24/7/365 Power for North America from Solar Towers
It is possible to run the entire North American grid just on solar power towers. I do not mean that the combined array would provide enough generation capacity. I mean that it would provide sufficient amounts of electricity to satisfy the needs of the entire grid every hour of the year, come rain or come shine—for the sunny subdivisions of Las Vegas, for cloudy Pittsburgh and everything in between. It can provide for all the residential; and commercial, and industrial needs.
I should know, I modeled this.
I ran a simulation on every temperature, cloud coverage, solar radiation, humidity, and wind parameter of every hour of the year in a number of key locations around North America: in all, about sixty-five different meteorological parameters for each hour of the year for each location. I used a meteorological data set that had been carefully chosen to typify the weather in given locations sampled over decades. I optimized the hourly power output for every hour of the day, for every month of the year, and for every one of five suitable regions. Then I combined it all into one energy-generation composite and reviewed the resultant hourly energy outcome against anticipated demand for every hour of the year. The damn thing works.
Now, it is possible to trim down the total land footprint required by introducing other technologies, such as wind and PV panels. Yet, my point in this paper is just to show that we can run the entire show on solar tower powers alone.
Solar tower powers have the incredibly useful capability to take in the sun's rays and provide energy at noon on an August summer day and also in the middle of a frigid night in January.
The mechanics of a power tower are straightforward enough.
Thousands of tracking, moveable mirrors—called heliostats—follow the sun’s path throughout the day. In tandem, they reflect the sun’s rays, directing them to a bank of tubes located on top of a central receiver tower at the heart of the installation. The tubes contain molten salt, which the converging rays of the sun heat up to a sizzling 565°C (≈1,050°F). Subsequently, the molten salt flows down into a storage tank. Later, the heat embodied in the salt is used to generate steam and generate electricity in the traditional fashion. However, in the interim, the tank stores the molten salt until it is time to generate electricity. This is a big deal. Essentially, this decouples power generation from the capture of solar energy. This makes it possible to have power on demand, both when the sun is shining and when it is not.
When electricity is to be generated by the solar power tower, the superhot salt is routed from the storage tank to heat exchangers. The resultant steam is then used to generate electricity in a conventional steam turbine cycle that is found in coal or natural gas power stations. The heat energy extracted from the molten salt in the exchanger brings it down to 290°C (≈555°F), a temperature at which the salt still remains molten. After exiting the steam generation system, the cooler molten-salt is routed to a second insulated storage tank where it waits. When it is needed, the salt goes up the tower via pipes for reheating to blistering temperatures again.
The solar power tower is to have dual salt storage units that together provide up to 17 hours of storage, 17 hours of reserve power. The salt used in a solar power tower is a mix of 60% sodium nitrate with 40% potassium nitrate. These minerals are abundant.
The molten salt can be kept in reserve to be used as needed in a molten state for at least one week before it would inch down to dangerously low temperatures and turn solid within the pipes. In the liquid state, salt has a viscosity and appearance similar to that of water and has several highly beneficial properties in solar power applications. First, liquid salt has highly efficient heat transfer properties, and it retains heat for long periods with minimal losses. Second, the salt can be heated to high temperatures without any degradation, resulting in efficient energy storage and electricity production systems.
In order for the solar installation to provide power around the clock, we will need two fully independent facilities working in concert. One facility would be tuned up and configured to generate energy during the night, the other would take care of the day—thus ensuring twenty-four hours of continuous power supply. To my knowledge, the idea of two solar towers working thus in tandem is novel. I am not aware that anyone came up before with nighttime and daytime installations configuration, coupled with storage units that provide continuous power
Using the reserves of molten salt, the charge of the nighttime installation would be to provide juice during the night time, with a distant second goal of producing energy during the day. The charge of the daytime installation is the reverse. It kicks into a high gear during the hours that the night installation is at low ebb. Together, the two installations complement each other. Together, they provide continuous power.
During the summer months, the tower installations would generate enough energy to come out of our collective ears, day and night. However, during the winter months, there is a need to control the amount of molten salt that is released in any given hour of the night, when the sun is not supplying the system with any additional source of heat.
It requires a creative and stringent regimen to coax electricity in the cold months from the relatively limited supply of pitch-hot molten salt that is in storage once the sun sets. Too much released at any one time will not leave molten salt in sufficient amounts for subsequent hours of the night. It is a balancing act.
If every day, or every second day, was sunny, the above scenario would be sufficient. However, this is not the case. There are many consecutive days that are cloudy, at which time the reserves of salt would run down and with them the power output. To assure a continuous power supply throughout the year irrespective of local weather, there is a need for a network of such tower installations, scattered over many hundreds or thousands of kilometers. It may be rainy for a few days in one location or in two. But it is not cloudy everywhere for days on end, at least not in arid or semi-arid regions.
Thus, between many thousands of installations, each with a seventeen-hour reserve of molten salt, we can achieve year-round energy for the entire North American grid. Between weather variability spanning vast distances and the ability to store energy for many days at a time, this network of power tower plants would do the trick
Numerous, sizeable regions bearing degraded land were tested in the model for suitability. In the end, the five regions that offer the most bang for the buck plus enough climatic variability to smooth out the weather fluctuations are the regions in the vicinity of Oklahoma City (OK), Lubbock (TX), Bakersfield (CA), Limon (CO), and Dallas (TX).
This would require a total of 597,745 square kilometers, but it can be done. If push comes to shove, it is possible to power the entire future energy needs of North America with five mega installations of solar power towers.
Researchers at MIT, in collaboration with RWTH Aachen University in Germany, have devised a new configuration of the mirror array, along the lines of spirals on a sunflower that cuts back on 20 percent of the land footprint. This means a revised total area of 478,196 sq km.
As it turned out, we have more than that degraded land that we can utilize.
 Corey J. Noone, Manuel Torrilhon, and Alexander Mitsos, “Heliostat Field Optimization: A New Computationally Efficient Model and Biomimetic Layout,” Solar Energy 86, no. 2 (2012): 792–803.
Other Posts by Daniel Rirdan
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