Compressors enable 100% recovery of input energy

The Carbon Zero innovation provides effective heat transfer (within specific operating conditions) so that a considerable proportion of electrical power input can be recovered in the form of hot water at 90degC. This provides significant benefits for industries employing hot water or steam within process operations such as food and beverage, pulp and paper, chemicals and power plants.

By using the compressor output hot water either directly or as pre-heated boiler feed water, the consumption of process heating fuel and its associated carbon dioxide emissions can be reduced or, in some cases, eliminated altogether.

The renowned, independent TUV institute (German Technical Monitoring Association) earlier in 2009 supervised tests on Atlas Copco's oil-free screw compressor series ZR 55-750, equipped with built-in energy recovery systems. The tests demonstrated that, under specific design conditions, 100 per cent of the electrical energy consumed could be recovered for heating water.

Using technology to save energy

Technologies incorporated within the ZR 55-750 compressors to enable 100 per cent energy recovery include highly efficient heat exchangers, high-efficiency components, an optimised oil-free compressor element design, and the use of variable-speed drives. [Note: because of the thermodynamics, none of the electrical energy is converted to stored energy in the compressed air, provided the compressed air is at the same temperature as the input air.]

Stephan Kuhn, President of Atlas Copco's Compressor Technique business area, comments: "We are very pleased to offer our customers a compressed air solution that can recover 100 per cent of the input energy. Today our customers face stringent targets to reduce carbon dioxide emissions. The Carbon Zero compressor will have a significant impact towards preserving the environment and also on our customers' bottom lines. Energy efficiency has been at the core of our product development for a long time. The global need to reduce CO2 emissions confirms we have been, and are, on the right track."

Energy saving in compressed air systems is important, since energy consumption typically represents over 80 per cent of a compressor's lifecycle cost. Compressed air systems, on average, account for approximately 10 per cent of industrial electrical power consumption; this energy can represent as much as 40 per cent of a plant's electricity bill.

How the carbon zero compressor works

Atlas Copco's ZR 55-750 compressors (rated form 55 to 750kW) are two-stage, oil-free, water-cooled units. There is an intercooler between the first and second compressor stages, plus an aftercooler and an oil cooler for the gearbox oil. In addition, both of the compressor stages have water jackets that also extract heat from the air as it is compressed.

Because the ambient air input to the compressor contains moisture, this is condensed in the compression and cooling process, releasing latent heat (or 'condensation' heat). This energy, transferred to the cooling water, results in the compressor generating both compressed air and hot water.

Tests on a ZR 55 compressor, using 121 litres/s of ambient air at 40degC and 70% RH, consumed 63.75kW of electrical energy. However, energy recovery (in the form of a flow of hot water) was 65.57kW, made up of 57.67kW of energy recovery without the latent heat and 7.9kW due to the latent heat. Atlas Copco says that energy recovery at other ambient conditions will always be at least 90 per cent (from the energy recovery excluding latent heat).

For a 10bar version of the ZR 55 with a free air delivery of 121 l/s, the nominal flow of hot water is 0.24 l/s. Not only could this save 165 tonnes of CO2 per year, but it could also save EUR37,400 in electricity and EUR38,829 in heating oil costs every year (assuming the compressor runs for 8500 hours per year, and making other assumptions about energy costs, boiler efficiency, etc).

Follow the link for more information about the Atlas Copco Carbon Zero compressors.