Conceptual Study of a High-Density Fluid Hydro-Power Energy Storage Project

Conceptual study summary of a high-density fluid hydro-power energy storage project at Parys Mountain.

Prepared by Anglesey Mining plc and RheEnergise Limited (Issue 1, 08/04/2025)

Background

In October 2024 the Department for Energy and Net Zero published its response to the Long Duration Electricity Storage (LDES) consultation, designing a policy framework to enable investment in long duration electricity storage. The report states that:

“The Department for Energy Security and Net Zero is at the heart of this government’s agenda,

leading one of the Prime Minister’s 5 national missions, to make Britain a clean energy

superpower with zero carbon electricity by 2030, and accelerating our journey to net zero.”

And went on further to explain:

“LDES assets will be vital in helping to not only decarbonise our power grid, but to also increase our energy security by allowing us to maximise the use of intermittent renewables, storing this energy when there is excess generation for use in periods of low generation. It is why we are seeking to support developers in deploying more of these assets.”

High-density pumped hydro storage was referred to as one of the novel enabling technologies:

“Pumped Storage Hydro (PSH) is the most mature LDES technology, but there are also more

novel technologies such as Liquid Air Energy Storage (LAES), Compressed Air Energy

Storage (CAES), gravitational, high-density pumped hydro, and flow batteries at varying stages

of commercial readiness and deployment.”

In February 2025 Anglesey Mining and RheEnergise (who are UK based, and the leaders in high-density hydro energy storage) signed a memorandum of understanding (MOU) to explore the potential deployment of such a system. Anglesey Mining recognized that the modern mothballed underground mine at Parys Mountain could be used to host the system, and its deployment would have the potential to generate sustainable cash-flow, commencing within two to three years. This cash-flow could be then used to support the commencement of underground polymetallic mining and processing at the site.

Business Objectives

Anglesey Mining is focused on delivering a polymetallic underground mine at Parys Mountain. To that end, Anglesey’s management are developing strategies to enable investment in the development of Parys Mountain to be incremental so far as practicable, thus allowing risks to be mitigated in stages before considering options for the next step of development.

Exploring the deployment of RheEnergise’s innovative High-Density Hydro® (HD Hydro) energy storage technology at the Parys Mountain mine site, using the mothballed underground workings and the Morris shaft which was excavated in 1989, fits with the incremental development approach.

Elements of the energy storage project scope, for example: the de-watering and refitting of the Morris shaft for material and personnel hoisting, the dewatering of the workings emanating from the Morris shaft 280m below the surface, the upgrading of the power-line to site, the on-going environmental and social studies and the deployment of impact avoidance, mitigation and compensation strategies, are each synergistic with the first steps of establishing a modern underground mine on Parys Mountain.

It is an essential and clear intent of this project that Anglesey Mining retains all the optionality that it currently has for the construction and commissioning of an underground mine at Parys Mountain, and that the hydro energy pumped storage project should not detract from those options over the medium and long term.

The energy storage project will initially market its products, which are energy balancing and storage and national electricity grid (the “grid”) stability services, to third parties. In the future the Parys Mountain mine may be in a position enter a long term energy offtake contract for the powering of the mine and processing plant, at that time mutually beneficial commercial terms will be explored as the energy storage supply and off-take will be in close physical proximity.

Project Description

The high-density fluid developed by RheEnergise will flow in a closed loop cycle between an upper and lower reservoir, storing and releasing gravitational potential energy which will be obtained from and discharged into the grid, in the form of electrical energy.

The lower reservoir will utilise the existing 280m (below surface) level workings originating from the Morris shaft at the modern Parys Mountain mine and the upper reservoir will be developed as a series of interconnected pipes, tanks and/or chambers on the surface adjacent to the Morris shaft headframe.

Vertical pipes of approximately 1m in diameter, either in the Morris shaft or bored into the rock nearby the shaft, will connect the upper reservoir with a ‘Power House’ which will be built in the region of the 280m level close to the line of the Morris shaft.

The Power House will be in a mined out chamber, containing:

A bespoke turbine and generator combination developed by RheEnergise, which will convert the kinetic energy of the falling fluid, under approximately 280m of head, into electrical energy.
A sump and pumps for returning the high-density fluid back to the upper reservoir
The instrumentation and actuators necessary for monitoring and remote control of the system
Power transmission connections to transmit the electricity generated by the turbine, via cable, up the Morris shaft to the surface and into the grid and vice versa from the grid into the electrical motors connected to the pumps.

Access to the Power House both during construction and operation will be by means of the de-watered, ventilated and re-fitted Morris shaft, with hoisting capability both for personnel, materials and equipment.

As well as the upper reservoir a temporary facility will be built on surface to manufacture the high-density fluid, which will have a processing circuit which will include crushing and grinding. This high density engineered fluid will utilise as large a percentage as possible of the waste rock material that was excavated from the Morris shaft and is currently stored on surface. Once the required fluid has been placed into the closed loop system the manufacturing facility will be surplus to requirements and can be dismantled and/or repurposed.

High voltage power transmission lines connecting the site to the National Grid will be permanently installed.

The Morris shaft has a cross-sectional area of 17.7m² and the proportion of this dedicated solely to the energy project (fluid delivery pipes, power transmission and control cabling) will be approximately 2m² (less if the fluid delivery pipes are raise bored into the rock). Subsequent use of the shaft by a mining operation at Parys Mountain, for ventilation, emergency egress or hoisting activities will not be compromised by this in any meaningful way.

While the existing 280m development level is in use as the lower reservoir for the energy project then mine development from this level down, in the immediate vicinity, will not be possible. Provision will be made for Anglesey Mining to have an option to replace the lower reservoir by providing a suitable alternative reservoir in another area which has, by then, been mined out. There will also be a negotiated option for Anglesey to make the power project financially whole through compensation; if that option were to be taken up by Anglesey, it would imply that the incremental mining of the area immediately below the 280m level would have been assessed to be more financially attractive than continuing to run the energy project.

Project Management

It is intended to develop the project through this conceptual phase, into pre-feasibility study (PFS) which itself will be split into two; PFS-A where options are explored (the divergent part of PFS) and PFS-B which converges to a single scope that will be taken forward into a Feasibility study (FS). During the FS the detailed engineering will be undertaken, resulting in a project deployment schedule and methodology with defined packages of work that can be tendered.

Conceptual Study Phase Objectives

This document is a summary of the Conceptual study phase. The objective of this phase is to set out the initial assessment that has been made of the project, to articulate the opportunity that has been found and the reasons why it is worth pursuing. A further objective is to document a high-level scope of the project, the business case for it and the risks (threats and opportunities) that have been thus far identified. Plans for the mitigation of these risks, in detail, will be addressed in subsequent study phases.

Project Objectives

To deliver a financial return to investors in the energy storage project with due regard to safe operating practices and in compliance with current social and environmental expectations.
To commission and operate the first commercial scale High-Density Hydro® (HD Hydro) energy storage technology project, in line with the requirements of Technology Readiness level 8, which is the next step in RheEnergise’s technical development journey. This will involve capturing the intellectual property (IP) and technical knowledge, such that there is increased confidence in subsequent deployments of the technology, allowing the IP to be monetised.
To incrementally advance the development of Parys Mountain to a profitable, operational poly-metallic mine.
To facilitate the more productive use of existing and new forms of renewable energy generation sources on the Isle of Anglesey, through the storage of the power when generation exceeds demand and the releasing back of energy into the grid during periods of high demand.
To provide local economic opportunities during the project’s construction and operation.

Project Capability

The maximum instantaneous generation capability of the project is determined by the flow rate in the pipeline passing through the Power House, the density of the fluid and the elevation change between the upper and lower reservoirs (the head of the system).

The energy storage capacity is limited to the volume of the fluid in the closed loop system, which in this case, is constrained by the volume of the lower reservoir. The volume of the existing workings emanating from the 280m level in the modern underground mine at Parys Mountain is approximately 13,500 m³. Using these constraints power generation of 5.5MW with 4.5 hours of storage capacity has been calculated to be possible.

The project will incorporate options for increasing the size of the lower reservoir by mining additional drives from the 280m level. The material removed will be hoisted to surface and will likely be crushed and ground for use in the manufacture of the high-density fluid.

Anglesey Mining will have the opportunity, outside of the bounds of this energy project, to take advantage of this material being crushed on surface. It is likely that mineral processing equipment could be justified to recover the contained minerals for the purposes of mine production product testing and reconciliation with the geological model. The cost of this processing test work and possible revenues from the sale of said minerals has not been assessed at this time and the net financial impact is not reflected in this conceptual study.

Increasing the volume of the lower reservoir by 32,500 m³ (approximately 80kt), for example, would allow power capacity and duration trade-offs to be made within approximately the following bounds:

High power / short duration Power at 15MW, with a duration of 4.5 hours

Lower power / long duration Power at 4.5MW, with a duration of 15 hours

Calculated Physicals:

Sources of Revenue

The major source of revenue is from pumping the fluid up from the bottom reservoir during the off-peak segments of the electrical demand cycle and allowing the fluid to flow back down during peaks, these are so called Arbitrage Revenues. The project will also be able to sell grid Balancing Services which include management services and to take advantage of Ancillary Revenues such as frequency response and power factor correction. Additionally, there is also the potential to exploit Capacity Payments.

Project Resources

The Project is dependent upon having (granted by Anglesey Mining) the unrestricted use of the underground development originating from the Morris shaft at the 280m below surface level and for that level to be mined out further to increase the capacity of the lower reservoir. Additionally, it depends on the ability to use land at the surface upon which the upper reservoir can be constructed.

In addition, licences and agreements need to be in place for Anglesey Mining to provide unhindered access to the underground development through the Morris Shaft, an allocation of real estate within the cross-sectional area of the shaft, and the supply of material (stockpiled arisings from the already dug out Morris shaft) for the manufacturing process of the high-density fluid.

Anglesey Mining will also be relied upon to provide the technical and environmental studies that have been undertaken to date as well as logistical support, construction laydown areas, offices, storage areas etc. Anglesey Mining has good relations with, for example, the community, local councils and heritage groups.

The Project is dependent upon RheEnergise’s technology, consulting services, and the solutions provision during the engineering procurement and construction phases of the project as well as technical knowledge. RheEnergise are in a position and would be contracted to supply the Project with operational and maintenance expertise during commissioning and ongoing operation of the Project.

Anglesey Mining and RheEnergise propose to form a jointly held entity, that will seek third party financial resources to progress the Project.

Sequence of Engineering, Procurement and Construction

The most immediate next steps are to:

Appoint a project manager, likely from within the existing internal resources of Anglesey Mining and RheEnergise to run the project through the pre-feasibility study phase of behalf of its two shareholders. Develop a project charter and establish governance protocols and a steering committee.
Explore the range of funding options for the project, including those from government agencies such as the National Wealth Fund.
Assess the extent of planning permissions required, leveraging the baseline environmental work that has been undertaken by Anglesey Mining on the Parys Mountain site, and then progress the required applications.
Apply for a grid connection licence.
Commence the engineering of high-density fluid.
Commence options analysis as part of the divergent phase of the pre-feasibility study (PFS), concentrating on key macro solutions for the overall layout and design, for example:
1. Determine the initial optimal size of the lower reservoir, and hence the energy storage capacity of the system.
2. Trade-off power generation capability and storage duration, within the physical constraints of the site.
3. Assess whether to place high-density fluid delivery pipes within the Morris shaft, bore the route down into the rock, or explore another solution.
4. Select the storage vessel options and layout of the upper reservoir: buried pipeline, surface tanks, bespoke lined and covered reservoir or another solution.
5. Establish the size and location of the underground Power House and its layout.
Refurbish the Morris shaft headframe and hoist winder that will be used to facilitate, firstly the dewatering of the underground workings (permits are already in place for discharging the water from the Morris shaft) and subsequently re-fit the shaft so that it can be used to construct then operate and maintain the underground reservoir and Power House.

The feasibility study phase will commence with a settled scope, so that detailed engineering work can be focused upon. RheEnergise will be a key solution provider during the detailed design phase specifically the engineering (and manufacture) of: the high-density fluid, the turbine generator, the pumps, combination and management systems in the underground Power House, and operating, monitoring and control systems.

The de-watering of the shaft and underground workings could be progressed prior to detailed design work being completed, as re-establishing a presence underground is likely on the construction critical path to project delivery. Once safe underground access is secured, pipes can be fitted in the shaft or pilot holes can be drilled from surface to allow raise boring of the fluid delivery hole(s) to be undertaken, with the waste rock being continuously cleared from the 280m development level and hoisted to surface. It should be noted that pre-investment in underground establishment would be undertaken with Anglesey Mining, fulfilling the duties of the owner as laid out in the Mine Regulations 2014.

Following a final funding decision, mining to extend the volume of the 280m level will then be undertaken based on detailed designs and in parallel with that the surface construction can commence.

There are two major surface work scope elements, the storage facility for the upper reservoir and the fabrication of the fluid manufacturing facility. Once those two major surface scope elements are complete, the manufacture of the high-density fluid can be started with the product stored in the upper reservoir. At the same time the underground Power House can be constructed and fitted out with bespoke pumps, turbines, motor and generator sets.

Power transmission connection to the national grid on surface and down to the Power House, via cabling in the Morris shaft will be completed, and will need to be installed together with a surface control room with remote interface to the Power House.

As the project progresses it is likely that a specialist, engineering, procurement and construction management firm will be appointed, with the Anglesey Mining and RheEnergise teams combining into an owners’ team.

The project will then move into the commissioning phase, an operational readiness plan and a commissioning protocol will be developed as part of the FS, and a budget for these activities will be laid out.

Next Stage Work Plan

The next study phase, that of pre-feasibility study (PFS), will likely be managed by the internal resources of Anglesey Mining and RheEnergise. The PFS phase will initially be divergent where a range of options are considered for the layout and size of the project as well as options for project delivery. Some of the questions to be answered in the divergent phase of PFS will include:

The capacity of the lower reservoir
The engineering, procurement and contracting strategy that it is best to deploy to deliver the project
The scope and resources of the owners’ team
An articulation of the options for staging the project
The level of pre-investment that might be required to ensure that elements of the project with a high fixed capital component, such as power transmisson, are sized with future expansion duly considered.
Ensuring the relationship between energy storage capacity and duration together with the Capacity Management framework can be modelled, with the uncertainty in models understood and described.

Risk Register

A risk register has been established to document the risks identified and the response plans put in place to: avoid, mitigate or compensate for undesired impacts. This risk-based management approach is essential and will be on-going as the project progresses.

Risk is defined as any activity or event that might cause the project costs to deviate either adversely (threats) or beneficially (opportunities) from the central value case. Risks have two components: (1) the Probability of deviating from a particular outcome, and (2) the Consequence(s) of not ending up with the intended outcome.

Deviation from the expected plan can be caused by quality, contractual, technical or schedule issues. It also includes any financial impact, as well as the cost of mitigation of risks, however caused.

Risk management is the practice of proactively dealing with threats and opportunities, prioritizing them so that the project’s resources: time, money and skills can be deployed in areas that yield the most value for the effort expended.

Description of risk	Conceptual mitigation plan	Likelihood rank 1 (low) to 5 (high)	Consequence rank 1 (low) to 5 (high)	Overall level, before any mitigation (Low, medium or high)
The waste mineral in storage at the mine can’t be engineered into the formulation of the high-density fluid. Meaning a significant portion of the material for the fluid has to be transported to the mine site	Test-work on the range of mineral stored at the mine site to be undertaken in the PFS. Unparalleled range of geological sequences available on the island. Port within 20km of the mine site allowing material to be shipped in in extremis.	4	2	High (threat)
Grid connection license not available to export power at time of commissioning	Application for connection license as soon as technical parameters are known. Leverage Anglesey County Council’s Anglesey Energy Island™ Programme. Seek to partner with existing, local, wind generation assets.	2	5	High (threat)
The high-density fluid technology and the integral turbine power generators do not successfully scale up from the demonstration units.	Consider the installation of intermediate power offtake points down the shaft, to reduce the head. Consider reducing the diameter of the pipe to reduce flow, back to a proved scale.	1	5	Medium (threat)
Exploration and trial mining from the 280m level proves up recoverable mineral resources to the extent that mine development from the 280m level becomes compelling by comparison to the energy storage project.	This is an opportunity for both the Parys Mountain mine and the energy project. If during project deployment financial returns were forecasted (from continuing to mine and explore from the 280m level) which were likely to exceed those of using it as the lower reservoir, then the mine would logically buy out the energy storage project at an agreed premium taking into account the expected returns of the project.	2	4	Medium (opportunity)
An accident or an unmitigated, unintended environmental harm incident occurs during construction or commissioning of the project.	From project inception, develop the systems and norms of a first-class Health, Safety and Environment (HSE) control and management plan. Driving an interdependent safety culture, with all the support and reporting mechanisms in place.	2	4	Medium (threat)
The energy market in Britain changes significantly during the time between now and the commissioning	This is recorded as an opportunity. The UK government’s net zero targets will require an increase in the use of electrical energy as a share of the UK total energy use mix, driven by de-carbonization of transport and home heating. The proportion of electrical energy generated by renewables (predominantly wind and solar) that are intermittent. Logically requiring more demand on energy storage technologies. Companies with existing hydro-energy storage in the UK, highlight expansion projects that are available under the correct energy pricing regime and expect changes that incentivize those projects in the near term.	2	3	Medium (opportunity

A PDF version of the above study can be viewed/downloaded by clicking HERE