Innovatium’s Simon Branch looks at how the company’s PRISMA Liquid Air Battery can increase the efficiency of compressed air systems in the cement sector, while reducing costs and CO2 emissions.
Compressed air is an essential service for cement producers around the world. Indeed, around 10% of all industrial liabilities are linked to it. However, the variable demands placed on compressors and storage systems often lead to considerable inefficiency, alongside high costs and CO2 emissions. With energy costs likely to rise even further in the future - as well as high CO2 permit prices in the EU - the onus is now on cement producers to operate compressed air systems as efficiently as possible.
The PRISMA concept
Over the past four years, Innovatium and its partners have developed the PRISMA system for compressed air, at the heart of which is the Liquid Air Battery. The system takes compressed air from the plant’s existing system and converts it into liquid form. The liquified air is then stored in a tank at -158°C under a pressure of around 16bar(g). Under such conditions, the air is 700 times more dense than ambient air and around 150 times more dense than the compressed air systems commonly used by the cement industry.
The conversion takes place over several steps, starting with compressed air entering the unit at about 7bar. This air, already dry and clean, is further cleaned in an initial step in order to remove any remaining moisture and dust. The air then passes to a tank, in which it begins the transition from gas to liquid.
The first part of the tank uses a packed bed, an approach often used in thermal storage. This comprises a number of columns that contain inorganic material. More commonly, packed beds consist of gravel media. However, the PRISMA process uses an adapted material that is protected by a forthcoming patent. At the top of the packed bed the air is at ambient temperature. At the bottom it is at -158°C. However, it is still a gas at this point.
The conversion from cold gaseous air to liquid air takes place in a second column at the bottom of the tank. Here, a solid phase-change material removes the latent heat from the air by converting it to a liquid. As it does this, the air reaches a density 150 times greater than normal compressed air storage systems.
Benefits of the PRISMA system
Compressing air to 16bar(g) takes more energy than compressing it to 7bar(g). However, the energy used to do this is paid back in spades by the PRISMA system in use. Most of the benefits derive from the tremendous flexibility provided by having the capacity to store a large mass of air.
First and foremost, it is possible to run the plant’s air compressors at the most efficient load for a much greater proportion of the time, doing away with the exceedingly inefficient practice of partly-loaded running. This is the main source of wasted energy in conventional compressed air systems.
With the PRISMA system, the excess air is diverted to the Liquid Air Battery. Instead of slowing down the compressors, they can continue to work at their most efficient rate, building up a huge store of compressed air for future deployment. The need to work at partial compressor load is virtually eliminated. This is analagous to how the battery on a hybrid car operates. The cement plant can run at the efficiency of a clear highway, even if the traffic is stop-start.
The PRISMA system also reduces costs, by allowing the cement plant to purchase the energy used for compressed air at times when the cost is low. The plant could charge the system overnight and turn off the compressors during the day. This applies not just to conventionally-generated power, but to any local or captive renewable power that may generate energy at the ‘wrong’ time of the day. Often it is excess solar or wind capacity that leads to such ‘grid events.’ The PRISMA system is an overspill to store this energy, lowering CO2 emissions from the network as a whole.
At other times, the ability to supply the plant directly from the PRISMA system could also help, as industrial users are paid by networks to reduce their consumption during times of peak demand.
Industrial PRISMA system at Cauldon
In late 2018 Innovatium and its partners were seeking an industrial end user to host the first demonstrator PRISMA system. Through the UK Department for Business, Energy & Industrial Strategy and The Carbon Trust, Innovatium met with representatives from Aggregate Industries (AI), the UK subsidiary of Holcim. AI indicated that the PRISMA concept would solve a number of issues across its operations and identified its Cauldon cement plant in Staffordshire, England, as a test bed.
Over the next three years, Innovatium and researchers from Birmingham University’s Engineering Department built and extensively tested a lab-scale system. Designs for an industrial-scale PRISMA demonstrator system capable of storing 200kg of air were then drawn up by the leading cryogenic vessel manufacturer Wessington Cryogenics, in consultation with Innovatium, Birmingham University and AI.
After some supply-related delays due to the Covid-19 pandemic, the system was ready for installation in early 2022. The team entered the plant in early March 2022 and completed the work in 12 weeks. The PRISMA system is connected to the plant’s pre-existing compressed air system by a single air line. Aside from this, the only utility it needs is electrical power. The system is co-located with the compressors, a concept that Innovatium intends to apply in future projects.
PRISMA in use
After connection to the rest of the plant, the PRISMA system was extensively tested and commissioned by Innovatium. As intended, the day-to-day operation of the Cauldon plant’s main business - making cement - is unchanged. The system simply offers the capacity for the plant to use its compressed air in a much more efficient manner than in the past. The flow of air between the systems is controlled so that the PRISMA takes over when needed. It continuously monitors the pressure in the main system, allowing the Liquid Air Battery to store and release compressed air as required. The system operates in the background, with limited need for human oversight. The PRISMA demonstrator at the Cauldon site has shown that, when fully deployed, the energy demand for compressed air and the associated CO2 emissions could fall by up to 25%.
On top of this, it has no moving parts aside from valves and motors, so maintenance is low and availability is high. The packed bed and phase-change materials are both chemically stable, with negligable degradation over time, ensuring a long service life. Early indications indicate very reliable operation at the Cauldon plant, despite tough operating conditions. This indicates that the system will be applicable across a wide range of industries.
PRISMA Roll-out
The immediate next step in the development of this technology is to install second-generation Liquid Air Batteries at the Cauldon site to further boost its capacity for compressed air storage. There is 1MW of compression across the site, which will require 4-6 Liquid Air Batteries, enabling a total storage capacity of 8 - 12t of air. It is thought that larger cement plants could use up to 10. Beyond this, there are many other industrial plants that can benefit from PRISMA systems, both within the cement sector and in many others that rely on compressed air.
Instant pay-back
Aware that increasingly few cement plants have the capital available to purchase such equipment up front, Innovatium plans to adopt a leasing model in which it underwrites the savings. This minimises the risks to the user as much as possible at a time when cash is tight. This means that the Cauldon plant would see a financial payback on the first day
of operation.
Innovatium and its partners anticipate that demand for the PRISMA system will rise rapidly among cement producers and other industrial users. It is already looking to set up a production line to meet future demand, which could reach into hundreds of units per year by 2030.
