First, not all energy is the same. It has different grades: the higher the grade, the higher that energy's environmental impact, the harder it is to generate, the more expensive it is to generate, and the more things you can do with it. For example, electricity (extremely high-grade energy) can be used to power a computer or drive a pump; heat energy (the lowest grade) can only be used to keep things warm.
Just as energy sources have a grading, so do building energy demands. At the bottom end, there is the need to keep rooms at a certain temperature, and at the top end are electrical appliances, which need high-grade power.
What's the payoff?
The advantage of thinking about energy in this way is that different kinds of energy can be matched with the most appropriate job. Specifically, the aim is to match the lowest possible grade of energy with the lowest grade of energy demand. This means that low-grade tasks, such as heating rooms, should be matched with low-grade energy sources, such as passive solar gain; whereas high-grade energy, such as that from photovoltaic panels, is only used for high-grade needs, such as powering computers.
For energy matching to be achieved successfully, buildings should be designed with grading in mind from the outset. What this means in practice is that designers will have more of an incentive to use natural daylight and to restrict the use of electrical lighting to night illumination.
The obvious benefit of matching grades is that it can dramatically reduce the demand for expensive, higher-grade energies of all types.
It questions the logic of using as much high-grade energy as we do to run pumps and fans to satisfy a low-grade energy demand such as controlling room temperature.
Energy grading reinforces the incentive for designers to use natural daylight and to restrict their use of electrical lighting
Energy matching also has an impact on the way we assess a building's energy efficiency.
Building energy consumption has been rising over recent years as a result of the increased use of mechanical systems, such as air-conditioning fans and pumps, that run continuously on electrical power. It can be argued that this increase is offset by a reduction in heat typically lost through the building's envelope, so that there is no overall increase in "delivered" energy.
But there's the rub. The term "delivered" energy assumes that each kilowatt is equal, but this is not so: 1 kW of electricity used to power a computer is not equal to 1 kW of gas used by a heating boiler to offset the heat lost through a wall.
Beyond the kilowatt
Strange as this may sound, the inefficiency of power stations means that 1 kW of electricity will put almost three times more carbon pollution into the atmosphere than 1 kW of gas (a fact that will be highlighted by the proposed changes to Part L of the Building Regulations, which suggest a change to the terminology for energy consumption from kilowatts to emitted carbon).
This problem has arisen because designers and architects have slipped into using watts and kilowatts as if they are complete energy measures, when they do not measure the usefulness of energy. The same is true for energy benchmarks and energy targets, which use kilowatt hours, on the basis that all kilowatt hours are equal.
Getting back to renewable sources of energy, the grading approach means that designers ought to satisfy most energy needs using solar collectors, wind turbines and other active renewable systems. Ultimately, it may allow us to achieve the "factor 10" reduction in the use of fossil fuels needed for long-term sustainability.
Postscript
Chris Twinn is an associate director of multidisciplinary consultant Arup.
