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Total energy efficiency

Total energy efficiency
Rising energy costs, tough standards for emissions and an increasing awareness of environmental issues make maximising energy efficiency a prime requirement for machines and engineered systems of all types. But, says Mark Dash, Manager, Automation Solutions of Bosch Rexroth, the only way to meet this requirement is to consider the application as a whole.

There's no doubt that today's specifiers and end users are demanding more energy efficient solutions, whatever the application. And, of course, suppliers are responding to this demand. Open any supplier's catalogue or check any website and the energy saving benefits of the products are certain to be highlighted. The use of energy-efficient products is an important step along the route to optimising energy efficiency.

It is, however, only one step - there is much more that can and should be done. The key is to consider not just the components in isolation, but the application as a whole, ensuring that energy efficiency is engineered into every aspect of the design. The details of how this can be achieved depend on the application but, in general, there are four areas to be considered.

Application considerations
The first step, as we've already mentioned, is to choose high-efficiency components. The second is to consider whether there are any areas of the application where energy that would normally go to waste can usefully be recovered. The third is to ensure that the equipment uses energy only when it actually needs to. The fourth step is to evaluate the proposed design, often with the aid of software simulation, to ensure that overall efficiency really is optimised.

Let's look in a little more detail at what each of these steps might entail, starting with the choice of components. Certainly it's a good idea to check the data sheets for candidate components and to compare their efficiencies, but thinking a little more laterally will often yield even better results. 

For example, would a modern linear motion system use less energy than a conventional motor driving a rack-and-pinion? In pneumatic systems, might compact valve carriers located close to actuators mean that a lower volume of air is needed than with a more conventional solution? Would the use of axial piston pumps, which regulate power without throttle losses, reduce energy usage in hydraulic systems? Also, when it comes to motors and drives, what about considering new developments, such as the innovative pole shoe technology now being used in some servo-motors?

Energy recovery
Moving on to energy recovery, it's certainly worth looking at any drive system that involves braking. In older systems, brake energy is simply dissipated in a resistor where it is converted into unwanted heat. Today, however, the best drive systems make provision for recovering this energy and either buffering it, or using it immediately to supply other motion axes. With large motors, it may even be possible to feed electrical energy back into the supply network.

Energy recovery is not, however, limited to electrical drives. Hydrostatic regenerative braking systems for vehicles, especially those with short work cycles such as fork lift trucks and waste disposal vehicles, can deliver energy savings of up to 25%. They work very simply - part of the braking system is a pump that, when the brakes are applied, is used to pressurise hydraulic fluid in an accumulator. When the vehicle later needs to accelerate, the pump acts as a motor driven by the pressurised fluid, reducing the load on the vehicle's engine and, therefore, its fuel consumption.

In industrial as well as mobile hydraulic systems the use of accumulators is also worth considering, as they offer a very effective and inexpensive way of storing energy that might otherwise be wasted, thereby improving overall efficiency.

Continuous running
Ensuring that energy is only used when it is actually needed sounds like a very obvious measure, but even today there are many installations where compressors run continuously even though the demand for air is only intermittent, and where cooling fans run at full speed irrespective of the temperature of the components they are supposedly cooling. Fortunately, there are many options for avoiding such scenarios.

Employing motors with frequency-converter variable speed drives is a good example, provided that they are used in conjunction with sensor-controlled feedback loops to ensure that the motor speed is always matched to demand. Another interesting option is provided by pneumatic valves with an energy- saving mode that ensures that electrical energy is consumed only when they are required to change state.

Though we have listed design optimisation as the fourth step along the route to energy efficiency, this shouldn't be taken as meaning that it's the last step to be carried out. In practice, the design will need to be checked and optimised at every stage to ensure that energy efficiency is being maximised. In particular, there is no place in today's designs for the over-engineering that was once so commonplace. 

Optimum sizing
Every drive and every component must be exactly the right size - no bigger and no smaller. To make this possible, many component suppliers now offer specialised software that simplifies product selection and removes the uncertainty from sizing. The best suppliers also provide simulation software that is an invaluable tool for confirming the energy efficiency of a design before it is actually built.

The benefits of a four-step total approach to energy efficiency are clear, but one question remains - how can these ideas best be translated into practice? After all, most suppliers will provide support for one or perhaps two of the steps, but how many are able to provide support for every step?

Rexroth's Energy Efficiency programme, or Rexroth 4EE for short, covers each of the four steps and ensures its customers are supported in the development of systems and solutions that not only save them money, but also help them to make the best possible use of one of the world's scarcest resources - energy.
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