Energy efficiency - tips for better measurement, control and efficiency
When it comes to their impact on the environment, Britain's water companies are most often in the spotlight for their performance in the area of water quality. However, in common with other areas of industry, the water operators are also major energy users and are making big efforts to keep energy consumption under control.
A good starting point for energy savings is the aeration plant. Pumping air to aeration tanks can account for over 70% of the energy consumption on a typical sewage treatment plant. Regulated dissolved oxygen levels optimise the rate of tank aeration and reduce pumping requirements. They also provide a stable environment for the micro-organisms that process the wastewater, which reduces sludge production and chemical usage.
Aeration can be achieved by mechanical agitation or by the use of diffusers to feed air bubbles into the mass of sewage. The object is to keep dissolved oxygen levels just above the critical 2mgl-1 needed to keep micro-organisms working at their peak. Raising oxygen levels beyond this wastes energy, so keeping a close check on dissolved oxygen levels using dissolved oxygen analysis equipment is a vital part of any energy efficiency initiative.
Information from this equipment can then be used to control variable speed drives on motors regulating the aeration tank air feed pumps or to control the speed of an agitator.
Where the source of the oxygen is compressed air, over-aeration can prove even more expensive. This is because compressors turn around 90% of the energy they use into waste heat, which effectively makes compressed air ten times as expensive as electricity.
Keeping track of compressed air usage is therefore paramount to controlling energy consumption. One way to measure compressed air consumption is to use thermal mass meters. These types of meters directly measure mass flow, making them cheaper and easier to use than volumetric alternatives. They also offer a turndown ratio of 150:1, provide excellent measurement accuracy. Alternatively, where installation space is at a premium, swirl meters may provide the best solution. At United Utilities’ Biddulph water treatment works, for example, these meters helped to save up to £10,000 on the cost of installation of two new aeration lanes.
Significant regional differences in water resources and water treatment requirements mean that some companies are finding it difficult to bring overall energy consumption down.
For some, energy consumption has increased as due to more stringent requirements on sewage treatment processes to meet environmental obligations, such as mandatory secondary treatment and disinfection at various coastal locations.
But the trick that all the water companies need to master is the ability to act simultaneously on both fronts, aiming for improvements in both water quality and operating efficiency. Though such improvements, by themselves, will often be incremental, when taken together they can actually present scope for significant energy cost savings.
One example is using UV monitoring equipment to control and regulate raw water nitrate levels. Nitrate level reduction is primarily achieved by blending a high-nitrate source of raw water with a low nitrate source water to bring the overall nitrate concentration down to an acceptable level. Where a low nitrate source is not readily available, it may be necessary to instead use water that has been treated either by reverse osmosis or ion exchange processes, both of which require large amounts of energy. Capable of providing much greater accuracy than conventional monitoring methods, UV monitors can help optimise the level of treatment and the blend ratios required, minimising both treatment costs and energy consumption.
Possibly the biggest potential for energy savings throughout any water network lies in the drives that control pumping operations throughout the network. Typically, reducing the speed of a pump from 100% to 80% can cut energy consumption by up to 50%. But every kW of energy consumption that is saved has a number of benefits, including operational cost, maintenance and environmental impact.
The best way to realise these potential savings is to use the latest variable speed drive technology. Many of today’s working drives were installed some 15 years ago. A modern drive will cause substantially lower energy losses in a motor than an older unit, so the whole system needs to be considered.
For Severn Trent Water, replacing three, 37kW AC drives on water process pumps at its works in Wanlip with the latest technology alternatives helped to achieve a 65% energy saving and a payback of only seven months. In another case, the company saved thousands of pounds in energy and maintenance costs by replacing two drives from the 1980s with new models at its Ladywood sewage pumping station in Ironbridge, Shropshire.
The greater part of the electricity used in industry, approximately two-thirds, is used by electric motors. This seems like a staggeringly high share, but the ubiquitous low voltage AC motor is fitted into virtually anything that moves. In the UK alone, some 10 million motors are used, consuming over £6.6 billion worth of electricity per year. Many of these run far less efficiently than they could, so there is vast scope for savings.
With hundreds of motors being used throughout the UK water network, taking steps to manage the performance of motor equipment is therefore also essential in any attempts to save energy.
A UK water company cut its motor electricity costs by 6% following the introduction of a motor management policy. Motor usage accounts for a large proportion of the company’s yearly £25 million electricity bill. By replacing its existing motors with high efficiency units, the company expects an energy saving of about 24,000 kWh/year, with a payback on investment of less than two years.
However efficient your motors, drives and pumps may be, it’s in the nature of the water business that the lion’s share of energy is used in simply pumping the water, waste water and sewage around the network. It’s also universally true that you can’t control what you can’t measure, so it follows that accurate flowmeters are a type of instrument that water companies should be committing a lot more investment to, if they seriously intend to keep pumping costs down. After all, why waste energy pumping water out of leaky pipes? Leakage reduction and control initiatives are often only considered as water saving measures or as a response to public pressures. However, there is a substantial energy saving to be made by reducing leakage. Even if networks are gravity fed, water will have been pumped somewhere in the cycle.
Pressure is being applied to water companies by Ofwat to more accurately identify and address leaks throughout their supply networks. This has already led some to employ new technologies providing more accurate monitoring and featuring additional capabilities, such as remote GSM communication, which enable leaks and other problems to be immediately traced and rectified.
“Electronic” flowmeters that work on the magnetic, ultrasonic and other principles and incorporate electrical, rather than mechanical sensing equipment, have been around in various guises for some time. Yet they have generally been slow to be adopted by the water industry, largely because they cost more up front than mechanical metering equipment. However, in most cases, payback on the installation of an electronic flowmeter could actually be achieved in a matter of days. Furthermore, with no mechanical moving parts that can be subject to wear and that will need replacement, electronic flowmeters not only offer constant accuracy throughout their service life, but also eliminate the cost, time and disruption associated with maintenance.
The starting point for a successful energy saving plan is to determine just where energy can be saved. Using instrumentation to monitor the efficiency of the processes outlined in this article is one key way of achieving this.
Where pumps, motors and drives are concerned, the best course of action is to carry out a regular energy audit. Typically, such audits should look at all aspects of operation, including the condition, age, efficiency, performance, usage habits and other parameters of the installed equipment. This information can then be used to identify potential areas for improved efficiency and performance.
The information from an energy audit can also be useful in assessing the potential payback that can be achieved by adopting the latest technology. This is particularly important as operators increasingly begin to embrace the idea of total cost of ownership, which takes into account all the costs incurred throughout the life of a flowmeter instead of just its purchase price. One reason for this is the growing trend toward companies outsourcing responsibility for both supplying and running equipment to contractors, which must ensure they opt for the solutions offering best value for money over the long term.
For example, replacing an average 1980s motor with a state-of-the-art high efficiency alternative could deliver a potential payback due to lower energy consumption of 12 to 36 months, depending on factors such as annual operating hours, energy price and the type of drives being used.
An added incentive to investing in energy efficient equipment is the availability of Enhanced Capital Allowances (ECAs). Introduced by the Treasury to encourage industry to find ways to cut energy consumption, ECAs are essentially a discount on energy efficient equipment. These enable companies to deduct the full cost of the investment against corporation tax in the year of purchase.
Compared to the usual capital allowances available, this has three advantages. Firstly, it allows companies to claim the full amount. Writing off equipment the traditional way won’t give tax relief on the full amount, as companies write off a percentage of a diminishing balance each year but never quite get down to zero. ECA saves capital cost by giving 100% tax relief. Secondly, it increases value of the tax benefit as this is calculated on this 100%. Thirdly, it improves cash flow as the benefit is gained in the first year rather than spread over a number of years.
The money stays in the recipients’ account and earns interest or is gainfully employed in their business. Depending on their tax rate and return on investment, the ECA is equivalent to a rebate of between 6 and 15%. This will reduce or even eliminate the price differential to less efficient products and shorten the payback time.
The Enhanced Capital Allowances are applicable to a list of specific items that includes CHP (Combined Heat and Power), boilers, motors, variable speed drives, lighting, refrigeration, pipe insulation materials and thermal screens, which meet the relevant energy efficiency criteria. Flowmetering equipment is also included on the Water Technology List, which is part of the ECA scheme and is aimed at promoting better control of water quality and consumption throughout industry.
A site-wide energy audit may also assess the fuel sources being used and propose alternatives where appropriate. Some audits even address the possibility of installing renewable energy technologies, such as solar panels and wind turbines, or the use of biomass, which can be especially relevant in sewage treatment plants.
Auditors also consider financial benefits in addition to actual energy savings. For example, the auditor may suggest operational changes that could potentially change the tariff paid by the company. This may be as simple as not using certain non-essential equipment during periods of peak demand. Depending on the energy pricing structure, implementing such changes can result in significant economic savings.
Water and waste treatment operators face significant challenges. For some, increasing demands make it difficult to keep energy costs under control. However, in common with the rest of industry, rising prices mean that it’s more important than ever to get to grips with energy.