Exploiting Waste Heat in Small and Medium-Sized Combined Heat and Power Plants Using Steam Injection

Abstract

Combined heat and power generation (CHP) is a way of providing both electric power and thermal heat for industrial and domestic facilities at high fuel efficiencies. Often small and medium sized gas powered internal combustion (IC) engines, rated at electric power outputs of 50–600 kW, are used for such applications. During the time when the available thermal heat is used, the fuel efficiency of such CHP plants is very high, but it drops to the efficiencies of simple power generation when there is no heat demand, e.g. during summer. In these cases, the exhaust heat is blown off, especially as CHP units are mainly heat-lead, i.e. designed to cover the heat demand rather than the demand for electrical power. Moreover, as the cooling water heat rejection is also more difficult at elevated ambient temperatures, these units are then operated at part load or even switched off, hence having a lower degree of capacity utilization. The approach of the work presented here is to replace the turbocharger system commonly used for IC engines and to use an electric driven compression device instead, while the turbine serves to generate additional electric power from the exhaust gas. Furthermore, for periods with low thermal heat demand, steam is generated from the turbine exhaust heat. The steam is injected in front of the turbine in order to increase the turbine work output further. Thus, at least part of the exhaust heat available is used and the power output as well as the electric efficiency is increased. In the present work, two configurations of the described setup using a medium sized gas powered IC engine CHP unit are modeled in order to assess the impact on plant performance and the characteristics of such a facility. In both cases the engine cooling circuit is integrated. Depending on the configuration used, the plant power output increases by up to 7% only because of the power turbine. Additional steam injection to use the waste heat increases the power output further. The relative electric efficiency increase with steam injection is in the range of 3–5%. Apart from the higher output of electric power, this approach allows longer operating hours to be achieved, as the exhaust heat available is utilized and the heat load for the cooling water circuit is reduced.