Book - School of Science and Technology

314 Fuels, storage and handlingstored using both old and new techniques. Those most relevant to the present subject aredescribed in an earlier chapter (p. 120).Tariff structuresBecause plant of overall higher efficiency operates at night, generation costs are lowerthen than during the day. If the costs of metering were of no consequence either to thesupply industry or to the consumer, then a price structure which took account of bothdemand pattern and energy use might be the universal and preferable tariff practice.For the larger consumer, a tariff including separate components for maximum demandand energy use is commonly used and a variety of such arrangements exists to suitdifferent circumstances. In each case, the charge for energy used may be at a single rateor be biased as to time of use. The family of tariffs used in the domestic sector may eithertake the form of a standing charge which reflects the cost of making the supply available,plus unit rates set at a level appropriate to recovery of both the demand- and energyrelatedcosts, or a rate combining these two elements of the charge. The unit rate may,again, be as a single rate for all units no matter when consumed; as a dual rate where unitsconsumed for a specific purpose during the night are charged separately or in simple day/night form where one rate applies during the day and a lower rate is charged for all useduring the night. For domestic consumers, the off-peak rate is commonly 50% or less ofthat charged for daytime use.Since the various supply companies throughout the country are independent organisations,the details of their tariffs are related to the particular supply and distributioncircumstances arising within the geographical area which they serve. In general terms,however, roughly seven hours of off-peak supply are available each 24 hours, commonlyfrom midnight (24.00 GMT) until 7 am. (07.00 GMT) and normally referred to as Economy7 (see also Chapter 7). During the summer months, when British Summer Time is inforce, the availability of the off-peak supply remains related to GMT and is thus out ofphase.Space heatingWhen a daytime tariff applies, electricity is an expensive source of energy but, in spite ofthis, it is used extensively as a result of its almost universal availability and the flexibility itoffers. It is, however, as a supply taken off-peak that electricity has rather more to offerthe user. At any time this supply offers:. transmissibility to any point regardless of physical levels or similar limitations. absence, local to the point of use, of dust from fuel and ash, fumes, etc.. reduction in on-site labour for plant operation. availability at actual point-of-use. relative ease of control for energy input.It is often argued that to produce electricity from a raw fuel involves much wastage sincemore than three-quarters of the calorific value of that fuel, as supplied to the generatingstation, may be wasted in cooling towers, rivers and canals or is lost in the distributionsystem. The contra-argument was that the grade of raw fuel used in generating stations isso low that it would be difficult if not impossible to burn it elsewhere and, whilst thisstatement is valid as advanced, the value of that low grade fuel as a chemical feed-stockcannot be ignored. The construction or conversion of a generating station designed to