An overview of GRANEX technology for geothermal power generation and waste heat recovery

This paper reports on the recent advancement of the GRANEX technology platform developed by our group for power generation from low-grade heat sources. The technology is particularly suited to applications involving geothermal power generation and waste heat recovery. By combining the concepts of heat regeneration and supercritical Rankine cycle into a unified process, GRANEX improves the thermal efficiency of the cycle and increases the net electrical output which can be recovered from a given low-grade heat source. The regeneration of the thermal energy in GRANEX is achieved through a novel heat regenerator invented and patented by our group in partnership with Granite Power Limited (GPL). Development of GRANEX dates back to early 2006 when a Research and Development Agreement was established between the University of Newcastle and GPL. In conjunction with a program of fundamental studies an applied program of work was undertaken for proof of concept and prototype development with the assistance of a REDI grant from AusIndustry (2007-2009). By 2008, a 1 kW prototype had been built and experimental trials of the system had been completed, demonstrating considerable advantages over conventional systems in terms of both thermal efficiency and power generation (about 40% improvement). This was followed by the design of a 100 kW pilot-plant in early 2009. The pilot-plant is currently under construction and is due to be commissioned by late November 2009

DEM simulation of aggregation of suspended nanoparticles

A DEM-based model was developed and examined for simulation of aggregation in suspensions of α-alumina nanoparticles. In the model, the random Brownian diffusion and the externally induced dielectrophoresis (DEP) motion were considered as the driving mechanisms for the transport of particles in colloidal suspension. To simulate particle interactions, the non-contact surface force and the contact force were taken into account using the well-known Derjaguin-Landau-Verway-Overbeek (DLVO) theory and the soft-sphere model, respectively. Specifically, the model was used to study the effects of pH, solid volume fraction and external AC electric field on α-alumina aggregate growth which was expressed in terms of coordination number, longest dimension, and fractal dimension. The simulations were carried out over a pH range of 4–10, solid volume fraction of 0.02–0.4, and a variety of AC electric fields. In relatively dilute suspensions, the aggregates predominantly exhibited chainlike structures, whereas at high solid volume fraction, aggregates with complex netlike structures were formed. It was also evident that, in concentrated colloidal suspensions, DEP had a negligible influence on aggregate growth over the examined conditions. The effect of DEP however, was found to be more noticeable on aggregate structure leading to the formation of more compact aggregates with a greater particle number density. The break-up and reattachment of sub-aggregates as well as the rearrangement of nanoparticles in the particle assemblies and subsequent curling of the loose network promoted by a strong AC electric field was deemed to be responsible for this structural transformation. Finally, the DEM-based model was used to predict the size of α-alumina aggregates over a range of pH. The predictions were found to be in good agreement with the published experimental data, particularly around the isoelectric point

Thermodynamic assessment of a novel concept for integrated gasification chemical looping combustion of solid fuels

A novel integrated gasification chemical looping combustion (IGCLC) process is proposed here for combustion of solid fuels, particularly coal. The proposed process incorporates an ex situ step for gasification of the solid fuel, but unlike conventional ex situ methods, the gasification process is fully integrated with the combustion process. This is achieved using a three step chemical loop for the production of hydrogen, combustion of gaseous fuels, and regeneration of metal oxides. A detailed thermodynamic chemical equilibrium assessment of the IGCLC process was carried out to evaluate its technical viability. The relevant analyses were performed using Aspen Plus process simulation software. The IGCLC process was found to be thermodynamically feasible. More specifically, it was uncovered that the gasification process can operate at an adequate temperature (above 1023 K) at thermoneutral conditions with high coal conversion (95%). It was also found that, to achieve the highest hydrogen production, the steam/hydrogen to carbon ratio (SHTCR) had to be set to 2, at which the gasification temperature was around 1070 K, coal conversion was 95%, gaseous fuel reactor (i.e., combustor) temperature was 1140 K, and H₂ productivity was 0.85 mol/mol carbon. Mass and energy balance calculations were also performed. It demonstrated that the proposed IGCLC system can achieve an electricity efficiency of 49.5% at SHTCR ~ 2 and feed temperature of 1100 K with some appropriate assumptions, which is 80% higher than a conventional coal-fired power station with carbon capture and storage (CCS) measures

An in-depth assessment of hybrid solar-geothermal power generation

A major problem faced by many standalone geothermal power plants, particularly in hot and arid climates such as Australia, is the adverse effects of diurnal temperature change on the operation of air-cooled condensers which typically leads to fluctuation in the power output and degradation of thermal efficiency. This study is concerned with the assessment of hybrid solar–geothermal power plants as a means of boosting the power output and where possible moderating the impact of diurnal temperature change. The ultimate goal is to explore the potential benefits from the synergies between the solar and geothermal energy sources. For this purpose the performances of the hybrid systems in terms of power output and the cost of electricity were compared with that of stand-alone solar and geothermal plants. Moreover, the influence of various controlling parameters including the ambient temperature, solar irradiance, geographical location, resource quality, and the operating mode of the power cycle on the performance of the hybrid system were investigated under steady-state conditions. Unsteady-state case studies were also performed to examine the dynamic behaviour of hybrid systems. These case studies were carried out for three different Australian geographic locations using raw hourly meteorological data of a typical year. The process simulation package Aspen-HYSYS was used to simulate plant configurations of interest. Thermodynamic analyses carried out for a reservoir temperature of 120 °C and a fixed brine flow rate of 50 kg/s revealed that under Australian climatic conditions (with a typical ambient temperature of 31 °C in summer) a hybrid plant would outperform stand-alone geothermal and solar power plants if at least 68% of its energy input is met by solar energy (i.e. a solar energy fraction of ≈68%). This figure drops to about 19% for reservoir temperatures greater than 170 °C. Case studies also showed that, for a mid-range reservoir temperature of 150 °C, the cost of electricity production can be reduced by 20% when a hybrid plant is used instead of the stand-alone Enhanced Geothermal System (EGS)

Assessment of geothermal assisted coal-fired power generation using an Australian case study

A systematic techno-economic analysis of geothermal assisted power generation (GAPG) was performed for a 500 MW unit of a typical coal-fired power plant located at the upper Hunter region of New South Wales, Australia. Specifically, the GAPG viability and performance was examined by investigating the impacts of reservoir temperature, resource distance, hybridisation scheme, and economic conditions including carbon tax and Renewable Energy Certificates (REC). The process simulation package, Aspen HYSYS, was employed for all simulation purposes. Thermodynamically, GAPG system was found to increase the power output of the plant by up to 19% under the booster mode whilst in fuel saving mode the coal consumption reduced by up to 0.3 million tonne/year decreasing the Green House Gas (GHG) emission by up to 15% (0.6 million tonne/year). Economic analyses showed that for a typical HDR resource with a reservoir temperature about 150 °C located within a 5 km radius from the power plant, the GAPG system becomes economically competitive to the stand-alone fossil fuel based plant when minimum carbon tax and RECs rates of 40 $/tonne and 60 cents/kW h are introduced. The figure of merit analyses comparing GAPG system with both stand-alone fossil fuel and stand-alone geothermal plants showed that an economically feasible GAPG system requires the use of HDR resources located no further than 20 km from the plants. Reference maps were also developed to predict suitable conditions for which the hybrid plant outperforms the stand-alone plants

Mixing and segregation of binary oxygen carrier mixtures in a cold flow model of a chemical looping combustor

In a typical chemical looping combustion process, the oxygen for fuel combustion is supplied by circulating metal based oxygen carriers between two interconnected fluidised bed reactors. The redox characteristics of oxygen carriers and hence the overall performance of the process can be significantly improved by utilising binary mixtures of oxygen carrier particles. The full potential of such multi-species particle systems however can be only realised when particles segregation is minimised. This study is concerned with gaining an understanding of the mixing and segregation behaviour of binary mixtures of oxygen carrier particles with different sizes and densities in a cold flow model representing a 10. kWth chemical looping combustor. The hydrodynamics of such systems were investigated and compared with a typical chemical looping combustion process where single species are used. This was followed by investigating the solids mixing and segregation behaviour in terms of segregation intensity and species weight percentage at each reactor as a function of operating parameters. It was shown that increasing the total solid inventory, particle terminal velocity ratio, composition, and air reactor superficial velocity increases the riser pressure, solid circulation rates, and riser solid holdup. Mixing and segregation regimes of the fuel reactor and the component segregation between the two reactors were also mapped. The results showed that, for mixtures of species with low terminal velocity to high terminal velocity ratios of greater than 0.7, a good mixing in the fuel reactor can be achieved by maintaining the superficial gas velocity to the mixture minimum fluidisation velocity ratio above 5. For the tested conditions, the component segregation between the two reactors was avoided by maintaining the ratio of the riser superficial velocity to the terminal velocity of the species with a high terminal velocity between 1.25 and 2

Chemical looping combustion of ultra low concentration of methane with Fe₂O₃/Al₂O₃ and CuO/SiO₂

This study examines the performance of two metal oxide species in oxidizing ultra low concentration of methane (below 1% in volume). The focus on low methane concentrations are driven by its practical importance in applications such as abatement of ventilation air methane (VAM) in mining operations. Two mixed metal oxides, Fe₂O₃/Al₂O₃ and CuO/SiO₂, were selected as oxygen carriers and prepared using dry impregnation method. The metal oxide loading contents are found to be 45 wt% and 48 wt%, respectively. The redox reactivity of the selected oxygen carriers were studied at various methane concentrations (i.e., 0.1%, 0.5% and 1% in volume) and temperatures between 873 K and 1073 K using a thermogravimetric analyzer. At low methane concentrations and low temperatures (below 1073 K) the conversion of Fe₂O₃ to Fe₃O₄ showed higher reduction reactivity than the reduction of CuO to Cu. The redox reactivity of Fe₂O₃/Al₂O₃ was also found to be quite stable even after 60 redox cycles at 1073 K. The respective weight percentages for oxidation and reduction were found to be 100% and 96.67%, corresponding to a full oxidized state Fe₂O₃ and a reduced state between Fe₃O₄ and FeO respectively. Moreover, the results for the global reactivity of reduction and oxidation (calculated at X = 0.5) showed that the reduction rates were temperature and concentration dependent, varying from 0.14%/s to 2.2%/s over the range of temperatures and methane concentrations of interest. The oxidation rates were much higher than their reduction counterpart. The values varied from 8.95%/s at 873 K to 10.65% at 1073 K

Comments on the effect of liquid layering on the thermal conductivity of nanofluids

This article provides critical examinations of two mathematical models that have been developed in recent years to describe the impact of nano-layering on the enhancement of the effective thermal conductivity of nanofluids. Discrepancy between the two models is found to be an artefact of an incorrect derivation used in one of the models. With correct formulation, both models predict effective thermal conductivity enhancements that are not significantly greater than those predicted by classical Maxwell theory. This study indicates that nano-layering by itself is unable to account for the effective thermal conductivity enhancements observed in nanofluids