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                                   SOLAR TIME CURVES
(Comparative analysis, “How Autoswitcher is more efficient than Time Switch”.)

As the name indicates, these are the graphs of sunset and sunrise times plotted against the days of a year
at a given place. It hardly matters where the place of interest is situated, as it is intended to explain the 
working of twilight controller and the energy saved by its use. For the purpose of explanation, the graph is
plotted against these timings at Pune, India.

 

From January onwards, sunrise times are gradually decreasing and sunset times are increasing upto the middle
of the year resulting in longer days and shorter nights. The trend is reversed for the later half of the year making 
the nights longer and days shorter. Ideally, streetlight controller should follow these curves for its on and off action,
i.e. lights should be on as per sunset curve and off as per sunrise curve. It is also common observation that 
sufficient ambient light is available for a few minutes (about 15 to 20 min) just after sunset and before sunrise.
This is the real twilight zone indicated by the area between solid and dotted lines in each curve. AUTOSWITCHER
is designed to follow the dotted lines for its on off action.

Let us analyse the graph further. A timer type switch controller needs manual setting for its on and off times.
It will go on controlling the lights as per setting irrespective of Solar Time Curves. Hence, we have to set these
timings regularly, say, for practical purposes, every 3 months. To avoid the darkness in early evening and early
morning, we have to select the earliest time for sunset and latest for sunrise in a given quarter of a year. Accordingly,
on and off times graph for timer switch is drawn in dashed lines. Note the area bound by the STC and
its respective dashed lines of time switch is straight away energy saved by Autoswitcher as
compared to Time Switch.

CALCULATIONS:

To find out the energy saved by Autoswitcher in a year as compared to its working vis-a-vis
Time Switch, let us first evaluate the time saved on quarterly basis.

1. First Quarter, January to March:

Sunrise for Jan 1 is 7.14 AMand for March 31, it is 6.35 AM. And the sunset timings for the same period are
6.11 PM and 6.51 PM. Time Switch has to be set at 7.14 AM for its off time and 6.11 PM for its on time
so that the setting is valid for the entire quarter.
The time saved by Autoswitcher will be-
Off Time Savings: 7.14 - 6.35 / 2 = 20 Min/day
On Time Savings: 6.51-6.11/2 = 20 Min/day
Twilight Zone (dotted area in the graph) saving for On and Off is 15+15=30 Min/day
Total savings per day = 70 Min
Total savings per month = 70x30= 2,100 Min i.e. 35 Hrs/month

Total savings in a quarter= 3x35=105 Hrs

Similar calculations are carried out for remaining three quarters of the year and total times saved are
listed below.

2. Second Quarter, April to June: 103.5 Hrs

3. Third Quarter, July to September: 115.5 Hrs

4. Last Quarter, October to December: 115.5 Hrs

Total savings in time in a year: 439.5 Hrs

A typical feeder pillar in a city feeds, say 25 poles of 400W sodium vapour lamps, about 10KW of load.

Total energy saved in a year will be 4395 KWH.

Calculate the amount of money saved in a year and do not be surprised that the 
cost of Autoswitcher is recovered in a few weeks!

 

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