Solar energy in public transportatio

As there have been many projects trying to use solar energy as a means of powering signs and other information systems, we decided to summarize the facts about and the possibilities of this type of power supply in this article. We also list the failure possibilities of this power supply. Our goal is not to define ways of calculating and dimensioning of such appliances but to point out the (in)appropriateness of this solution which is the reason why there is a certain “leeway” to the calculations listed in this article. However, the results correspond with our experience.

The article points out the properties of incident solar radiation and the possibilities of actually using it to power devices and connected accumulator usage.

One of the examples of a typical application is running an independent OAS 130 B-N beacon powered by a solar panel of “appropriate” size.

Properties of incident solar radiation

Facts about incident solar radiation:

1. The input power of solar radiation received by the surface of Erath’s atmosphere is about 1360 W/m² when the surface is in the perpendicular angle to the radiation. Based on this a “Solar Map” is created (Author: SolarGIS © 2011 GeoModel Solar Ltd.., CC BY-SA 3.0, source: https://commons.wikimedia.org/w/index.php?curid=15360397), which clearly shows the amounts of energy that are received by individual regions in a year.
2. Only about 17% of the energy of solar radiation can be successfully converted to electric energy by using photovoltaic panels. This constant has to be multiplied by the incident solar radiation.

Air pollution, cloud cover, altitude, morning mist and the inclination of a panel can all affect solar radiation and lower the amount of usable solar energy.

As we do not intend to produce energy to sell it but to power an autonomous device (e.g. a stop panel) we have to concentrate on other solar radiation properties:

• Direct radiation – solar radiation that is not reflected or absorbed and radiated again while going through the atmosphere. Its input power comes during sunny days. Full panel performance can be expected at such times.
• Diffuse radiation – Solar radiation that reflected from particles contained in the atmosphere (water, dust) and changed its direction. The wave length of this radiation remains the same as before its reflection. The amount of diffuse radiation depends on cloud cover and atmospheric pollution. The listed factors lower the amount of direct radiation and the amount of the produced electric energy.

This chart shows what is the ratio of sunny ad cloudy days in the course of a year and what amount of energy they provide. However, it does not show its time distribution during a month i.e.  in what interval sunny and cloudy days alternate.

This chart can be supplemented by another chart showing the number of sunny hours during a month (in this case in Brno):

This chart clearly shows that e.g. in December there are only 37 sunny hours with direct radiation in Brno. This means that in December there are approximately 5 sunny days and the rest is cloudy. This chart says nothing about the distribution of sunny days i.e. whether it was “nice” 5 days (direct solar radiation) in a row and the rest was cloudy (diffuse solar radiation) or the sunny hours came during partly cloudy days. If we go back to the chart no.3 we can see that these 5 sunny days “supplied” about 25% of energy during that month. The last solar radiation chart shows the progress of the impact of solar energy in the course of one day in various months. It clearly shows that most complex month in regards to solar power supply is December when the maximum possible supplied energy is about 22% of the energy supplied in June.

Pic. no. 5 describes the duration of total intensity of solar radiation on a horizontal surface with pollution coefficient Z=3 for an area of  50^O latitude. The individual curved lines show a characteristic day of a month during sunny weather. The dashed line depicts the duration of the intensity of diffuse radiation in June and December. Diffuse radiation in clear weather (from the point of view of the definition it happens always) is usually not higher than 100 W/m² which is about 10% to 15% of the total radiation.

Typické hodnoty intenzit celkového záření
Modré nebe	800 – 1000 W/m2	difúzní podíl 10%
Zamlžené nebe	600 – 900 W/m2	difúzní podíl až 50%
Mlhavý podzimní den	1002	difúzní podíl až 100%
Zamračený zimní den	50 W/m2	difůzní podíl až 100%
Celoroční průměr	600 W/m2	difúzní podíl 50-60%

Possibilities of using solar energy

If we work with the assumption that the device powered by solar energy will work year-round we have to concentrate on powering the device in December when there are the fewest sunny days and the shortest day in such a way that prevents device failures. We also have to consider the 17% efficiency of converting solar radiation to electric energy (other factors – e.g. the inclination of the panel – are not taken into consideration for the reason of simplification).

The following table lists possible input powers at different times of the day and for various types of radiation (direct or diffuse) for a 1 m² panel in December:

typ záření	Denní hodina											energie celkem*)
	8	9	10	11	12	13	14	15	16	17	18
přímé	0	4,25	14,88	25,5	40,38	42,5	38,25	23,38	12,75	1,275	0	180Wh
difúzní	0	0,85	2,975	5,1	8,075	8,5	7,65	4,675	2,55	0,255	0	34Wh

*) this estimation depends on the type of panel and its ability to process diffuse radiation.

The table shows that in December when a 1m² panel is used the supplied energy is 180Wh on a sunny day and 34Wh on a cloudy day. The input power has to be recalculated for different panel sizes.

In other words, if a solar panel is used the device powered by this panel can have maximum average input power – 1,4 W (in this case we do not consider losses in energy converters or the position of the panel (see pic. No. 2) and losses during accumulator charging) which is the supplied solar energy divided to cover 24 hours. The minimum accumulator capacity has to be sufficient for 26W which in the case of e.g. a 12V accumulator requires the minimum capacity of 2,2 Ah. If this would not be the case the device would fail because of insufficient energy.

This means that solar energy is not suitable for continuously powering LED or LCD panels at stops with the exception of LCD based on e-paper, however they still have to work in a “suitable” mode. However it can be used e.g. for powering command receivers for the vision impaired OAS 130 – see the first picture.

One of the ways to reduce this is to have a larger accumulator that would be able to “cover” the lack of energy on cloudy days. In the case of e.g. common lead accumulators we have to consider their life cycle. The capacity should be determined based on it:

• When 100% depleted                    – 250 cycles
• When  80% depleted                     – 350 cycles
• When  50% depleted                     – 550 cycles

E.g. when a 12V accumulator with 12Ah capacity is used the maximum depletion depth will be about 17% which would ensure a long lifespan.

An example of covering lack of energy – if we state that in December there will be one sunny day in 15 regular days (diffuse radiation) we get the energy:

E= 14*34 + 1*180 = 656 W supplied in 15 days,

This makes it possible for consumption to be increased to 1,8 W from 1,4W while the minimum accumulator value is 44 Ah. If we require higher device power consumption the accumulator capacity has to be increased significantly during winter months.

Conclusion

A solar battery powered device that has to work permanently has to be a low input power device or the operator of the device has to count with the fast that there will be device failures in winter months.

Bachelor thesis  „SOLÁRNÍ ZÁŘENÍ V LOKALITĚ VUT FSI V BRNĚ“ by Josef Horváa, supervised by  doc. Ing. Josef Štětina. Was used as a source.