Sunday, May 18, 2014

Shale Gas: Water, Water Everywhere, but Not a Drop to Drink (2 of 5)

I'll first start by saying this post is not to serve as a complete justification or vindication of shale gas and hydraulic fracturing; it is to provide context. Context can make the case seem better or worse than we expect, but either way we walk away a little more informed.

That said, I outlined my list of grievances (which I believe most concur with) in my earlier post. I'm going to spike out my major concerns that I think merit a closer inspection first. Let's first start with the 1st grievance, that concerning water.

1) Exorbitant use of fresh water.

As I mentioned before, fracking operations use between 4 and 6 million gallons of fresh water in their initial hydraulic fracturing run to break open the shale and release natural gas for roughly a year. I calculated this to be the equivalent of the annual water consumption of 4 US homes per well. There are 400,000 gas wells in the US [1], and 40% of natural gas now comes from shale source [2]. I'm assuming all wells are equal in capacity (not true, but for our purposes not a bad approximation). This means there are 750,000 homes-worth of water being consumed each year in shale operations. Vikram Rao, author of Shale Gas: The Promise and the Peril, believes that underground salt water can be used for the initial frack and that this should be mandated through regulations. Until best practices reveal the means by which salt water can be used, gas drilling companies will use millions of gallons of fresh water for fracking. I thought this was a lot of fresh water. Turns out it's only part of the story.

All thermal power plants require cool fresh water as a heat rejection mechanism. Power plants prefer fresh water over briny water for the same reason fracking wells do: lower risk of scale formation, less corrosion susceptibility, easier materials compatibility. The thing is, power plants use A LOT of water to keep cool. 49% of all water withdrawals in the US is for power plant cooling. That's 200 billion gallons a day. That's a little less than 2 Mississippi Rivers-worth of water. The next closest is irrigation to grow our crops at 31% [3]. I find this number staggering. Admittedly, this is for withdrawals which is different than consumption, and depending on the type of cooling system used, a lot of that water may be returned to the environment. 

Regardless, it's still a large number that dwarfs the water consumption used in fracking. A report from the Harvard Kennedy School estimates that water used in fracking constitutes less than 10% of the water consumed when shale gas is used in a high efficiency combined cycle gas turbine and low-water consumption recirculating cooling (the typical construction cases these days), and that this is a factor 2 lower water consumption/MWh than coal (which uses 2x as much water/energy content in washing coal than fracking) and factor 4 better than nuclear [4]. The Kennedy report is quick to add that hydraulic fracturing water consumption can stress water resources locally due to the short-duration/high-rate at which water is consumed, even though the gross consumption is relatively small. I'll add that the water consumption for wind and solar PV are essentially zero. 

There's a few things I think are important to point out here. The reason natural gas comes out so far ahead of coal and nuclear is mainly due to power plant thermal efficiency. If your power plant is less efficient, more thermal energy needs to be rejected for a given amount of work, so more water is necessary to keep things cool. Combined cycle gas turbine plants are basically two power plants in one (a gas turbine top cycle and steam turbine bottoming cycle), and as such run somewhere in the vicinity of 45% efficient. You can't run a gas turbine on coal (some people are trying though), so you can't leverage a topping cycle, so you're stuck with just one steam turbine cycle and an efficiency of about 33%. Nuclear is de-rated on efficiency for safety and process considerations, so comes in around 29% [5]. The other element I was surprised about was the water intensity of other energy extraction processes. As I mentioned earlier, coal mining is about 2x more water intensive that natural gas fracking, and uranium mining 10x more intensive. It seems to me that not only are we using too much fresh water with even our lowest demanding energy process, but we completely ignore significantly more demanding processes. I find that concerning. The silver lining about all this water research is the fact that 2 renewable energy generation methods, wind and solar PV, use no water at all. This point I believe should be emphasized. 

I think that water use in energy generation is actually a really big problem. It seems in this regard shale gas and hydraulic fracturing for combined cycle gas turbine plants are actually better than the conventional coal or nuclear power plants. While water is a large problem, it's not the only one. We'll look at other environmental impacts in subsequent posts.


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