Abstract
High performance buildings typically relies on extreme envelope performances in terms of insulation level, solar gains valorization and airtightness. The energy needs are limited and the system sizing is reduced to increase the recourse to renewable energy sources. The only exception to this trend is related to the ventilation, which is even more crucial because of the infiltration control. Mechanical ventilation is a common option to ensure an appropriate ventilation rate and a suitable indoor air quality. Heat recovery devices are widely used to save energy in buildings mechanical ventilation systems. Their contribution in the design or operating phases is typically assessed in terms of effectiveness, which expresses the ratio between the amount of energy – sensible and/or total – actually recovered and the maximum recoverable under ideal configuration. However the effectiveness of recovery devices is not a constant value and depends on the operating – outdoor and indoor –conditions and control strategy. In order to evaluate the energy and cost savings allowed by heat recovery systems, these aspects have to be considered, and their relative impact on different heat recovery technologies accounted for. In this work, some of them are analyzed in detail and their contribution to the device effectiveness is discussed. The objective is to understand the influence of the air conditions (fresh and exhaust) on the energy savings considering different sensible and total heat recovery devices and operative constraints which could affect the control strategy. The analysis starts from hourly weather data, representative of different climate types, to quantify the actual effectiveness of different heat recovery systems. The impact of indoor conditions on the sensible and latent recovery is analyzed, defining appropriate control strategies to prevent excessive moisture recovery and discomfort issues when the indoor humidity is considered. Moreover, the effect of condensation and frosting on the hourly and annual performance of each kind of devices is analyzed. Three different climatic conditions have been considered. Sensible fixed-plate cross flow heat recovery system have been compared with enthalpy wheels By means of a parameterization of the operating constraints and the expression of the main quantities in specific terms, depending only on the building use, it has been possible to generalize the main results, making them independent of a specific case study, and to compare different technologies and climatic contexts. Seasonal and annual energy and economic performances have been quantified, as well as their sensitivity to specific calculation assumptions, such as the building type, the investment and energy costs, providing a synthetic overview of the technical and economic viability of the considered technologies and a geographic mapping of their competitiveness.