CENTRALIZED GENERATION, DISTRIBUTED LOADS
We pay, however, particular attention to one form of electricity, in which large central stations generate synchronized alternating current and send it out to users over a network that includes long, high-voltage transmission lines. Since the 1880s, electricity systems based on this common technical model have spread all over the world, bringing electric light, motive power and other benefits on which modern society now depends. Large-scale centralized generation of electricity has become so important, and so dominates our thinking, that we have long tended to discount the many alternative forms of electricity generation that are smaller in scale and less centralized.
In recent years, however, these forms of generation have become harder to overlook. Based, for instance, on wind turbines, micro hydro, diesel engines, gas engines, Stirling engines, microturbines, fuel cells and solar photovoltaics, they tend to come in unit sizes much smaller than central station generators, usually less than 5 MW. Since individual units or clusters of units may be widely dispersed across an electricity system, rather than being centralized, these technologies have come to be called ‘distributed generation’, a key form of decentralized energy.
Although traditional electricity generation is centralized, the loads that use the electricity, such as lamps, motors, heaters, chillers and electronics, have always been widely distributed and dispersed. Except for the very largest loads, such as pot lines in aluminium smelters, loads are much smaller than central station generators, usually by many orders of magnitude. This mismatch in scale between generation and loads requires the network to divide up the large output of a generator into flows appropriate to the loads – that is, to distribute the electricity. Alternatively, of course, generation itself could be distributed, closer to loads in both location and scale.
Although traditional generation is centralized, the loads using the electricity have always been widely dispersed
The reason why it is not is historical, and overdue for reassessment. In the early decades of electricity, generating technologies were based on water power and steam power. The economies of unit scale of steam engines and turbines, water turbines and alternators meant that a bigger generator produced cheaper electricity. That was the premise on which Edison and his competitors set up the first central station systems. The savings on investment in larger generators more than made up for the extra investment in the necessary network. In the subsequent century this premise continued to prevail, up to generators of gigantic size and networks to match, entailing likewise gigantic investments. The investments were possible because the monopoly franchise made captive customers carry the risks which, by the 1980s, sometimes proved equally gigantic. Nevertheless the arrangement made electric light and other electric services available and affordable over much of the world.
A survey by province
Canada has a new association to promote cogeneration nationally. Here, President of COGENCanada Gordon Robb surveys the industrial cogeneration and wider energy scene by province, noting that woody biomass is perhaps the most important fuel sector.
The TransAlta 400 MW combined-cycle plant serves four petrochemical complexes in Sarnia, Ontario. The plant incorporates three 115 MW Alstom gas turbines and a 120 MW steam turbine
COGENCanada was created in mid-2004 to promote cogeneration and provide related training throughout Canada. The Association is also promoting cogeneration-based ‘Eco Industrial Networking’, a concept that involves using outputs and waste from one industrial process as inputs to other processes.
Canada’s provinces have jurisdiction over electricity within the country. The Federal Government deals with nuclear safety and, through the National Energy Board, licences exports and certificates export transmission lines.
In 1976, the Federal Government introduced an accelerated depreciation rate or capital cost allowance of 50%, reduced in 1994 to 30%, on a straight-line basis as an incentive to build cogeneration systems. To qualify for this incentive, a specified fraction of the electrical output from a plant must be true cogeneration. However, a much more effective incentive is required. There is a case for modifying the arrangement to provide an incentive to build power plants near heat loads, whether or not the electrical output is large relative to the heat load. There is also a case for deeming all components essential to the delivery of the electricity and heat to be eligible.
A decade ago, electricity was supplied throughout Canada by vertically integrated monopoly utilities. With two exceptions, they have not built cogeneration systems, and cogeneration systems were owned by steam users. Most provinces are now in the process of restructuring.
Forest industries account for a large fraction of the cogeneration systems in Canada. In recent years the lumber and pulp and paper industries have become integrated, with chips for making pulp coming from adjacent saw mills. Wood residues from the saw mills are used as fuel for steam generation and cogeneration by the pulp mills, as is pulping liquor. There are also many newsprint mills in Canada. Paper mills using recycled paper are ideal candidates for cogeneration. However, the increased use of thermo-mechanical pulping has reduced the scope for cogeneration because that pulping process generates steam for paper drying.