旭ダム排砂バイパストンネルによる石礫の排出機構および下流河道の瀬淵の回復;旭ダム排砂バイパストンネルによる石礫の排出機構および下流河道の瀬淵の回復;Study on Flushing Mechanism of DAM Reservoir Sedimentation and Recovery of Riffle-Pool in Downstream Reach by a Flushing Bypass Tunnel
In the Asahi River, riffles and pools had been scaled down in the downstream reach of Asahi Dam due to sediment deposition in Asahi Dam reservoir. To improve this environmental problem, a flushing sediment tunnel was constructed so as to bypass Asahi Dam reservoir. In this study, first, the authors demonstrated the recovery of riffles and pools and the grain size distribution in the downstream reach by using the data of cross-sectional surveying, air photographs and field investigation. Next, authors’ two-dimensional model was applied to a series of flood events in the Asahi River. As a result, the flushing mechanism of Asahi Dam reservoir sedimentation, the recovery process of riffles and pools and the change in grain size distribution in the downstream reach were clarified by the model.
Field calibration of bedload monitoring system in a sediment bypass tunnel: Swiss plate geophone
Under the impact of climate change, sediment transport in melting water draining from glacier basins and reservoir sedimentation tend to increase worldwide. As a consequence, three main problems arise: (1) the loss of reservoir storage volume for energy production, flood retention, water supply and irrigation; (2) increased hydro-abrasion at turbines and hydraulic structures; and (3) negative environmental impacts due to downstream sediment deficit. An effective countermeasure against reservoir sedimentation in small to medium-sized mountainous reservoirs is to route sediment around the dam by using a sediment bypass tunnel (SBT). A major problem affecting nearly all SBTs is severe hydro-abrasion on the tunnel invert due to high bed load transport rates in combination with high flow velocities. Depending on site-specific operating conditions and sediment properties, i.e. size, hardness and shape, invert abrasion can cause considerable refurbishment costs. For optimized operation of SBTs with respect to sustainable sediment management and cost efficiency, continuous real-time monitoring of bed load transport is necessary. Bed load transport can be monitored indirectly by using passive sensors like geophones or hydrophones. However, these techniques require a site-specific calibration depending on hydraulic conditions, particle-size and shape. This study deals with the field calibration of a so-called Swiss plate geophone system implemented at the outlet of Solis SBT located in Grisons in the Swiss Alps. The geophones with a sampling rate of 10 kHz are placed across the whole tunnel width and have an inclination of 10° against the invert slope. Three different particle size classes were tested 16-32 mm, 32-63 mm and 0-400 mm. The results indicate that calibration is independent from flow velocity due to the counter inclination of the geophones. However, a certain degree of signal saturation due to high bed load transport rate occurred and hence a further investigation is needed.
Techno-economic analysis of chemical looping combustion with humid air turbine power cycle
Abstract Power generation from fossil fuel-fired power plant is the largest single source of CO2 emission. CO2 emission contributes to climate change. On the other hand, renewable energy is hindered by complex constraints in dealing with large scale application and high price. Power generation from fossil fuels with CO2 capture is therefore necessary to meet the increasing energy demand, and reduce the emission of CO2. This paper presents a process simulation and economic analysis of the chemical looping combustion (CLC) integrated with humid air turbine (HAT) cycle for natural gas-fired power plant with CO2 capture. The study shows that the CLC–HAT including CO2 capture has a thermal efficiency of 57% at oxidizing temperature of 1200 °C and reducer inlet temperature of 530 °C. The economic evaluation shows that the 50 MWth plant with a projected lifetime of 30 years will have a payback period of 7 years and 6 years for conventional HAT and CLC–HAT cycles respectively. The analysis indicates that CLC–HAT process has a high potential to be commercialised.