Costante Mario Invernizzi

THE POTENTIAL ROLE OF ORGANIC BOTTOMING RANKINE CYCLES IN STEAM POWER STATIONS

Abstract Firstly it is observed that in a large number of existing steam power plants the energy potential of the cooling medium is not fully utilized owing to turbine limitation in exhaust volume flow-handling capability. A method is proposed by which a fraction of the low-pressure steam is extracted and fed to an auxiliary organic Rankine cycle (ORC) module of small capacity which, besides being perfectly suited to exploiting even the coldest cooling agent, improves the working conditions of the main turbine by reducing its exhaust volume flow. Through the implementation of an appropriate computer program the performance of a typical power station supplemented with an ORC system is analysed for different cooling situations. Alternatively, as an obvious reference option, the performance of the same plant is evaluated under the assumption that the turbine is provided with an additional exhaust section. The characteristics of the ORC module are then considered. Working fluid selection within the new classes of ambient friendly refrigerants is discussed. Particular attention is devoted to turbine optimization, leading to high-efficiency low-stress two- and three-stage turbine configurations. With reference to the extended geothermal experience in the use of low-temperature ORC conversion systems a preliminary economic analysis is performed, giving encouraging indications about the potential viability of the proposed method.

Prospects for real-gas reversed Brayton cycle heat pumps

Abstract Ideal-gas reversed Brayton cycles are shown to be intrinsically inefficient owing to the high level of turbomachinery losses. An appropriate selection of the cycle operating parameters leading to the location of the expansion process in the vicinity of the critical point, where specific volumes and turbine works are small, allows the design of regenerated gas cycles with efficiencies similar to those of conventional vapour compression cycles, at least in the generation of high-temperature heat. A number of working fluids are presented (both pure substances and mixtures) yielding a good conversion efficiency at various source/sink temperatures. Basic optimization rules are given for fluids of different molecular structure. Fluids with a simple molecule (Xe, CO 2 etc.) tend to produce heat at very high temperatures and with an excessive temperature change: compression staging is effective in correcting this trend. Moderate pressure ratios (2 to 4) are sufficient to yield a good cycle efficiency; however, operating pressures are intrinsically high, since a minimum pressure around p cr is in general requested. The main features of the real-gas heat pump cycle can be summarized as the large power density, the ability to operate at high temperature with a small pressure ratio, and non-isothermal heat generation. Whenever such characteristics are of particular value, as, for example, in the production of heat for a long-distance conveyance, as needed for urban heating systems or for industrial heat networks, the real-gas reversed Brayton cycle should be examined as a possible alternative to conventional heat pump cycles.

Supercritical heat pump cycles

Abstract Supercritical heat-pump cycles suited for high-temperature heat generation and in which heat is delivered in the form of sensible heat of a high-pressure fluid are examined and their energy performance is evaluated. The main variables governing the energy efficiency of the process and the temperatures of the heat produced are recognized to be the fluid critical temperature, the molecular complexity, the top cycle pressure and the amount of internal regeneration of heat. Two cycle configurations are examined: one featuring fluid compression after a regenerative preheating and one that also includes turbine expansion of a fraction of the high-pressure fluid in order to achieve a more effective regeneration. General diagrams giving the operating characteristics of a supercritical heat-pump cycle for any kind of fluid are reported. Some fluids are presented (SF 6 , C 3 F 8 , C 2 HF 5 , c -C 4 F 8 ), which exhibit a high level of thermal stability and are thermodynamically suitable for supercritical cycles: for each one a detailed performance chart is given. An example application in which a conventional high-temperature cycle is compared with two supercritical solutions is presented. The following conclusions summarize the findings of the thermodynamic analysis. (1) In supercritical cycles high heat-output temperatures are achievable with moderate compressor pressure ratios and with a comparatively simple cycle arrangement, while conventional cycles require a large pressure ratio and a complex cycle organization. Sub-atmospheric pressures, which may be required in conventional cycles, can be avoided. (2) As heat is available in supercritical cycles within a certain temperature range, applications implying the use of heat at variable temperature could benefit from the natural matching between temperature availability and process requirements. (3) The comparatively high pressures at which heat is produced in supercritical cycles could represent a drawback for small-capacity plants but are probably acceptable or even beneficial for large systems. (4) The internal regeneration of a sizeable amount of heat, which is requested in supercritical cycles, represents a definite cost item for this type of heat pump.

Potential performance of real gas Stirling cycle heat pumps

Abstract Heat pumps based on the reversed Stirling cycle are shown to be positively influenced by real gas effects, provided they are designed to operate in a proper region of the fluid state diagram. A simplified model of a Stirling heat pump, aimed at understanding the basic cycle thermodynamics is presented, which allows a first optimization of real gas cycles. Provided the expansion process takes place in a proper narrow region close to the critical point, efficiencies much higher than those achievable with an ideal gas and similar to those of vaporization-compression cycles are obtained. A number of zero ODP, safe fluids are considered (Xe, CHF3, C2F6, CHF3 + CF4 mixtures) allowing optimum operation in a wide range of heat source and heat production temperatures. Only mixtures, however, are recognized to permit a fine adjustment of the fluid properties to the heat source characteristics and to the users temperature requirements. In order to reach good energy performance, high-pressure operation (around 200 bar) and an efficient internal regeneration of heat are needed. Graphs are supplied that reveal the heat pump cycle performance for each fluid at a wide range of temperatures, pressures and cycle compression volume ratios. Loss analysis shows that fluids having a simple molecule yield the best efficiency and the minimum amount of heat regeneration. Stirling power cycles are also shown to benefit from real gas effects, with the result that at top temperatures around 400–450°C, which are probably acceptable for a number of organic fluids, a fuel to work conversion efficiency around 25–30% seems possible for a cogenerative prime mover. The performance of such motors, intended for heat pump drives, are given for C2HF5 and C3F8 fluids. Very high pressures are required to optimize the cycle performance. Preliminary information on the prospective characteristics of a fuel powered Stirling-Stirling low-grade heat generator is given.

Real gas Brayton cycles for organic working fluids

Abstract Organizing a closed Brayton cycle in such a way that the compression process is performed in the vicinity of the critical point where specific volumes are a fraction of those of an ideal gas yields performance indices particularly attractive, mainly at moderate top temperatures. Cycle thermodynamic analysis requires the development of adequate methods for the computation of thermodynamic properties above the vapour saturation curve about the critical point. Working fluids suitable for the proposed cycle can be found in the class of organics, in particular among the newly developed, zero ozone depletion potential, chlorine-free compounds. The numerous technical and environmental requirements which a fluid must meet for practical use combined with the peculiar thermodynamic restraints limit the number of suitable fluids. Mixing two substances of different critical temperatures yields an indefinite number of fluids with tailor-made thermodynamic properties. One such mixture 0.93 HFC23 + 0.07 HFC125 (molar fraction), having tcr = 30°C, at tmax = 400°C, pmax = 150 bar, gives an efficiency above 27 per cent with heat rejection temperatures between 89 and 33°C. With a different mixture composition with a 50°C critical temperature, at the same tmax and pmax, an efficiency of 25.1 per cent is attained in a combined heat and power generation cycle with heat available in the range 53-103°C. An experimental programme to test the thermal stability of organic fluids showed that top temperatures of 380-450°C are achievable with some commercially available fluoro-substituted hydrocarbons. In view of practical applications a conversion unit based on a reciprocating engine could handle without problems the pressures and temperatures involved. The use of turbomachinery would lead to power plant of large capacity for the usual rotor dimensions or to micro-turbines at high rotating speed in the low power range.

Thermodynamic performance of selected HCFS for geothermal applications

The thermodynamic efficiencies of selected HFCs (hydrofluorocarbons) have been investigated for applications in binary geothermal conversion systems. Following examination of thermodynamic cycles, interactions with sensible heat sources were considered. For every working fluid, there is a temperature range for optimal use. Each fluid is best applied with a heat source at a temperature somewhat above the critical fluid temperature.

Supercritical and real gas Brayton cycles operating with mixtures of carbon dioxide and hydrocarbons

Closed Brayton gas and supercritical cycles operating with mixtures of carbon dioxide and hydrocarbons, in particular mixture with a low content (5–15%) of benzene, were studied. Totally, supercritical cycles and condensation cycles were looked at and a comparison was made with pure carbon dioxide cycles with a minimal temperature around 40–50 °C (typical minimum temperatures of air cooled radiators). First and second law cycle efficiencies were considered and analysed. Critical point calculations of several mixtures were performed by means of an accurate model for the thermodynamic properties and compared with experimental data from literature. For the cycle calculations, a simpler model with classical mixing rules was used because the results were in sufficient agreement. The cycles operating with mixtures showed lower maximum pressures and higher cycle efficiencies compared to the pure carbon dioxide cycles. Taken into account, the high global warming potential of fluorinated fluids and the high flammability and high volume expansion ratios of comparable Rankine toluene cycles, mixtures of carbon dioxides and hydrocarbons exposed promising features.

Performance Analysis of OTEC Plants With Multilevel Organic Rankine Cycle and Solar Hybridization

Among the renewable energy sources, ocean energy is encountering an increasing interest. Several technologies can be applied in order to convert the ocean energy into electric power: among these, ocean thermal energy conversion (OTEC) is an interesting technology in the equatorial and tropical belt, where the temperature difference between surface warm water and deep cold water allows one to implement a power cycle. Although the idea is very old (it was first proposed in the late nineteenth century), no commercial plant has ever been built. Nevertheless, a large number of studies are being conducted at the present time, and several prototypes are under construction. A few studies concern hybrid solar-ocean energy plants: in this case, the ocean thermal gradient, which is usually comprised in the range 20–25 °C in the favorable belt, can be increased during daytime, thanks to the solar contribution. This paper addresses topics that are crucial in order to make OTEC viable, and some technical solutions are suggested and evaluated. The closed cycle option is selected and implemented by means of an organic Rankine cycle (ORC) power plant, featuring multiple ORC modules in series on the warm water flow; with a three-level cycle, the performance is approximately 30% better if compared to the single-level cycle. In addition, the hybrid solar-OTEC plant is considered in order to investigate the obtainable performance during both day and night operation; this option could provide efficiency benefit, allowing one to almost triplicate the energy produced during daytime for the same prescribed water flow.

General method for the evaluation of complex heat pump cycles

Abstract Whenever the fractional temperature lift ΔT/Tc of a heat pump is ⪆0.15 , simple cycles with one-stage throttling exhibit unsatisfactory energy performance. The adoption of multi-stage throttling, both in non-regenerative and regenerative cycles, is the most direct way of improving the cycle coefficient of performance (COP). The performance of these complex cycles is found to be a function of the molecular complexity of the working fluid, the reduced evaporation temperature, the fractional temperature lift and the number of stages of throttling. Furthermore, complex cycles are shown to be equivalent to a combination of simple cycles and their performance may be directly inferred by this route. Such calculations show that for a given fractional temperature lift an optimum molecular complexity (between that of R12 and n-butane) exists. Fluids with simpler molecules exhibit excessive vapour superheating during compression, and those with more complex molecules have excessive throttling losses. Also, with complex cycles, regeneration should be applied only to the cycle at the lowest temperature in order to improve the cycle COP and to prevent condensation during compression. As a general trend, however, complex cycles suffer a significant loss in performance compared to optimized simple cycles due to the adverse area of the two-phase diagram in which they work.

Binary and ternary liquid metal—steam cycles for high-efficiency coal power stations:

Abstract Since in the use of coal the direct recourse to combined cycles is impractical, binary alkali metal steam cycles are recognized as an interesting and feasible option. Past attempts to employ metal vapour conversion cycles for power generation are surveyed. After selecting potassium and cesium as possible candidate fluids, the binary cycle is optimized taking as variables the top temperature, the number of condensation levels of the metal vapour cycle, and the characteristics of the bottoming steam cycle. At vaporization temperatures in the range of 750—850 °C, metal vapour cycle efficiencies of about 20—24 per cent and binary cycle efficiencies of 57—61 per cent seem achievable. A survey of available building materials in the steel and in the super-alloy class showed that top temperatures of 800–850 °C could be reached with state-of-the-art alloys. Metal vapour turbines are recognized as a key issue of binary plant design in that exhaust volume flows are very large even for a moderate turbine capacity. For a double flow solution, limiting turbine dimensions to those of existing 1500 r/min steam low pressure stages leads to metal vapour turbine capacity of 120 MW for potassium and 170 MW for cesium. Assuming that in the future, better materials will be available allowing alkali metal vaporization temperatures in the range of 1400–1500 °C, a ternary solution is proposed which employs lithium, potassium, and steam as working fluids. At 1450 °C top temperature, a cycle efficiency in excess of 70 per cent is attained.