Michael Owen Univ. of Stellenbosch/Dept. M&M Eng./STERG A hybrid dephlegmator for incorporation into an air-cooled steam condenser Michael Owen Univ. of Stellenbosch/Dept. M&M Eng./STERG Energy Postgraduate Conference 2013

Overview Air-cooled condensers (ACCs): what, where, why? Hybrid (dry/wet) dephlegmator concept HDWD performance evaluation Concluding remarks

3 ACCs: what, where, why?

ACCs: what, where, why? (cont.) 4 ACCs: what, where, why? (cont.) Wet cooling using evaporative condensers Approximately ½ of all power plants in the U.S. use evaporative cooling¹ 36 of the 37 major operating solar thermal power plants are wet cooled² Water intensive: >30 000 litres per day per MW installed³ Typically accounts for > 80 % of the water consumption at a plant4 The use of alternative means of cooling therefore holds the greatest potential for decreasing water consumption related to electricity production

ACCs: what, where, why? (cont.) 5 ACCs: what, where, why? (cont.)

ACCs: what, where, why? (cont.) 6 ACCs: what, where, why? (cont.) ACCs use ambient air to cool and condense the process fluid No water is directly consumed in the cooling process ACCs are therefore an attractive option considering current global water security concerns ACCs allow for flexibility in plant location Location not constrained by proximity to water resources Plants can be built nearer fuel/energy supplies or load centers → increased reliability and reduced transmission costs ACCs experience a reduction in cooling effectiveness at high ambient temperatures Q α Tv - Ta A dynamic relationship exists between condenser and turbine performance Reduced cooling effectiveness → decreased plant output on hot days

Hybrid (dry/wet) dephlegmator 7 Hybrid (dry/wet) dephlegmator Conventional Replace conventional dephlegmator with a HDWD Originally proposed by Heyns and Kröger5 2 stages connected in series: 1st stage – ACC (inclined finned tubes) 2nd stage – hybrid plain tube bundle 2nd stage operation: Low ambient temperatures → DRY High ambient temperatures → WET Wet mode Both latent and sensible heat transfer Measurably enhanced air-side heat transfer coefficient Hybrid

HDWD performance evaluation 8 HDWD performance evaluation 3 street ACC with a HDWD (operating wet) provides the same turbine output benefits as: A 33% over-sized ACC (4 streets vs 3) - at a much reduced cost A conventional ACC with spray cooling enhancement – while consuming 20 % - 30 % less water and avoiding fouling and corrosion issues

HDWD performance evaluation (cont.) 9 HDWD performance evaluation (cont.) One-dimensional performance analysis to identify the best configuration for the 2nd stage HDWD bundle More rows of smaller diameter tubes in a multipass arrangement with a single tube row in the final pass Highest heat transfer rates (up to 35% increase for an ACC) Lowest ejector loading Slightly higher water consumption (although still negligible compared to an evaporative cooler) Higher steam-side pressure drops A B C D E F G H I J K Tube dia., mm 38.1 19.0 No. of tube rows 16 25 No. of tubes/row 156 314 No. of passes 3 12/3/1 2 13/2 14/1 1 15 20/5/1 20/5 38.1 19.0 16 16 25

10 Concluding remarks While ACCs offer water consumption advantages over evaporative cooling towers they experience decreased performance during hot periods Alternative and enhancement strategies have thus far proven problematic A novel hybrid dephlegmator concept is proposed that: Offers enhanced cooling performance While consuming only a small amount of water Is able to reduce the load on the ejector Simple and cost effective Experimental data is required to fully quantify the exact performance characteristics of the HDWD

11 References Carney B, Feeley T, McNemar A. NETL Power Plant Water Research Program. EPRI Workshop on Advanced Thermoelectric Cooling Technologies 2008; Charlotte NC. NREL 2010: http://www.nrel.gov/csp/solarpaces/project_detail.cfm/projectID=6 Gadhamshetty V, Nirmalakhandan N, Myint M, Ricketts C. Improving Air-cooled Condenser Performance in Combined Cycle Power Plants. J Energy Eng 2006; 132/2:81-88. DiFilippo M., Reclaiming Water for Cooling at SCE’s Mountainview Power Plant, Presentation: EPRI workshop on Advanced Thermoelectric Cooling Technologies, Charlotte, 2008. Heyns, J.A., Krӧger, D.G., Performance characteristics of an air-cooled steam condenser with a hybrid dephlegmator, R&D Journal of the South African Institute of Mechanical Engineers, Vol. 28, pp. 31-36, 2012.