The Efficiency Of Different Fuels

Press, Volume CVI, Issue 31349, 20 April 1967, Page 26

The Efficiency Of Different Fuels

(By J. W. H. KELLY, manager of the domestic heating division. Shell Oil New Zealand, Ltd.J The question of which fuel will suit your house and purse is not an easy one to answer, but if you are going to install central heating in either part of the house or whole house, oil heating is the most economical to run.

A fuel heats by virtue of the number of British Thermal Units that are stored in it. A British Thermal Unit, or B.T.U. for short, is the amount of heat required to heat one pound of water by one degree Fahrenheit. Normally, however, a B.T.U. is too small a unit, and we use the term “therm” instead, which is 100,000 B.T.U.’s. But, this is not all. Only a fraction of the heat contained in a fuel can be made use of. The rest is wasted. The efficiency with which a fuel can be burnt depends upon the nature of the fuel and the efficiency of the appliance. That is where the term “useful therm” comes in. We are, therefore, interested in the cost of a "useful therm” to us, that is, the cost of the heat that we actually get We buy electricity, in kilowatt hour units. One kilowatt hour Is equal to 3413 B.T.U.’s, so a therm of electric heat will cost you 29.3 d if you are using electricity at Id a unit On the other hand, electricity, once in your home, is always converted to heat at 100 per cent efficiency whether you use a cheaper, mass-pro duced, heater or an expensive one. Your useful therms are always equal to your actual therms.

The dry, ash-free calorific value of coal varies between 18.5 therms a hundredweight for anthracite and 15.6 therms for lower grade coal. Assuming normal ash and moisture content, let us consider an average heat content of 15 therms a hundredweight. However, coal can only be burned in the home at relatively low efficiencies. The worst is the old fashioned open fire which is supposed to give an efficiency of about 25 per cent but may be nearer 15 per cent What, then, is the cost of coal at 10s a hundredweight when burned in an open fire at 25 per cent efficiency? It could be 42.9 d a useful therm or about the same as an electric inset fire using electricity. But as it takes quite a long time for,an open fire to get burning, and as it makes dirt, it is more expensive than the electric fire. More efficient appliances mean, of course, that the cost a “useful therm” will drop. Closed fires or slow combustion stoves have far higher efficiencies than open fires and central heating units using coal would have even higher efficiencies. The cost a useful therm in a slow combustion stove is about 24.3 d depending on operating conditions.

Oil has a calorific value of 1.7 therms a gallon and can be burned in central heating appliances at about 75 to 80 per cent efficiency. The blended fuel, consisting of both illuminating kerosene and diesel, as used in vapourising pot burner units, costs Is lOd a gallon, so that the cost a useful therm comes to 18.2 d. The straight distillate diesel oil used mainly in the larger capacity gun burner models costs Is 4jd a gallon and the cost a useful therm comes to 13.5 d. Oil-fired heaters come in numerous models and capacities and can involve an outlay from £75 to £5OO and upwards, but they are cheaper to run for comparable heat outputs. The larger the unit the cheaper to operate. Although the price of gas is often quoted in therms, it varies far more than the price of other types of fuels. The charge, what is more, is often hidden by a variety of fixed charges. A typical cost is 12s 6d a 1000 cubic feet of gas (450 B.T.U. a cubic foot approximately). Gas central heating has again an efficiency of about 70 per cent and thus the cost a useful therm is about 41d. Gas fires are a little less efficient and the cost a useful therm has been estimated at 48 to 50d.