An even cooler thing about growing grasses

By Russ Conser
Dec 25, 2016

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In previous posts, I’ve pointed out that warm season grasses are cool, and that we waste an enormous amount of solar energy when we don’t capture that energy in growing plants.  Today, I want to connect those two points and talk about how the 1st statement is literally true, and the impact of the 2nd may be bigger than you think.

Thermodynamics is the science of energy flow and operates on a few fundamental ‘laws.’  These ‘laws’ essentially say we can convert energy, but not destroy it, and that the process of conversion leads to ‘lost’ energy that is harder to use again.

With respect to grass growing in your pasture, this means that the solar energy converted into chemical energy stored in the plant is no longer in the air itself anymore, so the air must, by definition, be at least a bit cooler.  It also means that the conversion process consumed energy, too, so the air is even cooler still.  A large fraction of this conversion energy goes into ‘pumping’ water through the plant and “evapo-transpiring” it into the air.

Well it turns out that the evaporation of water consumes a large amount of energy.  This is why you see steam coming off cooling towers for industrial power plants, and also how your air conditioner works.  Your air conditioner works by capturing heat from the inside of your house via evaporation of a ‘refrigerant’ fluid.  The heat taken from inside is then released when the fluid is condensed outside.  Water evapo-transpiring from growing plants does the same thing – it takes heat from the air near the ground, and then releases that heat higher up in the atmosphere when it condenses and comes down as rain.  Thus a growing plant is quite literally cooling the surface of the Earth just like your air conditioner cools your house. 

So let’s put some numbers to that to get a sense of how much cooling might actually be going on in your pasture.  This paper measured the evapotranspiration of a grazed prairie during peak growing season as 455 mm over a growing season of 175 days or 2.6 mm/day.  This reference documents that we consume 2.45 MJ/m2/day for every 1 mm/day of evapotranspiration.  So we multiply 2.6 by 2.45 to get 6.37 MJ/m2/day.  If we then convert the units of MJ/m2/day into kW/acre, we find that the evapotranspiration process of a grazed prairie is consuming roughly 300,000 Watts/acre (or 300 kW/ac or ~74 W/m2). 

So although it’s convenient to think in terms of ‘light bulbs’ – where this would be the equivalent of 3,000 x 100 Watt light bulbs - it may make more sense to think in terms of cooling capacity equivalents.  Home air conditioning units are sized in ‘tons’ of cooling capacity – an average home taking about 5 tons here in the south.   Using this simple calculator, we discover that 300kW is the equivalent of 85 tons of cooling capacity.  Therefore, we divide 85 by 5 and discover that, when your grass is growing, each acre of your land is performing like 17 average home-sized A/C units (5 tons) – all powered by sunshine growing grass! 

So that’s the theory, but how real is the effect?  Below is an infrared photo of ornamental C4 grasses in our garden.  See by the blue color how this tall bunch grass stands out as a cold spot, not only against the hot stones, but the other vegetation?  Remember from an earlier post, that warm season grasses actually perform better on hot days than cool ones.  This warm season grass really loves the hot Houston summers, and is strongly dissipating solar energy on a hot afternoon in August. 

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Thus, growing grass cools the Earth by capturing some energy directly in biomass, and also by consuming energy in the process of growing.  This is why your pasture is cooler than a parking lot.  In a parking lot, the energy is either absorbed as heat or bounces back into the air.  Maybe more importantly, the same applies to bare soil.  So back to my prior post, when we fail to capture sunshine in a growing plant, we’ve not only wasted the opportunity to make use of the captured energy, but we failed to enable growing life to cool the land.

By Russ Conser - Fulshear, TX

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