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Title: Experimentation on a co-generative system based on a micro-turbine. Authors: Luigi P.M. Colombo, Politecnico di Milano; Fabio Armanasco, CESI S.p.A.; Omar Perego, CESI S.p.A. Extended Abstract. In the last ten years energy technology has been more and more oriented to the so-called distributed generation. This deals with small-scale power generation technologies (typically in the range of 3 to 1,000 kW) located close to the end users to provide an alternative to or an enhancement of the traditional electric power system. It is generally accepted that centralized electric power plants will remain the major source of electric power supply for the future. Generated distribution, however, coupled with heat production, such as in co-generation systems, may further improve cost- effectiveness. On this subject, the lowest size currently available is characterized by an electrical power generation of about 30 kW: these systems are thus suitable both to replace boilers and to satisfy the electrical energy demand of a building. Distributed generation technologies may be classified with respect to the nature of the power generation system: • renewable energy generators; • fossil fuel generators, mainly methane and gas oil. In particular, dealing with the latter category, co-generative plants based on micro-turbines may present total efficiency higher than that of a traditional power system, in spite of its much lower electrical efficiency (usually less than 30%). In this paper an experimental characterization of a micro-co-generative plant will be presented. In the following, both a system description and the experimental protocols developed are summarized. The co-generative plant installed at CESI S.p.A. is mainly composed of a micro-turbine, a gas compressor, a dry cooler, a plate heat exchanger and a control panel. The micro-turbine, a Turbec T100, can produce up to 167 kW heat power, 110 kW electrical power and a maximum temperature of the heated water of 130°C, depending of course on inlet conditions, as temperature and mass flow rate. The water circuit consists of two separable rings: • the plate heat exchanger line, which connects the micro-turbine with the district heating of CESI S.p.A.; • the dry cooler line, which allows the simulation of a variable heat load. Figure 1 shows a simplified scheme of the plant. To perform a complete energetic characterization, the system is equipped of suitable measurement instrumentation integrated in a PLC control system. In particular temperature, pressure and mass flow rates were acquired and monitored respectively in seven, three and two measurement sites by resistance thermometers PT100 (class A), piezo-resistive transducers (class 0.5% std) and electromagnetic flow meters (accuracy ±0.25%). The whole arrangement allows the real-time calculation of the following quantities: combustion heating power, electrical power, co-generative heating power, heat load at the air dryer, electrical efficiency, thermal efficiency, first principle efficiency, second principle efficiency, PES (primary energy saving index) and thermal limit. Moreover the control system enables the definition of different Figure 1. Scheme of the co-generative plant. PUBBLICATO 06006384 (PAD – 749267)PUBBLICATO 06006384 (PAD – 749267)PUBBLICATO 06006384 (PAD – 749267)
operating conditions to simulate heat loads and study the consequent response of the micro-turbine. In particular the following programs are available: • program B (base or security program): it is an automatic procedure which ensures that the dry cooler dissipates all the heat power generated; • program A1: the electrical power produced and the water temperature at the outlet of the micro- turbine are set manually by an operator; all the heat is dissipated; • program A2: the heat load to be dissipated and the water temperature at the outlet of the micro- turbine are set manually by an operator: consequently, the micro-turbine is forced to regulate the electrical power to follow the required heat generation. We may distinguish the experimentation as follows: • tests to follow the electrical production, whose aim is the energetic characterization of the plant; • tests to follow the thermal demand, oriented to simulate variable thermal loads. The operative procedures A1 and A2, discussed above, were respectively used to run these tests, which will be briefly summarized here. During tests of the first type the electrical power generated was varied in the range 50 – 110 kW, with 10 kW stepping. For each step, the set point of the temperature at the outlet of the micro- turbine was varied in the range 60 – 80°C, with 5°C stepping; values lower than 60°C are unsuitable as thermal demand. All quantities listed above were evaluated and reported in diagrams as functions of the electric power, parameterized by the temperature outlet. As an example in Figure 2 we show the plot concerning the primary energy saving index (PES), calculated from the definition given by the Italian Energy and Gas Authority, which sets as reference parameters 0.38 for electrical efficiency for this size of plant using natural gas and 0.80 for thermal efficiency in civil applications: ( ) 1 thel 80.038.01PES − η+η−= . It is seen that the plant tested with the hypothesis of total heat recovering allows a primary energy saving index ranging between 18% and 30%, for the electrical power production varying from 50 to 110 kW. The thermal limit, defined as the ratio of thermal power to total power, ranges between about 59% and 65%. It is worth noting that the values of both parameters are much greater than those indicated by the Italian law for a power plant to be considered as co-generative, i.e. 10% and 15% respectively. We are currently performing tests of the second type, varying the set-point temperature at the outlet of the micro-turbine in the range 55 – 85°C, with 5°C stepping. For each step, the heating power dissipated is varied in the range 80 – 160 kW. In this case the turbine has to regulate its power generation in order to meet the heat demand. Furthermore, environmental impact has been investigated with respect to gaseous and acoustic emissions. In particular NO x , SO 2 , CO, CO 2 concentrations in the exhaust gases were measured at different regimes of the electrical power output. Nevertheless it should be noted that the Italian law considers a co-generative plant, such as installed, as not having a relevant environmental impact; so no limitations on emission hold. 15% 20% 25% 30% 35% 40 50 60 70 80 90 100 110 120 Electrical power [kW] PES [%] 60 °C 65 °C 70 °C 75 °C 80 °C Figure 2. Energy saving index versus electrical power production.
31 Dicembre 2006
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