Feasible and Practical Solar Street Lighting

I was asked recently if it was possible to use solar power in street lighting. As always new developments in technology makes what was once not possible feasible. In this case developments in high intensity discharge and led bulbs that use less power and stll provide equivalent luminousity as standard street lamps makes it feasible. However is it practical ?

Practicallity

1. A 20 watt HID or LED lamp will require 200 watts hours (Wh) for 10 hours of lighting.  So we need solar panel or panels that can produce more than 200 Wh a day. The next problem is to mount this panel or panels on pole we attach the street lamp to. Here the considerations of the height. strength of the pole and effects of wind sheer on the panels have to be taken into account.

2. On location where the panels are sited is vital to it's effectiness in producing power. If it is sheltered or shadowed these factors must be taken into account.

3. Next problem to solve is power storage. As sunlight is variable from day to day there will be days when power produced is less than the 200 Wh required. On these days we will have to depend on stored energy on days when we have excess to make up the slack.

4. A 40 ah battery stores about 500 Wh. If the worst day power production is 100 Wh this battery should provide enough for 5 days of uninterrupted lighting. Of course larger batteries will provide more uninterrupted days.

5, The problem with larger batteries is that they have to be mounted on the pole. A mid pole mount brings into question aestatics and wind sheer factors. In ground mount is not practical because of damp and flooding risks, An in pole mount is best but limits the size of batteries that can be used.

6. Instead of lead acid batteries we could use lithium ion. However a 40 ah 12 volt lithium ion battery would cost $1200 compared to a restored and reconditioned lead acid battery at $60.

So we see that what is feasible is not always practical. The ability to solve the practical problems is what divides those who succeed and the also ran.

Why amorphous panels perform well in low light

Multi-crystalline panel operating voltage is between 14 - 18 volts. If the voltage falls below 13.5 volts charging voltage, the battery cannot be charged because of back emf. Energy produced is dissipated by the controller, ie wasted.
Amorphous panel have peak operating voltage of 62 volts. As voltage drops in low light the voltage output it is still well above 13.5 volt. Energy produced can still charge the batteries.
Solar panel efficiency itself should not be the indicator when choosing panels. More important is the total useful power produced.

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Setting up a grid tie solar system

Solar Panels
Flush Mount Racks
Cables & Clips
DC Combiners
AC Dc Disconnects
3000 Watt Sunny Boy SB3000 Inverter

1. Solar panels
BD40 amorphous 48 volt amorphous panels, intended for grid tie systems, often have much higher voltages to accommodate the voltage windows of grid tie inverters.

2. Racking/Mounting System for PV modules
The mounting system for the PV modules includes the hardware to permanently affix the array to either a roof, a pole, or the ground. These systems are typically made of aluminum and are customized to the mounting surface and the model of module used. It is important to consider distance from roof for flush-type roof mount installations. Restricting airflow under the modules results in higher module operating temperatures that reduce power output.
With pole mounts wind loading must be considered and proper civil works must be done with the foundation for the pole as well as the possible addition of supplementary wind supports for the array frame. The cost of a mounting system can vary drastically based upon the number of modules and type of mount.

3. Combiner Box
A combiner box is an electrical box where series strings of PV modules are then spliced in parallel. This is also the place where the PV series string fuses or circuit breakers are located. This allows the installer to bring the separate strings together and combine them into one positive and one negative conductor, change wire types and leave the area of the modules in conduit. They are usually outside and weather rated, so they can be right next to the array.

4. DC and AC Disconnects
The DC and AC disconnects are manual switch units that are capable of cutting off power to and from the inverter. Some inverters have disconnects integrated into the unit with switches, others can have them integrated into a power panel assembly, and some inverters leave you on your own to provide suitable disconnecting means. The disconnects are used by service personnel or authorized persons (fire/police/electric workers) to disable power from a renewable energy system (in this case PV) so that there are no live electrical parts associated with the inverter, and that no current is going to the grid that could harm utility employees in the event that they are working in your area. Homeowners or authorized personnel can use the disconnects to de-energize a system for maintenance or service.

5. Grid Tie Inverter
The grid tie inverter is the device that takes the energy that the photovoltaic system produces in DC current form and turns it to AC current that is then sent (sold) to the electric grid. These inverters typically have a voltage input range from 100 to 500 volts DC and they convert it into 120 volts AC, 240 volts AC, or 208 volts AC. These inverters are especially sophisticated devices that must conform to special regulations in order to tie into the utility. When the power goes out in your area, it is important to know that the grid tie inverter will not allow power to be sold back to the grid. This is done to prevent electric utility workers from being injured or killed by working on power lines they thought were de-energized. When the power goes out, your power will go out as well. A grid tie inverter will not resume normal operations until the utility grid has established standard conditions for 5 straight minutes within strict parameters.

6. Cables, Connectors and Brackets