ABSTRACT

This paper investigates air-to-air (AAHX) and earth-to-air (EAHX) heat exchangers, to reduce the energy requirements of air conditioning systems for office buildings. The AAHX considered is of static type with a partition wall and allows recovering both sensible and latent heat. The EAHX technology exploits the capability of the ground as energy storage, through a system of earthed air ducts laid horizontally. In summer, the ground usually presents temperatures lower than outside ventilation air temperatures, while in winter it could have, in some hours/days, a temperature higher than the outside air. Three European cities are considered: Palermo (southern Italy), with mild winters and very hot summers; Milan (northern Italy), with cold winters and hot summers; Berlin (Germany), with very cold winters and quite hot summers. The analysis is performed by means of EnergyPlus, a dynamic building energy simulation software, with reference to the same, newly designed office building, with or without heat recovery systems. The air conditioning system is based on fan-coils and primary air, and the EAHX (or the AAHX) is inserted in the first part of the primary air-handling unit. The research shows that in winter the use of the AAHX reduces the energy requirement much more than the EAHX, while in summer the EAHX is preferable. Moreover, the annual analysis shows that the use of the EAHX is more convenient in hot climates. The case of coupled heat exchangers (AAHX + EAHX) is also investigated. The evaluated discounted payback is about 6–9 years for the EAHX (3–5 years if an incentive of 20% is considered) and 1–3 years for the AAHX (less than 1 year with the incentive).