Cold Ends in Geothermal Power Plants
The “cold end” of a power plant refers to the condenser and the systems required for condensing the steam, or binary vapor, flowing from the turbine and for maintaining the required pressure in the condenser. A paper named; Cold Ends in Geothermal Power Plants was submitted to the WGC taking place in Reykjavik in October 2021. The paper provides an overview of the available cold end designs along with discussion on the characteristics of the major types. The presentation takes place in an online session on Tuesday 15th June, 11:00 AM (UTC). [Room: Stream D] SESSION 29D: Power Generation 2 - Equipment.
The cold end designs are reviewed in the paper from the point of effective utilization of the resource, operation, water balance, gas abatement, cogeneration of heat and power and environmental issues. The review includes experience of operating different types of cold ends.
The authors are Kristinn Ingason, Kristín Steinunnardóttir and Gunnlaugur Ágústsson, all geothermal experts at Mannvit. The paper itself will be shared on mannvit.com after the WGC 2021. Below are short excerpts from the paper.
Design Factors
The design of the cold end for geothermal power plants is site specific and adapted to the conditions for each power plant. Important items are e.g. the characteristics of the resource, power generating process, ambient conditions and environmental requirements. The efficiency of the power generating cycle is highly dependent on the cold end, which consumes most of the parasitic load of the power plant and frequently cold end constructions are prominent and important for the appearance of the power plant.
Air Cooled or Dry Cooled?
A condenser is a heat exchanger where vapor from the turbine is transferred into liquid by removing the latent heat with the help of a coolant. As the operating pressure of the condenser is lowered, the enthalpy drop of the vapor coming from the turbine increases. As a result, the generated power increases. The coolant in geothermal power plants is either water (wet cooling) or air (dry cooling). Air cooled condensers are usually not an option for steam turbines but are often used in binary power plants. Wet cooling may though also be preferred for binary plant, since the investment cost is lower, the footprint is smaller, parasitic load is smaller and the output is not as dependent on ambient condition as is in dry cooled condensers. Dry cooling may however be necessary especially in areas of limited water resources.
Direct Contact or Surface Condensers?
Direct contact condensers have more efficient heat exchange than surface condensers due to the direct mixing of steam and circulating water and consequent lower temperature difference between the cooling water and condenser. Direct contact condensers also have lower cost and are less prone to fouling since they have no tubes. The gas cooling part may however be problematic due to sulfur precipitation.
Surface condensers can be figured to have lower parasitic load than direct contact condensers, since the circulating water pumps that must overcome the condenser tube side pressure drop, may require less power than hotwell pumps which must draw suction from the low direct contact condenser pressure.
Parasitic loads are also vastly different between direct contact condensers and surface condensers. The paper provides comparison of parasitic loads for the two condenser types which are generally used in power plants where geothermal steam is used directly in turbines and discusses pros and cons of each one.
Operating Experience
The output of power plants where cooling towers are installed, is relatively stable. Usually maximum wet bulb temperature is a design condition and speed of cooling fans can be reduced if the wet bulb is lower. In binary plants with air cooled condenser the output can fluctuate +/- 20 – 30% with ambient conditions and is also sensitive to wind.
Sulphur deposits from geothermal fluids which are rich is H2S and can cause operational problems in condensers and cooling towers. Sulphur exchanges between sulphate and hydrogen sulphide depending on its concentration, pH and temperature. There are cases of algae formation in wet cooled surface condensers. This can cause decrease in the cooling system efficiency and interfere with water flow. Shock treatment once or twice each week with chlorine, such as NaOCl solution, can be employed to inhibit biological growth and deposition in the circulating water system and associated equipment.
In some plants, deaerator system has been added with the purpose of reducing the gas in the condensate from a surface condenser. Normally the condensate from surface condenser is adequately deaerated, i.e. contains low concentration of geothermal gases, and secondary deaerator system is not necessary.
As mentioned above, the paper itself will be shared on mannvit.com after the WGC 2021.
Like to know more? Need consulting for geothermal energy plants?
Contact Kristinn (at) mannvit.is Section Manager Geothermal Dept. or call +354 422-3000.
Photo: Cooling towers at 303 MW Hellisheidi geothermal heat and power plant, Iceland. Copyright: Mannvit.