how energy efficient are led lights for sports fields

When we (those of us over 40) think of an LED we tend to think of a little glowing diode on a pc board or electronic toy and register the fact the LED draws very little power.  While this may be correct for a single diode, when we ramp things up 10W of power is the same whether you’re powering a halogen bulb or LED.

OK so where is this power-consumption advantage of LED then?  And will I not get any benefit from changing over my old 2kW Metal Halide sports lights?

The answers lies in a number of factors which have to work together to make LED effective.  The key elements that the manufacturer has to blend are the following:

  1. Identifying the application
  2. Selecting an LED
  3. Designing an optic
  4. Designing the heat sink or housing
  5. Control of usage and dimming

The first point to consider when designing a lamp is what the application is.

Designing for the application

In a warehouse, the size of the lamp is not an issue as the roof space is generally a void.  Because the lights are running constantly, the user is more concerned about the power usage or efficiency.  For this user a high lumens/watt ratio is ideal as it means using less electricity.  Lights are therefore designed to have a large surface area and use lots of LED’s. Although this costs a bit more initially, the pay-back in power consumption easily justifies it.

Whilst this is good for warehouse lighting, it is not necessarily so for sports lighting or vehicle lighting.  Having a large and heavy light at the top of a 25m sports pole causes problems with ‘sail area’.  Much like a ships sails, these lights will cause resistance to the wind and put a lot of strain on the pole.  For instance, if you’re wanting to change from Metal Halide to LED, the poles at your club may have been designed for holding 10 lights weighing 20kg/each with a sail area of 0.8m2.  These are then replaced with 10 LED lights weighing 30kg/each and a sail area of 1m2.  We don’t have to be mathematicians to realise that we could well have an issue on a windy day!  Likewise having a very ‘energy efficient’ 350mm-diameter driving light on your 4WD is not going to look very cool or be very fuel efficient.

So as we said, the application is very important when designing a light and there is no best-fit for all industries.

Selecting the LED

Selecting the LED for a specific application is a complex problem.  There are hundreds of options on the market from dozens of manufacturers, covering the spectrum from 0.2W up to COB’s which can be over 50W.  As a rule of thumb, the less hard you drive an LED the brighter it glows relative to the wattage you put in.  This efficiency factor is called lumens/watt. So for instance a 5W LED may be producing 100 lumens/watt at 5W but if you run it at 2W it will produce 130 lumens/watt (Lumens being the measurement of light output against how many watts are put in).  Although the overall lumen value is less, the actual efficiency (or light output to current-draw) is much higher. Therefore if you run 5W LED’s at 2W you will get a high efficiency (lumens/watt).

The LED however just produces an intense glow of light and in itself has no control. For this we need an optic or reflector which will guide the light where we want it to go.

Designing an optic

This step is done in conjunction with the choice of the LED as they have to work together and is the most critical aspect.  Each optic or reflector is therefore optimised for a specific LED and this combination will determine the energy efficiency and light control.

As mentioned above LED’s themselves just produce a ball of light.  Harnessing this and guiding it exactly we it needs to go is the work of the optic.  There is three main ways to do this a) Total Internal Reflection optic b) Chrome reflector c) Combination of both.  There are pro’s and con’s for all three and we won’t go into that in any depth here.  Sufficient to say this control of the light decides whether to beam is going to be 5° wide and go 1km down the road, or 100° wide for a wide flood etc.  Sports lighting uses a combination of beam patterns for different applications.  The most difficult has proved to be the asymmetric narrow beam needed for large, field based sports like baseball, football, soccer and cricket.  Most manufacturers have taken the easy route of having a round beam pattern in a set number of angles.  However in big-field sports this proves to be very inefficient and can require up to 50% more power to achieve the same result as using a 2kW metal halide fitting.  Given that LED’s tend to be more expensive than metal halide, this can result in the project costing a lot more than you perhaps budgeted.

So, when you’re looking into this, be sure to get an IES lighting plan done before committing to purchase.  Many manufacturers will do this for a nominal sum, or even free, and is very important to make sure that you can achieve the right lighting levels without overloading your existing poles or ending up using twice the amount of electricity.

Designing the housing

Essentially this should be the simplest part of the design but is still very important.  The LED’s are working away and the optics are putting the light where we need it but unless we remove the heat effectively, the light isn’t going to last for very long.  On the other hand if we space the LED’s out and make large cooling fins, it will run very cool but will be too heavy to replace metal halide fittings one for one.

Designers use special software to model heat build-up and ensure that there is a balance between too much aluminium and sail area (like a ships sail) and being too small and risk overheating. Many high-powered LED sports lights weigh 2-3 times that of 2kW metal halides but are no more efficient.  Care needs to be taken to ensure that the poles have been designed to cope with the extra weight and sail area.

Control of usage and dimming

With digital LED lighting more options are available to us to save energy.  App based controllers are now available to interface with almost any lamp and can be used for pay-as-you-play, dim for training, get energy usage reports and much more.  These systems can have a rapid pay-back and take a lot of hassle out of managing the day to day problems of responsibility and maintenance.  Your lighting supplier would either have their own bespoke system or will have alliances with third party providers to offer a range of packages from simple to very complex.

Conclusion

So, in conclusion if you are wanting to install digital LED lighting for the energy efficiency, make sure that you are able to achieve this as it cannot be taken for granted.  If you are unsure we would recommend investing a few hundred dollars in getting a third party to cross-check that the lighting layout provided is accurate, as once the lights are up it could be an expensive mistake.  Suppliers should be willing to provide the IES files so this can be done.  Also use a lux meter to test what lighting you currently achieve and ensure that you have come-back on the supplier if minimum standards are not met.