use once through cooling. Due to increasing concern about and regulation of water use for electric power production, most of them built after 1970 use cooling towers. These two types of condenser cooling systems account for about 84 % of steam power plants. Most of the rest use cooling ponds. Dry cooling has emerged as another option because of the water withdrawal and consumption rates for the other options, even with wet cooling towers. About 1 % of steam power plants use an air-cooled condenser (dry cooling system).

Economic Comparison

Cost and Performance Comparison for Cooling Systems
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There is a reason that once through cooling was used in almost all steam power plants built before 1970. It is the simplest system and has the least initial cost and least operating cost. Both the wet cooling tower system and the air cooled condenser (dry cooling) system have higher capital cost, higher power requirement (and thus higher operating cost), leading to approximately 1.9 % higher cost of electricity due to use of a wet cooling tower and approximately 4.9 % higher cost of electricity for air cooled condenser use, as shown in the table at the left. The information in the table came from a presentation at a California State Water Resources Control Board Workshop in 2005. (Maulbetsch, 2005).

References

1. DOE/NETL, 2008, Estimating Freshwater Needs to Meet Future Thermoelectric Generating Requirements - 2008 Update, DOE/NETL-400/2008/1339, Sept. 30, 2008.

2. EPRI, 2002, Water & Sustainability (Volume 3): U.S. Water Consumption for Power Production - The Next Half Century, EPRI, Palo Alto, CA, 2002, 1006786.

3. Maulbetsch, J.S., 2005, Power Plant Cooling -- What are the tradeoffs?, Presented at California State Water Resources Control Board Workshop, Oakland, CA, Dec. 7, 2005.

DOE/NETL, 2008,

About the Author

Dr. Harlan Bengtson is a registered professional engineer with 30 years of university teaching experience in engineering science and civil engineering. He holds a PhD in Chemical Engineering.

Related Reading

Power Plant Condenser Cooling - Part 1: Introduction to Alternatives - The steam power plant water going through the condensing unit requires cooling so that liquid condensate can be pumped back into the boiler. Cooling for the condenser may be supplied by once through cooling, a wet cooling tower, dry (air) cooling, a hybrid (wet/dry) system, or a cooling pond.

Next Article »
Steam Power Plant Condenser Cooling
The main use for steam power plant water is in the condenser. The alternatives for cooling are once through cooling, wet cooling tower, dry cooling tower (air cooled condenser), and cooling pond. Each of the steam power plant cooling methods has advantages and disadvantages.

Comments

Showing all 3 comments
 
Edwin Nov 24, 2010 11:05 PM
SJAE
Is the same (rates, size, components, etc) vaccum system (I.e. SJAE) for either air cooled condenser or wet condensing (surface condenser)?
Harlan Bengtson Apr 7, 2010 9:49 AM
Vacuum in steam condenser
The vacuum is drawn at the exit from the steam turbine in order to give a larger pressure difference across the turbine and thus more electrical generation. Since the steam leaving the turbine goes to the condenser, there will be a vacuum at the inlet to the condenser. The purpose for the vacuum is to increase the efficiency of the turbine in converting thermal energy to electrical energy, the vacuum at the inlet to the condenser is just there as a result.

Harlan Bengtson
PADMESH KUMAR SINGH Apr 7, 2010 7:06 AM
condensor
Why vaccum is created in process of steam convert in to water ?
 
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