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Lighting Efficiency: Mercury Vapour vs High Pressure Sodium Lamps for Outdoor Lighting
Green Zone - Posted: 2011-03-20 18:08:15
Author: H.l. Williams

Mercury vapour lamps are popularly used for security lighting in Guyana. Years ago they were also in common use as streetlights. More recently, however, mercury vapour street-lamps are being replaced by the more efficient high pressure sodium lamps. This is not the case for commercial and residential security lighting; mercury vapour lamps remain in common use for this application. The main reason for this seems to be the lower initial cost for the mercury vapour lamps.

I once advised a businessman, who was developing an office building, to use high pressure sodium lamps for the security lighting instead of mercury vapour lamps. I explained to him that the sodium lamps have a higher initial cost, but the energy savings he will derive over the life of the lamps will more than compensate for the higher upfront cost. Several weeks later I happened to be passing the site, when I noticed there were a number of mercury vapour lamps around the building. I stopped and asked the contractor why mercury vapour lamps were used. He said that the mercury vapour lamps were G$14,000 each while the high pressure sodium lamps were G$30,000 each. So, the businessman decided to use the former.

It means that I did not convince the businessman that using the high pressure sodium lamps was the more economical option. I guess, as a businessman, only figures will convince him. I gave him the advice while we were standing at the building site; I did not show him figures to illustrate my point. Let us now examine the facts and figures to determine if indeed high pressure sodium lamps are more economical to use than mercury vapour lamps.

Information from Pro Lighting shows that a 175-Watt mercury vapour lamp has a light output of 7,900 lumens, while a 70-Watt high pressure sodium lamp light output is 6,400 lumens. Both lamps have a 24,000+ lighting hours life expectancy.

The 175-Watt mercury vapour lamp is the most common size used in Guyana. Let us assume that a building is using five 175-Watt mercury vapour lamps. The total light output will be 39,500 lumens (i.e. 7,900 lumens x 5). If the lamps are operated for 12 hours per day, the total energy consumption per month will be 315,000 Watt-hours (i.e. 175W x 5(lamps) x 12 hours per day x 30 days = 315,000 Watt-hours). The utility company charges for electricity consumption by Kilowatt-hours (kWh). Therefore 315,000 Watt-hours become 315 kWh by dividing by 1000.

Conversely, if high pressure sodium lamps are used, seven of the 70-Watt lamps will give a light output of 44,800 lumens (i.e. 6,400 lumens x 7). This is more light than is given by the five mercury vapour lamps. If these lamps are operated for the same amount of time, the monthly energy consumption will be 176,400 Watt-hours (i.e. 70W x 7(lamps) x 12 hours per day x 30 days = 176,400 Watt-hours) or 176.4 kWh.

So, by using seven 70-Watt high pressure sodium lamps instead of five 175-Watt mercury vapour lamps, we can use 44% less energy (i.e. 176.4 kWh as against 315 kWh) for security lighting. This will also translate into 44% monthly cost saving on energy for security lighting. Now let us examine the payback.

A commercial consumer with the Guyana Power and Light Inc. will pay G$69.82 per kWh. Accordingly, the monthly energy cost for the five mercury vapour lamps will be G$21,993.30 (i.e. 315kWh x G$69.82). But for the seven high pressure sodium lamps, the monthly energy cost will be only G$12,316.25 (i.e. 176.4kWh x G$69.82), providing a monthly saving of G$9,677.05.

At G$14,000 per lamp, five mercury vapour lamps will cost G$70,000. On the other hand, at G$30,000 per lamp, the seven high pressure sodium lamps will cost G$210,000. This is a difference of G$140,000. With the high pressure sodium lamps providing a saving of G$9,677.05 per month, the difference in cost can be recovered in 14.5 months (or 1.2 years). This means there will be significant savings over the remaining life of the sodium lamps. With a life expectancy of 24,000 lighting hours, the 12-hour daily operation of a high pressure sodium lamp means that the lamp could last for approximately 5.5 years.

I hope this would convince people that high pressure sodium lamps are more economical to use for security lighting than the mercury vapour lamps; they are more efficient, giving more light per watt. Why do you think the government is replacing mercury vapour street-lamps with high pressure sodium lamps? The main disadvantage of the high pressure sodium lamp, however, is its somewhat yellow colour light. This results in poor colour rendering of illuminated objects. Hence, high pressure sodium lamp use is limited to outdoor applications where light colour is not important.

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