Electric power generation currently accounts for 36% of U.S. energy-related carbon emissions. Only 31% of the primary energy consumed in U.S. power plants emerges as usable electric energy. The remainder, which is primarily power plant waste heat, represents about 25% of U.S. energy-related carbon emissions. New electric-only gas turbine combined cycle power plants have a higher efficiency than the current average, but even the most efficient new electric-only plant produces significant quantities of waste heat, which can be recovered in cogeneration facilities (with an efficiency of 70-80%).
A significant opportunity for greenhouse gas reductions exists through recovery and distribution of power generation waste energy via district energy systems (DES). DES provide the infrastructure for the distribution of recovered thermal energy to buildings, in the form of chilled water, hot water and/or steam, to provide space heating, air conditioning, domestic hot water and industrial process energy. Recovered heat can be used for heating or for generating chilled water for air conditioning. There are an estimated 5,800 DES in the U.S., serving downtown areas, universities, military bases, hospital complexes and other groups of buildings.
District energy systems provide other environmental and economic benefits, including:
In the quantitative analysis, the engineering, environmental and economic characteristics of two types of cogenerated DES were contrasted with conventional approaches to providing the same end-use energy requirements (heating, cooling, and electricity. The analysis shows that greenhouse emissions can be reduced by 9.6 million metric tons of carbon equivalent (MMTCE) through the implementation of a mix of gas-fired and coal-fired cogenerated DES in place of a mix of gas combined cycle and coal fluidized bed condensing power plants. The conventional generating capacity dispaced equals 22% of the total projected power plant capacity additions between 1995 and 2010, or 13% of the projected new additions through the year 2015.
Debt service costs of DES are about 35% higher than with conventional approaches, but energy consumption is cut significantly. In essence, district energy systems substitute capital for energy consumption and environmental degradation. At current price levels, the total cost per unit of useful energy with DES is an estimated 6% lower than with conventional approaches. However, with projections for "source" fuel prices as provided in 1994 by the administration's Interagency Analysis Team, district energy provides, by the year 2030, net present value savings of $60-185 per ton of carbon reduced (discount rate 7% and 3%, respectively).
In the U.S., growing demand for electricity, combined with electric utility industry restructuring, will result in significant capital investment in new generating capacity. With already staggering energy losses (and associated carbon emissions) in generating electricity, and substantial growth projected, the U.S. should address opportunities to shift infrastructure investment toward technologies which reduce these losses.
The Intergovernmental Panel on Climate Change (IPCC) has identified cogeneration/district energy as a key greenhouse gas reduction measure, and the European Commission is now developing a European Union cogeneration/district energy strategy.
District energy potential can be realized through policies and measures to: increase awareness and knowledge; recognize the environmental benefits of district energy in air quality regulation; encourage investment; and facilitate increased use of district energy in government buildings.