FEATURE ENERGY RECOVERY SYSTEMS RECOVERING ENERGY FROM COMPRESSED AIR


The course to achieving optimised energy efficiency and increased cost savings is an ongoing effort requiring continual review. Here James Houseman, Product Specialist in Atlas Copco’s Compressor Technique Services division discusses how energy recovery is playing its part in achieving these goals


water for use in other applications. Most compressor installations feature a


two-stage design and come standard with an intercooler between the first and second stage, and an after-cooler, which is located after the second stage. The intercooler and after-cooler can either be air-cooled or water-cooled. As the air passes through either of these types of after-coolers, the heat is extracted and can be redirected to other uses. Air-cooled systems are more common in


F


romlow-energy light bulbs to automated heating, manufacturers are


acting in many ways to reduce their energy usage in the face of rising prices. But even after obvious steps have been taken, the challenge to find further savings remains. According to a survey this year by the


manufacturers’ organisation EEF, energy affordability is one of the most critical issues facing manufacturing today and is perceived as the biggest threat to growth. The survey of nearly 200 British industry


leaders confirmed that “much of the low- hanging fruit has already been plucked” and with energy prices remaining among the highest of all major manufacturing countries, it’s no surprise that more than half of the respondents identify the need to continue reducing usage of gas and electricity as the biggest energy challenge of the next two years. Addressing this challenge will require greater attention to less visible areas of energy usage such as compressed air, which, on average accounts for around 12% of total industrial electricity consumption in Europe. In some cases, the figure is as high as 40%. The potential is clear but fulfilling that potential will also require a greater willingness to consider innovative approaches to energy conservation such as energy recovery, a concept which enables waste heat from the compressor to be recycled as hot water. This can in some cases be used within certain manufacturing processes, or redirected into the domestic heating or hot water system of the building. Adoption of compressed air energy


recovery is building gradually in the UK and there are a number of examples that demonstrate the savings that can be obtained. However, the UK lags behind continental Europe in embracing energy recovery, perhaps


16 AUTUMN 2015 | ENERGY MANAGEMENT


as a result of poor awareness or doubts about the deliverable results. So let us deal with those two issues upfront. Firstly, what do we mean by compressed air


energy recovery? When air is compressed, the energy transforms from electrical energy to potential energy in the form of compressed air and thermal energy, or heat. After compression, air typically reaches a high temperature and before it can be distributed into the piping system and delivered to the end process, the heat has to be extracted. Unless heat is recovered, it becomes waste.


Figure 1: Autofill application


small and medium-sized compressors, and heat recovery is often limited to the cool seasons, when the recovered energy can be used as heated air and is rerouted to nearby interior spaces for supplemental heating. When discussing the potential of energy


recovery, it is vital to balance the perceived complexity of installation, which could be another reason for relatively low adoption, with the quantifiable return-on-investment that can be achieved. To illustrate the substantial financial


“Most compressor installations feature a


MANAGING HEAT LOSS As much as 94% of the electrical energy used by an industrial air compressor is converted into heat and lost through radiation in the compression process. The remaining 6% is unrecoverable due to the small amount of heat that remains in the compressed air or is dissipated into ambient air. Therefore, a properly designed heat recovery unit can recover from 50% to 94% of this available thermal energy as low-grade heat. This can be used to heat water, up to 90°C. The heat energy produced by compressing air


two-stage design and come standard with an intercooler between the first and second stage, and an after-cooler, which is located after the second stage”


savings and environmental contribution a ‘carbon zero’ compressor can make, a typical application scenario involves a 132kW compressor with a heat equivalent of 132kJ/s, running for 8,000 hours per year. In addition to the main duty of supplying compressed air, its hot water output can be utilised as pre-heated feed water to an oil-


fired boiler, thus saving boiler fuel. These calculations are based on a boiler efficiency of 90%, the calorific value of heating oil at 41,200kJ/l, and a fuel cost of 45p/litre:


• Heating oil saved = 132 x 3600/(41,200 x 0.9) = 13 litres per hour


• Heating oil saved over the course of a year = 104,000 litres


•Cost of fuel = £0.45/l x 13 l/h = £5.85 per hour • Yearly savings = £5.85 x 8000 hours per year = £46,800 per year


can be recovered and reused in other manufacturing processes. To understand how energy recovery works, it is important to remember the high temperature of the compressed air that is generated. To lower the temperature of the air, the heat is removed via either oil, water or air, depending on the compressor type. Oil injected screw compressors and oil-free water-cooled screw compressors offer the best potential for recovering heat energy from their cooling systems, and capturing it in the form of heated


† Energy conversion factor taken from Carbon Trust’s Food Industry Factsheet CTL018: Energy and Carbon Conversions


* Based on the carbon emissions of 3 tonnes of CO2 per year for an average car with average annual mileage


• Equivalent CO2 = 104,000 x 2.518kg CO2 per litre† = 261,872 kg or 262 tonnes of CO2 This carbon saving is approximately equivalent to taking 87 average cars off the road!*


Based on these calculations, the case for greater take-up of energy recovery technology is hard to ignore.


Atlas Copco www.atlascopco.co.uk 0845 601 0001


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