The United States wastes about 60% of the fuel burned for energy including electricity generation, space and process heating, and transportation. According to Lawrence Livermore National Laboratory, transportation of all sorts and electric generation each waste about 75% of their fuel input. The waste energy (heat) is rejected to air, lakes, and rivers. Surprisingly, if the heat generated by nuclear power plants in the US was 100% converted to electricity, there would be no need to burn coal or natural gas, at all, for electricity. One potential way to reduce this waste is combined heat and power (CHP).
Combined heat and power consists of a combination of electric and useful heat generation. The key word in that sentence is “useful”. For CHP to be cost effective, the end user must be able to use nearly all the rejected heat – heat that is needed anyway for processes and space conditioning. Consider two cases; a hospital and a food processor.
For commercial buildings, hospitals likely have the greatest need for steam all year, but more importantly, a substantial need in the heat of summer. The first chart shows what steam production from a CHP plant may look like for a hospital in three scenarios.
The blue curve represents the actual steam required for the facility, and it is naturally highest in the winter when space heating is the greatest. The red curve represents the steam produced by a combined heat and power plant that just provides the minimum power required by the facility. The peak in the middle represents cooling needs. The green curve/line represents the minimum steam required by the facility at all times – the probable lowest consumption rate of any hour of the year. To maximize cost effectiveness, the combined heat and power plant is sized to produce this amount of steam at all times.
Now consider the same steam curves for a food processor as shown in the second chart. Since process heating and cooling loads for both steam and electricity much more dominate the total consumption for a food processor, the minimum need for steam, and thus the minimum production of CHP electricity, is much greater. The CHP plant in this case can produce at least 60% of the plant’s required steam loads at any given time. For the hospital, this ratio is only about 20%.
The summations of electricity and steam production over the year for both the hospital and food processor cases are demonstrated in the third chart.
Reiterating, for maximum return on investment, a CHP plant should be operating at 100% capacity at all times with 100% of both gas and waste heat (steam in this case) being in demand. Theoretically, a typical food processor can generate with CHP all its electricity and most of its steam required. Due to their widely fluctuating steam loads, large hospitals can produce less, but substantial amounts of electricity and steam with CHP.
 Steady state production. Shutdowns for maintenance and holidays will require grid power.