Liquid Air Energy Storage: How Cryogenics Can Support a Greener Grid

by Richard Riley, Highview Power Storage,

Climate change is one of the most pressing issues of our time and it is presenting enormous challenges to the engineers who design and operate electricity systems around the world. The task is the progressive decarbonization of electrical generation, and it needs to be achieved while maintaining continued stability and reliability of the power supply upon which we have come to rely. The United States is taking a leading role, with a growing number of states setting aggressive renewable targets. At a municipal level, Madison WI, host of this year’s Cryogenic Engineering Conference, set a goal of 100 percent renewables and is due to publish a roadmap for achieving this in early 2018.

The cost of renewable energy has fallen rapidly in the past decade-80 percent in the case of solar-and renewables are increasingly becoming cheaper than traditional energy sources. However, renewable resources are largely intermittent in nature; the wind does not blow and the sun does not shine on demand.

The displacement of stable baseload power generation by intermittent renewables is forcing a change in how grid operators manage networks, and a key tool in achieving the change without relying on fossil fuels is energy storage. Energy storage can shift “wrong-time” energy to times of peak demand and provide the ancillary services required to reliably maintain the grid at the required frequency and voltage. In addition, energy storage can provide a cost-effective “non-wire” solution that alleviates constraints in an evolving electrical system by serving peak loads or absorbing peak production to avoid expensive upgrades of wires and transformers to accommodate just a handful of extreme peaks per year.

Figure 1: Energy storage projects by year. Data: US Department of Energy

Figure 1: Energy storage projects by year. Data: US Department of Energy

LAES process. Image: Highview Power Storage

LAES process. Image: Highview Power Storage

The past five years have seen a rapid increase in the deployment of energy storage systems (Figure 1) and have been dominated by the lithium-ion technology well known from smart phones and electric vehicles. Battery technologies are fast-reacting, and along with technologies such as flywheels and supercapacitors, are well suited to resolving the supply/demand mismatch on the shorter timescales measured in minutes to a couple of hours. However, despite a recent fall in costs, they are expensive for large-scale, long-duration applications.

Large-scale energy storage has been an integral part of the grid for many decades in the form of pumped hydro, used to absorb excess energy by pumping water uphill to reduce the inefficient and costly cycling of large thermal plants, such as nuclear facilities. However, these incumbent systems are inherently constrained by geography.

The value of energy storage is greatly dependent on location. As the need becomes more acute, an energy storage solution is required that can provide large-scale storage where it is most needed. Cryogenics holds an answer.

Highview Power Storage has spent the past twelve years developing a technology to store energy in bulk using the medium of liquid air. Liquid Air Energy Storage (LAES) is based on components proven through millions of operating hours over decades of use in the industrial gas, oil and gas and power generation sectors, and provides a low-cost solution for bulk-scale storage that is ready to be deployed today and can be freely located where the value is greatest.

The LAES system charges using an industrial air liquefier based on the Claude cycle used on the front end of existing air separation units. Electricity from the grid is used to produce liquid air at close to ambient pressure, thus avoiding the excessive cost of containing a high-pressure fluid in significant volume.

The low-pressure storage of liquid air in tanks sets LAES apart from Compressed Air Energy Storage (CAES), where high-pressure air must be contained at approximately 200 atmospheres. In order to achieve this cost-effectively, the few CAES systems in existence make use of underground caverns, which introduces a geographical constraint in common with pumped hydro systems and severely limits the viable market.

Energy is recovered from the LAES system by extracting the liquid air from the tanks, pumping it to high pressure and reintroducing heat to create a high-pressure gas. This gas is expanded through a multi-stage turbo expander to drive a generator and export electrical energy to the grid. At the right scale, the process can recover up to 60 percent of the electrical energy used to charge the system, or more when utilizing waste heat and cold.

In a standalone configuration, gas is warmed with heat recovered from the compressors of the air liquefiers and stored in hot water tanks. Similarly, the cold released from the phase change of liquid air to a gas during discharge is recovered in a dry air loop and stored in tanks containing a packed bed of gravel-a cheap and abundant medium for cold storage-and later used to assist in the cooling of liquid air during charging. Alternatively, as a thermodynamic system, waste sources of heat and cold-for example, power plant exhaust and LNG regasification-can be used to extract even more energy from the liquid air in the system.

The components comprising a LAES system are readily available from a well-established supply chain serving existing industries. The turbines, compressors, tanks, pumps and heat exchangers that make up a LAES system benefit from economies of scale and exhibit long lifetimes, meaning that Liquid Air Energy Storage demonstrates very low lifetime cost-indeed at the right scale, the lowest for a freely locatable energy storage system.

And as LAES is based on well-understood components and processes, the barriers to commercialization are few. To date, Highview has deployed LAES at pilot scale, operating a 350 kW / 2.5 MWh system since 2011, and is currently commissioning a 5 MW / 15 MWh project to demonstrate the operation of the LAES power recovery system as a commercial scale grid-connected asset.

Highview has developed designs for commercial systems from 10 MW to 200 MW, and leveraging its partners’ wealth of expertise in the construction of air separation units, LNG facilities and power plants, it aims to initiate the first commercial projects in the next 12 months.