Book - School of Science and Technology

Factors affecting choice 119Fuel Unit sold Heat per unit sold To produce 100 MJNatural gas kWh 0.036 27.77 kWhBottled gas (propane) litre 0.256 3.309 litreSolid fuel kg 0.279 3.584 kgLiquid fuel litre 0.356 2.808 litreElectricity kWh 0.036 27.77 kWhIf unit prices, roughly representative of a domestic situation (and reflecting the competitivepricing following privatisation of gas and electricity supplies) at the time of goingto press, are put against these figures, then the unit costs in each case per 100 MJ of heatinput potential, excluding adjustment for any imposition of VAT are:Fuel Unit price Units per 100 MJ Cost per 100 MJNatural gas 1.4 p 27.77 ˆ 39 pPropane 35.0 p 3.906 ˆ 137 pCoal (singles) 28.0 p 3.584 ˆ 100 pOil (class D) 16.2 p 2.808 ˆ 46 pElectricity(on peak) 5.9 p 27.77 ˆ 163 p(off peak) 2.6 p 27.77 ˆ 72 pExcept in the case of direct electrical heating and, to a lesser extent direct gas heating,these figures do not represent the cost of the heat as delivered to the point of use. In anindirect system there will have been a loss in the conversion of raw energy during thecombustion process in the boiler and to the chimney, say 20% with solid fuel and 15%with oil or gas. In addition, some heat will have been lost from the pipework system enroute to the space to be served ± mains loss ± which may amount to another 5% in certaincircumstances.Other factors affect fuel consumption such as controllability, no-load losses overnightand intermittent operation generally. For instance, an electrical storage system may havelosses during the night as it charges but some part of these may be absorbed in thebuilding structure; in contrast, boiler plant burning heavy fuel oil may carry an indirect 24hour penalty as a result of electrical consumption for fuel preheating or pumping and,similarly, gas firing may carry the penalty of pressure boosting as well as that of apermanent pilot for ignition. Each case merits separate evaluation.