In this paper, the reductions are investigated in energy use and environmental emissions achievable when cogeneration is applied using the facilities of electrical utilities. These reductions are illustrated for several cases when cogeneration is implemented by Ontario Hydro, the principal electrical utility in the province of Ontario, Canada. The work reported here relates to other recent studies by the author and others of the potential benefits of utility-based cogeneration in Ontario.

Here, cogeneration refers to the simultaneous production of two energy forms (electricity, and heat in the form of steam and/or hot water) from one energy source (e.g., a fossil fuel or uranium). Cogeneration has been used, particularly in industry, for approximately a century. Although a cogenerator can be a utility, an industry, a government, or any other party, this report considers cogeneration only by electrical generating utilities, particularly Ontario Hydro.

In thermal electrical generating stations (such as fossil fuel and nuclear plants), the energy content of a resource (normally a fossil or nuclear fuel) is converted to heat (in the form of steam or hot gases) which is then converted to mechanical energy (in the form of a rotating shaft), which in turn is converted to electricity. A portion (normally 20–45%) of the heat is converted to electricity, and the remainder is rejected to the environment as waste.

Cogeneration systems are similar to thermal electricity-generation systems, except that a percentage of the generated heat is delivered as a product (normally as steam or hot water), and the quantities of electricity and waste heat produced are reduced. Overall cogeneration efficiencies (based on both the electrical and thermal energy products) of over 80% are achievable. Other advantages generally reported from cogenerating thermal and electrical energy rather than generating the same products in separate processes include: reduced energy consumption, reduced environmental emissions (due to reduced energy consumption and the use of modern technologies in large, central installations), and more economic, safe and reliable operation. Most thermal systems for large-scale electricity generation are based on steam and/or gas turbine cycles, and can be modified relatively straightforwardly for cogeneration.

Two main categories of heat demands can normally be satisfied through cogeneration: (i) residential, commercial and institutional processes, which require large quantities of heat at relatively low temperatures (e.g., for air and water heating); and (ii) industrial processes, which require heat at a wide range of temperatures (e.g., for drying, heating, boiling in, for instance, chemical processing, manufacturing, metal processing, mining and agriculture). The use of a central heat supply to meet residential, commercial and institutional heat demands is often referred to as district heating. As well as satisfying heat demands, cogenerated heat can also drive chillers; this application is examined in detail in a related report, and could be particularly beneficial in Ontario where the 1991 peak electrical demand was associated with the summer cooling load.

Many general descriptions and studies of cogeneration systems have been reported, some of which have focussed on Ontario. Cogeneration systems are in use throughout the world (e.g., over 4000 are listed by the Association of Energy Engineers), and the basic technology is well understood and proven. Numerous examples exist of large cogeneration systems: (i) a steam turbine plant in Switzerland generates 465 MW of thermal power and 135 MW of electrical power, with an overall efficiency of 75%, (ii) a nuclear power plant in Michigan left incomplete due to lack of funding was eventually completed as a gas fired combined-cycle cogeneration plant having 12 heat recovery steam generators and gas turbines and two steam turbines, producing 1400 MW of electrical power and 285,000 kg/h of steam, and (iii) approximately 10 plants are used to generate 240 MW of electrical power and to supply 90% of the 1500 MW thermal demand for the city of Malmo, Sweden (population 250,000). In the last example, fuel drives two of the plants (an extraction steam turbine plant generating 110 MW of electrical power and 240 MW of thermal power, and a back pressure steam turbine plant generating 130 MW of electrical power and 300 MW of thermal power), while the remaining plants operate on waste heat from neighbouring industries (e.g., smelting, carbon-black production, sewage treatment and refuse incineration).

The size and type of a cogeneration system are normally selected to match as optimally as possible the thermal and electrical demands. Many matching schemes can be used. Systems can be designed to satisfy the electrical or thermal base-loads, or to follow the electrical or thermal loads. Storage systems for electricity (e.g., batteries) or heat (e.g., hot water or steam tanks) are often used to overcome periods when demands and supplies for either electricity or heat are not coincident. Cogeneration systems are sometimes used to supply only the peak portions of the electrical or thermal demands.

In heating plants, energy in the form of a fossil fuel or electricity is converted to heat (in the form of hot gases or another heated medium), often with an energy conversion efficiency of over 80%.

For its current electrical generation, Ontario Hydro relies mainly on nuclear and hydraulic energy and fossil fuels (almost entirely coal). Since most thermal plants operated by Ontario Hydro from a given energy source are conceptually similar, sample coal (Nanticoke) and nuclear (Pickering) plants from Ontario are considered here and assumed representative of other plants using the same energy source. Nanticoke and Pickering are proven, both having been in operation since 1971, and contain eight individual units having net electrical outputs of approximately 500 MW each. The units operate on a thermal cycle having four main steps: (i) heat is produced and used to generate and reheat steam; (ii) the steam is passed through a series of turbine generators, which are connected to a transformer; (iii) cooling water condenses the steam exhausted from the turbines; and (iv) the temperature and pressure of the condensed steam are increased in a series of pumps and heat exchangers. Fig. 2 shows that the overall station efficiency (based only on electrical energy) for Nanticoke is 37% and for Pickering is 30%, and that by far the largest energy loss is the heat rejected from the condensers in cooling water. Thus, efficiency can be markedly improved for both plants if the thermal energy rejected by the condensers is used, i.e. if cogeneration is implemented.

Few applications of cogeneration exist in the current electrical generation system in Ontario. For example, within electrical generating stations, small quantities of steam are extracted from various points on the turbines and used to preheat the liquid exiting the condenser before it enters the boiler. This internal use of cogeneration increases overall station efficiency by reducing the external energy input to the plant while reducing electrical output by a relatively smaller amount. Other examples include (i) the use of steam from the steam generator of the Bruce Nuclear Power Station for heating operations in the on-site heavy-water production facility, and in the Bruce Energy Centre, a nearby industrial park, and (ii) the Coolwater Farms aquaculture facility which is supplied by pipeline with warm water from the Pickering station condensers.