شبیه سازی سیستم ذخیره سازی حرارت و CHP اخراج زیست توده در سیستم های موجود حرارت مرکزی

The installation of a biomass-fired Organic Rankine Cycle (ORC) unit coupled to a heat storage system (HSS) in an existing district heating (DH) system is proposed and analyzed from both energetic and economic point of view. A real DH system is considered as case study, and the optimal layout configuration is investigated varying the size of the components. The analysis is carried out tuning the heat demand dataset obtained from real data of a different existing DH system with a 6-min time step and ten years of operation. The heat demand is used to match the production from different generation units. The overall efficiency of the system, the primary energy savings related to CHP production, as well as the pay back time of the investment are evaluated. Calculations show that for the considered case study the maximum size of the HSS that gives noticeable advantages is 150 m3/MWth. The optimal configuration is different when considering energetic or economic criteria. Moreover, the current Italian incentive tariff on electricity production from renewable sources appears to promote the choice of low efficiency layouts for the case study under consideration.

Energy production from renewable sources, together with energy efficiency and energy saving measures, is a key question in the limitation of greenhouse gases emissions (GHG) and in the diversification of energy resources. The European Union in its Climate Package has set a target of 20% of energy production from renewable sources by 2020, with further objectives for 2050 [1] and [2]. Energy production from renewable sources has increased in recent years up to 1660 Mtoe in 2010 [3]. Biomass is currently the most diffused and exploited renewable source all over the world. In 2010 about 75% of primary energy production from renewable sources was produced from biomass and renewable wastes [3]. However, research and planning activities are still required in order to improve overall sustainability and energy conversion efficiency of biomass to energy pathways. The use of wood-fired combined heat and power (CHP) and district heating (DH) systems can play an important role in improving a rational use of bioenergy [4], [5] and [6], when an accurate analysis of both availability of local biomass and thermal demand is performed. CHP plants can reach higher overall efficiencies due to the recovery of the waste heat resulting from electricity generation, even though wood-fired plants often work at lower performances than expected due to not optimal design and operational strategies. For these reasons, a careful design and operation of the plant based on integrated analysis of both thermal demand side evaluation and CHP unit, boilers and HSS performance can allow to enhance the overall efficiency of the system. Furthermore, reduced emissions of pollutants typical from a wood-fired combustion plant (above all particulate matter and nitrogen dioxide) can be obtained limiting part load operation of the plant and with proper operational and control strategies that are beyond the scope of the study performed so far. Several simulation codes have been developed in recent years to support DH planning [7], both in the short-term [8] and the mid-term [9] perspective. Furthermore, some studies have investigated CHP systems coupled with DH [10] as well as CHP performances at partial load [11]. However, there is often a lack of information about the data measured from DH systems during operation and consequently the real behavior can differ from the design hypothesis. This work represents the first stage of a study aiming to simulate the behavior of DH systems and to analyze energetic, economic and environmental aspects with a multi-criteria approach. The model proposed is based on the integration of computed and measured datasets.

The simulations performed show some key features of the operation of a wood biomass ORC unit coupled with a HSS in an existing district heating network. The installation of a HSS can lead to a significant increase of the overall efficiency of the system (up to 8.6%). In the present case study the highest efficiencies are reached for a HSS size of 150 m3/MWth, whereas for higher sizes the HSS provides no additional advantages. The current formulation of the national incentive on electricity production has a strong effect on the system layout and operation, and appears to discourage the more efficient solutions. The high value of electricity leads to a shift of the optimal configuration to a solution with no HSS and a corresponding lower overall efficiency. Moreover, the incentive has an influence also on the operational strategy, making in some cases the full year operation of the system (with heat dissipation during the summer season) more convenient. Finally, the discontinuity of the incentive at 1 MW of electric power induces a significant distortion in the trends. Considering the design of biomass-fired CHP units, energy efficiency and economic profit are still conflicting. Therefore a careful analysis of these two aspects, together with considerations about environmental and social aspects, is crucial in order to reach a global sustainability of the system and of the wood-to-energy pathway.