Matching analysis for on-site building energy systems involving energy conversion, storage and hybrid grid connections

Abstract

Under the background that all new buildings in EU should be nearly zero-energy buildings (nZEB) from the year of 2021, the energy and building industries are progressing towards the direction of decreased local building energy demand and enhanced on-site renewable energy production. This, on one hand, leads to the continuously decreased annual primary energy consumption/equivalent CO2 emission, whereas on the other hand it brings in the matching problem between the on-site generation and local building demand. Considering the fact that the renewable energy fraction in the hybrid grid networks in EU is not likely to reach 100% by the year of 2021, the undesirable mismatch is an inevitable side-effect of low-energy and zero-energy buildings. However, the scientific gap is that there is a lack of comprehensive methodology for the matching analysis of the increasingly complicated on-site hybrid energy systems involving all the energy forms, energy conversions, diversified storage types and hybrid grid connections. Therefore, the objective of this thesis is to set up a methodology to close the aforementioned gap. Correspondingly, six extended matching indices are defined for six aspects of on-site matching situation based on the extension of two basic matching indices. Furthermore, a topology is proposed for a comprehensive understanding and formulation of the extended indices. In order to show the applicability of these extended indices, a thorough matching analysis is conducted for the components of on-site hybrid renewable energy systems in two office buildings with distinct climate conditions. Moreover, in order to overcome the complexity brought in by the six extended matching indices, one evolved index is defined. By the mutual investigation of the evolved index and the extended matching indices, both the overall matching capability and the detailed specific matching aspects can be comprehensively illustrated, which has been proved in an example for a micro-cogeneration application.

With the aid of the methodology developed in this thesis, the matching caused by the diversified treatments of the excess on-site energy production can be quantitatively compared and analysed. For example, the matching capabilities can be compared between the two options for the treatment of excess on-site photovoltaic production: one is to directly export the excess production to the electrical grid, and the other one is to process the electrical-thermal energy conversion for recharging the hot water storage tank. Thereafter, a solution with better matching capability can be achieved. The general outcome shows that the methodology developed in this thesis is a powerful tool in aiding the analysis, design and control of the increasingly complicated on-site energy systems in buildings.