Turning heat into energy

09 July 2014

Heat is an inevitable by-product when producing compressed air. However, it is no longer inevitable that this heat must be wasted energy. Anyone looking to enhance efficiency can now use this heat and increase the efficiency of compressors to about 95% as a result. Mark Whitmore, general manager, BOGE Compressors, explains

Because almost the entire energy consumption from the supply net of a standard compressor is converted into heat there is a correspondingly high degree of energy available for heat recovery. Take for example an oil lubricated screw compressor: up to 94% of the electrical energy input to this equipment is available for heat recovery. This is especially efficient in plants where the compressor is within the vicinity of the area being heated, for example in a workshop where the compressor is stationed, or by the use of ducting where the compressor is in a more remote location.


Despite the powerful potential for enhanced efficiency, too few compressed air users are taking full advantage of heat recovery.


This may be because too few of them have realised the inefficiency of using compressed air as a utility. However, if they stop to consider the cost of compressed air they may be more eager to consider the highly attractive efficiency savings that can be made by adding heat recovery to the mix when it comes to generating energy. 

According to figures from The Carbon Trust, more than 90% of the electrical energy used by a typical air compressor is lost as heat. The ability to capture and reuse this energy for space heating, water heating or for other production duties therefore offers real benefits for businesses and is in step with the view being taken across industry that a broader, more holistic view must be taken when it comes to energy efficiency.

So, how do compressed air users take advantage of the potential to enhance efficiency using heat recovery? The best place to start is to look at the entire system and evaluate the parameters of each process; you can then identify the process that will make the best use of the heat produced by the compressor and thus make the maximum saving for the business. The choice for a plant that has a constant demand for heat throughout the year can be very straightforward; regular processes that take place in paint shops and electroplating operations need a constant supply of heat, as do many temperature controlled processes in the manufacture of chemicals and foodstuffs, and so there is strong potential here to save energy through heat recovery. And in a plant that does not require heat to carry out any process, the decision is also often a simple one: use the recovered energy to heat either hot water or the premises. At BOGE, we switch with the seasons. In winter, recovered heat is used for heating the premises; in summer, it is used to heat hot water. By using this combined process to optimise the efficiency of heat recovery, we save the business around 100,000kWh of heating power per year.

The next thing to do before investing in heat recovery is to calculate the potential energy and cost savings of implementing heat recovery by assessing the heat or hot water demand in areas, where practical, adjacent to the compressor installation. This assessment can then be compared to the average operating hours of the existing compressed air system, which will highlight the possible payback in terms of the immediate reductions in fuel, oil and gas costs.

As you might expect, the adoption of heat recovery for anyone purchasing a modern water-cooled compressor is a straightforward matter, as these new units are already well-prepared for easy integration of a heat recovery system into the cooling circuit. Retrofit solutions are available for plants running older compressor stations that are not prepared for heat recovery. For example, BOGE has recently introduced a new Duotherm system that is engineered to retrofit directly to existing screw compressors to provide an external heat recovery system — regardless of the year of construction, performance or manufacturer.  The BOGE Duotherm is engineered to allow more than 70% of the used energy to be recovered in the form of heat. The key element of this system is a plate heat exchanger, which can be connected to the oil system of the compressor and the service and process water network. It is engineered to increase the energy efficiency of the compressed air station and protect the environment at the same time.

With energy consumption typically the biggest target for engineering operators when looking to make savings and improve efficiency, designers and engineers have invested much time and effort in recent years into developing energy efficient compressed air solutions. This is a core mission at BOGE where the growing popularity of heat recovery has led to the recent extension of the Duotherm heat recovery product range, now available as an external, stand-alone model. As well as recovering up to 94% of the input energy used in compression in the form of heat, the Duotherm heat recovery system is compact, with a minimum space requirement.  



It is only possible to make significant, beneficial improvements when there is accurate information about the current performance of a compressed air application,


This is why a new standard has been introduced to set the requirements for conducting a compressed air system assessment.  Historically, the lack of any formal standard allowed suppliers with differing levels of expertise to conduct a compressed air audit, and with such a variance in the quality of each evaluation it was difficult to consider any analysis as a conclusive, comprehensive system assessment.  The globally accepted ISO 11011 has now created a framework for the compressed air system energy efficiency assessment and auditing process.

BOGE Compressors has been accredited to Compressed Air Energy Efficiency Assessment Standard ISO 11011. The achievement recognises BOGE’s commitment to providing comprehensive and detailed energy audits, which extend our customer support services and enable compressed air users to optimise their energy usage and identify actions that can reduce costs.