What are the limiting factors of an eco-friendly city

An eco-city is a city built from the principles of living within environment means. The ultimate goal of many eco-cities is to eliminate all carbon waste (zero-carbon city), to produce energy entirely through renewable resources, and to merge the city harmoniously with the natural environment; however, eco-cities also have the intentions of stimulating economic growth, reducing poverty, using higher population densities, and therefore obtaining higher efficiency, and improving health.