# Energy

A reliable energy provision will be one of the central challenges in the 21st century. Even though conventional sources, such as oil, gas and coal, will continue to play a considerable role, renewables will grow significantly. In the absence of large energy storage facilities, renewables provide a much more volatile energy source. This is likely to pose major challenges for our energy supply system. Here, we collect a few figures. A more detailed discussion can be found in David MacKay's book Energy - without the air .

## Units

In different contexts energy is usually described by different units, which makes comparisons difficult. All numbers on these pages are quoted in the unit kWh (kilo Watt hour), which should familiar to everyone from electricity bills. It is derived from the power unit Watt [W], which is defined as energy/time (1W = J/s, Joule [J] is a typical energy unit). 1 kWh is the energy used by a device with the power of 1000 W for one hour, for instance, a tumble dryer with 3kW running for 20 minutes. A common unit in physics from mechanical work is a Joule [J], $1J = 1 kg m2/s2= 1Ws$. Another well known unit from food consumption is the calorie, 1kcal = 4.2 kJ = 0.0012 kWh. A few useful examples for comparison are the following:

• Energy content of one litre of petrol: 10kWh
• Charging a phone or tablet (5-10W) for an hour: 0.005-0.01kWh
• Boiling water in a kettle for 20 minutes: 1kWh
• Typical food energy intake for a man in one day (2500kcal): 3kWh
In Energy - without the air the power consumption is analysed in terms of the average amount of kWh per day per person.

## Energy consumption

According to enerdata the total annual energy consumption in 2015 was roughly 13500 Mtoe = 1.57005e+14 kWh= 15700 TWh [The figures here are somewhat lower]. Assuming 7 billion people this corresponds to 61.5 kWh per day per person. It is generally higher in Europe (roughly 125 kWh/d see here) and North America (250kWh/d) and lower in many parts of Africa and Asia. The world energy consumption has been stable in the last 2 years. It is generally the questions, from which sources this energy can be provided. According to figures of the International Energy Agency (IEA) the energy sources are roughly split about into 40% oil, 15% gas, 11% coal, 12% biofuels, 18% electricity, 4% others.

A long distance flight with full occupation consumes roughly 3L/100 km per seat according to this . Thus a one-way flight from London to New York (5,585 km) amounts to 167 litres or 1670 kWh per person.

Two web pages which allow you to estimate the electricity consumption of household appliances can be found here and here.

## Renewable sources

Eventually, renewable sources will have to account for energy supply. In order to harvest large amounts for energy from, e.g., wind and the sun, large areas are needed. Current figures are:

• Solar PV (10W/m2)
• Wind (3W/m2)

In order to replace a large electric power plant with output of 1GW, solar PV needs an area of 100 km2. This is a bit less then half the area of the whole city of Frankfurt in Germany. Equivalently, for wind and area of 333 km2 is required. This implies that power generation will likely compete with other land uses, particularly for countries with relatively high population density. For instance, the inverse population density in the UK is 4000m2 per person.