Energy efficiency analysis of condensed waste heat recovery ways in cogeneration plant

Making full use of condensed waste heat is an effective approach to increase heating capacity and reduce air pollutant emissions of the cogeneration plant. In this article, the heating equivalent electricity method is adopted to evaluate energy efficiencies and applicability of different condensed waste heat recovery ways, such as the condenser, the single-effect lithium bromide absorption heat pump and the bleeding-steam-driven compression heat pump. The following discussion is based on a 300 MW water-cooling steam turbine heating system and a 300 MW air-cooling steam turbine heating system. Combining with the characteristics of the water-cooling steam turbine and the air-cooling steam turbine, main factors affecting energy efficiencies are analyzed. Applicability evaluations of these condensed waste heat recovery ways are made by comparing energy efficiencies with each other under design conditions. Analysis shows that direct heat exchange by the condenser should be given the first priority. The condenser is most suitable to provide basic heating load. For the heat source composed of several steam turbines, the corresponding condensers should be connected in series. The proportion of steam turbines with high backpressures should be controlled due to high energy consumptions and inflexibility of adjustment. When supply water temperature is low, a surplus of bleeding steam pressure causes large irreversible loss in the generator of the single-effect absorption heat pump. A waste of energy grade of the bleeding steam makes the energy efficiency of the single-effect absorption heat pump lower than that of the bleeding-steam-driven compression heat pump. However, when bleeding steam pressure matches with the single-effect absorption heat pump, the energy efficiencies of them are basically the same. Therefore, the bleeding-steam-driven compression heat pump is more suitable when bleeding steam pressure is high. Due to large irreversible loss, energy efficiency of the heating network heater is the lowest. (C) 2015 Elsevier Ltd. All rights reserved.