Sunday, September 27, 2009

Water-Demand makes Renewables Unsustainable.

It takes resources to get resources, especially those of water, one that is often overlooked in the various strategies of obtaining renewable energy. It is reckoned to take 2,500 gallons of water to grow sufficient corn to make one gallon of ethanol, against which the often quoted, but still sizeable, four gallons of water required to produce a gallon of ethanol from corn (or other source of sugar) is a mere drop in the ocean. Indeed, the intention to produce 36 billion gallons of ethanol per year by 2022, would use enough water to keep Chicago supplied for over 100 years. In Illinois, fields are adequately watered by rainfall whereas in more westerly and dryer regions it is necessary for farmers to actively irrigate their fields.

California has given a target of producing 1 million gallons of ethanol annually, but to grow enough corn to do so would need the entire volume of water that is currently diverted from the Sacramento-San Joaquin River Delta. Since this is the water that is used presently to irrigate 7 million acres of the Central Valley and provides water supplies for the cities of Southern California, it is debatable there is enough water to fulfil these purposes and that of wholesale ethanol production.

The problem of corn-ethanol, and its water demand, could be circumvented by making cellulosic ethanol instead but this technology is some years off from being one of large-scale production, and realistically, making ethanol in the quantities that are spoken of needs to get away from corn. In Brazil the sugar-cane ethanol industry is mature and is far less demanding in terms of water, since the crop is substantially supplied by rainwater.

I had not thought of there being a resource connection between solar-energy and water, but it seems there is. Photovoltaics are fairly independent of water, and generate clean (green) electricity with little demand once they are installed. The same is not true of solar power plants, for which a technology known as CST (Concentrating Solar Thermal) is more useful, in contrast to smaller, e.g. solar panels on the roof, type installations. CST employs an array of mirrors to focus sunlight onto a working fluid under pressure which is used to transfer heat to generate steam and then drive a steam-turbine to make electricity.

Solar thermal power plants (as do all power plants) produce waste heat, which is removed in cooling towers and released into the ambient atmosphere by the evaporation of water. In reality, beyond the initial stage that uses heat from the sun (and so is entirely renewable) the rest of the plant is that of any other kind of power plant and gets through huge quantities of water. Since it makes more sense to situate such CST installations is sunny spots (such as the desert Southwest), where there is year-round sunshine, there is an additional pressure therefore imposed on regions where securing adequate water supplies is already an issue, for example the Mojave Desert where it is planned to build 150 CST plants.

It is likely that heavy groundwater pumping would kill desert wildlife which depend on precious water from seeps and springs which would run dry. CST plants can be cooled by air but again the desert climate poses a problem, since higher outside temperatures decreases the efficiency of waste heat disposal, and wet-cooled plants are preferred because the heat-transfer is better, meaning they produce 5% more power and are 10% cheaper to build.

The problem is compounded by the effect of climate change which is blamed for a reduced flow in rivers out-west, and a consequent reduction in the amount of available hydroelectric power. The provision of water and energy are not independent agenda and need to be considered in a combined strategy.

Related Reading.
"When Renewable Is Not Sustainable," By Robert Glennon.


Mark said...

It is funny that you mention the amount of water necessary to grow enough plant matter to produce ethanol in California because we are experiencing a third year of drought with no end in sight. Plus there is the added problem that the pumps feeding water from the Sacramento River to the farms San Joaquin had to be heavily curtailed because enough water has to be left to keep an endangered fish, the delta smelt, from being decimated. Combine this with the fact that the Sacramento River feeds into environmentally sensitive areas of the San Francisco Bay where salt water intrusion is becoming a problem. Then there is the problem about allocations from the Colorado River which will soon be reduced so the water can go to Arizona and Mexico.

Things are getting so bad that some water districts like those in Long Beach, Marin County and San Diego County are actively pursuing desalination projects. I have heard positive and negative things about desalination but good or bad that appears the direction we are heading.

I guess the famous American writer Mark Twain was correct when he said "whiskey is for drinking and water is for fighting" because I predict there will be a lot more legal fighting over this precious resource in the future.

Professor Chris Rhodes said...

Yes, I remember Mark Twain's remark! Perhaps it will prove apposite?

Water does seem to be the unthought of or unmentionable resource for so many aspects of energy...



Unknown said...

Hi Chris,
There is a company in California called “Sol-Focus” which produces concentrating solar pv systems. These do not use any water for cooling purposes. Their products consist of arrays of parabolic mirrors, which focus the sun’s rays directly onto the pv unit. I understand that they have a plant somewhere in Spain.

Professor Chris Rhodes said...

Hi Peter,

thanks for this information. So this is concentrating PV rather than concentrating solar thermal which does seem to use cooling water. I know the latter are supposed to be more efficient in terms of energy recovery in the form of electricity, but there is the water issue.

Best regards,


Jane said...


On the topic of CPV, this technology now has efficiencies that typically beat 25%, and with higher manufacturing volumes have reached a levelized cost of electricity that in many cases is competitive with or better than concentrated solar thermal.
Coupled with very low water requirements and low land impact this makes this technology a very promising approach for high solar resource locations in general, and hot dry locations in particular, without having some of the drawbacks you mention for CSP.
Worth checking out...


Professor Chris Rhodes said...

Hi Jane,

that's very interesting and worth looking into further, which I shall do.

I think it is a case of "horses for courses" with all kinds of solar and other energy technology. So, as you say, CPV may prove best for arid climates but CSP for sunnier climes where there is water in reasonable access since the overall efficiency runs around the typical Carnot cycle value of about 35%.

Best wishes,


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