Development of a simulation model for the dynamic analysis of exergy flows in a heat distribution network of a LowEx cluster


Motivation
The impact of rising greenhouse gas emissions, as well as the finiteness of fossil fuels, is well known nowadays. A third of the final energy demand in Germany is required for the building sector. The bigger part of energy used for heating is won from highly exergetic fuels (e.g. coal, gas and electricity). Since living spaces are usually heated to around 20 °C, low exergetic fuels are sufficient. Economically viable energy sources with low exergy share already exist for this application. So called “LowEx”-systems with natural “LowEx”-energy sources are, for example, heat pumps (geothermal heat), solar collectors (solar radiation),  and district heating systems that use renewable energy. In addition to reducing emissions the unused fossil fuels are free to be used in processes that require these resources. If the main part or even all the buildings of a residential area are equipped with heating systems that guarantee a low exergy supply, then it is possible to design entire residential areas and supply chains according to the LowEx-principle.

Objective
The objective of this work is the modelling a building group (LowEx cluster) as well as the exergetic analysis of all occurring energy flows. The building model consists of 10 residential buildings with a high energetic standard, a low-temperature district heating network with transfer stations, a buffer storage as well as selected renewable (e.g. heat pumps and solar collectors) and fossil (gas boiler) sources for energy. Multiple renewable supply scenarios are constructed, modelled and simulated from an existing base model. Different renewable energy sources are examined, while the heating load of the cluster (LowEx cluster) and the district heating network remain identical. A decentralized fossil condensing boiler is used as a reference model. In addition parametric variations are performed in which the supply temperature as well as the pipe length of the supply network differs. To conduct exergetic analysis the system boundaries of the scenario that is to be analysed must be defined. Subsequently occurring energy and exergy flows can be calculated. Based on the exergetic analysis of the configurations a technological comparison is performed. For this comparison key figures such as the energetic efficiency, exergetic efficiency and the exergetic expenditure factor are used. The results are then evaluated and are compared with each other.

Results
The evaluation of the scenario analysis shows that renewable energy sources have far better results than fossil energy sources from an exergetic point of view, due to their usage of energy with a lesser quality. In return the energetic efficiency of renewable energy sources is lower. The best exergetic efficiency is achieved by the scenario in which the district heating network is supplied with solar energy. The exergetic efficiency of the heat pump is dependent on the electricity mix. The higher the part of renewable supply units, the higher the efficiency. Losses and mixing processes in the supply chain also have an effect on the exergetic efficiency and should be kept as low as possible. The supply temperature of the distribution network and the desired temperature of domestic hot water also play an important role in regards to exergetic efficiency. Although large investment costs are to be expected for a renewable distribution network, it can be asserted that solar heating is a promising technology since it uses cost-efficient and environment-friendly energy sources.

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