Performance enhancement of conventional combined cycle power plant by inlet air cooling,inter-cooling and LNG cold energy utilization

This paper has proposed an integrated advanced thermal power system to improve the performance of the conventional combined cycle power plant. Both inlet air cooling and inter-cooling are utilized within the proposed system to limit the decrease of the air mass flow contained in the given volume flow as well as reduce the compression power required. The latent heat of spent steam from a steam turbine and the heat extracted from the air during the compression process are used to heat liquefied natural gas (LNG) and generate electrical energy. The conventional combined cycle and the proposed power system are simulated using the commercial process simulation package IPSEpro. A parametric analysis has been performed for the proposed power system to evaluate the effects of several key factors on the performance. The results show that the net electrical efficiency and the overall work output of the proposed combined cycle can be increased by 2.8% and 76.8 MW above those of the conventional combined cycle while delivering 75.8 kg s^?1 of natural gas and saving 0.9 MW of electrical power by removing the need for sea water pumps used hitherto. Compared with the conventional combined cycle, the proposed power system performance has little sensitivity to ambient temperature changes and shows good off-design performance.     

Solid/vapour sorption heat transformer: Design and performance

A high temperature high lift solid sorption based heat transformer has been successfully designed and tested. The sorption reactor concept is based on a tube-fin heat exchanger where the heat exchanging fluids can flow through the hollow fins. The plates were brazed together with porous metal foam that was impregnated with either of the sorbents, LiCl and MgCl₂. The adsorbate is ammonia. The batch system was tested as to the power delivered at high temperatures, 150–200 °C. Peak power at 200 °C was about 0.8 kW, the average power about 0.4 kW. The thermal efficiency, COP, was calculated from the experimental results to be 0.11. This is only 40% of the expected theoretical value and can largely be attributed to the thermal mass of the reactor.     

Characterization of melting and solidification in a real-scale PCM–air heat exchanger: Experimental results and empirical model

This paper describes the experimental studies carried out to test thermal cycling of a real-scale PCM–air heat exchanger at ambient temperatures. To achieve this goal an experimental setup previously designed and used for testing real-scale prototypes of PCM–air heat exchangers is modified. The PCM used is commercially available, organic, and paraffin based. The total energy exchanged during melting and solidification, as well as the time elapsed until total melting/solidification are determined from the power curves experimentally obtained. The influence of the inlet air temperature and air flow is studied, and results show that the continuous thermal cycling of the unit is a repetitive process: running experiments with similar conditions leads to the same thermal behavior, no degradation in the PCM properties is noticed. Pressure drop is measured for different air flows. Depending on the inlet air temperature, full solidification of the PCM could be achieved in less than 3 h for an 8 °C temperature difference between the inlet air and the average phase change of the PCM. Average thermal powers of up to 4.5 kW and 3.5 kW for 1 h are obtained for melting and solidification stages, respectively. An empirical model is developed from the experimental results, which could be a useful designing tool for applications that use such technology: green housing, curing and drying processes, plant production, HVAC, and free-cooling.     

The externally-fired gas-turbine (EFGT-Cycle) for decentralized use of biomass

The externally-fired gas turbine unites two advantages. On the one hand, the utilisation of the waste heat from the turbine in a recuperative process and, on the other, the possibility of burning “dirty” fuel. In particular, the EFGT opens a new option to utilise biomass for combined-heat-and-power and contributes to reduce greenhouse-gas emissions. A micro-gas turbine with 100 kW electric-output is chosen as an example to study the effects of temperature difference and pressure loss in the gas-to-air heat exchanger on cycle efficiency and power. The simulation calculations are performed with the code AspenPlus. In addition to cycle optimisation, the effect of low-calorific biogas on the combustion air ratio and the possibility of solar energy as a heat source for the EFGT are studied. For combusting biomass in an EFGT-Cycle, two alternatives are possible: First, a special, well-designed combustor for solid biomass, with a cyclone to reduce particles in the exhaust gas. Secondly, a gasifier with gas cleaning and a standard gas-burner. Waste heat from the process can be used for the gasification process, especially for drying and preheating the biomass. The detailed results are presented in the [Kautz M. Auslegung von extern gefeuerten Gasturbinen für dezentrale Energieanlagen im kleinen Leistungsbereich. Dissertation, Universität Rostock, Fakultät für Maschinenbau und Schiffstechnik; 4.11.2005.].     

Potential of geothermal heat exchangers for office building climatisation

Low depth geothermal heat exchangers can be efficiently used as a heat sink for building energy produced during summer. If annual average ambient temperatures are low enough, direct cooling of a building is possible. Alternatively the heat exchangers can replace cooling towers in combination with active cooling systems. In the current work, the performance of vertical and horizontal geothermal heat exchangers implemented in two office building climatisation projects is evaluated.A main result of the performance analysis is that the ground coupled heat exchangers have good coefficients of performance ranging from 13 to 20 as average annual ratios of cold produced to electricity used. Best performance is reached, if the ground cooling system is used to cool down high temperature ambient air. The maximum heat dissipation per meter of ground heat exchanger measured was lower than planned and varied between 8 W m^?1 for the low depth horizontal heat exchangers up to 25 W m^?1 for the vertical heat exchangers.The experimental results were used to validate a numerical simulation model, which was then used to study the influence of soil parameters and inlet temperatures to the ground heat exchangers. The power dissipation varies by ±30% depending on the soil conductivity. The heat conductivity of vertical tube filling material influences performance by another ±30% for different materials. Depending on the inlet temperature level to the ground heat exchanger, the dissipated power increases from 2 W m^?1 for direct cooling applications at 20 °C up to 52 W m^?1 for cooling tower substitutions at 40 °C. This directly influences the cooling costs, which vary between 0.12 and 2.8€ kW h^?1.As a result of the work, planning and operation recommendations for the optimal choice of ground coupled heat exchangers for office building cooling can be given.     

Analysis of effectiveness and pressure drop in micro cross-flow heat exchanger

A theoretical model that predicts the thermal and fluidic characteristics of a micro cross-flow heat exchanger is developed in this study. The theoretical model is validated by comparing the theoretical solutions with the experimental data from the relative literature. This model describes the interactive effect between the effectiveness and pressure drop in the micro heat exchanger. The analytical results show that the average temperature of the hot and cold side flow significantly affects the heat transfer rate and the pressure drop at the same effectiveness. Different effectiveness has a great influence upon the heat transfer rate and pressure drop. When the micro heat exchanger material is changed from silicon to copper, the thermal conductivity changes from 148 to 400 W/m K. The heat exchanger efficiency is also similar. Therefore, the (1 1 0) orientation silicon based micro heat exchanger made using the MEMS fabrication process is feasible and efficient. Furthermore, the dimensions effect has a great influence upon the relationship between the heat transfer rate and pressure drop. Therefore, the methodology presented in this paper can be used to design a micro cross-flow heat exchanger.     

Synthesis of multistream heat exchangers by thermally linked two-stream modules

A simple yet very efficient algorithm has been devised for the analysis of multistream heat exchangers. In this approach, an n-stream heat exchanger is considered as a stack of (n ? 1) two-stream exchangers separated by diabatic partitions. Starting from the analysis of the basic two-stream units the entire heat exchanger can be designed non-iteratively. The validity of the algorithm has been established through a number of examples, comprising of two-steam exchanger with external heat leakage, three-stream and four-stream exchangers. Excellent agreements with experimental results and published simulation have been observed. Finally the most crucial assumption made in the present work has been assessed critically and its justification has been provided.     

Air heat exchangers with long heat pipes: Experiments and predictions

This paper presents measurements and predictions of a heat pipe-equipped heat exchanger with two filling ratios of R134a, 19% and 59%. The length of the heat pipe, or rather thermosyphon, is long (1.5 m) as compared to its diameter (16 mm). The airflow rate varied from 0.4 to 2.0 kg/s. The temperatures at the evaporator side of the heat pipe varied from 40 to 70 °C and at the condenser part from 20 to 50 °C. The measured performance of the heat pipe has been compared with predictions of two pool boiling models and two filmwise condensation models. A good agreement is found. This study demonstrates that a heat pipe equipped heat exchanger is a good alternative for air–air exchangers in process conditions when air–water cooling is impossible, typically in warmer countries.     

Single-phase heat transfer in the high temperature multiple porous insulation

An experimental study of steady state flow and heat transfer has been conducted for the multiple plate porous insulation used in the reactor pressure vessels of ‘Magnox’ nuclear power stations. The insulation pack studied, consisting of seven dimpled stainless steel sheets and six plane stainless steel sheets, was of the type installed in the Sizewell A plant. A large scale experimental test facility, based on the guarded hot plate method, was used for measuring the effective thermal conductivity of Magnox reactor pressure vessel insulation, which consists of alternate layers of plain steel foil and dimpled foil. The measurements were made both with the fluid within the insulation pack nominally stationary and with an imposed flow through it, simulating leakage through the insulation pack. The experimental conditions corresponded to a heat flux of 75–1000 W/m², fluid pressures of atmospheric to 5 bar gauge, pack orientations in range of 0°–45° relative to the horizontal, leakage velocities ranging from 0.05 m/s to 0.20 m/s and inlet air bulk temperatures ranging from 18 °C to 290 °C. Local values of effective thermal conductivity of 0.04–0.23 W/m K were obtained for the above experimental conditions. The heat transfer modes in the insulation pack were conduction through the contacting metallic foils, thermal radiation across the gas gaps, and conduction and convection in the air. The effective thermal conductivity of the porous insulation increased with increasing air pressure, inclination angle, and air velocity. Buoyancy effects increased with increasing inclination angle and air pressure.     

Experimental and computational evolution of a shell and tube heat exchanger as a PCM thermal storage system

A combined experimental and numerical study has been designed to study thermal behavior and heat transfer characteristics of Paraffin RT50 as a phase change material (PCM) during constrained melting and solidification processes inside a shell and tube heat exchanger. A series of experiments are conducted to investigate the effects of increasing the inlet temperature of the heat transfer fluid (HTF) on the charging and discharging processes of the PCM. The computations are based on an iterative, finite-volume numerical procedure that incorporates a single-domain enthalpy formulation for simulation of the phase change phenomenon. The molten front at various times of process has been studied through a numerical simulation. The experimental results show that by increasing the inlet HTF temperature from T_H = 70 °C to 75 and 80 °C, theoretical efficiency in charging and discharging processes rises from 81.1% to 88.4% and from 79.7% to 81.4% respectively.     

Enhanced heat exchanger design for hydrogen storage using high-pressure metal hydride – Part 2. Experimental results

Hydrogen storage systems utilizing high-pressure metal hydrides (HPMHs) require a highly effective heat exchanger to remove the large amounts of heat released once the hydrogen is charged into the system. Aside from removing the heat, the heat exchanger must be able to accomplish this task in an acceptably short period of time. A near-term target for this ‘fill-time’ is less than 5 min. In this two-part study, a new class of heat exchangers is proposed for automobile hydrogen storage systems. The first part discussed the design methodology and a 2-D computational model that was constructed to explore the thermal and kinetic behavior of the metal hydride. This paper discusses the experimental setup and testing of a prototype heat exchanger using Ti_1.1CrMn as HPMH storage material. Tests were performed to examine the influence of pressurization profile, coolant flow rate and coolant temperature on metal hydride temperature and reaction rate. The experimental data are compared with predictions of the 2-D model to validate the model, calculate reaction progress and determine fill time. The prototype heat exchanger successfully achieved a fill time of 4 min 40 s with a combination of fast pressurization and low coolant temperature. A parameter termed non-dimensional conductance (NDC) is shown to be an effective tool in designing HPMH heat exchangers and estimating fill times achievable with a particular design.     

Multifunctional heat exchangers derived from three-dimensional micro-lattice structures

A micro-scale cross-flow heat exchanger is constructed from a hollow nickel micro-lattice structure, which is fabricated by conformally electroplating nickel onto a sacrificial polymer micro-lattice formed from self-propagating photopolymer waveguides. The periodic unit cell of the hollow nickel micro-lattice structure tested here includes lattice members with a diameter <1 mm and a nominal pore size <9 mm. The heat transfer performance of the micro-lattice-based heat exchanger is analyzed in terms of thermal conductance per unit volume, which is equal to the value of overall heat transfer coefficient multiplied by surface area to volume ratio. Calculated values range from 0.84 to 1.58 W/cm³K for Reynolds number ranges of between 3400 ± 200 and 6500 ± 500 for hot water flow inside the hollow lattice members and 85 ± 6 and 240 ± 20 for cold water flow around the lattice members. Based on a developed correlation, the experimental heat transfer data is used to predict the thermal performance of larger and smaller micro-lattice-based heat exchangers, as well as various micro-lattice feature dimensions that are tunable with the fabrication process (node-to-node spacing, inner diameter, etc.). The micro-lattice heat exchanger was tested under quasi-static compression and the results illustrate the multifunctional capability for load bearing and energy absorption applications. This work demonstrates a multifunctional heat exchanger with a fully-scalable fabrication process which is useful for size and weight constrained heat transfer applications, including those in the automotive and aerospace industries.     

Thermal performance of a novel rooftop solar micro-concentrating collector

Concentrating solar thermal systems offer a promising method for large scale solar energy collection. Although concentrating collectors are generally thought of as large-scale stand-alone systems, there is a huge opportunity to use novel concentrating solar thermal systems for rooftop applications such as domestic hot water, industrial process heat and solar air conditioning for commercial, industrial and institutional buildings. This paper describes the thermal performance of a new low-cost solar thermal micro-concentrating collector (MCT), which uses linear Fresnel reflectors, and is designed to operate at temperatures up to 220 °C. The modules of this collector system are approximately 3 m long by 1 m wide and 0.3 m high. The objective of the study is to optimise the design to maximise the overall thermal efficiency. The absorber is contained in a sealed enclosure to minimise convective losses. The main heat losses are due to natural convection inside the enclosure and radiation heat transfer from the absorber tube. In this paper we present the results of a computational and experimental investigation of radiation and convection heat transfer in order to understand the heat loss mechanisms. A computational model for the prototype collector has been developed using ANSYS–CFX, a commercial computational fluid dynamics software package. The numerical results are compared to experimental measurements of the heat loss from the absorber, and flow visualisation within the cavity. This paper also presents new correlations for the Nusselt number as a function of Rayleigh number.     

Assessment of thermoelectric module with nanofluid heat exchanger

For applications such as cooling of electronic devices, it is a common practice to sandwich the thermoelectric module between an integrated chip and a heat exchanger, with the cold-side of the module attached to the chip. This configuration results thermal contact resistances in series between the chip, module, and heat exchanger. In this paper, an appraisal of thermal augmentation of thermoelectric module using nanofluid-based heat exchanger is presented. The system under consideration uses commercially available thermoelectric module, 27 nm Al₂O₃–H₂O nanofluid, and a heat source to replicate the chip. The volume fraction of nanofluid is varied between 0% and 2%. At optimum input current conditions, experimental simulations were performed to measure the transient and steady-state thermal response of the module to imposed isoflux conditions. Data collected from the nanofluid-based exchanger is compared with that of deionized water.Results show that there exist a lag-time in thermal response between the module and the heat exchanger. This is attributed to thermal contact resistance between the two components. A comparison of nanofluid and deionized water data reveals that the temperature difference between the hot- and cold-side, ΔT = T_h ? T_c ≈ 0, is almost zero for nanofluid whereas ΔT > 0 for water. When ΔT ≈ 0, the contribution of Fourier effect to the overall heating is approximately zero hence enhancing the module cooling capacity. Experimental evidence further shows that temperature gradient across the thermal paste that bonds the chip and heat exchanger is much lower for the nanofluid than for deionized water. Low temperature gradient results in low resistance to the flow of heat across the thermal paste. The average thermal contact resistance, R = ΔT/Q, is 0.18 and 0.12 °C/W, respectively for the deionized water and nanofluid. For the range of optimum current, 1.2 ? current ? 4.1 A, considered in this study, the COP ranges between 1.96 and 0.68.     

Experimental analysis and FEM simulation of finned U-shape multi heat pipe for desktop PC cooling

This paper presents the performance analysis of a finned U-shape heat pipe used for desktop PC-CPU cooling. The experiments are conducted by mounting the system vertically over a heat source situated inside a rectangular tunnel, and force convection is facilitated by means of a blower. The total thermal resistance (R_t) and heat transfer coefficient are estimated for both natural and forced convection modes under steady state condition, by varying the heat input from 4 W to 24 W, and the air velocity from 1 m/s to 4 m/s. The coolant velocity and heat input to achieve minimum R_t are found out and the corresponding effective thermal conductivity is calculated. The transient temperature distribution in the finned heat pipe is also observed. The experimental observations are verified by simulation using ANSYS 10. The results show that the air velocity, power input and heat pipe orientation have significant effects on the performance of finned heat pipes. As the heat input and air velocity increase, total thermal resistance decreases. The lowest value of the total thermal resistance obtained is 0.181 °C/W when heat input is 24 W and air velocity 3 m/s. The experimental and simulation results are found in good agreement.     

Space characteristics of the thermal performance for air-cooled condensers at ambient winds

Ambient winds may lead to poor fan performance, exhaust air recirculation and mal-distribution of the air across the tube bundles of the air-cooled condensers in a power plant. Investigations of the impacts of the ambient winds on the air-cooled condensers are key area of focus. Based on a representative 2 × 600 MW direct dry cooling power plant, the physical and mathematical models of the air-side fluid and heat flow in the air-cooled condensers at various ambient wind speeds and directions are set up by introducing the radiator model to the fin-tube bundles. The volumetric flow rate, inlet air temperature and heat rejection for different air-cooled condensers as a whole, condenser cells and fin-tube bundles are obtained by using CFD simulation. The results show that the thermo-flow performances for the air-cooled condenser as a whole, condenser cells and heat exchanger bundles vary widely in space. The thermal performances of the air-cooled condensers, condenser cells and fin-tube bundles at the downstream are generally superior to those at the upwind. It is of use for the upwind fan regulations and the A-frame condenser cell geometric optimization to investigate the space characteristics of the thermal performance for the air-cooled condensers in a power plant.     

Influence of packed honeycomb ceramic on heat extraction rate of packed bed embedded heat exchanger and heat transfer modes in heat transfer process

Under the condition that the transient oxidation heat extraction process of coal mine ventilation air methane (VAM) is equivalent to a series of steady state process, the steady state heat extraction experiment platform is built. The influence of the honeycomb ceramic packed in heat extraction zone and its two-side space on heat extraction rate and heat transfer modes is investigated. The experimental results show that the honeycomb ceramic packed in heat extraction zone two-side space can always strengthen heat extract ion of heat exchanger by increasing gas physical flow velocity in bed and radiation heat exchanging area and disturbing heat exchanger leeward side flow field. The contradictory dual characteristic of the influence of the honeycomb ceramic packed in heat extraction zone on heat exchanger heat extraction rate determines that the honeycomb ceramic has no great influence on heat extraction rate and doesn't always strengthen heat exchanger heat extraction. Contribution of heat transfer modes on packed bed embedded heat exchanger heat extraction is investigated using the method of coating heat exchanger outer surface silver; the experimental result shows that 55% contribution of packed bed embedded heat exchanger heat extraction rate is from radiation when gas mass flow rate is 0.15 kg·s^− 1·m^− 2 and its temperature is 1113 k; with the gas temperature being increased further, radiation will become the main way of packed bed embedded heat exchanger heat extraction.     

Working fluids for low-temperature heat source

The performance of different working fluids to recover low-temperature heat source is studied. A simple Rankine cycle with subcritical configuration is considered. This work is to screen working fluids based on power production capability and component (heat exchanger and turbine) size requirements. Working fluids considered are R134a, R123, R227ea, R245fa, R290, and n-pentane. Energy balance is carried out to predict operating conditions of the process. Outputs of energy balance are used as input for exergy analysis and components (heat exchanger and turbine) design. The heat exchanger is divided into small intervals so that logarithmic mean temperature difference (LMTD) method is applicable. R227ea gives highest power for heat source temperature range of 80–160 °C and R245fa produces the highest in the range of 160–200 °C. There is optimal pressure where the heat exchanger surface area is minimum. This optimal pressure changes with heat source temperature and working fluid used. The least heat exchanger area required at constant power rating is found when the working fluid is n-pentane. At lower heat source temperature (80 °C), the maximum power output and minimum heat exchanger surface area for different working fluids is comparable.     

MINLP optimisation model for increased power production in small-scale CHP plants

In this work we present a mixed integer nonlinear programming (MINLP) model for increasing the power production in small-scale (1–20 MW_e) CHP plants based on a steam Rankine process and using biomass fuels. Changes that could increase the power production in these plants are, for instance, a steam reheater, a feed water preheater, a two-stage district heat exchanger, and a fuel dryer. In the model we also consider the integration of a gas turbine and a gas engine into the CHP process by using the oxygen remains of the turbine or engine exhaust gases as preheated combustion air in the biomass boiler. The developed MINLP model was tested with four existing small-scale CHP plants. The results showed that there are profitable possibilities to increase the electrical efficiencies and power-to-heat ratios of these plants with the addition of a two-stage district heating exchanger, a feed water preheater, a steam reheater, and a fuel dryer. Furthermore, the integration of a gas engine increased the efficiencies significantly. Overall, the MINLP model gave good results for the example cases, but the model could be still improved by developing its mathematical formulation to a more convex model and by adding the operational changes in the district heating network to the model with multiperiod modeling. The current model gives new possibilities to the design planning and optimisation of the small-scale CHP plants, and it also provides a good basis for the future design modeling of the CHP plants and their optimal integration to the district heating network.     

A study on the thermal contact conductance in fin–tube heat exchangers with 7 mm tube

The thermal contact resistance has been frequently neglected in the process of design of heat exchangers because of the difficulty of measurement and the lack of accurate data. However, the thermal contact resistance is one of principal parameters in heat transfer mechanism of fin–tube heat exchangers. The objective of the present study is to investigate new factors such as fin types and manufacturing types of the tube affecting the thermal contact conductance and to find a correlation between the thermal contact conductance and the effective factors in fin–tube heat exchangers with 7 mm tube. The thermal contact conductances in the 22 heat exchangers with 7 mm tube have been investigated through the experimental–numerical method. A numerical scheme has been employed to calculate the thermal contact conductance and the portion of thermal resistances using the experimental data. As a result, the thermal contact conductance has been evaluated quantitatively, and a new correlation including the influence of new factors such as fin types and manufacturing types of the tube has been developed in the fin–tube heat exchanger with 7 mm tube. Also, the portion of each thermal resistance has been evaluated in each case.