Total Energy Potential of Solar
The world faces three energy crises today.
The first crisis is that the exploitation of fossil fuels and biomass for energy do significant damage to the environment at almost all stages of extraction and use, and that most people in the world depend on fossil fuels either entirely or in some way. The second crisis is that we have consumed about half of the worlds total recoverable oil reserves and deforested significant areas of the world; neither of these practices are sustainable. The third crisis is that even with this global exploitation of fossil fuels and biomass for energy, large populations of humanity lack even the basic energy access to water for disinfection. [1]
And yet the earth has energy. According to Wikipedia, "Mostly thanks to the Sun, the world also has a renewable usable energy flux that exceeds 120 PW (8,000 times 2004 total usage), or 3.8 YJ/yr, dwarfing all non-renewable resources."
1 terrawatt = 1 trillion watts
In the first image we see boxes representing different forms of energy on the earth. Solar energy is 86 000 terrawatts, significantly greater than wind energy at 870 terrawatts, geothermal at 32 terrawatts, and human consumption at only 15 terrawatts. This is not surprising, as physics can tell us nearly everything that happens on the surface of our planet is powered by the sun.
In the second image we see the distribution of solar energy on the planet [2]. Most people on the planet live where there is a lot of sun: South and Central America, Africa, Middle East, South Asia (India, China, Indonesia), and Southern Europe all receive significant amounts of solar energy. The solar energy hitting the blue dots in image 2 if converted at only a 9 percent efficiency would equal 18 terrawatts!
With solar systems closer to home and used as much as possible directly for any heating needs, we would require even less surface area, less infrastructure and can use land we already occupy.
So why aren’t we there yet?
The sun represents so much energy that the puzzling question is why solar energy is not already the world’s primary energy source?
The reason is because sunlight is not already conveniently concentrated for us by nature. Or in fact it is, but only indirectly. Strong winds, waterfalls, large trees, oil wells, all represent naturally concentrated solar energy. Humanity has developed by simply going and taking naturally concentrated stores of solar energy. Though we didn’t have the technology to reflect or refract light to concentrated it ourselves for most of human history, even now that we do the inertia of habits, thinking and industry keep us on a path of exhausting energy sources long after it is intelligent to do so, such as the case of biomass, hydro and fossil fuels.
For instance, trees concentrate energy in chemical bonds that we can unleash by burning them. It is relatively easy to cut down a tree, it can burn at a high temperature, and we can store it for use at any time. Making a fire is relatively simple. These qualities make fuel wood a more attractive choice over direct solar energy systems ... at first. Over time however, if too much wood is cut down there is not only significant environmental problems such as land erosion and biodiversity loss, but the trees become farther and farther away. With the passage of time, the added effort needed to collect fire wood and manage environmental problems is significantly more than the effort it would take to build and maintain solar capturing machines. Though the payoff for converting to direct solar can be very high, since trees, when used for energy, are simply a very inefficient solar technology[3].
The same story can be told of fossil fuels and massive hydro-electric damns.
Though life is easier when nature does the task of concentrating solar energy for us, if we take too much nature becomes profoundly imbalanced.
Fortunately, It is not difficult to concentrate sunlight ourselves. Practical demonstrations on essentially all scales are already in operation all around the world. A low-tech solar concentrator is as about as complicated as a door or bicycle, and a high-tech solar concentrator is about as complicated as a car. It would be much easier to create a solar concentrator for everyone on the planet than to build the 600 million cars in circulation in 1997! [4]
2. Efficiency of concentration
Concentrating energy is important because it makes life much easier: not only are tasks easier with more potential heat or power available, but a concentrated amount of energy is usually easier to store, transform, and move around. Also, having access to higher concentrations of energy rarely excludes low energy tasks.
For instance, when the sun shines outside we can use it to dry fruit and clothes. When solar energy is concentrated we can cook, boil water, bake ceramics, melt metals, make steam, store the energy for later use ... as well as dry fruit and clothes since we can always dilute concentrated energy (or use the waste heat from one process to power another lower temperature process). The more concentrated, the higher the temperature, and the more potential applications.
The difference lies in temperature and efficiency. At high temperatures and high power more different kinds of tasks can be done with the same amount of total energy. A simple example is if you place a newspaper in the sun it reflects the suns light and allows you to read it, the light hitting it functions as illumination. However, if you place a magnifying glass in front of the paper you can concentrate some of the solar energy onto a smaller point, and light the paper on fire, the same amount of light is now used to start combustion.[5]
Another important point is efficiency. The greater the concentration the greater the potential energy efficiency. For instance:
1. To capture solar energy we must enter it into our application through an opening, window, or heat exchange. Unfortunately we end up losing energy through the same window. The bigger the window, the more heat will be lost. If we can pass the same amount of energy through a smaller window, the same amount of energy will enter our system but we’ll lose less, which means our net energy capture, the temperature of our system, and efficiency will all be higher. In short, we’ll be able to do things faster and do more kinds of things with concentrated energy.[6]
2. Another place where concentration greatly affects efficiency is in converting solar energy into mechanical energy. To do this always requires a temperature difference. A hot side that moves towards a cold side — the wind being a simple example where hot air masses move into cold air masses — and in the middle something like a turbine or Stirling engine that produces mechanical energy. The bigger this heat difference the more efficient the generator can be. [7]
Costing
Energy efficiency of course does not necessary mean cost effective. To determine cost effectiveness the intended task, environmental factors, and available materials and skills must be taken into account, not simply "energy conversion efficiency". We need further concepts to determine what’s useful. [8]
3. Technical feasibility of solar energy
Almost all necessary human energy use is in the form of heat, mostly for cooking, boiling water and heating spaces and processes. Most people in the world can live comfortably with a source of food, water, air, shelter and heat. Other forms of energy such as electricity certainly make life easier but can’t really be called "necessary to live". But even with electricity in industrialized temperate zones an average of 60 percent of domestic energy is used to heat the home, 15 percent to heat water, 5 percent for cooking, and 5 percent for washing and drying, which makes nearly 85 percent for heat applications. Though we often see electronics and lighting more they do not represent the bulk of actual energy consumption, which is what we want to reduce (or transform) to save the planet. [9]
This is extremely important to keep in mind when thinking about energy policy: though electricity means gadgets, heat means survival and stability. Though electricity is very convenient to use, an electric economy would not be efficient from the perspective of basic physics, nor remotely realistic in the short term we must act. In short, everyone needs access to heat, so much so that we’ve come to take it and its sources for granted. Heat access forms the energy base of society; once this energy base is stable, efficient and sustainable, it is easy to build other forms of sustainable energy on top of it, such as electricity and transportation.
Solar energy is the best, perhaps the only, candidate to replace fossil fuels and biomass as a global heat-energy base: The sun shines almost everywhere in the world almost everyday so most of the distribution system needed is already in place; sunlight transforms into heat at high efficiencies; and solar concentrators are very simple. No other source of energy in use today compares to solar energy in even one of these points.
4. The Solar Concentration Economy
A "solar concentration economy" would be one in which the primary energy source is solar concentration, though wind, photovoltaic, other renewable energies and even fossil fuels could play roles, most energy on a global scale would be direct solar. These solar concentrators would exist in a local form, for local heat purposes, direct mechanical power and even electricity production, and also in a central form for industrial purposes.
Domestic and community (local scale) solar concentration is adapted for most heat applications on the globe. When solar concentration is placed locally and the heat generated from solar energy is used directly, efficiencies of 60 percent are realistic.
Cooking, roasting, boiling, baking (bread, bricks, ceramics etc.), space heating, paper and plastic recycling, metallurgy, and anything else requiring heat can all be done locally or regionally with solar concentration for most populations in the world. This has several benefits:
1. The highest possible efficiency for using solar energy for heat applications can be attained: since energy conversions and transport are a minimum. This means much lower land use and infrastructure (both in volume and sophistication).
2. Domestic and local solar concentrators can be built and maintained locally, almost anywhere in the world. Engineers and specialized technicians are not a limiting factor. Mechanics, handypeople, and enthusiasts can build and maintain most non-industrial scale concentrators. This is a significant characteristic few energy sources have. Though engineers and specialists can build larger or special concentrators, there is no shortage of talent to implement solar concentration on a significant global scale in a short amount of time.
3. An extremely stable energy system results. The more communities and people can, when necessary, function independently, the more stable the energy system is, and the more stable society is. Exchange between communities increases stability, but only if the original potential for independence remains.
Local and central renewable electricity production can easily power an electric grid. This has several advantages:
1. Electricity is ideal for mass transportation, notably electric trains, electric cars and compressed air vehicles.
2. Electricity is necessary for electronics and extremely efficient for mechanics.
3. Allows a large number of generation units (local and central) to share excess energy, which increases stability and decreases waste.
However, solar to electricity production is done at a high of 14% efficiencies today, and a practical maximum of 20% total efficiency seems realistic in the medium term [10]. After, we can transport that electricity over an electricity grid with another loss of about 14 to 20 percent. [11]
Considering these losses, it makes little sense to build such an electric system to turn heat into electricity, transport it over a grid, and then then turn it back into heat at the end user. We should completely forget about a "grid based economy". Even at central solar stations we could imagine direct industrial applications, such as metallurgy. And wherever a solar concentrator does produce electricity the waste heat should always go to powering other applications. In fact, solar concentrators should always be viewed as a source of thermal power and installed for some thermal purpose and electricity viewed only as an added bonus: 10 to 20 percent of thermal energy can be practically converted to electricity leaving 90 to 80 percent of the energy for thermal uses.
From here we can start to see a realistic basis of a sustainable energy system. When the sun shines, domestic and community solar concentration is used for heating, cooking, and other heat applications. Excess solar concentration energy can be used to make biochar (charcoal from wood) which can be burned in non-sun hours or to power solar-charcoal hybrids for high temperature applications such as ceramics. When the thermal load is taken care of, and transportation volumes and distances decreased by more local production, it becomes fairly easy for a combination of renewable energy can power an electricity grid for electronics and transportation purposes. As the technology develops more and more direct uses become possible, which can efficiently replace fossil fuels.
5. Policy
On the political front is is important to note that solar energy is so abundant there is little sense in a "war for sunlight", as there could be for oil and uranium when or if they are the primary energy sources. Though solar may not solve all global conflicts, it is unlikely to cause more of them. Because there is universal access, both industrial and non-industrial countries mutually benefit from any advancements, which will only serve to increase mutual understanding and policy, as well as economies of scale for solar technology. [12]
All the technical skills and knowledge exist to take advantage of solar energy for almost every thermal application; it is even proven to be cheaper than fossil fuels in many places in a relatively short amount of time, as the "fuel" is free. The cost of solar concentration is little as no exotic materials or skills are necessary, and they can last for dozens or even hundreds of years with maintenance.
The benefits of solar energy are enormous: it is abundant, there is little environmental impact, and it is universally accessible providing a solution to energy poverty and water born diseases.
Eerik Wissenz May 2008
6. FOOTNOTES
[1] Image 1 appears in the wikipedia article on solar energy representing the energy fluxes on earth based on figures from an MIT professor, click here for source image. Image two also appears on the wikipedia article on solar energy, representing the insolation averages on earth (the blue dots together equal 18 terrawatts at 9 percent conversion efficiency), click here for source image.
"In 2005, total worldwide energy consumption was 500 EJ (= 5 x 1020 J) with 86.5% derived from the combustion of fossil fuels, although there is at least 10% uncertainty in that figure.[1] This is equivalent to 15 TW (= 1.5 x 1013 W) of power. Not all of the world’s economies track their energy consumption with the same rigor, and the exact energy content of a barrel of oil or a ton of coal will vary with quality. [...] Most of the world energy resources are from the sun’s rays hitting earth - some of that energy has been preserved as fossil energy, some is directly or indirectly usable e.g. via wind, hydro or wave power. The term solar constant is the amount of incoming solar electromagnetic radiation per unit area, measured on the outer surface of Earth’s atmosphere, in a plane perpendicular to the rays. The solar constant includes all types of solar radiation, not just the visible light. It is measured by satellite to be roughly 1366 watts per square meter, though it fluctuates by about 6.9% during a year - from 1412 W/m2 in early January to 1321 W/m2 in early July, due to the earth’s varying distance from the sun, and by a few parts per thousand from day to day. For the whole Earth, with a cross section of 127,400,000 km², the power is 1.740×1017 W, plus or minus 3.5%. [...] The estimates of remaining worldwide energy resources vary, with the remaining fossil fuels totaling an estimated 0.4 YJ (1 YJ = 1024J) and the available nuclear fuel such as uranium exceeding 2.5 YJ. Fossil fuels range from 0.6-3 YJ if estimates of reserves of methane clathrates are accurate and become technically extractable. Mostly thanks to the Sun, the world also has a renewable usable energy flux that exceeds 120 PW (8,000 times 2004 total usage), or 3.8 YJ/yr, dwarfing all non-renewable resources"
Source: Wikipedia article World energy resources and consumption
"1.4 PW - geo: estimated heat flux transported by the Gulf Stream.
4 PW - geo: estimated total heat flux transported by earth’s atmosphere and oceans away from the equator towards the poles.
174.0 PW - astro: total power received by the earth from the sun."
Source: WikiPedia articleOrders of Magnitude (unfortunately, this article is not sourced)
[2] [top] Source of photo: "Solar power systems installed in the areas defined by the dark disks could provide a little more than the world’s current total primary energy demand (assuming a conversion efficiency of 8 %). That is, all energy currently consumed, including heat, electricity, fossil fuels, etc., would be produced in the form of electricity by solar cells. The colors in the map show the local solar irradiance averaged over three years from 1991 to 1993 (24 hours a day) taking into account the cloud coverage available from weather satellites."
Source: Total primary energy supply: Land area requirements.
[3] It is only in at low population densities that biomass is sustainable. Any time organic matter is taken from an ecosystem, the ecosystem is disrupted. Ecosystem productivity is dependent not just on surface area, water and sun, but quantity and quality of the soil. Healthy soil requires continuous dead organic material, and is significantly eroded by the removal of trees and other plants (which protect it). The Sacred Balance by David Suzuki (1997, Allen & Unwin) is an excellent introduction to ecology.
[4] Practical examples of solar energy devices are found on this site (see Technique) and other websites (see Links), and many are resumed in the wikipedia article on solar energy. "There are over 600 million motor vehicles in the world today. If present trends continue, the number of cars on Earth will double in the next 30 years." Cars Emit Carbon Dioxide. Global Warming, Focus on the Future, 1997.
[5] The amount of energy is actually a bit less in the case of the magnifying glass as some light is reflected. A bigger example would be passive (plate) solar collector for heating water. Passive solar water heaters can be up to 60% efficient, which is pretty good. However, these systems generally produce a maximum temperature of 100 C° and cannot realistically produce steam under pressure. With the same surface area of incoming sunlight a solar concentrator can easily reach temperatures of over 1000 C°, and very precise concentrators reach 2500 C° (very close to the theoretical maximum), yet the total amount of incoming energy is the same.
[6] In general all solar thermal devices act as an oven occupying a volume surrounded by a surface (walls or sides). Though the geometries may differ, the more insulated the sides of the oven are the more energy can be trapped in the volume. However, if the oven was 100% insulated no energy would be able to enter, as there is no mechanism that allows a perfect one-way energy flow. So, assuming high insulation for the rest of the oven, significant losses will be observed through the energy window proportional to its surface area, nature and the energy difference between the two sides of the opening. Though certain techniques can increase efficiency: 1. glass lets most light pass through but does a good job of blocking hot air and infrared radiation from getting out, 2. the opening can be placed at the bottom of the oven so that the hot air generated rises to the top where there is better insulation, there is no perfect solution, so the smaller the opening the less there is loss. When the sunlight is concentrated at extreme ratios, the opening can simply be tolerated (since it is very small compared with the oven itself). In the case of lower concentration ratios a window/insulator can be used, but these are generally more expensive than the oven walls. For instance, ceramic glass and vacuum sealed glass tubes are harder to find and more expensive than the widely available forms of insulation; furthermore, they are less efficient: ceramic glass does not insulate better than an oven wall, and vacuum sealed glass tube does not insulate better than vacuum sealed metal tubes or thick oven walls, and are much more fragile. Use of these materials should therefore be kept to a minimum (the limit being concentration ratios reaching temperatures where any sort of would-be-cost-effective window breaks or melts, in which case the opening must simply be tolerated).
See Exell’s Notes for Students for a basic introduction to the Principles of Solar Thermal Conversion. [7] What’s the highest efficiency a Solar-thermal power system could have?
[8] Common things that affect cost effectiveness are too high a concentration and commonly available materials, such as steel, melt. Clearly, the more accurate the design, the more costly. High wind speeds for instance can un-calibrate heliostat designs, and not enough direct sunlight can make solar concentration impractical (such areas would require a different solution).
[9] Source: WikipediaDomestic energy consumption , Eandismagazine, wegwijs in energie, page 4
[10] "Therefore, the annual system efficiency of a today’s solar thermal trough power plant varies between 10 % and 14 % for the considered irradiation range." - Contribution of concentrated solar thermal power for a competitive sustainable energy supply. However, though electric production doesn’t compare to local concentration for heat applications, Solar Concentrating Power (SCP) is cost competive with fossil fuels and Nuclear for producing electricity (in sunny regions). It can even be produced in high solar radiation areas and transported long distances too less sunny places. "Satellite-based studies by the German Aerospace Center (DLR) have shown that, using less than 0.3% of the entire desert areas of the MENA region [Middle East, North Africa], solar thermal power plants can generate enough electricity and desalinated seawater to supply current demands in EU-MENA [Europe, Middle East, North Africa] and anticipated increases in those demands in the future. [...] Solar and wind power can be distributed in MENA and transmitted via High Voltage Direct Current (HVDC) transmission lines to Europe with transmission losses that would be no more than 10-15%." - DESERTEC.
[11] A maintained electricity grid loses around 7 or 8 in resistance (grid) loss. However, a buffer must always exist between electric production and electric consumption; since the moment consumption tries to overtake production the grid fails. Since power consumption is somewhat unpredictable, to avoid a grid failure requires some power stations to run as a buffer. A typical buffer in industrial countries is 5 percent of total energy production. This loss must be added to the resistance loss of the grid, making a total of 13 or 14 percent. However, in non-industrial countries where the equipment is less sophisticated losses can be higher.
"Older power plants in many developing countries consume from 18% to 44% more fuel per kilowatt hour of electricity produced than those in OECD countries ... Transmission and distribution losses represent about 31% of generation in Bangladesh, 28% in Pakistan, and 22% in Thailand and the Philippines. (In the United States only 8% of electricity is lost during transmission, in Japan 7%.)" - 1992 World bank development brief (figures are out of date, but still serve to give a general idea).
[12] Research, development and implementation in high-tech (industrial) solar concentration benefits low-tech (non-industrial) solar concentration. Low-tech designs can benefit from research in materials, geometries and environmental factors. Likewise, research, development and implementation of low-tech solar concentration benefits high-tech, since it pioneers domestic use, and local applications, all of which high-tech machinery scaled down can also perform in more industrial or affluent areas. Also, many components are the same in both low and high tech (such as mirrors); industrialized and non-industrialized demand for these components lowers prices and increases availability. "For solar thermal parabolic trough power plants the progress ratio is about 0.88 /3/. In other words, a price reduction by 12 % can be expected when doubling the market volume."Contribution of concentrated solar thermal power for a competitive sustainable energy supply. And of course, everyone benefits from clean energy on a global scale and geopolitical stability. Non-industrial countries can easily take advantage of solar energy, compared to other energies such as wind and nuclear.
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