The powerful case for compressed air energy recovery

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From low-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, says James Houseman, Product Specialist in Atlas Copco’s Compressor Technique Services division

The powerful case for compressed air energy recoveryAccording 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 per cent of total industrial electricity consumption in Europe — and in some cases the figure is as high as 40 per cent. 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 as a result of poor awareness or doubts about the deliverable results.

Thermal energy recovery

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.

As much as 94 per cent of the electrical energy used by an industrial air compressor is converted into heat and lost through radiation in the compression process. The remaining six per cent 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 per cent of this available thermal energy as low-grade heat, and this can be used to heat water up to 90degC.

Heat energy produced by compressing air can be recovered and reused in other manufacturing processes. A large number of industries can benefit from energy recovery, though industries with higher running hours and continuous duty applications, such as the use of process water, are able to reach a higher level of savings in a shorter amount of time. In other cases, the heat can be directed into the heating or hot water systems of the facility itself, which can reduce reliance on external sources of electricity and gas, for example.

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 water for use in other applications.

Air or water-cooled options

Most compressor installations feature a two-stage design and come standard with an intercooler between the first and second stage, then 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 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. This type of heat recovery installation offsets the energy required to perform the same function and can help facilities save thousands of pounds on annual heating bills.

Water-cooled systems operate through one of three main principles: open systems without circulating water, open systems with circulating water, and closed systems with circulating water. The most recommended system is the closed system with circulating water, whereby water continually circulates between the compressor and some form of external heat exchanger which then transfers the recovered heat to the intended process.

There are many benefits to a correctly implemented closed-water energy recovery system. Primarily, it requires little supervision and has low maintenance costs. Unlike an open-water system, where water is supplied by an external source such as a municipal water main, the closed-water system uses treated water. Therefore, there will be little to no mineral deposit build-up in the cooling media components, which enhances the compressor operating conditions, reliability and service life.

Once the compressed air has passed through a water-cooled system, the water now contains the energy that was first used to compress the air. This heated water can then be used throughout the facility to preheat process water, and so the energy that would normally be used in these processes is offset. Imagine your costs to heat process water disappearing. With energy recovery, the savings quickly add up, resulting in a short return on investment and ongoing cost savings from year to year.

Financial and sustainability benefits

There are numerous examples of how beneficial this technology can be from both financial and sustainability perspectives. One of these is Autofil, a Midlands-based manufacturer of textiles for the automotive industry. The company achieved average energy cost savings of £37,000 a year and reduced annual CO2 emissions by 260,000 tonnes by adopting Atlas Copco’s energy recovery technology. By recovering heat produced by the air compressor used in the manufacturing process, the company is able to raise the temperature of the process water it requires, which reduces overall energy usage.

This is made possible by the ER-S5 energy recovery unit, whereby hot oil from the compressor’s air end is diverted through a stainless steel plate heat exchanger. Heat from the oil is transferred to cool water on the opposite side of the exchanger, producing a continuous flow of hot water. Based on the compressor running for 8424 hours per year, the combination of the GA 250 and ER-S5 unit is recovering more than 1.4 million KWh of energy a year.

Meanwhile, in Northern Ireland the production plant of Greiner Packaging found a very different use for the excess heat from its compressor system, donating it to a neighbouring college as a means of reducing its £40,000 annual heating bill. Dungannon Integrated College, a secondary school with 500 pupils, receives the heat into its central heating system via underground pipework, not only reducing costs but saving 200 metric tons of CO2.

The opportunity to generate heat for the college is fully exploited by the design of Atlas Copco’s Z-range of oil-free screw compressors with energy recovery capability, whereby the total energy recovered as hot water can be over 80 per cent of the electrical input energy, depending on the site conditions.

Balancing complexity against return

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 savings and environmental contribution a ‘carbon zero’ compressor can make, a typical application scenario involves a 132kW compressor with a heat equivalent of 132 kJ/s, running for 8000 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 per cent, the calorific value of heating oil at 41,200 kJ/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
  • 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.

 

 

 
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