Field behaviour of a flat panel ground heat exchanger

An experimental plant has been devised to investigate the behaviour of a novel type of horizontal ground heat exchanger (GHX), aiming to improve the performance of ground-source heat pumps for space heating and cooling. The GHX system is composed by hollow flat panels, which have been installed edgeways in shallow trenches two meters deep in soil. The hydraulic closed loop and the surrounding soil have been equipped with several digital sensors to monitor the ground temperature distribution and the plant in real-time. The behaviour has been tested for two years in several operating carried out especially in summertime. The specific power of heat transfer for surface-unit achieves considerable values, and no over-heating conditions were measured at the soil surface. Moreover, the GHX showed to be able to involve a large soil volume, and this behaviour enables high energy performance, at least in cooling mode. After few months of inactivity, the natural ground heat transfer erased the memory of the energy exploitation carried out by the GHX. Thus, unlike with the vertical systems, long-term subsurface thermal energy build-up or depletion wouldn’t be expecting by shallow GHXs.Alternative Technologies Ltd., Energy Investment Ltd, JMV Vibro Blocks Ltd., Solar Engineering Ltd. and Solar Solutions Ltd.peer-reviewe

Innovation in ventilated tiled roofs : the HEROTILE European project

In ventilated roofs, the so-called Above Sheathing Ventilation (ASV) helps dissipating the excess heat in summer, thus reducing the cooling energy requirement. The ASV can be enhanced by increasing the air permeability of the tiled covering through the development of new tile shapes. This is the purpose of the Life HEROTILE European project, of which this work presents the preliminary analysis. The air permeability of a novel Marsigliese tile is analysed in comparison with the standard tile. The new design is improved with a higher sidelock and a new headlock pattern. A CFD model is then used to simulate the airflow through the tiles, solving the steady-state, incompressible fluid flow, in a 3D domain by means of the RANS-based standard k-ε model. A parametric study is conducted to analyse the variation in the air flow passing through the tile for different speeds and directions of the incident wind. The reference and new tile designs are compared in terms of air pressure drop and volumetric flow rate trough tiles. The novel shape increases the air permeability up to 100%; contrary to the standard shape, the new design allows also an increase of the air flow rate as the wind blows sideways.peer-reviewe

Experimental analysis and numerical simulation of a flat-panel ground heat exchanger

Le recenti politiche ambientali, volte alla riduzione del fabbisogno energetico in edilizia ed alla riduzione delle emissioni di gas clima alteranti, hanno supportato il diffondersi delle tecnologie ad energia rinnovabile. Tra queste, le pompe di calore geotermiche si sono affermate come soluzione alternativa ai tradizionali sistemi per il riscaldamento e raffrescamento degli edifici, in virtù della loro provata affidabilità e dell’elevata efficienza. Sono attualmente disponibili diverse tipologie di pompa di calore geotermica che possono essere in primo luogo classificate in due sottocategorie: a circuito aperto e a circuito chiuso. Le pompe di calore geotermiche a circuito chiuso sono maggiormente diffuse e sono termicamente accoppiate al terreno, che è la sorgente/pozzo termico, per mezzo di scambiatori geotermici. Questi sono generalmente costituiti da un sistema di tubazioni in materiale plastico, che può essere installato in perforazioni verticali (fino a 200 m di profondità) o in posizione orizzontale all’interno di appositi sbancamenti e trincee superficiali (solitamente fino a 2 m). Le pompe di calore geotermiche accoppiate a scambiatori verticali beneficiano della favorevole temperatura del terreno alle basse profondità. All’incirca a 10 m dalla superficie infatti il terreno ha una temperatura quasi costante pari alla temperatura media annuale dell’aria, che aumenta all’aumentare della profondità secondo il gradiente geotermico locale. Al contrario, nel caso di scambiatori orizzontali, la pompa di calore è accoppiata termicamente ad una sorgente (il terreno superficiale) la cui temperatura oscilla stagionalmente al variare delle condizioni ambientali. In virtù di ciò, gli scambiatori verticali offrono prestazioni mediamente migliori, tuttavia l’elevato costo, rende comunque competitiva la più economica soluzione orizzontale per applicazioni residenziali di piccola taglia. Negli ultimi decenni, un considerevole sforzo è stato fatto per l’ottimizzazione delle prestazioni delle pompe di calore geotermiche, sia in ambito accademico che industriale. Di recente sono state sviluppate nuove configurazioni e geometrie per gli scambiatori orizzontali con l’obiettivo di aumentarne l’efficienza di scambio termico. Questa tesi si inserisce in questo ambito, essendo dedicata ad un innovativo scambiatore geotermico di tipo Flat-Panel, inventato e sviluppato presso l’Università degli Studi di Ferrara. L’analisi delle prestazioni di scambiatori Flat-Panels è stata condotta sia per via sperimentale sia impiegando tecniche di modellazione numerica, nell’intento di fornire indicazioni approfondite sul loro utilizzo in accoppiamento a pompe di calore geotermiche. Allo scopo è stato allestito un apparato sperimentale equipaggiato con due prototipi di Flat-Panel, presso il Dipartimento di Architettura dell’Università di Ferrara. Sono stati condotti diversi test simulando il funzionamento di una pompa di calore geotermica in differenti condizioni operative (riscaldamento e raffrescamento) e in diverse modalità (funzionamento continuo, discontinuo e pulsato). Con rifermento ai più comuni scambiatori orizzontali, il Flat-Panel ha fornito prestazioni molto buone sia in riscaldamento che raffrescamento. In particolare, un’ottima prestazione è stata ottenuta durante i test estivi, in virtù della maggiore differenza di temperatura tra il fluido termovettore ed il terreno termicamente indisturbato. Come riportato in letteratura in merito agli scambiatori orizzontali, anche per i Flat-Panels non sono stati osservati fenomeni di deriva termica nel terreno superficiale, indipendentemente dall’energia scambiata. L’analisi numerica ha riguardato la modellazione dello scambio termico nel terreno per mezzo di scambiatori di tipo Flat-Panel. Allo scopo è stato impiegato un modello numerico agli elementi finiti risolvendo lo scambio termico in regime transitorio in un dominio bidimensionale. Nel dominio di calcolo la particolare geometria del Flat-Panel è stata ricondotta ad una condizione al contorno. È stato inoltre sviluppato un modello del bilancio di energia alla superficie del terreno (condizione al contorno del terzo tipo) al fine di simulare dettagliatamente la variazione giornaliera e stagionale della temperatura del terreno superficiale, che è determinante per le prestazioni degli scambiatori orizzontali. In considerazione di ciò, l’analisi è stata approfondita con ulteriori simulazioni per valutare l’effetto sulla soluzione numerica di differenti condizioni al contorno alla superficie del terreno: il modello del bilancio di energia, un flusso termico equivalente ed infine una temperatura superficiale equivalente. I risultati indicano che l’utilizzo del modello del bilancio di energia è l’approccio da preferirsi, senza che questo comporti un particolare aggravio in termini di tempo di calcolo. L’utilizzo di una temperatura equivalente è una ragionevole semplificazione, sebbene la sua stima corretta sia piuttosto complessa. I risultati del modello numerico proposto sono stati confrontati con i dati sperimentali ottenuti durante i test in diverse condizioni operative. Complessivamente il modello si è dimostrato affidabile nel calcolo della variazione di temperatura nel terreno determinato dall’effetto combinato dello scambio termico alla superficie del terreno e allo scambiatore. Infine, è stata svolta un’analisi di sensitività per valutare l’effetto della variazione della conduttività termica del terreno.Worldwide, the innovation and environmental policies for energy saving in buildings and the reduction of greenhouse gas emissions have widely supported renewable energy technologies. Ground-source heat pumps (GSHPs) are regarded as a reliable technology and may represent an efficient and cost-effective solution for space heating and cooling, when the investment for ground heat exchangers is reasonable. In ground-coupled heat pump (GCHPs), a subset of GSHPs, a ground heat exchanger is required to thermally couple a heat pump with the ground. The ground heat exchanger usually consists of a piping system installed in vertical boreholes or in shallow diggings. Vertically coupled heat pumps benefit from the relatively stable temperature in the deep ground and uses geothermal energy from the earth. A horizontally coupled heat pump uses the seasonal heat storage in shallow soil therefore, the performance of horizontal ground heat exchangers (HGHEs) is strongly dependent on climatic conditions due to the low installation depth. A considerable amount of research has been devoted to the performance optimisation of GCHPs, in the last decades. More recently, a number of studies have dealt with the development of new configurations and new geometries for HGHEs, aiming to improve their efficiency. As part of these efforts, this thesis was dedicated to an innovative HGHE, called Flat-Panel, which was invented and developed at the University of Ferrara. This study dealt with the experimental analysis and the numerical simulation of Flat-Panels and it was intended to provide guidance on the behaviour and the performance of this novel HGHEs. The experimental analysis was carried out by means of a dedicated experimental setup equipped with two FlatPanel prototypes. Tests were conducted simulating the operation of a GCHP in different operating conditions (heating and cooling) and modes (continuous, discontinuous and pulsed). Very good performance was reached for both heating and cooling mode in comparison with the widespread installations of straight pipes or slinky coils. The performance was higher in heating mode due to the higher temperature difference between the working fluid and the undisturbed soil in summer. Moreover, according to other studies, seasonal thermal drifts were not measured for HGHEs, regardless of the amount of energy exchanged. The numerical analysis dealt with the simulation of heat transfer in soil due to Flat-Panels. A finite element numerical code was applied solving the unsteady-state heat transfer problem in a 2D domain. In view of this, the FlatPanel shape was modelled as a boundary condition. In order to further delve into particular aspects of HGHEs behaviour, the different heat transfer processes at the ground surface were modelled on the basis of the surface properties and a comprehensive weather dataset. Furthermore, the effect on numerical simulation of HGHEs of different boundary conditions at the ground surface was analysed. The ground surface energy balance model (GSEB), the equivalent surface heat flux and temperature were assigned as boundary conditions of the 1st, 2nd and 3rd kind in three different simulations, respectively. The results indicate that the use of the GSEB model is the preferable approach to the problem, not affecting the calculation time. The equivalent surface temperature could be considered as a reasonable simplification, although its correct estimation is a major issue. The results of the numerical simulation were compared with multiple experimental data sets in different operating conditions. Overall, the model produced a good agreement in terms of ground temperature variation due to the combined effect of the HGHE operation and the heat transfer process at the ground surface. In addition, a sensitivity analysis was carried out to evaluate the effect of variations in soil thermal conductivity

On the heat transfer through roof tile coverings

In hot climates, ventilated pitched roofs with tiled coverings reduce the heat transfer across the roof, due to the ventilated air layer between tiles and roofing underlay, which is formed by the arrangement of battens and counter-battens. This so-called Above Sheathing Ventilation (ASV), which depends on the air entering and leaving at the eaves, ridge and the gaps between the tiles, has been enhanced by increasing the roof air permeability by means of novel roof tile shapes. This study analyses the air permeability improvement of a novel tile and its effect on the heat transfer induced by an external heat source (e.g. solar radiation). A CFD model is used to simulate the fluid flow and heat transfer through the tiles, solving the steady-state incompressible fluid flow in a 3D domain by means of the standard k-ε model. A parametric study is conducted to analyse the variation in the air flow passing through the tile and the tile temperature for different incident air flow conditions. A significant increase in flow rate is observed with the novel tile, which produces a lower temperature of the tile and of the air flowing through the tiles. This would help dissipating the excess heat in summer

Heat transfer analysis of underground thermal energy storage in shallow trenches filled with encapsulated phase change materials

The use of a horizontal ground heat exchanger may represent a reliable and cost effective option for ground-source thermal applications. This study presents the thermal performance analysis of a drainage trench used as ground heat exchanger (GHE) coupled with underground thermal energy storage (UTES). The trench is dug in shallow soil and filled with encapsulated phase change materials (PCMs) as granular filler. Two types of PCMs with different melting points are supposed to operate in summer and winter. Fluid flow and heat transfer in porous media are solved via a 2D finite element model to perform a yearly simulation under hourly-scale boundary conditions. The equivalent heat capacity approach is applied to consider the latent heat of the PCMs. The results show a significant capacity of the trench to smooth thermal waves produced by the heat pump. The effect of the PCMs is analysed by comparing with the corresponding case using coarse gravel as filling material instead of PCMs. The case without PCMs still shows good performance, but PCMs offers the advantages of a seasonal UTES and smoothing thermal wave as well. The proposed solution can be therefore considered as an advanced alternative to other widespread common GHEs

Experimental analysis of an innovative tile covering for ventilated pitched roofs

This study investigates the thermal and fluid-dynamic behaviour of two ventilated roofs equipped with a new tile shape in comparison with a standard Portuguese tile. The novel tile shape was designed to improve the air permeability of coverings, thus producing a further Above Sheathing Ventilation enhancement as proposed in the ongoing LIFE HEROTILE European project, in which original roof tiles with innovative sidelock and headlock patterns have been designed, produced and are now under testing with real operating conditions. The heat transfer in the different roof layers and the ASV fluid flow have been experimentally analysed by means of a dedicated real scale mock-up. The results of the preliminary testing period are presented in this paper, they are referred to a first period with open eaves and to a second one in which eaves were closed to observe the only effect of tile air permeability. The data analysis shows that the ASV is strictly linked to the external wind conditions and grants better performances in HEROTILE roof. Moreover, the new tile design produces a significant increase of the ASV, so that better thermal performances are achievable in reducing solar heat gain during hot periods. As a consequence, the mock-up with HEROTILE ventilated roofs need less energy for air conditioning and allows high energy saving when compared with the standard one

Small-scale lab analysis of the ground freezing effect on the thermal performance of a Flat-Panel ground heat exchanger

Shallow ground heat exchangers are increasingly studied due to their advantages in cost and long-term energy performance stability when coupled with heat pumps for space heating and cooling. As for borehole heat exchangers, the backfilling material affects significantly the operating efficiency of the whole system, mainly driven by the low thermal diffusivity of the soil. To enhance the heat transfer, the mixing of the backfilling material with phase change materials (PCMs) is a novel strategy still partially investigated, especially with regards of the heat pump on/off cycling. This study presents the results of experimental tests carried out at labscale to analyse the performance of a shallow Flat-Panel ground heat exchanger (FGHE) coupled with water-sand mixture. Firstly, the comparison between FGHEs coupled with dry sand and water-sand mixture is performed; then, the impact of latent heat resulting from freezing is further studied in three on/off operating modes. A maximum of 31.6% increment in heat transfer efficiency is observed in wet conditions and for the highest on/off frequency. Therefore, coupling FGHE with water-sand mixture enhances the heat transfer, especially in icing interval and when combined with a suitable on/off operating frequency

Analisi numerica dell’accoppiamento di materiali a cambiamento di fase a scambiatori geotermici

L’impiego di materiali a cambiamento di fase (PCMs) è una soluzione diffusamente utilizzata per l’accumulo di energia termica, nell’ambito dello sfruttamento di risorse rinnovabili caratterizzate da una certa discontinuità della disponibilità energetica. Un’ulteriore possibilità di utilizzo è rappresentata dallo smorzamento della richiesta termica istantanea degli impianti di climatizzazione a pompa di calore, in particolare quelle geotermiche (GSHPs). In questo lavoro, un modello numerico agli elementi finiti è utilizzato per risolvere lo scambio termico nel terreno e valutare le prestazioni di scambiatori geotermici a installazione orizzontale (HGHEs) accoppiati all’utilizzo di materiali a cambiamento di fase. L’installazione proposta prevede che i PCMs siano miscelati direttamente con il materiale di riempimento delle trincee per l’alloggiamento di HGHEs, o in alternativa alloggiati in appositi contenitori posti a contatto con la superficie dello scambiatore. I risultati ottenuti mostrano che l'uso dei PCMs in accoppiamento a HGHEs è in grado di soddisfare parzialmente i picchi di richiesta per riscaldamento delle pompe di calore, smorzando le sollecitazioni termiche all’interfaccia scambiatore/terreno. Di conseguenza, i valori di picco della temperatura del fluido termovettore ne risultano ridotti e pertanto, è possibile raggiungere migliori rendimenti (COP) per le pompe di calore. Inoltre, l’utilizzo di PCMs rende possibile lo stoccaggio stagionale di energia termica per impianti con scambiatori geotermici orizzontali

Prestazioni termiche estive: confronto fra tetti a falda e coperture piane

Si indaga il comportamento termico estivo di un edificio a falde a confronto con uno a tetto piano, mediante un modello ad elementi finiti. A parità di condizioni rispetto alla copertura piana quella a falde ventilata necessita di una potenza inferiore del 60% con conseguente risparmio energetico The traditional forms of building insulation and their role in energy savings are well recognized in cold climates, while energy performance optimization of the building envelope in hot climates is often misunderstood, such as the opportunity to have a ventilation layer in pitched roofs. This feature is commonly referred as: Above Sheathing Ventilation (ASV), an eaves-ridge open cavity present under the waterproof layer thank to the laying of the tiles over a batten and/or counter-batten support system. Air enters both at eaves section and through the air-permeability of the overlapping tiles, and flows to the ridge, sinking the heat transfer generated by the solar radiation. This study surveys the thermal behaviour during the summer season of a building in which varies the air-permeability between tiles, compared to a concrete flat roof building. Several studies have demonstrated the performance of a pitched roof, but it is not well yet investigated the impact of air-permeability of the external waterproof surface over the chimney effect, because several factors contribute to the complexity of the problem, such as the increasing mass flow rate and the Buoyancy-driven forces. The analysis has been approached by means of a numerical model, solving the fluid-dynamic and the heat transfer problems in unsteady state. Time series for wind, solar radiation and indoor space cooling were introduced to simulate realistic boundary conditions, taking into account different air-permeability of the waterproof surface and ASV thickness of the pitched roof

CFD analysis of roof tile coverings

In hot climates tiled pitched roofs significantly reduce the heat transfer across the roof structure, due to the ventilated air layer between tiles and roofing underlay formed by the arrangement of battens and counter-battens supporting the tiles. This so-called Above Sheathing Ventilation (ASV) depends on the air entering and leaving at the eaves, ridge and the gaps between the tiles. With a view towards higher energy savings in space cooling, the natural and forced convection occurring in ASV could be enhanced by increasing the roof air permeability by means of novel tile shapes, as here analysed in two stages. The first stage of designing the new tile shapes was to measure the air permeability for a type of existing tile (Marseillaise style) using an experimental test rig, by monitoring the volumetric flow rate through the tiles over a range of pressure differences across the tiles. Then, a three-dimensional CFD model was implemented to replicate the full test rig geometry, and this was calibrated against the experimental data. In the next stage, the calibration was used to support the design of novel Marseillaise tile shapes, and to compare their performance against existing tiles. Finally, in order to analyse the variation in air flow under typical wind conditions for a pitched roof, a parametric study was undertaken, consisting of 72 scenarios varying wind speed, direction and angle of incidence. An increase in volumetric flow rate through the tiles was found to be related not only to an increase in the open area between tiles, but also to the design of the tile locks. By redesigning the geometry of these locks, whilst still giving consideration to their primary purpose of preventing the ingress of driving rain, it was possible to yield an improvement in air permeability of up to 100% in comparison with the original designs. Additionally, these novel designs were shown to increase the air flow rate as the wind angle moved from being directly up the roof slope around to the side, in contrast to the decrease seen with existing tile shapes