The processes which deliver energy from an energy source to the wheels of a vehicle can be broadly summarised by the ‘well to wheels’ energy flow. Obviously, this term has been developed with the crude oil supply chain in mind. A more generalised summary can be developed to include a wide range of fuel types, as indicated below.
As can be seen, any thorough energy analysis of a vehicle’s operation should start at the energy source and end with the desired forward motion of the vehicle. This ‘well to wheels’ efficiency is described here as ‘source to wheels’ and in addition to accounting for a vehicle’s drive cycle, it accounts for the entire fuel cycle.
Going one step further than a combined fuel and drive cycle analysis is the concept of a complete life cycle assessment. Energy and material costs do not reflect their entire social costs, and as a result, the process of life-cycle assessment is an attempt to look beyond the current prices of materials and fuels. In addition to accounting for processes during vehicle operation, it also includes vehicle manufacture, service, disposal and re-use.
To overcome the complexities of a complete assessment, a limited life cycle assessment can be conducted by considering only energy inputs and outputs. A complete energy assessment allows for the comparison of existing and emerging vehicle technologies whilst still accounting for the important issues of embodied energy, operation and maintenance, and waste management.
The concept of life cycle assessment is destined to become increasingly important as externalities gain significance to all stakeholders, and the component of resources used during vehicle operation diminishes. A Massachusetts Institute of Technology study titled On the road in 2020 notes that the embodied energy of present vehicles accounts for about 7% of life-cycle energy, whereas the same parameter for emerging vehicle technologies will account for up to 18%. This is based on the widely accepted notion that vehicles will become lighter in weight, and that vehicle power trains will become more efficient.
Load cycle efficiency is yet another way of examining vehicle energy use. The load cycle uncovers the efficiency of carrying out the intended task of transportation; that of getting something or someone from origin to destination. During a typical test cycle about 17% of the energy in fuel reaches the wheels of a vehicle (decreasing further if accessories are being operated), and considering 95% of that energy is used to propel the vehicle not the driver, the efficiency of actually transporting the driver is less than 1%.
This is an edited excerpt from Opportunities for Vehicle Integrated Photovoltaics.