Efficiency of Wholes Not Parts

Solar Power Design Manual

Do It Yourself Solar Energy

Get Instant Access

Critics of renewable energy typically make five erroneous assumptions. They assume electricity must be generated in a power plant; that existing generating facilities run 24 hours a day; that existing facilities are highly efficient; that renewable energy is insufficient to power big industries; and that price is all that matters. Based on these beliefs and, often, a dose of economic denial about the subsidies for conventional fuels, renewable energy is dismissed.

Solar energy is seen as weak, just as electric cars are seen as wimpy. But unlike conventional engines, electric motors can deliver full torque at any speed. A race between a conventional Grand Prix car and an electric car wouldn't be a contest. With no transmission and precise power control the electric car would lap the field. Similarly, few notice the extraordinary flexibility of solar technology. It can be used in dispersed form to heat water or generate electricity, and it can be concentrated to melt through steel. The sun generates enough energy to power us and all the plants we eat; all the trees our houses were made of; all the ships and animals that carried our ancestors; and all the wind, waves and clouds that define the lives of the trillions of organisms that populate earth — hardly a wimpy performance.

Renewable energy is often criticized as being inefficient, a characterization that adds to the wimp factor, with the implication that existing technologies are very efficient. Citing the low efficiency of photovoltaics in converting to electricity a seemingly meager 12 percent of the light that strikes the cells, critics dismiss the technology. They compare such low efficiencies with that of a big power plant, where more than 50 percent of the fuel con sumed might be transformed into useful electricity. This is a dubious comparison.

Efficiency is a measure of economy. A machine is said to be efficient to the degree it uses the least resources to accomplish the most work. But we often look at machines in isolation. Much like assuming cars are cheap without counting the cost of roads, proponents of one or another technology often claim their concept is very efficient at generating electricity, but neglect to note the energy lost in the whole system of distributing electricity

The electric-utility system in North America and much of the world is based on largely centralized power plants transmitting power via long-distance powerlines and local distribution grids. On average the distribution system loses about 60 percent of the power in transmission. Then consumers waste about half of what they receive in low-efficiency structures and appliances. Industrial and commercial customers waste less, but often maintain old and inefficient facilities. Aged distribution systems in many countries probably lose more than 75 percent.

Coal under the high plains of Wyoming is potential electricity. But first it needs to be mined, and that takes energy in the form of big shovels. Second, it needs to be moved, and that takes the energy of many trains. Third, it needs to be burned in power plants, and that loses energy in wasted heat. Fourth, the electricity needs to be transmitted to consumers via the grid, and that loses energy by the resistance in wires. In the end, after subtracting all the energy consumed, it's likely that only about a third of the original potential energy in Wyoming actually arrives at one's toaster in the kitchen in Chicago. On average the means of producing and distributing electricity consume about two-thirds ofthe energy.

If the majority of homes converted to independent or neighborhood-scaled local-grid systems, with hydrogen and/or batteries used for storage, the sun could power all structures. The amount generated, primarily by photovoltaics plus wind turbines in some areas, would range from 2.5 to 3.5 times maximum consumption on any day, and could recharge hydrogen tanks and/or battery packs for night and cloudy periods. Assuming new technologies are used, the future home would use less electricity than it does today, thus reducing the size and cost of the energy equipment, and representing less total generating capacity

Which is more efficient, a system that consumes more than half the energy it generates in machinery stretched out over the landscape, or a system that consumes a tiny quantity of steel and silicon to produce electricity from a roof only feet from the toaster ? Does it matter if photovoltaics are only 10 or 15 percent efficient if a typical home can be powered with PVs covering half the roof ? New photovoltaics achieve 18 percent efficiency, and others in development prove efficiencies exceeding 25 percent. There's no shortage of sunlight.

Presuming photovoltaics must be concentrated in a power plant, critics envision vast remote fields devoted to PVs. They succeed in transforming a solution into a problem.

The notion that electricity is made in a factory is couched in a 19th Century vision of power generation, where noisy smoke-belching power plants were far away on the other side of the tracks. Renewable energy technologies, especially photovoltaics, don't need to be isolated because they just generate electricity, nothing else. The very qualities that make them valuable make them unnoticeable.

Roofs are an under-utilized resource covering more than half the area of cities, and they are right over our heads. The potential of electric roofs is obvious from the window on any flight approaching any airport into any city. Roofs dominate the city landscape. The vast majority of buildings are one-to three-story homes, apartment buildings, retail complexes and industrial structures. Photovoltaic roofs, with solar-thermal systems on the roofs of larger industrial and office buildings, can provide ample power for the vast majority of structures.

Roofs can be more than just shelter. The flat urban roofs of many homes could be part photovoltaics and part grass, with the latter creating a cool roof with perhaps a deck. Instead of being just an expense, the roof becomes a means of keeping warm, generating electricity, collecting rainwater, oxygenating the air, and perhaps visiting with friends. This is maximizing your asset value.

Renewable energy is as practical for powering a factory as a watch. Judging by the sheer size of many industrial and commercial structures it would seem they consume a vast quantity of electricity. Yet San Francisco's Mos-cone Center, a massive two-block-square underground convention center, receives half its electricity from photovoltaics covering a quarter of its roof. Where higher power densities are required, roof-mounted solar-thermal technologies can supply two to four times the energy in the same area. The strategies would vary by building, but in the vast majority of cases all needs could be met on or near the structure.

Solar-thermal power generators are ideal for producing heat and electricity. Contemporary systems use mirrors to focus sunlight on pipes carrying oil and are generally being built by utilities as large power plants. The hot

It is often said conventional utility systems are efficient and photovoltaics are not. This is a questionable assumption. Electric-utility systems are built around centralized power plants transmitting power via grids and consuming a fuel. Obtaining the fuel, processing it, moving it, burning it and distributing the product, electricity, can consume from 75 to 90 percent of the original energy value of the fuel. Thus the entire system is only about as efficient as typical photovoltaic cells, which can be located where the power is needed and use fuel delivered daily — light.

oil is continuously circulated through a boiler to generate steam, which is used to spin a turbine to drive an electric generator. The same system can heat a space or use hot water for an industrial process. Solar-thermal generators could be mass-produced in several sizes, from a small plant the size of a standard gasoline generator to larger units designed to be installed atop factory or office building roofs or over parking lots.

There is a specific type of solar thermal collector, called a non-imaging concentrator, that exemplifies the simplicity of renewable technologies. Its purpose is to concentrate sunlight to achieve higher temperatures. To do that it's been assumed that the device must be centered on the sun constantly; thus a small guidance system, analogous to celestial drives that move a telescope to follow a star, would be required to track the sun by day and season. By contrast a non-imaging concentrator is a mirror shaped to a precisely calculated curve of changing radius, much like the inside of a small teacup, and because sunlight striking that particular curve will be reflected downward, no matter what time of year or day, no tracking system is required. Non-imaging concentrators can be cone-shaped or long troughs aligned east to west, but in either form they need only be open to the sunlight. Light striking interior walls will be reflected like water spiraling down a drain. A small non-imaging concentrator, say a metal cone about one foot tall, with the interior wall coated with chrome, can focus about 860 watts on a ceramic disc half the size of a thumbnail. It would be white hot.

Photovoltaics are viable for the majority of residential, small commercial and stand-alone purposes, while photovoltaics plus solar thermal, wind or small-scale hydroelectric power would be required for larger industrial applications and major complexes of buildings. Solar-thermal facilities are the most attractive option for large-scale structures and industrial complexes because they concentrate so much energy in a small area, with the capability of generating both electricity as well as heat as hot water or steam.

Was this article helpful?

0 0
Solar Power

Solar Power

Start Saving On Your Electricity Bills Using The Power of the Sun And Other Natural Resources!

Get My Free Ebook


Post a comment