Drives for Change

05 November 2010

As energy costs rise and new legislation encourages companies to cut carbon emissions, building operators are seeing the advantages of using the latest variable speed drives in their HVAC systems as Steve Ruddell explains

BUILDINGS SUCH AS hospitals, offices and shopping centres are among the greatest
users of motor driven fans and pumps. A HVAC system can use hundreds of such devices to maintain temperatures and humidity at the best levels for comfort, as well as other parameters such as air pressure, number of air changes and carbon dioxide content.
Yet, an HVAC system is not always working in the most efficient way possible and older
installations that have not been refurbished and modernised may be wasting thousands of pounds worth of energy a year. A traditional control system for a ventilation plant uses a mechanical arm raising and lowering a damper to control air flow. This system is costly, inefficient, delicate and prone to disturbances. It is a brute force method of providing changes of air and temperature and by its nature can take no account of the prevailing weather, the needs of the occupants or how many people are using the building. Even small differences in efficiency can make big differences to a motor’s life cycle costs - a 11 kW motor costing about £700 can consume over £67,000 worth of electricity over a ten-year operating life.
The most modern HVAC systems make extensive use of VSDs (variable speed drives),
which adjust the output of a fan by controlling the speed of the fan motor, resulting in significant energy savings and improved control possibilities. Supply and return air flow rates can be controlled by adjusting the fan motor’s speed by using signals from various sensors connected directly to the HVAC drives. The drives can also control other equipment such as air duct dampers and water circulation valves.
As a retrofit item, the VSD can be one of the most effective factors for producing energy
savings. Insulating a building can give a return on investment over 30 years, whereas the drive achieves this in two years or less. The impact is immediate. By reducing fan speed by a little, energy consumption can be reduced by a lot.
Flow control
Using VSDs to control the flow of air and water in a HVAC system can save over 50 percent of the energy compared to using traditional control methods such as throttling valves and dampers. This is because a small reduction in speed can make a big difference in energy use, as the power required to run a centrifugal pump or fan
changes with the cube of the speed. A pump or fan running at 80 percent speed consumes only half as much energy as a unit running at full speed.
Because many pump and fan systems run at less than full capacity for much of the time, VSDs can produce huge savings. If a 100 kW pump is throttled by 20 percent, for example, the investment in installing a VSD will have a payback of typically six months based on continuous operation.
Hospitals are extremely energy intensive buildings, requiring a lot of pumped water and clean air, electricity can account for over 50 percent of a hospital’s total energy costs. At Coventry Hospital, matched pairs of ABB high efficiency motors and VSDs will reduce energy consumption for the ventilation system in the hospital’s block A by over 1,400 MWh per year.
Another example is the Hammersmith and Charing Cross NHS Trust, which has retrofitted more than 70 high-efficiency electric motors and VSDs at its two London hospitals, resulting in improved HVAC efficiency with savings of over 25 percent.
Reducing the number of air changes per hour can drastically cut the energy bill. VSDs are ideal in this scenario as they can be used to implement demand controlled ventilation – if nobody is in the room, far fewer air changes are needed. If there are changes in the use of the area throughout the day, it might be possible to divide
the building into zones with different levels of air movements, with control methods and time schedules for switching HVAC equipment on and off.
VSDs make HVAC applications easier to realise because they give much greater scope for control. This makes them ideal for providing a more comfortable indoor climate with reduced input of energy – the type of indoor climate that users demand in the 21st century. As well as energy savings, VSDs also bring other benefits, allowing fans, ducts, pumps and pipes to be smaller, cutting installation and running costs, while reducing mechanical stress and noise levels and allowing more accurate control of the process.
Technology does not stand still and even the great efficiencies possible with VSDs are being improved upon all the time. ABB’s current drives offer over ten percent higher efficiency than its most popular drive from the 1980s. Many installed drives are reaching the end of their lives, more and more are due for replacement as users seek the best energy savings possible as well as keeping serviceability up to required levels. One of the major reasons for replacement is reliability. The reliability of even the best maintained drives will decline after about 15 years of use and a cost-effective replacement will be a priority for many drives users before they start to experience unacceptable levels of downtime.
The market for replacement drives continues to grow. In 2010 it is expected that 40 percent of all new drives purchased will be replacing existing drives. One of the biggest drivers of this replacement market is the increasing emphasis placed by governments on reducing energy consumption in order to meet their climate change commitments commitments such as the CRC Energy Efficiency Scheme. Among the biggest impacts on energy saving can come from applying VSDs to many of the electric motors used in buildings. Although the latest, most efficient drives are bound to save money and often have a reasonably short pay back time, funding their purchase still entails a fairly large capital outlay for many organisations, particularly in a large office block or a major public building like a hospital.
To help ease the impact of this initial investment, the Carbon Trust has a pot of £100m to invest in energy saving equipment such as VSDs and high efficiency motors, with interestfree loans that are available from £3,000 to £500,000. Salix provides funding for these schemes for public sector bodies through loans and ring-grants to help these bodies make effective, long-lasting carbon saving projects.
Enhanced Capital Allowances to give a further financial incentive for users to choose energy efficient products. VSDs in pump and fan applications qualify, along with high efficiency motors in all applications, enabling companies to write off the full cost against tax in the year of purchase. Capital allowances reduce the amount that businesses have to pay in income or corporation tax - 100 percent first year capital allowances are granted for energy saving investments in the private sector. Firms making qualifying investments will be able to deduct the full costs of those investments when arriving at their corporation tax or income tax bills.Normally, capital allowances are given at 20 percent on a reducing balance basis. Over 10 years, almost all of the allowances will have been claimed. The ECA scheme enables users to claim 100 percent allowances in the first year.
Scrappage or swappage schemes also help. One of these is run by ABB and allows companies to trade in their old VSDs and motors from any manufacturer for new ABB drives and motors.
The scheme offers at least 17.5 percent discount off published list prices for new drives from 0.12 kW to 400 kW and new motors from 0.75 kW to 710 kW when old equivalent products from any manufacturer are traded in. The first stage involves assessing a drive and motor application for its energy saving potential. This could involve a free on-site energy appraisal, looking at suitable applications that could produce the most significant savings.
Measurements of the application’s energy use will be analysed to show what energy savings the user can expect to achieve, together with the equivalent amount of CO2 saved, with the results helping a user target their investment so that it produces the highest possible savings and gives the best return. This evidence is essential to make use of the interest free loans offered by the Carbon Trust.
Energy efficiency is increasingly seen by many building owners and operators as an inescapable part of managing their bottom line and many are appointing energy managers to keep their use of this expensive commodity within reasonable limits. Pressure and encouragement from government, combined with the help and expertise of private industry, are making VSDs the technology of choice for cost conscious building managers.
● Steve Ruddell is Division Manager, Discrete Automation and Motion, ABB Ltd
energy@gb.abb.com; www.abb.co.uk/energy