Renewable energy from the sun—which includes solar, wind, and water energy— can meet all of our energy needs and will allow us to eliminate our dependence on fossil fuels for electricity production. At least, that is the “Siren song” that seduces many people. Amory Lovins, the head of the Rocky Mountain Institute, has been one of the strongest proponents of getting all of our energy from renew­able sources (what he calls “soft energy paths”) (1) and one of the most vociferous opponents of nuclear power. A recent article in Scientific American proposes that the entire world’s needs for power can be supplied by wind, solar, and water (2). Is this truly the nirvana of unlimited and pollution-free energy? Can we have our cake and eat it, too? Let’s take a critical look at the issues surrounding solar and wind power.

SOLAR

Photovoltaic (PV) Solar Power

Let me be clear that I am a proponent of solar energy. I built a mountain cabin a few years ago that is entirely off the grid. All of the electricity comes from solar photovoltaic (PV) panels with battery storage. The 24 volt DC is converted to AC with an inverter and is fed into a conventional electrical panel. It provides enough energy to power the lights, run a 240 volt, three-quarter horsepower water pump 320 feet deep in the well, and electrical appliances such as a coffee pot, toaster, and vacuum cleaner. But I am not implying that all of my energy needs come from solar. The big energy hogs—kitchen range, hot water heater, and a stove in the bedroom—are all powered with propane. Solar is not adequate to power these appliances. In 2010 I also had a 2.5 kW solar PV system installed on my house that ties into the utility grid. When the sun is shining, I use the electricity from the solar panels, and if I use less than I generate, it goes out on the grid to other

users. If it does not produce enough for my needs, then I buy electricity from the grid. When I am generating more than I use, the utility buys electricity from me. Because my house is entirely electrical, including heating, I generate only enough electricity to provide about 20% of my total electrical power needs. I am a pro­ponent of solar and I live in an area where it makes sense to use solar power. But there is more to the story than meets the eye.

The most important thing about solar is that it is a diffuse, intermittent energy source that is highly variable both geographically and temporally. It is just com­mon sense that there is more solar energy available in hot, sunny areas such as the southwestern United States and less in cloudy areas such as the northwestern and eastern United States. We also know that even in sunny areas the clouds some­times block the sun, the intensity of the sun varies from dawn to dusk and, of course, it does not shine at night. Even in sunny areas, there are only about 9-10 hours of useful sunlight in the summer—depending on latitude—which does not cover the early morning and late afternoon and evening hours when people use the most electricity. And, of course, in the winter there are fewer hours of sunlight and the intensity of the sun is less during the daylight hours compared to the summer. Since people still want to use electricity when the sun is not shining, solar power cannot be relied upon as a primary source of energy unless there is adequate storage of energy—such as in batteries, as I do in my solar cabin—or it is backed up with the grid that gets a stable baseload source of power from a conventional power plant, as in a grid-tie system like the one I use for my house.

The National Renewable Energy Laboratory (NREL) in Golden, Colorado, produces a solar map that shows the solar availability for the United States (Figure 4.1). A glance at this map shows that the best solar resources by far are in the Southwest, especially states such as Arizona, Nevada, New Mexico, and south­ern California, and are good over much of the western United States. Solar energy is a less attractive option in the eastern half of the nation. Except for southern California, the best solar resources are in areas where there is a low population density.

The most common use of solar power is to produce electricity using photo­voltaic (PV) cells. These work by the photoelectric effect,1 but in this case, the photons from the sun do not have enough energy to ionize atoms. PV (solar) cells are made of layers of silicon crystals with small amounts of other elements added, very similar to the material used in semiconductors for computer chips. Electrons in the silicon are normally bound to atoms in the material, but when they absorb a photon they get enough energy to jump into a conduction band in the silicon material, creating a current that is collected through wires. Thus, PV cells directly convert photons from sunlight into electricity, but the process is quite inefficient. Solar cells are individually small but can be put together into modular solar panels that are 1 or 2 meters on a side. Besides the silicon that is at the heart of solar cells, they are covered with an antireflection coating consisting of silver, titanium, pal­ladium, and silicon layers. Most solar panels have an efficiency of only 15-16%, with the best in research labs at about 23% for single-crystal cells that are very

United States Mainland
Germany
large area
Hawaii (USA)
suns rays
long distance
	Hawaii (USA)
equator
short distance
United States Mainland
small area
Geographic regions are not shown to scale.
NATIONAL RENEWABLE ENERGY LABORATORY

F igure 4.1 Solar resource map of the US and Germany.

source: Reproduced by permission from the National Renewable Energy Laboratory.

expensive (3). Of course, this is an active area of research, and new materials and processes may increase the efficiency somewhat.

Let’s crunch some numbers to put things in perspective, starting with my grid-tie system (here comes the math!). I have 12 solar panels, each of which are rated at 210 watts and have a surface area of 1.61 square meters. In order to stan­dardize things, let’s think about the area to produce 1 kW of power, which is 7.67 square meters or about 5 panels. A very important point about solar panels is the rated power output, which is the maximum power the panel is capable of produc­ing. In reality, it will only produce this at the peak time of the day when there are no clouds. Suppose that my system could generate the full kW for every hour of the day. The amount of energy produced in kWh (kilowatt hours) would be 24 kWh, which is about what an average Coloradan uses each day. But, of course, it can’t actually do that, since it is cloudy sometimes during the day, the intensity of sun varies during the day, and it doesn’t shine at night. So how can you determine what to expect?

An excellent tool for analyzing solar power is a program developed by the NREL called PVWatts.2 This program gives the amount of average daily solar energy falling on a square meter of land at many locations in the United States and around the world. It then calculates the amount of AC energy you can expect to generate for a given size of solar system; it will also calculate how much money that will save you. The yearly average solar radiation in Boulder, Colorado (the closest location to my home in Fort Collins) is 5.56 kWh per square meter per day. Of course, it is less in the winter (4.43 kWh/m2/day in January) and more in the summer (6.24 kWh/m2/day in August). Five of my solar panels, with an area of 7.67 square meters that produce 1 kW maximum DC power, could theoretically produce 42.6 kWh per day on average, but over two years of actual use, I gener­ated an average of 4.57 kWh per day for an overall efficiency of 10.7%. Over a year, PVWatts predicted that I should generate 1,459 kWh, but I actually generated 1,650 kWh. The predicted energy generation takes into account average or typi­cal actual weather on a daily basis throughout the year, which reduces the total energy generated compared to a theoretical value based solely on the solar irradi — ance. Because of vagaries of weather, the predictions of PVWatts are expected to be within 10% for annual predictions and 30% for monthly predictions.

Why am I able to utilize only about 11% of the available solar energy? Do I have a particularly bad solar system? In fact, my system and its various components are new and efficient for commercially available systems. My solar panels are rated at 15% efficiency, but they become less efficient when they get dirty or are cov­ered with snow, amounting to about a 5% loss. The inverter that converts the DC coming from the solar panels into AC that can be used in my house is about 96% efficient, so there is another 4% loss. There are also small losses in the wiring and other electronic components. Overall, PVWatts estimates a derating factor of 0.77 in converting the DC power to AC power, giving an efficiency of about 12% over­all for the system. As solar panels age, they also get less efficient, losing efficiency by about 1% per year, according to the NREL and the solar panel warranty. That may not sound like much, but it means that after 20 years they produce only 80%
as much power as when they were new. That is why the typical useful lifetime (and warranty) is 20 years.

Colorado State University (CSU), where I teach, is one of the top universities in the country for using solar power. CSU has developed a 30-acre site filled with high-efficiency solar panels—about one-third of which track the sun, improving the overall efficiency. The 23,049 solar panels produce 230 W DC (maximum) per panel, for a total capacity of 5.3 MW (megawatts). The annual energy output is about 8.5 million kWh, which works out to be slightly better than the efficiency of my system—about 12%. This amount of energy is sufficient for about 1,000 homes using 8,500 kWh per year, the average for Colorado but much less than the overall US average of 10,900 kWh per year. Average energy output is not the most important information, however. What really matters is what is available when users need it, and this varies greatly by hour, day, and month. If it is cloudy or stormy and the system is not generating electricity, the grid has to have sufficient power to make up the difference.

An electrical grid system has to have sufficient baseload power to handle con­stant needs 24 hours a day and other sources of power that can be switched on or off to meet the intermediate and peak loads that vary during the day (Figure 4.2). Intermittent sources such as solar can only contribute toward the intermediate loads during the day, but there still has to be sufficient capacity from other sources to make up for the loss of power when the sun isn’t shining. Peak loads can vary rapidly and require a source of energy that can be quickly ramped up and down, typically gas-fired power plants. Peak loads in the summer tend to occur during the late afternoon and evening when people get home from work and turn on the air

source: Reprinted by permission from Enerdynamics Corp (Understanding Today’s Electricity Business, 2012).

in the morning when people get up and turn on lights, heat their houses and cook, and another peak in the late afternoon and evening (4).

Fort Collins is a pretty good location for solar power, but there are better and much worse places. Table 4.1 lists different locations in the United States and other countries with their solar resources. It gives the monthly maximum, mini­mum, and average solar radiation in kWh/m2/day and the monthly maximum and minimum—as well as yearly—expected energy production in kWh for a 1 kW DC system.

Existing utility-scale PV installations in the United States are mostly in the Southwest, with a few scattered in New England, Florida, and other states. These plants are relatively small, mostly below 20 MW. The total PV solar installations in the United States by the end of 2012 had a rated DC value of 5.9 GW (5), but remember that is only the peak value, and the actual average amount of power generated is less than 15% of that. Solar power currently accounts for only 0.04% of the electricity generated in the United States (see Chapter 2). There are about 125 pre-operational PV installations under development in the United States, concentrated in California, Nevada, and Arizona, and a few of these plants are up to several hundred megawatts (6). Even so, solar electricity production is pro­jected by the EIA to be just 5% of the non-hydropower renewable energy portfolio by 2035 in the United States (7).

One of the worst places for solar energy is Germany, but they are one of the leaders in solar energy production in the world, thanks to large subsidies to the industry and to users (8). The map in Figure 4.1 shows that Germany’s solar resources are worse than Alaska—about half as good as they are where I live. The subsidy arrangement in Germany is the feed-in tariff—a guaranteed price that utilities have to pay for solar power—that makes solar panel installations profit­able at the expense of electricity consumers who pay high prices (9). While renew­able energy rose from 6% of German electricity production in 2000 to 16% in

2009, PV solar contributed only 6.2% of that (for a total of 1% of electricity gen­eration), yet it was subsidized at a rate of 43€ cents ($0.57) per kWh in 2009. That amounts to a subsidy that is about 9 times the cost of electricity produced by the power exchange (4.78€ cents [$0.06] per kWh in 2009). Altogether, the subsidy for PV solar amounted to €35 billion ($46 billion) by 2008 and was projected to be €53 billion ($70 billion) by the end of 2010 (10). Germany installed 4,300 MW of solar in the first half of 2012 before a 30-40% reduction in government subsidies in July. That brings the contribution of solar to 5.3% of its total electric power (11). The cost for abating carbon dioxide (CO2) by using solar energy for electricity is over €700 ($928) per metric ton of abated CO2, an incredibly high cost (10). The subsidies amounted to about 2 cents/kWh in 2012 and cost German consum­ers an extra $5 billion annually to the cost of their electricity (11). Clearly, the extremely high subsidies for PV solar in Germany make no economic sense and simply take away from other less expensive options. Germany is not the model to follow for solar power.