Pumped Storage Hydro

Small pumped storage at core of neighborhood project combining solar, wind, battery

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Small pumped storage at core of neighborhood project combining solar, wind, battery

By Richard Baldwin

The unique Jacksons Creek project, when completed, will supply electricity to a subdivision of 12 rural lots in Porirua City, New Zealand, that were converted from plantation forest. The primary energy supply for this project is solar and wind. In this city, the minimum rural lot size allowed is 5 hectares. Some lots are twice this size, with areas sequestered for re-establishment of indigenous bush, and there is plenty of space for ground-mounted solar arrays. The core of the Jacksons Creek project is a small pumped hydro storage system.

The Jackson’s Creek micro-grid network is a private secondary network, operated as a customer network where consumers buy and sell from the body corporate. The customer network is configured to maximize generation efficiency and minimize cost and connection requirements to the parent network. This significantly increases the reliability of electricity supply for consumers, as well as reducing connection costs to the parent network.

The customer network will support voltage on Wellington Electricity lines and may increase reliability of the network if the immediate section of the network can be islanded (e.g. storm or disaster), as well as defer operating and upgrade costs to customers.

The first call on energy use is community demand, the second call is local storage and the third is export to the national grid.

Insight into the project

The pumped hydro storage system is located in energy easements on several of the lots that offer maximum altitude difference. It uses 2.5 million litres of water at 235 metres of head between upper and lower reservoirs. Annual generation is estimated at 60 Mwh, which is around 30% of actual capacity. The advantageous geography of this steep to rolling hill country lends itself well to the project, and the new use for the land has not compromised food production, which is an important consideration for farmland conversion to rural residential purpose. Grid connection to a nearby 11-kV line provides an export portal for surplus electricity.

The entire enterprise comprises a microgrid spanning domestic distributed solar and common wind generation, located on three small rises on the main transecting land ridge, selected for optimum production and separation from house locations. They aregrid connected but can continue as an island, with the pumped hydro generator black starting, shifting its primary function during external grid failure.

The energy community has the potential to supply immediate neighbors, and renewable surplus is significant, standing at 150 kW peak, 750kWh/day, seasonally variable. Each of the 12 lots connects its contributing 10-kW solar plant to the common microgrid. Domestic metering is at the house border in the conventional manner. The three vertical-axis wind machines provide up to 30 kW to the microgrid via a buffering 55-kW three-port bidirectional inverter and 200-kWh battery. A supplementary group of solar arrays provides an additional 80 kW of capacity.

During the development of control systems, a diesel-powered synchronous generator has allowed live simulation and testing while the pumped hydro storage facility is being completed.

Planning and action to reach the current 90% civil works completion and new house builds under way on the 12 lots has taken six years from inception.

The mature system with all its components in place will be owned by the community in equal shares and managed for common benefit. Revenue from surplus electricity exported to the grid is distributed according to surplus export at house borders.

System details

Transportability of the Jacksons Creek concept is centred on the control process. Repeat project principles will be much the same except for the influence of the terrain on hydraulic specifics and will require recalculation in most cases but can be accommodated relatively easily in the choice of turbine and pump for head and stored volume.

The economics of the total enterprise are a fine balance. Requirements for a minimum 10-kW entry-level solar plant must be met by house builders so do not load the developer. There are several mitigations that have reduced the extra margin of direct cost added to the project by pumped hydro storage dam, penstock, generator, pump and control system.

The development has taken place over 1.7 km, considered a long distance to extend a low-voltage single-ended supply network. Normal grid supply for a development of this geographical spread would require a medium-voltage feeder and one or two transformers. The medium-voltage system would have been owned by the lines company and the entire cost levied upfront, the cost of medium-voltage services and underground distribution exceeding NZD1 million (US$608,512). Stripping out the medium-voltage components and using community owner infrastructure for distribution allowed a large proportion of this expenditure to be redirected to the pumped hydro project. Household-located generation and voltage support from the buffering inverter help to keep voltage losses within national guidelines for low-voltage networks.

Primary energy storage is 2.5 million litres of water in reservoir impounded by a repurposed farm dam. Natural head is 235 m to the turbine and pump house, reducing by 11 m of head under full flow conditions.

The farm dam that acts as the upper reservoir was drained for lining.

The penstock runs from the invert level of the dam through a 110-m-long tunnel drilled under terrain to an East-facing gully, the outlet control valve head and route for the balance of the penstock. The valve head is actuator-controlled and will shut down if penstock water velocity exceeds a preset rate. At the dam outlet valve, the penstock is vented to ambient air as part of surge protection. High-density polyethylene pipe for the penstock is used in two pressure ratings, PN16 and PN25, to cope with increasing water pressure as the penstock descends. The penstock enters the turbine floor to a thrust block leading to a main valve.

Horizontal drilling in progress under the East-West ridge for penstock installation.

The valve opens fully over a 30-second period, when the 60 kW turbine is called upon to start. Water is fed to a turbine spear jet valve with a preset opening to start the machine and run it at just below generator synchronous speed, after which further control action is handed over to the Woodward control system. The lower prefabricated reservoir captures spent water for pumped return to the high dam.

Storage volume is less than that of the high dam but allows for estimated demand for 12 hours, after which overflow to an adjacent waterway takes place through a system of automated valves. This also reduces high dam tidal ebb and flow. Initial water charge and top-up of reservoirs occurs with natural precipitation of 400,000 litres each month, less estimated evaporation of 140,000 litres. Water is also available if required from an existing farm water supply.

The farm dam, which had provided seasonal stock water for many years, was drained and recontoured to maximize capacity and is compliant with small dam guidelines. Drainage is installed in the dam floor to ensure natural ground water and water entering behind the dam lining is led away. Otherwise, the dam liner would become an unstable “boat” as the dam cycles daily through its water level variations.

The Pelton turbine was manufactured in Christchurch and drives a four-pole 58-kW Stamford synchronous generator and 80-kg flywheel. For speed over-run emergency, an automatic deflector is operated by loss of pneumatic pressure, causing a truck brake actuator under spring load to move a flap into the path of the water jet, controlled by an independent speed sensor and the Woodward synchroniser. The pump is a progressive cavity type chosen for positive displacement, ability to work at high head and amenable to speed and output modulation when energy surplus is not able to supply the full rated pump motor requirement. Both turbine and pump are selected to provide optimum round-trip efficiency.

The control system is built around Woodward, Micrologic programmable logic controllers and SCADA software. Control of the process includes pump, generator, export flows and internal domestic sub loads. Exercise of robust control is most critical in an islanded situation when weak grid conditions prevail. A “donkey” motor is coupled to the Pelton machine shaft to dry-run the machine for adjustments and demonstration.

Location of the upper controls and accommodation for the battery energy storage system.

System stability is reduced in an islanded situation when the micro-grid is operating as a “weak network.” The hydro synchronous generator assumes the excitation role of the network voltage source, replacing the temporarily absent national grid. All solar inverters must be grid-connected, externally excited for operation, shutting down if it fails. It is essential that control is maintained over generator speed, thus frequency, and it must always be operated to deliver power. Key to this is central control of all distributed solar inverters, scaling back power to ensure minimum hydro generator power delivery sufficient for network stability. Control is exercised over a fiber-based Transmission Control Protocol network for engineering and meters at critical locations. Customers have the option to take an internet feed off the same network.

Associated with system stability is the rate of hydro output variation, which has a naturally slow hysteresis. This is complemented by the bidirectional inverter and battery, which offers fast hysteresis and transient accommodation during rapid load variation.

A combined static Volts-Amps Reactive (VAR) generator and active harmonic filter is installed in the network to deal with VAR, harmonic distortion and phase balancing. The concentration of inverter-based sources benefits greatly by this addition, allowing grid export to be “cleaner” than what is offered if importing energy.

Pumping water to the elevated reservoir is a particular challenge. The required head precludes the use of any form of turbine reversed as a pump. A Pelton turbine is the ideal generator driver machine, being naturally efficient and maintaining efficiency at less than full output, will not work as a pump.

Pump criteria, in order, were efficiency, ability to deliver under less than full-drive power and total cost of ownership. The choice is a progressive cavity pump with variable speed drive. The positive displacement pump allows delivery of water when less than full power is available, which occurs during the day when renewable generation, reduced by local demand, leaves less than the required 55 kW for full pump drive. This contrasts with centrifugal pumps, which must operate above a certain point, do not offer flexibility and are less efficient.

The pump controller is modulated by the ratio of export to import measured at the point of common coupling to the national grid, to ensure imported energy is not used for routine pumping. At a future point, buy-in of grid energy for pumping when the spot market is around a few cents per unit, then returning energy to the grid during peak market rates, is being considered but will not be taken further before all domestic solar installations are operational.

Wind power is drawn from three 10-kW vertical axis machines. From each, wild frequency three-phase output is fed to its rectifier, clamp and dummy load. Resultant 750-v direct current is managed for injection to the microgrid by the three-port bidirectional inverter and battery energy storage system buffer. Vertical-axis machines were selected, despite lower efficiency compared to horizontal types, for reduced maintenance and lower level of operating noise. All towers are fitted with a hinge and hydraulic system to allow easy lowering for inspection and maintenance.

Much of the project has used off-the shelf skills and hardware. Exceptions are the Pelton machine, tailored to head and flow volume, and modifications to wind turbines to better suit the application. Primary innovations are in systems developed around the microgrid to provide essential stability, particularly during islanded operation.

During preliminary research and planning, the developers found a yawning gap in available knowledge of small-scale pumped hydro storage and no established work to refer to. In the systems work-up, for example, there was a barrier in understanding from those engineers who were used to run-of-river hydro projects and found it difficult to agree when the developers rejected dump loads as a method of turbine stabilization. Retained water is gold, money in the bank.

The network was energized in late 2021 using renewables, as work proceeds on final touches to the civil works and completion of the pumped hydro system, which is expected to be operational in the last quarter of 2024. An extension to connect immediate neighboring properties is possible. The systems and control package is able to be transported and established in any location that has land space for solar arrays and geography suitable for water reservoirs.

Richard Baldwin is technical advisor for Jacksons 2019 Ltd and Horokiri Electric Light and Road Company Ltd. and is responsible for civil works and design.