(579c) Optimization of the Economic and Environmental Performance of An Ammonia-Water Absorption Cooling Cycle

Actually, it seems clear that a drastic change in the energy structure should to be made in the developed countries, and energy efficient technologies and renewable energies have to be more promoted. Cooling and refrigeration represents an important contribution to the total energy consumption. Most of the installed units are based on the mechanical vapour compression systems driven by electrical motors. As a consequence, the electrical demand has been rising, which worsens the trouble for both the generation and distribution networks, along with the associated environmental issues.

A more sustainable concept, known as thermal solar refrigeration, is based on the use of an absorption unit driven by low grade heat from industrial processes as waste heat or cogenerations or solar thermal panels. The idea of producing cold from solar thermal energy is not new at all. However, despite of their benefits, both environmental and economical (for their low operating cost), their deployment in the market is null. The reason for such a low market impact could be attributed to high purchase cost and low integration of the devices involved in such installation, and making a custom design for each application necessary.

Among the absorption systems, mainly two different working fluid mixtures are used: ammonia-water of water-Lithiumbromide (LiBr). The water-LiBr working pair is suitable for cooling applications above 0ºC and shows in general a higher coefficient of performance (COP) than ammonia-water. Ammonia-water can work at subzero temperatures and the heat dissipation temperature is not limited by crystallization. Thus air-cooling can be considered and no wet cooling tower is required.

The objective of the present work is to provide a quantitative decision-support tool for the optimal design of environmentally conscious absorption cycles. The approach presented relies on the development of a multi-objective formulation that simultaneously accounts for the minimization of cost and environmental impact at the design stage. The latter criterion is quantified by the Eco-indicator 99, which follows the principles of life cycle assessment (LCA). The design task is formulated as a bicriteria nonlinear programming (NLP) problem, the solution of which is defined by a set of Pareto points that represent the optimal trade-off between the economic and environmental concerns considered in the analysis. These Pareto solutions can be obtained via standard techniques for multi-objective optimization. The main advantage of this approach is that it offers a set of alternative options for system design rather than a single solution. From these alternatives, the decision-maker can choose the best one according to his/her preferences and the applicable legislation.

The capabilities of the proposed method are illustrated in a case study problem that addresses the design of an ammonia-water absorption cooling system. As driving energy both steam from fossil fuel and solar thermal energy will be considered.