1,721,109 research outputs found
Metal hydride energy systems performance evaluation. Part B: Performance analysis model of dual metal hydride energy systems
No full textA model for performance evaluation of dual metal hydride energy systems is presented. The complete model has been implemented by assembling the dynamic model of heat and mass transfer between two coupled hydrides, previously developed by the author, with the model developed here to describe all feasible cyclic operations occurring in metal hydride energy systems, such as heat pumps, temperature upgrading devices, refrigeration plants, etc. It seems that a reasonable compromise between accuracy and simplicity, suitable for practical use, has been attained: the peculiarity of lumped-parameters models allows a rather fast resolution, so that several parametric analyses may be developed, while the accuracy is ensured by taking into account all major determinants of the dynamic behaviour of the heat and mass transfer process. In this regard, a validation of the heat and mass transfer model has been carried out previously, and this has pointed to a good agreement between calculated and experimental results. Thus, the present model provides a realistic estimation of the overall system performance: cycle time, amount of cyclic transferred hydrogen, input and output thermal power, effective attainable temperature levels, efficiency, etc. Moreover the model may be usefully employed to analyse the influence of the operational parameters, heat transfer and heat exchanger characteristics on the overall system performance in order to improve the design of the present metal hydride energy systems. © 1993
Performances of metal hydride heat pumps operating under dynamic conditions
No full textIn this paper a procedure for the evaluation of the performance of metal hydride heat pumps under dynamic operation is presented. This procedure, based on a limited number of experimental results, allows the evaluation of the performance of coupled hydride beds, operating in heat pump mode (heat "upgrading" mode) or in refrigerant mode, for a wide range of working parameters of the cycle. The initial amount of hydrogen to be charged into the hydrides to obtain the maximum thermal power output may be also evaluated by this procedure. This allows the setting of the experimental apparatus to the most suitable initial conditions avoiding a large number of experimental tests. The procedure developed here has been applied to the LNA/MNA (LaNi4.7Al0.3/MmNi4.5Al0.5) hydrides operating in "upgrading" mode. The results of this application, which are in good agreement with the experimental results obtained by other authors [29-33], are presented and discussed. In particular the results show that the thermal power output attains a maximum value for a definite cycle time and for a definite hydrogen amount initially charged into the hydrides while the heat output and the efficiency always increase by lengthening the cycle time in consequence of the increase of hydrogen transfer. © 1989
Metal hydride energy systems performance evaluation. Part A: Dynamic analysis model of heat and mass transfer
No full textThis paper presents a numerical model to evaluate the dynamic behaviour of heat and mass transfer in metal hydride energy systems. At first, with reference to each metal hydride reactor (i.e. heat exchanger filled with metal hydride), both the equations which describe the hydrogen absorption-desorption and the heat transfer are derived. Then, the "core" of metal hydride energy systems, i.e. coupled metal hydride reactors, has been considered and, by the use of the previous correlations and by further equations taking into account the transient phenomena in the gaseous hydrogen line linking the reactors, the complete numerical modelling has been carried out. This model, founded on a typical thermodynamic approach, is a lumped-parameters model, and thus its numerical resolution is rather easy and fast. The numerical model is able to take into account the fundamental determinants of coupled metal hydride operations: the dynamic of hydrogen transfer between paired metal hydride containers and the dynamic of the heat transfer process in the containers. Thus the model allows us to know and to foresee the effects of operational and design parameters on the mass and heat transfer performance of the metal hydride energy systems. The validation of the model has been carried out by comparison with experimental results obtained by other authors and the results of this validation procedure, which are in good agreement with the experimental ones, are presented and discussed. © 1993
Improving the OTEC power plant performance by metal hydride energy systems
No full textA new system to improve the present OTEC (ocean thermal energy conversion) power plant performance is here presented. This is a metal hydride energy system operating as a “temperature upgrading” device which allows an increase of the OTEC plant working fluid temperature at the turbine inlet. The integrated MHTUP (metal hydride temperature upgrading)-OTEC plant has been investigated, taking into account the dynamic operations of MHTUP system and the OTEC pumping power increase due to the water circulation in the MHTUP system. The results show an increase in the OTEC net power of about 20 percent and the technological feasibility of the proposal. The large amounts of metal hydride and of heat transfer surface required by MHTUP system involve a critical situation from an economical point of view. Then further analysis, particularly regarding the performance optimization and new plant arrangement of the MHTUP system, have to be developed in order to attain the economical feasibility of the proposal. © 1997 by ASME
Solar electricity generation in hybrid thermal power plants
No full textThe EU Directive on renewable sources, Directive 2009/28/EU, requires European countries to increase renewable energy uses until they reach 20% of gross final energy consumption. This target has been distributed among the member states which have put incentives on renewable energy uses and renewable electricity generation.
Among renewable sources, solar energy is one of the most interesting but it presents a key issue: it is a non-dispatchable renewable energy source. For this reason, when solar energy is used to produce electricity, concentrated solar thermal power plants are often integrated in conventional fossil power plants (hybrid power plants); in this case it is very important to distinguish the amount of electricity produced by this renewable source from the one produced by fossil source, since the two heat inputs can contribute to electricity production in different measure. It is possible to elaborate numerical models able to quantify performance of hybrid power plants and to allocate the total electricity for each energy source: for example, assuming a constant fuel consumption and adding solar heat, these numerical simulations allow evaluating the extra electricity generated, that is really the electricity from solar energy. However, such an evaluation is impossible during power plant operation.
This paper presents a simple, but effective, methodology able to distinguish electricity generation of each energy source during hybrid power plants operation: it is necessary to know (by measures) only heat input from fuel and solar energy, initial and final temperatures of working fluid during heat addition and rejection, and the factor of internal losses. This last parameter is very important and this paper not only demonstrates that it doesn't depend on solar energy share but it also proposes its evaluation. Finally the paper compares results of this methodology with those of numerical models here elaborated showing their perfect correspondence in solar electricity evaluation
Hybrid thermal power plants: Solar-electricity and fuel-electricity productions
No full textIn response to global climate target, over the last decade renewable electricity generation from solar energy has grown rapidly and hence measures to deal with its non-dispatchable nature. Integrating concentrating solar plants in conventional fossil power plants is a widely researched solution to tackle solar energy intermittency. This results in hybrid power plants whose total electricity production consists of two different contributions (solar and fuel-electricity) that can be evaluated separately by implementing numerical methods based on the so-called "with and without solar energy" approach. Nonetheless, such evaluation cannot be carried out in actual power plants operation where fossil and renewable contributions are not discernible from each other within the overall production.To overcome this limitation, and consequently allow the total electricity generated to be properly partitioned among energy sources, this study proposes and validates an alternative "on-line and real-time" method to quantitatively assess solar and fuel-electricity by subdividing the overall cycle efficiency into subsequent ones related to the different energy conversion processes during real plant operation. Required input can be derived from available operating data except for factor of internal losses that, however, showing negligible dependence on solar energy and power plant load, can be reasonably assumed constant and estimated in dedicated calculations
A procedure for monitoring and control of condenser losses in steam power stations
No full textIn this paper a method for evaluation of losses caused by condenser efficiency reduction in steam power stations is presented. The procedure allows the evaluation of the effects caused either by the reduction of cooling water flow rate or by decrease of condenser heat exchange (fouling); moreover it allows the evaluation of the consequent variation in the condensing pressure (temperature) and the increased energy losses as heat consumption increase of the thermal process. The method is based on the comparison of the real (measured) data with design data 'corrected' by derating conditions
Repowering of steam power plants for medium-high increase of power generated
No full textThe repowering of steam power plants by overlapped gas turbine units is discussed. The analysis has been performed on a particular arrangement of the two power plants: steam at medium pressure is produced in a proper recovery boiler fed by the exhaust of a gas turbine unit and it expands into the medium-low pressure steam turbine of the pre-existing plant in addition to the main steam flow. This solution involves a simple layout, very few modifications to the pre-existing components and a good compromise between performance and efficiency for medium-high increase of power generated (power ratio GT/ST about 0.3 ÷ 0.5). The thermodynamic effects produced by these operations and the behavior and performance of the repowered plant are analyzed, and the results are presented and discussed
On the effects of heat recoveries discharged into steam power plants
No full textIt is noted that energy saving operations realized at steam power stations connected with industrial plants as well as steam plant repowering involve the discharge of heat recoveries and their interaction with preexisting plant components. The authors present some criteria for the analysis of steam plant performance characteristics and the respective results on the effects produced by these heat recoveries. A parametric analysis was carried out, taking into account the main schemes of steam power systems and layout, size, and design performance. In particular, the influence of the following parameters was analyzed: preexisting layout and size of plant; quantity, quality (i.e., temperature), and location of heat recoveries discharged into the plant; and plant working conditions (nominal and partial load)
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