ECN Eye On Efficiency
Hermetic Feedthroughs In Flywheel Energy-Storage Solutions
The phrase “everything old is new again” certainly applies to today’s flywheel technology. Forget the mechanical bearing, standard atmosphere 5,000 RPM steel behemoths of the past, many of today’s flywheel designs feature compact carbon fiber composite rotors on magnetic bearings, turning in a vacuum at up to 60,000 RPM. As flywheel technology continues to improve, flywheel energy storage (FES) systems are gaining in use across a wide variety of applications, from frequency regulation in power utilities to energy recovery in trains and industrial equipment to rack-mounted uninterruptible power supplies. With demand rising for reliable, cost-effective, and environmentally friendly energy storage, especially to support the growth of green power solutions like wind and solar, FES is quickly coming into its own. Compared to other energy storage solutions, FES systems have long lifetimes with minimal maintenance requirements, high energy densities (~ 200 kJ/kg), and large maximum power outputs. The round-trip efficiency (ratio of energy out per energy in) of flywheels can be as high as 90%, with power output capacities ranging from 2 kWh to 133 kWh, and an FES system can typically reach full charge in as little as 15 minutes. Compared to batteries with low capacity, long charge times, heavy weight, and short usable lifetimes, FES presents a bright spot in tomorrow’s clean energy solution.
These new generation flywheels are made possible by advances in material science for rotor technology, as well as the application of magnetic bearings running in a vacuum environment. While rotating a flywheel in a vacuum is an obvious way to get rid of the windage friction losses, mechanical bearings alone won’t stand up to operating in a vacuum, nor to the high speed requirements of the new flywheel designs. With the advent of magnetic bearings and magnetic-mechanical hybrids, FES engineers gained bearing solutions with very low and predictable friction, the ability to run without lubrication, and the capability of high performance in a vacuum – the ideal bearings for high-speed, vacuum applications.
However, with the requirement for operation in a vacuum comes one of the critical design challenges facing today’s FES engineer - ensuring the vacuum integrity of the flywheel housing, while meeting the needs for noise-free monitoring and high power inputs and outputs. Any breach in the vacuum environment of the rotor could lead to FES failure, making hermetically sealed feedthroughs a critical engineering component for FES development. FES designers often try to significantly reduce the system size, making it as small as possible, while taking into consideration the co-location of associated electronic and control systems and how the essential feedthroughs will be successfully situated. Thus, control and power feedthroughs that fit into tight areas, turn corners, and still maintain vacuum, require custom housing designs, often with unique geometries and specialty materials.
Massachusetts-based Beacon Power uses hermetic vacuum feedthroughs to optimize the performance of its Smart Energy 25 FES systems, which are being deployed on the utility grid to provide frequency regulation. The feedthroughs provide transfer of power and signal data from the control system on the atmospheric side to the internal volume of the vacuum-sealed flywheel chamber.
“Our Gen4 flywheel design relies on hermetic feedthroughs in order to reliably maintain vacuum inside the chamber during operation,” says Dick Hockney, chief engineer at Beacon Power. “This capability is critical for reducing windage, which increases efficiency and prevents the high-speed rotor from overheating.”
For control systems, speed, temperature and vibration all need constant monitoring via numerous thermistors and other sensors, often incorporating shielded and/or twisted wires to maintain signal integrity. For power transfer, copper post studs or heavy gauge wire feedthroughs must be accommodated, depending on current requirements. In all cases, small and high density feedthroughs provide less risk of leakage than multiple connectors. In the case of flywheel chambers that are submerged in a heat transfer fluid, these feedthroughs must also be leakproof and resistant to whatever fluid is in use.
Oftentimes the vacuum environment and heat transfer fluid requires that special attention be paid to material selection. Understanding parameters such as outgassing, permeability and material compatibility is critical in developing solutions that will perform as desired over the 20+ years of operation that most of these units require.
While the potential for FES solutions is tremendous, these projects are often at risk when the challenge of getting signals and power into and out of the vacuum environment is underestimated. Consulting hermetic feedthrough experts during the design phase can ensure that these small, but necessary components do not become the failure points for an otherwise successful project.
High Performance Electro-Optical Systems
Efficiency is a key buzz word in today's “greening” economy. As the use of technology increases across the globe – seen in the electro-optical industry with better medical diagnostic & treatment equipment and more sensitive pollution detection systems – pressure to increase efficiency without sacrificing performance also increases. Especially in handheld, battery-operated devices, battery life is a driving specification. Military contracts have been won and lost on battery life performance.
Anytime energy is converted from one form to another, there will be losses. For example, in a laser-based spectroscopy system, there are several layers of conversion. Typically, AC power is converted to DC power to control electronics that condition the drive current to a laser diode. The optical power output of the laser diode is conditioned with optics and delivered to a sample. Additionally, a thermoelectric is used to maintain the laser die at a stable temperature and converts electrical current to heat transfer. The laser diode linewidth must be narrow and at a fixed wavelength in order to meet part-per-billion sensitivity specifications. At the detector, signal-to-noise ratio is dependent on how much optical power can be delivered to the sample, the responsivity of the optical detector, and the associated electronics. The efficiency of a single chip semiconductor laser diode can be as low as 10%. The lost electrical power is translated into heat. The thermoelectric used to control the temperature of the laser die is usually about 10% efficient. This means the drivers of these devices have to deliver 10 times the required optical output power in electricity. The drivers themselves regulate power delivery to the laser and thermoelectric, involving additional losses of efficiency. Efficiency for a PWM system can reach 50-80%, while a linear control system may deliver 20%. Wall plug efficiency is a frighteningly low single digit number at best.
Performance and efficiency are locked together as design trade-offs. Temperature stability of a PWM system is usually one to two orders of magnitude less than a linear control system. That can mean the difference between a laser line being stable enough for use in a communications system or not. PWM noise in a laser diode spectroscopy system can mean orders of magnitude of loss of sensitivity. Power, instead of being delivered right at an absorption wavelength, is dispersed across a band, and the band can move with temperature variation.
In many electro-optical applications, the demand for performance – better medical diagnostic limits, better sensitivity to measure pollutants – outweighs the desire for efficiency. Designers simply expect to have to dissipate heat and plug into a wall to power the device.
Portable military, automotive, and consumer applications are driving the need for efficiency in electro-optic systems. Material science research has led to new laser technologies such as Quantum Cascade lasers, OLEDs, SLEDs, or fiber lasers that can achieve better than 36% efficiency. New experiments with nanotechnology are making the first steps toward higher figure of merit materials that can increase efficiency in thermoelectrics for the first time in decades. Control technology is merging classical linear techniques with more efficient (frequently switching based) means of delivering power to a transducer. In electro-optical systems, design for efficiency requires an understanding of the entire process – every transition of energy. There is no single point that is responsible for all the losses.