One of my readers asked about inverter efficiency and suggested that it would be helpful to the Off Grid Ham community if I did an article about overall solar efficiency. Since that’s not the first time the question has been raised, I’ll accept the invite. In this two part series we’ll go through a hypothetical power system step by step and examine solar efficiency, then fit the pieces together to make the picture complete.
Changing your perspective of solar efficiency.
When you buy a car, or a new furnace for your house, or any large appliance, you carefully consider the energy cost and hopefully also the environmental impact of the energy that runs the device.
But you would probably care a lot less about energy efficiency if the fuel was free, virtually unlimited in supply, and emitted no pollution. Who wouldn’t want a dream deal like that? A lot of hams who are considering going off the grid with solar (and some who already have) incorrectly see it that way.
Although the fuel is free, there is a technical and financial cost associated with turning sunlight into useful electricity. We cannot simply overlook the distinction between a household appliance that consumes energy and a solar power system that produces energy. There are two exact opposite dynamics in play here, but the end goal is the same: To get the most out of the input energy.
The point of this little mind bender is to make you think less about the fuel source and more about the process of converting that fuel into something useful. The off grid ham should care about solar efficiency because of the expense and trouble involved with making that conversion, not because of the fuel supply.
The Law of Conservation of Energy.
When we produce energy, we are not really “producing” it. We are only converting it from one form to another. And every time energy is converted or is passed through a physical medium (such as a copper wire) or a device (such as a charge controller), some of that energy is lost, usually in the form of heat.
The catch is that energy cannot be “lost”. It may be turned into a form that is not useful, but it never just disappears. Those who were paying attention in high school physics class will recognize this principle as the Law of Conservation of Energy.
All the energy that ever existed since the universe was created is still out there somewhere. Energy truly is immortal.
What does this mean to the off grid ham?
For the purposes of solar efficiency, we theoretically can get all the free energy we want but our capacity to turn it into electricity is limited because of losses (inefficiency) in solar power systems. Our concern is how much it costs, in both money and technology, to make that conversion. Having an unlimited supply of energy is not helpful if too much of it is lost in the conversion process.
Solar panels: The sun funnel.
Current solar panels are in the 10%-15% efficient range. That doesn’t look too great on paper, but the sun is so powerful and solar panels are so inexpensive that you can give up 85% of the available energy and easily have plenty left to work with. And the future is looking better: The latest generation panels are testing in 21%-23% efficient range.
What you need to know: Solar panels have a fixed efficiency and there isn’t anything the radio amateur can do to improve it. We have to accept it for what it is. The good news is that panels are so inexpensive that it doesn’t really matter, and their efficiency is greatly improving.
Copper wire feedline: Don’t overfill the pipe.
The feedline that brings the DC power from the panels to the controller is often overlooked as a point of power loss. Unlike solar panels, the off grid ham has a great deal of control over the efficiency of the DC feedline.
The limitations of copper manifests itself on home solar power systems when the feed wires from your panels are too small for the current they are carrying. Energy that cannot be handled by the wire is turned into heat (per the Law of Conservation of Energy), and in a worst case scenario can be a fire hazard.
Solving this problem is simple: Just use the appropriate size wire. This circuit wizard from Blue Sea is a valuable tool for determining what size wire is needed for any application.
One way to increase copper wire solar efficiency is to increase the voltage. This will in turn decrease the current and help you get more out of your wire. Two 12 volt panels wired in series to supply 24 volts has more solar efficiency than the same panels in a 12 volt configuration.
What you need to know: There is no good reason to accept poor DC feedline solar efficiency. Always use the correct size wire and allow for future expansion.
Charge controller tradeoffs.
The charge controller is a common point of solar efficiency loss, and unfortunately, minimizing the loss requires a better, more expensive controller.
There are two types of controllers: Pulse width modulation (PWM) and multi point power tracking (MPPT). Choosing one or the other is a matter of compromises. PWM is inexpensive and simple but not particularly efficient. Expect to lose about 10-30% of your power through this type of controller.
MPPT controllers are a big boost in solar efficiency compared to PWM. They pass over 90% of the input electricity along to your batteries or other load. But MPPT is more expensive and technically complicated than PWM.
Controllers will consume a small amount of power to support their own internal circuitry. This is known as tare power, tare loss, vampire power, or some variation of these terms. The tare loss is usually indicated in the user manual or data sheet. If it is not listed, assume it to be less than two watts.
What you need to know: The main takeaway is that charge controllers contribute to the inefficiency of any solar power system. I won’t go into a lot of detail about the function and types of charge controllers here because I’ve already covered the topic in great detail in this Off Grid Ham article from a few months ago. If you feel a bit lost trying to figure out charge controllers, please refer my previous article and I’m sure you’ll gain a better understanding.
Coming up in part 2 of the Off Grid Ham series Solar Efficiency: It’s Not What You Think, we’ll continue with a discussion about batteries and how they fit into the picture. We’ll also go through a complete solar power system from beginning to end and examine how much energy is lost and what the radio amateur can do to minimize the losses. The series will conclude with some thoughts about the practicality and usefulness of renewable energy.
If there are any questions so far, let me know and I’ll address them as best I can.
Hope you’ll join me next time.