Providing enough power for 1.5 billion humans to live and work at a high standard of living is a massive challenge. A large reason for the challenges associated with transitioning the world's energy grids to using lower-carbon energy sources stems from the fact that humans can consume a large amount of power in their daily lives and generally speaking people in developed economies have gotten used to the idea that they can microwave their burrito whenever they want. While the human desire for electricity seems insatiable renewable energy technologies are already starting to compete against more "conventional" (*cough* carbon-intensive *cough*) energy sources. According to several sources, including this Forbes post, the world's energy needs could theoretically be met by covering about 1% of the Sahara in solar panels, assuming we could conveniently store all of that energy for times when the sun wasn't shining.
Cool, we've solved our desert urbanization goal, and we didn't even have to go into much detail. Lots and lots of solar panels combined with some magic way to efficiently store the surplus for the night time. Boom, blog post complete, Obie can shut up. If you want the high-level explanation then yes, you can consider yourself done with class, for those interested in some more nuanced solutions, please follow along.
Solar power is fast becoming one of the best options for more and more of the world's energy needs, solar is not quite the silver bullet that will solve all of our energy problems. Even in places like the Sahara, the Arabian Penninsula's Empty Quarter, or the Australian Outback, where the sun shines for more than 3000 hours per year aka really sunny there will still be night time and a random storm system that blocks your panels. For your non-sunny periods, you are going to need some ability to store that energy, there are several technologies that can be used to store solar energy and they all have one thing in common, no matter how good your storage technology is, you are only going to get a part of the input energy back. Engineers describe the energy losses of energy storage as round trip efficiency, the worse your round trip efficiency the less energy you can use later.
The most common way for power grids to store surplus electricity is to pump water uphill this technology is called "Pumped Hydro". Pumped hydro is amazing in its simplicity when power is cheap pump it uphill when there isn't enough power to meet the grid demand you let the water go downhill to move a turbine. The round trip efficiency for pumped hydro is generally estimated to be about 70-80+% which means that if you had a megawatt-hour of surplus power you would get about 700-kilowatt hours of power out on the other side. As of 2019 pumped hydro represents roughly 95% of the world's large scale energy storage capacity. In regions where you have the necessary geology and climate pumped hydro makes a lot of sense, unfortunately, our desert cities are unlikely to be built near a river that isn't already being used for something else. Fortunately for our future city developers, we clever humans have come up with many ways to store electricity.
Stealing from the Environmental and Energy Study Institute's table here are some of the alternative energy storage methods (I am also adding a few additional technologies and will provide appropriate links otherwise info came from here).
Technology Efficiency
Pumped Hydro 70-85%
Compressed Air 40-70%
Molten Salt 80-90%
Li-ion Battery 85-95%
Flow Battery 60-85%
Liquid Air Energy Storage 60-100%*
While this information is cool and hopefully starts you thinking about ways to store the energy our cities will need, it doesn't tell a complete story. The round trip efficiencies listed above assume you are using the power stored by the respective technologies under optimal conditions. For example molten salt sounds really awesome, engineers can get up to 90% of their energy back into the grid, what's the problem? In the case of molten salt, our challenge is heat loss, the longer something is stored the more energy you lose, and at some point, you can't cost-effectively produce electricity from the remaining heat. If our cities are only trying to produce enough electricity for their day to day needs the efficiencies of the above storage technologies might be sufficient. For our cities to be future economic powerhouses they will need to be able to produce enough energy to export to the nations of the world and for that grid-storage might not be sufficient. Instead of just electricity from photovoltaics and batteries, our city planners might want to consider some additional technologies.
Come back Friday for Powering It All part Dos, where we will look at other ways to power our city to make sure it is as robust as possible. I hope this article was interesting if you have any questions or feedback feel free to ping me or leave a comment.
*Liquid Air Energy Storage is a bit wonky as its round trip efficiency depends on whether or not it is working independently as energy storage or if it is working in concert with other systems. If they have surplus heat coming from some other facility that increases the efficiency by causing the liquid air to expand more. If there is a surplus of cold coming from another facility then the liquid air energy system uses less energy to cool the air to a liquid state. The article linked below provides some detail
