A large oak tree produces 2.2 metric tons of wood over a 50 year growth lifespan. This wood has a heating value of 20Mj/kg the tree has a caopy of 10 meters in diameter and this canopy collects solar enegy over a 6 month growing season each year at an average rate of 177W/m^2 ; assume no solary energy is collected druing the other 6 months when the tree is dormant. Estimate the tree's efficiency for converting solar energy to wood heating value.