Can We Reduce the Temperature of Stormwater Runoff by Geothermal Cooling?
I was first introduced to the idea of cool pavements several years back at a United States Green Building Council event that discussed heat island effect and the solar reflective index of different pavement surfaces. The basic idea is that asphalt is bad because it’s black and absorbs heat (approximately 95% heat absorption and temperatures exceeding 140 ºF), concrete is better due to its lighter color of grey (Approximately 65% heat absorption) and permeable pavements, depending on the mix design, color and region would be the best of the pavement options due to their ability to store water in their subgrade and evaporate water through the pavement system. This summer when watching the weather, notice how much hotter the temperatures are in the urban habitat compared to sub-urban and farming areas. The temperatures can increase by as much as 22ºF according to the EPA
The environmental issues with pavement heat aren’t strictly air temperature, but water temperature as well. Tests have shown that pavements that are 100ºF can elevate initial rainwater temperature from roughly 70ºF to over 95ºF.4 This heated stormwater generally becomes runoff, which drains into storm sewers and raises water temperatures as it is released into streams, rivers, ponds, and lakes. Studies over the last decade have been conclusive that the impacts on downstream ecosystems have been devastating to fish populations and their abilities to reproduce, getting them outside of their natural cycles. So, in conclusion, heat on pavement is bad for humans and it’s bad for fish, so what can we do about it?
I’m not sure I have the answer, but I do have an idea that might allow us to think about the problem in a different way. Everything I have seen to date is passive technologies. Changing the pavement color or providing subsurface storage to allow evaporation to cool the surface, but what if we looked at active systems to cool pavement.
Last week I spent a week in snowy Indianapolis where they are using a new permeable pavement system called PaveDrain. Because PaveDrain was designed to be cabled and laid in sheets, it has ducts running through the blocks about 2” below the surface. Those ducts are large enough to run ½” Pex Tubing through the ducts. That Pex can then convey water that is heated geothermally through the ducts, heating the pavement and melting the snow.
Temperatures within ten feet of the earth’s crust are a constant 50 ºF to 60 ºF . They have to get the surface of the Pavedrain blocks to 34 ºF to get snow to melt, and pushing 60 ºF water through the conduits within 2” of the surface does this very effectively. Because it’s permeable, the snow melts, and all pollutants associated with that snow wind up in the stone storage reservoir below.
It’s an incredible process, but I’m from Houston where it takes a cold day in hell for there to be snow. But, it got me wondering if we could reverse the process for the 100 ºF summer days we frequently get. Could we use the same approach, but instead of warming the pavement on a freezing day, could we cool the pavement by using geothermal cooling to combat the suns heat as it absorbs into the block.
Any of us that have any experience with geothermal systems know that its an expensive process, but as new technologies come on line and the industry grows, this might not be that far fetched of an idea. At one point, a hundred years ago, I’m sure someone thought “wouldn’t it be great if we could design our pavements to melt snow?”.