FAQ Compressed air heating systems

A compressed air heating system (or CAHS in short) is a compact small power plant that is delivered as a finished module in a sound-insulated housing (similar to an air compressor, CHP or heating system) and simultaneously provides compressed air (“power engine, power plant”) and heat (“heating”).

CAHS can also be used for special applications and particularly high compressed air qualities. Here, two-stage dry-running compressors are used, as they are also used in electric compressors. By combining high compression and exhaust temperatures, hot water temperatures of up to 108°C can be permanently decoupled, which can then be used for thermal processes.

A CAHS is economical if it has relatively long running times at a stretch. It should be dimensioned so that it is switched on and off as rarely as possible. The CAHS is thus designed for the compressed air demand in the base load and should achieve running times of e.g. 120 operating hours per week (Monday to Friday) or 168 operating hours per week (Monday to Sunday). The heat generated at the same time should be used for heating purposes (process heat, heating heat).

First and foremost, the economic advantage is decisive because the purchase of a CAHS promises a very attractive return. The payback period is between 2 and 4 years, after which a profit is made because the total costs for compressed air and heat generation are significantly reduced by operating the CAHS.

A short answer is, by significantly increasing energy efficiency. But in detail: Firstly, the temperature level is higher than with a classic CHP and also than with an air compressor, which is important and good for industrial applications, and secondly, the thermal efficiency is almost 90% (CHP at max. 50%) So a CAHS achieves a similarly high efficiency as a modern heating system. At the same time, however, compressed air is still generated. The expensive electrical energy otherwise required for this is minimised to almost zero. This means, as a next short formula: The costs for heat remain the same and the previously high electricity costs for compressed air generation drop to (almost) zero.

Compared to a classic combined heat and power plant (CHP), the CAHS does not generate electricity, but compressed air. Here, a natural gas engine is connected to a directly driven compressed air compressor. Through the air’s compression process, the gas engine’s complete mechanical power is converted into heat. This thermal energy represents the first stage of heat utilisation. Furthermore, the heat from the engine cooling and from the exhaust gas is fully utilised. Compared to the separate generation of compressed air and heat, CO2 emissions are reduced by approx. 42 percent.

Compared to separate generation (compressed air and heat), up to 42% CO2 can be saved, depending on the degree of utilisation (especially of the heat). Assuming a running time of 6,000 operating hours per year (approx. 120 operating hours per week) and a degree of utilisation of 100% for compressed air and heat, this means approx. 75 tonnes of CO2 per year for the smallest unit, the Föhn, and approx. 900 tonnes of CO2 per year for the largest unit, the Tornado.

The standard design is natural gas with a calorific value of 10 kWh/m³. The flow pressure should be greater than 20 mbar and less than 100 mbar. The following fuels are also possible, but require appropriate design and project planning:

• Special gases such as propane, biogases, wood gas and digester gases from a sewage treatment plant.; 
• Bio-methane;
• Liquefied petroleum gas, e.g. LNG, LPG;
• hydrogen.

Fuel supply connection, usually to the gas supply (yellow line), please never forget the gas meter! The Compressed airline: A cyclone separator is already installed at the CAHS compressed air outlet or is supplied separately. Then there is a decoupling of vibrations from the rigidly laid compressed airline, either via a compensator or a hydraulic hose, to the fixed compressed airline. In principle, a CAHS has the same compressed air connection as an electrically operated stationary compressor. Connection for heating or heat extraction via the Heating flow and Heating return connectors. Exhaust gas: The (mostly) supplied silencer is installed on-site at the exhaust gas outlet. Then the actual exhaust gas pipe is installed. Condensate: First, condensate is discharged (condensate from the compressed air) below the cyclone separator via an electronic float trap (usually Bekomat). The condensate from the exhaust gas stream is drained from the hot exhaust gas into the (usually) supplied condensate collection box. Depending on the installation conditions and sound requirements, appropriate supply and exhaust air facilities (possibly with sound insulation options) must be installed to allow the combustion air from the engine, cooling air from the compressed air aftercooler, and the intake air from the compressor, to flow in, or the heated cooling air to flow out. Electricity: In general, each CAHS has two electric pumps (electronically controlled and variable speed) and an electric control cabinet with integrated control, and an electrically driven fan. Consequently, a CAHS requires an electrical connection of varying sizes (between 2 and 15 kW), depending on the size of the machine.

To have the possibility of a tax refund on the energy tax to be paid for the natural gas used, the CAHS must be registered with the main customs office and later proof of the high degree of utilization achieved (more than 60%) must be provided using appropriate measurement technology. altAIRnative advises on both the application and the necessary measurement equipment for the plant and supplies the appropriate measurement equipment.

In principle, none. Ultimately, the following is recommended for your energy management and a possible tax refund:

• Gas measurement (calibratable)
• Heat quantity measurement (including emergency coolers)
• Compressed air quantity measurement
• Connection to higher-level compressor control (if available)

Compared to standard compressors in the 20 to 300 kW shaft power range, CAHS’s are energy power plants that are not built in large numbers for a broad compressed air market. CAHS’s often undergo individual adjustments for the corresponding customer situation, e.g. regarding the provision of heat. Besides the low quantities (Economy of scale) and the customized adaptations, it is however the gas engine and the more extensive control system, which makes a CAHS more expensive compared to an electric compressor. Here, it makes more sense to compare the CAHS with a CHP. But again, the comparison is flawed because the BHKW does not have the cost-intensive compressor of an electric compressor. In this respect, the investment costs for a CAHS adds up as follows: BHKW price minus generator plus compressor stage plus additional heat exchangers.

The standard design of all CAHS‘s is 90°C flow and 70°C return temperature. With the heat exchangers designed with reserve, flow temperatures as high as 95°C can be realized. For the series of oil-free compressing CAHS’s namely Orkan OF, Hurrikan OF, Taifun OF, and Tornado OF, it is also possible to run flow temperatures of up to 108°C. Customized solutions, e.g.

• first stroke feed water increase of 15-30°C
• heat extraction at two temperature levels, e.g. 105/85 and 80/60 °C
• Steam generation from exhaust gas heat
• Thermal oil heating
• Additional utilization of the condensation energy from the exhaust gas stream (utilization of calorific value)
• Generation of drying air by exhaust gas/air heat exchanger
• etc.

are also possible.

If no heating or processing heat is required throughout the year, the provision of cooling in the summer and transitional months via absorption chillers can be a sensible solution. AltAIRnative has absorption chillers and re-cooling units specially designed for its series.

If there is a high demand for heat and nitrogen in the customer’s company, but the demand for compressed air is comparatively low and approaches zero at the weekend, the generation of nitrogen from the compressed air with appropriate nitrogen generators is an option. Since compressed air becomes a by-product of the CAHS, decentralised nitrogen generation can be a lucrative alternative to the conventional supply of liquid nitrogen.

If there is a high demand for compressed air in the customer’s company, but there are no heat sinks (even in the winter months for heating purposes), the use of downstream ORC modules (Organic Rankine Cycle) for power generation can be an interesting variant of heat utilisation from an economic point of view (ROI of approx. 5 years).

In general, a CAHS operates with compressed air. This means that when a CAHS goes into operation according to the correct design, it operates accordingly in its working range (60-100 %) and thus covers the baseload demand for compressed air. If the corresponding heat cannot always or simultaneously be taken from the compressed air production, the surplus heat is transferred to an emergency cooler. The installation of an emergency cooler is not bad or uneconomical per se. It rather leads to a safer operation of the CAHS.