A Review of Technologies Employed on Some Recent UK Power Plant Projects to Mitigate the Impact of the Cooling Water Intake on Aquatic Life

International Fish Screening Techniques 2011 191

A review of technologies employed on some recent UK power plant projects to mitigate the impact of the cooling water intake on aquatic life

N. R. Rogers Directory of Technology, Ovivo Water UK Ltd., UK

Abstract

The abstraction, circulation and discharge of large volumes of water is essential for the generation of electricity by power plants utilizing open circuit, once through, cooling. However there is increasing awareness of the impact of this process has on aquatic life through entrainment of fish and other biota in the water intake channels and impingement on the screening elements. This paper reviews some technologies applicable to large cooling water intakes to mitigate the impact of the abstraction and screening (filtering) of cooling water and this is illustrated by two example power plant projects recently constructed in the UK, encompassing abstraction flows up to 40 m3/sec. In particular, the key features of Fish Recovery and Return (FRR) systems for Band and drumscreens is described and the benefit of combining screens fitted with a FRR system with an upstream “behavioural screens” based on Acoustic Fish Deflection (ADF) technology will be discussed. An outline of some areas for future development work will be given. Keywords: cooling water intakes, screening, bandscreen, drumscreen, fish recovery and return, acoustic fish deflection, behavioural screens.

1 Introduction

The reliable screening of “raw” water abstracted from lakes, rivers or oceans for the subsequent use by process cooling, desalination or for consumption is common to many industrial processes. Whilst it is not a complex technology, it is crucial to the operation of the downstream process and its efficient operation has

WIT Transactions on State of the Art in Science and Eng ineering, Vol 71, © 2013 WIT Press www.witpress.com, ISSN 1755-8336 (on-line) doi:10.2495/978-1-84564-849-7/016 192 International Fish Screening Techniques 2011 a significant bearing on the overall efficient operation of the plant. Power plant that utilises once trough, open circuit, cooling can extract very large volumes of water and there is growing concern regarding the impact of such water intakes on the aquatic ecosystem (a comprehensive list of studies of the impingement of fish on cooling water intakes is provided in [1]). Legislation in the USA and, to a lesser extent, in Europe (for selected species), requires measures to be under taken to minimise the entrainment and impingement of fish and other biota. There is also operational advantages in minimising the impingement of fish as there is a cost for disposal and potential for disruption to generation due to the loss of cooling water as a result of massive inundations of schooling species or jellyfish [2, 3].

2 Power plant cooling water requirements

Table 1 shows typical ranges of water abstraction rates per GW of electricity generated for plant generation type and cooling technologies (adapted from [4]). Once through (open circuit) cooled power plant abstracts a much greater volume of water per GWe than “closed” cooling water systems for similar generating technologies. (Notably, open circuit cooled Nuclear Power Plant extracts significantly more cooling water then fossil fuel plant; this is because of the lower steam temperatures used and thus it is less thermodynamically efficient.) As the entrainment/impingement of aquatic specifies is clearly g closely correlated to the volume of water extracted, so the question arises as to why direct cooled power plant continues to be constructed (particularly outside the USA).

Table 1: Typical cooling water requirements per GWe for differing generating and cooling technologies.

Water Withdrawal Plant and Cooling System Type per GWe (m3/sec) Fossil/biomass/waste-fuelled steam, once through cooling 21 to 25 Fossil/biomass/waste-fuelled steam, pond cooling 0.3 to 0.6 Fossil/biomass/waste-fuelled steam, cooling towers 0.5 to 0.6 Nuclear steam, once-through cooling 26 to 63 Nuclear steam, pond cooling 0.5 to 1.2 Nuclear steam, cooling towers 0.8 to 1.2 Natural gas/oil combined-cycle, once-through cooled 7.9 to 21 Natural gas/oil combined-cycle, cooling towers ~0.2 Natural gas/oil combined-cycle, dry cooling ~0

In the UK Environment Agency publication Cooling Water Options for the New Generation of Nuclear Power Stations in the UK [4] one of the considerations was the relative merit of differing cooling technologies on power plant efficiency, complexity and visual impact (Table 2). Whilst the difference in generating efficiency may at first sight be insignificant, an efficiency gain of even a fraction of one per cent translates into significant economic savings and reduced environmental impact: less cooling water abstracted, less heat rejected, less fuel has to mined and transported; less ash, carbon dioxide, sulphur dioxide and nitrogen oxides are emitted. The report conclude that “that direct cooling may be the best environmental option for large power stations sited on the coast or estuaries, subject to current best planning, design and operational practice and mitigation methods”.

3 Intakes for once through cooled plant

The majority of cooling water (CW) intakes for once through cooled commercial power plant utilises onshore screening technology where the screening equipment is located in one or more open civil work channels that may be fed directly from the sea or via either a closed conduit from an offshore intake feeding an open forebay or direct from the sea adjacent to the coastline (Figure 1).

Table 2: Comparison of cooling options [5].

Offshore intakes are used at marine and estuarine sites to minimize the water recirculation and to improve water quality by extracting water away from the shore line where the level of silt and other contaminants are increased and the water temperature is higher. There is often a capping arrangement, referred to as a “velocity cap” that is known to reduce fish ingress by eliminating vertical water currents which fish are ill-equipped to avoid. Some form of coarse screening is provided at the initial point of intake preventing the entry of large items which might otherwise obstruct the inlet tunnels.

Figure 1: Main features of an offshore and onshore water intake utilising onshore screening equipment [6].

Onshore intakes become viable where deep water is found immediately close to the shore. From the intake entrance, water passes to a fore bay protected by a coarse screen which normally has provision for the insertion of stop logs for isolation and maintenance. From this point on the system associated with both intake types are similar: water drawn from the forebay passes through a series of parallel culverts. Fitted across these culverts are vertical bar screens to prevent drifting materials from being entrained into the system. Typically these have apertures lying between 30 to 100 mm. The trash rack can be equipped with an automated raking mechanism to remove accumulated debris. These parallel culverts typically feed into screen chambers where the fine debris is removed by travelling screens fitted with a screening aperture lying between 1–10mm. Here either Band or drumscreens can be employed (Figure 2).

4 Fish protection systems

4.1 Behavioural screens

The document “Screenings of Intake & Outfalls: a best practice guide” [7] published by the UK Environmental Agency provides guidance for designers and operators of UK plants including measures to minimise the impact on aquatic life. For large water intakes (>5m3/sec) located on coastal or estuarine waters the cited best practice is a combination of technologies consisting of a “behavioural screen” in the form of an Acoustic Fish Deterrent (AFD) followed by a travelling screen with a proven Fish Recovery and Return (FRR) facility. There are a number of different types of “behavioural screens” each of which provide a sensory stimulus to the fish that encourages an avoidance reaction to divert them away from the point of water abstraction. Commonly used stimuli include sound, strobe lights, bubble curtains (sometimes combined with sound), electric fields and Louvre screens. Electric barriers are affected by water

Figure 2: A typical screening system an onshore intake showing the various types of equipment commonly employed (courtesy of Ovivo Water UK Ltd). conductivity and are unsuitable for marine or brackish water environments and their use gives rise to a potential safety issue to the public [7]. Sound has the advantage of penetrating turbid waters and research has established that a varying frequency within the range of 10 Hz–3 kHz is most effective. For power plant CW intakes an AFD typically consists of a number of special loud speakers, termed sound projectors (SP), arranged underwater to form a Sound Projector Array (SPA). There may be 6 to 90+ SPs depending on the size of the intake. These in turn are coupled to dedicated amplifiers, a diagnostic unit and signal generator. The number and location of the SPs are optimised by software specially developed to model underwater sound fields (named “PRiSM”). This takes into account the reflections of sound from the seabed, sea surface and structures in the immediate vicinity (Figure 3). The SPs are normally located at the point of abstraction, typically just upstream of the coarse bar screen (Figure 4) and are mounted on rails attached to the civil structure allowing them to be raised for annual maintenance (Figure 5). Not all fish species are equally sensitive to the acoustic or other stimulus nor is the acoustic barrier 100% effective even on the sensitive species [7]. The more sensitive marine fish tend to be pelagic fish (mid-water swimming silvery fish) and they also tend to be the most susceptible to damage from contact/handling. The more robust bottom feeders (flat type fish) and sinuous fish (eels, lampreys, etc) tend to be less sensitive to the AFD but less susceptible to injury through physical contact or handling. Thus, to protect the fish that bypass the behavioural screen, the travelling fine screen incorporates modifications that provide a FRR system designed to collect and return the fish back to the water body with the minimum of stress. Typical overall efficiency for the combination of the AFD and FRR, when applied to bandscreens, is shown in Table 3.

Figure 3: Example of a PrISM acoustic field model for a SPA; levels in dB (courtesy of Fish Guidance Systems Ltd).

Figure 4: Typical installation of a Figure 5: An SP raised for SPA on two channel maintenance, Doel intake (courtesy of Fish Nuclear Power Guidance Systems Ltd). Plant, Belgium (courtesy of Fish Guidance Systems Ltd).

4.2 Fish recovery and return systems

4.2.1 Dual flow bandscreens Bandscreens are designed for installation onshore in the water intake channels downstream of the coarse bar screens where they provide an efficient means of continuously removing floating and suspended solids. The screens are commonly fitted with mesh apertures ranging from between 2mm and 10mm (in 2 dimensions). Design capacities generally fall within the range of 0.5 to 15m3/sec

Table 3: Summary of average fish deflection efficiency for a SPA and the survival rates for bandscreens fitted with a FRR system and the total efficiency (collated from data from a number of trials).

Acoustic fish Sensitivity of deflection Survival rate fish to acoustic system for fish return Expected overall system efficiency system efficiency High 85% 10% 86.5% Medium 50% 65% 82.5% Low 20% 80% 84.0%

per screen. There are three commonly used flow patterns termed Through Flow (TF), Centre Flow (CF) and Dual Flow (DF). The DF flow pattern is explained in Figure 6.

Figure 6: With a DF bandscreen the raw water enters either side of the screen, debris is trapped on the outside surface of the moving screening band, which carries the trapped solids to deck level where it is backwashed into an external debris hopper, which conveys the solids to a central collection area. The DF bandscreen produces a converging flow which makes it more suited to close coupling to the CW pump, thus this can reduce footprint and subsequent civil work costs.

Regardless of the flow pattern the control principle of the screen is similar with the operation being initiated by headloss across the screen (typically starting at 100–150mm) and over ridden by a cyclic timer. Bandscreens are normally two-speed or variable speed, operating between 3 to 12 m/minute; the speed increasing with rising. The fish elevator profile was developed using full scale models in test flumes to provide a stable area for the fish to shelter whilst under the surface (Figure 7).

Figure 7: A schematic of a FRR bandscreen fish capture, elevation and sluicing process. The elevator provides a sheltered region for fish to settle and then they are subsequently elevated to deck level in a water pool and then gently sluiced into a flooded return trough (courtesy of Ovivo Water UK Ltd).

When the elevator rises above the surface it provides a pool of water for the fish. The bandscreen head section is modified to provide two washing stages. The first stage entails a sluicing action using low pressure washwater (0.7 bar) that washes the fish and some of the trash into a flooded launder trough for return to the sea/lake. A second stage washing using high pressure (2.0 bar) removes the more persistent debris which is either collected in a self draining debris basket or is returned to the water body. The location of the discharge of the fish return is critical so as to minimise the recirculation of fish (and some debris) back to the intake and measures need to be taken to prevent predation even at the lowest low water levels. Currently Ovivo has about 22 sites utilising FRR bandscreens totalling about 60 units (mainly USA, some in Canada). There is an installation at Barking Reach Power Station, UK located on the Thames where 5 bandscreens fitted with FRR have been operating since 1992 (total flow 21.3m3/sec).

4.2.2 Double entry drumscreens Drumscreens provide an alternative to bandscreens for the fine screening of an intake flow. Screening aperture typical ranges from 1.0 mm to 10mm. Capacity per machine can exceed 40m3/sec per screen, more than double that of bandscreens. There are 4 drumscreen flow patterns in commercial use, the most common of which (outside France) is in-to-out double entry, single exit flow pattern commonly referred to as a “Double Entry” (DE) drumscreen (the DE flow pattern is explained in Figure 8). The control principle is similar to that of bandscreens.

Figure 8: With a DE drumscreen the flow enters the screen via two ports in the civil work where it turns through 90 degrees and flows radially out through the screen. A proportion of the flow passes under the screen and then all the flow exits via a single outlet port. The debris accumulates on the inside surface and is backwashed off at deck level by externally mounted washwater jets and collected by hoppers located inside the screen.

Figure 9: Backwashing and debris collection arrangement for a non-Fish recovery DE drumscreen

On power plant applications it is common for the concrete volute CW pump to be close coupled to the outlet of the screen thereby minimising plant foot print. FRR on DE drumscreens has a relatively recent history; the first trial unit was installed on one drumscreen operating at Longannet Power Station (LPS), a 2,400 MWe Coal fired plant operated by Scottish Power and it is currently the UK’s largest direct cooled plant. LPS CW intake has four 16.5m diameter drumscreens, fitted with 8 mm mesh, each designed to pass a flow of 22.7m3/sec. LPS is a prescribed process under the Schedule of the Pollution Prevention and Control (Scotland) Regulations 2000 and, as part of the review process for the operating permit a number of improvement conditions were imposed, including the objective that “By March 2008, the Operator shall review the techniques available for minimizing fish entrainment at water intake. The review shall include proposals for implementation and shall be recorded and reported to SEPA.” In response to this requirement, one drumscreen was modified in 2007 by Ovivo to incorporate a prototype FRR system and a trial was conducted by Scottish Power in the summer of 2007. This involved c and recording the impinged fish species discharged by the screen and monitoring their survival rates after 4 days retention in a holding tank. Scottish Power commissioned a report which was presented in 2008 reviewing options and summarising the outcomes of the drumscreen FRR trial The results were not made public but it is understood that overall survival rate achieved by the prototype FRR, including those fish that were dead on arrival at the sampling point (predominately herrings), was around 30% at the end of the retention period. Following the trial, number improvements were made to the elevators, flushing system and hopper geometry and the FRR modification was then implemented on the remaining 3 drumscreens in 2009 together with a permanent 200m+ long fish and debris return trough engineered to discharge below the LWL.

Figure 10: Modifications to add a Prototype FRR system to LPS drumscreen.

5 Recent UK power projects utilising AFD combined with FRR

5.1 A bandscreen based CW intake – Marchwood CCGT

Marchwood Power Station is an 842MWe natural gas combined cycle gas turbine (CCGT) power plant built on the site of an old power plant, located on the estuary of the River Test, opposite Southampton dock, UK. Marchwood Power Station began generating electricity in December 2009 The intake and outfall structures are those remaining from the old power station, refurbished as part of the construction phase.

Figure 11: Location of Marchwood CCGT power plant, Southampton, UK.

Figure 12: Plan of Marchwood CW intake screening plant.

Figure 13: Overall view of Marchwood CW intake showing a mono-rail mounted raking machine in the foreground and the heads of 3 DF bandscreens in the background

The plant extracts 15m3/sec of CW fed by an onshore intake via three intake channels each fitted with a raked 50mm gap coarse bar screen and a Dual Flow bandscreen fitted with 5mm aperture mesh. Located at the inlet of the intake are 8 FGS Type 30-100 sound projector arrange in 4 columns and linked by an underwater cable harness to amplifiers and signal generators are located at deck level. The average flow velocity at the point of abstraction is 0.37 m/sec. The DF bandscreens operate continuously at a creep speed of 2m/min and have a have a FRR system that returns the fish via an elevated GRP launder which descends in stages to the LWL in front and to the west of the intake. There is also a facility for the division of the fish return flow to a collection basket/tank for surveying purposes.

Figure 14: Diversion in the fish Figure 15: Showing the separate return trough to a debris and return troughs collection tank for for the FRR bandscreens. sampling of returned fish.

5.2 A drumscreen based CW intake – Pembroke CCPP

Pembroke is a new 2,000 MW combined cycle power plant (CCPP) being constructed in Pembrokeshire, S. Wales, on the site of a previously demolished oil-fired Power Station. It will be the UK’s largest CCPP. At the time of writing the plant is in the process of being commissioned. The CW intake utilises the existing intake civil structure where 3 duty plus one standby DE drumscreens are installed, each designed to pass 13 m3/sec.

Figure 16: Location of Pembroke – CCPP.

Figure 17: Plan of Pembroke CW intake screening plant.

Figure 18: CAD model of the 2 stage washing and the fish return and debris hopper arrangement for the drumscreens employed at Pembroke CCPP.

Located immediately upstream of the un-raked coarse barscreen are 72 SILAS™ (Synchronised Intense Light and Sound) arranged in 18 columns supplied to Alstom as part of Ovivo’s contract for the CW intake. These newly developed units combine a sound projector with a circular high intensity white flashing LED light to divert fish species insensitive to the sound field. Each drumscreen is fitted with an FRR system, the design of which is a development on that installed at LPS. This incorporates two stages of washing with separate collection hoppers for fish return and debris capture. The fish return feeds into a large 6m square holding tank which discharges via 200m long pipe into the estuary. The drumscreen and fish recovery backwash operate continuously when there is intake flow, with the screen speed ramping up in response to an increase in head loss.

6 Areas for future work

The Eels (England and Wales) Regulations 200 requires the placement of screens that will protect eels within a diversion structure “capable of abstracting at least 20 cubic metres of water through any one point in any 24-hour period” This will have implications for many CW intakes and other water abstraction sites which currently have travailing screens (Band, Drum of Cup type) and no FRR system. Firstly, the challenge will be to develop a fish recovery elevator that is effective in retaining writhing eels and test its effectiveness in field trails. Secondly, the fish recovery elevators and associate backwashing system will need to fit within the constraints of the existing screen geometry.

For DE drumscreens, used primarily for thermal and nuclear power plant, the hydraulic characteristics of fish elevators needs further refinement to minimise the turbulence induced inside the elevator when submerged at various angles of incidence in the vicinity where majority of the flow passes through the screen. Finally, increased feed back from operators of power plant on the performance of FRR systems would be welcomed as this essential to aid there continued improvement. To date it has proved to be very difficult to obtain any data from UK plant owners although it is known that studies have and are being undertaken.

References

[1] Martinez-Andrade, F., Baltz, D. M., Marine and Coastal Fishes Subject to Impingement by Cooling-Water Intake Systems in Northern Gulf of Mexico: An Annotated Bibliography”, U.S Department of the Interior, August 2003. [2] Turnpenny, A.W.H, Operational and Environmental Effects of Fish Inundation in Cooling Water Systems and ‘Best Practice’ Solutions, Conference Proceedings, PowerGen Europe, 3–5 June 2008. [3] Leel, J., Choi, H., Chae, J., Kim D. S., and Lee, S., Performance Analysis of Intake Screens in Power Plants on Mass Impingement of Marine Organisms, Ocean and Polar Research, Vol. 28(4):385-393, December 2006. [4] Goldstein, R., Smith, W., Water & Sustainability (Volume 3): U.S. Water Consumption for Power Production – The Next Half Century, ERPI Topic Report, March 2002. [5] Turnpenny, A.W.H., Coughlan, J., Ng, B., Crews, P., Bamber, R.N., Rowles, P., Cooling Water Options for the New Generation of Nuclear Power Stations in the UK – SC070015/SR3, Environment Agency, June 2010. [6] Jenner, H. A., Whitehouse, J. W., Taylor C. J.T., Khalanski, M., Cooling Water Management in European Power Stations Biology and Control of Fouling. [7] Turnpenny, A.W.H. and O’ Keefe, N.J. Screening for intakes and outfalls: a best practice guide. Environment Agency, Science Report SC030231, 2005. ISBN: 1 84432 361 7, February 2005. [8] Fletcher, R.I., Gathright, T. T, Marcellus K. L., On the Redesign of Vertically-Travelling Barrier Screens equipped with Fish Recovery Apparatus: Flume experiments and Field Trials, Proceedings: Fish Protection at Steam and Hydroelectric Power Plants, March 1988.