Low and medium temperature heat source exergy analysis for key processes and equipment in HVAC plants

Valorization of low and medium temperature heat is a very relevant topic to be considered when designing nearly zero-energy buildings (2010/31/UE Directive) and also when considering 2009/28/EC Directive on the promotion of the use of energy from renewable sources. From the thermodynamic point of view, heat recovery covers a lot of technologies: vapor compression chillers and heat pumps, sorption machines, power cycles, cogeneration. A first aim of this paper is to analyze the main options for the above mentioned technologies from the energetic and exergetic point of view, elaborating on the impact of the heat source, the heat sink and the environment temperatures. In a more technical approach, humid air as heat source in the main psychrometric processes is studied. As in moderate climates new and energy retrofitted buildings are going to have more and more cooling demands with respect to heating ones, direct and indirect evaporative cooling air conditioning plant schemes are studied from the exergetic point of view.