Options to Use Solar Heat to Enhance Geothermal Power Plant Performance

ABSTRACT Many future geothermal plant locations are located in dry regions of the world. This is especially true for future EGS (Enhanced Geothermal Systems) locations. These are areas also blessed with high levels of solar incidence. The solar heat can be captured at different temperatures using different solar collector technologies. This makes it possible to consider a number of scenarios in which solar heat can be used to boost the geothermal plant performance. The Queensland Geothermal Energy Centre of Excellence (QGECE) of the University of Queensland is investigating some of the most promising methods in which solar and geothermal heat can be combined. Most of these methods have been considered with a focus on future EGS plants but they may also be applicable to more conventional geothermal applications. The combination amounts more than the sums of the individual components in this instance. The paper will describe how solar and geothermal heat can be combined to generate more value than what can be generated by each source individually. The applications will include the following: boosting geothermal fluid temperatures to increase cycle conversion efficiencies; enhancing the performance of natural draft dry cooling towers by using solar heat; and solar chilling of future supercritical CO2 EGS plants to maintain higher efficiencies

Heatline and Energy-Flux-Vector Visualization of Natural Convection in a Porous Cavity Occupied by a Fluid with Temperature-Dependent Viscosity

Temperature-dependent viscosity effects in buoyancy driven flow in a porous-saturated enclosure is studied numerically, based on the general model of momentum transfer in a porous medium. The exponential form of the viscosity-temperature relation is applied. Both cases of viscosity increase and decrease versus temperature are considered. Application of the effective Rayleigh number concept and the reference temperature approach are investigated. Use of heatlines and the energy flux vectors are illustrated for a more comprehensive analysis of the problem

Heat transfer and entropy generation optimization of forced convection in a porous-saturated duct of rectangular cross-section

We investigate analytically the first and the second law characteristics of fully developed forced convection inside a porous-saturated duct of rectangular cross-section. The Darcy-Brinkman flow model is employed. Three different types of thermal boundary conditions are examined. Expressions for the Nusselt number, the Bejan number, and the dimensionless entropy generation rate are presented in terms of the system parameters. The conclusions of this analytical study will make it possible to compare, evaluate, and optimize alternative rectangular duct design options in terms of heat transfer, pressure drop, and entropy generation. (c) 2006 Elsevier Ltd. All rights reserved

Application of high porosity metal foams as air-cooled heat exchangers to high heat load removal systems

A numerical study has been conducted to investigate the fluid flow and heat transfer of an air-cooled metal foam heat exchanger under the high speed laminar jet confined by two parallel walls for which the range of the Reynolds number is 600-1000. Two independent numerical solvers were used and cross-validated being a FORTRAN code and the commercially available software CFD-ACE. The effects of local thermal non-equilibrium, thermal dispersion, porosity, and pore density on the heat transfer augmentation are examined for different Reynolds numbers. Application of energy flux vectors, for convection visualization, is also illustrated for a more comprehensive analysis of the problem. Finally, the performance of the metal foam heat exchanger is compared to that of conventional finned design. It is observed that the heat removal rate can be greatly improved at almost no excess cost

Selected papers from the 17th IAHR (International Association for Hydro-Environment Engineering and Research) International Conference on Cooling Tower and Heat Exchanger

The special issue of Heat Transfer Engineering published selected papers from the 17th International Association for Hydro-Environment Engineering and Research (IAHR) International Conference on Cooling Tower and Heat Exchanger, held in Gold Coast, Australia from September 7–11, 2015. The authors were invited to revise/upgrade their manuscripts and resubmit them according to the reviewers’ comments and the journal requirements. The special issue started with a paper on heat exchanger performance by Dai and others from Xi’an Jiaotong University, China. The authors reported the results of a thermoeconomic comparison of a basic organic Rankine cycle (ORC) with a parallel double-evaporator organic Rankine cycle

Dragline Field Testing

Draglines are the most expensive pieces of equipment used in coal mines at a cost of $50 M to$100M each. Improving their productivity will produce major benefits to the coal mining industry. The dynamic behaviour of the dragline structure has a significant effect on the fatigue life of the main components of a dragline and related maintenance costs. This paper describes the field tests conducted on the dragline DRE23 at the Peak Down coal mine, Queensland, Australia. Sixteen accelerometers were installed on the dragline boom and mast. Three different excitation methods were used in the test: 5.4-kg impact hammer, dragline bucket impulse and ambient excitations produced during normal operation. The aim of the modal testing was set to explore the six global modes for the dragline boom structure. The results showed that the impact hammer excitation was not adequate to excite any of the global modes. The excitation produced by bucket impulse was powerful but was difficult to control. The output-only identification using the response to the ambient excitation was promising but it was difficult to identify all targeted global modes

Experimental study of cold inflow effect on a small natural draft dry cooling tower

The heat rejection rate of natural draft dry cooling tower, as well as the operating performance of a power plant, can be affected by numerous ambient factors. The cold inflow is an unfavourable air turbulence at the top of the cooling tower and has a significant negative effect on the performance of natural draft cooling towers. In the present research, results are given for a 20 m high natural draft dry cooling tower experimental system tested at different ambient conditions. Several events of cold air incursion into the top of the cooling tower are identified and the detailed experimental data are presented. The experimental data show that this effect could seriously impair the thermal performance of the cooling tower. The water outlet temperature of the cooling tower has increased by as much as to 3 °C in these tests because of the cold inflow effect. The mechanism and the solution are discussed based on the experimental data. The findings in this paper can lay an important foundation for future small natural draft cooling tower design and operation

Simulation of the UQ Gatton natural draft dry cooling tower

Natural draft dry cooling tower (NDDCT) is an effective cooling technology which can be utilized in most of geothermal and concentrated solar thermal (CST) power plants. The experimental studies of the full scale cooling tower, especially the small size NDDCTs, are still not extensive. To fill this gap, Queensland Geothermal Energy Centre of Excellence (QGECE) at The University of Queensland has built a 20m high NDDCT. In this paper, the 1D analytical model and the 3D CFD model of this cooling tower were developed and its cooling performance was investigated at different ambient temperatures, hot water temperatures and velocities of cross wind. The result shows that the small size NDDCT is suitable for 2~3MW CST power plants. The cooling performance decreases with the increase in the ambient temperature and the decrease in the hot water temperature. In terms of the cross wind, the heat rejection ratio decreases with the increase of the cross wind velocity when cross wind velocity is low. However, when velocities of the cross wind become large enough, the heat dumped at the bottom of the tower can compensate some loss caused by cross wind. The results found in the present paper give reference for planed future experiments

Influence of ambient conditions and water flow on the performance of pre-cooled natural draft dry cooling towers

A simplified heat and mass transfer model in cellulose medium was developed to predict the air outlet temperature and humidity after evaporative cooling. The model was used to simulate the operation of pre-cooled Natural Draft Dry Cooling Towers (NDDCTs) by a validated MATLAB code. The effects of supplied water flow rate to the media, ambient temperature and humidity on the performance of pre-cooled NDDCTs were investigated. It was found that the effect of the selected water flow rates on tower performance is negligible. Both ambient temperature and humidity affect the tower performance

Techno-economic analysis of supercritical carbon dioxide power blocks

Developing highly efficient power blocks holds the key to enhancing the cost competitiveness of Concentration Solar Thermal (CST) technologies. Supercritical CO (sCO) Brayton cycles have proved promising in providing equivalent or higher cycle efficiency than supercritical or superheated steam cycles at temperatures and scales relevant for Australian CST applications. In this study, a techno-economic methodology is developed using a stochastic approach to determine the ranges for the cost and performance of different components of central receiver power plants utilizing sCO power blocks that are necessary to meet the Australian Solar Thermal Initiative (ASTRI) final LCOE target of 12 c/kWh