Abstract
In this paper the energy performances carried out by an investigation conducted on a Heating, Ventilation and Air Conditioning (HVAC) system, composed by an Air Handling Unit (AHU) for the primary air coupled with a horizontal-pipes Ground-to-Air Heat eXchanger (GAHX) and fan-coil units, for an office building, supposed to be placed in four different cities (Rio de Janeiro, Dubai, Naples, Ottawa) belonging to four different worldwide climatic areas, according to the Köppen climate classification. The investigation is performed by means of a two dimensional numerical model, experimentally validated, of an GAHX solved with finite element method. The results introduced in this paper are carried out by varying the length of the pipes and the air flow velocity; thus, also the number of tubes forming the GAHX is varied since the volumetric flow rate to be provided to the AHU must be kept constant. The energy performances of the above-mentioned HVAC system are analyzed, both in summer and in winter operation modes. Specifically, the reduction of the power of the heating and cooling coils in the AHU due to the pre-treatment of the air operated by the GAHX, the efficiency of the GAHX and the inlet-outlet temperature span are evaluated through a sensitivity analysis. These results lead to say that an GAHX for the geothermal pre-treatment of the air to be introduced into the AHU is energetically very convenient since it leads total thermal power savings in all the investigated cities, that makes this solution competitive. Specifically the best value of power reduction (61.5%) for a 100 m pipe-length GAHX is obtained in Ottawa, a city belonging to continental climate zone. The worst results in terms of power reduction are registered in Rio de Janeiro, a city belonging to the tropical or equatorial climates: this reduction, for a 100 m pipe-length ground to air heat exchanger, is 23.9%.
Highlights
Air conditioning is widely used in the construction sector, both in residential and in commercial buildings: it plays a key-role in ensuring the achievement and permanence of comfort conditions for the occupants of an environment, contributing to the health and well-being of people
The results introduced in this paper are carried out by the simulations on the Ground-to-Air Heat eXchanger (GAHX), performed by varying the length of the pipes and the air flow velocities; the number of tubes forming the GAHX is varied since the volumetric flow rate to be provided to the Air Handling Unit (AHU) must be kept constant
The coupling of a double approach, i.e., the rigorous solution of the heat transfer problem related to the GAHX, and a more practical approach tending to evaluate the benefits obtaining for the air conditioning system; the positioning of the GAHX upstream of the AHU, which represents a system configuration rarely analyzed in the previous literature works [14,16,17]
Summary
Air conditioning is widely used in the construction sector, both in residential and in commercial buildings: it plays a key-role in ensuring the achievement and permanence of comfort conditions for the occupants of an environment, contributing to the health and well-being of people. Considering the previously mentioned state of the art, with particular reference to the progresses concerning the development of hybrid solutions of mechanical ventilation or air conditioning systems coupled with GAHX, the investigation introduced in the present paper shows the following main elements of innovations: the coupling of a double approach, i.e., the rigorous solution of the heat transfer problem related to the GAHX (through the development and experimental validation of a two-dimensional numerical model based on the finite elements method), and a more practical approach tending to evaluate the benefits obtaining for the air conditioning system; the positioning of the GAHX upstream of the AHU, which represents a system configuration rarely analyzed in the previous literature works [14,16,17] In this way, the GAHX is not a simple pre-treatment of the air to be introduced into the building by means of a mechanical ventilation system, but a component inserted in the air conditioning system; the evaluation of the power saving associated to the HVAC system heating and cooling coils, deriving to the placement of the GAHX upstream of the air-handling unit (with consequent significant decrease of the overall energy consumption). In the last section, the main conclusions about the results collected are drawn
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