An important concept in this discussion is exergy, or the quality of energy. This follows from the ‘laws of thermodynamics’. The first law states that energy can never be lost, that it will remain. The second law introduces the notion of quality: although energy cannot be lost it loses quality and entropy is created when used (google for exergy and ‘laws of thermodynamics’ to learn more).
Using resources – energy, water, materials, etc. – in an efficient way means using these resources while trying to limit waste, trying to do things in the right manner. The concept of ‘Trias Energetica’ will be used to explain and some examples will be given. Trias Energetica was developed by Lysen and Duijvestein (1997). It consists of three, consecutive steps:
- Limit energy demand and energy use;
- Use renewable energy sources;
- Use fossil fuels as efficiently and cleanly as possible to fulfill remaining demand.
The first step is the most important, because each amount of energy that is saved does not have to be produced. An example of this first step is to insulate dwellings properly so less energy is needed to heat the dwellings. Remaining demand should be fulfilled by applying renewable energy sources, like solar or wind. The third step talks about the efficient use of fossil fuels. For example, the introduction of fuel-efficient cars or even hybrid cars. These cars still need fossil resources, but they use these resources more efficiently, see graph 1.
Graph 1: Fuel-efficiency of some car types, gasoline use
It is important when looking at efficiency and energy-saving measures that they do not result in a rebound effect. So, e.g., changing non-efficient light bulbs with efficient ones (LED, fluorescent light bulbs) is a good measure. The pitfall though is that people leave the lights on, because they know it is more efficient, so they think that it does not matter. This results in the end in more energy use anyway.
For a true sustainable system, the Trias Energetica should be adapted. Measures to save energy may not result in loss of comfort or health problems. A system can only be sustainable if it uses renewable resources. The rate in which we use fossil fuels, is not renewable. The resources of oil, coal and gas are not regrowing. Another point to consider: is it nowadays cheaper to invest in more insulation or to invest in renewable energy production systems? The importance of re-use, re-cycle and manufacturing, keeping the end of products in mind, is also growing. Therefore, a new concept has been developed ‘the New Stepped Strategy’ (Dobbelsteen and Tillie, 2009). In this strategy, the last step is replaced:
1. Reduce consumption without loss of comfort and health;
2a. Exchange and re-use waste energy systems;
2b. Use renewable energy sources and ensure waste is re-used as food.
Applying those steps to a city or region towards sustainability can be seen as a sign of effectiveness. This means trying to use resources in the right way, trying to reach the result, to do the right things. Think with the result or purpose in mind and do not start from the means. For example, do you need your laundry cleaned or do we need the best, most energy-efficient laundry machine to clean our clothes? The question is ‘what is the most resource effective way to clean our clothes’ (we can come back to that another time)?
Reduce consumption still is the most important step. The New Stepped Strategy introduces also the importance of different scales, from dwelling level to neighborhood to city level. Before a decision is made, it has to be studied what is the most effective step to take at which level. Some things can be arranged very effectively at dwelling scale, like insulation, but others will be more effective at a larger scale, like cascading remaining qualities. An example is the remaining heat of a power plant or industry. Nowadays, it will be the remains after fossil fuel burning, but in the future it may well be the remains of renewable fuel burning or use. The idea is that the remaining heat of this industry is not a waste product, but can still be useful for another purpose. For example as processing heat for an industrial facility that needs only heat of lower temperatures. After use in this industry, it still has some heat quality remaining that can be used in, e.g. green houses. A last step can be heating of houses that needs only a low energy (in the form of heat) quality, see graph 2.
Graph 2: Example of a heat cascade in an urban system
So, in order to reach sustainable cities, it is important to reduce energy consumption and to apply the local available renewable and residual resources in an effective way. The urban metabolism has to evolve to a circular metabolism in which any waste product is seen as a remaining quality that can be used by another function within the city. This will decrease dependency on foreign resources. It will increase the search for local potentials and characteristics. Cities are multi-functional entities. The different functions should and need to be connected and in close proximity to effectively use the local potentials towards sustainable cities.
Dobbelsteen, A., van den, Tillie, N., 2009. Towards CO2-neutral Urban Planning: Presenting the Rotterdam Energy Approach and Planning (REAP). Journal of Green Building, 4.
Duijvestein, C.A.J., 1997. Drie Stappen Strategie. In editors D.W., Dicke, E.M., Haas, Praktijkhandboek Duurzaam Bouwen. WEKA, Amsterdam, pp. (20) 1-10.