چارچوب مدل سازی ساعات روز: ارزیابی بهره برداری منابع انرژی و انتخاب مبدل انرژی و سایزبندی در ساختمان ها

The promotion of the exploitation of renewable sources in the built environment has led to the spread of multi-energy systems in buildings. These systems use more than one energy source in various energy converters to overcome the limitations that may be characteristic of each source. However, the design of the optimization of such systems is a complex task because the number of design variables is high and the boundary conditions (climate, operation strategies, etc.) are highly variable, so the system simulation has to be performed in the time domain. In this work an original hourly model to optimize multi-energy systems is presented and applied on a case study. It is an evaluation method to assess, in an integrated fashion, the performance of a building system as a whole and the viability of the exploitation of various energy sources. This tool is intended to take into account the variation of the conversion efficiency as a function of the design power, part load, boundary and climatic conditions. The relations that can model the energy converters of the case study (standard boiler, condensing boiler, various types of chillers and others) from the energy performance and from the financial points of view are also presented. This model represents a valuable alternative to currently available tools for hybrid systems simulation because of the optimization approach and of the detail in the thermal energy converters performance. Ultimately, the theoretical and applied knowledge of this contribution aims also at promoting a more conscious use of renewable and non-renewable energy in the built environment.

It is well known that the increase in the energy efficiency of the building systems is one of the most promising strategies to reduce the energy use in the built environment without penalizing the indoor comfort and the satisfaction of the final uses. This can be done not only by using highly efficient energy converters (or components) at both full and part loads, but also – in a broad sense – by the exploitation of natural resources and by matching the local energy supply with the building energy demand. There has been a great development of small-scale renewable energy systems, and this caused a gradually developing integration of various energy sources into systems that can be called hybrid systems [1] or multi-energy systems. The second term is used to stress the fact that these systems are fed by a mix of sources and use more than one energy converter to cover one load, contrarily to conventional systems that, for each load, are fed by one energy source that is used in one energy converter. In fact, one of the main reasons of the spread of multi-energy systems refers in fact to the use of various renewable sources, that are characterized by an intermittency that causes a mismatch between the energy demand and the energy supply that affects the system reliability and can be overcome by an integration of various converters working at the same time. This is why hybrid systems, originally used in remote applications [2] and [3], are recently used also in the building sector, as can be seen from the theoretical and experimental studies [4], [5], [6], [7], [8], [9] and [10]. The definition of the lay-out and the sizing of the energy converters and components of this type of system is however quite a complex task, since it involves the assessment of highly variable quantities as the building energy demand and the energy sources, especially renewable sources, and the characterization of the energy converters. The design of a multi-energy system, both in terms of components sizing and definition of operation strategies, can be specified as the determination of the energy demand and supply profiles and in the optimization between the energy demand, the energy sources, the energy converters, the storages and the back-up components. In the literature this problem is addressed with reference to specific system configurations (e.g. geothermal heat pump coupled with solar collectors, trigenerators, etc.) as it is – for example – in the works [11], [12], [13], [14], [15], [16], [17], [18] and [19], but not by means of an integrated tool that may make it possible to compare a great number of different design scenarios. Such a tool is the scope of the research work that is presented hereinafter.

An original, synthetic and integrated hourly modelling framework for the design of multi-energy systems was developed and applied to a case study. It can be a valuable mean to assess the building system and the mix of energy sources, both conventional and renewable, that should be preferred from the energy savings, economic viability and environmental impact points of view. Such a tool implies that the system of a building is integrated, that is the thermal, cooling and electricity loads are covered by the same unitary set of energy converters, and this is surely the tendency of recent and future energy systems in the building sector. This tool is also of considerable interest because a progressive integration of renewable sources in the built environment will definitely take place in the near future, in order to accomplish the requirements of a carbon free energy production. Therefore, evaluation methods to assess, in an integrated fashion, the performance of a building system as a whole and the viability of the exploitation of the various energy sources will be increasingly used. The method that is the object of this work follows another one developed by the same authors [27] and based on a steady-state balance of the building system that can be used to determine the system lay-out and to size the energy converters at the design concept stage with a small amount of input data. On the contrary, this hourly method can be used only when the hourly energy demand profiles of the building (at lest heating energy, cooling energy and electricity, or more than that if the thermal level of heating and cooling energy must be considered) and the detailed characteristics of the energy converters are available. If this is the case, this method represents a powerful alternative to the conventional analysis tools for multi-energy systems in buildings currently available (e.g. software tools like TRNSYS, EnergyPlus, HOMER). This is because in the software tools a finite number of system configurations is simulated at a time, while in this hourly optimization framework the set of all possible system configurations – that the user defines in the coupling matrix – are investigated. Even if the numerical optimization does not guarantee the finding of the absolute minimum, this is in any case a great change for an improvement in the system design procedures. In fact, the design power of each energy converter is variable, and the system lay-out is open (that is to say that the dispatch factors of the loads between the various energy converters can assume any value between 0 and 1). Furthermore, as a result of the sectorialization of each software tool, not all the energy converters that are used in buildings can be simulated in the above mentioned software tools and not all the energy fluxes can be taken into account (e.g. the optimization model for distributed power HOMER can model some thermal converters only at a simplified stage). Finally, contrarily to the simulation approach, this procedure is based on the optimization approach, which is performed on the year-round energy environmental and financial performance of the system, and should be carried out with a suitable optimization technique. As in an hourly method also quadratic or cubic relations between the variables of the optimization problem are used, no optimization method can assure the finding of the absolute minimum. This is why a further research activity is currently being carried out by the authors on the subject of the optimization technique and the objective functions to be coupled to this modelling framework, in order to determine the most suitable optimization algorithm to be used as a function of the objective criterion formulation. Others issues that are to be addressed are connected to the thermal and electric energy storage and the relative components that can be adopted to realize a better matching between the energy demand and the energy supply, and the variability of working strategies during the operation. This last issue may lead to a time variation of some dispatch factors.