What is Energy Recovery in Compressor Installations?

Many installations that produce compressed air offer significant and frequently underutilized energy saving possibilities in the form of waste energy recovery. In large industries, energy costs can amount to 80% of the total cost of compressed air production. However, a lot of this energy can be recovered, which in turn will save you a lot of money.

What is waste energy recovery in compressor installations?

When air is compressed heat is formed.

Before the compressed air is distributed into the pipe system the heat energy is extracted, and becomes waste heat. For each compressed air installation, the issue of sufficient and reliable cooling capacity for the installation must be addressed. Cooling can take place either by means of the outdoor air or a cooling water system that uses municipal water, stream water or process water in an open or closed system.

A compressor central plant in a large industry that consumes 500 kW over 8,000 operating hours per year represents a yearly energy consumption of 4 million kWh. The possibilities for recovering substantial amounts of waste heat via hot air or hot water are real. As much as 94% of the energy supplied to the compressor can be recovered, for example, as 90°C hot water from oil-free screw compressors. This fact illustrates that saving measures quickly provide substantial return.

The return on investment for energy recovery is usually as short as 1–3 years. In addition, energy recovered by means of a closed cooling system enhances compressor operating conditions, reliability and service life due to an equalized temperature level and high cooling water quality, to name but a few advantages. Nordic countries are somewhat of a forerunner in this arena and energy recovery has been standard practice for quite some time now for compressor installations. Most medium to large compressors from the major suppliers are now adapted for fitting with standard equipment for waste heat recovery.

How do we calculate the recovery potential?

The laws of physics dictate that nearly all energy supplied to a compressor installation is converted into heat. The more energy that can be recovered and utilized in other processes, the higher the system's overall efficiency.

Recovered energy in kWh/year:

Savings per year: (€)

TR = Time of recovered energy demand (hrs/year)

K1 = Part of TR with loaded compressor (hrs/year)

K2 = Part of TR with off-loaded compressor (hrs/year)

Q1 = Available coolant power with loaded compressor (kW)

Q2 = Available coolant power with off-loaded compressor (kW)

ep = Energy price level (€/kWh)

η = Normal heat source efficiency (%)

In many cases the degree of heat recovery can exceed 90% if the energy gained by cooling the compressor installation can be utilized efficiently. The function of the cooling system, the distance to the point of consumption, and the degree and continuity of the heat requirement are all decisive factors. With large thermal flows, selling the recovered heat energy is a possibility that should not be ignored. The electrical energy supplier could be a potential customer, and investment, sub-order and delivery could readily be negotiated. An opportunity for savings also exists by coordinating energy recovery from several processes.

How to recover energy in an air-cooled system?

Energy recovery from compressed air installations does not always result in heat when it is required and oftentimes, not in sufficient quantities. The quantity of recovered energy will vary over time if the compressor has a variable load. In order for recovery to be feasible, a corresponding relatively stable heat energy demand is needed. Recovered waste heat energy is best utilized to supplement energy supplied to the system. That way the available energy is always utilized when the compressor is operating. Options for air-cooled compressors, which produce a high hot air flow rate at a relatively low temperature, are direct building heating or heat exchanging to a preheating battery. The heated cooling air is then distributed using a fan.

When buildings do not require additional heat, the hot air is evacuated into the atmosphere, either automatically by thermostat control or manually by controlling the air damper. A limiting factor is the distance between the compressors and the building that needs to be heated. This distance should be limited (preferably the distance between adjoining buildings). Furthermore, the possibility of recovery may be limited to the colder periods of the year. Airborne energy recovery is more common for small- and medium-sized compressors. Recovery of waste heat from compressor air cooling systems results in only small losses from the distribution and requires little investment.

How to recover energy in a water-cooled system?

The cooling water from a water-cooled compressor with a temperature up to 90° can supplement a hot water heating system. If the hot water is used instead for washing, cleaning or showering, a normal base load hot water boiler is still required. The energy recovered from the compressed air system forms a supplementary heat source that reduces the load on the boiler, saves heating fuel and could potentially result in the use of a smaller boiler.

Prerequisites for energy recovery from compressed air compressors differ in part depending on the type of compressor. Standard oil-free compressors are easy to modify for energy recovery. This type of compressor is ideal for integration in a hot water heating system since it provides the water temperature (90°C) required for efficient energy recovery. On oil-lubricated compressors, the oil, which takes part in the compression process, is a factor that limits the possibilities for high cooling water temperatures. In centrifugal compressors, the temperature levels are generally lower because of the lower pressure ratio per compression stage, thereby limiting the degree of recovery.

Waterborne waste energy recovery is best suited to compressors with electric motor power over 10 kW. Waterborne recovery of waste energy requires a more complex installation than airborne waste energy recovery. The basic equipment consists of fluid pumps, heat exchangers and regulation valves.

Heat can also be distributed to remote buildings using relatively small pipe diameters (40-80 mm) without significant heat losses using waterborne energy recovery. The high initial water temperature means that waste energy can be used to increase the temperature of the return water from a hot water boiler. Therefore, the normal heating source can be periodically switched off and be replaced by the compressor's waste heat recovery system.

Waste heat from compressors in the process industry can also be used to increase the temperature of the process. It is also possible to use air-cooled oil-lubricated screw compressors to apply water-borne waste energy recovery. This requires a heat exchanger in the oil circuit, and the system will provide water at lower temperatures (50° - 60°) than with oil-free compressors.

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