101 CHP Definition and Overview

Combined heat and power (CHP) systems capture the heat energy from electric generation for a wide variety of thermal needs, including hot water, steam, and process heating or cooling. Figure 10.1 gives an example of the efficiency difference between separate and combined heat and power. A typical U.S. CHP system converts 80 out of 100 units of input fuel to useful energy — 30 to electricity and 50 to heat. By contrast, traditional separated heat and power components require 163 units of energy to provide the same

* The U.S. Department of Energy's CHP Web site, www.oit.doe.gov/chpchallenge, and the U.S. CHPA Web site, www.nemw.org/uschpa, contain numerous references.

** For an excellent review of small-scale CHP worldwide, see Major, G., Small Scale Cogeneration, CADDET Energy Efficiency Analysis Series, 1995.


CHP versus separate power.

amount of heat and power. Thus, with today's technologies, CHP can cut fuel use nearly 40%.*

10.2 Available Technologies

Commercially available CHP technologies for DG include diesel engines, natural gas engines, steam turbines, gas turbines, microturbines and phosphoric acid fuel cells.** Table 10.1 summarizes the characteristics of commercial CHP prime movers. The table shows the wide range in CHP capacity — from 1 kW Stirling engine CHP systems to 250 MW gas turbines. All are projected to have lower costs and emissions and higher efficiencies due to incremental technology advances.

10.2.1 Reciprocating Engines

For CHP applications, the two principle types of combustion engines are fourcycle spark-ignited (Otto cycle) and compression-ignited (diesel cycle) engines. CHP projects using reciprocating engines are typically installed for $800 to $1500/kW. The high end of this range is typical for small capacity projects that are sensitive to other costs associated with constructing a facility, such as fuel supply, engine enclosures, engineering costs, and permitting fees.

* Based on a paper by Roop, J. M. and Kaarsberg, T. M., Combined heat and power: a closer look, Proceedings of the 21st National Industrial Energy Technology Conference, Houston, TX, May 1999. These are national averages for existing installed boilers and central generating plants.

** Nearly all the 171 PAFCs installed in the U.S. obtained a federal government subsidy of as much as $1000/kW; thus, in what follows, they are not included in cost estimates.

TABLE 10.1

Comparison of DG/CHP Technologies

Diesel Engine

Natural Gas Engine

Gas Turbine


Fuel Cells

Stirling Enginea

Electric efficiency (LHV)

Part load Size (MW) CHP installed cost ($/kW) Start-up time Fuel pressure (psi) Fuels



Diesel, residual oil

Uses for heat recovery Hot water, LP steam, district heating 3400



10 sec 1-45

Natural gas, biogas, propane

Hot water, LP steam, district heating 1000-5000


(combined) Poor 3-200 700-900

10 min-1 hr 120-500 Natural gas, biogas, propane, distillate oil Heat, hot water, LP-HP steam, district heating 3400-12,000



0.025-0.25 500-1300

60 sec 40-100 Natural gas, biogas, propane, distillate oil Heat, hot water, LP steam



H2, natural gas, propane

60 sec

Hot water, LP-HP Direct heat, hot steam water, LP steam

500-3700 140-700

3000-6000 500-1000

a Expected to be commercial by 2005.

Source: ONSITE SYCOM Energy Corporation, Market Assessment of CHP in the State of California, draft report to the California Energy Commission, September, 1999 (except for Stirling data).

Solar Stirling Engine Basics Explained

Solar Stirling Engine Basics Explained

The solar Stirling engine is progressively becoming a viable alternative to solar panels for its higher efficiency. Stirling engines might be the best way to harvest the power provided by the sun. This is an easy-to-understand explanation of how Stirling engines work, the different types, and why they are more efficient than steam engines.

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