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 plant-to-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; 1- electricity only, 2- electricity
and heat with or without temperature peaking or equipment oversizing, 3-
electricity, heat, and cold. Comparisons showed that the choice primarily
depends upon the district size, district-to-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.