Characteristics of Alternate Energy Sources

There are a ton of resources on the web describing every aspect of alternate energy technologies and it is simply not possible to explore them all in depth here. I have listed a few popular sites if you want to explore further. On this page I am only providing my summary and opinions.  

 

Any relation between my opinions and actual reality is pure conjecture.

Density - maximum solar energy is often given as a bit over 1 kw per square meter, which is close to 100 watts per square foot. This only occurs with the sun directly overhead with a very clear, dry atmosphere. As the sun approaches the horizon the energy must travel through more and more of the atmosphere with a corresponding lose of energy. Moisture, dust and haze also reduce available energy (and of course clouds really reduce energy). Practical collectors can only gather a fraction of the energy which falls on them.

 

Availability - the most obvious fact of solar energy is often overlooked in casual analysis: it is unavailable the majority of the time. Over a year, a tracking solar collector (one that moves to keep pointing directly at the sun) can produce most of its rated power about 45% of the time if the sightlines are clear to the horizon. A fixed (non-moving) collector will produce most of its rated power only about 25% of the time (more in summer, less in winter). This is a theoretical maximum based on geometry, before you make any allowance for cloud cover. You might get close to this value in the desert, but in a place like New England you will only get near rated power 1/6 to 1/8 of the time averaged year round. The key question is: what do you do when the sun isn't shining?

 

General Solar LINK

 

 

Idea: This is the first form of solar energy many of us saw as the 1970's energy crisis led to solar collectors sprouting on roof tops across the country. This was most commonly a black flatplate collector under glass with either air or some kind of fluid running across it so as to pick up the heat of the suns rays. It sometimes was used to provide some space heating but the favored use was to help provide hot water.

 

Issues: poorly designed and built collectors looked crappy and broke before they ever payed back the energy and money used to create and install them.

 

My Take: in sunny climates, newer,  properly designed systems can effectively provide most of the energy required to produce hot water (which is often the second highest energy use in the home).

 

Idea: Orient the house to align with the sun, put the correct amount of windows on the south side, add the proper amount of thermal mass and the home can collect a fair percentage of its heating needs with no moving parts.

 

Issues: excessive glazing, lack of shading and insufficent thermal storage often led to spaces that overheated during the day, leaked heat at night.

 

My Take: given a well designed, properly insulated house, passive solar can provide much of the heating required spring and fall. Usually you still need auxiliary heat during mid-winter when the sun is lowest. A well integrate passive solar component can be the best way to utilize solar energy, but it sometimes isn't recognized as such because it doesn't make the power meter spin.

 

Idea: Focus the suns rays on a boiler to produce steam to generate electricity. A thermal plant can sometimes capture a higher percentage of the suns power than most photo-voltaic cells, although photo-voltaic cell may eventually catch up in that department.

 

Issues: Complicated to do this efficently on a large scale. Requires bright, direct sunlight.

 

My Take: Might make some sense in the desert where land and unobstructed sun are plentiful. Best opportunity may be in combined fuel plants which incorporate some thermal storage to cover short interruptions (passing clouds) and a natural gas burner (typically) to take over at night. This lets the same turbine/generator run full time using whichever mix of energy is available at that moment.

 

Idea: Build panels that use the Photo Voltaic effect (basically photons of light knocking electrons out of atoms in a semiconductor material) to generate electricity. There are several common types of PV cells. Crystaline silicon cells are currently around 15% efficient. Non crystaline cells are in the range of 8 to 10% efficient but cheaper to produce.

 

Issues: PV cells produce DC current, require an inverter to produce house power or tie to power grid. Storage or backup source required. Cost is an issue although it is coming down. Contrary to common belief, PV cell do not last forever, but degrade over time. There is lots of exciting research into new production techniques but increasing efficiency, reducing cost and making a long lasting cell is hard to do all at the same time.

 

My Take: A favored technology because it produces power with no moving parts and no noise and can be integrated right into the roof if a building is properly oriented. Current cost is around $4 per peak watt. People have been waiting for the big breakthrough on cost for decades, which hasn't exactly happened. What has happened is a continuous incremental improvement. Cut costs 10% per year and after 10 or 20 years it starts to add up to a big breakthrough. However, even if the cells themselves suddenly became free, solar electricity has issues simply because it isn't there the majority of the time. Storage or backup generation is a major cost, unless the load is  perfectly matched to availablity.

Idea: Use windmills (wind turbines) to extract energy from winds to pump water or generate electricity. At 15 MPH (a typical average speed for a good windy site), a wind turbine can generate about 6 watts per square foot (64 watts per square meter) of swept area. Most turbines produce their rated power at around 30 MPH. The power is proportional to the cube of the wind speed. At 15 MPH a turbine only generates 1/8th as much power as at 30 MPH. So there is usually a big difference between the rated (maximum) power of a wind turbine and the actual average power produced. Understanding the distribution of wind speeds at a particular site is critical.

 

Issues: It takes a LOT of BIG wind turbines to equal one large conventional power plant. To produce 500 MW of average power it might take: 1,000,000 turbines of 10 foot diameter or 40,000 turbines of 50 ft diameter or 1600 turbines of 250 ft diameter. People object to impact on birds, noise from turbines, visual impact. Wind is variable. At many sites, most of the power is concentrated in "energy winds" that average about 2 days out of 7.

 

My Take: Currently the fastest growing AE source of electricity. In a good wind site (and with good tax breaks) cost per kw-hr can be competitive with conventional power plants. As with solar power, the biggest problem is the fact it is intermittent so storage or alternate generation is required. Click here for the America Wind Energy Association website.

Idea: There are many ways to use natural plants, cultivated crops or biological organisms to produce useable energy. The use of wood for heating is probably the oldest form of energy known to man. Many crops or agricultural by-products can simple be burned to generate energy. Other options include using fermentation to produce alcohol (ethanol) from plant material. Pressing some plants can produce vegitable oils than can be used in diesel engines. Methane from decomposition of organic material (ranging from municiple waste to cow poop) can heat buildings or generate electricity.

 

Issues: There is much discussion over which processes actually produce a net surplus of energy. Click here for a good Wikipedia article summarizing this. (Producing ethanol from corn seems to be one of the less energy efficient options but farm state politics makes it popular.) All of these forms of energy are basically solar energy captured by the plants. While it is a big plus that energy storage is inherent in the plant material, the percentage of solar energy that gets captured and converted is rather low. Therefore it takes a LOT of land area to grow an energy crop on a useful scale. I've read estimates of 100 to 500 square miles of wood lot to support a 1000 MW wood fired electric plant. So there is a large effect on the local environment, lots of water required, etc.

 

My Take: There are really two major categories of bio-energy. The first is utilization of waste material we already produce. Methane from land fills can be captured and used to generate electricity. People with bio-diesel cars often fill up at McDonalds taking the used oil from the fryer. Did you know that Maine gets almost 30% of its electricity from wood burning power plants that run on the left overs from the paper and lumber plants? All forms of organic waste can be looked at as an energy resource. This often has a big energy payback since the energy to produce it has already been expended for other reasons. This can be as close to the mythical "free energy" as you can get.

 

The second category is crops grown specifically for the energy they can produce. This takes more thought to get right. Ideally an energy crop should grow fast without taking too much water or fertilizer. It should yield a product with high energy density (liquids are preferable for convenience) and the energy input for processing must not be too high. 

 

Some of the analysis showing corn base ethanol took more petroleum to produce than it displaced is unsurprising. Many of the processes used were created 80 or 100 years ago when oil was plentiful and cheap and the energy efficiency was not a consideration. Just because one particular process is inefficient doesn't mean there can never be a useful biofuel process. We are just starting to figure this out. One interesting link examines 29 different options and that is only scratching the surface.