Power Generation and Distribution. 469 



Even to-day there are few electric power stations in England 

 consuming less than 6 lbs. of coal per unit sold. Of the 26 electric 

 supply undertakings in the London Metropolitan area, supplying 

 over 150,000,000 units a year, the average cost of coal in 1905 

 exceeded o.5d. per unit sold. Assuming the average cost of coal to 

 be 15/- per ton, this corresponds to an average consumption of 6^ 

 lbs. per unit sold. The average thermal value of the coal probably 

 exceeds 13,500 B.Th.U. per lb., on which basis 84,375 B.Th.U. 

 are thus expended per electrical unit sold in London, w^hereas an 

 electrical unit corresponds to 3,438 B.Th.U., consequently the overall 

 thermal efficiency, that is to say, the proportion of the latent heat 

 energy contained in the coal, which is recovered in the form of 

 electric energy at consumers' premises throughout London, is 

 practically 4 per cent. 



By employing a small number of correspondingly larger engines 

 of the most economical type, this efficiency ought, in the light of our 

 present knowledge, to be greatly improved, but it is doubtful whether, 

 with the most economical steam plant, and with the highest load 

 factor obtainable under practical conditions, it is possible at the 

 present time to generate and distribute electric energy 24 hours per 

 day over a large area — involving extra high pressure mains and the 

 consequent transforming losses — with a higher overall thermal 

 efficiency than 8 per cent. 



The following figures show approximately the distribution of 

 losses, giving this result : — 



rtesultiiig 

 efficiency. 



Boiler efficiency ... ... ... 70%) 12-88°/ 



Engine and dynamo 16 lbs. steam per unit ... l8'4%3 '° 



Increased steam consumption due to engines 



being at times uneconomically loaded ... 10% ll"59% 



Losses by radiation from steam pipes, blowing 



off boilers, &c. ... ... ... 10% 10-43% 



Power absorbed by station auxiliaries ... 5% 991% 



Losses in electric distribution including trans- 

 formers ... ... ... ... 20% 7 '93% 



Say ... ... 8% 



In view of the last-mentioned loss, it may at first sight be difficult 

 to realize that a power-user, developing his own power direct by 

 modern engines, could under any circumstances be supplied from a 

 distant .steam-power station at a lower price than his own cost, with 

 profit to tiie supplier, for the following reasons : — 



{a) The Power Distributor must (generally speaking) 

 generate his own power in similar manner, and can 

 only obtain comparatively small advantage in the 

 higher efficiency of larger generating units. 

 {b) The Power Distributor has to incur the additional 

 losses of converting his mechanical power as given off 

 by the engines, into electrical power. He has also to 

 incur losses in transmission, and must cover the con- 

 sumers' losses in reconversion of the electrical energy into 

 mechanical energy. 



