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20 Antonino La Rocca et al. plant (e.g., saving of fossil energy resources, reduced greenhouse gas emission, etc.). As far as it pertains to regasification, considering prudentially the whole cost of production only attributable to electricity production, regasification costs are included in a low range because, actually, only the incidence of regasification of LNG using the OR back-up system during the period of its operation is to be considered apart. Considering the two different kind of innovative modular CHP plants, operating respectively with Helium or with Nitrogen, for the same quantity of LNG regasified (requiring a thermal energy of 740…750 kJ/kg of LNG) a modular plant working with Helium produces an amount of electric energy of 0.38 kWhe/kg of LNG regasified while a modular plant working with Nitrogen produces 0.29 kWhe/kg of LNG regasified (24% less). Cryogenic heat exchangers for regasification of LNG have the same nominal duty (45 MWt) and their cost is almost the same (5% less for Nitrogen plants, note that Nitrogen works at higher pressures and its molecular weight is higher than that of Helium, but this last has better thermo-physical properties and the mass flow rate required is less: 0.5 kgHe/kgLNG; while for Nitrogen 1.8 kgN2/kgLNG). The duty of a compressor using Nitrogen gas is less than 55%, compared with that using Helium, and a gas turbine using Nitrogen gas has a duty less than 44% compared with that using Helium. As a result of a thorough feasibility study, done by the Authors and pertaining to conceptual design of machines to be used in CHP plants working with Helium or with Nitrogen), on the contrary, it results that CHP modular plants working with Helium, for the same quantity of LNG regasified, can produce 24% more electric energy. OR technology for a nominal regasification capacity of 2040 MStm 3 /y (287 days/y, 24 h/day; 2.6 months/y not in operation) requires 11.728 GWh/y of electric energy for pumping sea water (it is considered a mean decrease of 8 °C of sea water temperature from inlet in OR to exit) and for auxiliary services of facility. The CHP plants proposed when working both with Helium or Nitrogen have a mean electric production efficiency ηe = 0.43 (a little more when Helium is used). Note that in Italy, now, combined cycles of electric utilities (top gas turbines and bottom steam turbines) have been built using modern equipment and have a mean electric production efficiency of 52%. But, modular CHP plants proposed for LNG regasification based on Brayton cycles working with inert gases allow a saving of 11.728 GWh/y (equivalent electric power need of 7.7MWe for pumping sea water when OR technology is used). Thus, to have a figure for comparison of order of magnitude, a mean overall electric efficiency can be accounted for 49% working with Helium and 47% working with Nitrogen. Considering the whole amount of electric utilities operating in Italy, penalty paid results in 3% working with Helium and 5% working with Nitrogen.
Bul. Inst. Polit. Iaşi, t. LVIII (LXII), f. 4, 2012 21 4.4. Thermodynamic Analysis Thermodynamic analysis is based on a careful numerical simulation which has been performed with a software developed at DREAM, using as a main tool the EES, Engineering Equation Solver, of S.A. Klein and A. Alvarado, Professional version 6.567. The results obtained by exergetic analysis are presented separately for the plant working with Helium and the plant working with Nitrogen. The regasification facility is a CHP (cogeneration system), which has the aim to produce electric power and heat to regasify LNG. It can be defined a process efficiency based on the first thermodynamic principle (energetic efficiency) and a process efficiency based on the second thermodynamic principle (exergetic efficiency) by two different point of view. 4.4.1. Energetic Efficiency: (a) First definition: The whole facility (see Fig. 9) can be considered a CHP system producing electric energy and heat. Then, the input energy is represented by heat furnished in the top cycle burning natural gas in the top turbine gas generator - CC. The outputs are: electric power generated in the top and bottom Brayton cycles and heat supplied to LNG which regasifies in cryogenic regasifiers. This approach allows to define the first thermodynamic principle energetic efficiency with η NHP Peltop + Pelbottom + Qreg = . (6) Q NGburned (b) Second definition: Heat furnished in top cycle, the energy of cold furnished by LNG to be regasified and the pumping power of LNG stream in liquid phase, which regasifies at hypercritical pressures, are considered as input items. As output items they can be considered: the electric power generated in the top and bottom Brayton cycles and the exergy available in the NG regasified at a pressure suitable for direct transfer in pipeline network. So, a COP can be defined as a first thermodynamic principle energetic efficiency parameter with COP CHP Pel + Pel + Ex = Q + Q + W top bottom NG NGburned OLNG pLNG The parameters defined by the relationships of Eqs.(6) and (7) for the CHP plants analysed are: ηCHP COPCHP Working fluid in bottom cycle: Helium 0.69 0.51 Nitrogen 0.72 0.64 4.4.2. Exergetic Efficiency. (a) First definition: This kind of analysis (second thermodynamic principle) considers the plant as a CHP plant . (7)
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