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Future for Regenerative Energy

Page history last edited by Malcolm 12 years, 8 months ago


Nikkei Electronics Asia -- January 2008

Regenerative Power on the Rise

Dec 19, 2007 02:24 Nikkei Electronics Asia

An expanding variety of machinery equipped with motors is tapping the advantages of regenerative power technology, from hybrid cars and power-assisted bicycles to elevators, trains, subways, streetcars and other rail transport; and industrial applications like machine tools, robots, manufacturing equipment, and construction cranes. The power generated when the brakes are applied to a motor is collected and reused, with the objective of improving the overall energy utilization efficiency of the machinery.

Regenerative power technology was first put into use in machinery with large-capacity motors, but it is beginning to penetrate low-output equipment as well (Fig 1). The original application was recovering the electricity generated in motors when the brakes were applied in railway cars, and returning it to the overhead lines, from the late 1980s. Since then, it has gradually spread to a wide range of industrial machinery, machine tools and other applications with outputs of over 100kW.

For example, FANUC Ltd of Japan, a manufacturer of machine tools, industrial robots and other items, sells about one million units a year, and all of them are said to utilize regenerative power technology. It is common practice to recover the power generated by elevators when the cage drops as electricity, and supply it to the facility.

The release of the Prius hybrid automobile by Toyota Motor Corp of Japan in 1997 made the technology famous. By using regenerative power technology in a commercial hybrid vehicle, Toyota Motor expanded the range of application to the automotive industry, and accelerated its development into power assisted bicycles and other fields with outputs of only several hundred watts. In recent years AC-DC converters handling regenerative power have been showing up not only in motors, but also in switching power supplies, marking the first applications in industrial machinery power supplies with outputs of about 300W.

Tougher CO2 Regulations
Regenerative power technology seems to be slowly spreading, but it is more than possible that there will be an explosion in new adoptions in the near future, thanks to a global trend toward tougher regulations on CO2 emissions.
Many nations are working on implementing "post-Kyoto Protocol" legal frameworks designed to cut CO2 emissions in 2050 to half of what they are now. When their efforts move a bit closer to actual implementation, energy efficiency will become a key specification item for equipment manufacturers. Equipment offering high energy efficiency may well receive some sort of incentives, while those with low efficiency may be penalized.

Regenerative power technology is in the spotlight. While it may appear pretty quiet at first, its effect can be dramatic. Take hybrid cars, for instance - a representative example. Instead of using conventional mechanical brakes to slow a gasoline-fueled automobile, the vehicle is decelerated using a motor, and the electricity generated through deceleration is stored. By using that energy when next the vehicle accelerates, energy consumption is reduced.

The key point is that regenerative power utilizes energy that would formerly have been discarded. In the past the most common approach to reducing energy consumption was to boost the energy efficiency while driving, in components like motors and inverters. Regenerative power adds a new effect by improving the energy efficiency of the entire system. Regeneration efficiency is also improved if the energy efficiency of motors, inverters, etc, can be improved, providing significant overall effect.

Lower CO2 Emissions
What sort of effects can be expected when introducing regenerative power technology into equipment? It is difficult to quantify, but a look at the hybrid car is revealing.

In addition to regenerative power technology, the hybrid car only uses the engine in drive regions with high efficiency, has a body weight significantly lower than that of a gasoline-fueled car, and uses other measures to improve mileage. Fuel mileage is said to be 20% to 30% higher than that of similar sized conventional gasoline-fueled cars. Assuming that regenerative power accounts for about half of this gain, this would mean that just adopting regenerative power reduces fuel expenses 10% to 15% below those of gasoline-fueled cars.

This is significant. Total greenhouse gas emissions in Japan in 2005, expressed as CO2, reached 1,293 million tons. Of these, CO2 emissions from transportation accounted for 256 million tons. If all the vehicles covered within this transportation sector were hybrid, electric or similar designs with regenerative power systems, CO2 emission could be slashed by 25 million tons or more (Fig 2).

In Consumer Equipment
Regenerative power technology, of course, is effective in applications other than hybrid cars, too: it is useful in almost anything with frequent starts and stops. As a result, it would not be unreasonable to expect the technology to show up in a rapidly expanding variety of industrial equipment and household appliances drawing large amounts of power, such as washing machines.
An engineer who works on washing machines pointed out that, during spin-dry, these machines generate 100W or more for over ten seconds during braking. Existing washing machines are not equipped with power storage systems, however, or mechanisms to regenerate electricity. The regenerated power is, the engineer said, "...just dissipated as heat in the motor windings, to prevent it from adversely affecting the circuit components."

If the cost of electricity storage systems drops in the future it will no doubt become possible to adopt regenerative power technology. And that would mean recovering power from the spin-dry cycle used in drying, along with smaller washing machine motors, higher outputs and other advantages.

In addition to products using motors, AC-DC converters have appeared to boost energy efficiency by 1% or 2% by recovering the surge voltage that occurs during switching in switching power supplies. The market for these power supplies is yet quite small, and limited to industrial equipment, but a source at a power supply manufacturer revealed that automobile manufacturers and other companies are quite interested. As CO2 regulations grow tighter, these power supplies may be used in a host of vehicular systems.

Regeneration Simple
The range of application for regenerative power technology has spread to include AC-DC converters, but the major use is still recovering power in motors. Depending on whether or not a storage device is present, this utilization can be split into two broad categories.

The first supplies the regenerated electricity to an external system (Fig 3). No storage device is needed, and the electricity is used immediately as a power source in the plant, for the building or whatever. The second returns the recovered electricity to a storage device in the equipment, keeping it available for local use when needed.

The former approach has been in use for a longer time, especially in trains, machine tools and similar applications. Storage devices have become more popular only recently, with the appearance of hybrid vehicles.

In either case, all that is required is some method of converting between AC and DC (such as inverters and converters), allowing current to flow efficiently between the power supply and the motor (Fig 4). This is enough to benefit from an energy saving effect through power regeneration.

Eliminating Resistors
Regenerative technology supplying electricity to external systems was originally adopted in railway cars as a means of reducing brake system cost, rather than as a way of reusing electric power. Traditionally, the power generated when the brakes were applied to a train was consumed as heat in a resistor; but by returning it to the overhead power line it was possible to eliminate the cost of the resistors. The technology was widely adopted for this reason, and today regenerated power is being used by other trains, improving energy savings for the network.

In machine tools and other industrial machinery, however, adoption was driven by two factors: cost reduction achieved by eliminating resistors, and cutting electricity charges (Fig 5). Electricity charges can be reduced both in terms of savings in electricity bills as a result of using regenerated power in other equipment in the same plant, and in terms of the indirect savings resulting from, for example, the need for less air conditioning to absorb the heat generated by the resistors. Such savings could represent significant amounts of cash.

As a result of these effects, most industrial equipment using motors supplies regenerated electricity to external systems. Some manufacturers offer an external motor system supporting power regeneration, usable when plant equipment is not provided with its own organic regenerative power functions.

More Hybrid Vehicles
There has been considerable development recently in systems which first store regenerated electricity in a storage device, and then make it available as needed for external consumption.

It is often difficult for equipment designers to build storage devices into the equipment itself, not only because of the high price of storage devices, but also because of reliability issues with rechargeable batteries, capacitors, etc. "In many cases," explained an engineer at a firm manufacturing industrial equipment, "the first things to break are the capacitors, batteries and other storage devices. We want to keep them out of our products as much as possible."

Toyota Motor achieved the breakthrough with the release of its Prius hybrid car. Unlike trains, automobiles cannot use external power supplies; the only method of regenerating power is through an onboard storage device.

The appearance of the Prius has stimulated research and development into storage devices for high-output applications, including NiMH and Li-ion rechargeable batteries, large-capacity capacitors and more. Hybrid cars are more expensive than their gasoline-fueled competitors, but surging gasoline prices have resulted in some estimates showing total cost is about the same after a decade of driving. CO2 emissions, of course, are significantly lower.

A rising number of power regenerating products with storage devices are reaching the market. Sanyo Electric Co Ltd of Japan, for example, used the NiMH rechargeable batteries common in hybrid vehicles to develop a power-assisted bicycle with power regeneration, in 2001. Until then, while power-assisted bicycles mounted storage devices, they had no regenerative functions. Power regeneration reduces the need for recharging at home and provides power assistance for longer periods of time, which has significantly increased its popularity. A source at the firm commented that while it is a bit more expensive than other power-assisted bicycles, sales volume continues to grow.

Offering New Value
Hybrid vehicles, power-assisted bicycles and similar items are boosting sales by emphasizing energy-saving effectiveness, but storage devices are still expensive, and in many cases the energy-saving effect gained through power regeneration is not enough to make commercialization possible. In many cases, therefore, manufacturers have been finding ways to add new value.
In 2001, Mitsubishi Electric Corp of Japan began selling a small elevator using the exact NiMH rechargeable batteries used in hybrid cars. Small elevators installed in condos and similar buildings can regenerate power, but there is no external equipment to utilize it, for which reason the electricity is stored and used to drive the elevator itself. The elevators require storage devices and controllers, of course, which boosts cost by about Yen1 million, but these elevators can be used for about ten minutes even in the event of a power outage, and this feature has earned the product strong demand for application in hospitals and in other types of building.

The East Japan Railway Co of Japan (JR-East) began trialing a prototype hybrid car mounting Li-ion rechargeable batteries in July 2007, simultaneously with prototype trials of a construction tower crane using electrical double-layer capacitors by Takenaka Corp of Japan and Ogawa Factory Corp of Japan. While the hybrid car reduces fuel consumption by about 10% and the tower crane by about 7%, these products offer other value in addition.

Hybrid automobiles generate power by directly coupling the diesel engine to the generator, which means the same inverters, drive motors and other components that are used in trains can be used. Not only is maintenance the same as for trains, but there is less need for the replacement of consumable parts, such as that which might result from mechanical brake wear in electric vehicles.
Building capacitors into the tower crane makes it possible to reduce the maximum voltage supplied from an external source, which in turn means that the power reception facility can be smaller. As a result, the tower cranes can now be used in medium-scale building construction projects where power reception facilities cannot be erected.

Kawasaki Heavy Industries Ltd of Japan is developing a next-generation streetcar, using NiMH rechargeable batteries. Storage devices are mounted in the streetcar, making it possible for it to run for 10km or more without receiving any power from the overhead lines. As a result, infrastructural investment, such as cabling, can be kept to a minimum.

by Koji Kariatsumari

FANUC: www.fanuc.co.jp/en
JR-East: www.jreast.co.jp/e
Kawasaki Heavy Industries: www.khi.co.jp
Mitsubishi: global.mitsubishielectric.com
Sanyo Electric: www.sanyo.com
Takenaka: www.takenaka.co.jp
Toyota Motor: www.toyota.co.jp/en


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