The Science Behind The Simplest Type Of Solar Power

Garden hoses are the devil. They lay there, quietly baking in the hot summer sun, just waiting for some dumb schmuck to come along and try to take a sip — then BAM! A faceful of scalding hot water.

However, this painful phenomenon is not without benefits — hoses turn into miniature geysers thanks to solar thermal energy (STE), one of the oldest and greenest renewable energy sources known to man. Unlike photovoltaic technology, STE harnesses the sun's rays to generate heat rather than electricity. In residential use, STE usually boosts the efficiency of water heaters, heat pumps, and absorption chillers. In large-scale installations, it can be used to generate electricity. Here's how it works.

The use of STE dates back hundreds of years. In 1760, Swiss naturalist Horace de Saussure devised a solar oven built out of a half-inch tall pine box, topped with glass pane. Placed in sunshine, the glass pane and enclosed area worked in tandem to concentrate incoming solar energy and prevent its escape (aka the greenhouse effect). It was effective enough to boil water to a temperature of 228 degrees F. "It is a known fact," Saussure said, "that a room, a carriage, or any other place is hotter when the rays of the sun pass through glass."

STE-harnessing technology continued to develop into the 20th century when, in 1881, Charles Kemp patented the first passive solar water heater, today known as "batch water heaters," and sold them as the Climax System. To make domestic hot water, it used a trio of water tanks set in a sunny greenhouse on the south wall of a home. In 1909, William Bailey improved upon this design by separating the heating element — panels filled with copper water pipes — from the storage tanks. This allowed the system to continuously heat larger quantities of water for a houshold throughout the day. Bailey sold 4,000 of his Day and Night Solar Hot Water Heaters over the next nine years. But an adundance of cheap fossil fuels was already reducing the demand for this technology.

Reducing — but not eliminating. Even as the world economy exploded with the advent of the internal combustion engine, STE technology continued to evolve. Today, low-temperature collectors make up a vast majority of STE technology in America. These systems, commonly used to heat pools, operate at or below the surrounding air temperature. They're basic: A black plastic mat, placed over the pool, transfers the sun's energy into the pool and holds the heat in the water.

Medium-temperature collectors, which operate at temperatures significantly higher than the ambient air, have also found a niche heating homes and businesses. Solar water heating, like Kemp's Climax, uses solar water panels to continually heat a large supply of water. They can be passive, like Kemp's, but can also be pump-driven, active systems. Systems like these can provide domestic hot water, or they can be used with heat pumps to efficiently heat and cool entire houses. Some of the more creative new applications of these systems can be seen at the biennial Solar Decathlon competition, a contest among college students to design and build a home that achieves net-zero energy consumption.

High-temperature collectors are another beast entirely. These systems are used exclusively in energy production. They occupy square miles of land, and reach temperatures into the thousands of degrees. The solar furnace at Odeillo in the French Pyrenees-Orientales tops out at nearly 3800C. These plants rely on a huge number of reflective collection plates, which can be flat, trough or parabolically shaped, to concentrate the sun's rays on a single point to super-heat a fluid — like synthetic oil, molten salt or pressurised steam — which then drives a Sterling, steam or gas turbine engine. Unfortunately, as much as 70 per cent of this heat is lost before it can be put to work by the engine.

The US built nine such power plants, between 1984 and 1991, in the Mojave Desert in California. Still in operation today, they together produce about 354 megawatts annually, enough to power a half million homes. Sandia National Labs also runs a solar farm just outside of Albuquerque, New Mexico. The only testing facility of its kind in the US, the Solar Thermal Test Facility at Sandia National Laboratories can focus six, 11.5m parabolas, each comprised of 82 mirrors, at a seven-inch wide target and heat it to over 650 C. "That'll melt almost anything known to man," Sandia engineer Chuck Andraka told Popular Mechanics. "It's incredibly hot."

While such solar farms take up a large amount of space, they have the capability to deliver an equally large amount of energy. Industry experts like Fred Morse, a senior advisor to Spanish power utility, Abengoa, believe that were the Southwest's 226,000sqkm of open, flat and sunny land were used for a solar farming, it alone would generate 11,200 gigawatts of power. But the current, reasonable estimates put the total installed STE-collection base at just 35 gigawatts by 2020. Still, the entire country generated 1,138 gigawatts in 2010. Even that lower estimate significantly alter the amount of electricity generated by renewable sources.

[How Stuff Works - Popular Mechanics - CNet - Greentechnica - Wikipedia 1, 2, 3, 4 - Radiant Solar - California Solar Center - ECS Solar]

Image: tangencial/Shutterstock


    My personal favourite use for solar-thermal was actually not covered - the solar updraft tower:

    Not terribly efficient, but decidedly low-tech (i.e., lower cost of construction/maintenance) and more importantly can be done in tandem with solar-pv/high temperature solar thermal.

    I love the idea of this tech. Watched a show on discovery channel ages ago about the parabolic mirrors heating molten salt. Its a brilliantly simple concept that unfortunately hasn't caught enough traction yet. The glass for the mirrors is expensive to produce and producing glass also uses a crap load of energy.

      Producing glass for the mirrors does use a lot of energy. But then you're interested in how much payback you get - the metric used to measure this is the Energy Return On Energy Invested, or EROEI figure. It calculates the ratio between the energy output by the power plant and the total of all energy used to build it, including mining, refining, manufacturing, construction, etc.

      Some sample EROEI figures:
      Solar collector (what we're talking about here): 1.6
      Wind turbines: 18
      Nuclear power plants: 50 (I think that's once-through for the fuel, meaning you throw away about 97% of it unburnt - with reprocessing this could be well into the hundreds)
      Hydro: 100
      Coal: 80

      From an EROEI perspective, solar thermal doesn't look so hot (pardon the pun).

        I should note, I just pulled those EROEI figures from Wikipedia - there are probably better, more up-to-date figures around, looking at different types of solar thermal. Some of these technologies might get better performance.

    Make an excellent bong lighter.

    Australia has LOTS of space, most of it desert.

    Last edited 05/12/12 8:09 am

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