Hi,
The solve function returns an empty object if there isn't any solution to the equation. For instance if the equation is changed as shown below,
E2 = 730 == To+(Tcz-To)*(1-exp((h*h*a*90)/(k*k))*(1-erf((h/k)*(sqrt(a*90)))))
It returns the following solution:
htc =
-555.33097392244426184617865401959
Hope it helps!