The Thermal Spectrum of Low-Temperature Energy Use in the United States

Abstract

A detailed analysis of the U.S. energy consumption was performed as a function of its utilization temperature from 0 to 260°C. The study highlights the changes that have occurred in U.S. energy use since the 1970s and suggests how renewable energy could provide a large fraction of energy now used for direct use at low end-use temperatures that is currently mostly supplied by high grade fossil fuels. For example, most of the energy we need for water and space heating used at low temperatures is provided by combusting natural gas and oil at very high temperatures. This downgrades the thermodynamic potential of the fossil fuels for generating power resulting in large reduction of its exergy or availability. By focusing attention on the thermodynamic losses inherent to our current energy system, we suggest a paradigm shift in the way we view and use energy by strategically matching the source providing the energy to the end-use temperature of the application. Thermal energy below 260oC could be supplied more sustainably without large exergetic or availability losses by geothermal or solar thermal energy resources, or by waste heat. In addition, direct thermal use of geothermal energy also has economic advantages over using it to generate electricity by avoiding the inherently large 2nd Law losses in converting geothermal energy to electricity.
Using the U.S. Energy Information Administration database as a primary source of information, we found that the total thermal energy from 0 to 260°C used in 2008 was 33.5 EJ (31.7 quads), about one third of the entire U.S. demand. More than half of the thermal energy demand below 260°C (55%) comes from the residential sector, while the rest comes from the industrial (24%) and commercial (21%) sectors. Also quite importantly, almost 80% of 33.5 EJ is used to provide heat below 150°C. Space heating and

water heating have end-use temperatures of 40 to 60°C and are responsible for 38% of the thermal energy consumption below 260°C in the residential and commercial sectors.