Stone mix concrete slabs are used to absorb thermal energy from flowing air that is carried from a large concentrating solar collector. The slabs are heated during the day and release their heat to cooler air at night. If the daytime airflow is characterized by a temperature and convection heat transfer coefficient of T infinity = 200 degrees celcius and h= 35 w/m^2 K, respectively, determine the slab thickness 2L required to transfer a total amount of energy such that Q/Qo = 0.90 over a t= 8-h period. The initial concrete temperature is Ti=40 degrees celcius.