A Feasibility Study of Using Waste Thermal Energy from a District Heating System for Absorption Cooling

Abstract

Waste incineration-based district heating plants in Norway struggle from energy efficiency drawbacks during the warmer months of the year from May - September due to the reduced demand in heating. As a result, thermal energy gets discarded to ambient air, which affects the energy efficiency of the plants. This study aims at presenting an overview of the waste incineration industry in Norway and the current options for surplus energy from waste incineration. Additionally, the aim is to analyze and identify the energy efficiency difficulties in the current system of a selected waste incineration plant and propose a more energy efficient alternative system. A waste incineration plant Returkraft AS in Kristiansand, located in southern Norway is selected as a case study and the thermal energy production process in this plant is analyzed. The analysis of Returkraft´s production data revealed that averagely 10 836 MWh of thermal energy gets discarded monthly to ambient air during the warmer months. An alternative system where the otherwise wasted thermal energy is used in absorption cooling is put forward, and simulations are carried out on ASPEN PLUS to present how much cooling can be achieved with the available waste heat. The results indicate that up to 2.33 MW of cooling can be achieved, producing chilled water at a temperature of 7.01C. A COP of 0.72 was achieved which is above the average COP of a single effect commercial absorption chiller. The results further present the energy utilization improvement from 41% with the current system to 95% with the proposed system during the warmer months. Despite investment costs being high with a possibly lengthy payback time, waste incineration plants in Norway are encouraged nonetheless to make energy efficiency improvements to their plants. The massive benefits of waste heat recovery with a wide range of applications available make the investments worthwhile long-term.