Wednesday, 23 March 2011

Electrics and low-energy lighting

In terms of lighting, there are three factors to efficiency. First is the type of appliances used, second is the way that they are laid out, and third is the way they are used. In terms of appliances, LEDs and compact fluorescents are going to use less energy than incandescents and candles. If the light illuminates what needs illuminating to the degree it needs illuminating, then you will use less energy. Finally, if the lights are switched off when they are not being used, this could make a bigger difference than the first two points.

As well as the obvious energy efficiency benefits of low-energy lighting and energy-efficient electrical appliances, in a highly insulated, airtight house, they will mean a cooler summer. Energy efficiency is a measure of how much energy comes out in the form you want it, for example how much power is coming out of a motor or how much light is coming out of a lamp. For example, the luminous efficiency of a candle is something like 0.04%.  In other words, for 10,000 units of energy, 4 of them will turn to light, and 9,996 will come out as heat. Next time you see somebody using candles and calling themselves green, bear this in mind. The rest of the energy is turning into heat, so inefficient electrical appliances mean more heat in the summer. In the winter, this may not be a bad thing, and in fact having a few lamps with incandescent bulbs will make very good back-up heaters for those really cold winter nights.  

Incandescent lights are over 50 times more efficient than candles. Facilitated by Edison's flair for publicity, that's why people stopped using candles and switched to electric light (image of light bulb coming on above head). They are still only 2%-5% efficient, so of those 10,000 units of energy, around 200 are now coming out as light, 9,800 are coming out as heat. The hotter the filament, the more efficient the light is, so very bright lights, such as projector lights, reach 5%, while most bulbs are around 2%. Halogen bulbs are not much more efficient than incandescents. 

Fluorescents light and LEDs are similar in luminous efficiency, around three times better than incandescents. The best fluorescent seems to be Panasonic's spiral tube, reaching 18% The theoretical limit for LED efficiency is up to 40%, and the most efficient currently being manufactured are around 20% efficient.  So now, 2,000 units of energy are coming out as light, and 8,000 as heat. These "lights" are still working better as heaters than illuminators.

For reference, the sun has 12% luminous efficiency.

The advantage of LEDs over fluorescents is that the light is all pointing in the same direction, which is often down rather than into fittings, walls or ceilings. This can also be seen as a disadvantage but generally it makes LEDs two or three times more efficient than fluorescents. LEDs are also smaller in size, use fewer materials and are simpler to produce so ultimately should be cheaper to manufacture, and they are likely to take over the market.

This leads onto the design of lighting. Generally speaking European lighting, with spotlights and ambient lighting is more efficient by design, while in Japan gains made by use of energy efficient fluorescents are partly lost because everything is bathed in bright light. Work surfaces, such as desks or kitchen tops, need bright light to be able to read or to distinguish between carrots and fingers on the chopping board.  Ceilings, walls, stairwells and corridors generally don't need as much light, so a design based on efficiency would use modest light in corridors, so we can see where we are going, use some ambient light on walls and ceilings, so we don't feel we are in a cave, and focus bright light around chairs, desks, tables and work tops. Flexibility is needed as we don't know exactly where the desks or chairs will be, so adjustable spot lights seem like a good idea.

One of the energy efficiency features of the Panasonic spiral tube is that it will change its brightness depending how much light is in the room. In bright daylight, it will turn off; dim daylight it will come on weakly and at night it will come on fully. As well as light sensors, use of motion sensors, for example in entrances, storage spaces and toilets, will likely stop lights being left on and save electricity. In terms of design and installation, they should also remove the need for light switches on walls, so may be cheaper. This remains to be seen!