Meanwhile, in a large saucepan, heat the oil. Add the shrimp shells and cook over high heat, stirring, until pink, about 3 minutes. Add the coarsely chopped onion and cook over moderately high heat, stirring, until softened and starting to brown, about 3 minutes. Add the tomato paste and cook, stirring, until it starts to caramelize, about 2 minutes. Add the remaining 8 cups of water and bring to a simmer, scraping up the browned bits on the bottom of the pan. Simmer over low heat for 30 minutes. Strain the broth and return it to the saucepan.