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Solar Energy—Getting Started

FIGURE 1 Commercial solar installation (Source: Sun Solar)
Written by Stuart Ball

With green energy in vogue, I thought it might be useful to cover the basics of solar energy. I want to focus on some of its practical aspects. This article may be useful to someone designing a solar-powered product, or to a senior EE student who is working on a green senior project, or just someone who wants a little more depth than you get from the popular press.

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  • Basics of Design
    Topics Discussed
    How does solar energy work?
  • How effective are solar panels?
  • What should I know when installing solar panels?

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  • Solar cells

With green energy in vogue, I thought it might be useful to cover the basics of solar energy. I want to focus on some of its practical aspects. This article may be useful to someone designing a solar-powered product, or to a senior EE student who is working on a green senior project, or just someone who wants a little more depth than you get from the popular press.

Solar energy is produced by converting sunlight to electricity. There are multiple ways to do this, but this article will focus on the direct conversion of sunlight to electricity using solar cells. A solar cell is a silicon semiconductor device. It consists of a P-type layer with “missing” electrons (called holes) and an N-type layer with excess free electrons. When the two types of material are bonded together and exposed to sunlight, electrons are ejected. If the two sides are connected with wires or through some load, the electrons flow through the wires to the other side of the cell, producing an electric current. The voltage produced by a single solar cell is about 0.6V, the voltage of the PN junction. But most solar panels have multiple cells in series to produce a higher voltage.

You might be thinking that in describing a solar cell, I’m describing a diode: a P-type material and an N-type material bonded together. And you’re right, if you connect an LED to a voltmeter and expose it to bright light, it will generate a small voltage. I tried this with a generic green LED with a clear lens and got about half a volt. Solar cells and transistors and LEDs are all semiconductor devices with semiconductor junctions; solar cells are just optimized to generate electricity. And you could build a circuit that uses an LED as a light sensor.

BUILDING-SIZE SOLAR INSTALLATIONS

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Figure 1 shows a solar power installation on top of a building. This is typically what comes to mind when we think of solar power. But there are other applications for solar energy. You can purchase devices that use solar energy to charge a battery that can then be used to recharge your smartphone. You can get solar panels for charging a laptop. You can even get a solar panel that sits on your car’s dashboard and trickle charges the battery. Solar power is used to charge the batteries in warning lights on highways, ocean buoys, remote irrigation systems and even recreational vehicles.

FIGURE 1
Commercial solar installation (Source: Sun Solar)
FIGURE 1
Commercial solar installation (Source: Sun Solar)

Figure 2 shows the typical installation of a residential or commercial solar system. The top figure is a DC coupled system where the DC output of the solar cells goes to a charge controller that manages the charging current to the battery (or batteries, as there may be a bank of them) to ensure that they are charged but not overcharged. The DC output of the batteries goes to an inverter that converts the DC voltage to AC voltage for the building. In an application where you don’t need AC voltage, such as charging your laptop, you don’t need an inverter, but you may need to step the DC voltage up or down.

FIGURE 2
Diagram of typical residential solar installation
FIGURE 2
Diagram of typical residential solar installation

Figure 2 also shows an AC coupled system where the solar panels connect to the inverter which then connects to a switch that manages current into the battery or the building. A second inverter charges the battery or draws battery current to supply the building. This configuration requires two inverters.

The configurations shown in Figure 2 are for a system that is connected to the power grid. If the system was installed someplace where power wasn’t available, say a mountain cabin, then the power grid connection would not be used.

SUNLIGHT AND POWER

Any form of solar energy is dependent on the available sunlight. Current silicon solar cells are about 15% efficient, meaning the output is approximately 15% of the solar energy to which they’re exposed. I used a small solar cell about 2” square to measure solar power throughout the day. Figure 3 is a photo of the solar panel with a load resistor attached, and Figure 4 is a schematic. The panel is attached to a wooden board with grommets to prevent cracking the cell with the attachment screws.

FIGURE 3
Photograph of solar cell fixture
FIGURE 3
Photograph of solar cell fixture

FIGURE 4
Schematic of solar cell fixture
FIGURE 4
Schematic of solar cell fixture

For these measurements, I loaded the panel with two 270Ω resistors in parallel, producing a 135Ω load (Figure 3). To measure the power output, I measured the voltage across the resistors and calculated the power. Power output = voltage2/135. To make sure I wasn’t just measuring the maximum voltage the panel could produce, I measured open-circuit voltage in direct sunlight. It was about 3.8V.

With the load resistor connected to the panel, two measurements were made: one with the panel horizontal and one with the panel aimed directly at the sun. I measured one day that was cloudless and then measured a single time on a day that was partly cloudy.

You can see in Table 1 that there is approximately five hours of full power when the panel is horizontal versus approximately eight hours (10am to 6pm) when the panel is pointed at the sun. Pointing directly at the sun was accomplished by manually adjusting the angle of the panel for maximum output when making the measurement—equivalent to motorized systems that always keep the panels facing the sun. As Table 1 makes clear, solar panels at a fixed angle produce full output power for significantly fewer hours. On the cloudy day, the output was down to about 36% of the maximum from the cloudless day.

TABLE 1
Solar cell power using 135Ω load over a single day
TABLE 1
Solar cell power using 135Ω load over a single day

The cell I used is an old one that I purchased years ago for another project, so I don’t know its exact characteristics. But the point is that if you are going to use solar cells in a real design, you must account for the amount of time it gets sunlight. And for a mobile application, such as a cell phone charger clipped to a hiker’s backpack, that can be quite variable.

SERIES/PARALLEL CONNECTION

Solar cells can be connected in series in a panel, like the one I used for the power measurements. Panels can likewise be connected in series—the positive lead of one panel connected to the negative lead of the next—to produce higher voltage. But the disadvantage is that if one panel is in shade, the output of the entire array goes down. The current through the array will be limited to the current through the weakest (shaded) cell. Solar panels can also be connected in parallel, with all the positive connections common and all the negative connections common. This produces more output current but lower output voltage.

Whether you need series or parallel connection depends on your application. If you want to charge a smartphone or laptop, you will likely want series-connected panels to simplify regulating the voltage to that needed for the appliance. Or, you can use a lower-voltage parallel connection and a voltage booster.

LOADED VOLTAGE

The panel I used in my example had a 3.8V open-circuit output, but even in full sunlight, the output dropped to about 2.74V when loaded. So, your solar power design has to take into account the loaded output of the panel, at whatever current you are using. Say you wanted to use the panel I used to charge a 3V battery. Under full load, the output of the panel may not be high enough to charge the battery, so a voltage boost circuit would be needed. This specific panel was only generating about 20mA, so it’s not a good example of a battery charger. But the basic principle still applies: You have to design for the loaded panel output, not the unloaded output. The datasheet for the panel should provide information on unloaded and fully loaded output voltage and power.

PREVENTING BATTERY DRAIN

When there’s no light, a solar panel produces no output voltage. But even worse, it becomes a conductor of electricity. The small panel I used for the power measurements draws about 10mA from a 1.5v battery when there is no light. Not much current, but then this isn’t a very big panel. If you’re using the panel to power a circuit that doesn’t have a battery, this may not be a problem. But if you’re using it to charge a battery, you may need to prevent the panel from discharging the battery when there’s no light. A small solar cell used as a light sensor would potentially have the same issue.

Figure 5 shows a schematic of a solar cell charging a battery through a limiting resistor. A diode, D1, is in series with the positive lead of the solar cell to block reverse current when there is no light. This is a simple circuit that you might use for something like trickle-charging a NiCad battery. For other types of batteries, you would need a more sophisticated charging circuit. But the principle is the same: If there is a discharge path from the battery through the solar cell, it needs to be blocked with a diode, a switched-off transistor, or some other mechanism. Note that the blocking circuit will reduce the output voltage somewhat—in the case of the diode it is approximately 0.6V. Also, the blocking circuit must be able to handle the full forward current of the circuit when the solar cell is generating electricity.

FIGURE 5
Solar cell blocking diode
FIGURE 5
Solar cell blocking diode

LATITUDE AND SEASON

In full sunlight a square meter of the Earth’s surface at the equator receives up to 1000W of energy on a cloudless day. But not all latitudes receive the same solar energy. A solar energy system needs more panels to produce a given amount of power as you move north or south from the equator. At 40° latitude it’s about 600W per 1m². So, calculations for the solar panel area needed to power your product need to account for the latitude (or latitudes) where it will be used. A solar-powered irrigation system will need a smaller panel size at the equator than it will in Maine.

Seasons also affect solar power on the Earth’s surface. In the northern hemisphere, July and August will result in more power output from a solar cell than December and January. This is due to all the factors you would expect: days are shorter, the sun is at a more oblique angle, and winter months (in the northern hemisphere) are more prone to cloud cover. In places where it snows, a fixed installation will have reduced output when the panels are snow-covered.

Latitude and season don’t make solar installations impractical. But it does affect how much solar panel area is needed for a given power output. See Circuit Cellar’s article materials page for a link to a set of charts that show seasonal variations in the energy in different portions of the United States [1].

SITE-SPECIFIC ISSUES

Say you have a solar-powered gate opener or irrigation system at a remote cattle pasture where there is no connection to the power grid. In the summer, there is more solar energy available, but there may also be tall grass or shade trees that partially block the panels. The angle of the sun changes, so the angle of the solar panels may need to be different in winter. Maybe you have to use a compromise position that gives less than full power in the summer but a little more in the winter. In a marine location, you may have a problem with roosting seagulls and their excrement. In an inland city, it’s pigeons.

There is a hiking trail I sometimes use and the road to the trailhead is in the mountains, in an area where there is no cell service, and along the way you see houses with firewood stacked outside because they heat with wood. In one spot on the road, there is a house with a solar panel array next to a fence. The road is between two tree-covered hills, so the effective horizon is high on both sides. I doubt those solar panels get more than an hour of direct sunlight each day. The point of all this is that if you’ve got a fixed installation, the specifics of the site must be considered as well as the latitude and season.

A SOLAR CAR EXAMPLE

How does all this translate to a practical application? Let’s look at a simple example. Let’s say you commute 20 miles each way to work every day. You decide that you’re going to make an electric car with a solar cell on the roof to charge the battery while you’re at work. No need to ever plug it in for charging. You’ll test the prototype by driving it to and from work 20 miles each way, every day. Is this feasible?

From freshman physics, we know that F = ma, force is equal to mass times acceleration. A car moving at a constant speed isn’t accelerating, but force (power) is still needed to overcome forces that are resisting forward motion; for a car on level ground, these are air resistance and the force needed to overcome the rolling resistance of the wheels, which is a function of the weight of the car and the friction of the wheels. If the car is traveling up an incline, additional force is needed to overcome gravity. We’ll pretend you have a flat road (no hills) just to make the math easy. Ignoring the air conditioner and heater, the formula to calculate the power needed to move the car 20 miles is given by:

This equation holds whether the car is gas-powered or electric, although for gas an additional calculation step is needed to convert watts to gasoline.
For our hypothetical car we’ll use the following specifications:

  • Weight = 3000lb = 1361kg (less than the average car)
  • Speed = 40mph = 65kph ≈18 m/s
  • Rolling resistance = 0.011 (See article materials for the range [2])
  • Drag coefficient = 0.3 (See article materials [3])
  • Front surface area = 8ft2 = 0.74m²

Plugging these values into the formula in the equation gives 2641 + 874 = 3515W.

A 20-mile commute will take an hour a day at 40mph (30 minutes to work, 30 minutes home).

We’ll say the motor is 90% efficient, meaning that the actual power needed for the round trip is 3515W/0.9 or about 3905W-h (3.905 kW-h). If you live near 40° latitude, a 1m2 solar panel (about 10.7ft2) on the roof of the car will produce about 0.45kW-h per day in the summer (600W x 5 hours x 15%). That’s on a cloudless day. You would get a little more than that since the hours when the panel isn’t producing maximum output will still produce some. But to provide 3.9kW-h, you need approximately 8.7m2 of solar cell, about 94ft2. That’s roughly twice the size of a king-size bed, and certainly bigger than a car roof.

Remember that we’re ignoring acceleration, power to go up hills, lights, air conditioning in the summer and heat in the winter, and lower power output in winter. Further, this calculation is done with a car that weighs less than the average car, driven at a sedate 40mph, not at 75mph highway speeds. And we haven’t even considered what happens if you have three days of cloudy skies. Driving on gravel or sand requires more power. Driving into a headwind requires more power. Anything that increases the forces that resist forward motion requires more watts. And as for highway speeds, note that the increase in air resistance in the equation goes up exponentially with speed.

LIGHTYEAR

While writing this article, I learned about a new car, the Lightyear, that has 5m² solar panels on the roof and claims to need no charging from the grid—if driven about 21 miles per day in the summer, about half of the hypothetical example I used. According to the manufacturer’s website, it has a 0.2 drag coefficient (smaller than I used in my example). The website doesn’t describe the speed at which the commute is driven; I queried the company about that but got no response. That’s not to say that the Lightyear concept doesn’t have value. A solar panel on the roof can supplement charging from the grid, so that less power is required to charge the batteries at the end of the day. And if you had a large solar installation at home to charge a house-sized battery, then you could potentially charge a car for free by charging the home battery during the day and then using that to recharge the car at night. But that only works if you take it home to charge every night. And, like any fixed solar installation, you would want to know how long the payback is on that hypothetical solar/battery configuration to determine if it makes financial sense.

OTHER APPLICATIONS

Although a solar-powered car is just one specific example, the larger point is that if you want to use solar to charge batteries—whether it’s an EV car battery, a battery for a remote monitoring station, or a cellphone battery when hiking—you need enough panel area to generate the needed power. You need to account for cloudy days, time exposed to the sun and all the other things that reduce the power output of the solar cell. If you look at pictures of the cars entered in the Australian World Solar Challenge, they are typically slow, single-passenger, lightweight, wide to accommodate many solar cells, and have a minimal frontal area and narrow wheels. This energy usage analysis I’ve provided is exactly why. Unfortunately, at current solar cell efficiencies, a solar-powered car is not a practical replacement for a standard vehicle.

CONCLUSION

It may seem that I’m focusing on the limitations of solar energy here. But any time you use a technology or a part or a process, you need to understand its limitations to ensure that the design is feasible and functional. Solar energy is limited both by the technology itself and by the environment where it operates. Like any electrical generation method, you must design within those boundaries. Hopefully this discussion has provided some illumination on the subject. 

REFERENCES
[1] Chart of solar power received by region and season in the U. S.:
https://www.solar-electric.com/learning-center/solar-insolation-maps.html/
[2] Rolling resistance: https://www.engineeringtoolbox.com/rolling-friction-resistance-d_1303.html
[3] Drag coefficient, Wikipedia: https://en.wikipedia.org/wiki/Automobile_drag_coefficient

SOURCES
Power to move a car, Colorado State U: https://www.engr.colostate.edu/~allan/fluids/page8/page8.html

PUBLISHED IN CIRCUIT CELLAR MAGAZINE • JUNE 2023 #395 – Get a PDF of the issue

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Stuart Ball recently retired from a 40+ year career as an electrical engineer and engineering manager.  His most recent position was as a Principal Engineer at Seagate Technologies.

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Solar Energy—Getting Started

by Stuart Ball time to read: 13 min