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Abstract: The winter operation of ground-source heat pumps is expected to lead to temperature decrease in the zones surrounding the vertical ground heat exchangers. The following thermal drift phenomenon occurring over the years is still deeply studied from both the experimental and the theoretical viewpoint. A moving aquifer tends to oppose the thermal drift, due to the advection groundwater phenomena. For this purpose, a modelling study on the quantitative influence of the groundwater flow on the thermal drift is performed. Then, the analytical solution of the moving infinite line source is applied. Different forms (closed analytical, asymptotical or tabled one) are identified for this solution depending on the value range of the thermal physical parameters of the system. Referring to a particularly critical case study, an easy-to-use application of the model based on this analytical solution is then presented. An evaluation is carried out as well on the interaction between the existing borefield and a nearby one, being operated afterwards, in presence of either still or moving aquifer. Finally, the influence of the actually existing flow upon improving the effective operating conditions of the heat pump plant is assessed. Moreover, when groundwater flow is taken into account, the decrease of the total borehole length is evaluated. [Copyright &y& Elsevier]

Abstract: In this paper a numerical model to analyze the thermal behaviour of vertical ground heat exchangers with a helical shaped pipe is presented. This type of configuration can be a suitable alternative to conventional ground heat exchangers, especially when the heating and cooling loads of the building are very low. The model describes the heat transfer problem by means of a network of interconnected thermal resistances and capacitances. Moreover, as the investigated ground heat exchanger is usually installed in shallow depth, the model takes into account the interaction between the ground and the ambient environment which affects the fluid heat carrier temperature into the heat exchanger and, as a consequence, the energy efficiency of the heat pump. After a sensitivity analysis on the mesh parameters, the presented model is compared with experimental data and the simulation results show good agreement with the measurements. Finally, analyses to investigate the influence of the weather conditions, of the axial heat transfer and of the pitch between the turns of the helical pipe for two types of ground are carried out. [Copyright &y& Elsevier]

Cool supply air flowing through the underfloor plenum is exposed to heat gain from both the concrete slab (conducted from the warm return air on the adjacent floor below the slab) and the raised floor panels (conducted from the warmer room above). The magnitude of this heat gain can be quite high, resulting in undesirable loss of control of the supply air temperature from the plenum into the occupied space. These warmer supply air temperatures can make it more difficult to maintain comfort in the occupied space (without increasing airflow rates), particularly in perimeter zones where cooling loads reach their highest levels. How to predict plenum thermal performance is one of the key design issues facing practicing engineers – evidence from completed projects indicates that excessive temperature rise in the plenum can be a problem. One of the recommended strategies for addressing temperature rise in UFAD systems is the use of ductwork (flexible or rigid) within the underfloor plenum to deliver cool air preferentially to perimeter zones or other critical areas of high cooling demand. Several experiments were carried out in a full-scale underfloor plenum test facility, in order to characterize all the phenomena that take place in an underfloor plenum equipped with a fabric or metal duct. Experimental data were collected for validation of a computational fluid dynamics (CFD) model of the plenum. This paper describes the first part of a more comprehensive work, whose aim is to use the validated CFD plenum model to conduct simulations of a broader range of plenum design and operational parameters. This work proves that using ductwork within the underfloor plenum reduce the temperature rise in the plenum. [ABSTRACT FROM AUTHOR]

Highlights: [•] Short helical shaped pipe and double U-tube heat exchangers are compared. [•] The thermal behaviour in both long and short terms is investigated. [•] The models consider the interaction between the ground and the environment. [•] The axial heat conduction affects the fluid heat carrier temperature. [•] The helical heat exchanger has better thermal performance than the double U-tube. [ABSTRACT FROM AUTHOR]