Power generation technology's impact on water supplies.

[skids]

"Most of the electricity generated around the world is produced using water intensive processes. Thermoelectric power plants use fossil and nuclear fuels to heat water in large boilers where steam is created to drive large turbine generators. Water is circulated throughout the power plants in huge quantities to cool the turbines, clean scrubbers and boilers, and perform a number of other tasks. Chemicals are often added to the water to extend the life of the machinery and prevent biological growth in the cooling towers. And many power plants return no water to the source - it is all lost to evaporation. Other power plants return water to the source at greatly elevated temperatures, destroying or radically altering the local ecosystems."

...

"Billions of gallons of water can be saved every day through the use of solar, wind and other renewable energy systems. According to a United States Geological Survey report published in 2000, 195 billion gallons of water are withdrawn every day from our aquifers, lakes, rivers, and oceans to cool thermoelectric power plants in the US. This water withdrawal represents 48 percent of total water withdrawals in the United States."

http://renewableenergyaccess.com/rea/news/story?id=35664

[phantom power]

1. There's an angle I hadn't heard of before.

That could be helpful in the coming droughts.

[hunter]

2. Water conservation is a very hot issue in power plant design.

It's entirely an economic issue. Large dry cooled power plants are more expensive to build than water cooled plants. It's much less expensive to dump waste heat into a large body of water or use evaporative coooling than it is to dump waste heat directly into dry air.

The use of dry cooling in large power plants is increasingly common in places where the supply of cooling water is restricted.

[BeFree]

3. The inefficiency

...of these plants is pointed out very well here.

It takes heat to generate the power, but so much of the heat is just wasted away.

Once again pointing out that the chemists and engineers who designed these plants are not taking into consideration all the factors involved. We simply can't trust them to do the right thing unless we are allowed full involvement in the whole process, from design to construction.

Many people have won the rights of citizen involvement in the process leading to such things as stopping many nuke plants from being built. That is an example of democracy in action overcoming greedy private interests.

[hunter]

4. Modern power plants are very efficient heat engines.

In a heat engine producing electricity the amount of waste heat is determined by the second law of thermodynamics.

If you want to google this yourself the words you want are "Carnot Cycle" or "Carnot efficiency."

Here's one page I came up with, which I thought it was interesting because it includes a carnot efficiency calculator:

http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/carnot.html

In a modern combined cycle power plant fuel is burned in a turbine which turns a generator. The turbine is basically a large jet engine. The exhaust of this turbine is then used to boil water which powers a steam turbine.

http://www.powerfrontiers.com/ngccplants.htm

The upper level carnot efficiency achieved by these plants is mostly limited by the ability of the gas turbines to withstand heat. Even with perfect materials the upper temperature would still be limited by the temperature of the burning gas.

The efficiency of a modern combined cycle plant is about 60%, which means 40% of the energy in the fuel must be rejected as waste heat. (An automobile wastes over 90% of the energy in the fuel.)

Some sort of electrochemical fuel cell might theoretically achieve better-than-carnot efficiency, but first someone has to figure out how to do that.

[skids]

5. Wasted heat, not waste in thermoelectric conversion.

It's not the efficiency of the thermoelectric conversion. It's the efficiency of throwing out the heat because there is noone to use it.

In a micro-CHP application, the waste heat is used for hot water, so the total efficiency of the system is much greater, simply because there is someone there to make use of it.

http://www.whispergen.com/main/products/

[hunter]

6. Bathtub warm water is not a particularly valuable resource.

Micro-cogeneration projects need to be compared to such alternatives as geothermal heat pumps.

http://www.energystar.gov/index.cfm?c=geo_heat.pr_geo_heat_pumps

In many cases the overall thermal efficiency of very large and very efficient central power plants coupled with geothermal or solar assisted heat pumps can exceed the thermal efficiency of "micro-CHP" cogeneration schemes.

But the sorry truth is that we live in a society where such optimizations are exceedingly difficult. In most domestic households heating is accomplished using simple gas, oil, or electric resistance heaters.

Furthermore, most households and businesses do not have appropriate levels of thermal insulation.

[skids]

7. Oh, I certainly agree.

CHP as designed burns fuel. I suppose if it were biodiesel it would be sustainable.

However in some situations geo and solar heat need a little "top up" help from fuel or electricity anyway -- and in those cases where fuel must be used you might as well feed that preheat stream into a CHP unit and get electricty as well.

The point I was trying to make, though, is that it is inefficient to throw away the waste heat at a central location.

(EDIT: I don't agree with the comment that warm bathtub water is not valuable, though. Look at how much money and carbon load we use to produce it. That's how valuable it is.)