This paper provides a reference set of indices based on emergy, for the evaluation of ecotechnological processes and whole economies. Indices of emergy yield ratio (EYR), environmental loading ratio (ELR), and emergy investment ratio (EIR), among others, are stressed, and a new index the emergy sustainability index (ESI) is defined. The emergy indices for a given system are shown to be functions of renewable, nonrenewable and purchased emergy inflows. Indices are given for several ecological engineering activities including oil spill restoration, land reclamation and wastewater recycle through wetlands, several production systems, and several national economies to demonstrate their usefulness. Ecological engineering involves both natural and engineered systems and the flows of renewable and nonrenewable energy, the appropriate amounts of which are important to determine if they are to result in sustainable use of resources. The sustainability index can be used to evaluate appropriate nonrenewable investments in eco-technology to maximize their performance. Sustainability of economies is shown to be a function of the net yield of the economy and its 'load' on the environment. The trends of these indices can be monitored over time and provide useful information about the dynamics of economic systems within the carrying capacity of the environment in which they develop. When a particular sector or production process is focused on, instead of a national economy, emergy based indices can provide insights into the thermodynamic efficiency of the process, the quality of its output, and the interaction between the process and its surrounding environment. (C) 1997 Elsevier Science B.V.
