1.4.6.Mechanical energy storage

The main energy storage technologies which store energy as mechanical energy are:

• pumped hydro storage which stores energy as gravitational potential energy in water reservoirs;

• compressed air energy storage;

• flywheel energy accumulators which store the energy as kinetic energy in flywheels.

Pumped storage systems pump the water from the lower reservoir to the upper reservoir during periods when electricity demand is low (Fig. 7). When electricity demand is very high, the systems work as a classic hydroelectric power, the water flows out of the upper reservoir driving the turbines to generate electricity.

Figure 7. Hydro pump storage system.

The stored energy depends on the height of the waterfall (h) and the volume of pumped water (V) as follow:

[J] (3)

where: ρ - density of water (1000 kg/m³);

g – acceleration of gravity (9.8 m/s²);

Pumped storage systems can be divided in to two categories [5]:

• long term storage, when the storage time period varies between 1 and 24 h;

• short term storage, when the storage time period is one day or more.

The life time of Ppumped storage systems p is around (30–50) years, with an efficiency of (65–80)% and power capital costs of (500–1500) $/kW and (10–20) $/kWh [3].

They are useful in controlling electrical network frequency and in provision of reserve generation. They can respond to sudden changes in electrical demand within seconds (Table XVII) [5].

17. The response time of sudden changes in electrical demand for power plants [5].

Power plant type	Response time of sudden change in electrical demand
Hydropower plants	(3-5) min.
Pumped storage hydropower plants	(3-5) min.
Fuel oil plants	3 h
LNG – Natural gas plants	3 h
LNG – Combined plants	1 h
Coal fired plants	4 h
Nuclear plants	5 days

In a conventional gas turbine installation approximately 60% of the mechanical power generated by the turbine is consumed by the compressor supplying air to the combustion chamber.

A compressed air energy storage facility separates the air compression and power production processes in time, to use electrical power during the low load hours for air compression, and then to produce, during the peak hours the power. The stored compressed air is later time injected, after its preheating using the heat contained in turbine exhaust gases, into a simplified combustion turbine comprising a combustion chamber and an expansion turbine (Fig. 8).

Figure 8. Compressed air energy storage system [1].

As a compressed air energy storage unit must be coupled with a gas turbine, the technology is a hybrid storage-generation technology.

The net efficiency of storage is limited by the heat loss during compression. The overall efficiency of energy storage is about 75% [2]. The storage of compressed air is achieved at high pressures (40–70 bar) and at ambient temperature in appropriate underground mines, caverns created inside salt rocks or in aquifers.

The largest compressed air energy storage plant is the Iowa Stored Energy Park, with 2700 MW of turbine power, in Norton, Ohio.

Advanced-adiabatic compressed air energy storage technology is an evolution of traditional compressed air energy storage technology which offers higher efficiency. The heat removed during the air compression process is stored separately in an oil tank and then re-injected into air prior the combustion stage. The round-trip efficiency of the process increases to over 70%.

A flywheel energy storage system consists of a massive or composite flywheel which is supported on a stator by magnetically levitated bearings (that eliminate bearing wear and increase system life) and set inside a housing with low vacuum (to reduce drag) and which is coupled with a motorgenerator.

The energy stored in a flywheel is calculated with the following equation:

[J] (4)

where: I - moment of inertia of the flywheel (kg·m²);

ω - rotational velocity (s^-2);

k – inertial constant depending on the shape of the rotating flywheel;

m - mass (kg);

r - radius (m);

ρ - density (kg/m³);

V - volume (m³).

One can note from eq. (4) that the angular speed and/or the mass have to be increased in order to maximize the energy capacity of a flywheel.

The main advantages of flywheels are: low maintenance, a great cycling capacity (up to 100000 cycles), a 20-year lifetime and high instantaneous energy efficiency (up to 90%). The low-speed, high-mass flywheels (relying on moment of inertia for energy storage) are typically made from steel, aluminum, or titanium and the high-speed, low-mass flywheels (relying on the angular speed for energy storage) are made from composites such as carbon fiber [2].

They are useful in voltage regulation and uninterruptible power supply (UPS) capabilities due to their low energy density and specific energy.

Fig. 9 shows the power generating unit ENERGIESTRO which consists of a Diesel engine (1), a flywheel energy accumulator (2), an alternator (3), a vacuum casing (4), a ball bearing (5), a clutch (6), an inverter (7), a vibration isolation (8) a fuel tank (9) and a soundproofing (10) [6]. The unit can operate independently or being connected to the grid.

Figure 9. The power generating unit with flywheel acumulator – ENERGIESTRO [6]

At off-grid operation, the unit operates continuously as long as the power demand is high and when the power demand is low, it operates intermittently at variable speed with alternating the charging (engine operates, flywheel-generator accelerates) and discharging periods (engine stops, flywheel-generator decelerates).

At grid connected operation, the engine stays stopped and for power disruptions longer than one minute the engine starts operating.

Unlike a conventional gen-set which must operate continuously to deliver power to an isolated consumer, the ENERGIESTRO engine stays stopped 90 % of the time.

This leads to much increased lifespan and reduced fuel consumption at low power.