A general thermodynamic model for cooling devices is derived and applied to absorption chillers. Observing that finite-rate mass transfer dominates irreversibilities in absorption chillers, we derive how chiller coefficient of performance should depend on cooling rate and key system variables. Model predictions are compared against performance data from journal articles, manufacturer catalogue data, and our own experimental measurements, with favourable results.
A comprehensive mathematical model for the simulation of a pressurized circulating fluidized bed combustor will be presented. The model consists of a combustor model describing the combustion chamber, the cyclone and the external heat exchanger as well as of a gas turbine model. The results of the simulation for the combustor at full load and different pressures and for the combined cycle power plant at full and part load are presented in form of temperature-, flue gas composition- and heat transfer-profiles in the combustor. Especially, energy fluxes from the combustor to the water-/steam cycle and the output of gas- and steam-turbine will be shown. The validity of the model will be shown by comparative simulation of an existing plant for the special case of atmospheric conditions.
The Pinch Design Method identifies when stream splitting is necessary in order to obtain a Minimum Energy Requirement network. The method does not provide any guidance regarding which streams should actually be split and what the split stream flowrates should be. The situation is rectified in this work. A number of CP-matrices are developed. These matrices provide for the systematic development of stream split networks.
Process intensification has the potential to change the state of the chemical and process industries. The polymer film compact heat exchanger (PFCHE) is a new type of intensified heat exchanger. The potential market is seen as being large [1] but as yet the unit has not been adopted by industry. The advantageous heat transfer characteristics of such a unit have been shown in previous work [2]. This work investigates the dynamic behaviour of the PFCHE and the process control problems that may arise.The PFCHE available at Newcastle University has been used to generate dynamic temperature data. The data have been employed to formulate and validate time series type models. These models were then used in simulated process control studies. The dynamic behaviour of the unit appeared to be linear and response times were quick. It was found that the responses of the model to disturbances in inlet temperatures could be controlled well using a digital form of PI control. There were, however, doubts as to the ability of the model to completely replicate the system.
In combined power-desalting (plants, high available steam (at high pressure temperature) is expanded first in a steam turbine (and thus produces work) before its extraction (from the turbine) as a heat source to the desalters. The amount of energy consumption charged to the predominantly used multi-stage flash (MSF) desalter in this combined heat and power plant is a question of great concern in the Gulf area. The following are among the methods used to answer this questions (i) the available energy of the heat supplied to the desalter; (ii) work loss from the lower pressure stages of the steam turbine due to steam extracted to the desalter; (iii) energy charged if a separate boiler was used to supply the desalter with its required heat; and (iv) the excess energy supplied to the combined power desalting plant as compared to a single purpose power plant producing the same power output. There would be a different rating method of the power producing process associated with any of the above mentioned charging methods. In this paper, the MSF desalting method and its power consumption are outlined, together with the rating method of the power-desalting plants and the energy charged to the desalter methods. These rating methods are applied to real cases of dual purpose plants working in Kuwait.
An approximate method is presented for calculating the effective length of a flat heat pipe when a strip heater is partially covering the evaporator section. For a specific width of the strip heater, the optimal position to achieve a minimum effective length is found when the heater is placed symmetrically at the centre of the evaporator section. In this position, a higher value of the capillary heat transport limit can be achieved as compared to the case where the heater is placed on one side of the evaporator section.
Ahlstrom Pyropower, Inc. (API) pioneered the development of atmospheric circulating fluidized bed (ACFB) technology. The sustained success with ACFB and continued research in ACFB systems has led Ahlstrom towards the development of pressurized circulating fluidized bed (PCFB) technology. Since the mid-1980s many components of the PCFB combined cycle power plant were developed. Based on the component development experience, a 10 MW(th) PCFB pilot plant was built to support the design of commercial size PCFB units.The PCFB pilot plant has operated over 4500 hours since its commissioning in mid 1989. Five types of coals and five types of sorbents have been tested. Emissions and load following performance of the pilot plant have demonstrted that the PCFB performs significantly better than required under the current stringent environmental standards. The sulfur capture has been 95 to 98%; the NO(x) level with SNCR is less than 25 mg/MJ; the ash generated from the PCFB is less basic than the ash from conventional systems.API, under the U.S. Department of Energy's Clean Coal Technology III program, was awarded a contract to build a nominal 80 MW(e) PCFB combined cycle repowering unit. The unit is scheduled for commissioning in early 1997. API has also performed design studies for larger capacity units in the range of 100 - 400 MW(e). These designs include a second generation PCFB technology combined cycle system for the use of high ash coals. The second generation PCFB technology involves partial gasification of coal and the use of the syngas to heat up the PCFB exhaust flue gas to 1300-degrees-C at the gas turbine inlet. This paper, in addition to presenting the pilot plant results, summarizes the salient features of PCFB combined cycle plants for various fuels including high ash Indian coals.
In a previous work, a packaged commercial heat pump was coupled to a geothermal brine purification system. Subsequently, a new compact heat pump-assisted purification system was farbricated which involved the elimination of two heat exchangersi and a reduction in the amount of tubing; this resulted in higher efficiencies due to lower temperature differences in the heat exchangers. The quality of the distilled water obtained from the geothermal brine was similar to commercially available distilled water with respect to chlorides and silica. A coefficient of performance (COP) of 4.5 was achieved with a brine boiling temperature of 63°C. Higher COPs could be achieved with a higher compressor efficiency. The process shows considerable promise for future development.
The research and development of LMMHD energy conversion (EC) systems which started in the 1960s has already come a long way and is heading towards commercialization. Design and development of such systems has to deal with a number of questions relating to single- and two-phase flows of molten metals, including different patterns of two-phase flow, interphase phenomena, heat transfer, performance of LMMHD components and compatibility of liquid metals with other fluids and with confinement materials. Liquid metal MHD (LMMHD) power conversion systems proposed many years ago are gaining increasing attention in their various proposed modes, consisting of single-phase or two-phase fluid Row for a wide range of heat sources, e.g. solar energy, waste heat, nuclear energy, etc.Liquid metal MHD (LMMHD) power systems have been recently proposed for direct electrical energy conversion of low grade thermal sources of energy, like solar energy. Solar-powered LMMHD power generation systems are very attractive regarding efficiency and cost per unit of installed power. Theoretical and experimental investigations carried out in the various aspects of these systems are presented. A state of the art review of activities in the solar-powered LMMHD power systems field which have taken place so far is described here.
This paper presents a performance analysis of the newly developed Combined Cycle Fully Integrated Air/Air Heat Pump (CFIA). The CFIA System combines the proven efficiencies of heat pump technologies with heat regenerative technologies to bring the most efficient heating/cooling system with capabilities to produce domestic hot water. The CFIA uses environmentally sound refrigerant blends. Extensive laboratory testing showed that the CFIA yielded an impressive coefficient of performance, higher than 2.7 at -15 degrees C outside temperature. Furthermore, field testing revealed an average COP of 2.4 at -13.5 degrees C outside temperature.
A thermodynamic analysis of a combined cycle power plant using pressurized circulating fluidized beds for partial gasification and combustion of coal has been made on the basis of both first law and second law. The Redlich-Kwong equation of state is used for evaluation of properties of air at high pressures in the topping gas turbine plant. A dual pressure steam cycle is considered in the bottoming plant for reducing irreversibility in heat transfer from gas to water and steam. The effects of pressure ratio and peak cycle temperature ratio of the gas cycle and the lower saturation pressure of the steam cycle on the overal performance of the combined plant have been evaluated.
The detailed thermodynamic cycle of the NH3NaSCN absorption refrigeration unit is presented, based on the thermodynamic properties of the working media. Correlations are developed, which express the coefficient of performance and the cooling capacity in terms of the required evaporation temperature, Tev, and the available ambient temperature, Tamb. A nomograph is also presented, which shows in a compact form the behaviour of the NH3NaSCN system and allows direct estimation of its main characteristics. It is concluded that if (Tamb − Tev) varies from 0 to 40°C, the theoretical coefficient of performance decreases linearly from 95 to 77%. For the same range of (Tamb − Tev) the theoretical cooling capacity varies from 1150 to 1300 kJ/kg NH3 if Tev varies from 0 to −15°C. Under the conditions examined, for Tamb − Tev > 23°C, the coefficient of performance of the NH3NaSCN system becomes higher than that of the NH3LiNO3 system. The observed increase reached 4% at Tamb − Tev = 40°C.
The full repowering of an existing fossil fuel steam power plant is investigated. This study aims to introduce and examine the most important parameters, i.e., the design of the heat recovery steam generator (HRSG) and selection of the gas turbine (GT). The repowered cycle is studied in perspectives of energy, exergy, economic, and environmental views. Considering the pressure levels of the HRSG and the number of HRSGs in the new cycle, six different cases are evaluated. The produced power of the original units is 200 MW. The results of this study show that using two 180 MW GT and two HRSGs results the maximum possible thermal efficiency. By using a high-capacity GT (380 MW), the maximum efficiency will be obtained in case of using one HRSG. Moreover, in all cases, the CO2 production rate is reduced. Introducing new nondimensional parameters named x, y, and α, the balance of the axial force of the steam turbine shaft and the capacity of cooling system are considered. The effect of various parameters on these three parameters are analyzed.
Application of two-phase closed thermosiphons to heat recovery systems has led the authors to investigate the performance of thermosiphons at medium temperatures. Two-phase closed thermosiphons working under various conditions have been tested and their thermal performance has been measured for mean evaporator wall temperatures between 100°C and 250°C. A description of the design and construction of the test facility is included. Aspects of safety of container materials have been investigated for water as the working fluid. It was found that copper-nickel alloys and carbon-manganese stainless steel are suitable container materials for the range of temperatures considered. The critical heat flux and dry-out limit were observed in experiments with a 13.2 mm diameter thermosiphon. Boiling heat transfer phenomena and overall thermal conductance have been experimentally investigated. It was found that an increase in thermosiphon diameter changes the boiling mechanism from saturated film boiling to nucleate boiling. The test results show a good agreement with published correlation criteria. Further experiments are needed to determine the optimum pipe diameter for the applications under consideration.
In this study the ratio of the annual cost and the annual energy gain has been calculated for two-pass solar air heaters with single and double covers above the absorber. The cost-benefit ratios of the collectors are examined over a wide range of design and operational parameter (ṁ, L, D1 and D2) and compared with those of single-pass collectors with no cover, a single cover and a double cover, as studied by the authors previously. For shorter duct lengths and lower air mass flow rates, the performance of the two-pass air heater with a single cover is found to be most cost-effective, as compared to the other designs.
The optimization of the design of a zeolite-water adsorption heat pump is presented, using a recently published model. Attention has been focused on the optimization of the energy fluxes between the machine components and the user. A modification of the system is proposed to achieve a constant heat flux from the external heat exchanger. The influence of several parameters, including the global bed heat transfer coefficient, on the performance of the system was analyzed.It was shown that by optimizing the design of the heat pump system, good improvements in performance can be achieved. A constant power of 18.6 kW released during 70% of the cycle time with a COP of 1.4 can be obtained with a two reactor regenerative system using 152 kg of zeolite in total. On the way to more efficient and economic regenerative systems, future research attention should be focused on improving the heat transfer inside the machine.
Forced flow air-cooled heat exchangers (ACHEs) as found in the petro-chemical, process and power industries use fans arranged in single or multiple fan rows to force air over finned tube bundles. Any flow disturbances or distortions experienced at the inlets of these fans tend to reduce the effectiveness of the ACHE.The reduction of the air flow rate through ACHE models having different numbers of fan rows is investigated experimentally. By varying the distance between the fan platform and the ground level, significant changes in air flow rate are observed and the results are correlated by an empirical relation which can be applied to the design of ACHEs. Fans along the edge of an ACHE are most affected by the distorted inlet air flow pattern. Furthermore, the influence of a walkway along the edge of the fan platform on the air flow rate through an ACHE is also investigated.
The exergy efficiency, as well as the charging and discharging rates, in a latent heat storage system can be improved by use of the PCMs having different melting points. The melting point distribution of the PCMs has substantial effects on the exergy efficiency. The optimum melting point distribution of the PCMs has been estimated from numerical simulations and also from simple equations. The fast charging or discharging rate leads to high exergy efficiency.
Combustion turbines and combined cycles have become a dominant mode of new capacity addition in most parts of the world. However, to maximize the benefits of a combined cycle, it must be designed to take into account site-specific technical, economic, and environmental considerations. This paper outlines the important design considerations that must be addressed in the early stages of a project's development.
Thermal storage systems utilising the latent heat of fusion of a phase change material (PCM) are simulated for symmetric-balanced/unbalanced conditions in terms of the conventional thermal effectiveness and newly proposed performance characteristics factors. A unified two-phase model is formulated to accommodate the performance prediction of cyclic thermal regenerators with encapsulated PCM or conventional packing. The effects of flow channelling and transverse thermal condition on the performance of such systems are also discussed.
