Tests of prototype PCM ‘sails’ for office cooling

Abstract

PCM modules, constructed from a paraffin/LDPE composite, were tested in an occupied London office, in summer. Design variations tested the effect on heat transfer of a black paint or aluminium surface, the effect of different phase transition zones and the effect of discharging heat inside or outside.The modules' temperatures were monitored along with air flow rate, air temperature and globe temperature. Their small size meant any effect on room temperature was negligible.Using DSC measurements of the PCMs' thermophysical properties, in conjunction with the environmental measurements, semi-empirical models of the modules were constructed in FLUENT using an enthalpy porosity formulation to model phase change. Good validation was obtained using the temperature measurements with notable divergence noted when maximum liquid fraction was reached.The models were validated by the temperature measurements and used to generate mean liquid fraction and surface heat transfer rate profiles for performance comparisons.The broad phase transition zones of the PCMs results in wasted latent heat capacity. Black modules transfer heat and exhaust latent storage capacity significantly quicker than aluminium modules, due to radiant exchange. Discharging heat outside leads to an increase in thermal storage capacity and a higher rate of heat absorption.2

A study on the effect of ground surface boundary conditions in modelling shallow ground heat exchangers

“…Further active PCM systems include; Cool-Phase TM , which can reduce 90% of HVAC energy, replacing traditional air conditioning systems (Monodraught Ltd, 2014), PCM 'sails' (Susman et al, 2011) and a PCM HVAC diffuser (Gowreesunker et al, 2013). Using a limited control 22% energy savings were simulated by the diffuser.…”

Improved simulation of phase change processes in applications where conduction is the dominant heat transfer mode

“…Previous observations show that these are not always the case, and can result in discrepancies between experimental and numerical results [4,7,9].…”

“…This method is inspired by similar works performed on evaporation/ condensation [8]. The concept relies on the addition of a heat source term S E in equation (4) to mimic the melting and solidification process. The works in ref.…”

“…This, therefore, is a semi-empirical method requiring the experimental enthalpy-temperature relation to be determined though thermal techniques such as Differential Scanning Calorimetry (DSC) [4] or the T-history method [13].…”

“…Computational Fluid Dynamics (CFD) simulation environments usually possess a default melting/solidification model but this does not provide the flexibility to vary the enthalpytemperature relationships and the possibility of introducing temperature hysteresis [4]. The default enthalpy-porosity method used, developed from the Stefan Problem, does not explicitly track the solid-liquid interface, but rather, a parameter known as the liquid fraction (β), which indicates the fraction of liquid in a specific cell in the modelling domain [5].…”