Cooling America with PDRC's

I’m a little obsessed with PDRCs (Passive Daytime Radiative Cooling Materials) I won’t deny that. I mean how can you not get at least a little excited about a material that helps to reduce how much energy you need to cool a building. Recently we looked into the potential of using PDRCs to help cool off the planet’s oceans, in that case, we would need something like a million square kilometers to help vent off all of that surplus heat within a decade. Our question for today is: How many square kilometers of PDRCs would you need to replace all of the electricity the United States uses to keep things cool?

The first piece of information we need is, how much energy does the United States spend on refrigeration and air conditioning?

According to the Energy Information Agency the United States consumed over 3.8 Trillion kilowatt-hours of electricity. Just a bit over 20% of that energy production went into generating cooling, about 768.5 billion kilowatt-hours of electricity.

Next, we will need to factor in the Coefficient of Performance, the measure of how much cooling you get for a given unit of energy. If I have a refrigerator with a COP of 1 for every kilowatt-hour of electricity I use I will be able to remove 3.6 MegaJoules (MJ) of heat from the refrigerator. A COP of 2 means that for each kilowatt-hour of electricity you would remove 7.2 MJ of heat. In the case of the US’s cooling infrastructure, we will assume that the COP will be somewhere between 2 and 4.

For a COP of 2 the US would need to replace 5.54 trillion MJ of cooling

For a COP of 3 the US would need to replace 8.30 trillion MJ of cooling

For a COP of 4 the US would need to replace 11.1 trillion MJ of cooling

Our final variable will be the cooling ability of PDRCs. As PDRCs are not yet commercially available it will be necessary to estimate performance in the best and worst cases of performance. In this instance, we will assume a worst-case performance of 12.5 watts/m^2 of cooling and a best case of 125 watts/m^2. That means that if you had one square meter of best-case PDRC providing cooling, it would provide 3,900 MJ of cooling per year.

Now we can estimate how much land our PDRCs would need to replace all of that air conditioning (assuming 100% of that cooling energy is magically transported to where it needs to be)

Worst CASE

  1. (COP of 2) 5.54*10^12 MJ/year /390 MJ/m^2/year= 14 billion square meters

  2. (COP of 3) 8.30*10^12 MJ/year /390 MJ/m^2/year= 21.1 billion square meters

  3. (COP of 4) 11.1*10^12 MJ/year /390 MJ/m^2/year= 28.1 billion square meters

Best Case

  1. (COP of 2) 5.54*10^12 MJ/year /3900 MJ/m^2/year= 1.4 billion square meters

  2. (COP of 3) 8.30*10^12 MJ/year /3900 MJ/m^2/year= 2.1 billion square meters

  3. (COP of 4) 11.1*10^12 MJ/year /3900 MJ/m^2/year= 2.8 billion square meters

This means that if we wanted to eliminate 100% of the electricity demand associated with cooling things down* we would need to cover at least 2.1 billion square meters of land. To put things in perspective 2.1 billion square meters is 2,100 square kilometers or just over half of the land area of the state of Rhode Island (now the article image makes sense, I hope). This number assumes cooling demand is perfectly averaged out year-round and that all of that cooling capacity can perfectly get to where it needs to go.

Demand variability is one of the biggest challenges in transitioning our world’s economy towards more sustainable solutions. Our massive field of PDRCs wouldn’t make sense in replacing refrigeration and air-conditioning, but they do make sense as a way to reduce overall cooling demand. SkyCool Systems is developing rooftop PDRCs that improve the performance of airconditioners by up to 12% (the stat is at the 9:50 mark in the video). If this technology was just applied to US residential consumers, we would save 25 billion kWhrs of electricity each year, the equivalent of 2.2 million American homes no longer drawing power from the grid. This number gets even more exciting when you realize that you no longer need as many power plants to meet all of that demand.

There are some interesting secondary questions that are harder to answer like, what is the impact of agressively raising the albedo of two thousand square kilometers of land?

Thanks for reading, as always comments and questions are welcome.

*Assuming the US’s cooling infrastructure has an average COP of 3

Obadiah KopchakComment