Sun is the only source of energy. Producers convert solar energy into chemical energy in the form of food.The primary consumers (herbivores) depends on producers for food.According to the energy transfer law, only 10% of energy is transferred from one trophic level to the other.Therefore, the energy that is captured by the producers does not come back to the Sun and the energy transferred to the herbivores does not come back to the producers.It just keeps on moving to the next trophic level in a unidirectional way.That is why the flow of energy in the food chain is always unidirectional.