Solar Energy

With at least five billion years of sunlight left before our local star burns out, solar energy is probably the most abundant renewable energy available to humanity. Estimates are that roughly 250 watts of solar energy from the sun impact every square meter of the surface of the earth each day (24 hour period). Using that value and some math, it is possible to determine that the sun provides the Earth with about 3,850,000 exajoules of solar energy per year (give or take depending on solar flares, brightness, etc.). If humans use about 550 exajoules of energy (2010 estimate), then that means there are another 3,849, 450 exajoules left. In other words, there is more than enough solar energy from the sun to meet human needs.

Another way to look at the topic is in average power use. The average household in the United States, which has the highest per capita energy usage in the world, uses about 29 kilowatt-hours of energy each day. Each square meter of surface receives about 6 kilowatt-hours of solar energy per day, which means solar panels covering about 5 square meters are required to produce enough solar energy for each person in the world.

Insolation

Given the above information, it seems almost implausible that every person in the world doesn’t simply have a solar energy array in the garden that generates all the power they could possible need. Further, you think, if each home just added one more square meter of solar paneling, then people could power their electric cars and all would be well. Unfortunately, the answer is more complicated than that.

The phenomenon of the sun impacting the surface of the planet is termed ‘insolation’. As it turns out, insolation is not constant the world over for several reasons as listed here:

Insolation varies with angle. When sun shines directly onto a surface, impacting it at 90 degrees (perpendicular to the plane of the surface), then insolation is at its maximum. As the angle changes, however, solar energy drops. At 30 degrees, the amount of solar energy reaching a square meter of surface is cut in half. This means that regions near the equator receive more solar energy than regions near the poles.
The idea of angle has another complication, which is the change of seasons. Because the earth is tilted on its axis in reference to the sun, the direction at which sunlight impacts the surface changes with the seasons. For regions near the equator, the change is minimal, but for regions near the poles, the change can be dramatic. For instance, the average daily solar energy in Northern Europe during the winter is only 0.8 kWh/m2 in the winter compared to 4 kWh/m2 in the summer.
Other impacts on insolation result from atmospheric anomalies such as cloud cover, pollution, dust, etc.

These differences mean that some areas receive much more sunlight than others on average. For Central Europe, the average is 1000 kWh/m2 while in the Mediterranean the average is closer to 1700 kWh/m2. In equatorial Africa and parts of Australia, the average is closer to 2200 kWh/m2. If the average American uses about 11,000 kWh of electricity per year, then that person would have to cover 11 square meters of space in Northern Europe, but only about five square meters in Australia. The bottom line is that location greatly affects insolation, which affects the amount of solar energy that is available.

Solar Energy Capture

Of course, all of the solar energy available to a solar panel is not captured. There are always inefficiencies in any energy capture or conversion process. Current solar panel technology is, at best 30% efficient (a value that is very generous). There are laboratory tests that have reached a little over 40% efficiency, but that is in controlled settings with state of the art equipment.

On average, current commercial solar panel technology will yield about 21% efficiency, which means 21% of the sunlight that reaches the panel is converted to useable electricity. Assuming that the process from that point to consumption is perfectly efficient (it is not), the area needed to generate a given quantity of electricity jumps five-fold. For the average American discussed above, that means a total of 55 square meters of solar paneling are needed in Central Europe and 25 square meters are need in Australia.

To put that into perspective, the United States is home to about 300 million people. If they all switched to solar energy, they would have to cover an area of about 7.5 billion square meters or 7,500 square kilometers in Australia to produce enough electricity. That is an area roughly 9.5 times the size of New York City. Is that doable? The answer is yes, with reservation. The sections on cost and storage will explain why solar energy hasn’t quite taken off yet.

Estimates are that about 500,000 square kilometers of land area would need to be covered in solar panels in order to produce all the energy currently consumed across the planet. Of course, solar panels are only one solution for capturing solar energy.

Capturing Solar Energy

There are two basic ways to capture solar energy from the sun; convert light into electricity or convert heat into electricity. Each approach has advantages and disadvantages, and the technology is drastically different.

Solar panels, called photovoltaic panels, work on a principle of physics in which photons can dislodge electrons from certain materials (like silicon) to create and electric current. This phenomenon is known as the photoelectric effect and it is central to the operation of photovoltaic panels.

The current impediment to solar panel efficiencies is material sciences. Some materials are simply better at absorbing photons and releasing electrons than others. Right now, the best efficiencies have been achieved by cells called multi-junction solar cells, which are made of special types of silicon arranged in particular patterns. These cells have been reported to reach 44% efficiency, but are complicated and thus expensive to produce. Commercially available cells range from 14% to 22% efficiency.

Whereas photovoltaic science is a technically challenging and complicated field, the other method of capturing solar energy is very basic. Heat from the sun is redirected using mirrors onto a single area of water. That water becomes superheated and generates steam, which is used to move a turbine and produce electricity. This is referred to as concentrated solar power.

Concentrated solar energy is about 31% efficient, which makes it better than the currently available PV technology. It also does not require complicated manufacturing and is relatively straightforward in terms of construction. It is limited, however, by the need for suitable locations and the fact that it is most efficient when built on large scales rather than distributed (individual) installations.

Solar Energy Issues

It may appear that humanity has the technology that it needs to make solar energy a reality for every person. After all, the technology to capture the energy is available, the land area required is not overwhelming, and calculations indicate it can be done. The problem is, there is a catch…well actually there are three catches.

The first catch is cost. It isn’t the overall cost of solar energy that makes it prohibitive, but rather the initial cost of construction that prevent it from being more viable. In the United States, energy costs an average of 11.2 cents per kilowatt-hour using conventional generation means. This is a whole lot less than the 27 cents per kilowatt-hour of solar thermal and the 20 cents per kilowatt-hour for photovoltaic.

The next problem, which is related to cost, is transmission. Recall that solar energy is best captured in only a few locations around the globe, most of them near to the equator. This means that the energy would need to be moved great distances to where it is needed. Current transmission lines either don’t traverse the needed spans or, if they do, are not equipped to carry electricity efficiently over long distances. The bottom line is that new transmission infrastructure is needed and it is prohibitively expensive to build. Building solar energy technology closer to the consumer is not a more affordable option because the decrease in capture efficiency increases the size of installations and offsets the gains from decreased transmission infrastructure.

The problems above are relatively minor compared to the biggest issue facing solar power – storage. The sun only shines part of the day (12 on average, though more or less depending on season and location) which means that there is a time each day during which solar systems cannot produce electricity. The solution is storage, but storing that much energy is impossible with current technology. Solutions so far are only able to store enough energy to supply a limited amount of energy for upwards of 6 hours. Of course, 12 hours worth of storage or more is needed in some locations. The problem of storage is the largest for solar energy. If this problem could be overcome, cost and transmission would be far less problematic.

Solar Heating

Lest anyone think that solar energy is only useful for generating electricity, it is important to note that solar energy can also be used directly for its heat. It can be used either to heat water or to heat living spaces. Passive and active solar solutions to both space heating and creating hot water have been used for hundreds, if not thousands of years, even if we didn’t really understand it at the time.

The Future of Solar Energy

Solar energy has a bright future (pun intended). Though it won’t immediately replace traditional fuel sources for the reasons mentioned, it is becoming a popular supplement to those sources and a way to reduce dependence on more traditional forms of energy production. The cost of solar energy has dropped roughly 80% in the last decade, making it more accessible than ever before. Solar will slowly increase its share of the energy market as governments and citizens look for ways to reduce environmental impact.