Given the difficulties of building efficient electric batteries (for automobiles), I wonder what happened to the idea of using mechanical flywheels?
A flywheel stores energy very efficiently-the problem is in making them strong enough to that they do not self-destruct.
Has the concept been tested to the point that people know that is cannot work?
Or are there still people working on it?
the problem is, that’s a big problem. In order to store enough useful energy, the flywheel(s) have to have a lot of mass, spin at high speeds, or both. If there’s any manufacturing flaw in the wheel that causes it to spontaneously disassemble itself, people are going to get killed. Yes, you can encase it in a scattershield, but that’s going to be extraordinarily heavy itself. When the linear velocity of the disc’s edge is 900 km/h, that’s dangerous.
then there’s the packaging angle to it; ideally you would want two counter-rotating flywheels to cancel some of the gyroscopic forces. Then the question becomes “where do you fit the darn things?”
the concept works, like anything else the complication lies in making it feasible for broader applications.
Flywheels are used for energy storage in some niche areas. At best, their energy density is comparable with batteries – Wiki quotes up to 360 kJ/kg, which if you dig into the sources is only true for the best high-speed composite flywheel prototypes. Those same sources state that typical energy densities with a steel flywheel system are more like 20 kJ/kg.
For comparison, lead-acid batteries are around 100 kJ/kg, lithium-ion batteries are around 720 kJ/kg, and gasoline is a whopping 47,200 kJ/kg.
You can buy a race-spec Porsche 911 GT3 R Hybrid with a flywheel where the passenger seat would be.
http://www.roadandtrack.com/racing/motorsports/going-flybrid-porsche-911-gt3-r-hybrid-2.0
Especially in a car. Imagine an accident knocking a fully revved up flywheel loose.
Previous threads:
Machine Design magazine, had an article about new developments in flywheel technology in its Aug 11, 2011 edition. According to the article Beacon Energy funded by ARPA-E has a 25kW-hr unit and is working on a 100kW-hr unit.
One thing I discovered when looking at flywheel vs. batteries for off-the-grid power storage is that flywheels are typically built to produce an extremely large amount of power over an extremely short time. That’s not to say you couldn’t build a flywheel to produce a consistant 500 Kw but I don’t think they are off the shelf.
There is still work being done on both automotive flywheels and industrrial fly wheels for power storage.
Formula 1 has allowed the use of hybrid kinetic energy recovery systems (KERS) in the past few seasons. One team (Williams) tried to go the flywheel route but I don’t think the system ever saw the track in a race, instead all the teams use batteries to hold the power at present. Williams are still chasing the flywheel as a solution and the Porsche I linked above uses their flywheel technology apparently.
Have they tried it in NASCAR, where the gyro problems would actually be an advantage? 
There is a trial happening on London buses at the moment. The article mentions that it is based off F1 technology.
Inertial energy storage technology advances along with other areas of technology. As bearings and materials improve so do the flywheel systems.
A lot of the publicity has centered around the use in vehicles. In this case flywheels are very efficient at powering wheels because it is a directly linkage with minimal loss. But there is additional stress on bearings that in a flywheel in a moving vehicle, and a safety problem with a heavy object spinning at high speed that could disintegrate in operation or from the impact of an accident.
There other applications in fixed units that may have potential for energy storage from intermittent sources like solar cells and wind power. A very large flywheel could operate in a containment structure for safety and convert inertia into electricity with high efficiency (at least as far as powering a generator, the generator efficiency would be the key factor).
A few years ago I saw some research done on a flywheel battery which had large permanent magnets in an outrunner brushless DC motor configuration that made the magnets and housing the flywheel. The motor then became the generator in the discharge mode. The initial tests looked promising, but haven’t seen anything else about that experiment. As long as the potential uses exist and the costs of oil increase, these devices will show promise as an alternative energy storage system.
Flywheels are used as part of the UPS systems in some data centers. They’re very efficient, but only provide a minute or two of power at most - just enough for diesel generators to ramp up.
fiddlesticks mentioned the Williams F1 team. Williams Hybrid Power is their commercial offshoot promoting flywheel power storage.
ATM: short term at best power storage. I can see it in use for a transit bus as they stop and then accelerate very often. Plus, if they are within the city they will never go very fast. So, using the flywheel to effectively brake the bus, like when you engine brake, would work. Then engage it again for the initial acceleration when the engine is at low-rpm, under a higher-load, and at its point of least efficiency. So, yeah, that could totally work out well.
The weight of the flywheel on a bus would also be a small factor as buses are heavy and take on lots of people. On a car though, I don’t really see it as the weight of the flywheel would probably outstrip whatever gain it might provide unless all cars are that Porsche hyrbid, which is a prototype it seems, and expensive.
Related, but not flywheel, is using hydraulic pressure to store energy. This was developed and is being used in Boulder or some other place in Colorado for trash trucks. As they are constantly starting and stopping very short distances. So, as the truck “brakes” the hydraulic pump is turned on, charges the fluid via increasing pressure within an accumulator (a tank design to hold hydraulic fluid at great pressure). Then to start, the pump is now a motor, and helps accelerate the truck. There are some losses involved as both hydraulic motors and pumps have parasitic losses. But, the system is fairly easy to adapt and is somewhat compact. Plus, you can put the accumulator someplace farther away and run hydraulic lines to and from. The actual transfer of energy of hydraulic fluid, IIRC,is relatively efficient from one place to another. And, you can easily multiply the force or have graduated control.
Eaton and Ford have been faffing about with that for a bit. I think UPS was looking into it as well for their delivery trucks.