1 Introduction

Tw ice a day the River Mersey undergoes the second largest tidal shiftin the UK.As a result,a large volume of water flows in and out of the River Mersey estuary providing a vast untapped source of tidal energy that could be extracted using a range of tidal energy technologies.

从当前我国的国家标准中规定，在高速公路跨度小于50m的情况下，通常要选择使用标准跨径装配式混凝土桥梁，而该种形式的桥梁被大量的应用到实践中，所产生的经济效益是非常可观的。

Previous studies have been conducted by the Mersey Barrage Company between 1988 and 1992,and the Mersey Tidal Barrage Group between 2006 and 2011.Both these studies examined the uses of a tidal barrages to control the flow in order to power tidal turbines,in order to produce a reliable and predictable source of energy.Both reports concluded that the optimal location for a tidal barrage would be between New Ferry and Dingle and could potentially produce up to 920 GWh of energy per year.However,the estimated construction costsalone had a staggering£3.5 billion price tag and,as a result of low energy prices,the project has been unable to secure funding from investors(Peel Energy 2011).

There are a number of alternative methods of extracting tidal energy that have been under development and testing.They include the SeaGen turbine in Strangford Lough,which is in operation since 2007 and became the first tidal stream generator to be connected to the National Grid in 2008(supplying 1.2 MW for 18 to 20 h each day—equivalent to an annual supply of 6 GWh).This was a key milestone for tidal stream generators and proved their viability as a reliable source of power generation(Marine Current Turbines Ltd 2008).

When examining the geographical terrain of Strangford Lough,it became apparent that it shared a number of key geographical characteristics with the River Mersey,such as a narrow bottle neck leading to a large inlet and a large tidal range.An initial investigation into the potential for the positioning of tidal turbines in the River Mersey has been carried out previously(Kelly 2015).This resulted in a very simplistic analysis for a small section at the river mouth between Perch Rock and Gladstone Lock,which concluded that there was the potential to produce 13.6 MWh per lunar month from a single rotor multidirectional turbine located at site F situated in the middle of the river mouth.The output for this site was severely constricted due to the shallow depth and the relatively slow flow at the identified site in the river.One of the major concerns about positioning a turbine at site F was its close proximity to marine traffic routes,which can be seen in Fig.1.It was concluded that a turbine situated at site F would present asignificant marine hazard to ferry services,as well as vessels berthing at Liverpool cruise terminal and using Gladstone Lock to enter Liverpool container port.

Fig.1 River Mersey mouth showing the area analysis between Perch Rock and Gladstone dock and identifying site F(Google Maps 2017)

This research has set out to validate the initial feasibility study,by conducting a holistic assessment of the Mersey estuary in order to identify additional sites that might be better suited to tidal power generation.In order to achieve this,the initial aim of producing a computer model of the river basin from the original report was revisited using the specialist tidal flow simulation software package,M IKE 3,which has been developed by DHI Group and is currently used in the coastal and fluid Engineering sector.

2 Development o f the Numerical Analysis Model for Tidal Power Assessment

The software developed by DHI has been widely used in industry as a simulation tool for a variety of different hydrodynamic conditions.The software is extensive and broken down in to a number of subprograms in order to stream line the simulation process depending on the type being conducted.In order to conduct a tidal flow simulation,the following main subprograms were to be used:

M IKE Zero:modelling and mesh generation

M IKE Toolbox:global tidal model data

M IKE 3:simulation software for flow modelling of costal marine areas

Below is a description of the subprograms that are used for the development of the numerical models presented.

3 MIKE Zero

In order to generate a realistic model,accurate data for three boundaries conditions have to input to identify shoreline,river depths and river inlet location.The shoreline and river depth data have to be imported into the model in the format of text files,in order to provide the required data for the software tool to produce the boundary conditions for the model.The inlet boundary could then be specified using a tool in the software.This tool identifies the section that will drive the simulation process using data obtained from the global tidal model.

3.1 Shoreline Boundary Condition

The shoreline data is required by the model to provide a boundary restricting the flow of the river in the X and Y directions.To obtain the data for the River Mersey shown in Fig.2,the Admiralty Chart 3490(United Kingdom Hydrographic Office 2012)and the Ordnance Survey Map Sheet 108 were used to find the longitude and latitude values,for points along the banks of the river starting at Perch Rock working the way around the Basin to Gladstone Lock.The software could then be used to generate a solid boundary between these points by assigning each value a connectivity of one.This identified to the software that each point was connected to the next.In order to simulate the exit to the sea at the river mouth,the last data point at Gladstone Lock was assigned a connectivity value of zero indicating opening between Gladstone Lock and Perch Rock to the software.

Fig.2 River Mersey(Google Maps 2017)

On a spring tide,the effects that the tide has on the River Mersey can be observed as far up stream as Howley Weir in Warrington.However,for the purpose of reducing the complexity of the model,the river basin was effectively cut off at Runcorn Bridge.This was chosen due to the constriction in the river at this point and the absence of depth data for the area upstream.

3.2 River Depth Boundary Condition

The river depth data file was used by MIKE Zero software to create a boundary condition in the Z direction.Using the Admiralty Chart,the longitude and latitude for known depth values could be recorded and the data added to the model.

Regrettably,the Admiralty Chart only provided river depth data for the area between Perch Rock and the entrance of the Manchester Ship Canal(the main shipping route).Additional data could not be sourced for this area due to the constantly shifting river bed conditions that occur due to sediment transportation.In order to provide enough data for the model,estimates for the depths in this area had to be made using estimations based on known data from the chart.They were used in conjunction with known data from the chart in an attempt to provide a realistic prediction for river depths in this area.

In order for the depth data obtained to be used in the model,it had to be adjusted from chart datum to mean sea level(MSL).This involved adding 4.93 m,the MSL specified by the chart,to each of the depth values which had been recorded from the lowest astronomical tide.

“三入”桃花岭 “三拉”近距离（董雄华等） ..............................................................................................4-57

3.3 Inlet Boundary Condition

In order to specify an inlet in the model,the last shoreline data points at Perch Rock and Gladstone Lock had to be selected using the inlet tool in the M IKE software.This tool could then be used to specify the area between as the inlet boundary.

For the purpose of this study,the flow of water was simulated using only the change in tidal height data.As a result,the volume of water due to the natural flow of the river itself was not considered.This assumption can be considered as valid,since the flow of the river contributes just 1%of the flow exiting the estuary,and therefore only accounts for a relatively small amount when compared to the volume that flows in and out due to the tidal shift(The National Oceanography Centre 2016).

3.4 Conversion of Longitude and Latitude Values

After initial modelling attempts failed to situate the model in the precise location within the global model,it became clear that the model was not able to handle the format in which the longitude and latitude data had been recorded.In order to rectify this problem,the longitude and latitude data for both the shoreline and river depth boundary conditions had to be converted from the standard format of degrees,minutes and seconds into decimal degrees using Eq.(1)(Rapid Tables 2016).

3.5 Meshing Process

Once the boundary conditions had been incorporated into the M IKE Zero software,an initial mesh is produced and then refined if needed.By refining the mesh,the number of nodes was increased adding further detail to the model.The Redistribute Vertices tool was used to increase the number of nodes that provided the boundary conditions for the river bank,by using a predictive modelling tool to insert extra data points every 200 malong the banks.This resulted in a slight change to the profile of the river since the software is used to deal with curves and meanders of most typical shorelines,and not the straight edges of the river Mersey.As a result of this refinement process,there will be a slight error in any data recorded close to the banks;however,this was considered appropriate because turbines would not be positioned in these areas.Further refinement was carried out in order to adjust the conventional mesh between data points and produce a refined mesh for the riverbed in the main channel.

The mesh refinement process was limited due to the 1000 node limit imposed by the student licence of the M IKE 3 software.Considering that the initial data accounted for 472 nodes,there was very little room for refining the mesh;however,the final mesh consisted of 964 nodes.The final mesh can be seen in Fig.3 which also shows the boundary condition for the inlet(green line),shoreline(red dots)and depth points overlaid on the grid of longitude and latitude values.

4 MIKE Toolbox

Once the inlet boundary conditions had been specified using M IKE Zero,the M IKE Toolbox application is then used for finding the tidal flow expected at the precise location using data from the built-in global tidal model.In order to validate the output data,tidal data for the period between 21 February 2015 and 23 March 2015 that corresponds to the period of one lunar month is used as input to the corresponding analysis.

融合不仅包括制度性的整合，也包括主观性的融入。如何从创造共同利益发展为凝练共同认知，进而塑造共有认同，将是一个复杂的系统工程。现代国家认同包括政治认同和文化认同两个层面，经济社会融合只能为国家认同的建构创造条件，而不能代替政治及文化认同建设本身。国家统一主体性的建构仍需要国家认同建设的推进及维护。

In order to validate the data taken from the global model,a comparison was drawn against the tidal data that was predicted to occur at Gladstone Lock over the same period examined.An example of this comparison can be seen in Fig.4 which shows the predicated and recorded data for the 22nd of February 2015.The graph indicates a strong correlation between the model and the recorded data;however,a slight discrepancy of around 0.4 m was observed between the high and low tide values.

5 MIKE Software

Using the mesh file created in M IKE Zero and the data obtained from M IKE Toolbox,an initial simulation could be run using M IKE 3.

5.1 Initial Simulation

Initially,a simple area analysis simulation was conducted to visualise the flow through of the model.This analysis also provided preliminary water depth,velocity and direction of the flow data;the last could then be used to confirm that the simulated flow through the model was as in line with expectations.In order to achieve this,a simulation for the 30-day period was carried out using a time period of one iteration per minute,resulting in a simulation period of 43,200 iterations.This produced a minute-by-minute analysis of the changing flow rates and river depths,which was displayed using a polychromatic contour plot overlaid on the model.Some of the results of which can be seen in Fig.5.

This initial simulation confirmed that the model worked as expected and simulated different flow conditions in and out of the river,as the tidal height at the inlet varied over the time period.However,two initial problems were identified:(1)discrepancies between the simulated flow velocities and the data provided by the Admiralty Chart and(2)recirculating flow at the inlet boundary during periods of low velocity.

5.2 Validating the Model

Due to the concerns brought about by the initial area series analysis,further tests were carried out in order to validate the model.This was done by comparing data produced during the simulation to known velocity data provided ateach of the tidal diamonds on the Admiralty Chart.The tidal diamonds on the chart indicate flow speeds at hour intervals for 6 h before and after a spring and neap tide.In order to make this comparison,data had to be gathered from the model using a point series analysis,allowing data to be gathered from the model at the precise location of each of the tidal diamonds.

Despite attempts to fix the turbulent flow at the model inlet,no further improvements could be made.As a result,any data generated by the model for the section between the inlet and Tower Groyne was considered corrupt.This meant that data generated for this section of the river was discarded,resulting in the failure to complete the initial aim of validating the original report.

Fig.3 Final model mesh

Fig.4 Comparison of tidal data for 22 February 2015

In order to validate the remaining area,a simulation was conducted for a 6-h period before and after high tide for a spring tide on the 21st of February and a neap tide on the 27th of February 2015,in order to find the velocity at the specified tidal diamond points B,C and D.The data could then be compared to the Admiralty Chart data for the tidal diamonds B,C and D.Point A was discarded from the process due to its location in the area affected by recirculating flow,which corrupted the data.See Table 1 for precise locations of each tidal diamond.

It was immediately apparent that the data for the initial model was drastically different from the expected values.Further examination and simulations revealed that this was due to a restriction in the volume of water that could flow into the estuary,which was not sufficient to produce the expected flow velocities.This was caused by the conservative estimates that had been made for the unknown depth values which,as a result,restricted the volume of water that could be contained in the estuary.

Fig.5 Various flow conditions in the Mersey.a Flow in.b Flow out.c Change of the tide.d Recirculation at low velocities near the entrance

In order to adjust the amount of water flowing into the estuary,the estimated depth values in the initial model were altered to increase the volume of water flowing into the model,and the comparison process repeated.This procedure was repeated a number of times until a close correlation between the simulated and expected values was achieved.

Figure 6 shows the comparison between the simulated and known values for the final model,which clearly suggests that the model corroborated the data to a respectable degree of accuracy validating the altered depths of the model.

Table 1 Location of tidal diamonds A to D

-3°01′78″W 53°26′82″N B-3°00′98″W 53°25′52″N C-2°59′78″W 53°23′02″N D-2°58′48″W 53°22′12″N Tidal diamond Latitude Longitude A

Fig.6 Comparison of flow velocity at tidal diamond C for flow data generated during the simulation and data obtained from the Admiralty Chart for the spring and neap tidal flows

5.3 Optimal Locations

The power that can be extracted for the case of a tidal stream generator is given by Eq.(2).Due to the assumptions that the density of sea water remains at a constant 1025 kg/m3,and that the efficiency of the turbines at each potential site would remain the same,only two parameters in the equation can be varied to affect the power produced by the turbine:the swept turbine area and the velocity of the flow.Both of these are constricted by the profile of the river.

where ρ is the density of sea water(kg/m3),A is the swept area of the turbine blades(m2),V is the velocity of water flowing through the turbines(m/s)and CP is the turbine power coefficient.P is the power generated(W).

她是悄悄离开的，行李不多，就带走了这几年收藏的几十双鞋。她搬了新的出租房，做着邵南的女友之一，很乖，很听话。

In order to pinpoint the location for a turbine in the river,the data produced by the analysis was examined using these two key criteria points to identify potential turbine sites.This was done to determine if a deeper location,which could accommodate a larger swept area,would be better than a faster flowing section of the river,which normally has shallower depths that restrict the size of the turbines swept area.

世界历史不是历史学中的概念，它不是对世界范围内的历史事实进行澄清和研究，而是建立在普遍联系的基础上，用理性的抽象的思维去把握世界历史的本质和价值，从而为其发展趋势提供一种科学的研究方法。在前资本主义社会真正的世界历史并未形成，这一概念只是存在于理论的抽象中，在资本主义兴起确立并在世界范围内取得统治权后，世界历史终于变成了经验性的事实。因此，世界历史是人类历史不断发展过程中的阶段性产物，劳动作为它产生与发展的深层原因，为其奠定历史的起点，推动历史的转变，预示历史的未来。

The rated power of a turbine is the maximum power output that can be achieved by a turbine.In order to calculate the rated power of the turbines at each site,a simple power calculation was conducted to find the power produced when the velocity of the water was 2 m/s.Due to the variation in turbine diameters,there was a difference in the rated power calculated for each site.The turbine located at site 3 was an 11.9-m diameter turbine rated to 208 kW,where as site 8 encompassed a 17.2-m diameter turbine rated at 430 kW.

Fig.7 a Location of turbines sites 1 to 5 from left to right,in the fastest flowing section of the river.b Location of turbines sites 6 to 10 from top to bottom in deep areas

Figure 7b indicates the location of turbine sites 6 to 10,which were selected based on a depth analysis that identified some of the deepest locations in the river.

The simulation will analyse the uses of a single rotor turbine positioned at each site.

In order to calculate the swept area of each turbine,the minimum depth at which water speeds which were in excess of 1 m/s was identified from the data.In order to account for the clearance between the turbine blades and the seabed,the depth value was modified by-1.5 m before the swept area was calculated.

Table 2 shows the results of the initial average power outputs that can be expected from the ten potential sites identified.The comparison identifies site 8 as the most efficient location for the positioning of a turbine with the potential to produce an average of 221.3 kW over a lunar period.Site 3 was also identified as another potential location producing 201.9 kW during the same period.Since each turbine site had initially been selected using the different identification criteria,it was decided that both sites should be examined in greater detail in order to determine the optimal location in case the initial location criteria affected the potential outputs of the turbine.

As a result of this,a second area series analysis is conducted.This allowed for the identification of areas of interest through the examination of the velocity and the depth data generated from this simulation,so that potential turbine locations could be identified.

6 Power

In order to compare sites 3 and 8 in greater detail,the velocity data for the entire lunar period was examined to account foractual conditions at each location.To provide a realistic power output that could be expected at each site,a number of assumptions had to be made.Due to the availability of data that exists on tidal turbines,some of the assumptions were based on the data recorded at the SeaGen turbine in Strangford Lough.

Table 2 Average power available at each turbine location

Turbine site Swept area(m2) Average velocity(m s-1)Power(kW)74 1.54 138.5 2 105 1.53 192.7 3 110 1.53 201.9 4 104 1.54 194.7 5 54 1.67 128.9 6 191 0.93 78.7 7 192 1.18 161.7 8 232 1.23 221.3 9 211 1.19 182.2 10 140 1.15 109.1 1

6.1 Assumptions

By multiplying the power values calculated for each phase by the percentage of time,the average power produced during that phase could be calculated as seen in Table4 for the turbine at site 3 and Table 5 for site 8.

Multidirectional turbines were simulated to account for power generation from the flow in any direction.

The turbine was assumed to be a fixed-based horizontal axis turbine with the foundation structure resting on the river bed.

A point series analysis was then conducted in order to gather precise information,at each of the identified turbine locations for an entire simulated lunar cycle.This allowed for data to be gathered over a range of different tidal conditions.In turn,this allowed the power outputs for each site to be calculated by using the average velocity obtained during this period.In order to find the potential power at each point,a 100%efficient multidirectional turbine was simulated.

Rated power for the turbine in each location was limited to the power produced by a flow of 2 m/s through the sweptarea.

运动塑料专家igus扩展了其drylin W直线模块化系统，现在还提供直线滑块配合以玻璃纤维增强塑料制成的直线导轨。从汽车制造到实验室技术，这种无金属的替代方案能够帮助用户在整体结构中降低成本、减轻重量。

Turbines required water speeds in excess of 1 m/s in order for the turbine to become operational.

Efficiency of the turbine was taken as 45%to account for losses.This value is the lowest operating efficiency that has been observed at the SeaGen site since it became fully operated in 2008(Martin Wright 2010).

6.2 Rated Power

Figure 7a shows the location of turbine sites 1 to 5 in the area identified as being the fastest flowing section of the river which,as expected,was the narrowest section of the river.

6.3 Power Produced

In order to calculate the power produced at each location,the velocity data for the two sites was examined and organised into three flow phases specified as:

12次测定结果偏差均处于标准值-20%～+20%，符合LS/T 6115-2016[5]中关于X射线检测仪器校准与测量准确度的要求。

Non-operational—velocity less than 1 m/s

Rated power—velocity over 2 m/s

Cut-in—velocity between 1 to 2 m/s

Since data had been gathered in 1-min increments,it could be used to calculate the percentage of time that the turbine was operating for each of the specified phases during the lunar period.The results of which can be seen in Table 3.

In order to calculate the average power generated at any given time by the turbine,the power calculated for each of the specified phases was found using the velocity values.To find the power produced during the rated power period,a velocity value of 2 m/s was used,due to the initial assumption made limiting the power of the turbines.The non-operational period was calculated using a velocity of 0 m/s.This was due to the assumption that flow rates of less than 1 m/s were not strong enough to power the turbine resulting in a zero power output during this period.In order to calculate the power produced during the cut-in phase,the velocity was calculated using the average of all the data between 1 and 2 m/s.

典型的MS/RS主要由高密度存储货架、提升机、多层穿梭车、轨道、控制系统和仓储管理系统(Warehouse Management System，WMS)构成。其中，多层穿梭车在货架每层轨道上运行，负责货物的水平运输;提升机安装在巷道口，负责货物的垂直运输，而DMS/RS的提升机单次可以运输两个货物。单个货架的示意图及俯视图如图1和图2所示。

The following assumptions were made for the given reasons:

The average power for each phase could then be combined to find the average power produced over the lunar period,resulting in an output of 105.2 kW at site 3 and 127.7 kW at site 8.This confirmed the initial calculations identifying site 8 as the most efficient location in terms of produced power for a tidal turbine between ten different locations.

7 Discussion

7.1 Site 8

Site 8 is situated mid-river between Morpeth Dock and Albert Dock.A single 17.2-m rotor multidirectional turbine located there would be able to produce an average output of 127.7 kW over the lunar period;however on average,the turbine will benon-operational for 30.5%of the time due to the low velocities of the flow at this point.This means that the turbine would be operational for an average of 17 h a day,during which the maximum capacity of 430 kW would be achieved for a period of one and a half hours.The average power produced during the operational phase is 183 kW.In total,the turbine would have the potential to produce 91 MWh per lunar month,which equates to an annual output of 1.12 GWh.

Table 3 The percentage of time spent operating in each period over the lunar month

Group Site 3(%) Site 8(%)Rated power 11 7 Cut-in 67.4 62.5 Non-operational 21.6 30.5

However,due to the site location in one of the busiest sections of the river,a turbine is unlikely to be deployed at this location due to the increased hazard that it would pose to any vessels that are navigating to one of the many surrounding areas,including the Liverpool Cruise and Tranmere Oil Terminals and the docks at Cammell Laird Shipyard and Brunswick.

7.2 Comparison to Initial Feasibility Results

Even though the results of the initial feasibility study could not be verified,a comparison could still be made between the annual power outputs calculated for the sites identified in each report.The first report found that an 11-m turbine located at site F(situated mid-river between Perch Rock and Gladstone Lock),would be able to generate an average of 20.6 kW over the lunar period,which equates to an annual output of 0.18 GWh.When compared to the output of 1.12 GWh per year of site 8,it is clear that site 8 is significantly better suited for tidal power generation.This is as a result of the higher flow rates observed as well as the ability to encompass a larger diameter turbine at this location.

In order to increase the power output at site 8,a multiturbine array such as the one used at the Strangford Lough site could be employed(Marine Current Turbines Limited 2016).This would effectively double the power output calculated for the single rotor turbine to 2.24 GWh per year.This figure can be compared to the 6 GWh per year output of the existing SeaGen turbine.The significant difference between these two outputs is due to the larger swept area and faster tidal flow observed at the Strangford Lough site.In order to assess whether a turbine will be an efficient form of power production,a feasibility study would need to be undertaken to determ ine the cost-effectiveness.

7.3 Modelling and Simulation

Through the modelling and simulation process,a number of discrepancies and problems were observed due to limitations of the software or lack of available data,all of which will have had an effect on the final results.

Due to the lack of river depth data for the area between New Ferry and Runcorn,data had to be generated in order to complete the model.This initially resulted in large discrepancies between the measured and known data for each of the tidaldiamond positions.Using atrial and error approach,data for this area of the model was modified,in order to change the velocity profiles of the flow to within a respectable degree of tolerance to the data supplied by the Admiralty Chart.Despite this,there was still a degree of error between the data obtained from the model and the chart.If further data had been available,a more precise model could have been generated for this section of the estuary and as a result,a better comparison between the model and the actual conditions could have been achieved.

对于摩擦作用对材料变形的影响，已有不少学者做了研究。李达人等[7]通过数值模拟方法确定了W-40%Cu粉末烧结材料在热加工数值模拟过程中的摩擦因子。邓华红等[8]通过数值模拟研究了叶片精锻过程中摩擦的作用，发现摩擦对温度场和载荷形成曲线均有较大影响。马勇等[9]采用有限元软件分析了不同摩擦条件对7075铝合金等通道角挤压过程的影响，发现随着摩擦因数增大，载荷峰值明显增大甚至成倍增长，且载荷值波动加剧，等效应力应变分布不均匀。本文结合Deform 3D有限元软件，研究了摩擦系数对2024铝合金的热模拟压缩过程的影响。

Table 4 Power outputs for different periods at site 3

Time(%) Velocity(m/s) Power(kW) Average power over time period(kW)Rated power 11 2 208.6 22.9 Cut-in 67.4 1.67 122.2 82.3 Non-operational 21.6 0 0 0

There was a discrepancy of around 0.4 m between the tidal height predicted by the global tidal model and the predicted tidal height for Gladstone Lock.However,the effects of this error will have been minimised during the process to validate the model.

During the simulation analysis,a problem with recirculation of the flow was identified at the entrance to the river.This phenomenon in the model was later attributed to the process of the inlet boundary conditions drying during low tide as the tide dropped below these abed height.This occurred during low tide for a number of different tidal conditions and led to the objective to validate the initial repot being dropped due to the turbulent results near the boundary.In order to resolve this,the depth data at the entrance could have been edited;however,this would have led to further disruption to the data resulting in the same outcome.Further studies could be done using data obtain from the Admiralty Chart 1951,which details the approach to Liverpool.This was not carried out in this study as it would have increased the complexity of the model pushing the 1000 node limit imposed by the student licence for the software.

During the meshing process,the shape of the riverbank was altered through the use of the redistribute vertices tool.This resulted in a rounding of the edges of the model compared to the relatively straight edges of the River Mersey.As a result of this process,there will have been a slight change in the flow characteristics close to the river banks.However,the effects this had on any of the potential sites identified were considered insignificant,since none of the turbines were situated within 200 m of the banks.

7.4 Assumptions

The assumption that the power rating of the turbine would be limited to 2 m/s was made based upon the rated power of the SeaGen turbine achieved at water speeds of 2.2 m/s.However,further work will be required to identify the optimal power rating for a turbine at site 8.This would require the undertaking of a cost-based analysis to determine whether or not the price of increasing the power rating of the turbine can be offset.It will require calculating the extra power produced while factoring in the decreased duration of time that this higher output could be achieved.

通过打开喘振控制回流阀，调节压缩机入口流量，避免压缩机因流量太小使操作点到达S1，因喘振工况对压缩机造成损坏。

It should be noted that once the rated power of a turbine is reached,the power output is constant—providing a steady supply to the National Grid—whereas the power produced between the cut-in period and the rated power level fluctuates over time.This results in an unsteady rate of supply and creates difficulties when exporting power to the grid.

1.3.2 不良反应的评定 根据患者明显感觉到的不适及需要对症处理的临床症状，分别记录服药过程中恶心呕吐、腹痛腹胀和里急后重等不良反应。

超重者峰值摄氧量明显高于体重正常和肥胖患者，分别是（27.6±8.0） ml/kg/min、（24.4±5.9）ml/kg/min 、（22.4±3.7） ml/kg/min，差异有统计学意义（P＜0.01）。见表4。

An efficiency rate of 45%was used in order to calculate the power produced by the turbine.This value is the minimal operating efficiency associated with the SeaGen turbine.However,since its installation,advances have been made to improve tidal stream generator efficiencies and current technology is boasting of efficiencies in excess of 55%.It should be noted that there is currently no source of data to corroborate this under operational conditions.However,the potential to increase the turbine at site 8 operating efficiency from 45%to 55%would increase the annual power output by 1.12 to 1.37 GWh a year.

7.5 Turbine Type

The turbine simulated was a fixed-based multidirectional turbine.This type of device was chosen due to its popularity within the industry;however,it is not the only type of turbine that could be used to produce power from the river Mersey.Currently,there are seven different classifications for tidalturbines as described by the European Marine Energy Centre in Orkney(EMEC 2017):

Table 5 Power outputs for different periods at site 8

Time(%) Velocity(m/s) Power(kW) Average power over time period(kW)Rated power 7 2 428.7 29.9 Cut-in 62.5 1.42 156.4 97.8 Non-operational 30.5 0 0 0

Horizontal axial turbines

Vertical axis turbines

Oscillating hydrofoils

Tidal kites

Venturi turbines

从提起公诉到审理完结，只用了仅仅4天时间，李凌正是这“4天奇迹”的缔造者。为了将这起简单刑事案件又快又好地审理完结，李凌主动出击，从被告人的基本情况入手。李凌多次联系将邹某逮捕的娄星区公安局的办案人员，通过查卷宗以及询问办案人员的方式，最后发现被告人邹某曾有过前科。在掌握了这些情况的条件下，李凌认为该案可以依法适用简单程序，实行独任审判。

Archimedes screws

Alternative designs

Some of the turbines above may provide a viable option to extracting tidal power from the river limiting the danger to marine traffic.However,the designs are still being developed,tested and improved,so the decision was made to focus on the traditional fixed-based horizontal axis turbine,which is beginning to be deployed commercially.

7.6 Errors

Due to the theoretical nature of the investigation,a number of errors could have occurred.In an attempt to reduce the errors,the model was validated against known tidal speeds.Initial testing found drastic differences between the two;this resulted in adjustments being made to the model.Further testing then found that there was a close correlation between the values produced by the model and known flow speeds at specific points in the river.As a result,the model was considered to be validated despite the errors.

It is important to remember that the report objective was to identify the optimal location for the turbine and provide an assessment of the potential power outputs that could be expected.This is the first stage of a larger feasibility study as it locates the position for further investigation,allowing a targeted approach to real-world testing of the site before any decision would be made about turbine deployment.

8 Conclusion

There is the potential to produce a minimum of 1.12 GWh per year from a 17.2-m single rotor turbine at site 8 with the potential to double this to 2.24 GWh through the uses of a multi-turbine array.However,due to its location in the centre of one of the busiest sections of the river estuary,it is unlikely that a turbine will ever be situated in the River Mersey at this site,due to the increased risk that it would pose to marine traffic in the area.

The tidal shift in the river Mersey cannot be ignored as a potential source of power.However,the common problem seems to be the hazard that tidal turbines poses to marine traffic in the area.As a result,further investigations should be carried out to identify if there is a way of reducing this hazard;this could be achieved by moving the location of the turbine to a lessoptimal position outside of the shipping lanes,or by changing the type of turbine used.

Acknowledgments The authors would like to thank DHI for granting access to the M IKE software and John Carrier for providing technical comments.The authors would also like to thank Liverpool John Moores University and Royal Academy of Engineering for supporting this study.

References

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