Most industries waste substantial amounts of heat in their manufacturing processes and in operating their buildings. Recovering at least a part of this waste heat is increasingly becoming a major focus of many comprehensive energy management programs. Heat recovery (HR) strategies that deliver sustained and growing benefits are relatively rare. This is less from a lack of technical choices, but more from the need for improved analysis and management.
As a general rule, investing time, effort, and money recovering heat makes no sense if it cannot be economically used. This obvious fact is all too often overlooked as teams become engrossed in evaluating different HR technologies and forget that using the heat will be the key to success.
All good HR assessments should begin with a comprehensive mapping of the site’s needs and costs for heating and cooling for both process and building functions. The variations of these thermal needs by both changes in production volumes and seasonality should be factored in. In effect, we first need to understand the potential “market” for recovered heat. Since waste heat can also be converted into electricity, the electrical needs and costs of the site also should be mapped.
In the next preparatory step, all of the potential waste heating and cooling sources on the site should be mapped in terms of capacity, again by seasonality and production loading. The visually intuitive Sankey diagrams are a commonly used tool to understand how energy uses, sources, and waste flow across the site. The picture of 70% wasted energy in compressors will often communicate the recovery potential better than the number alone.
|Peter Garforth heads a specialist consultancy based in Toledo, Ohio and Brussels, Belgium. He advises major companies, cities, communities, property developers and policy makers on developing competitive approaches that reduce the economic and environmental impact of energy use. Peter has long been interested in energy productivity as a profitable business opportunity and has a considerable track record establishing successful businesses and programs in the US, Canada, Western and Eastern Europe, Indonesia, India, Brazil and China. Peter is a published author, has been a traveling professor at the University of Indiana at Purdue, and is well connected in the energy productivity business sector and regulatory community around the world. He can be reached at firstname.lastname@example.org.
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This mapping of thermal use and waste frequently highlights that the uses and potential sources are inconveniently located relative to each other. If heat recovery is viewed as a point source serving a specific need, this results in one of two reactions. The first is to kill the project because it is simply too much hassle. The second is to analyze it as a stand-alone project with dedicated thermal distribution, which in the majority of cases will kill it economically.
Once these spatially awkward projects have been eliminated from the potential, we are usually left with a small list of projects that may well be worthwhile on a stand-alone basis. However, their total contribution may be so small relative to the site’s overall energy challenges, that they are not deemed worth the bother when compared to other priorities. This is doubly true if the heat recovery or reuse involves touching the core manufacturing process and may be judged as a risk to production quality.
At this point, the heat recovery program has been reduced to a handful of localized low-risk small projects. In parallel, the possibility of using recovered heat to make electricity using steam turbines or organic Rankine cycle (ORC) units may be evaluated. In the past, these usually made no sense economically; with inexpensive natural gas in the United States, reducing equipment costs, and rising electricity prices, this may be a more attractive option in the future.
So, is it possible to break this pattern and capture broader benefits from heat recovery? As with so much of good energy management, the key is integration. A site that embarks on a systematic integration of its heating and cooling systems will be able to share sources and uses across the entire factory. This in turn will allow inconveniently located waste heat sources to economically serve multiple end uses. Less efficient heating and cooling capacity can be shut down or mothballed as more efficient capacity is shared across the system. The petrochemical industry has understood this for decades; it is now time for other sectors to get on board.
Thermal integration also allows easier cascading of end uses. High-quality recovered heat can first be used where it has the most value — typically in manufacturing, absorption chilling, or electricity generation. The residual heat can be used for less demanding tasks such as space heating or domestic hot water.
The technology to support HR solutions continues to become more reliable and lower cost. Heat exchangers are available in a wide variety of types and materials to cope with difficult conditions. Absorption chillers, ORC units, and steam turbines that convert waste heat to electricity and cooling are substantially more efficient and lower cost than even a few years ago. Standardized low-cost heat recovery packages are available on air compressors, gas turbines, and CHP engines. We are on the edge of seeing electro-thermal materials finding a place in the market.
It is fair to say there's no shortage of available heat recovery tools waiting to be incorporated into a sensible integrated thermal plan for the entire site. An integrated approach demands longer-term infrastructure planning with the associated multi-year investment planning. Done right, this will not only create substantial energy cost benefits, it will improve system reliability and enhance operating flexibly.