LAEN, which was an outcome of the report, will serve as the global hub where new ideas are demonstrated and shared, and promote liquid air as a potential energy solution among researchers, technology developers, manufacturers, energy producers and consumers, and government.

Air can be turned into a liquid by cooling it to around -196 °C using standard industrial equipment; 700 liters of ambient air becomes about 1 liter of liquid air, which can then be stored in an unpressurised insulated vessel.

The expansion of the liquid air when heat is reintroduced can be used to drive a piston engine or turbine to do useful work. The main potential applications are in electricity storage, transport and the recovery of waste heat. Since the boiling point of liquid air is far below ambient temperatures, the environment can provide all the heat needed to make liquid air boil.

The low boiling point also means the expansion process can be boosted by the addition of low-grade waste heat (up to +100° C), which other technologies would find difficult to exploit and which significantly improves the energy return. Sources of low-grade waste heat include power stations to factories to vehicle engines.

Despite their considerable industrial heritage, cryogenic liquids are a relative newcomer to the debate around energy vectors, as we explore in the papers that make up the Full Report. But it is this application of established technology to new challenges that makes liquid air such an attractive proposition, especially to a mature industrial nation such as the UK.

...More immediately, liquid air can prove its worth in specific sectors, including providing the cooling in refrigerated lorries, serving as the primary fuel for vehicles where zero-emissions are critical such as warehouses or mines, and exploiting sources of waste heat or coolth found in industrial processes or LNG terminals.

—Dr. Jonathan Radcliffe and Prof. Richard A. Williams, University of Birmingham

Liquid Air in Transportation Applications

Ricardo, which has been researching a split-cycle engine with cryogenic injection (earlier post), wrote the report chapter focusing on the potential of liquid air for transport applications.

Liquid air offers significant potential benefits as a future energy vector, both for use in light duty propulsion and as an enabler for other promising low carbon powertrain innovations, particularly waste heat harvesting. It is clearly worthy of further research and development effort to create a better understanding of its potential alongside more widely recognized potential future low carbon technologies such as advanced battery systems and hydrogen.

—Professor Neville Jackson, Ricardo chief technology and innovation officer

In its chapter, Ricardo considered a number of potential roles for liquid air considered in which the air is used as a heat sink and then as a working fluid within a heat engine. The heat engine can be deployed as either the prime mover or in a supporting role to recover waste heat from a conventional engine or fuel cell. In this secondary role, the liquid air device can either be used to produce shaft power to reduce the load on the primary engine, or to power auxiliary functions such as refrigeration.

Among the transportation uses of liquid air outlined in the report are:

• Prime mover. A cryogenic engine such as the Dearman piston engine produces zero emissions at the point of use; has low greenhouse gas emissions provided the liquid air or nitrogen is produced from low carbon electricity; has energy and power density on a level with battery electric technology; and has the potential for rapid refueling. It potentially is attractive for use in small cars and vans for short range urban use, scooters, short range marine craft, forklift trucks and mining equipment.

• Heat hybrid. A cryogenic engine such as the Dearman engine could also be used as a heat hybrid in combination with an internal combustion engine or hydrogen fuel cell to convert waste heat into additional shaft power at high levels of efficiency, reducing both fuel consumption and emissions. This approach would be viable in passenger ferries, commuter trains, heavy duty trucks and urban buses, and could also deliver ‘free’ cooling for passengers or goods.

• High efficiency internal combustion engine. Heat recovery could also be achieved using the Ricardo split cycle engine, a novel internal combustion engine design that incorporates liquid nitrogen to capture exhaust heat and increase fuel efficiency. Detailed modeling of this approach undertaken through the Technology Strategy Board-funded “CoolR” project has suggested that efficiencies of more than 60% are possible, compared to around 40% for current diesel engines.

  Nitrogen could be supplied from a modest-sized onboard tank or an onboard liquefier driven by the engine and boosted by regenerative braking. This approach would be suitable for heavy duty trucks and container ships, and potentially rail locomotives, other commercial vehicles and even larger passenger cars.

• Refrigerated food transport. Some food delivery vehicles already use liquid nitrogen as a heat sink to provide refrigeration, which cuts noise, complexity and carbon dioxide emissions substantially compared to conventional diesel- powered refrigeration. However, current systems fail to capture any additional shaft power from the nitrogen evaporation process.

  The report calculates that a vehicle food refrigeration system using liquid nitrogen or liquid air to provide both additional shaft power and cooling would cut emissions from 47 tonnes per truck per year (diesel refrigeration) to 10 tonnes, a reduction of almost 80% on the basis of current grid average electricity. The same approach could also provide refrigeration or air conditioning for passenger ferries, cruise ships, freight trains and buses, with greatest benefits in hot climates.

The Dearman engine. The Dearman Engine Company Limited (DEC) is developing a novel, zero-emission piston engine that runs on liquid air (or liquid nitrogen); the exhaust is cold air. With Ricardo as a partner, DEC is tracking to deliver a test engine by end 2013 for testing and demonstration in Q1 2014.

The company says that it also intends to deliver a proof-of-concept vehicle by Summer 2014 to prepare for full application specific field trials and is working with MIRA ((Motor Industry Research Association) on this program.

The Dearman Engine operates by the vaporization and expansion of cryogenic fluids. Ambient or low grade waste heat is used as an energy source with the cryogen providing both the working fluid and heat sink. The Dearman Engine process involves the heat being introduced to the cryogenic fluid through direct contact heat exchange with a heat exchange fluid inside the engine.

Prior cryogenic expansion engine have worked on an open Rankine cycle akin to a traditional steam engine but operating across a different temperature range. The cryogenic fluid is pumped to operating pressure and vaporized through a heat exchanger, before expansion in the engine cylinder.

This approach has a number of drawbacks, DEC suggests, as the heat exchanger must be large to cope with the heat transfer rates and heavy to withstand the high pressure. Additionally, little heat transfer occurs in the expansion stage (near adiabatic expansion) reducing the work output.

Specific work available from an expansion over a variety of pressures for isothermal and adiabatic cases. Dashed lines indicate the specific work rom the expansion net of pumping work. Source: DEC. Click to enlarge.

The Dearman Engine instead uses a heat exchange fluid (HEF) to facilitate extremely rapid rates of heat transfer within the engine. This allows injection of the liquid cryogen directly into the engine cylinder whereupon heat transfer occurs via direct contact mixing with the HEF. The heat transfer on injection generates very rapid pressurization in the engine cylinder.

Direct contact heat transfer continues throughout the expansion stroke giving rise to a more efficient near-isothermal expansion. With the pressurization process taking place in the cylinder, the amount of pumping work required to reach a given peak cylinder pressure is reduced.

After each expansion cycle the heat exchange fluid is recovered from the exhaust and reheated to ambient temperature via a heat exchanger similar to a conventional radiator.

Policy recommendations. The report notes that policy support for early stage transport technologies such as liquid air is “somewhat insensitive to the potential of real disruptors and the needs of the small companies that typically develop them.” To counter this, the report recommends considering the following changes to transport technology policy:

1. Grant funding calls should offer appropriate opportunities for disruptive technologies, and make allowance in their structure for a less widespread level of understanding of those technologies; objectives should be set but the means should be technology-agnostic where possible.

2. New technologies should be supported by a process of pre-clearance, to establish their basic scientific feasibility. This pre-clearance should then be publicly available, so that fund assessors can quickly verify the unfamiliar technology’s credibility. The costs of pre-clearance should be grant funded.

3. A rigorous review should be undertaken periodically of existing visions for longer term CO₂ abatement, to quantify progress against targets and identify emerging roles for disruptors. In the context of liquid air or nitrogen, this would need to embrace not only its role as a main or supplementary fuel in some applications, but also its energy-chain interaction with electricity grid buffering and with bulk LNG evaporation.

4. Support mechanisms such as research and infrastructure grants should evolve to embrace the increasingly complex interaction of energy systems—for example, some of the liquid air vehicle-fuel systems described could involve vehicles, refrigeration, grid buffering, the industrial gas industry, and bulk LNG supply within a single concept.

5. A specific program should be developed to support the field trial and deployment of technologies that replace or reduce diesel use in refrigerated food transport, which would be equally open to batteries and hydrogen fuel cells.

Resources

• Liquid Air in the energy and transport systems: Opportunities for industry and innovation in the UK