Energy Saving Potential in Existing Compressors

The Compressed Air Sector (CAS) is responsible for a relevant part of energy consumption, accounting for a mean 10% of the world-wide electricity needs. This ensures about the importance of the CAS issue when sustainability, in terms of energy saving and CO2 emissions reduction, is in question. Since the compressors alone account for a mean 15% of the industry overall electricity consumption, it appears vital to pay attention towards machine performances. The paper deals with compressor technology and it discusses the energy consumptions, on the basis of a comprehensive analysis of data for existing machines, mainly provided by the Compressed Air and Gas Institute (CAGI) for the US scenario, and PNEUROP, on the European compressors market. Data referring to different machine technologies, were processed to obtain consistency with fixed reference pressure levels and organized as a function of main operating parameters. Saving directions for different compressor types, screws & rotary vanes, have been analyzed. Main factors affecting overall efficiency have been split and all different efficiency terms (adiabatic, volumetric, mechanical, electrical, organic) considered separately. This has allowed a term-by-term evaluation of both the margin for improvement and the impact of each term on the “step change” in energy saving, leading to the evaluation of how efforts in the CAS contribute to the 20-20-20 policy emissions reduction targets. If a negligible growth in efficiency is achievable by further increase of volumetric and mechanical terms (few tenths percent), wide margins for improvement come from an upgrade of the transformation, through the adoption of a dual-stage intercooled compression. Its potential has been compared to that of an internal cooling strategy, with a fine oil spray injected within the flow: if the former solution requires the use of dedicated cooling devices between stages, the latter has its main drawback in the presence of cooling medium vapor phase within the flow, leading to a growth in compression work. Since the heat provided by oil cooling is available at a temperature (70-90°C range) that allows the conversion into mechanical energy by means of an Organic Rankine Cycle (efficiency range 8-10%) and considering that the thermal power from the oil and the mechanical power absorbed by the compressor are of the same order of magnitude, energy recovery is interesting as well. This measure, coupled with that of a multi-stage compression, has the potential to overcome the globally shared goals on energy and carbon saving

modeling and characterization of molten carbonate fuel cell for electricity generation and carbon dioxide capture

Abstract The growing electricity request and more severe commitments on emissions led to the research of more and more efficient energy transformation processes. The use of Fuel Cell in order to improve energetic and exergetic efficiency is well-assessed and a number of advanced processes and highly functional materials are currently under investigation, advising a high potential of these systems for the future development of sustainable energy technologies. In particular, the capabilities of integrating high temperature fuel cells within energy conversion systems having medium and high grade thermal sources (flue gases) has resulted in a renewed interest in Molten Carbonate Fuel Cells (MCFC). In fact, they operate at temperatures in the range of 600-700°C and they could be fed by unreformed gas, internally integrating a methane-steam reforming section (direct or indirect). In this paper, the Authors present a theoretical activity finalized to the design and characterization of the integration of a MCFC in a coal fired power plant: a physical model of the fuel cell has been developed, where the energy and chemical processes are represented for the cell stack and geometrical and electrical parameters have been taken into consideration. The model has been applied for system analysis with respect to multiple steady states, sensitivity and stability behavior. Both direct and indirect internal reforming cases have been compared each other, evaluating the energetic and environmental performances of the use of the MCFC as CO2 remover

A New Conversion Section for Parabolic Trough - Concentrated Solar Power (CSP-PT) Plants☆

Abstract One of the most important challenges facing our future is the balance between energy needs and production, in the framework of the CO 2 commitments almost universally adopted. The total energy consumption is still a prerogative of fossil fuels, with a share close to 90%, [1]; renewable energy, apart from the energy production from hydro, photovoltaic, biomass, waste and others, counts 3-4% since many years ago. This discouraging result calls for new conversion technologies based on renewables, if the concept of sustainability is really adopted. Concentrated Solar Power (CSP) plants technology could make the difference with respect to the other renewable technologies, thanks to "bridity"n combining the concentrated solar energy source and the conventional power generation (actually steam turbine plants as energy conversion section). In the sector of energy production, parabolic trough (PT) type is more promising. Recently, the Authors showed in [2,3] how convenient could be the utilization of gases as Heat Transfer Fluid (HTF) with advantages from a technological point of view in the heat collector section and, mainly, from the conversion section point of view, having the possibility to use gas turbines in which the HTF directly expands. In this work, the Authors discuss some thermodynamic and engineering aspects concerning the use of gases as HTF, limiting the attention to air and CO 2 and they further discuss the performances of an innovative gas turbine power plant. It is based on a sequence of compressions and expansions, intercooled and reheated (inside linear solar receivers) respectively, in order to increase cycle specific work and efficiency. The paper focuses the attention on the optimum number of compressions and expansions: when it changes, pressure levels change too, requiring a series of reheating processes which operate in parallel, so increasing the overall solar receiver length and, definitively, investment costs. The optimization has been done adopting as design parameter the power per unit of collector length [kW/m], which is the most sensible parameter defining investment cost

experimental assessment of engine charge air cooling by a refrigeration unit

Abstract Following the increasing awareness on the global warming, international governments have set up severe targets on CO2 emission in transportation sector: the overcoming of these limits produces a fine which directly influences the market value of the vehicle. Moreover, concerning the traditional pollutants, they still remain targeted by future EURO6(b-c-d) limits. Charge air cooling, in turbocharged diesel engines, is widely used to increase air density, improve cylinder filling and, definitively, engine volumetric efficiency. This is usually done through a heat exchanger fed by environmental air positioned after the charge air compression: the cooling is strictly related to vehicle speed, dedicated radiator positioning, engine operating point and environmental temperature. All these factors lead to an in-cylinder intake air temperature in the 40-70°C range. A refrigerating unit, featuring an evaporator, suitably placed inside the intake manifold, could provide an additional cooling. Such an option is not so difficult to be implemented considering that a conditioning unit is already present on board for cabin comfort. This unit often is over-designed and frequently under-employed. In this paper an evaporator was placed on the intake line of a turbocharged diesel engine, available on a test bench, and the effects of the under-cooling of the charge air have been experimentally assessed. The evaporator is fed by an air refrigeration unit present on vehicle board for cabin conditioning. Fuel consumption saving has been observed as well as a sensible pollutants reduction, taking obviously into account the mechanical power required by the compressor

solar thermal based orc power plant for micro cogeneration performance analysis and control strategy

Abstract The paper deals with the performance assessment of a small scale cogeneration system for building applications, featuring an Organic Rankine Cycle-based plant bottoming a solar collector array for combined heat and electricity generation. A sliding vanes rotary expander and a water cooled condenser are employed in the recovery section. A comprehensive MATLAB® model accounts for the dynamic of each component, as both a stand-alone device and a plant-integrated unit: a parametric study is presented and an off-design analysis is performed to properly assess the performances of both the heat exchanger and the expander. Heat availability to the ORC heat exchanger is evaluated, based on solar availability, thermal losses in the pipes and plant requirements, in terms of operating temperature and pressures, having the collection area, the mass flowrate for the fluid in the solar collector branch and the fluid type in the recovery section as main variables. Due to the need for DHW production, a storage unit for hot water is present, upstream the recovery branch: dependently on the ability the fluid at the collector outlet has to meet the ORC requirements for proper operation (about 110°C), the ORC evaporator is fed and the recovery section enabled. Both continuous and unsteady operation underwent an in-depth analysis, as well as the benefits associated with different discharge times for the storage unit: dependently on whether the electrical output or the thermal one need to be maximized, a different control logic for the whole system comes out (e.g. either a flash or a progressive tank discharge). The virtual platform allowed the setting-up of a pilot plant, for direct performance assessment and model validation

Experimental and numerical characterization of a positive displacement vane expander with an auxiliary injection port for an ORC-based power unit

Abstract In the present work a novel technology based on a dual injection vane expander has been introduced. The component works on a power unit fed by the exhaust gases of 3L turbocharged diesel engine. The new device was tested in a wide range of operating conditions and its numerical model was validated on the experimental data. The performances of the new machine were compared to those of the original one. The results showed that the dual injection expander provided an increase of the indicated and mechanical power up to 50% and 30%. Mass flow rate can be increased by 30% and this widens the performances of the power unit; this aspect is particularly suitable for a recovery unit fed by the widely changing exhaust gases flow rates in ICEs

Effects of Oil Warm up Acceleration on the Fuel Consumption of Reciprocating Internal Combustion Engines

Abstract The homologation cycle of vehicles for private passenger transportation or for light duty applications considers a cold start from ambient temperature. The most part of harmful substances (≈ 60-65%) are produced during the thermal engine stabilization which occurs in the very of the driving cycle. This strongly influences also engine efficiency, i.e. fuel consumption. The more recent commitments on CO2, therefore, reinforce the concept of reducing warm up time encountering it in the low carbon engine technologies. Due to this importance, engine thermal management has been the subject of a huge interest opening the way to new components, technologies and control strategies. This regards not only the coolant fluid, which undoubtedly influences engine warm up, but also the lubricant:an its heating acceleration produces much faster benefits.. The purpose of this paper is to assess the effect of a faster oil heating during the homologation cycle on the fuel consumption. An experimental campaign has been done on an 3L Iveco F1C engine mounted on a dynamometer test bench operated in order to reproduce the NEDC. The engine OEM has been characterized and the effect of the oil temperature has been studied according to: (a) an external heat source which brings the oil at its stabilized temperature value before engine start, (b) an internal heat source represented by the exhaust gases which almost immediately reach a temperature value able to heat-up the oil. The effects on CO2 emissions during the cycle have been evaluated. The benefits are noteworthy and justify some oil circuit modifications

csp pt gas plant using air as heat transfer fluid with a packed bed storage section

Abstract Concentrated Solar Power technologies represent an important alternative able to replace in a medium/long term fossil fuel sources. Current technology has several drawbacks which prevent a large diffusion: the principal one is the choice of the Heat Transfer Fluid which involves a certain complexity, including the heat storage section. Conventional plants in operation, consider diathermic oil and, more recently, molten salts. The potential of gases as working fluid has been underestimated till now and its use has not still fully exploited. Using gas would determinate a simpler conversion section increasing reliability. The gas, as proposed by the authors, can expand directly in a series of inter-reheated turbines after a series of intercooled compressions, reaching an acceptable overall global efficiency of the conversion section. The paper describes the optimum choice for the thermodynamic cycle which approaches an Ericsson cycle, integrating it with a comprehensive mathematical model for the heating section of the gas inside the solar receiver. A Thermal Energy Storage section based on the use of a packed bed of rocks has been considered, merged at the plant to insure production continuity. The overall software platform for the plant can be used as design tool in order to set up most important alternatives related to the plant characteristics and specific parameters

Performance Enhancement in Sliding Vane Rotary Compressors through a Sprayed Oil Injection Technology

In Sliding Vane Rotary Compressors, as well as in most of positive displacement machines, the oil is injected to fulfill sealing and lubrication purposes. However, the oil injection could produce an additional effect during the compression phase with a great saving potential from the energetic point of view. Being the air inside the cell at a higher temperature than the oil injected, a cooling effect could be achieved so decreasing the mechanical power required for the compression. At the moment, the oil is introduced inside the compressor vanes through a series of simple holes that are only able to produce solid jets. In this way any effective heat transfer is prevented, as demonstrated by p-V measurements inside the cells during the compression phase. In the current study, a theoretical model of a sprayed oil injection technology was developed and further experimentally validated. The oil was injected along the axial length of the compressor through a number of pressure swirl atomizers which produced a very fine spray. The conservation equations, solved with a Lagrangian approach, allowed to track the droplets evolution from the injection until the impingement onto the metallic surfaces of the vanes. The theoretical approach assessed the cooling effect due to the high surface to volume ratio of the droplets and a reduction of the indicated power was predicted. The model validation was carried out through a test campaign on an mid-size sliding vane compressor equipped with a series of pressure swirl injectors. The reconstruction of the indicator diagram as well as the direct measurements of torque and revolution speed revealed a reduction of the mechanical power absorbed close to 7 % using an injection pressure of 20 bar. The model is in a satisfactory agreement with the tests and it also confirms the experimental trends available in the literature. A parametric analysis on the injection pressure and temperature and on the cone spray angle was eventually carried out in order to identify an optimal set of operating injection parameters

CFD Analysis of an ORC Vane Expander using OpenFOAM Solver

Recent studies on the use of 3D Computational Fluid Dynamics (CFD) for the analysis and design of sliding vane machines has proved beneficial for the detailed evaluation and optimisation of the vane expanders for a given working fluid and operating condition. The authors have earlier developed a customised rotor grid generator for integration with commercial CFD solvers and validated it for use in typical small-scale ORC expanders for waste heat recovery. In this paper, this customised grid generation is extended to an open source CFD solver OpenFOAM, by using a connectivity methodology originally developed for roots blower and twin-screw machines. The control of the rotor grid deformation is through a user code integrated within the flow solver. A case study of the reference ORC expander operating with R245fa was used for validation. The available experimental data for three operating conditions are compared with the results calculated with ANSYS CFX and OpenFOAM-v1912 solvers. During the filling and expansion process, the internal pressure traces are accurately captured by both the solvers and the difference is within 0.05 bar with measurements. However, between the outlet port closure and inlet port opening process the pressure and temperature prediction with OpenFOAM solver is considerably different from the ANSYS CFX solver. It was observed that the OpenFOAM solver is resulting into a non-physical low temperature zone upstream to the tangency region of the rotor and the stator that goes below 80℃. Overall, CFD solution obtained with the commercial solver ANSYS CFX is much more stable and robust than the open source OpenFOAM solver. The generic nature of the deforming grid generation used with an open source CFD solver presented in the paper allows broadening of the utilisation of CFD modelling tools for the design of vane machines