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Delegates are invited to meet and discuss with the poster presenters in this topic directly after the session 'Storage & grid integration' taking place on Wednesday, 12 March 2014 at 16:30 -18:00. The meet-the-authors will take place in the poster area.

Kasra Zarisfi Impressive Engineering Limited, United Kingdom
Kasra Zarisfi (1) F P
(1) Impressive Engineering Limited, Aberdeen, United Kingdom

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Energy storage, the missing technology for wind energy


A gravitational based Energy Storage (ES) underneath a Wind Turbine (WT) which utilises a gigantic Hydraulic Cylinder (HC) made of a borehole excavated underground can match irregular wind energy with daily fluctuations in energy demand.

The low price energy produced at off-peak hours can be stored and used later at high price to meet high demand peak hours. This revenue covers the cost of ES.

The WT’s rotor drives a pump which injects water into the HC to lift a Heavy Solid Mass (HSM) for storing potential energy. The stored pressurised water then runs a generator by a turbine.


HCs are widely used for lifting heavy weights in industry. In HCs a driver liquid acts to push a piston through a cylinder. The liquid must be fully sealed within a chamber by a dynamic seal to be pressurised and creates a mechanical force to displace the piston and consequently lift a mass.

The dynamic seal requires smooth machined surface to be able to seal and move with low friction. A WT equipped with ES (WT+ES) requires a gigantic HC. Accurate machining is not practical considering depth (~500m) and bore (~8m) of HC.

Main body of abstract

The machining issue cannot simply be resolved by using an elastomeric (rubber) pressure seal due to friction, material constrain and integrity concerns. A unique Liquid-Driver-Liquid-Mass (LDLM) hydraulic mechanism is invented to make such a dynamic seal feasible.

LDLM function can be illustrated by an example where two insoluble liquids with different densities like oil (driver liquid) and mercury (mass) are poured into a U-shape tube. The mercury can be lifted in one tube by injecting the oil in the other tube. Mercury and oil pressures are equal at the interface point so they stay separated.

Considering a rubber disk at the liquids interface, neither fluid is found to leak to the other side since the pressure is equal on both faces of the rubber. Therefore the rubber disk acts only as a separator and not a pressure seal.

WT+ES’s HC uses the same concept. The HSM above the piston is submerged in a heavy density liquid to provide LDLM. The liquid is a suspension comprising fine iron powder particles dispersed in oil which is slightly denser (2800kg/m3) than the HSM (2600kg/m3). This concept has been tested and proved by a small scale prototype.

A WT+ES with the same rotor size as a Conventional Wind Turbine (CWT) can harvest more wind and generate more electricity since it can have higher Rated Output Speed. WT+ES’s pump is cost effective with higher range of wind speed compared with CWT’s generator and other associated electrical components.


WT+ES reduces capital cost, increases electricity production and provides predictable energy.

WT+ES which stores energy 6 hours during night and generates electricity 18 hours per day has the capacity factor of 18/24=75% unlike CWT typical capacity factor of 35%.

This reduces the capital cost of electrical component like generators, transformers, array cables, export cables, sub-stations etc. An offshore wind farm with 100x6MW turbines can be de-rated from 600MW to 280MW using WT+ES while the annual production is kept within similar rate (600MWx35%=280MWx75%).

Deletion of gearbox and convertor and using the ES structure as turbine foundation considerably reduces the cost.

Learning objectives
The following will be reviewed in the presentation:

• Essence of energy storage for wind turbines;
• Time based electricity market;
• WT+ES capacity factor;
• WT+ES efficiency;
• WT+ES rated output speed;
• ES capital cost and revenue;
• WT capital cost reduction;
• Drilling technology for mono-piles and large diameter deep bore holes;
• Use of hydraulic mechanisms in wind turbines;
• Innovative dynamic seal mechanism and a short video of the small scale prototype function.