Analysis of displacing natural gas boiler units in district heating systems by using multi-objective optimization and different taxing approaches

Abstract

District heating systems are proven to be effective way of increasing energy efficiency, reducing environmental impact and achieving higher exergy efficiency. In research papers, district heating multi-objective optimization usually takes into account minimization of the total discounted cost and environmental impact, while exergetic objective function is rarely introduced. Most of the times, economic and ecological objective functions are studied as a single objective optimization problem through internalization of the cost related to carbon dioxide emissions tax. This paper presents novel approach since additional tax, related to exergy destruction, has been introduced. The influence of these two taxing systems on a single and multi-objective optimization results of district heating system has been carried out. Two approaches have been proposed. In the first one, multi-objective optimization has been used where objective functions were defined as economic and ecological or exergetic. In the second approach, single-objective optimization has been used where cost function also includes both carbon and exergy destruction tax. It has been shown that inclusion of carbon tax causes convergence of Pareto fronts after specific exergy destruction has been reached. On the other hand, if all technologies are available, increase of exergy tax doesn’t reduce carbon dioxide emissions. The most important outcome of this paper is analysis of the impact of exergy tax on natural gas consumption in heat-only boilers. Acquired results show that exergy, together with carbon tax, can effectively reduce natural gas consumption in heat-only boilers. If there are no back-pressure CHP technologies available, these taxing systems can completely push out its consumption. Finally, the analyses with carbon emissions allocation in CHP units has also been carried out. Acquired results have shown that with increase of carbon tax, exergy efficiency of the system could be increased.