Comments on: Storing energy with flying metal objects

Pentadyne twirls weights in the air, and it makes electricity.

[gbryanthudson]

unclear on math

"Kinetic energy equals mass times velocity squared. So doubling mass doubles energy storage, but doubling the rotational speed increases energy storage exponentially, the company's Web site reads."

NO, the energy increase quadratically and this is WAY LESS than exponential. I wouldn't advise investing in these silly people.

[suyts2]

Thanks for clearing that up

for me. I was looking for the facts when I saw you post. Saved me some reading. I believe it is just another silly start-up created to scam people with their newly found "green conscience".

[lhiggins30]

Response to comments from 'gbryanthudson'

For a complete and interesting treatise on the subject of flywheel energy storage there is no substitute for the 1985 publication by G. Genta titled "Kinetic energy storage: Theory and practice of advanced flywheel systems." This book is no longer in print, but I refer you to the Linda Hall Library of Science, Engineering and Technology stack reference as follows, should you wish to borrow this reference:

http://leonardo.lindahall.org/ipac20/ipac.jsp?session=11LS308905R73.6657&profile=lhlopen&uri=full=3100001~!27174~!1&ri=1&aspect=subtab852&menu=search&source=~!davinci#focus

The complete picture of the flywheel energy equation is:

E = ½ (J) { (max speed squared) ? (min speed squared) }

While it is true that energy available from a flywheel is partially defined by the speed squared, my response is "so what?" The reason for my dismissal of this fact lies in several issues that are not raised by this simplified point of view.

The premise of the statement "doubling the mass doubles the energy, but doubling the rotational speed increases the energy storage exponentially" implies the factor J remains constant. This is seldom the case. The factor J is called the moment of inertia of the flywheel and is a property of the shape of the flywheel and the density of the flywheel material. Related to the shape of the flywheel and the material density is the mechanical stress generated within the rotor when it spins. This limits the J that can be employed in any given rotor design. So, why the interest in composite rotors?

Composite materials are typically used in the aerospace industry where weight is of particular concern. This is also why composite flywheels were of interest when they were originally being considered for use in hybrid electric vehicles (HEV) ? weight matters. The idea of flywheels for HEVs never caught on commercially and most auto manufacturers have turned their interest to supercapacitors or advanced batteries.

But what about flywheels for power quality? Does weight matter? There are some conveniences during transportation and installation, but the two issues of primary concern to purchasers of UPS equipment are reliability and the cost of achieving that reliability, including total cost of ownership. Whether one uses a steel flywheel or a composite flywheel with their UPS, the reliability will far exceed that of lead-acid batteries. Lead acid batteries are subject to rapid performance degradation when subject to temperatures other than 25 degrees Celsius or frequent discharge. Topping this off, the reserve capacity of batteries is difficult to monitor making them more prone to undetected failure modes that leads to "failures on demand" (i.e., battery fails to deliver power when needed during a utility outage, etc.). Many users have experienced the frustration and cost of frequent battery maintenance in strict accordance with the manufacturer?s recommendations only to be subject to demand failures at the time of critical need.

When considering the cost of flywheel systems, the purchase price of composite and steel flywheels for use with double-conversion UPS are comparable both in terms of cost per rated kW (kilowatt) and cost per unit energy delivered. However, purchase price is only one factor in the total cost of ownership. UPS with integrated flywheels offer substantial electrical efficiency improvements over existing double-conversion UPS types that will lead to energy cost savings typically sufficient to pay for the equipment in two to four years. Typical double-conversion UPS efficiency is in the range of 88 to 92 percent. Adding a stand alone DC flywheel will further reduce the net UPS efficiency. Integrated flywheel UPS systems are available with efficiency approaching 98 percent which includes flywheel losses.

Flywheels for power quality applications are gaining momentum (pun intended). A 2007 market study by IMS Research shows Active Power UPS using integrated flywheel energy storage garnering 4.3 percent of the UPS market in the range of >1000 kW in a total market that is estimated to be approximately $6 billion. With more than 1,700 flywheels in service and cumulative run time exceeding 37 million hours, at least one ?silly start-up? has made significant inroads with its flywheel products. The idea that faster is better just doesn?t stand up to scrutiny with flywheels in the power quality market. High reliability at a reasonable cost is paramount. Gunta sums this up nicely in his introductory chapter:

"If the efforts in [flywheel] development are maintained, flywheel energy storage systems may come to be used in industrial practice because their advantages can be exploited in certain applications. ...it is useless, for many applications, to seek for the 'optimum flywheel' or the 'very-high-performance flywheel.' A reliable, safe, well-designed and well-built medium energy density rotor is enough for most applications."


David E. Perkins
Chief Technical Officer
Active Power, Inc.

[azerthoth]

Nothing New

Rotary UPS are nothing new, the maglev idea is a neat twist, but overall they are only good for extremely short periods of time. Such as the 5 to 10 seconds it takes for backup generator to come online, stabilize and take the load.

Good only for short brown out conditions or as a transitional power supply, they can hardly be counted as a UPS in the traditional sense. A cheaper and more effective solution would be an online active UPS.

[rickealm]

Very kool stuff

Anyone have links to other articles

[chrisbrandow]

ben rosen

also started a flywheel energy storage company ~10 years ago that ultimately flamed out. That is certainly relevant in his involvement here, I would think.

http://money.cnn.com/magazines/fortune/fortune_archive/1996/09/30/217438/index.htm