Using Water Well? Studies of Power Stations and the Aquatic Environment
Improvement in the quality of treated effluents discharged has become increasingly important to permit the subsequent use and re-use of river water downstream. Moreover, as the environmental needs for water are becoming more clearly understood, increasing regard has to be paid to the quantity and quality of water in our water bodies. To give some indication of the pressure on water resources, consider the following global statistics.
Freshwater is only 3% of the global water resource, the rest being seawater. Moreover 78% of freshwater is locked up in snow and ice and 21% is below the earth's surface as groundwater. Only 1% of freshwater is present as surface water and only 57% of this small fraction is contained in rivers and lakes. Thus, crude calculation shows that less than 0.2% of the earth's total water resource is readily available freshwater -and much of that is located far from centres of population. This very small percentage has to meet all the domestic, industrial and agricultural needs of the world's population and, as is now increasingly being realised, the needs of our environment to maintain or, in some cases, improve its present quality.
The natural water cycle is distorted to meet our water needs, for example:
- here, in the UK, our domestic water and sanitation pattern of use averages about 200 l/h/d (litres per head per day); this compares with 600 l/h/d in Phoenix, Arizona and 30 l/h/d in Lodwar, Kenya.
- industrial water use varies widely. For example, to make 1 tonne of cement 3600 1 of water are used; 1 tonne of steel can use as much as 12000 1 and a complete motor car requires about 38 000 1. Through reuse and recycling, net water required is often significantly less than the total water used.
- a typical tower-cooled power station in the UK uses 360 1 of water to provide the energy to power a 100W light bulb continuously for one month. Moreover, more than half of that water is returned to the water body from which it was abstracted.
In the UK, we take more or less for granted the effective regulation of man's use of the aquatic environment. This may not be the case elsewhere in the world and other societies may place differing priorities on environmental protection. The following illustrate the environmental impact of the global population's use and abuse of water resources:
- .Over-abstraction of surface water and groundwater has caused damage to flora and fauna (diminishing flora means less food and shelter for fauna and hence less food is available to predators, including fish). In the longer term, therefore, this will have an adverse effect on the human food chain. It should also be remembered that some 60% of the world's largest rivers are dammed or otherwise obstructed to allow abstraction to take place.
- .Half the world's rivers are seriously depleted and polluted, either by agriculture, (eutrophication or salination), or industry (through the organic load or toxic chemicals discharged, or both).
One of the principal problems, currently under investigation, is to decide how much we can afford to spend to satisfy the quantity and quality needs for water of all types of user, including those of the environment. This will affect the future development of water resources to meet those requirements and the cost of that water to the users. The unit price will, of course, have to include a proportion of the cost of meeting the environmental needs. This may be considerable, but it is a requirement of the latest EU Water Framework Directive, (2000), that the flow of rivers shall be maintained, with water of appropriate quantity and quality to satisfy the needs of their flora and fauna.
Industry is the largest single class of user of water in most developed countries and, of all the industries, power generation is usually the largest user of water. It is not, however, necessarily the largest consumer, since most of the water abstracted is returned to the water body from which it was abstracted. As water resources become more intensively used in the UK, methods by which less water can be used through re-use, recycling and better management techniques, become more and more important. A good example of this in the UK is the power industry; in 1967, 10% of all available fresh water passed through electricity generating stations, whereas twenty years later, this figure had been reduced to 2%.
The participants in the Joint Environmental Programme (JEP) are, therefore, to be congratulated on taking their responsibilities seriously, to both the community and its environment by:
- minimizing the impact of power generation's use of water on the population's other -uses of water and
- minimizing any adverse effect on the environment.
Such responsibilities must, however, be kept under continuous review as social, economic, environmental and scientific thinking develops and new problems, or priorities, emerge. Clearly then, an iterative process has been started which, if carried out as far as is reasonably practical, can only be of significant benefit to the population I and the environment of this country, its neighbours and those further afield. This monograph describes both the state of knowledge and the power industry's involvement in this process.
Foundation for Water Research Marlow
- Electricity is such a feature of modern life that few people wonder where it comes from. Those that do are surprised to find that even nuclear power is still rooted in the "steam age".
- Electricity generation takes over 50% of all water used in the industrialised and developing world, much of it for cooling steam -electric (thermal) power stations. A large (2000 MWe) direct-cooled station abstracts over 4 million cubic metres of water daily and discharges it some 10°C warmer.
- Electricity demand increased rapidly in the 20th century, exhausting the capacity of rivers to accept heat from more and larger stations. This was overcome by re-circulating water through cooling towers and dissipating heat in the atmosphere, enabling a station to reduce its abstraction by over 95%.
- Efficiency of conversion of fuel to electricity is limited, in theory, by the laws of thermodynamics and in practice by economics and materials. Modern steam turbines have theoretical efficiencies around 60% but losses at each step, from boiler to generator, reduce the overall efficiency to about 40%. By using a combined cycle (gas turbine/steam turbine) this can be increased to 52% or higher.
- These efficiencies still look poor and the power industry is often criticised for them. However efficiency pre-1918 was just 11% and it only reached 20- 25% during the 1950s. For comparison the efficiency of the last (1950s) railway steam locomotives was around 12% and today's highly developed "state of the art" petrol and diesel car engines are only 26-30 % efficient.
- Increased efficiency is not just good economics. Less cooling water is needed, less heat is rejected and, by reducing the fuel burn, atmospheric emissions and other waste streams are reduced.
- Despite huge technical advances the perception remains that power stations needlessly throwaway vast quantities of "waste heat". The heat in cooling water is unavailable energy, in the same way that the unavailable energy of petrol ends up as heat in a car's radiator. It has defeated most efforts to put it to use.
- Although steam-electric stations can be designed to operate without using water for cooling their energy conversion (efficiency) is lower; there is a trade-off between reduced water use and higher greenhouse gas emissions.
- The abstraction and discharge of cooling water has been the subject of considerable research, environmental concern and legislation. However, as in many other situations, some of the greatest reductions in the impact of power stations on the aquatic environment have come about through improved technology and efficiency, rather than through legislation.
- The environmental effects of water cooling can be divided roughly into three: impingement, entrainment and thermal effects. At the point where the water is abstracted fish and other organisms may be impinged against the screens that are installed to keep trash out of the cooling circuit. Impingement mainly affects juvenile fish and several studies have been carried out to determine the magnitude of any loss and its effect on fish stocks. Even where there are small local populations, the effect of this cropping by a power station is insignificant and is minute in comparison with the discards and by-catch of the commercial fleet.
- Entrainment describes the fate of plants and animals (mainly planktonic) that are small enough to pass through the screens at the intake. These organisms are subjected to temperature and pressure shocks as they go through the pumps and condensers, and may be exposed to biocide that has been injected to protect the circuit from fouling. The effects appear to be site-specific and range from near total mortality to almost complete survival. The larger, more elongate and more complex organisms tend to fare worst. Again, no study has detected any significant effects of this mortality in the environment.
- Around the cooling water outfall (discharge) is a zone that is temporarily or permanently warmed. Areas of significantly or permanently warmed water tend to be small. Motile organisms such as fish can enter or avoid this zone at will; organisms living in or attached to the bottom cannot. In some cases the effects can be beneficial - extended growing seasons, improved survival overwinter and earlier spawning -and in some cases it is a source of additional stress. There is no evidence that such zones form "thermal barriers" to the passage of migratory fish.
- Current UK and European regulation requires an integrated "multi-media" approach whereby the impact of a power station on one environment (e.g. air) cannot be reduced at the expense of another (water or land), unless this achieves the least impact overall.
- This book explains why power stations need cooling water and, by drawing upon over 50 years of research, examines the many ways by which its abstraction and discharge can affect the aquatic environment. Later chapters discuss methods of reducing these impacts, starting at the station planning and design stage.
- Copies of this report can be obtained from:
- Using Water Well?
Windmill Hill Business Park
Swindon SN5 6PB
- Price £10.00 (with order) including postage and packing
- © Innogy plc 2003