Exploitation of Low-Grade Heat in Site Utility Systems

Low-grade heat is available in large amounts across process industry from temperatures of 30 degrees C to 250 degrees C as gases (e.g. flue gas) and/or liquids (e.g. cooling water). Various technologies are available for generating, distributing, utilizing and disposing of low grade energy. The integration of these technologies with the site has not been fully studied, with regards to engineering and practical limitations for retrofit, the use of non-conventional sources of energy in energy generation, and consideration of variable energy demand loads. The identification of cogeneration potentials is one of the key performance indicators for screening various energy-saving technologies using low-grade heat and evaluating the integration-ability of these technologies to the overall site. The work in this study is therefore carried out to improve heat integration models which can systematically identify realistic cogeneration potentials, and provide the most appropriate strategies for exploiting low-grade energy technologies for the viewpoint of system analysis. An improved model has been proposed for the evaluation of power output by a combination of bottom-up and top-down procedure for the evaluation of steam header temperature and steam flow rates respectively. The applicability of the developed model is tested with other existing design methods and STAR (R) software through a case study. The proposed method is shown to give comparable results, and the targeting method is used for obtaining optimal steam levels. Identifying optimal conditions of steam levels is very important in the design of utility systems, as the selection of steam levels heavily influence the potential for cogeneration and energy recovery for the site. In this work, the optimization of steam levels of site utility systems has been carried out in the case study, in which the usefulness of the optimisation framework is clearly demonstrated for reducing the overall energy consumption for the site. Heat loads and steam levels can then be further used for subsequent evaluation of design options for low grade heat integration.