Curly or straight hair and the thermodynamic differences? Does curly hair make a thermodynamic difference on the amount of sunlight or heat reaching or escaping the scalp? 

 A: If by "thermodynamic difference on the amount of sunlight or heat reaching or escaping the scalp" you mean whether curly and straight hair have different thermal insulating properties then the answer to your question is "yes".
It's not really the hair itself that's a good heat insulator but rather the air that is trapped between the hair strands that provides the insulation properties. Think how other insulating materials like expanded polystyrene ("Styrofoam"), expanded polyethylene, rock wool, knitting wool etc all insulate well against both heat and cold, because the air (or other gases) contained between the material's solid strands is a poor conductor of heat.
As curly hair tends to trap more of the poorly conducting air, curly hair provides somewhat better insulating properties, compared to straight hair. All other things being equal that is a reasonable assumption.
This table shows the heat conductivity values of a number of common materials, notice how porous materials mostly have lower heat conductivities.
A: There is some research that shows that curly hair keeps the head cooler in warmer climates.
https://daily.jstor.org/why-do-some-people-have-curly-hair-and-others-straight/
Whereas straight hair keeps the head warmer.  I would say this matches an observation that ethnicities primarily found in colder climates tend to have straight hair.
A: Oils from the skin would reach to the end of the hair. With strait hair when it rains it tends to draw the water on the outer layers down and away while keeping the inner layers warm and dry in colder climates. On the other hand when you sweat the water is drawn out by the down grain of the hair to open air taking heat and moister with it. In curly hair sun light hits the outer curls and the heat generated is drawn buy wind while the inner layers stay in the shade with more air between them than strait hair, but fairs less in rain or snow in retaining heat. I conclude that Strait hair retains heat better, but curly hair expels heat better.
Feel free to add or object and why.  
