LOCATION AND DESIGN OF COOLING WATER INTAKE AND OUTLET STRUCTURES FOR POWER PLANTS IN GERMANY S COASTAL AREA

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1 LOCATION AND DESIGN OF COOLING WATER INTAKE AND OUTLET STRUCTURES FOR POWER PLANTS IN GERMANY S COASTAL AREA Tillmann Baur IMS Ingenieurgesellschaft mbh, Hamburg Abstract: Planning of cooling water structures for coal fired power plants actually experience a renaissance in Germany. The locations along the German shoreline are beneficial for two reasons. Firstly, economic transportation of coal to the sites can be guaranteed from sea. Secondly, the often applied oncethrough cooling technology requires the withdrawal of large quantities of cooling water. The paper summarises the concepts for location and design of the intake and outlet structures of a cooling water system and presents a project example carried out for a site at the Norderelbe in Hamburg. I. INTRODUCTION After governmental decisions about the nuclear phase-out, the standard operating life of nuclear power plants in Germany is limited to 32 calendar years from the first day of commercial production. The oldest of the 19 reactors in operation was shut down under the terms of the agreement between the government and the power industry in late 2003, the newest will be shut down in Based on the atomic energy compromise, coal fired power plants actually experience a renaissance in Germany. According to an expert study [1] coal has an excellent competitive position in power generation in the European Union such as in Germany. In the north of the country, 12 new coal fired power plants with newest technology are planned. The project sites with access to the sea are located in the federal states of Lower Saxony (LS), Schleswig-Holstein (SH), Mecklenburg-Vorpommern (MV) and Hamburg (HH). Table 1 gives an overview based on data of reference [2]. In several sites more than one power station are planned due to favourable boundary conditions. TABLE I. PLANNED POWER PLANTS IN GERMANY S COASTAL AREA Power Station Hamburg- Moorburg Federal State Capacity in MW HH Kiel SH Brunsbüttel SH Brunsbüttel SH Lubmin MV Dörpen LS Emden LS Stade LS Stade LS Wilhelmshaven LS Start operation Power Station Federal State Capacity in MW Wilhelmshaven LS Start operation The locations along the German shoreline are beneficial for two reasons. Firstly, economic transportation of coal to the sites can be guaranteed by bulk carriers from sea. Secondly, the often applied once-through cooling technology requires the withdrawal of water that passes the condenser and is then discharged back into the water body. Because of the large quantities of cooling water required, and to prevent adverse thermal impact on the ambient water, large power plants with oncethrough cooling are preferably located along sea and estuary coasts. The paper gives an overview on the design concepts for cooling water intake and outlet structures published earlier (e.g. [3], [4]), and describes a hydraulic engineering project where the concepts have been applied. Note that the design concepts apply to all power plants where oncethrough cooling technology is required. II. LOCATION OF COOLING WATER STRUCTURES IN TIDAL WATER BODIES The cooling water system consists of the intake, the pumping station, the cooling water pipes and the outlet where the warm water is released to the receiving water body. The locations along sea and estuary coasts may cause warm water recirculation from the outlet into the intake as a result of tidal current reversal. Additionally to the direct warm water recirculation between outlet and intake, in tidal water bodies also the indirect recirculation has to be considered when the warm water released in an earlier tide passes the intake after changing the flow direction. Not only is the transport of the released cooling water with the current of importance, but also the decrease in temperature achieved by initial mixing with the ambient water (see Fig. 1). 1

2 For the location of intake and outlet structures in tidal water bodies, the following options with regard to avoiding warm water recirculation are given: 1. Discharge of the warm cooling water in an area with strong currents and large water depths. This guarantees intense initial mixing, decrease of the temperature and convective transport of warm water. In this option the outlet needs to be away from the shoreline, whereas the intake is close to the shoreline. 2. The cooling water discharge close to the shoreline is possible, when the released water is transported with the ebb or flood current downor upstream respectively, so that it cannot reach the intake which is located offshore. 3. If both structures need to be located close to the shoreline due to given constraints, direct recirculation can only be avoided by increasing the distance between both structures. The intake should be located upstream of the outlet during the prevailing flow direction. Additional supporting elements that guide the released cooling water off the intake structure, e.g. sheet pile walls or dams, can be helpful. The decision for one of the mentioned options has to be made according to the given hydrological, bathymetrical, morphological and nautical boundary conditions. The problem of the warm water recirculation during slack water can only be solved by a sufficiently large distance between the outlet and the intake. To avoid direct recirculation during slack water, the withdrawal of water from the deeper layers of the water column can be helpful. With low flow velocities stratification of the warm water above the cooler ambient water occurs. III. Figure 1. Example of warm water discharge plume with temperatures in C from [1] DESIGN OF COOLING WATER STRUCTURES A. Intake structures The cooling water is delivered to a cooling system consisting of an intake, water filtration equipment, pumps and other supporting instrumentation. An intake should provide the plant with an adequate quantity of water of suitable quality, i.e. mainly low temperature and free of sediments. Mainly the required discharge and the local conditions determine the intake type. Basic types of intakes can be classified in submerged and in open channel intakes. At the same time the intake can either be placed onshore or offshore. Open channel intakes are not suitable in tidal water bodies and are therefore not discussed further in this paper. A submerged intake which draws in water below the surface is required in situations where the intake must be placed in an area that will be covered by the thermal plume at some time. It is consequently used in tidal water bodies. An example is depicted in Fig. 2. Figure 2. Submerged offshore intake structure for moderate discharges An onshore submerged intake is only suitable when there is sufficient depth for the intake to be raised slightly above the bed not to draw in sediment and also to be adequately submerged not to draw in surface water. These intakes are rare as the ambient conditions are usually unfavourable. Ideally, water depths should be as given for offshore intakes. Where the water depth and the bed slope are shallow, it is necessary to locate the intake offshore. Water is drawn in through pipes. The intake opening should be at least 0.5 m above the bed. The exact location of the intake will partly be determined by the predicted spread of the thermal plume. However, the intake itself may affect the spread. The intake should therefore be designed in combination with the outlet. The intake does not necessarily need to be combined with the pumping station in a single structure. The location of intake and pumping station can either be: onshore intake in combination with pumping station offshore intake, intake channel and onshore pumping station offshore intake in combination with pumping station. Two separate structures are beneficial when the intake needs to be offshore and the access to the structure is hindered as the distance to the shoreline 2

3 is large. Two structures are also beneficial when a combined offshore structure with larger dimensions is too expensive or not possible due to navigational constrains. In this case the water is delivered to the pumps through an intake channel which can be freesurface or submerged. The water discharges into the free-surface chambers upstream of the pumps so that the suction head is reduced to an allowable level. Fig. 3 shows an example. Figure 3. Onshore pumping station separated from intake If the intake channel is long, the offshore intake structure usually has a free-board above the maximum water level in order to provide boat landing and access to the channel from both endings. The structure needs to be visible for navigation and be protected against collision. B. Pumping stations The pumping station contains water filtration equipment, the pumps and other supporting instrumentation like stop logs that provide isolation for maintenance work. The depth of the structure is dependent of the lowest water level and the required minimum Net Positive Suction Head (NPSH) of the pumps. The other dimensions are defined according to the instrumentation applied, mainly the filtration system which very often consists of coarse bar screens and skip-rake fine bar screens. When the pumping station is located offshore in combination with the intake, the technical equipment should be automated to reduce the requirement for personnel to approach the structure during operation. Otherwise a transport bridge for frequent access is required. C. Cooling water pipes The economic diameter of the cooling water pipes can be calculated from the ratio of investment costs and operation costs. Small diameters are less expensive at time of the investment but cause high flow velocities correlating with large hydraulic losses and consequently higher operation costs. For maintenance works the pipes should be big enough to allow for manual inspection. A diameter of more than 1.50 m is recommended. D. Outlet structures The outlet structure must adequately release the process water without negative impact on operation and the local environment. The outlet structure discharges water with a higher temperature and thus a lower density than the receiving water. The design of the outlet structure can affect the hydraulic conditions of the tidal current. The often jet-like directed discharge into the receiving water changes the local flow field. It is therefore a main design criterion to adapt the discharge velocity to the local conditions of the receiving water. For evaluation of the design numerical modelling is often applied (sea below in Section V). Outlet structures can be designed either for surface or submerged discharge. Before a comparison of surface and submerged discharges can be made, the legislation controlling thermal discharges and the reasons behind it should be considered. 1) Submerged discharges Submerged outlets serve only for the release of relatively small amounts of water. Submerged discharges may either be through a single or a small number of ports. The ports can be oriented vertically or horizontally. To maintain the water pressure in the system, the ports of submerged outlets are generally located below the lowest water level. In tidal rivers very often the difference between the lowest design water level and the highest point of the cooling water system can reach a critical level so that the pressure drop may cause cavitation. In this case, a siphon pit (pressure control tank) is used at an appropriate elevation combined with a free weir overfall so that the cooling water system always experiences a positive pressure head. Downstream of the overfall, the releasing cooling water pipes guide the water into the discharge port. Another type of submerged discharge may through multi-port diffusers located at the river bed. This type has not been used in German water bodies due to possible hindrance of navigation and due to negative effects of sedimentation on operation. Multiport diffusers are very popular in the US. They are described more in detail in [5]. Submerged diffuser discharge schemes provide rapid mixing and much lower maximum surface temperature than are possible with surface discharge. However, surface discharge is preferable due to the higher rate of surface heat dissipation. 2) Surface discharges Surface discharge provides the easiest way of discharging cooling water in the receiving water body. The most popular type of outlet is a free weir overfall downstream of the siphon pit. The weir height depends on the required pressure head in the cooling water system and on the natural water level fluctuations in the tidal water body. In order not to affect navigation by high discharge velocities, the relatively strong current in the channel has to be reduced within the outlet structure. The latter consequently very often needs to be equipped with calming elements that reduce the flow velocity to the allowed level. Typical outlet structures can be divided by their hydraulic concepts of natural or forced spreading of the flow. Natural spreading occurs due to diffusion of turbulence with an angel of about 10 related to the 3

4 axis of the discharged jet. The mean velocity is reduced accordingly. When the outlet structure opening is steeper than this diffusion angel, no reduction of the flow will take place (see Fig. 4). In this case calming elements are required. v without calming v with calming Figure 4. Velocities at outlet structure with surface discharge according to [4] Forced spreading can be achieved with guiding walls that guide the flow towards the outer sides of the flow field. Another option is the construction of transverse walls located at the bottom or at the water level. Transverse walls can also be formed by sheet piles with openings, so that the discharge crosssection is smaller in the centre of the jet and larger at the outer sections of the jet (see Fig. 5). guiding walls transverse walls Figure 5. Hydraulic concept of forced spreading at outlet structure with surface discharge according to [4] IV. ENVIRONMENTAL ASPECTS Impact on the local environment may occur either by the withdrawal of the cooling water or by its release. Discharge of cooling water heated to levels significantly above temperatures of the receiving water body can alter aquatic ecosystems. This is why in German controlling legislation the following temperature limits for operation are often defined: max. temperature: 30 C max. temperature rise: 10 K The limits can be individual for different project sites and are predefined during the planning approval by environmental authorities. To reduce negative impact, special demands can be given by legislation also with respect to the concentration of dissolved oxygen of the released water. Appropriate design concepts as by IAHR [6] need to be applied. The impacts of cooling water withdrawals are characterized as entrainment, where small aquatic organisms are carried by the cooling water into the power plant and killed by heat, and as impingement, where the cooling water intake traps larger organisms against the intake screens. The detailed design of the intake must protect fishes which are drawn towards the intake. Ideally, the intake should be designed so as to reduce the number of fishes attracted towards it, and remove safely any fish that do become trapped on the screens. Elaborate methods of fish protection are necessary. So called behavioural barriers use a behavioural response of fish to avoid entry into the intake flow. They include sound and light barriers, air bubble curtains, and electrical barriers. In Germany mostly, electrical barriers are applied. However, they sometimes were determined to not have demonstrated effectiveness for a wide range of species and environmental conditions, especially in brackish water conditions. V. MODEL INVESTIGATION A. Plume modelling Generally, modelling studies are carried out related to the hydraulics of the cooling water with special regard on the prediction of the thermal discharge plume at the outlet. Especially the increasing concerns by legislation require more sophisticated approaches on this field. In the planning phase of cooling water structures hydraulic modelling is consequently always applied. Plume modelling is mostly undertaken with numerical models. For the correct schematisation of density driven flows 3D-simulations are required. It is common practise that institutions which operate and maintain sophisticated flow models perform also plume modelling studies as consultancy services. B. Modelling of approach flow to pumps For the investigation of intake structures including the pumps, very often physical scale models are used in detailed design studies [7]. In these models, the following hydraulic parameters, that have negative influence on the performance of the pumps, should be tested: - the occurrence of dead water zones, flow separation or reverse flow - vortex building and air entrainment in the pump compartments - pre-rotation in the pump suction pipes. For the optimisation testing, the design is fitted with local modifications, such as splitters (bottom and back wall), crosses below the suction pipe bell mouths and corner fillings [8]. The modelling of pump intakes with computational fluid dynamics (CFD) is still on a research level. Both, single-phase and multi-phase simulations are 4

5 performed. The stochastic character of small-scale vortices, resulting in wall tornadoes and air entrainment, requires a transient calculation on a sufficient fine grid and a highly sophisticated treatment of surface tension. Although CFD may not yet replace physical model testing, it can be used to evaluate the effects of design modifications, and thus replace the more qualitative desk studies. It may also be used to evaluate a final design, and replace the rather expensive physical scale model testing [9]. VI. PROJECT EXAMPLE: POWER STATION PEUTE, HAMBURG The author has been involved in the planning procedures associated with locating and designing of cooling water intake and outlet structures for several power plants in coastal areas of Germany. They have energy outputs between 100 and 1600 MW and require discharges of sea water between 11 and 60 m³/s. In the following the results of the first planning phase for the cooling water structures of the planned power station Peute, Hamburg, are described. A. Operation conditions In 2006, a feasibility study was carried out with respect to the cooling water intake and outlet structures for a new 100 MW power plant located in Hamburg at the northern branch of the river Elbe called Norderelbe. The electricity is intended to be used by local industry only. The power plant will not be fired by coal but by so called refuse derived fuels (RDF) of wastes. However, the design concepts apply also to power stations with smaller capacities such as in this project. For the summer and winter season, a cooling water discharge of 40,000 m³/h with a temperature rise of less than 5 K and a discharge of 20,000 m³/h with a temperature rise of less than 10 K is planned respectively. Both conditions guarantee a maximum output temperature of less than 30 C as predefined by legislation. B. Boundary conditions The Norderelbe is a tidal water body with a mean river discharge of 1,000 m³/s. The mean flow velocities are about 0.7 m/s in both flow directions. The maximum current is 1.0 m/s. The bathymetry of the river is between 5.0 m (western bank) and 6.5 m (eastern bank) below mean sea level NN. The mean tidal water levels range between NN m (MThw) and m (MTnw). For the cooling water operation a design lowest water level of NN m was defined. Other boundary conditions were given by a neighbouring highway bridge and a jetty, transport pipes and navigation in the river, flood protection as well as the soil conditions. C. Planning alternatives Realising the above mentioned design concepts, different planning alternatives have been elaborated based on the local boundary conditions. To improve the oxygen conditions in the river Elbe, it was decided to use a free weir overfall and release the warm cooling water as surface discharge. To reduce the effect of hydraulic recirculation it was decided to withdraw the water from the deeper layers of the water column. From a set of 5 different locations for the inlet and outlet structures, 10 possible combinations were defined and evaluated. In the following, the two most promising solutions are described: Option 1: The offshore submerged intake is located at the western bank of the river and with the closest possible distance to the highway bridge foundation in order to provide minimum additional hindrance to navigation. The pumping station is separated from the intake structure to improve access for operation. The outlet is located downstream of the intake in an onshore structure combined with a dredged channel in the flat area that leads to the main river (Fig. 6). The distance to the intake is as large as possible under the given boundary conditions. O initial dredging I Option 1: intake SW - outlet MV 40 m Figure 6. Location of intake (I) and outlet (O) structures in option 1 The outlet structure does not affect navigation by lateral currents due to the diffusion of the flow velocities in the flat area of the river. All structures can be placed on the area of the power station. The investment costs are relatively small and the approvability is evaluated as positive. The intake structure design is presented in the longitudinal sections of Fig. 2 and 3. Option 8: The offshore submerged intake is located at the eastern bank of the river where the water depth is high and the intake is even less affected by the warm water released at the opposite western bank than in option 1 (see Fig. 7). The pumping station is combined with the intake structure. The water is guided through an intake pipe to the power station. The closest possible distance to the bridge foundation also provides minimum additional hindrance to navigation here. Disadvantages of this option are the cost intensive pipes through the river and the location of the intake which is outside of the area of the power station. 5

6 Both, investment costs and approvability are evaluated not as good as in option 1. However, the likelihood of hydraulic recirculation is smaller than with option 1, and safe operation can be guaranteed. The outlet structure for free surface discharge that can be used in both options is shown in Fig. 8. Option 8: intake SE - outlet NW O siphon 40 m I Figure 7. Location of intake (I) and outlet (O) structures in option 8 Figure 8. Outlet structure for free surface discharge with siphon pit and free weir overfall.conclusions After governmental decisions about the nuclear phase-out in Germany, new coal fired power plants are planned in the North of the country for operation with once-through cooling technology. For the location and design of the cooling water intake and outlet structures numerous options exist under the given boundary conditions. As the location in tidal water bodies may cause warm water recirculation from the outlet into the intake numerical modelling is very often applied to predict the warm water discharge plume. Besides this, certain design concepts need to be considered as it is presented in this paper for a project site at the river Norderelbe in Hamburg. ACKNOWLEDGMENT The presented project was carried out for KPP Kraftwerk Peute Projektmanagement GmbH & Co. KG. The author appreciates the opportunity to publish the findings in this paper. REFERENCES [1] Prognos: The Future Role of Coal in Europe, Final Report, June [2] Deutsche Umwelthilfe: Genehmigte, geplante, zurückgestellte und aufgegebenen Kohlekraftwerke in Deutschland (Approved, planned, postponed and resigned coal fired power plants in Germany), Stand Feb [3] Miller D.S. & Brighouse B.A.: Thermal Discharges a guide to power and process plant cooling water discharges into rivers, lakes and seas, British Hydromechanics Research Association, [4] DVWK: Querströmungen und Rückgabebauwerke an Wasserstraßen (Lateral currents and outlet structures at waterways), Schrift Nr. 67, Paul Parey, [5] H. Jirka, T. Bleninger, Design of multiport diffuser outfalls for coastal water quality protection, Proceedings XXI Congreso Latinoamericano de Hidráulica, IAHR, ed. A.M. Genovez, São Pedro, Brazil, October 18-22, [6] I. R. Wood, Air entrainment in free-surface flows, Hydraulics Structures Design Manual No. 4, IAHR, A.A. Balkema / Rotterdam, [7] Schäfer, F.; Hellmann, D.-H.: Optimization of Approach Flow Conditions of Vertical Pumping Systems by Physical Model Investigation, ASME 2005 Fluid Engineering Division Summer Meeting and Exhibition June 19-23, 2005, Houston, TX, USA, ASME [8] J. Knauss, Swirling flow problems at intakes, Hydraulics Structures Design Manual No. 1, IAHR, A.A. Balkema / Rotterdam, [9] WL Delft Hydraulics, 6