Motoaki Utamura

A generalised mean temperature difference method for thermal design of heat exchangers

A Generalised Mean Temperature Difference (GMTD) method instead of the Logarithmic Mean Temperature Difference (LMTD) method for heat exchangers is proposed. Unlike the LMTD method, the GMTD method allows the performance analysis of heat exchangers with distributed physical properties. GMTD is calculated by a new method to take fluid temperature profiles as a function of heat load. Mathematical proof is made that the formula of the LMTD can be derived from that of the GMTD, in the case of constant physical property. The GMTD method is applied to experiments of a hot water supplier with supercritical carbon dioxide as a heating media that works through its pseudo-critical point where specific heat behaves anomalously. The empirical correlations of the local Nusselt number and pressure loss coefficient based on the GMTD gives an accurate prediction of the overall heat transfer coefficient, with a 5% error rating, whilst an error two times larger appears in the case of the LMTD.

Demonstration of Supercritical CO2 Closed Regenerative Brayton Cycle in a Bench Scale Experiment

Power generation with a supercritical CO2 closed regenerative Brayton cycle has been successfully demonstrated using a bench scale test facility. A set of a centrifugal compressor and a radial inflow turbine of finger top size is driven by a synchronous motor/generator controlled using a high-speed inverter. A 110 W power generating operation is achieved under the operational condition of rotational speed of 1.15kHz, CO2 flow rate of 1.1 kg/s, and respective thermodynamic states (7.5 MPa, 304.6 K) at compressor and (10.6 MPa, 533 K) at turbine inlet. Compressor work reduction owing to real gas effect is experimentally examined. Compressor to turbine work ratio in supercritical liquid like state is measured to be 28% relative to the case of ideal gas. Major loss of power output is identified as rotor windage. It is found the isentropic efficiency depends little on compressibility coefficient. Off design performance of gas turbine working in supercritical state is well predicted by a Meanline program. The CFD analysis on compressor internal flow indicates that the presence of backward flow around the tip region might create a locally depressurized region leading eventually to the onset of flow instability.Copyright

A Solar Gas Turbine Cycle With Super-Critical Carbon Dioxide as a Working Fluid

Solar thermal power generation system equipped with molten salt thermal storage offers continuous operation at a rated power independent of the variation of insolation. A gas turbine cycle for solar applications is studied which works in a moderate temperature range (600–850K) where molten salt stays as liquid stably. It is found that a closed cycle with super-critical state of carbon dioxide as a working fluid is a promising candidate for solar application. The cycle featured in smaller compressor work would achieve high cycle efficiency if cycle configuration and operation conditions are chosen properly. The temperature effectiveness of a regenerative heat exchanger is shown to govern the efficiency. Under the condition of 98% temperature effectiveness, the regenerative cycle with pre- and inter-cooling provides cycle efficiency of as much as 47%. A novel heat exchanger design to realize such a high temperature effectiveness is also presented.© 2006 ASME

Thermal-Hydraulic Characteristics of Microchannel Heat Exchanger and Its Application to Solar Gas Turbines

Incorporating “integral method” proposed here, a set of empirical correlations of local heat transfer coefficient and pressure loss coefficient are newly derived based on experiments using microchannel heat exchanger (MCHE) with supercritical carbon dioxide as heating medium and water as coolant. They are expressed in dimensionless forms of polynomial of Reynolds number and Prandtl number. The same correlation of Nusselt number is found applicable to both fluids and its value is almost two times larger than Dittus Boelter correlation. It was also shown that the above form is applicable to a wide range of geometry with the values of constants changed. Accuracy of both correlations is confirmed within 5% errors for MCHE with S-shaped fins in the range of pressures 9∼12.5MPa and temperatures 280∼390K. Based on the correlations, sizes of heat exchangers are evaluated contained in gas turbine cycle with super critical carbon dioxide as a working fluid applicable to solar power.Copyright

Optimal Heliostat Layout for Concentrating Solar Tower Systems

A methodology to give an optimal layout of a group of heliostats has been developed for concentrating solar tower systems. Given the maximum solar power together with optical parameters, the method determines an optimal configuration of a heliostat field around a tower where reflected beams are focused and a resultant annual solar energy collected. Various optical losses such as cosine factor, and an optical interference such as shadowing and blocking are taken into account in the calculation. Furthermore, spillage is considered due to dilution of light at a receiver collector that is derived from effects of a finite solar disk and various errors inherent in optical hardware and control. It is found the spillage becomes significant at heliostats located in a distance from the tower farther than four times of tower height and dominates the configuration of the optimal heliostat layout.

Empirical Correlations for Predicting Key Performance Parameters Due to Inlet Fogging

Empirical correlations to predict (1) drain mass flow rate, (2) temperature depression of intake air and (3) power increase to a given ambient and operating condition are derived. The cooling efficiency and drain ratio are expressed using separation of variables, i.e. relative spray amount, ambient wet bulb depression and fogger configuration (droplet spectrum, geometry of fog rack etc.). The correlation for power increase obtained as function of net specific spray water flow can be applied to overspray mode of operation. It is confirmed that the power increase predicted matches field data within error less than 10% in the range of net specific water amount up to 0.4% overspray and ambient wet bulb depression from 3.35 to 13.8deg.Copyright

Test Plan and Preliminary Test Results of a Bench Scale Closed Cycle Gas Turbine With Super-Critical CO2 as Working Fluid

Development of a closed cycle gas turbine using supercritical carbon dioxide as a working fluid is underway to generate power from industrial waste heat sources of a low or intermediate temperature range. Its demonstration test plan using a reduced scale turbomachine is described herein. Principal specifications include the following: net power output of 10 kWe and recirculation CO2 with flow rate of 1.2 kg/s under given turbine inlet conditions of 550 K and 12 MPa. The optimized ranges of compressor inlet temperatures and pressures are investigated in this study. Given these inlet conditions, primary and auxiliary component development is done. Coupled with cycle analysis, the design rotational speed of the co-axially aligned turbomachine was determined as 100,000 rpm. Aerodynamic CFD analyses were conducted for the centrifugal compressor considering real gas properties. Preliminary test results show indirect evidence of compressor work reduction inherent to the supercritical CO2 gas turbine concept.Copyright

Some Alternative Technologies for Solar Thermal Power Generation

Two advanced optical systems and a highly efficient thermal cycle suitable for beam-down power tower with thermal storage are presented. (1) To increase field efficiency, the “cross beam” heliostat array concept is proposed. Using continuum optical model, the characteristics of the cross beam concept and its economy were investigated. (2) To protect the central reflector (CR) against wind force, a “multi-ring CR” concept is proposed. The concentration performance of multi-ring CRs is calculated using the ray-tracing method. It shows no worse results than the case with a single hyperboloid mirror. (3) The potential of a closed gas turbine cycle with supercritical carbon dioxide as a working fluid was investigated. An optimal cycle configuration involves a regenerative cycle with pre-cooling and inter-cooling cycles, in which theoretically achievable cycle thermal efficiency is 47% at the turbine inlet temperature of 800 K and turbine inlet pressure of 20 MPa. Detailed thermal design of a critical component, regenerative heat exchanger (RHX) is carried out using a newly developed printed heat exchanger (PCHE). It proved to be a feasible design.Copyright