Environmental Engineering Reference
In-Depth Information
What Does It Take to Run … a Fridge?
The fridge, an appliance that has become an indispensable part of most households in the
industrialised world, exemplifies how abstract our relationship to energy has become. One
hundred years ago, the main energy requirement for a household was fuel (usually wood or
coal) to heat and cook. This was purchased or harvested, then stored in or next to the home. A
typical farmhouse in northern Europe would include a storeroom filled with fuel to sustain the
family throughout the winter. The energy used to run modern household appliances is abstract
because it only appears as a figure on an electricity bill. We tend not to concern ourselves
with the fuels used to generate that power. But if we were to run a domestic fridge ourselves,
how much fuel would we need?
A medium-sized fridge (200 litres) requires about 160 watts of power. If well insulated,
it will need to run for only about four hours per day, or 1,460 hours per year. To calculate
how much energy it consumes over a year, we multiple 160 watts (or 0.16 kilowatts) by the
number of hours it runs for. This gives us 230 kilowatt hours or units of electricity consumed
per year.
The average chemical energy content of coal is 6,150 kilowatt-hours per ton or 6.15
kilowatt-hours per kilo. Although coal-fired power plants are quite efficient, they are still
limited by the laws of thermodynamics. Therefore, only about 40 per cent of the chemical
energy in the coal is converted into electricity. So, a kilo of coal can generate 2.46
kilowatt-hours of power (0.4 x 6.15 kWh). To calculate how many kilos of coal we would
need to run a modern fridge, we divide 230 kilowatt-hours by 2.46 kilowatt-hours. That gives
us 95 kilograms of coal per year. If we now add a television, computers, lighting, heating,
an electric cooker, a vacuum cleaner and other electrical appliances, we would need a large
additional room just to store the fuel needed to power our appliances and heat our homes.
Most electricity is generated in power plants using coal or gas. So, figuratively speaking,
we all have our storehouse of coal, just not inside our homes. But what if we switched
to renewables? A house provides a ready-made base for one of the most promising of
small-scale renewable sources, solar photovoltaic (PV) panels. A modern domestic PV plant
in a temperate climate generates an average of 150 kilowatt-hours per square metre per
year. This means that a single 1.5 square meter PV panel can provide sufficient energy to
run a modern fridge. Twenty panels would meet the electricity needs of a typical European
household.
