Hi I'm Kristen Averyt. I'm a researcher here at the University of Colorado Boulder, with the Cooperative Institute Research Environmental Sciences. I'm standing here at the University of Colorado Boulder power plant. Today, what we're going to be talking about is the energy-water nexus. The thing about energy and water is that they're intrinsically linked. Water requires energy and energy requires water. First, let's start with the amount of water that's required for energy. Every part of the energy spectrum requires water, it's required for extraction of our resources, it's require for refining, it's also required for the generation of power at power plants like the one that I'm standing next to you, it's also required further on in the process. But we're not going to talk about that today. We're going to actually focus on the amount of water that's required to generate electricity, because over 80 percent of all the water used by the energy sector is used for thermoelectric power generation. So what is thermoelectric power generation? At many power plants, in fact, 90 percent of plants across the United States, the power it doesn't just require coal or natural gas, it actually requires water. The way these power plants work is that they have a reservoir of water in this power plant. You burn a fuel say coal or natural gas or through nuclear processes, and that actually heats up that reservoir of water, generates steam and that steam turns a turbine, that turbine generate electricity. But the important part of this process is that steam has to be recondensed. The best and most efficient way to do this is to bring cold water into that power plan. This cold water or cooling water is why generation of electricity requires so much water. You must have a continuous supply of cold water. Now, there are two major ways in which you can cool a power plant then you can provide cooling water. One process uses evaporative cooling. What that is, you have water coming through the power plant, you recondense the water and that water is cooled through evaporation through a cooling tower maybe it sits in a pond and you have water that evaporates into the air. The other way is that you have the power plant has a continuous supply of cold water, say that it sits on a river, sits on a stream or on a gigantic reservoir. The water continuously comes through that power plant. But there are trade-offs with these two processes, in an evaporatively cooled plan, what you're going to have is you're going to consume more water than you do relative to a once-through plant consumptive meaning that you're going to evaporate more water, have more evaporative loss. At the other type of power plant, a once-through facility you're actually going to be withdrawing more water than you would add an evaporative plan. So there are trade-offs, withdrawals versus consumptive use and the choice of what type of cooling technology is actually used at those individual power plants. The other thing to consider is that at a once-through plant, the temperature of the water that comes into plant when it's spit back out into that reservoir stream or lake on average 17 degrees Fahrenheit warmer. There are power plants where the effluent temperatures exceed 90 degrees Fahrenheit and in some cases a 100 degrees Fahrenheit. You might be okay with that if you're a bass but if you're a trout you might not be too thrilled about it. So there have been challenges with this around the country particularly in the East coast, where we have a lot of once-through plants. The second thing that's really important in terms of determining how much water is used at a power plant is actually the fuel that's burned. So say you have a coal-fired power plant down the street from your house. I do. It's just called the Belmont generating station. It's a couple of miles away. Say that power plant is going to be decommissioned. If I'm going to decommission that power plant I run a major utility and I want to replace it with say, nuclear power or even concentrated solar power but using the same cooling technology, believe it or not I would actually use just as much if not more water per unit of electricity than I would convert compared with the coal-fired power plant. So the important point here is, nuclear, concentrated solar, they are low carbon technologies but they are not low water technologies. Again, trade-offs. So say I go back to the drawing board and I'm thinking, ''Hey, you know, I really wasn't me that's low carbon and low water.'' Well, what about natural gas? If I was to replace my coal-fired power plant with natural gas I could reduce the carbon emissions by about 50 percent as well as the water-use. When I'm saying water-use I'm talking about both consumption and withdraw. So that seems like a win-win situation, but could I even reduce both even further? Well, there are options such as PV which similar to what you have on your house. They actually develop that at utility scale not just in the United States but across the world or you could invest in wind technologies. Low carbon, low water, win-win. That said, there's not necessarily a silver bullet when it comes to future electricity generation. We need power, it's really what sustain us, it's what's really helps support our economy, but we really need to think about the multiple trade-offs. For example, when we think about PV and we think about wind technologies, there are large land use footprints. There is not necessarily the perfect solution. We just have to think about the broad scope of options that we have available. Now one reason that the energy-water nexus and particularly the water that we require for thermoelectric generation has become so important in recent years is because of so-called collisions. These are times at which electricity generation at power plants has had to be in either curtail or completely shut off, and this has happened for one of three reasons. The first, if your incoming temperature, the temperature of water coming in your power plant is too warm the efficiency of the power plant absolutely tanks. As a consequence it's not efficient to generate electricity. You have to have that cold water. In the case of a nuclear power plant it's flat out dangerous to be bringing hot water into the plant. The second is if your estimate temperatures are too high. I mentioned those 90 degree and a 100 degree effluent temperatures, in some states across the United States there are regulations where temperatures cannot exceed certain thresholds as in order to protect aquatic ecosystem. Then the third reason is that there's just flat out not enough water. You have to have that water available in order to make it run. So during times of drought we've actually seen power plants that have had to shut down because they just flat out don't have enough water. So most of the collisions that we've seen at the energy-water nexus have occurred back East. But there's one very important collision or a potential collision that we're realizing here in the Western United States, and that has to do with a hydroelectric generation on Hoover Dam. Hoover Dam blocks off the Colorado River and create Lake Mead. At it's fullest Lake Mead has an elevation of 1,220 feet. We've seen the bathtub ring. Imagine if it was full, that's the highest elevation. When that level drops below 1,050, Hoover Dam almost has to completely shut down. That's the level at which there's too many bubbles coming into Hoover Dam and we can't generate electricity anymore. Right now, we're almost at a critical elevation. Today, we're at a 1,085 feet elevation in Lake Mead. At that elevation, 1,220 minus 1,085, for every foot decline we're losing 5.7 megawatts of potential generation on Hoover Dam. Right now, Hoover Dam is operating at about 60 percent of the total capacity for which it was actually originally licensed. That's very important when we're thinking about energy, water, drought as well as implications of climate change here in the Western United States. So let's transition now and let's think about the energy or power that's required for our water system. Average across the United States, the best estimates are that between 10 and maybe 13 percent of our entire electricity supply goes to powering our water systems. That's pumping our water up from groundwater, it's treating wastewater, it's making sure that we have fresh water delivered to our homes, it also includes heating the water that we use in our homes. But in the Western United States we use a disproportionate amount of our electricity for our water systems. In fact, in many states across the Western United States, over 20 percent of the electricity that we use goes to power the water sector. So the map that you're seeing on your screen right now what it's showing are the relative trade-offs between locally occurring natural supplies and local demands. In places where you see shades of yellow, orange, and red, these are places that are necessarily running out of water. These are places that rely on reservoir storage, conveyance or overpumping of groundwater or on wastewater treatment in order to ensure that there are adequate freshwater supplies. So if you live in the Western US, what this picture tells you is that you are really relying on pumping and conveyance and large-scale infrastructure to ensure that there is adequate water. But all of those processes that I just named they have take an incredible amount of electricity. For example, the Central Arizona Project pumps water from Lake Havasu up to Phoenix and then onto Tucson. Three things you need to know about that system, it cost over three billion dollar to construct, it travels over 3,000 foot of elevation, and every year it uses on average three million megawatt hours of electricity. That's a tremendous amount of electricity. The Central Arizona Project or CAP is the single largest user of electricity in the state of Arizona, and what powers CAP? It's a coal-fired generating station that sits on Lake Powell in Page, Arizona. That power plant over 40 percent of the electricity that it generates goes to power the Central Arizona Project. So you live in the state of Arizona and you get your water from CAP and you're worried about your carbon footprint, you might want be considering where your water is actually coming from. But CAP is not the only large scale projects across the Western United States, there are several. There are some that pump water, for example, over the Tehachapi Pass and down into Southern California. That project requires a lot of energy. For those for example living in Northern California, it probably takes about 1.3 mega watt hours of electricity per acre foot of water that you use in your house. One megawatt hour that's how much a family will little bit more than a family might use in the year. So hopefully that gives you an idea of what those numbers are. So again, Northern California 1.3, Southern California because of transporting the water over Tehachapi Pass, over four megawatt hours of electricity per acre foot delivered is required. Compare that with some of our friends back East, in New York City your water supply takes less than a megawatt hour to provide one acre foot of water. So depending on where you live anywhere in the United States there's going to be a very different amount of electricity embedded in your water supply. So if you care about carbon and if you're thinking about your electricity use, then you really need to be looking at where you get your water. But this isn't just an important issue now. We also have to think about the future. Right now, there are many additional infrastructures being considered across in particular the Western United States in order for us to be able to deal with future drought events as well as the potential for climate change. Some of these systems have tremendous electricity demands. This is very important that we think about what's happening with respect to our water systems and what the implications are for energy demands and to circle back and this energy-water nexus. If we're powering our water systems with thermoelectric generation, we're using water for that thermoelectric generation. Now granted we do have other options, there are hybrid technologies and dry cooling options that are being employed across the West, because we know in the West we are innovators. We really want to think about what we're doing. But we really are looking at a very different future and how we deal with our water supplies and ensure that we all do have safe drinking water, we do have fresh water available, we can support agriculture and ranching. We need to really be careful, we need to think more holistically, can't just be thinking about water, can't just be thinking about energy, can't just think about carbon alone, we need to tie those all together. So I hope that you found this really interesting. Energy-water nexus, not something everybody thinks about. But hopefully, I was able to drive something home that you'll consider in the future. Thanks.
