Hydropower: A closer look at Archimedes Screws. Dr Peter Walker APEM Senior Aquatic Scientist

Transcription

1 Hydropower: A closer look at Archimedes Screws Dr Peter Walker APEM Senior Aquatic Scientist

2 Why am I here? To talk about Archimedes screw hydropower Topics covered: A brief, general overview of hydropower UK small scale hydro schemes Advantages of hydro power Disadvantages of hydro power Brief comparison of different schemes In-depth look at Archimedes screws Pro s and con s Research and development

3 What is hydropower? Hydro power = water power Water located at altitude has potential energy due to gravitational forces The power contained within water can be observed during extreme events such as flash floods As water falls this energy is released Humans have developed several means of harnessing this energy and subsequently using it Hydro power has been used to pull or lift objects, turn mill wheels and stones In modern times the primary use of hydropower is to rotate turbines which subsequently produce electricity

4 What is hydropower? Hydro-turbines therefore convert water pressure into mechanical shaft power which drives an electricity generator The power available to any scheme is dependent upon the available head and the flow rate General formula for a hydro system s power output is: P = ɳ ρ g Q H P = mechanical power produced at the turbine shafts (Watts) ɳ = hydraulic efficiency of the turbine ρ = density of water (1000 kg/m 3 ) g = acceleration due to gravity (9.81 m/s 2 ) Q = volume flow rate passing through the turbine (m 3 /s) H = effective pressure head of water across the turbine (m) Flow Head

5 Why is there a drive for hydro development? Energy demands in the UK are increasing year on year Traditional sources of energy are finite therefore alternatives are needed UK Climate Change Act 2008 set target of 80% reduction in greenhouse gas emissions by Renewable Energy Strategy suggests that >30% of electricity should be from renewable sources by 2020 in 2011 it was only 6.7% Hydropower has been highlighted as a limited but important contribution to meeting renewable electricity generation targets Feed In Tarrifs are serving as an incentive for the development of small scale (micro/pico) hydro schemes

6 Hydropower in Scotland Within Scotland there is a long history of hydro power First documented scheme providing power to the public in Scotland = monks at Fort Augustus Abbey in 1890 Fort Augustus scheme powered the monks organ and street lighting for 800 villagers In the early 1900s estates started building small schemes to power estate houses and castles 1950 s & 1960 s = emergence of the national grid and investment in largescale schemes Some large schemes are still being built Majority of new schemes will be small and probably low head/run-of-river Photograph taken from National Geographic

7 UK Small scale hydro schemes Only a small number of different schemes are used for hydropower production in the UK The choice of scheme is dependent upon multiple factors Schemes are primarily determined by site topography as being: High head = >50m Medium head = 10-50m Low head = <10m Run of river = variable head but typically low Majority of small hydro schemes are run-ofriver making use of existing small dams/weirs Water is not stored for run-of-river schemes and thus BHA states they do not have the same kinds of adverse effects on the local environment as large-scale hydro

8 Advantages of small-scale hydro-schemes BHA identify the following advantages of hydropower over other renewable technologies: High efficiency (70-90%) better than other technologies High capacity factor (typically >50%) Solar = 10% Wind = 30% High level of predictability varies with annual rainfall patterns Slow rate of change output power varies only gradually from day to day (not minute to minute) Good correlation with demand (i.e. output is highest in winter) Uses a long-lasing and robust technology schemes can last >50years potentially

9 Dis-advantages of hydro-schemes Reduced fish passage Impoundment (hydromorphological impacts) Entrainment Impingement Reduced flows result in depleted reaches Reduced flows/velocities can impact on other resident flora and fauna (e.g. water quality) Predation rates upon fishes can be increased Concentration in passes/delays Disorientation due to turbulence in turbines Cumulative effects

10 There are various options for mitigating against the negative impacts of hydro schemes. For example: Fish passage facilities Establishment of environmental flows Fish friendly turbine designs Screening of hydropower off-take structures All of the above have been shown to have varying degrees of success Mitigation

11 Lets go compare What types of small hydro schemes are there? Pelton and other impulse types Crossflow Francis Kaplan Traditional water wheels Archimedes screw

12 Pelton Only used for high head sites (usually >20m) Not fish friendly but fine screening prevents entrainment of fish which might damage or clog the jets

13 Crossflow Typically used for sites with a head >4m In its simplest form they are a drum with slots cut into them and remaining metal angled to catch water Not considered fish friendly and thus require adequate screening Image taken from wikipedia website.

14 Francis Only used for medium to high head sites (usually >10m) Not fish friendly but screening usually prevents entrainment of fish Image taken from

15 Kaplan Expensive due to complex engineering Highly efficient Relatively fine mesh screens required Not fish friendly but fine mesh screens are required to prevent fish entering turbine Image taken from Image taken from

16 Traditional water wheels Of 10,000 historic mill sites >2,000 are potentially able to produce electricity using water wheels 2 types Overshot (2-7m head) Undershot (<2m down to 0.3m) More fish friendly than other schemes but impact damage is still an issue, especially for larger fish Usually restrict fish passage

17 Archimedes screws Originally used for moving water up not down! Used in reverse allowing water to turn turbines to create energy Considered to be fish friendly Only suitable for low head schemes

18 The best of the best? Are Archimedes screws the best of the best?

19 Fish friendliness Virtually every hydro scheme represents a hazard to fish Level of threat posed depends on the type of scheme (see table) Mitigation measures (e.g. screens, fish deterrents etc.) can reduce or eliminate threat posed to fish Important to note that screens can also pose a threat if not appropriate (e.g. impingement) Turbine type Mortality rates Francis 20% Up to 100% Kaplan 12% 5 and 20% 46% Archimedes screws References Odeh (1999) Larinier (2008) Pavlov et al (2002) Larinier (2008) Ruggles (1980) 0% Fishtek - various

20 Archimedes screws Are they really fish friendly? Are there any environmental issues?

21 Are Archimedes screws really fish friendly? Assumed to be due to slow rotational speeds, no significant shear forces or pressure changes Several studies have concluded that they generally allow fish to pass unharmed

22 Are Archimedes screws really fish friendly? Tests conducted with brown and rainbow trout ( 8-63cm long) at an AS on the River Dart (Devon) Over 1,000 fish passages were recorded (video footage) Various turbine speeds were also assessed Study showed no damage caused by passing through the turbine Images and data used with permission from Fishtek

23 Fish in Turbines

24 Are Archimedes screws really fish friendly? Naturally migrating smolts were also monitored and 1.4% if fish exhibited limited and recoverable scale loss. Not known if this occurred before or during passage A pinch point was observed where the leading edge overhung the trough Fish exiting the turbine showed no disorientation and thus no increased risk of predation Upstream migrating adult salmonids did not try to ascend the turbine but some were attracted to the outflow Average residence time in the outflow was less than 8mins so unlikely to delay migration significantly Images and data used with permission from Fishtek

25 Are Archimedes screws really fish friendly? Images and data taken from Fishtek 2007

26 Can Archimedes screws damage fish? Fishtek (2009) stated that the leading edge of the turbine was the only area that can potentially harm fish Main issues are pinch points and impacts Impact is bigger issue for bigger fish and higher impact speeds Fishtek showed that fish <40cm were very rarely hit Kelts = 20-30% risk of being hit Large eels = almost 100% chance of being hit Eels most at risk to pinch points but risk is very low, especially if turbine is designed/installed properly Images and data used with permission from Fishtek

27 Can risks be mitigated? Short answer = YES! If screens are installed to prevent entrainment of large (>1kg) fish and large eels then leading edges may not need extra protection Pinch points can be minimised/eliminated by appropriate design and installation Compressible rubber bumpers can reduce the risk of damage to most fish

28 Bumpers Unprotected steel leading edge (5-10mm thick) caused damage to fish >1-2Kg at speeds >3m/s (typical for small/medium AS) Severe injury was observed when speeds were increased to 4m/s Hard rubber bumpers offer reasonable protection for fish <4Kg at speeds up to 3.5m/s At speeds >3.5m/s hard rubber bumpers do not offer adequate protection Compressible bumpers reduce impact to approximately one fifth compared to steel edges and half that of hard rubber edges With compressible bumpers impact was below the damage threshold even at speeds of 5m/s well above the max speed existing large AS

29 Bumpers Steel edge Significant damage Hard Rubber Edge Some damage Compressible Bumper No damage

30 Environmental issues Other issues associated with Archimedes screws Similar issues as with other schemes Depleted reaches Weir pool habitats / spawning habitat Aesthetics Installation of additional physical (concrete) structure in the river and on the banks Encouraging the retention of in-river anthropogenic structures for many years Screens can restrict access to area of the river channel Rivers with high debris loads require bigger screens Are screens needed with Archimedes screws? Good for downstream passage but what about upstream passage More monitoring required What about cumulative impacts

31 Summary Range of hydro schemes available Different schemes present different levels of risk to fish Risks depend on scheme and site specifics Majority of risks can be reduced or eliminated using best practice guidelines Responsibility of regulators to assess and enforce mitigation requirements Archimedes screws considered to be fish friendly but more monitoring should be undertaken Risks to fish are just one consideration Risks to wider environment equally as important Cumulative impacts require careful consideration Archimedes screws carry similar additional environmental risks as many other schemes All risks need to be assessed on a project by project basis and it is important for regulators to impose appropriate restrictions and mitigation

32 Thank you for listening and..any questions?