The design of the trailer has had several revisions since the start of our project. The initial plan was to use flexible solar panels that can easily be rolled out of the trailer:
The first design (end-2010), based on Ascent's flexible solar modules.
However, there were a couple of issues with this design though, the most important being that no electric cars are rated for towing yet. Although EVs have great torque and are therefore excellent at towing, it would shorten their range even further. Perhaps in the future more EVs will come out that have been tested for towing, etc. Besides this constraint, Columbia wanted us to have a (gasoline) backup vehicle, in case we get stranded in a place like Death Valley in the middle of the summer!
This left us with one way out: we would have to tow the trailer with a conventional car. Unfortunately, the idea of being completely autonomous did not work any longer, but the primary goal of educating the public about solar and EV technologies could still be reached. So, the design changed from using flexible, lightweight modules to heavier First Solar modules:
Final design (2012): two CAD (Computer Aided Design) models of the setup. The sheets of modules roll out as drawers from a desk.
The trailer is purchased from Kaufman, a company that specializes in manufacturing custom trailers. It is 20ft long and 7ft wide, giving us plenty of room to store the solar array. While driving, the panels are stacked horizontally in a system similar to that of a desk with drawers.
Special rails slide out of the trailer so that we can take the (twelve) sheets of six modules out easily. Once positioned, we connect the cables to the charge controllers and the sun does the rest of the work! Here is an outline of the electrical setup:
Outline of the electrical interconnections between modules and electrical components. Strings of two modules are connected to create an open circuit voltage of 122 Voc (each module has 61 Voc).
We are planning on using electrical components by Outback, this includes the inverter/charge controller, and maximum power point tracker. More details on the electrical system will be uploaded soon. A complete design and installation guide (with videos!) of smaller arrays can be accessed here.
So lets do some calculations (for those who are interested!). Per panel (~0.75 square meters), we get an average irradiation of 5.1 kWh per day (Go to "sizing your array" for more info). From this, we can convert approximately 0.68 kWh to electricity with our First Solar panels (13.5% efficiency). The current generated then goes through three more steps where tiny bits of energy are lost.
1. Charging the external battery: An extra battery on board the trailer serves as a buffer for when clouds block the sun. There is a loss associated with charging/discharging, which is approximately 10-15% of the energy that goes into the battery. This is often referred to as "round-trip efficiency". Not all generated electricity goes through this step, as some of it will go straight to the inverter. So from the 0.68 kWh generated by a single panel, we get around 0.64 kWh of electricity going to the next step: the inverter.
2. From DC to AC: the inverter. The inverter turns Direct Current into Alternating Current with an efficiency of 90-92%, leaving 0.58 kWh from the 0.64 kWh we had before. The electricity then goes to the Level 2 Charger...
3. The Level 2 charger makes sure the electric car battery is properly charged. The efficiency is very high (>99%) so hardly any energy is lost in this conversion step.
One panel gets us approximately 0.58 kWh of electricity going into the EV battery. 72 panels generate ~42 kWh of electricity going into the EV battery.
So how far can we drive on this? Electric cars typically get a mileage (not mpg, but miles per kWh!) of 4-5 mpkWh. The faster you go, the lower your mileage. We will take it easy and aim for 4.5 mpkWh. So with 42 kWh of juice, we can drive 42*4.5 = 189 miles. Knowing that there are less-than-average sunny days, we planned stops every ~180 miles. We try to maintain a low speed, as air friction is the main force the motor will be counteracting.
The EPA reports 0.174 kWh/mile = 5.7 mpkWh (plug-to-wheel) for the Tesla, or ~230 miles on a single charge. Owners of the Tesla get a lower range though.