Study Finds Models For Assessing Energy Systems Inadequate

Despite the multitude of benefits of installing combined heat and power (CHP) systems, relatively few tools exist for estimating the displaced emissions, or for predicting how CHP affects constrained transmission systems or distributed emissions implications, according to a study.

On Jan. 3, the American Council for an Energy-Efficient Economy (ACEEE) released a report, "Survey of Emissions Models for Distributed Combined Heat and Power Systems." According to EPA, CHP is an efficient, clean, and reliable approach to generating power and thermal energy from a single fuel source. "By installing a CHP system designed to meet the thermal and electrical base loads of a facility, CHP can increase operational efficiency and decrease energy costs, while reducing emissions of greenhouse gases that contribute to climate change," EPA states.

The ACEEE report reviews existing models' attempts to quantify or describe the emissions effects of combined heat and power (CHP) systems. The models vary in design, scope and detail, but they all commonly seek to capture the functions of an energy economy and use knowledge of economic interactions to simulate the effects of economic and policy changes.

"CHP systems offer huge improvements in system efficiency over traditional electricity generation. However, the difficulty in analyzing the emissions displaced by both the heat and power outputs of these CHP systems has hampered the development of effective policies. Quantification of 'displaced emissions' would be immensely helpful," said Anna Shipley, co-author and an ACEEE industry program research associate.

The Integrated Planning Model (IPM), Average Displaced Emissions Rate (ADER), Market Allocation (MARKAL), All Modular Industry Growth Assessment (AMIGA), Oak Ridge Competitive Electric Dispatch (ORCED) and National Energy Modeling System (NEMS) models are all reviewed in the report. EPA's CHP emissions calculator also is profiled. While the approximate operation of each model is discussed, this survey seeks to specifically explain how these models handle emissions and how CHP and thermal energy are considered within the models.

Although a variety of well-established energy and emissions models exist, few tools capture the collective local and remote emissions benefits of a CHP system. "The absence of a comprehensive tool to estimate displaced emissions penalizes CHP by not allowing the full appreciation of the environmental benefits," said Neal Elliott, co-author and ACEEE industry program director.

The creation of such a tool might be approached by following the framework discussed in the report, initially providing coarse guidance and subsequently allowing the determination of specific benefits attributable to a CHP system. The CHP Emissions Calculator published by EPA is a good effort toward addressing some of these goals. However, the tool is too simplistic to allow geographical resolution of the displaced emissions, ACEEE officials said. It is hoped that a more comprehensive tool, or the integration of the Emissions Calculator methodology into a more sophisticated model, will be performed.

"Hopefully this study will encourage the modeling community to attempt a more robust and accurate assessment of CHP emissions," Shipley said.

The "Survey of Emissions Models for Distributed Combined Heat and Power Systems" is available online for free download at

This article originally appeared in the 01/01/2007 issue of Environmental Protection.

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