District energy systems provide tremendous opportunities to strengthen our infrastructure in a way that increases energy efficiency, reduces air pollution, combats global warming and decreases emissions of ozone-destroying chlorofluorocarbon (CFC) refrigerants.
Recovery of these currently untapped sources of energy, particularly from electricity generation, would substantially increase energy efficiency. Power plants are responsible for 36% of U.S. primary energy consumption. The average efficiency of U.S. power plants is 31%, with most of the remaining energy dissipated in the plant cooling towers or smokestack.
This wasted energy represents one quarter of U.S. energy consumption (Figure 3) and presents an unparalleled opportunity for conserving energy resources. Another 4% of energy use becomes waste heat in industrial processes. Some of this heat could also be recovered and distributed as district energy.
Figure 3. Primary energy use by sector 3
Widespread recovery of waste heat requires district energy systems because they provide the infrastructure for distribution of this energy to many energy users. Through district energy/cogeneration systems, efficiencies can be boosted to 70-80%. In contrast, current power plants have an average efficiency of 31%, and new gas turbine combined cycle plants can achieve efficiencies of 50-55%. (Figure 4)
The inefficiency of our current power plant infrastructure is due not to poor industry performance but a lack of policies necessary for widespread implementation of cogeneration. Significantly, there usually are no means of connecting potential users with power plants. District energy systems fill this critical gap.
Figure 4. Cogeneration plant efficiency compared to conventional power plants 3,5,
The environment in general and air quality in particular continue to be a significant public concern. Numerous surveys of the general public and of registered voters indicate strong support for the environment even if cleaner air means higher costs.
District energy systems decrease air emissions by increasing energy efficiency and effectively managing emission control systems. Economies of scale and highly trained staff enable district energy systems to install, maintain and operate systems for superior control of air emissions from a variety of fuels, including coal.
With cogeneration, district energy systems can bring even more dramatic reductions in air pollution compared to conventional technologies. For example, gas-fired district heating/cogeneration can cut in half emissions of nitrogen oxides (a precursor to ground-level ozone, i.e., smog) when compared to separate production of: 1) electricity in a new gas-fired combined cycle power plant; and 2) heat for commercial buildings based on the average mix of building heating technologies. (Figure 5)
Figure 5. Nitrogen oxides (NOX) emissions with district heating/cogeneration compared to conventional technologies
Cities which extensively employ district energy have lower levels of locally generated air pollution. For example, Stockholm, Sweden reduced its sulfur dioxide emissions by 95% and particulate emissions by 82% in 25 years, largely as the result of increased efficiency and fuel flexibility from expanded implementation of district energy. (Figure 6)
Figure 6. Emission reductions and growth of district heating in Stockholm, Sweden, 1965-1990 8
Global warming could have enormous human and environmental costs resulting from inundation of coastal areas due to rising sea levels, more severe droughts and floods and significant shifts in agricultural productivity.
There is now a solid international scientific consensus that global warming is a real threat which requires international action to reduce emissions of greenhouse gases. In addition, the public sees global warming as a serious threat. A survey conducted in Dec. 1995 indicated that 71% of registered voters nation-wide feel that global climate change is either "very serious" or "somewhat serious." (Figure 7)
The U.S. has committed to stabilizing greenhouse gas emissions at 1990 levels by the year 2000, as have 160 other nations who have signed the Framework Convention on Climate Change. Unfortunately, U.S. energy-related carbon emissions continue to climb, and were 5% higher in 1995 compared to 1990. The gap between our commitment and our performance continues to grow.
In fall of 1997 the Framework signatories plan to adopt a protocol establishing the measures which will be taken to achieve the goal of greenhouse gas stabilization. With U.S. greenhouse gas emissions continuing to increase, it is critical that we examine opportunities for shifting our energy infrastructure toward sustainable approaches with multiple energy, environmental and economic benefits.
District energy systems can reduce greenhouse gas emissions and provide beneficial impacts on stratospheric ozone, regional air quality, national energy security and economic development. As described in Section 4, a conservative estimate indicates a realistic greenhouse gas reduction potential of 9.6 million metric tons of carbon. This potential compares favorably with estimates for program groups described in the Climate Change Action Plan11 (Figure 8)
The Intergovernmental Panel on Climate Change (IPCC) -- the international scientific panel which jointly examines the climate change issue -- has specifically identified cogeneration as a key "promising approach" for reducing greenhouse gas emissions.9 The European Commission is now developing a European Union cogeneration and district energy strategy in 1997 as part of its overall climate change strategy.
Figure 7. Responses to public survey question on the seriousness of global warming 10
Figure 8. Greenhouse gas reduction potential of district energy/cogeneration compared to measures in the Climate Change Action Plan
International agreements have phased out the production of CFCs as of January 1996 and have scheduled the phase-out of hydrochlorofluorocarbon (HCFC) refrigerants by 2030. Hydrofluorocarbons (HFCs) and ammonia, which are also used as refrigerants, are not restricted by international protocols.
District cooling can be a key strategy for accomplishing an economical and environmentally wise phase-out of harmful refrigerants. Through better staffing and operational practices for monitoring and control, district systems are better able to control emissions of whatever refrigerant is used. District cooling systems also provide excellent opportunities to use cooling technologies with zero impact on the ozone layer, such as ammonia or steam absorption systems. (Figure 9)
Figure 9. Ozone depletion potential (ODP) of refrigerants or chiller types
Ammonia is a very effective refrigerant but requires close monitoring and control. In absorption chillers, water acts as the refrigerant which, in combination with harmless lithium bromide, is used to produce chilled water using heat as the driving energy. These technologies are being effectively and safely employed in district systems but are less feasible in individual buildings. For example:
District cooling eliminates significant capital investments for buildings which must otherwise modify or replace chillers in order to use non-CFC refrigerants. This capital avoidance is an important benefit in urban centers, which are often struggling to compete with suburban areas, as discussed below.