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A worthwhile investment

A worthwhile investment
In the push to reduce energy consumption and emissions, variable speed drives could pay for themselves within days, says ABB's Steve Ruddell.

Variable-speed drives (VSDs) control the speed of machinery, pumps, mixers, fans and compressors to match the needs of the process. In many applications VSDs save so much energy that their economic payback time is only a matter of months. A new approach to the assessment of environmental impacts shows that the environmental payback time of VSDs can be even shorter, in many cases as little as one to two days.

VSDs operate by converting the fixed supply from the network to a variable voltage and frequency in response to an electrical control signal. The change in frequency results in a corresponding change in the speed (and torque) of the motor coupled to the drive. This means that the motor speed, and therefore the speed of the equipment being driven, can be set on the basis of external parameters, such as flow rate or temperature. Speed control can significantly boost the efficiency of the entire motor driven system. In the case of conventional systems such as pump and fan applications, for example, the electric motor drives the pump or fan at full speed and then the desired flow rate of liquid or gas is achieved by restricting the output by means of valves, vanes or similar "throttling" devices.

Running the system at full speed and then restricting the output is obviously very inefficient. Some applications have proven that even a modest decrease in motor speed will considerably reduce energy consumption. According to the affinity laws, which govern the performance of pumps and fans, a pump running at 80% speed, for example, uses only 64% of the energy and slightly more than 50% of the power than one running at full speed. 

There is a huge scope for energy and emissions savings through speed control by VSDs. Pumps, fans, compressors, extruders and other motor-driven applications account for two-thirds of industrial electricity consumption, which in turn represents 40% of the overall electricity use in the world. However, less than 10% of motors are operated by drives and, of motor-driven applications less than 2.2kW, as many as 97% have no form of speed control at all.  In Europe it has been estimated that if VSDs were used in motor-driven systems throughout the continent, annual savings in electricity consumption totalling 50 million MWh could be achieved. This is equivalent to 25 million tons of carbon dioxide, or roughly a quarter of the total annual emissions of Finland.

The economic benefits of VSDs are relatively easy to calculate, as the investment cost, reduction in energy consumption and cost of power are known. This in turn enables the payback time to be readily calculated. Quantifying the environmental effects of VSDs, however, is somewhat more complicated. The methodology generally used to study the environmental impacts of manufacturing, use and disposal of products is life-cycle assessment (LCA). LCAs are designed to cover all the phases of a product's life cycle: from the production of raw materials and components to its disposal. Data is collected on all the relevant inputs and outputs, and this is then related to environmental impact categories such as global warming and ozone depletion. The areas of highest impact are identified, and then addressed in order to reduce the overall environmental burden. 

The information (or results) obtained from LCAs forms the basis of Environmental Product Declarations (EPDs). EPDs describe a product's most important environmental impacts during the manufacturing, usage and disposal phases. They can be certified by an impartial third-party organisation, which gives the data additional credibility. EPDs enable users to directly compare the environmental performance of different products, because the data is shown in terms of a functional unit. EPDs also support manufacturers' efforts to enhance their products because they establish an environmental performance benchmark.

Environmental impact
Even though EPDs focus on environmental impacts, they ignore the environmental benefits of using products like drives instead of a less efficient solution. For example, EPD information would show that an ABB industrial drive creates the greatest environmental impact during its usage phase. In fact a VSD can easily halve energy consumption in many applications, compared with the alternative of running the motor at full speed and then restricting output. Unfortunately the energy saved and emissions avoided by using a drive are not taken into consideration in the EPD in any way. Given the scale of the benefits - it is estimated that in 2008 alone the worldwide installed based of ABB drives saved around 170 TWh or about 142 million tons of carbon dioxide - this is a significant drawback to EPDs. One suggestion to address this shortcoming is to determine the ecological payback of products.

Determining ecological payback is a new approach to the assessment of the lifetime environmental effects of products, which takes into account both their positive and negative environmental impacts. Natural capital - ie natural resources - is consumed both in the manufacturing and disposal phases. However, by using eco-efficient products and processes such as drives in place of older, inefficient solutions, it is possible to considerably reduce the overall load on the environment. The ecological payback value shows how long a product has to be used in order to compensate for the one-time environmental burden of its manufacture and disposal. 

The table shows ecological payback in days for three different ABB drives. The payback times are short, decreasing with increasing power ratings. Emissions data from the EPD of an ACS800 250 kW drive shows that its manufacturing carbon footprint is 3.65 kg CO2/kW or 912.5 kg CO2 in total. The information given in the table for the same drive indicates that ecological payback in terms of global warming potential (GWP) is 0.5 days. 

In other words, by operating the drive for just half a day it is possible to avoid enough emissions to fully compensate for the carbon impact of manufacturing. The footprint then "turns negative" as the drive will continue to benefit the environment by saving emissions throughout its lifetime. In fact, an ACS800 industrial drive will typically provide a total lifetime savings of around 7,500 MWh or 3,800 tons of carbon dioxide emissions. In a world where AC induction motors are still the undisputed "workhorses of industry," VSDs can play a very important role in reducing energy costs and carbon emissions.
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