Computational investigation on daily, monthly and seasonal energy performances and economic impact through a detailed 2D FEM model of an earth to air heat exchanger coupled with an air conditioning system in a continental climate zone

In this paper the results of a wide investigation on a geothermal-integrated air HVAC system are reported: in the first part, the energy performances of an Earth to Air Heat eXchanger (EAHX) operating in Pomezia (a locality of the area of Rome, Italy) are carried out through a comprehensive numerical two-dimensional model based on the finite element method and the system is forced under real time-changing weather conditions over 1 year. The model is able to reproduce the time- and space- dependent thermo-fluid-dynamic behavior of the EAHX system, the humid air flux and the surrounding soil, as well as to account for the mass of condensed water in the tubes. In the second part of the investigation, the EAHX is coupled to an air handling unit of a company building for office use and tested while operating both in cooling and heating mode. The hourly results, in terms of power savings and energy consumption were reported for representative days of all the months belonging to the cooling or the heating working seasons, monthly and seasonal assessment. The convenience in terms of seasonal energy savings is +40.8% in cooling mode and +44.1% in heating mode. Then, the economic impact in terms of payback time and net present value was investigated through a cost analysis to examine the return of the economic investment under two possible soil morphologies and different prices of electrical energy in Italy. The simple payback with the real price of energy in Italy (0.50 €/kWh) is 3.3 years for the tufa-soil and 5.8 years for soft rock-soil; the economic return in terms of net present value is around 500 k€ for both the morphologies (560 k€ for the tufa; 446 k€ for the soft rock), per effect of the energy saving and, consequently, for a reduced expense of electrical energy. Therefore, in all the considered cases the installation of EAHX systems in support of traditional systems would generate both energy and economic advantages.