IEA EBC Annex 64: Optimised Performance of Energy Supply Systems with Exergy Principles (Working period 2014-2018)

In order to reduce CO2 emissions in cities, a holistic understanding of energy conversion processes is necessary. If alternative heat sources such as waste heat, ambient heat, solar and geothermal energy shuld be used, it is essential to consider the lower temperature level or exergy content of these heat sources. Accordingly, urban energy systems were analysed and optimised using the exergy approach within the framework of the EBC Annex 64. This includes the analysis of possibilities for cascading heat use, the identification of potentials for the integration of heat pumps as well as the comparison of national and international case studies and evaluation methods. The main results of the Annex are best practice examples and guidelines for the exergy-optimised design of urban energy systems, which have been summarised in a comprehensive guidebook, taking into account exergy principles and Austrian expertise.

Initial situation and motivation

Cities are responsible for more than 2/3 of final energy consumption. In order to reduce CO2 emissions, it is essential to increase energy efficiency in consumption and the share of renewable energies. A holistic understanding of consumer structures and conversion processes is necessary to increase the potentials in the heating and cooling sector. In particular for the use of alternative heat sources such as waste heat, ambient heat, solar and geothermal energy, the consideration of the lower temperature level or "exergy" content of these heat sources is important. So far, however, energy planning in cities has essentially been based on the balancing of energy demand and generation capacities.

Aim and methodology

The aim of the IEA program EBC ("Energy in buildings and communities") is to integrate energy-efficient and sustainable technologies in buildings and communities through research and innovation. Within the framework of the EBC Annex 64 (https://www.annex64.org/), urban energy systems are analysed with the help of the exergy approach with regard to economic efficiency, CO2 emissions and efficiency. Within the EBC Annex 64, AIT has participated representing Austria and leaded Subtask C on "Model Cities", which includes the comparative analysis of all Annex 64 case studies. AIT also carried out the following activities: The mathematical analysis of possibilities for cascading heat use within and between characteristic building types (Subtask A); the elaboration of potentials for the integration of heat pumps in low-temperature district heating networks (Subtask B); the comparison of different evaluation methods and optimisation algorithms and the integration of exergy indicators (Subtask D).

Results

The analyses of the case studies within the IEA EBC Annex 64 and the intensive discussions with the other Annex participants and national stakeholders show that exergy can in principle be helpful in identifying optimisation potentials in thermal systems. In any case, it is important to assess the value of a low temperature levels when designing and optimising heating networks. In this context, the resulting increasing potential and efficiency of generation units (especially renewable energy sources and heat pumps) due to the lower temperature levels are essential, but also increasing storage capacities and lower heat distribution losses have to be considered. On the other hand, however, there are investments required on the building side for handling low supply temperature, which can be very high especially for supply temperatures below the domestic hot water preparation temperatures. The use of existing high-temperature supply units can increase the efficiency of the systems by carrying out any necessary temperature boost. Finally, it is important to emphasise that exergy-optimised low-temperature networks are an important "enabler" of sector coupling, as the COPs of the heat pumps are significantly increased compared to high-temperature systems and electrical temperature boosters represent additional coupling points.

The results of the IEA EBC Annex 64 are summarised in design guidelines for urban energy systems, taking into account exergy principles and Austrian expertise, and are published in a guidebook.

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