We must install the photovoltaic street-lamps if we love the Mother Earth.
Renewable energies: solar energy, aeolian energy, photovoltaic systems, energy drawn out from hydrogen. In these days, especially due to the cost of the petrol, very often we listen to people talking about new energy sources. Italy is moving the first steps in this area and, as a consequence, is behind the other European countries. Italy, for its geographic position, could exploit the solar energy very well: better than Germany and Sweden that have done the energy saving their hobby-horse. Often, unfortunately, we see out of order street-lamps, sometimes whole roads are in the dark. So, why do not use photovoltaic street-lamps? They are:

- Inexpensive
- Easy to install
- Do not pollute and respect the Kyoto protocol
- Easily movable

Moreover:

- No light bill
- They work even during a black out situation
- Very little maintenance

A street-lamp breakdown does not influence the functioning of the other street-lamps

The photovoltaic solar lighting system has some peculiarities:

- The energy is got from the sun only
- It does not need any operator presence
- It does not need to be connected to the powerline
- The system is greatly reliable

Overall vision

The photovoltaic street-lamp is an isolated system where the photovoltaic modules capt the energy and load the accumulator. The energy is used to supply a high efficiency lamp during the night. The number of the required lighting hours and the lamp power are the factors that determine the surface size of the photovoltaic modules : due to this a good solution would be using modules equipped with solar pursuit devices. This solution would allow the lamp to last for a longer time or to reduce the photovoltaic modules surface that should be installed at the street-lamp top. Generally the lamps, used for photovoltaic street-lamps, use sodium vapors at low pressure (also called SOX). They emit monochromatic yellow light and they have a very high efficiency. In a photovoltaic street-lamp the efficiency must be at the maximum level because the available energy is limited and it must be used at the best in order to reduce the system cost or to increase the winter night lighting time. Also both the driving electronic circuit of the lamp (ballast) and the charge regulator that supervises the accumulator recharge must have a very high efficiency and the timer programming also is a key point for efficiency.

Technical description

Generally a photovoltaic street-lamp consists of:
- Photovoltaic module(s) with a variable total power from 80 Wp to 250 Wp.

Photovoltaic module

- Electronic control unit for the system control, programmable according the maker specifications. It has a switch programmable timer and a switching system with micro-controller for lead accumulator at 12V.
- Accumulator (12 V) with variable capacity from 100 Ah to 150 Ah, according the night lighting request. These accumulators are dedicated for the photovoltaic systems and they do not need a specific maintenance.

Accumulator

- Airtight ceiling fitting with sodium vapor lamps at low pressure (18W - 1.800 lumen, 26W - 3.600 lumen or 36W - 5.800 lumen). These lamps lifetime exceeds 8.000 working hours, the luminous cone has 9/10 meters of diameter and they can be oriented in every direction. The lamp is placed approximately at an height of 6 meters and generates on the surrounding ground a luminous flux of ~ 5.5 Lux. The lighting values expected from C.I.E (Commission International de L'Eclarage) are equal to 5-10 Lux for the roads with a medium traffic, equal to 5 Lux for the roads with low traffic.

- Metallic post with circular section to support the photovoltaic module(s)
- Metallic arm with circular section to support the ceiling fitting.
- Metallic holder to contain the control unit and the accumulator in case they are fixed at the post top

Control unit inside the metallic holder

Below there is a generic scheme of a street-lamp.

The electric energy production is assured from the photovoltaic module(s). Every street-lamp is supervised by a control unit that manages the accumulator recharge and at the automatic activation of the lamp, which switches on at twilight and switches off after a predetermined time (anyway according to the timer regulation). Lamp, photovoltaic module(s) and control unit are always fixed over a steel post of about 7 meters (~ 1 meter is in the basement). The accumulator can be also fixed as the other components (see the above drawing ), but it is better to fix it underground, in a suitable holder. This second solution saves the accumulator from big thermal excursions, moreover sheltering it from winter night low temperatures that can reduce its real capabilities hence decreasing the street-lamp lighting time. What we have shown above is a standard system, a further improvement has been done from the Western CO. This company has designed and produced a transceiver radio system, placed on every street-lamp and a radio interface, enough for 199 street-lamps, to monitor the electric parameters, the parameters setting and the time scheduling: everything has done staying at the post base, without climbing it. In this way a periodic control allows to set the parameters according the whether conditions and the costs are significantly reduced.

Project tentative

To conclude this subject let's try to project a street-lamp. Considering that the luminous cone at the base of a street-lamp has approximately an area S=75 m² and a total utilization factor K included between 25% and 50%, it comes out that the real utilized flux is included between 1/4 and 1/2 of every street-lamp nominal flux Øn. So it is possible to calculate the lighting medium value Em by the formula:

Using lamps of 18W (1.800 lumen) we get a lighting between 6-12 Lux that it is enough for our purposes (in the "Technical description" we have told that the expected lighting value for roads with medium traffic is 5-10 Lux). Using lamps of 26W (3.600 lumen) we get a lighting between 12-24 Lux, with lamps of 36W (5.800 lumen) we get a lighting between 19-38 Lux.

N.B. There is not a direct connection between Watt and Lumen because it depends on the source. An incandescent light bulb and a fluorescent light bulb that absorb the same electric power (P) have definitely different luminous fluxes (F): they have a different efficiency E=F/P. The incandescent light bulb has approximately 10 lumen per watt, the fluorescent one approx. 80 lumen per watt.

DEA/SOX Photovoltaic street lamp