Taking control of HID lighting

Faced with the need to reduce energy consumption wherever possible, schools can take advantage of new technologies to dim HID lighting. David Barnwell of Holophane explores the benefits

Cancellation of so many new build and refurbishment projects planned under the Building Schools for the Future (BSF) initiative has left many schools needing to reduce energy consumption, but unable to achieve this through major capital projects. These schools and those that were not included in the BSF programme now need to look at other, lower cost energy-saving options.

In many cases one of the obvious quick wins with a relatively low investment is to upgrade the lighting and, indeed, many schools have already taken such measures with the fluorescent lighting used in internal spaces. This has been relatively easy because dimmable fluorescent lamps and control gear have been available for several years.

However, there are often spaces with high ceilings in schools, such as sports halls and multi-purpose halls that make use of high intensity discharge (HID) light sources such as metal halide and high pressure sodium. Until recently, the options for controlling these have been severely limited because they could not be dimmed and once they have been turned off there is a delay before they can be turned on again. The result is that many of these are left on at full light output throughout the day, irrespective of natural daylight levels and occupancy.

Similarly, there are many exterior spaces that make use of HID lighting, ensuring good visibility in car parks, entrance and exit routes and pedestrian walkways between car parks and buildings. Again, these have tended to lack controllability and can potentially waste energy.

As a result, even schools that have introduced measures to increase the efficiency of lighting may have restricted those measures to fluorescent lighting and not addressed the HID lighting.

In all of these cases, the potential for wasting energy has increased as schools have become more closely integrated with their local communities. With school facilities being made available in the evenings for local people, both exterior and interior lighting are left on for longer periods, thus exacerbating the problem.

Fortunately, new control technologies combined with HID electronic dimming ballasts have opened the door to optimising the use of HID lighting in a wide range of situations. These work by enabling the lighting to be dimmed as and when necessary and field trials have shown that energy savings can be as high as 65% in certain applications compared to uncontrolled HID lighting.

The advantage of dimming is that the lighting can be ramped up again immediately, thus avoiding the warm-up delay associated with HID light sources. For example, in a sports hall or multi-purpose hall there may be extended periods during the day when the space is not in use at all, or perhaps just part of the space is in use. Here, if the entire space is empty the lighting can be dimmed throughout. Or, if the lighting is zoned, individual spaces can be dimmed while others remain fully lit. This control can be carried out manually through suitable switching, or the lighting can be connected to occupancy controls so that it happens automatically.

Similarly, many such spaces have roof lights or extensive wall glazing and receive high levels of natural daylight at certain times during the day. By linking the lighting to photocells, the HID lighting can be dimmed whenever there is sufficient daylight. In zoned spaces there is also the potential to dim different spaces as the sun moves around the building.

Another characteristic of such spaces is that they are used for a wide variety of activities that have different visual requirements. For example, sports activities that involve a fast-moving ball will benefit from high light levels, while lower levels will be adequate for less visually demanding sports and other activities in the space such as cleaning.

With the ability to optimise the lighting usage, the lighting levels can be altered to suit each activity, perhaps by using scene setting controls.

Cost of ownership

At the same time, it’s important to minimise the initial outlay, as capital budgets are very restricted. It therefore makes sense to use a system that minimises installation time and materials.

Obvious examples include use of a simple two wire bus system with one control cable per cable luminaire and one control cable for all control devices, thus reducing overall cabling costs. Similarly, any such system should be able to make use of standard containment systems such as trunking conduit, or any manufacturer’s busbar system, so that installers are not required to buy special, higher-priced materials.

Innovating with light

At a new high school in North Tyneside, the ability to dim HID lighting is put to a particularly innovative use, exploiting the unique design of the building. The building is an oval shape with a 22m high sports hall at the centre, surrounded by teaching and other areas. In order to achieve the required lighting levels, therefore, Holophane Prismatron high bay luminaires are mounted at heights of between 16m and 20m.

A further challenge was that it was necessary to be able to dim the high bay lighting to meet the head teacher’s desire to alter lighting levels during the day. This strategy was developed in the light of recent research showing that teenagers have different circadian rhythms to adults and are more lethargic and listless in the mornings.

Consequently, the luminaires are dimmed most of the time to give an illuminance of around 300-500 lux in teaching areas, but during the first couple of hours of the teaching day these are ramped up to give 1000 lux. Lux levels are also elevated during examinations.

Take it outside

Clearly, the ability to dim HID lighting can also be applied to exterior lighting so that, again, the lighting is controlled in relation to demand. For instance, during winter when pupils are leaving the school in the dark the exterior lighting needs to be on at full output to deliver high visibility and ensure maximum safety. This applies to both pedestrian areas and those for motorised traffic. Typically, staff will leave later than the pupils and not all at the same time, so the lighting of car parks and roadways could be dimmed to reflect the lower risk of accidents while still maintaining safe illuminance levels.

Later in the evening, there may be an event such as a play or a concert where large numbers of people arrive at about the same time, ready for the beginning of the performance. There will then be a period of a couple of hours where there is very little activity in the school grounds, before the mad rush when the event has finished. Dimming the lighting during this quieter period will maintain adequate illumination without wasting energy.

The use of dimming in this way simply follows the established best practice for other types of lighting, namely that the lighting of spaces should be aligned to the demand for light. The fact that this same principle can now be applied to HID lighting greatly increases the opportunity to extend energy saving initiatives to areas of schools and other buildings that have hitherto not been able to benefit from enhanced controllability.

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