Relay enhanced cellular system is a promising infrastructure redesign to enhance the performance of next generation mobile networks. The use of intermediate nodes to relay information from source to destination is efficient in eliminating black spots, extending coverage region, prolonging battery lifetime of users. Recently, energy efficiency of mobile networks has drawn research attentions. Relay deployment from the perspective of energy efficiency is an interesting issue which has not been deeply addressed. In this paper, we investigate energy-efficient relay deployment by determining the optimal number and location of relays in next generation cellular networks. We establish a mathematic model for analyzing energy efficiency measured by `Joule per bit' in relay aided cellular networks, where the pathloss status related to the cell traffic busy level is considered. We conduct intensive simulation experiments based on the model with realistic broadband channel propagation conditions. Our findings suggest that introducing appropriate number of relay nodes with proper locations into the cellular system can improve system energy efficiency without compromising system throughput.