Energy Recovery

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Energy recovery

A fundamental fact about energy use in modern society is that huge quantities are lost or wasted in almost every field and application. For example, the series of processes by which nuclear energy is used to heat a home with electricity results in a loss of about 85 percent of all the energy originally stored in the uranium used in the nuclear reactor. Industry, utilities, and individuals could use energy far more efficiently if they could find ways to recover and reuse the energy that is being lost or wasted.

One such approach is cogeneration , the use of waste heat for some useful purpose. For example, a factory might be redesigned so that the steam from its operations could be used to run a turbine and generate electricity. The electricity could then be used elsewhere in the factory or sold to power companies. Cogeneration in industry can result in savings of between 10 and 40 percent of energy that would otherwise be wasted.

Cogeneration can work in the opposite direction also. Hot water produced in a utility plant can be sold to industries that can use it for various processes. Proposals have been made to use the wasted heat from electricity plants to grow flowers and vegetables in greenhouses, to heat water for commercial fish and shell-fish farms, and to maintain warehouses at constant temperatures. The total energy efficiency resulting from this sharing is much greater than it would be if the utility's water was simply discarded.

Another possible method of recovering energy is by generating or capturing natural gas from biomass . For example, as organic materials decay naturally in a landfill , one of the products released is methane , the primary component of natural gas. Collecting methane from a landfill is a relatively simple procedure. Vertical holes are drilled into the landfill and porous pipes are sunk into the holes. Methane diffuses into the pipes and is drawn off by pumps. The recovery system at the Fresh Kills landfill on Staten Island, New York, for example, produces enough methane to heat 10,000 homes.

Biomass can also be treated in a variety of ways to produce methane and other combustible materials. Sewage, for example, can be subjected to anaerobic digestion , the primary product of which is methane. Pyrolysis is a process in which organic wastes are heated to high temperatures in the absence of oxygen. The products of this reaction are solid, liquid, and gaseous hydrocarbons whose composition is similar to those of petroleum and natural gas. Perhaps the most known example of this approach is the manufacture of methanol from biomass. When mixed with gasoline , a new fuel, gasohol , is obtained.

Energy can also be recovered from biomass simply by combustion . The waste materials left after sugar is extracted from sugar cane, known as bagasse, have long been used as a fuel for the boilers in which the sugar extraction occurs. The burning of garbage has also been used as an energy source in a wide variety of applications such as the heating of homes in Sweden, the generation of electricity to run streetcars and subways in Milan, Italy, and the operation of a desalination plant in Hempstead, Long Island.

The recovery of energy that would otherwise be lost or wasted has a secondary benefit. In many cases, that wasted energy might cause pollution of the environment . For example, the wasted heat from an electric power plant may result in thermal pollution of a nearby waterway. Or the escape of methane into the atmosphere from a landfill could contribute to air pollution . Capture and recovery of the waste energy not only increases the efficiency with which energy is used, but may also reduce some pollution problems.

[David E. Newton ]