Cogeneration

In this report I will discuss cogeneration, also known as combined heat and power (CHP) generation, which is one of the more thermally efficient techniques of fuel utilization in comparison with conventional fossil-fuel power plants. The topics to be covered are the basics of how cogeneration works, the pros and cons, as well as publicly available data on cogeneration in different countries.

Basics of Cogeneration

In most thermal power plants more than half of the thermal energy released from burning fuel is not exploited for electricity generation but is lost as excess heat due to the fundamental laws of thermodynamics. This heat is usually released through cooling towers.

On the other hand, CHP plants have a heat recovery unit which captures this excess heat and supplies it to nearby buildings usually in the form of hot water or steam. A large CHP plant can supply heat and power for an entire town. It is also possible to use the excess heat for purposes of cooling by installing absorption chillers. Plants producing electricity, heat and cold are sometimes called trigeneration plants. In addition to fossil fuels, the fuel source used in CHP plants can also be biomass, biogas, industrial or municipal waste.

Pros and Cons of Cogeneration

The benefits of cogeneration are:

• Fuel use efficiency of up to 85% compared to best thermal plant efficiencies of 55%.
• Net reduction in the greenhouse emissions, air and water pollution.
• In cold climates, cogeneration is one of the most cost-beneficial techniques of lowering carbon emissions for heating.
• Due to the proximity of CHP plants to where the power is needed, massive production outages are less probable.
• Shorter transmission lines are needed and transmission power loss is reduced for the same reason.

Some of the downsides of cogeneration are:

• In order for CHP plants to be most viable a certain match between electricity and heating needs is required.
• While larger CHP plants are most cost-effective in terms of electricity cost, because of rapid heat loss over distance and high cost for insulating pipes, smaller CHP plants with more expensive electricity generation are constructed.
• The capital and maintenance cost for a CHP plant construction is higher than those for a conventional plant.
• Regulatory barriers in some countries, e.g. in the U.S., present a major issue for spreading of CHP systems.

Fig. 1: CHP share of total national power production per country. A histogram constructed from data in [1].

Cogeneration in the World

As a result of growing environmental awareness many countries have initiated government programs and policies facilitating wider spread of cogeneration systems.

In February 2004, cogeneration promoting CHP directive 2004/8/EC was put into force by European Union. The three most cogeneration intensive countries in the EU and in the world are Denmark, Netherlands and Finland. In Denmark, more than 50% of electricity generation comes from CHP plants; the existence of district heating networks and strong government support were important factors in cogeneration success.

In Russia, combined heat and power stations also have a large share of electricity generation due to district heating networks and cold climate in which CHP plants are most viable.

Historically, the first cogeneration plant was constructed in the U.S. Despite this fact, cogeneration wasn't adopted widely mostly due to regulatory barriers. These barriers emerged in 1900s because the government sought rural electrification through centralized power generation model. In some states it was even made illegal to sell power for anyone other than the electric utilities. However, in 1978 Congress recognized the need for improving power plants efficiency and passed Public Utility Regulatory Policies Act which facilitated cogeneration in the U.S. Later the U.S. Department of Energy (DOE) established Clean Energy Application Centers to develop the required technology application knowledge and promote combined heat and power, and waste heat recovery. DOE set a goal of achieving 20% of cogeneration share in national power production by the year 2030.

To summarize, cogeneration systems use fuel more efficiently than conventional plants. They are more environment-friendly and reduce total energy cost for end users. However, due to higher capital cost and geography-related viability, government support seems to be essential for substantial increase of cogeneration share in national power generation.

© 2010 Daulet Askarov. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.

References

[1] T. Kerr, "Combined Heat and Power - Evaluating the Benefits of Greater Global Investment," International Energy Agency, OECD/IEA, 2008.

[2] T. Hammar, "The Case of CHP in Denmark - and Perspectives to Other Countries," Workshop on Energy Supply Side Policies and Measures, OECD, 10 Sep 99.

[3] "Testimony of Sean Casten before Senate Subcommittee on Energy, Natural Resources, and Infrastructure," 24 May 07.