Carbon Saved by Taking the Stairs instead of a Lift
Modelling lift behaviour can be quite a complicated and involved process. The physics involved depends on many different factors, like the type of lift, load capacity, the weight of the car, its speed, the power of the motor, how heavily it is used and so on.
There are also losses to consider due to the efficiency of the motor, the age of the lift installation and friction losses inherent within the mechanical system. However it is possible to get an idea of the amount of energy consumed by lifts (and therefore the carbon emitted) by using averaged data available from lift manufacturers. In order to make use of this data we have also incorporated some basic assumptions into our calculations.
However it will first be helpful to know a little more about lifts.
There are two main types of lifts: hydraulic which are used in low-rise buildings (less than 7 storeys), and traction lifts for mid to high rise buildings. Traction lifts can be geared or gearless. Geared systems are normally used in mid-rise buildings (between around 7 to 20 storeys), but tend to be less efficient than gearless systems, with smaller, less expensive motors. Gearless traction lifts are used in high rise buildings (greater than 20 storeys) as they have faster travel speeds (2-4m/s).
Hydraulic lifts are considered to be around 25% less efficient than traction lifts.
According to ACEEE (American Council for an Energy-Efficient Economy), there are around 700,000 lifts in the US, more than two thirds of which are hydraulic lifts. In the UK there are estimated to be around 250,000 lifts.
Lifts, once in, are usually in for the long haul and are replaced every 20 to 30 years. It is thought that at least half of the 250,000 lifts in the UK are over 25 years old. It is reasonably safe to assume that most offices are low to mid rise buildings (the majority will have fewer than 7 storeys) and so will use hydraulic systems.
A 15 second lift journey consumes as much energy as a 60W light bulb does in an hour. According to lift manufacturer Kone, a lift uses an average of 30 Wh of energy per start (where per 'start' is per door opening, so two starts per complete journey).
We are assuming that the average person might consider walking up to the 2nd or perhaps even 3rd floor, but would normally take a lift to go any higher. Hydraulic lifts are typically slower than traction lifts, travelling from around 0.3m/s to 1.0 m/s,
so we have taken a speed of 0.5m/s.
The average occupancy rate of a lift is 20% of the nominal load.
So that means for a 10-person capacity lift, the lift on average is delivering two people to their destinations. On to the calculations:
Let's assume that when you call for your lift in the morning, you are on the ground floor whilst it is on an upper floor delivering other passengers. Assuming one floor is 3m in height, at a speed of 0.5 m/s the lift will take 6 seconds to travel one floor. So travelling say 4 floors to get to you will take 24 seconds. If we assume you are also heading for the 4th floor, the whole round trip will take 48 seconds. Factoring in that there are passengers being delivered elsewhere, doors opening and closing, and the lift stopping to let passengers on and off, let's assume the whole process adds another 20 seconds. Your entire journey from calling the lift to being delivered to your destination therefore takes about 68 seconds.
As we saw earlier, a 15 second journey will use about 60Wh of energy. So for a 68 second journey, the lift will use (68/15) x 60Wh = 272Wh of energy = 0.272 kWh per journey
Using our carbon conversion factor of 0.527 kg CO2 emitted per kWh
, this is equivalent to: 0.527 x 0.272 = 0.143 kg CO2.
If we assume that in a day one person will use the lift 4 times (once each at the start and end of the day, and twice at lunch time when leaving and entering the building), the carbon emitted will be:
0.143 x 4 = 0.57 kg CO2 per person per day, or 0.6 kg approx.
Another method of determining energy usage uses daily energy consumption data. A study published by the technical team at EnergyIdeas.orgshows the average daily energy consumption of various traction lift systems.
Hydraulics use around 25% more energy than the mid-range Silicon Controlled Rectified (SCR) system. For a lift serving up to 5 to 10 floors the SCR system uses around 53 kWh of energy, so a hydraulic system would use 25% more than this:
Average daily energy usage of hydraulic system = (125/100) x 53 = 66.25 kWh
So on average it consumes 66kWh / 8 hours = 8.25 kWh of energy in an hour.
Which is 8.25kWh / 60 = 0.14kWh per minute.
So for a round trip that lasts 68 seconds, the average amount of energy consumed will be:
0.14 x 68/60 = 0.16 kWh
So the average amount of CO2 produced during your trip is 0.16 x 0.527 = 0.1 kg which is 0.4kg for 4 trips in a day.
So the CO2 savings from each method are around the 0.4kg to 0.6kg CO2 mark, giving us an average saving of 0.5kg CO2 by taking the stairs (or not taking the lift).
In reality as we mentioned earlier lift systems operate across a spread of energy values, and their energy consumption will vary according to the efficiency of the system, patterns of usage and the lift technology itself. However using averaged values as shown gives a reasonable indication of the carbon savings that can be achieved.
Standby Consumption
An elevator just on standby can consume 2kW,
which is about 10,000 kWh per year if left on for 5000 hours per year (this is a conservative estimate - if out of use for 16 hours on weekdays, 5 days a week and 48 hours on the weekend, this is actually 128 hours per week or 6656 hours per year).This would waste over half a ton of carbon just by being on standby. Standby consumption can account for between 25% to 80% of a lift's total energy consumption.
So there is a good argument for having fewer, better used lifts in a building, rather than a larger number just sitting around idle.
