SM

I did a Google search on the Kendall Power Station as you suggested. I thought from your post that it was nuclear, but it is coal fired and so not really relevant to your point that geothermal uses three times as much water as geothermal. Incidentally, the Johannesburg Electricity Department website refers to the "station's six steam turbines". I the assume that the steam comes from water, so there must be some water usage.

I don't agree that hot dry rocks are only option, so I guess we'll have to agree to disagree on this one.

My error - it should read three times as much water as nuclear".

Loxton,

It is clear from your posts that you have very little idea of how electrical power is generated.

Nearly all electrical power (PV solar excluded) involves spinning a turbine. For converting heat to power, the most effective way is to convert the heat to high temperature high pressure steam and use it to spin the turbine. The low pressure steam is then condensed releasing a portion of the heat energy, and then returned to the boilers again.

The water generating the steam circles in a loop, so the ongoing consumption is relatively small. The large consumption is used in expelling the heat.

This applies to hot rocks, nuclear, coal etc.

Kendal uses coal, but due to the shortage of water, uses air cooling instead of the water based cooling towers. These are vastly more costly.

These are proposed for the hot rocks to reduce the water consumption, but can also be use for nuclear, or any other system when water is not available.

The hot rocks also loses water into the granite, etc.

Au contraire, I understand perfectly well how electricity is generated. Moreover, I understand that nuclear power stations use a lot of water. This paper talks water use by nuclear power stations http://www.efmr.org/files/07rn12.pdf and concludes that "Per megawatt existing nuclear power stations use and consume more water than power stations using other fuel sources. Depending on the cooling technology utilised, the water requirements for a nuclear power station can vary between 20 to 83 per cent more than for other power stations."

I have also followed up on your stuff about air-cooled nuclear power stations and found Pebble Bed reactors which are gas coooled. However, there don't seem to be any that are actually operational.

I repeat that hot dry rocks are not the only geothermal option for Australia, in my opinion, but taking hot dry rocks for a moment. they equally do not have to use water. Other fluids are feasible including CO2 of all things. If they use water, it can be condensed and re-used which is waht they are doing with Geodynamics test hole and how it can operate in the middle of the desert.

As I said, I find it difficult to beleive that geothermal will use three times as much water as nuclear, but I am willing to be proved wrong.

Loxton,

I am a power engineer and have built several generation systems.

In all steam cycles cooling is required to condense the steam. The ratio between heat expelled and heat converted to electricity is defined by the Carnot limit as follows:

Efficiency = (Ts-Tc)/Ts

Where Ts is the steam temperature, Tc is the condensing temperature in degrees Kelvin.

Ts is determined by the heat source and the safety limits of the boiler.

For modern coal fired boilers 550C (or 823K) the max efficiency is 58%.

For existing nuclear reactors, safety limits are stricter, and Ts is about 450C For this the max efficiency is about 52%

While no system gets near the carnot efficiency, Modern coal fired boiler systems get a thermal efficiency in the order of 35% and nuclear 30%.

Putting this simply Nuclear power stations need to expel about 20% more heat per unit of power generated. (for older test reactors running at very low temperatures this can get up to 80%)

Water cooling is by far the cheapest and most effective. However, where fresh water is an issue, sea water, or through flow can be used.

In extreme circumstances (such as Kendal), air cooling can be used.

Considering that 99% of all power stations use water for cooling, this is where the figures come from.

For geothermal, the Ts is in the order of 250C giving a max efficiency of 33% and a real thermal efficiency of about a third of of a coal system.

Considering that modern nukes with modern metallurgy run at temperatures in excess of 500C, their thermal efficiency is much improved.

Geodynamics has tried to alleviate the water usage by using air cooled systems, unfortunately hot rocks also has the problem of water loss into the granite, which consumes fresh water.

While nukes can be built at the sea and consume no fresh water, (and with condensation towers generate a vast supply of fresh water from sea water) Hot rocks can not.