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Searching for Order in this World of Entropy

A note about tonnage and energy figures

It is customary for the waste treatment capacity of WTE plants in North America to be rated in terms of short tons per day. In Europe, metric tons per year is the typical metric for plant capacity. Maximum ratings of the furnaces are generally stated in metric tons per hour. The annual tonnage accounts for downtime of the incinerators for maintenance, while the daily and hourly tonnage values do not. The WTE database will store all plant ratings as metric tons per annum (year), and therefore the true annual capacity of American plants entered using tons per day will be inflated slightly, because it assumes that the facilities are running constantly through the entire year.

The same applies to energy figures, where American plants typically produce only electricity and are rated in terms of power (megawatts). This megawatt rating is usually the nameplate capacity and again does not account for maintenance or reductions in capacity. European plants usually produce heat as well and are often rated in gigawatt-hours per year (GWh/a) and this usually takes into account downtime and reductions from maximum capacity. The database stores the plant capacity as GWh/a.

50,000 Homes Per Day?

Many utility firms' websites and popular media reports mention the capacity of plants in terms of the number of average households that they can serve. However, the units used are often dubious. For example, the 'X' homes per day or 'X' homes per year is common. This unit is meaningless. What they meant to say was 'X' homes. The origin of the other fudge words may be due to the fact that the "homes per day/year" value is based upon the amount of power consumed by a home averaged over the course of a day or year, making for a specific number of watts rather than the wildly spiking instantaneous consumption which occurs when we turn devices on and off throughout the day.

If an average household uses 12,000 kilowatt-hours of electricity in a year, we can take this and divide it by the number of hours in one year (8760), giving around 1400 watts. A power plant rated at 14 megawatts (14,000,000 watts) could thus serve 10,000 homes.

A watt is a unit of power - how many lightbulbs/houses can the plant light up?

A watt-second (also called a joule) is a unit of energy - how long can I keep those lightbulbs/houses lit up? Watt-seconds are fairly small amounts of energy, so we use watt-hours, which is 3600 watt-seconds, 3600 being the number of seconds in one hour. This is still a fairly small unit for personal consumption, so it is common to use units of 1000 watt-hours: the kilowatt-hour. Very large power plants (like Itaipu Dam) are measured in trillions of watt-hours or terawatt-hours, of which they can produce many per year.

The Database of Waste-to-Energy Plants stores all figures of energy/power production in units of gigawatt-hours per year (GWh/a). This is a very fitting unit for the WTE industry, few plants produce less than 1 GWh and the largest ones top out at around 700 GWh.

Energy and power are not measured in units such as a "megawatt per day", "kilowatt per hour", "50,000 homes per year" or "50,000 homes per day"

Electricity is billed in kilowatt-hours (kWh), not kilowatts or the ever-prevalent "kilowatts per hour"

kWh is not the same as kW/h

 

Last Modified: September 28, 2010. 13:07:20 pm