Fig 3 Simplified Model

The "GENSSIS" Project

The underlying physics of this system are not complex and are based on the well-established principle of E=mgh, the main technical challenges are associated with the construction of a utility scale installation, as follows.

  • The drilling of a suitable borehole, including the calculation of diameter and depth referenced to costing.
  • The selection, installation and operation of surface level equipment.
  • The challenges associated with heavy weight handling and transportation.
  • The attachment and deployment of the mass solution into the borehole.

Concept Validation

Concerning this concept, a viable industrial capacity of energy storage can only be achieved through sufficient up-scaling of its fundamental components and as this is where the main technical challenges exist there would be little point in developing a small scale prototype. To that extent the following prototype specification is proposed using existing commercially available, proven technologies, in particular those developed in the offshore oil and gas industries and combining these components with innovative solutions.

Prototype Specification
Gross Storage Capacity 1.2 MWh’s.
Power 3.4 MW
Total mass including all rigging 470 tonnes
Borehole depth 1,500 metres.
Finished internal bore diameter 500mm / 19.75inches

Below Surface, The Suspended Mass

As the overall weight of the mass must be in the order of several hundred tonnes, a practical solution to facilitate the transportation, installation and reliable operation of the mass within the borehole has resulted in the following arrangement for the design concept.

A synthetic rope of sufficient specification suspended into the borehole and running on the sheaves of the traction winch, with the un-loaded portion of the synthetic rope to be constantly auto coiled and payed-out via an adjacent storage reel system during operations.

The suspended mass would be comprised of numerous modules, each having at least one pair of cast steel cylindrical half-sections. (see fig1b opposite)

The cylindrical half-sections would be bolted together around a steel cable to form a single independently attached unit of approximately 3.25 tonnes each. The combined mass would be the sum of the required quantity of these units.

The diameter of the steel cylinders relative to the borehole diameter would be sufficient to allow free vertical movement of the suspended mass within it.

The control of the mass descent velocity; potentially high efficiency levels could be achieved using generator load sensing. The generator loading could be governed electronically as generator loading conditions would in turn transmit an increased or decreased load through the traction winch thereby reducing/increasing the pay-out velocity of the rope.

In concept a significant advantage of this system would be the enhanced ability to store energy at a variable rate of input and then retrieve the stored energy at highly stabilised rates of output, effectively stabilising the production outputs of adjacent renewable power sources.

Gravitational Potential Energy Storage, (GPES)

The Borehole & Energy Generation

In order to investigate the technical and commercial feasibility aspects of this concept a prototype specification is outlined as follows; a vertical borehole of approximately 500mm finished diameter would be drilled or recovered to a depth of 1500m using available drilling technology.

The viability of capitalising on existing disused mine shafts should be investigated, potentially they could be transformed into valuable resources of stored electrical energy.

With a proposed prototype borehole depth of 1500 meters the installed mass would then have an overall length of 360m and a total weight of 383 tonnes when fully extended into the borehole. The gross power output and storage capacity of the proposed prototype as 3.4 MW for .4 hours with a capacity of 1.2 MW h.

Fully commercialised versions could have significantly increased power outputs and capacities by varying the borehole diameters and depths.

Round trip efficiency expectations would be in the order of 70-80% based on consultations with manufacturers of similar systems.

Gravitational Potential Energy Storage (GPES): Primarily this concept is intended as a means of providing stability to electrical power outputs from highly variable renewable energy resources such as wind, solar & wave power. The benefit of Energy Storage Technology (EST) when integrated with sustainable resources is measured by the amount of conventional generation that is offset by its introduction.

Black-start Capability: Co-located systems would provide turbines with the ability to Black-start independently of grid provided energy. Reactive power which must be provided externally to energise or excite certain generators can be required after a complete blackout situation.

Defer Generation System Upgrades: it can also be used to defer investment and upgrades to the T&D equipment (Eyer & Corey, 2010). T&D equipment upgrade deferral is a particularly good application for energy storage because often upgrades are needed to compensate for relatively modest power and energy increase requirements.

Round trip efficiency expectations would be in the order of 70-80% based on consultations with manufacturers of similar systems.

Could be capable of providing local reactive power, in the event of voltage dips,local reactive power capabilities are crucial for voltage stability which is a key issue for system security.

Above, GPES Concept Scaled Up to Multi MW Installation

Technical Details

The design concept sets out a system that could enhance the stability of electrical energy production from a number of sources essentially however its main focus would be the management of electrical energy generated from renewable sources such as wind/solar. Commercially the system could have global relevance and be capable of providing utility scale energy management via an arrangement of modular installations.

At the core of the concept is an electro-mechanical installation which would be surface-mounted at the head of a deep vertical shaft or borehole. Attached to this electro-mechanical system, by means of synthetic rope technology, would be a sizeable mass suspended vertically into the borehole.

Electrical energy from any source can be converted into gravitational potential energy when power is diverted to an electrically powered winch system enabling the suspended mass to be raised within the borehole. The restoration cycle of the electrical energy would be achieved when the raised mass is subsequently allowed to perform a controlled descent under gravity, a procedure which would mechanically activate an electrical generator installed at the traction winch gearing.

Fig 4 Surface Installation

Surface Components

A “Traction” type winch and storage reel system designed to handle synthetic rope, this system would enable the rope to operate on the winch sheaves under a stabilised, controllable velocity with zero slippage.

The concept is designed to function with the bulk of its components below ground so that the construction of an industrial type unit, 15m x 20m, to enclose the surface installation would have a minimum visual/environmental impact. Where necessary an underground ‘bunker’ system could be constructed to reduce visual impact even further.

A software-based control system acquiring data from numerous sources, synchronising the winch operation throughout the storage and energy regeneration cycle.

Gravitational Potential Energy Storage, (GPES)

Fig 2 Storage Cycle

Fig 3 Regeneration Cycle

As in FIG’s 1 & 2 above the GPES system would ideally be physically located on site or adjacent to a renewable electrical power source.

Unique Design Points:

The system is designed to have a small physical footprint to enable it to be co-located on-site with most types of renewable energy sources.

Short Ramp-Up Time: The system could be called into service very quickly, “despatched” within time limits of milliseconds as would apply to Primary control units.

The concept is a modular based design: Capacities could be expanded over time to correspond with operational and financial objectives. Initial capital expenditure can be balanced with the requirements and financial capabilities of individual Load Serving Entities LSE’s

Onshore Renewable Sources: The complete system could be installed on-site/close to Wind Farms, Solar Farms, and non-base load Enhanced Geothermal Systems.

Offshore Renewable Sources: The power production from offshore Wind and Wave Energy Convertors could be diverted to on-shore facilities located. This could be a major technical boost for wave energy devices by smoothing the highly variable power outputs associated with WEC’s.

Zero Loss of Stored Capacity: Between operations & while in standby mode, the system would not suffer any stored capacity loss as the suspended mass could be mechanically “braked” indefinitely suspended at any given level in the borehole.

Grid Penetration: The capability to significantly increase renewable energy grid penetration levels by limiting the effects of the non-synchronous power production characteristics associated with current renewable energy sources.

Energy Storage can provide reserve capacity that is readily available without consuming fuel.

Increased Commerciality: The potential to provide a long term solution which could lead to increased commerciality for renewable energy generating system developers/operators.

Curtailment Wastage: Reduce/eliminate by providing on-site energy storage for Wind Turbines into which electrical energy can be diverted during high production/low demand scenarios.

Load Levelling or Commodity Storage/ Arbitrage: Power which can be generated more cheaply during off-peak periods can be sold into the market again during high price hours.