Energiestro was founded in 2001. Their initial project was to develop an innovative generator, incorporating flywheel energy storage, capable of supplying power to buildings (electricity and heat) in a renewable way. Solar photovoltaic energy has become the least expensive renewable energy, but with a large need for storage, which this technology can now satisfy.
Energiestro innovates with a flywheel made of a low cost material. Before, flywheels were made of high performance, very expensive materials: carbon fibers or high tensile steel. The new material will decrease by a factor of ten the cost of energy storage. The company innovates again with a special alternator with low idle losses. Conventional alternators have a good efficiency at full power, but high losses at low power, causing the discharge of flywheels in tens of minutes. Energiestro aims to store energy for tens of hours, in order to release during the night the solar energy collected during the day. This alternator needed to solve the challenge of cooling in the vacuum. In order to reduce cost, they do not use expensive magnetic bearings, but simple ball bearings, assisted by a passive magnetic thrust bearing, which needed to solve the problem of lubrication in a vacuum.
The figure shows the technology: the flywheel comprises a cylinder (1) that can resist a high rotational speed in order to store kinetic energy. A motor/alternator (2) transfers electrical energy to the flywheel (acceleration) then recovers it (braking). Upper (3) and lower (4) bearings are ball bearings. A passive magnetic thrust bearing (5) bears the weight of the flywheel. A sealed chamber (6) maintains the flywheel in a vacuum to remove air friction. An electronic converter (not shown) transforms the DC voltage at the terminals of the flywheel into a high frequency AC voltage for the motor/alternator.
Energiestro has tested its flywheels by powering several pilot sites for several years: a house, two GSM antennas and their factory. They have accumulated tens of thousands of hours and hundreds of thousands of cycles. It stores energy like a stationary battery, but with the added benefit of unlimited life. A flywheel will operate more than 20 years and one million cycles, whereas a battery lasts a few years and thousands of cycles. The cost of energy storage is much lower than with a battery.
The company plans to produce a range of flywheels with storage capacity from 5 to 50 kWh. The power rating of a flywheel can be chosen independently from its storage capacity. As a standard, they offer a power rating enabling a charge or discharge in one hour (for instance the 5 kWh flywheel has a 5 kW power rating, as the table show). The flywheels will be available in low voltage (48 V) or high voltage (from 500 to 1,000 V) in order to connect to standard battery inverters.
The advantages of flywheels over batteries are, therefore:
- Unlimited life: battery life is limited to a few thousand cycles
- Resistance to extreme temperatures: batteries do not tolerate them
- No toxic or strategic materials: batteries contain lead, cadmium, lithium, etc…