The Two CHPs

August 1, 2016 By Carl Weinschenk

cogenerationCombined heat and power is a good deal: It is a fine way to drastically reduce waste and derive two uses from a single investment.

Combined heat and power (CHP, also known as cogeneration), is familiar to energy managers and building owners. The U.S. Energy Information Agency (EIA) last week posted a very interesting article that digs more deeply into various types of CHP systems and the industries that most often benefit from each.

The post says that there are two types of CHP: topping cycles and bottoming cycles. The difference is explained very well by The Office of Energy Efficiency and Renewable Energy (EERE). Topping cycle approaches use fuel first to generate electricity or mechanical power in a “prime mover” such as a gas turbine or reciprocating energy. Exhaust energy is captured and used for a secondary purpose, such as heating or cooling the facility.

Bottoming cycle systems – which the piece says also is known as “waste heat to power” – heat first is used “to provide thermal input to a furnace or other high temperature industrial process,” with a portion of the heat used for a secondary purpose. This, typically, is a waste heat boiler/steam turbine system, the post says.

EIA2

Source: U.S. Energy Information Administration (July 2016).

The EIA piece has a very interesting chart depicting the industries using CHP and, further, the breakdown between bottoming and topping cycle approaches. In general, bottoming cycle are use in a slightly wider number of industries (chemical; paper, primary metal; petroleum and coal products; food, nonmetallic mineral; crop production; wood product and miscellaneous).

However, topping cycle approaches produce far more energy where they are used. For instance, in the chemical sector, last year bottoming cycle CHP only produced about 1 GW of power, while topping cycle CHP produced almost 85 GW. Likewise, in the petroleum and coal products sector bottoming cycle CHP approaches generated far less than 1 GW, while topping cycle CHP generated more than 4 GW.

Organizations considering CHP should be aware of the differences between the two approaches. The EIA post refers to a study by the EERE and suggests that bottoming cycle CHP has the most upside:

According to the study, the greatest potential for expanding bottoming-cycle CHP is in energy-intensive industries, such as iron and steel, glass, and cement. These industries have high-temperature waste streams that can provide the input to generate electricity. Technological advances that allow the use of lower temperature waste streams can increase the potential for bottoming-cycle CHP. These new technologies with lower temperature requirements can also help to expand the bottoming cycle for non-energy-intensive industries such as wood products, transportation equipment, and fabricated metal products.

CHP – bottoming cycle or topping cycle – continue to gain adherents. “The number of topping cycle generating units have been growing much more quickly than bottoming cycle units, so over time the share of topping cycle units should increase. In fact, there has been only been one bottoming cycle generating unit added since 2008,” wrote Kelly Perl, Ph.D., the Industrial Team Leader U.S. Energy Information Administration in response to questioned emailed by Energy Manager Today.

Last week, for instance, the Frontiersman reported on a project in Wasilla, AL. The story says that operation of a Yanmar 35 KW mCHP Energy System in the TransAlaska building began in mid-July. The system is designed to reduce energy use in the 17,000-square foot office building.

The story does not specify, but the description suggests that it is a topping cycle implementation. That to this point the system has reduce energy costs in the building from 19 cents to 10 cents per kW hour.

Moving to a CHP system is a significant investment. Of course, energy managers and facility managers will do their homework if called upon to directly sign off on an investment or to advise the people to whom they report. It isn’t necessarily an easy call. “I think it’s hard to say whether topping or bottoming is more efficient,” Perl wrote. “CHP can be very installation dependent. A larger CHP plant servicing a manufacturing plant that runs 24/7 and can make use of all the heat and electricity will be more efficient than a smaller CHP plant servicing a manufacturing plant that shuts down occasionally. Also, the analysis for waste heat starts with the question of “How can we use waste heat efficiently?” and installing CHP is one option among several. For example, waste heat can also be used to heat water in boilers and displace fuel use.”

Understanding the differences between bottoming and topping systems – and where technical innovation may take them – is an important first step in this learning process.

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