Optimal Integration of District Heating, District Cooling, Heat Sources and Heat Sinks

Abstract

Large-scale HPs and refrigeration plants are essential technologies for decarbonising the heating and cooling sector, while using power generated from renewable energy sources. Thereby, intermittent power can be used to supply district heating (DH) and district cooling (DC) efficiently in combination with thermal storages. The representations of the coefficient of performance (COP) and economics of HPs and chillers in energy planning tools are often very simple and considerations of the heat source, heat sink, heating supply and cooling supply are barely taken into account. However, temperatures of these streams often vary during the year, so that simplified approaches may lead to wrong investment decisions. This could lead to a suboptimal exploitation of sources, resources and investments. This PhD thesis aims at analysing how the considerations of different heat sources and heat sinks and their characteristics influence planning decisions regarding the supply of DH and DC based on large-scale HPs and refrigeration plants. For this purpose, an optimization model was developed based on mixed-integer linear programming, which is able to identify production and storage capacities, heat sources, heat sinks and hourly operation for the most economical, sustainable or energy efficient supply of DH and DC using electricity. Detailed knowledge of a wide range of heat sources and sinks were obtained and applied to the optimization model, which included hourly temperature profiles and certain capacity limitations. In addition, linear correlations of investment costs for large-scale HPs depending on the used heat source were developed based on the experience of existing and planned HP projects in Denmark. The optimization model was applied to the new development district of Copenhagen, Nordhavn, to supply DH and DC and to the existing DH network of Tallinn, Estonia. The optimization model was also used to investigate how COP estimation influences the model results. Four different methods for estimation of HP COPs were investigated based on constant COP, Lorenz efficiency, exergy efficiency as well as a method presented by Jensen et al. The COP estimation methods were compared to the COPs calculated with a thermodynamic HP model for four heat sources. The main findings of the PhD thesis are that the investment costs of large-scale HP projects, in a range of 0.2 MW to 10 MW HP capacity, can be specified for the use of individual heat sources. Cost correlations were developed for using mbient air, industrial excess heat, flue gas, sewage water, groundwater and district cooling. It was found that the costs of the HP unit itself is 38 % to 56 % of the total investment costs of HP projects. The estimation of COP based on the ensen et al. method resulted in optimization results that were very similar to the ones obtained with COPs based on the thermodynamic model. This method showed lower sensitivity of the optimization results for uncertain input parameters compared to the other three methods. The use of a constant COP is not suitable for varying heat sources and heating supply temperatures. Applying the optimization model to the case studies showed that a combination of different heat ources and sinks within one system is competitive to the use of a single heat source/sink. A HP that uses the DC network as a heat source to supply DH is very efficient and economical. Groundwater and sewage water were proposed as heat sources and heat sinks for an economically optimal supply of DH and DC. Seawater was constrained by a large distance to the plant. The Pareto frontier showed that a large reduction in annual CO2 emissions is possible for a relatively small increase in investments.