MINIMISING Water CONSUMPTION
Reducing the drain on water resources
1 April 2008The environmental consequences of power production are increasingly attracting attention. Improved water management can reduce the drain on scarce supplies and help maintain sustainable power generation.
Water is used and consumed in power plants for auxiliary processes – cooling, fuel treatment, steam production and emission control technologies, as well as for washing and sanitary purposes. In steam turbine plants it is also used as a process medium. In the medium size single-cycle power plant range, typical output 10–200 MWe, the most efficient technology available is generally accepted as being the internal combustion engine. This article compares the water consumption of ICEs and competing technologies, with particular reference to the Wärtsilä power plant range.
Water use & consumption
The terms ‘water use’ and ‘water consumption’ are both used in connection with power plants, especially in reference to the requirements of cooling systems. In the cooling circuit of a power plant, water can be either used or consumed, whereas in other power production related operations water is normally consumed. Water ‘use’ indicates that the same volume of water that is taken from the source into the plant is directed back to that source. Water use takes place, for example, in once-through cooling systems. Water ‘consumption’, on the other hand, means that a certain amount of water is actually consumed in the operation. In cooling applications, the consumption of water indicates that only part of the water used for cooling is directed back to the water source, with the remainder being lost through evaporation and drift.
The scale of water consumption in a power plant is very much dependent upon the cooling method applied. Consumption for sanitary use of course relates directly to the number of people working on site, which again depends on the type and location of the plant. On the whole plants in developed counties have fewer staff.
The level of water consumption through auxiliary processes depends on the fuel and plant type. Consumption in gas and LFO (light fuel oil) plants is lower than in HFO (heavy fuel oil) plants, since no fuel treatment is needed. In addition to sanitary and auxiliary process water consumption, local operating methods and habits also play a role. Water consumption in operations such as area cleaning and parts washing can, for example, vary considerably from plant to plant. However, all of these water-consuming factors become insignificant within the larger picture if the cooling method applied in the plant is of a water consuming type. The main cooling methods in power production applications are once-through cooling, tower cooling, and closed-loop radiator cooling. The principles of these cooling methods are shown in Figure 1. Tables 1, 2 and 3 compare typical cooling performance in three different 100 MWe ICE powered plants.
Once – through cooling
The flow of water can be very high if temperature limitations have been placed on the discharged water. When the permitted temperature increase at the discharge point is limited, when for instance the warm water discharge is considered to have an adverse impact on the local ecosystem in the receiving body of water, the water flow is often much greater than actually needed for the plant’s cooling system, resulting in pumping and investment costs for the cooling water intake and discharge systems also being higher.
Cooling tower
The majority of the cooling tower make-up water is evaporated. To be able to keep the water quality in the cooling tower circuit acceptable, part of the cooling tower water has to be discharged as ‘bleed off’, which has a far greater concentration of impurities than the cooling tower make-up water. For proper operation, the cooling tower needs a continuous dosing of chemicals to prevent scaling and fouling within the cooling circuit. Limitations on the effluent composition affect the selection of chemicals since they are also concentrated into the bleed off water. Here too, the warm and saline water discharge may have an adverse impact on the local ecosystem within the receiving water.
Radiator cooling
In a closed circuit cooling system, water consumption is negligible. The heat is transferred to the airflow forced through the radiators by electrically powered fans.
Once-through cooling systems are typically applied in large-scale centralised power plants using steam turbines, and are rarely employed in decentralised engine and gas turbine installations. In Wärtsilä power plants, dry cooling by radiators is by a clear margin the most common solution. In engine powered plants, cooling is necessary in order to maintain the charge air, lube oil, and jacket cooling water temperatures at the required levels. With gas turbines, on the other hand, the above mentioned cooling methods are used for intercooling, or for secondary cooling of the mechanical refrigeration and absorption chiller systems used for inlet-air cooling.
Regardless of the power plant size and type, water use and consumption always have both economic and environmental impacts, especially when groundwater is used for cooling in drought sensitive areas. While large, centralised plants using once-through cooling systems have to be located near water sources, de-centralised plants using radiator cooling have a very moderate water demand and can be located in areas with limited water sources available.
Consumption of air-cooled plants
Water consumption of Wärtsilä power plants is, like all air cooled plants, lower, because cooling is normally achieved via air-cooled radiators. In some rare cases cooling towers may be used and this will naturally increase the water consumption, but even in this kind of installation water consumption is relatively low.
The water consumption of an engine powered plant has been estimated for the European Commission’s reference document on best available techniques for large combustion plants. According to this document ‘an engine driven power plant usually preserves its water’. However, consumption in a Wärtsilä power plant is lower than the document states. For example, for a 130 MWe power plant, the same size as that referenced in the document, water consumption is around 150 m3/h using a cooling tower, whereas in the reference document it has been estimated to be 220 m3/h.
As a comparison, the document gives a value of 500 m3/h for a steam turbine plant of the same size. In the case of radiator cooling, the water consumption is negligible in comparison to these figures. The estimated total water consumption, including all the water needed in the plant’s operations, is around 9 litre/MWh for a Wärtsilä HFO (heavy fuel oil) plant, and around 2.7 l/MWh for an LFO/gas plant. These values are based on the assumption that cooling is via radiators, which is the standard solution. The breakdown of water consumption in these plants is shown in Figure 2.
In HFO plants, water is consumed by the process water, the boiler water and sanitary water. The process water consumption includes water used by the fuel oil and lube oil separators, turbo washing, the oily water treatment system, and workshop operations. Some minor losses may occur in engine cooling and by evaporation. Another consumer group is boilers. HFO plants have their own (consuming) boilers that are used to produce steam for plant operation purposes. The boilers consume make-up water, and also in cooling blow-down from the boiler. The third and smallest consumer is sanitary usage.
In contrast to HFO plants, in LFO and gas plants the majority of the water is consumed by the cleaning system. The process water consumption in these units mainly consists of that used in maintenance operations. As with HFO plants, engine cooling may also consume some minor amounts through leakages and evaporation. However, cooling is not the main factor in the water consumption of either type of plant, in contrast to many other power plant types. A diagram showing the principal water flows for a Wärtsilä HFO plant is shown in Figure 3.
Comparison with gas turbines
Wärtsilä power plants have the reputation of being good performers, even in difficult conditions such as high ambient temperatures or altitudes. The performance of the plant remains high whether in tropical or arctic conditions, or if the plant is located at high altitude.
But the installation’s location has no significant effect on the plant’s water consumption. With gas turbine installations things are different. Gas turbine performance is normally given in ISO conditions, meaning a dry bulb temperature of 15°C, a relative humidity of 60%, and an atmospheric pressure of 1 bar (sea level). However, most gas turbine installations are not operating in such conditions. And the performance of a gas turbine decreases the more actual conditions differ from these norms. Comparisons of the de-rating factors due to site conditions between a Wärtsilä gas engine and a competing gas turbine plant are shown in Figure 4.
Changes due to local conditions do not have a marked effect on the de-rating factor of a Wärtsilä engine, while at high altitudes gas turbine performance suffers significantly. The same thing occurs in hot conditions, but this can be prevented in part by inlet-air cooling.
Inlet cooling
Several methods are used for inlet-air cooling in gas turbine plants. Possible options include media-type evaporative coolers, fogging systems, mechanical refrigeration systems, and absorption chillers. Of these methods, mechanical refrigeration systems and absorption chillers are the most effective, since their function is not limited by the ambient wet-bulb temperature. However, they have high initial capital costs, high operation and maintenance (O&M) costs, and relatively long delivery and installation times. Moreover, they require expertise to operate and maintain the plant.
As an example, the initial capital cost of chillers is in the magnitude of 1 million USD for a gas turbine with an output of 41 MW ISO.
A comparison of capital costs of the different inlet-air cooling systems is shown in Table 4.
The preferred solutions are in many cases media-type evaporative coolers and fogging systems, because of their relatively low capital and O&M costs, quick delivery and installation times, and easier operation. A media-type evaporative cooler consists of a wetted honeycomb-like medium, through which the inlet air is pulled. As the air flows through the medium, water is evaporated from the surfaces, the inlet air being cooled by giving up latent heat. The fogging system is also based on cooling air through the evaporation of water, but instead of using a medium, the water is atomised into an aerosol. The media-type evaporative cooler is the most widely used cooling method, and the fogging system the second most widely used. However, these methods also have drawbacks, namely the limitation on capacity improvement, and the high dependence of performance on the ambient wet-bulb temperature. And last but not least, both these methods consume relatively large quantities of water. For fogging systems demineralised water is always needed, whereas for evaporative coolers, the requirements on water quality are less stringent, although bleed off is needed to remove concentrated impurities.
A fogging system supplier has published typical water consumption values for 11 °C inlet air cooling. When the ambient temperature is 38 °C and the wet-bulb temperature 20 °C, the water consumption for an industrial gas turbine with 41 MW ISO output is 30.3 l/m. Without the inlet air cooling the output would be as low as 25.31 MW, whereas with cooling it can be increased by 8165 kW (32.3%) to 33 475 kW. However, as the figures show, even with cooling the output remains clearly under the ISO value. The water consumption of a media-type evaporative cooler is of the same magnitude.
A water consumption of 30.3 l/min corresponds to around 1800 l/hour. In the form of specific consumption, the value is 54 l/MWh. As mentioned earlier, the estimated total water consumption for a Wärtsilä HFO plant is around 9 l/MWh, and for a LFO & gas plant, 2.7 l/MWh. A comparison of these values shows that the water consumption in Wärtsilä engine plants is markedly lower than in a gas turbine installation using a wet cooling method. Moreover, the consumption figure in the gas turbine installation is only for inlet-air cooling, and is therefore ignores other consumption such as for sanitary facilities, the workshop, washing water etc.
When compared to gas turbine installations, the main benefits of engine powered plants are their high tolerance to extreme conditions, and their low water consumption regardless of the prevailing conditions. The more conditions deviate from ‘standard’, the greater the difficulty for competing gas turbine plants to maintain output of the plant at a satisfactory level. To minimise the drop in output, they are obliged to use inlet-air cooling systems. Such systems are either expensive and difficult to operate, or they consume significant volumes of water. It seems that in gas turbine applications, water consumption is closely related to the economical optimisation of the plant. Currently, water consuming cooling methods are more common because of their lower initial capital and O&M costs, but now, high water consumption is environmentally questionable. In this regard ICE powered plants offer a big advantage. Because of their low water consumption, they can be operated in locations with restricted water supplies and will, irrespective of the cooling solution, have a higher electrical efficiency.
Other benefits
Low water consumption plants have a minimal discharge of waste water. Another factor is their low usage of water treatment chemicals, which leads therefore to only minor risk of chemical spillage. The amount of waste water produced in an HFO plant is approximately 4 l/MWh.
This process waste, usually called oily wastewater, is produced, for example, in fuel and lube oil separators, and in plant area washing and is treated within the plant to comply with World Bank guidelines for thermal power before being discharged. The separated oil is collected in a sludge tank and then utilised or disposed of in an environmentally sound way. In a gas plant there is essentially no process wastewater produced.
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