Pnu tricks: compressed air energy storage

PEi visits the headquarters of Energetix, a UK firm specializing in low cost energy efficiency products, to find out more about Pnu Power à‚— a pneumatic battery that is set to challenge lead acid batteries as the energy storage medium of choice for uninterruptible power supplies for high ambient temperatures.

Tim Probert, Deputy Editor

Adrian Hutchings thinks his company’s technology is old hat. He is happy to tell you that his new products do not utilize the latest cutting-edge, state-of-the-art technology. That they are more Ford than Ferrari. In fact, he is rather proud of it.

Energetix, a UK manufacturer of energy efficiency products, specializes in the ingenious modification of existing technologies to create low-cost energy efficiency products. Pnu Power, pronounced ‘new power’, is one such product. Pnu Power is a pneumatic battery that provides an uninterruptible power supply (UPS) solution and back-up power using compressed air.

The Pnu Power energy storage system represents an ingenious modification of an existing technology
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Energetix engineers have taken the standard scroll compressor, first patented in 1905, and operated it in reverse à‚— as an expander. On detecting a mains outage, the system opens a fast-acting solenoid valve to allow air from the cylinders to pressurize downstream air regulators. The air then passes into the scroll expander, the heart of the Pnu Power system, where the expansion process turns a generator, which provides power to the DC bus that can then be converted for any voltage requirements.

The philosophy of Energetix is not to spend vast fortunes developing unproven technology, but to take a low-risk approach using mass-produced, easily available components. Not only does this approach bring down costs à‚— being able to negotiate between different manufacturers of the same components is a huge plus à‚— but using well-proven technology also leads to a more robust final product, says Hutchings, Energetix’s chief executive.

Intellectual property

Given the lack of cutting edge technology in Pnu Power, one could be forgiven for believing, at least at first glance, that anyone could come along and offer the same product. Not so, says Hutchings. He said: “Whilst I talk about robust engineering rather than esoteric science, we do a lot of patenting and building intellectual property as well. We have granted system patents, we have minefield patents around that, but the most important strand is the development of intellectual property derived from our own knowledge of the components.

“There are millions of scroll compressors out there, but no one else has ever developed or designed a scroll expander before. But it’s not as simple as just running the compressor in reverse.

“You can just roll a scroll compressor in reverse, and we do that with our first generation product, but with our next generation products, because we’ve learned so much about the process à‚— upping efficiency, reducing component counts and so on à‚— we are actually building our own intellectual property and patents into the component design. Manufacturers can now produce scrolls to our own designs.”

Hutchings had looked at flywheels, something he knew a great deal about from his time as managing director of International Energy Systems à‚— a subsidy of British Nuclear Fuels Limited (BNFL), as an energy storage solution for the UPS and distributed generation markets. It was not until Energetix developed Genlec, a domestic boiler that first used the reverse scroll compressor concept to generate both heat and power from natural gas, that he realized that compressed air could provide an alternative to valve regulated lead acid batteries (VRLA).

Energetix has targeted three main areas for Pnu Power à‚— telecommunications firms that require battery-free bridging power for a short duration (1-8 hours), while at the same time providing cool air à‚— thus eliminating 90 per cent of generator start. Pnu Power can also provide standby power for utility circuit breaker and protection relay applications in the transmission and distribution industries. It also has a use for industrial firms that need a reliable and consistent UPS system in harsh environments.

Energetix sees great potential for Pnu Power in the southern United States. Indeed, it has set up a business à‚— Energetix Technologies Incorporated à‚— in Fort Lauderdale, Florida. Hurricane-prone Florida and other states in America’s deep south are an ideal markets for Pnu Power à‚— power outages are relatively frequent and despite only lasting for a second or two, they are a big hit on lead acid batteries.


Energetix says the reliability of Pnu Power is not compromised by temperatures between -20 à‚ºC and 40 à‚ºC irrespective of the load profile. Every degree above 25 à‚ºC halves a VRLA battery’s life, says Hutchings. So with a five-year battery in Florida at an ambient temperature of between 30 à‚ºC and 32 à‚ºC, a VRLA will barely last six months. Pnu Power is unaffected at such temperatures: using an ISO freight environmental testing chamber at the Capenhurst testing facility in the UK, Energetix was able to demonstrate Pnu Power operation at 40 à‚ºC and at 95 per cent relative humidity, replicating the most challenging conditions in Florida.

The ability to operate in high ambient temperatures eliminates the need for expensive environment control systems. The known level of stored energy, which can be monitored locally or remotely, will also provide confidence of operation and accurate run time remaining. For example, in South Africa, Eskom operates Pnu Power via a remote monitoring system.

Energetix has conducted 30 000 stop starts à‚— equivalent to 700 years of operations. Hutchings says that Pnu Power has ten times the lifecycle of a lead acid battery. Of course, Pnu Power has higher upfront costs than VRLAs, but Hutchings is confident that the advantages of compressed air outweigh any short-term factors. He said: “Pnu Power has ten times the life of a lead acid battery. You need licenses for lead acid batteries, plus with VRLA systems you need to change and dispose of the toxic batteries.”

Neil Bright, Energetix’ group operations directors, who joined the firm from Exide Technologies, one of the world’s largest producers of lead-acid batteries, says that Pnu Power eliminates the need for batteries. He says that Pnu Power is unique in being completely battery free.

The Pnu Power system is modular; it consists of the power conversion module (PCM), the energy store and a recharge compressor. Being modular, many PCMs can be connected to a common energy store to meet increased power requirements.

The unit is scaleable and additional duration can be provided simply by adding more compressed air cylinders. Typically, ten air cylinders would provide a 20 kW system with ten minutes of back-up power.

The light inertia of the scroll, combined with an ultra-fast response time of the electro-pneumatic regulator, enables Pnu Power to respond to step changes in power instantaneously by varying the control signal. As well as light inertia, the scroll has a very low component count, which improves reliability. The scrolls are also designed to be operate lubricate-free, which greatly reduces the need for maintenance à‚— it also means no exhaust filtering is required.

From standby mode, Pnu Power takes less than 300 milliseconds to get up to full power. The compressed air is stored at 300 bar, a similar level to a diving tank. The noise emissions from the system, with silencers, have been recorded at 70 decibels at a distance of one metre.


Pnu Power was initially launched in 2007 and after successful trials Energetix has recently bagged its first large order à‚— a $380 000 deal from US aerospace and defence firm P&E Automation à‚— to be installed at a site in the state of Utah. Pnu Power also has won orders from a telecommunications firm in Florida and Telecom Italia in Italy, plus South Africa’s state-owned utility Eskom. It is also used to back-up Energetix’ computers at Capenhurst!

Florida’s M/A-COM is testing a 5 kW Pnu Power system for a State Law Enforcement Radio System site in Brooksville
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In April, Energetix received an order to supply a 5 kW Pnu Power compressed air back-up energy system to M/A-COM Incorporated for a trial installation at a State Law Enforcement Radio System (SLERS) site in Brooksville, Florida, USA. M/A-COM has indicated that the performance of the Pnu Power product will be evaluated to assess its ability to provide a high-reliability DC power back-up system in an application that requires 99.999 per cent uptime and which is subject to the disruption of extreme lightning strikes.

A successful trial that validates the Pnu Power technology is likely to lead M/A-COM to consider Pnu Power for the expansion of the SLERS network. On 19 August, Energetix announced that it would supply a unit for National Grid’s 275 kV switchyard in Capenhurst, Cheshire. The trial system will evaluate the performance of Pnu Power in providing back-up power in a real-life application on relay and circuit breaker switching, which requires high levels of system integrity.


Pnu Power’s ability to generate cold air à‚— effectively the exhaust gas from the generation of electricity à‚— has a potentially very important use for data centres, which are energy intensive and generate a great deal of heat. Hutchings claims that up to 1 kWth of cooling is generated for every 1 kW of electrical power produced. To this end, Energetix is developing a 50 kW prototype for use in data centres. In the long term, Hutchings expects to sell a number of units “in the low hundreds” of Pnu Power in 2010 and then shift to what he calls “real volume” in 2011.

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