Low exergy (LowEx) heating and cooling systems for sustainable buildings and societies

Heating, cooling and lighting appliances in buildings account for more than one third of the world's primary energy demand and there are great potentials, which can be obtained through better applications of the energy use in buildings. In this regard, the building sector has a high potential for improving the quality match between energy supply and demand because high temperature sources are used to meet low-temperature heating needs. Low exergy (or LowEx) systems are defined as heating or cooling systems that allow the use of low valued energy, which is delivered by sustainable energy sources (i.e., through heat pumps, solar collectors, either separate or linked to waste heat, energy storage) as the energy source. These systems practically provide heating and cooling energy at a temperature close to room temperature while the so-called LowEx approach, which has been and still being successfully used in sustainable buildings design.The present study comprehensively reviews the studies conducted on LowEx heating and cooling systems for establishing the sustainable buildings. In this context, an introductory information is given first. Next, energy utilization and demand in buildings are summarized while various exergy definitions and sustainability aspects along with dead (reference) state are described. LowEx heating and cooling systems are then introduced. After that, LowEx relations used to estimate energy and exergy demand in buildings and key parameters for performance assessment and comparison purposes are presented. Finally, LowEx studies and applications conducted are reviewed while the last section concludes. The exergy efficiency values of the LowEx heating and cooling systems for buildings are obtained to range from 0.40% to 25.3% while those for greenhouses vary between 0.11% and 11.5%. The majority of analyses and assessments of LowEx systems are based on heating of buildings.