PhD defence Henrik Pieper
Optimal Integration of District Heating, District Cooling, Heat Sources and Heat Sinks
Large-scale heat pumps 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 and cooling efficiently in combination with thermal storages. The representations of the performance and economics of heat pumps 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 district heating and cooling based on large-scale heat pumps 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 district heating and cooling 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 heat pumps depending on the used heat source were developed based on the experience of existing and planned heat pump projects in Denmark. The optimization model was applied to the new development district of Copenhagen, Nordhavn, to supply district heating and cooling, as well as to the existing district heating network of Tallinn, Estonia. The optimization model was also used to investigate how the performance estimation of heat pumps influences the model results.
The main findings of the PhD thesis are that the investment costs of large-scale heat pump projects, in a range of 0.2 MW to 10 MW heating capacity, can be specified for the use of individual heat sources. Cost correlations were developed for using ambient air, industrial excess heat, flue gas, cleaned sewage water, groundwater and district cooling. It was found that the costs of the heat pump unit itself is 38 % to 56 % of the total investment costs of such projects. Assuming a constant performance throughout the year 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 sources and sinks within one system is competitive to the use of a single heat source/sink. A heat pump that uses the district cooling network as a heat source to supply district heating is very efficient and economical. Groundwater and sewage water were proposed as heat sources and heat sinks for an economically optimal supply of district heating and cooling. Seawater was constrained by a large distance to the plant. It was further shown that a large reduction in annual CO2 emissions is possible for a relatively small increase in investments.