Cycling and Showering

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Are cycle commuters, who take an extra shower, doing more damage to the environment than those who commute by car?

Big Ideas: 
  • The equivalency of energy used in radically different ways. Apples and oranges can be compared through the common currency of joules, kWh or grams of CO2 emitted.

Are cycle-commuters who take a shower doing more damage than car-commuters?

A recent letter to the Vancouver Sun claimed that commuting by bicycle was worse for the environment than commuting by car. The reason given was that if a cycle-commuter takes an extra shower at work (because cycling makes you sweaty), that burns the energy equivalent of a litre of gasoline, which is more than a car would burn on an average commute. Fact or rubbish?

The shower

A typical shower uses 30 litres (i.e. 30 kg) of water - 6 litres/minute for 5 minutes. You can check how much water your shower uses by timing how long it takes to fill a litre bottle and noting how long you actually spend in the shower. Water enters a building at about 10°C (the mean year-round temperature in Vancouver is 11°C, and it is a little less at the elevation of our reservoirs). A comfortably hot shower temperature is 40°C. Therefore we need to calculate the energy required to heat 30 litres (i.e. 30 kg) of water through 30°C.

The specific heat capacity of water is 4.2 kJ/kg/°C

Total heat = specific heat capacity x mass x change in temperature

Total heat =(4200 J/kg/°C)(30 kg)(30°C) = 3.8 MJ = 1.05 kWh

Let's assume your water heater is only 85% efficient, i.e. 15% of the heat leaks into the building, or goes up the flue. This means the shower requires about 4.5 MJ of energy.

The commute by car

The energy content of gasoline is about 36 MJ/litre, so our 4.5 MJ is actually equivalent to only 4.5/36 = 0.125 litre. This implies our Vancouver Sun correspondent was wrong by a factor of 8. However, there are other issues to be considered.

Uncertainties

  • A single litre of gasoline will only take an average car about 13 km in the city, or about 6.5 km each way, which is a short commute, even for a bicycle.
  • As for greenhouse gas emissions, a lot depends on how you heat your water. Hydro or nuclear electricity are the cleanest energy sources, but if you shower in the day, you may be causing your electricity supplier to import electricity from a distant coal-fired plant (which is the case in BC), which is the dirtiest energy source. Natural gas is the cleanest fossil fuel, at 190 gCO2 per kWh; it is cleaner than gasoline at 240 gCO2 per kWh. Coal is highly variable, but typically produces about 300 gCO2 per kWh1
  • In the summer, when cyclists get the sweatiest, the cold water entering a building is warmer than 10°C (at the time of writing, late July 2009, "cold" water was 14°C in Vancouver) and showers don't need to be so hot. Hence our calculation above is an over-estimate.
  • It is not unknown for teenagers (and non-teenagers) to take showers of more than five minutes duration. An analysis of gas usage in the author's home shows that in the summer months natural gas is being burnt at the rate of 60MJ/day. Some of this heats water for washing clothes and dishes, but if we assume 3/4 goes for showers, that is still 50 minutes of showers per day. This is consistent with observations made while waiting for a turn in the bathroom.
  • There is a non-zero environmental cost to delivering water, even in Vancouver where an enormous quantity of the stuff falls out of the sky. In many parts of the world, even the developed world, water is a lot more costly to come by.
  • It is possible to get from point A to point B on a bicycle without needing a shower.
  • Car drivers take showers too.

  The following student-made video was inspired by a debate about cycling and showering in Shaun Dychko's Physics 12 class at Point Grey Secondary School in Vancouver BC. Whilst we are in complete accord with the spirit of this video, we do not recommend that you ride your bicycle in the manner depicted therein .

 

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