NetNews Usenet Archive 1992 #27/NN_1992_27.iso/spool/sci/energy/5482

Text File | 1992-11-17 | 7.6 KB | 149 lines

Newsgroups: sci.energy
Path: sparky!uunet!charon.amdahl.com!pacbell.com!decwrl!sdd.hp.com!zaphod.mps.ohio-state.edu!swrinde!emory!kd4nc!ke4zv!gary
From: [e-mail] (Gary Coffman)
Subject: Re: Renewable Energy - solar
Message-ID: <[e-mail]>
Reply-To: [e-mail] (Gary Coffman)
Organization: Gannett Technologies Group
References: <[e-mail]> <[e-mail]>
Date: Sat, 14 Nov 1992 18:54:09 GMT
Lines: 137

In article <[e-mail]> [e-mail] (Richard Stead) writes:
>In article <[e-mail]>, [e-mail] (Sue Pang 10-16-91) writes:
>>     One form of renewable energy in the nonsolar category is
>> "Hydro energy", which is a form of energy using water power as a driving
>> force to turn the hydraulic turbine to create electricity. Its cycle
>> is very inefficient, because 50% of the input energy is lost in the
>
>I would point out that 50% recovery is generally not considered inefficient.
>Most power options operate at much less than 50% efficiency.

I'd also point out that "hydro energy" *is* indirect solar energy. It's
the heat from the Sun that evaporates water forming clouds that then rain
or snow on uplands supplying the water at a higher gravitational potential
which then runs back to the sea through those nice dams. 50% conversion
efficiency *is* quite good, about 4 times better than solar cells, and
about twice as good as solar thermal.

>> process while generating electricity. Another form is "Ocean thermal
>> energy", which is a limited heat source with only about 2% to 3%
>> efficiency. It is determined by the temperature difference of warm
>> ocean surface water and the cold ocean floor water. If it is used for
>> a large scale generation, the aquatic life in the ocean will
>> certainly be disturbed from the water flow while electricity is being
>
>Agreed, ocean thermal does not seem very practical.  Beyond efficiency
>problems, the size of the generating systems is a problem, as are major
>storms.  Getting the power back to shore is a problem, too.

Ocean thermal is yet another form of indirect solar energy. The temperature
difference between surface water and deep water is a direct result of solar
heating of the surface layers. The heat storage is huge, but the temperature
delta is small resulting in poor Carnot efficiency.

>> generated. Unlike "Ocean thermal energy", "Geo-thermal energy" comes
>> from the heat of the earth and decay of radioactive minerals. It is
>> used to generate power at hot springs, Geysers, and near volcanoes,
>> and it is quite dangerous. For instance, if the underground pressure
>> is not maintained properly, the earth's surface might cave in. Also,
>
>I think you give geothermal an unfair shake here.  First, it is merely
>cheapest and easiest to implement in shallow volcanic environments.
>But it is certainly not very dangerous.  It is probably less dangerous
>than coal mining, or many of the other efforts associated with other
>energy sources.  The "Geysers" operation in California produces
>gigawatts and has done so safely for decades.  Iceland operates almost
>entirely on geothermal power, and it is very important in New Zealand
>and Italy.  I don't know where you heard of a collapse danger, but it
>is simply not true.  There could be some subsidence risk if fluid is
>not re-injected, but this is a very slow process and is ususally measured
>in millimeters to perhaps a few centimeters.  Subsidence risk is larger
>for oil recovery and for recovery of fresh water from aquifers.

That's correct. To keep the system operational, fluid must be reinjected
in geothermal wells. The fluid balance remains nearly constant, unlike
oil and gas, or water, recovery where the fluids are removed and *not*
recycled.

>> the impurities in the steam from the earth can corrode and clog
>> pipe-lines while bringing up this energy, making it economically
>> inefficient. "Wind energy", on the other hand, generates electricity
>
>True, the brines recovered due tend to clog the pipes, but that is no
>problem - at Geysers, they just open the system full throttle once in a
>while and blast the pipes clean.  However, the impurities are a problem
>for the turbines.  I doubt they have solved this problem.  One answer
>is to use heat exchangers, thus running only clean steam through the
>turbines.  However, even with replacement of turbine blades, Geysers
>provides some of the cheapest power on the California grid.

In deep *dry* heat reservoirs, by far the largest geothermal potential,
there are *no* brines. The system is very clean and very closed loop.
In brine systems, stainless steels, and nickel plated steels are used
to prevent corrosion and heat exchangers are used to keep the turbines
clean. Geothermal has enormous potential as a relatively cheap and clean
energy source.

>> using wind power, but the wind can be unpredictable; long transmission
>> lines will make this renewable energy quite expensive. Building
>> a power plant using wind to generate electricity requires special
>> planning. Its usage also requires backup generating system.
>
>Hold that thought.

Wind is, of course, indirect solar again. Differential solar heating
causes wind. It is *also* usually more efficient than direct solar
cells or direct solar thermal (though thermal is close in some areas).

>> Renewable resources such as hydro, ocean thermal, geo-thermal, and
>> wind energy achieve very low efficiency, and it is economically
>> expensive to construct and to operate on a large scale basis. An
>
>And that one, too.
>
>> alternative renewable energy source would be the use of solar
>> energy.
>
>Now, what did you just say?  First you complain about wind energy being
>unreliable.  But solar is reliably unreliable.  Guaranteed to be offline
>exactly half the year.  Ever hear of night?  Beyond that, not every place
>is the California desert - lots of places get clouds.
>
>Ooo, and inefficient - that describes solar better than any power source
>I know of.  Very inefficient.  And would you have us destroy the country
>side by carpeting all our open space with collectors?  Not very practical.
>Photovoltaics are the worst - consuming tremendous energy to manufacture,
>and producing boatloads of toxic waste in the process.  Solar thermal
>is the only sane approach.

Actually, all the approaches listed, with the exception of geothermal,
*are* solar energy. And *all* have better efficiency than *direct* solar.
However, the *most* efficient collectors of solar energy remain *plants*.
Burning plants, freshly dead, or concentrated under tons of rocks for
eons, is still the most efficient use of solar energy. And there is very
little capital cost involved with setting up the "plants."  :-)

>> Electricity can be produce during cloudy periods or in the evening
>> after sunset, using a two-tank (hot and cold) storage system. This
>> storage system will produce 99% efficiency, the ratio of its useful
>
>Whoa!  A thermal system with 99% efficiency.  Better tell the nearest
>thermodynamicist.  Sounds like the free-energy machine argument to me.

Indeed!!!

>>     The solar energy the sun sends to earth is free and everlasting.
>
>In the same sense, all energy sources are "free".  The cost is in tapping
>them and converting them to useful forms.  The heat of the earth (geothermal)
>doesn't cost anything.  Nor does the flow of rivers.  Nor does coal or oil
>sitting in the ground.  All of the cost is in the recovery, as it is for solar.

That's also correct, and is why direct solar remains uncompetitive as an
energy source. The cost of recovery is too high for anything but modest
space heating. The heat is too low grade and diffuse to produce decent
Carnot efficiency.

Gary