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Home > What's New > Air-cooled condenser
  • Air-cooled condenser2010-07-12
  • Deutsches Zentrum fur Luft- und Raumfahrt e.V. (“DLR”), a German government research agency, presented a study in 2007 comparing a particular dry cooling technology, the Heller system, with wet cooling for CSP plants in Spain and in the California desert. Water consumption was reduced by 97%, and the performance impact was quite minimal. Indeed the impact on performance in the higher desert temperatures of California was overwhelmed by the benefits of better annual insolation. They also noted that the potentially negative impact of high daytime temperatures is mitigated by the use of thermal storage, which uses energy collected during peak daytime insolation to produce electricity when temperatures are considerably lower. One interesting aspect of the DLR study was their focus on Heller systems over more familiar (at least in the US) direct dry cooling systems, and that is worth a closer examination.

    Two basic types of dry cooling systems have long been employed where necessary -– “direct” air cooling (usually called an air-cooled condenser” or “ACC”) and “indirect” air cooling (often referred to as the “Heller system”, after Laszlo Heller, the Hungarian thermodynamics professor who pioneered this approach in the 1950s). In ACC systems, the saturated steam from the steam turbine exhaust is carried directly to a very large array of A-framed fin-tube bundles, where large mechanical fans force air over the tubes, convectively condensing the steam.

    ACC system

    In Heller systems, the steam is condensed by spraying water directly into the exhaust flow in a ratio of about 50:1 (called “direct contact jet condensing”), creating a large volume of warm water, some of which is pumped back to the boiler as the working fluid and the rest of which is pumped to bundles of tubes arrayed at the base of a natural-draft hyperbolic cooling tower. The warm water circulating around the base of the tower and the cooler air at the top of the tower, combined with the tower’s hyperbolic shape, stimulate a powerful updraft that draws ambient air over the tube bundles, thereby convectively cooling the water before it is returned to the condenser. Both are closed systems.

    Heller system

    While the Heller system has been widely used elsewhere, there are none in the US. This is probably because the much lower auxiliary power requirements of Heller systems come with the visual impact of a large hyperbolic cooling tower (typically 150m high and 120m in base diameter), often a difficult sell given that most fossil power stations are located in the vicinity of the populated demand centers they’re intended to serve. The auxiliary power required to run an ACC system is roughly twice the power required run a Heller system, and the Heller system is considerably quieter, but these have apparently been considered prices worth paying for the lower profile (a typical ACC system can be 40m high), particularly when it was cheap coal-fired power. Simple lack of familiarity could be another factor in the hidebound world of US power utilities.

    The Electric Power Research Institute has kicked off a comparative study of indirect dry cooling (due to be completed in mid 2010), on the theory that it is the most economic dry cooling solution for large-scale thermal applications. The prospect of large amounts of CSP being built in the world’s deserts calls for a reconsideration of the relative merits of these two approaches, since it would require dry cooling to be deployed in a different application and to a far larger extent than has ever been the case.

    They also note that the footprint of an ACC system is larger than that required for a Heller system, though specific data is not offered. Overall system efficiency of a Heller system is in the range of 2% better than an ACC system. That performance improvement meant one thing in a fossil power plant in the bad old days of cheap dirty power, but when it means 2% less land area covered by solar collectors, and lower auxiliary consumption of much more costly power, it takes on a much greater significance. The same sources note that since the Heller systems are mechanically far simpler than ACC systems, maintenance is much less of an issue and system availability is significantly greater. In the remote areas where these plants will be located, and given the large land areas over which they will spread, these are far more significant considerations than they were for compact fossil power plants located close to the populations they served. Another factor noted in these sources is that an ACC must be located next to the steam turbine it serves, because of the cost of transporting saturated steam over any distance, whereas the Heller system has much more flexibility in where the cooling tower is located. This will be much more important to CSP, where one can envision clusters of power tower complexes in a given area each with its own steam turbine, than it was with fossil plants. And finally, the feature that most worked against Heller systems in US fossil plant applications – visual impact – should be far less of an issue in remote desert sites, especially with solar power tower complexes where the central towers will likely be of similar height.

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