Pumped Hydro Storage In The Usa

Improving Energy Storage in the USA:
An Action Plan for Pumped Hydro Storage

Luis Martinez-Montoya

Introduction

The increasing amount of energy supplied into the grid by renewable energy sources pose a challenge for utilities and project developers. That is, renewable energies such as wind and solar are intermittent because of their natural cycles. For instance, solar energy can beused only during daylight. Whereas wind speed variability can suddenly increase or decrease the supply of energy at different times of the day. The lack of a 24-7 steady supply of renewable energy compromises grid security and reliability in systems with a large share of intermittent sources.

Using storage technologies is the only way to mitigating, if not, eliminating, the adverse effects ofintermittent energy sources. Storage technology increases grid reliability by gathering energy at low-demand hours to be released when demand for electricity peaks. A number of technologies are expected to provide storage for grid-scale applications in the future. Such technologies include: PHS, (Pumped Hydro Storage), CAES (Compressed Air Energy Storage), Batteries and Flywheels.

Most of storagetechnologies are at an early stage of their R&D processes. To date, PHS is the only one that has proven successful for providing grid-scale storage. PHS is clean, reliable and highly efficient. It is also cost effective when a market for ancillary services is present (not the case for the US).

Increasing the amount of renewable energy in the US´energy mix, will increase the need of storagefacilities to guarantee grid reliability. Nevertheless, technology “lock-ins” and the valley of the death hypothesis, can prevent PHS to becoming the source of storage needed. The next few pages contain an analysis of the prospects for PHS in the US. Additionally, an assessment of the challenges and opportunities for PHS systems is provided. Finally, policy alternatives to spur deployment of PHStechnologies are proposed.




A glance to pumped hydro storage

Pumped hydro storage (PHS) refers to the modified use of conventional hydropower technology to store and manage energy or electricity. On a typical PHS facility, water flows between an upper and lower reservoirs. During off peak electricity demand time, water is pumped to the upper reservoir and its stored as potentialenergy. Then, when demand for power peaks, the water is released to be converted in electricity through a turbine like in any conventional hydro plant.

Source: Alstom Power

PHS is the only technology commercially proven for grid-scale energy storage. It is highly efficient ranging between 70 - 85% per cycle. And, when equipped with adjustable speed turbines PHS can provide fast ramping andfrequency regulation services.

The U.S. has roughly 40 PHS plants providing over 22 GW of storage. Largest projects are located in Michigan, Virginia and California. Additional 43 GW of PHS projects are in queue for licensing and permitting in the country. The driving force behind PHS projects has been the increasing use of solar and wind energy in states energy mix.

Although PHS is considereda mature technology, it has room for innovation in two main areas:
Variable/adjustable speed of turbines.- PHS systems are equipped with a reversible single-stage “Francis” pump-turbine, which acts as a pump in one direction and as a turbine in the other. Nevertheless, turbines have performance limitations while in the pump mode. Traditional devices pump water at constant speed and require aconstant energy input. Therefore, a power plant providing a steady supply of energy is needed to operate in the pump mode. Commonly, thermal plants burning oil or gas serve to this purpose.

Variable/adjustable speed turbines do not require a constant supply of energy. They adjust pumping speed depending on energy input. This flexibility will allow PHS plants to work with intermittent energy...