The electrical system is the most important system in the bus conversion - it powers everything except the actual movement of the bus. Here, we talk about some of the overall design choices for the electrical system- how we sized everything, and how we selected our components. If you would like more specific and technical information about putting the system together, check out the projects related to electrical in the construction tab. We wanted our system to be efficient, maintenance-free, and, most importantly, powerful. We anticipate that most of our time in the bus will be spent in moderate, relatively sunny locations. Our system will enable the bus to go off the grid for long periods of time, freeing it as much as possible from developed areas and allowing us to spend more time in nature.
First, general notes on energy for those who can't remember high school physics. A Joule (J) is a unit of energy. This can be thought of as your potential to do work. Raise something off the ground, and that system now has potential energy - there is the potential that the thing will fall, and by attaching things to this system, you can make other forms of energy. Think of a dam: the energy of a tall body of water is converted to electricity as it falls through the turbines. A Watt (W) is a unit of power, and its units are actually J/s. This is a measure of energy per time, or how fast your energy is being used or converted. To get some perspective on how big these units are, you can check out this video and this one. A joule is a fairly small amount of energy, so we usually measure energy in kilowatt-hours, or kWh. This is a weird form of measurement - if we multiply a watt by some time unit, we get Energy * Time / Time, or just energy. Intuitively, this makes sense. Multiplying the rate at which we use power by the time that rate is being used, we find total energy. One kWh is equal to 3,600,000 J. Insolation is a time-averaged number for the amount of power in the form of light that comes from the Sun. It is usually measured in energy per area, kWh/m^2. This number changes with time frame, latitude and longitude, and collector orientation.
Sizing the System
We need size our system. We want to make sure that, every day, we will be able to make from our solar panels the same amount of energy we consume. This is determined, more or less, by the amount of energy that is going to be used in the bus, location and time of year, and any other parameters that may be constrained. For example, we cannot put more panels on the bus than what fits on the roof. Insolation maps (two great resources from NREL are here and here) are important in sizing our solar panels. Latitudes closer to the poles have much less insolation; they also have more yearly variance with the seasons. We wanted our bus to be self-sufficient as long as possible, so we assumed that we would go no further north than about 45 degrees. We also made some other assumptions: the number of people onboard would usually be less than 2, that we probably would not be that far north during the winter, and that most of our heating during the winter would come from the wood stove. We assumed the efficiency for power generated by solar panels to the battery was 100%. MPPT chargers are usually about 95% energy efficient, so this represents an insignificant source of error in our ballpark estimates. We also assumed our solar panels would be flat, since they are rigidly mounted to the roof. Future improvements may include tilting panels, but to avoid always having to park the bus oriented East-West, the panels would have to tilt on two axes, which is somewhat complex.
Daily energy use was a little tricky to figure out. There are estimates available in various places online. According to the eia, An average American uses about 240 kWh a day (while the worldwide average is about 60 kW-h/day), which is a lot of energy. For perspective, our nominal battery capacity is 5.3 kWh. 240 kWh a day is too much, so what is our energy use? This can be calculated very simply. Find the typical wattage of appliances you use around your home, and multiply by the time you use them in hours. Add up these numbers for your total energy used, in kWh. You can check out our estimate in the open source tab; we arrived at about 1.3 kWh for one person for one day, and 4.0 kWh for 4-5 people per day using power carefully. Obviously, these numbers may change significantly with weather and type of use.
Keeping in mind our minimum daily power generation would probably occur at latitudes far from the equator during winter, we aimed for a minimum daily power generation of 2 kWh. Using the resources above, daily average insolation at about 45 degrees North during the month of February was between 2 and 3 kWh per m^2 per day. Solar panels convert energy at about 15% efficiency, so we can assume that we will get at least 0.3 kWh per m^2 in Michigan in the dead of winter. This means we need about 6.6 m^2 of panels to make back 2 kWh per day. To compare, those same panels in the summer in a place like California will generate almost 8 kWh per day.
Adding more panels to generate more power during the winter would be nice, but you lose deck space on the roof, solar panels cost a lot of money, and a large battery and large solar controller would be needed to take advantage of extra power generated in anything other than worst-case scenarios. It becomes a compromise between comfort, cost, and complexity. For the vast majority of situations the bus will find itself in, this system is more than adequate. Your mileage may vary.
Our 5 solar panels are 1.3 m^2 each, are 14.5% efficient, cost 48 cents/W, and get us very close to our target of 2 kWh/day. Small increases in efficiency usually mean disproportionately large increases in cost, but you do save roof space for the same energy production.
Sizing the battery ends up being much easier. We wanted to be able to run on minimal power (our estimate of 1.3 kWh) for 3 days with a worst-case scenario of no input from the solar panels (ie a rainstorm that blocks out an astonishing 100% of the Sun). Assuming 85% depth of discharge, our battery would have to have a rated capacity of 4.6 kWh. Even with worst-case supplemental power of 1.885 kWh per day, and a worst-case daily maximum use of 4.4 kWh (accounting for 10% losses and 4 kWh use per day) we still get 1.5 days of power from a full battery. If off-grid in demanding conditions is a greater priority, more batteries (and, for that matter, more solar panels) are recommended. We eventually decided on a 5.3 kWh battery, which would give us 2 days of power in the above worst-case scenario.