The electrical consumption in most industrial and commercial facilities can easily account for 50 -75% of the total utility costs. Hence, special attention must be paid into evaluating electrical systems.
Electrical system audit cover the Lighting systems, Motors & drives and Power factor.
Lighting system audit
Lighting accounts for a significant portion of electrical energy, usually between 15% – 35% of the energy usage in commercial facilities.
Usually,the first area to focus on. As Iighting is the easiest system to measure, analyse and implement in the auditing.
Energy can be saved in a lighting system by reducing the illumination levels, improving lighting system efficiency, curtailing operating hours and by taking advantage of available daylighting.
Benefits of Lighting Improvements
In addition to saving on energy and subsequent cost savings.
Improving a facility’s lighting system efficiency comes with the following benefits;
- Lower maintenance – Replacement of lighting sources with longer lasting alternatives (for example fluorescent lasts 10 to 20 times longer than incandescent).
This means that fewer bulb replacements would be made, saving labor and costs.
- Lower cooling costs– Each kW of lighting energy reduced, results in a heat load reduction of 3,142 Btu/Hr. Thus if the lighting efficiency improvement reduces the demand by 3.5kW, the air conditioning load drops by one ton, or 12,000 Btu/Hr.
- Better productivity – Improving your lighting can help people see their work better, reduce eye strains and fatigue. Better lighting increases productivity.
- Increased sales and image – in commercial retail shops, better lighting systems help to sell merchandise better. And also used to accent the image the store wishes to project.
- Increased safety – Proper lighting systems and levels help illuminate dark or hazardous areas etc.
- Saves money
Lighting basics
Light is a form of radiant energy. So are ultraviolet waves, microwaves, x-rays and radio waves. The distinguishing factors are frequency and wavelength but only light is the only form of radiant energy visible to the human eyes.
Understanding the basics of lighting design is fundamental to the several ways and means of improving the lighting efficiency of facilities.
Measuring Light
There are two common lighting methods in use;
- Point by point method
- Lumen method
Point by point method
The point by point method, calculates the lighting requirement for the task in question.
This method makes use of the inverse-square law, which states that the Illuminance at a point on a surface perpendicular to the light ray is equal to the luminous intensity of the source at that point divided by the square of the distance between the source and the point of calculation, as shown below
Where
E – Illuminance in foot-candles
I – Luminous intensity in candles.
D – Distance in feet between the source and the point of calculation
Lumen method
The lumen method assumes an equal foot-candle level throughout the area. This method is used frequently by lighting designers because of its simplicity. However it wastes energy. It is the light “at the task” which must be maintained and not the light in the surrounding areas.
Lumen method is the Illuminance on a surface of one square foot in area having a uniformly distributed flux of one lumen. From this definition, the lumen method is developed as shown below
Where,
N – Number of lamps required.
F1 – Required foot-candle level at the task.
A foot-candle is a measure of illumination; one standard candle power measured one foot away
A – Area of the room in square feet.
Lu – lumen output per lamp.
A lumen is a measure of lamp intensity, its value is found in the manufacturer’s catalogue.
Cu – coefficient of utilization.
Which represents the ratio of the lumens reaching the working plane to the total lumen generated by the lamp. The coefficient of utilization makes allowances for light absorbed or reflected by walls, ceilings, and the fixture itself. Its values are found in the manufacturer’s catalogue.
L1 – Lamp depreciation factor. It takes into account that the lamp lumen depreciates with time. It value is found in the manufacturer’s catalogue.
L2 – Luminaire (fixture) dirt depreciation factor. It takes into account the effect of dirt on a luminaire and this varies with type of luminaire and the atmosphere in which it is operated.
The lumen method formula illustrates several ways lighting efficiency can be improved.
Faced with the desire to reduce the energy use, the lighting designer has four options to consider.
- Reducing light levels
- Purchase more efficient fittings.
- Provide light when needed at the task at the required level.
- Introducing control and reducing lighting loads automatically.
The multitude of equipment options to meet one or more of the above, permits the consumer and lighting designer to consider the trade-offs between the initial and operating costs based upon product performance (life, efficacy, color, glare and color rendering).
Some definitions
Lumens – the amount of light that a bulb produces. Unit of output; technically “luminous flux”
Foot candles – units of light density striking a surface. One footcandle is defined as one lumen uniformly distributed over one square foot. The footcandle is used to measure light levels and is measured with a light meter.
Lux – units of light density striking a surface. One lux is defined as one lumen distributed uniformly over one square meter. A lux is used to measure light levels and is measured with a light meter.
[Note: 1 foot candle = 10.752 lux]
IES ILLUMINANCE CATEGORIES AND VALUES FOR GENERIC INDOOR ACTIVITIES.
Illumination Engineering Society (IES) has specified and published recommended light levels for various tasks and areas. A partial list of these levels is shown below.
| ACTIVITY | CATEGORY | LUX | FOOT CANDLES |
| Public spaces with dark surroundings | A | 20-30-50 | 2-3-5 |
| Simple orientation for short temporary visits | B | 50-75-100 | 5-7.5-10 |
| Working spaces where visual tasks are only occasionally performed | C | 100-150-200 | 10-15-20 |
| Performance of visual tasks of high contrast or large size | D | 200-300-500 | 20-30-50 |
| Performance of visual tasks of medium contrast or small size | E | 500-750-1000 | 50-75-100 |
| Performance of visual tasks of low contrast or very small size | F | 1000-1500-2000 | 100-150-200 |
| Performance of visual task of low contrast or very small size over a prolonged period | G | 2000-3000-5000 | 200-300-500 |
| Performance of very prolonged and exacting visual tasks | H | 5000-7500-10000 | 500-750-1000 |
| Performance of very special visual tasks of extremely low contrast | I | 10000-15000-20000 | 1000-1500-2000 |
IES Recommended Light Levels
A-C – Illuminance over a large area (i.e. lobby space)
D-F – localized tasks
G-I – extremely difficult visual tasks
Contact us
Victor Oyedu, FNSE, FNIEEE, CPQ.
Power Quality and Energy Management Specialist.
Publisher at Afrienergyonline.com &
CEO, FullSpectrum Energy Solutions Limited, Nigeria.
