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T-5 fluorescent lighting and lighting economics

Kevin Janni, Professor and Extension Engineer
Published in Dairy Star November 10, 2012

Dairy producers have more lighting choices to consider when selecting new or replacement lights. Lights include the fixture, lamp, electronics and housing. Lighting economics becomes more complicated when comparing different lighting options (ex. light emitting diode (LED) vs. fluorescent vs. metal halide) because of different useful life estimates, initial costs and electrical energy use for producing similar lighting levels.

Proper lighting is important for optimum cow performance and providing a safe and pleasant work environment for workers. Fluorescent, metal halide, high pressure sodium and LED lights are being used by dairy producers in milking centers, animal housing and freestall barns.

Cows and heifers are commonly recommended to have 20 foot-candles of light and 16 hours of light per day for optimal growth and milk production. Workers need sufficient light for observing cows, performing cow care tasks and assessing cow cleanliness before milking.

New fluorescent bulbs and lamps

Water tight fluorescent luminaries are commonly used in dairy facilities because they are energy efficient and have good color rendition index values, 70 to 95%, with 100% being the maximum value. They also have useful lamp lifetimes ranging from 20,000 to 30,000 hours.

New smaller diameter and more energy efficient fluorescent bulbs, T-5, (5/8 inch diameter) are available. T-5's are roughly 10% more energy efficient than T-8 bulbs (1 inch diameter). In the 1990's T-8's replaced T-12's (1.5 inch diameter) because the T-8's produced over 50% more light per watt of electrical energy used. The 10% increase in T-5 lighting efficiency suggests that it may not pay to replace T-8's if the existing fixture works. T-5 bulbs have a smaller diameter and produce more lumens so they can cause glare if they are close to eye level. Lamp diffusers and proper placement can be used to reduce glare from T-5 bulbs.

Economic considerations

Lighting system economics becomes more complex as the number of barn designs, lighting options, light performance characteristics and initial and operating costs increase. For example, producers with cross-ventilated and tunnel ventilated barns with solid walls must provide the recommended 16 hours of light per day (5840 hours per year) using lights to meet animal needs. Producers with curtain-sided freestall barns can use photocells and timers to turn off lights when natural sunlight provides light. There are roughly 4450 hours of natural daylight per year when supplemental lighting can be reduced. For the following analysis, 2190 hours of supplemental lighting per year was provided assuming an average of 6 hours of supplemental lighting per day; more in the winter and less in the summer.

Barn size and lumen output per light impacts the number of lights needed, which impacts the initial capital and installation costs and electrical operating costs. Some lighting companies offer lighting design services to layout efficient and effective lighting systems.

Light characteristics that impact annual capital and operating costs include light output (lumens per luminaire), electrical power (kilowatt used per hour), fixture life, useful lamp life, initial cost, and labor costs to change lamps. For example, LED lights cost much more but have longer useful lamp life. Electrical rates ($ per kilowatt hour) are also important. The average rate for Minnesota is $0.11 per kWh.

An economic analysis was done to estimate annual capital and operating costs of different lighting systems for a 200-foot by 110-foot freestall barn. Four lighting systems were compared: LED, fluorescent, metal halide and incandescent. The analysis assumed that all systems provided at least 20 foot-candles over the entire barn, and electrical rates and fixture and lamp replacement costs had an annual 2% inflation rate.

Figure 1. Annual capital and operating costs and annual electrical costs for three lighting systems for a 200-ft by 110-ft freestall barn.

Results in Figure 1 show that 20 LED lights, with an expected useful life of 60,000 (60 k) hours, had the lowest annual capital and operating cost compared to systems with 24 fluorescent or 20 metal halide lights. Reducing annual light operating hours from 5840 needed in cross-ventilated barns to 2190 hours reduced annual costs by 47 to 56% depending on the lighting system. For comparison purposes incandescent lights, which are not recommended, had annual capital and operating costs over $19,530 for 5840 hours of light per year and $7,580 for 2190 hours of light per year.

The annual energy costs in Figure 1 are a bigger part of the annual capital and operating costs of the fluorescent and metal halide systems. The annual electrical costs for incandescent systems were $15,400 and $5,780 for 5840 and 2190 hours, respectively.

These results indicate that lighting costs are variable for different systems. Many assumptions were made. The assumptions impact the results so each design needs to be analyzed. Technology changes and changing prices for electricity and lighting equipment will continue to change the economics of lighting systems for dairy barns. More on lighting economics will be covered in future articles.

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