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While moving towards 100 per-cent renewable district energy systems at low temperatures, the exergy of the district energy may decrease below the pumping exergy requirement, which eliminates the benefits of using low-exergy renewables. Because such a possibility may not be revealed by the First Law, an exergy-based holistic model for district energy systems was developed. Four tiers, namely renewable energy resources, energy conversion and storage, main district network, and the low-exergy district are identified. Each tier is indexed to the optimum plantto-district distance for maximum exergy-based performance with minimum CO2 emissions responsibility. This model further optimizes the temperature peaking with heat pumps versus HVAC equipment oversizing and determines the optimum mix of renewables. Three alternatives of conveying and distributing exergy to the district were considered, namely; 1electricity only, 2electricity and heat with or without temperature peaking or equipment oversizing, 3electricity, heat, and cold. Comparisons showed that the choice primarily depends upon the district size, districtto-plant distance, climatic conditions, local availability of renewable energy sources, optimum supply temperature, and thermal condition of the buildings. Another algorithm optimizes the thermal insulation thickness in terms of equipment oversizing and temperature-peaking.

An exergy-rational district energy model for 100% renewable cities with distance limitations