This paper reports on the experimental and numerical analysis of the optical and thermal aspects of PCM (Phase Change Material) RT27 when subjected to an irradiation source of energy. The analysis is separated into the thermal and optical characterisation of the PCM using the T-history method and spectrophotometry principles, respectively, and the experimental and numerical performance evaluation of a PCM-Glazed unit. Various relationships describing the variations in the extinction, scattering and absorption coefficients with respect to the liquid fraction/ temperature within the phase change region were developed from the experimental setup, and were validated in a numerical CFD model. The results show that: i) during rapid phase changes, the transmittance spectra from the PCM are unstable, while under stable conditions a visible transmittance value of 90% and 40% are obtained for the liquid and phases, respectively; ii) the radiation scattering effects are more prominent in the solid phase of the PCM, while radiation absorption dominates in the liquid phase; iii) the optical/ radiation performance of PCM can be successfully modelled using the liquid fraction term as the main variable, in a similar way that the enthalpy porosity method is employed for phase change processes; iv) relative to a standard double glazed unit, the addition of PCM improves the thermal mass of the unit during phase change, but depending on specific
One of the primary reasons for the application of cool materials is their energy and associated environmental impact on the built environment. Cool materials are usually applied on the roof of buildings to reduce cooling energy demand. The relative benefits of this reduction depend on the construction of the building, external weather conditions and use of the building. This paper examines the impact from the application of a reflective paint on a flat roof in a naturally ventilated office building in the area of London, UK where the climate is moderate with high heating demand by buildings.The environmental conditions (internal/external air and surface temperatures) of the building were monitored before and after the application of the cool roof during the summer. It was found that internal temperatures were reduced after the application of the cool roof. The building was modelled using TRNSYS and the model was calibrated successfully using the measurements. A parametric analysis was carried out by varying the reflectivity and insulation of the roof and ventilation rate; the heating and cooling demand for a year was calculated using the Summer Design Year for London as the weather file. It was found that cooling demand is significantly reduced, heating demand is increased and the total energy savings vary between 1 and 8.5% relative to an albedo of 0.1 for the same conditions. In free floating (naturally ventilated) buildings summer comfort is improved but there is a penalty of increased heating energy during the winter. Thermal comfort can be improved by an average of 2.5 o C (operative temperature difference for a change of 0.5 in albedo) but heating demand could be increased by 10% for a ventilation rate of 2 air changes per hour. The results indicate that in the case of temperate climates the type, operation and thermal characteristics of the building should be considered carefully to determine potential benefits of the application of cool roof technology. For the examined case-study, it was found that a roof reflecticity of 0.6-0.7 is the 2 optimum value to achieve energy savings in a cooled office, improve summer internal thermal conditions in a non-cooled office (albeit with some heating energy penalty). It indicates that it is a suitable strategy for refurbishment of existing offices to improve energy efficiency or internal environmental conditions in the summer and should be considered in the design of new offices together with other passive energy efficient strategies.
The food chain comprises agricultural production, manufacturing, distribution, retail and consumption. In the UK it involves approximately 300,000 enterprises, it employs 3?3 million people and accounts for £188 billion in consumer expenditure. The food chain is also responsible for 18% of total UK energy use, 176 MtCO 2 e emissions and 15 Mt of food waste. Estimates of energy consumption and emissions from the food chain vary between sources, primarily due to differences in the assumptions made, but the general consensus is that reduction in energy consumption and resource use in the food sector is not only vital for the profitability of the sector but also for food security and meeting the Government's greenhouse gas emissions reduction targets. This paper reviews the literature on energy consumption and emissions from each stage of the food sector and outlines approaches and technologies for demand reduction. It will provide the basis for further refinement of the estimates and development of methodologies and projects to effect significant reductions of energy and resource use as part of the End Use Energy Demand Initiative of Research Councils UK. Energy Volume 167 Issue EN3 Energy demand and reduction opportunities in the UK food chain Tassou, Kolokotroni, Gowreesunker et al.
This paper reports on the energy performance evaluation of a displacement ventilation (DV) system in an airport departure hall, with a conventional DV diffuser and a diffuser retrofitted with a phase change material storage heat exchanger (PCM-HX). A TRNSYS-CFD quasi-dynamic coupled simulation method was employed for the analysis, whereby TRNSYS® simulates the HVAC and PID control system and ANSYS FLUENT® is used to simulate the airflow inside the airport terminal space. The PCM-HX is also simulated in CFD, and is integrated into the overall model as a secondary coupled component in the TRNSYS interface. Different night charging strategies of the PCM-HX were investigated and compared with the conventional DV diffuser. The results show that: i) the displacement ventilation system is more efficient for cooling than heating a space; ii) the addition of a PCM-HX system reduces the heating energy requirements during the intermediate and summer periods for specific night charging strategies, whereas winter heating energy remains unaffected; iii) the PCM-HX reduces cooling energy requirements, and; iv) maximum energy savings of 34% are possible with the deployment of PCM-HX retrofitted DV diffuser.
This paper reports on a validation study of CFD models used to predict the effect of PCM clay boards on the control of indoor environments, in ventilated and non-ventilated
Anaerobic digestion (AD) plants enable renewable fuel, heat, and electricity production, with their efficiency and capital cost strongly dependent on their installed capacity. In this work, the technical and economic feasibility of different scale AD combined heat and power (CHP) plants was analyzed. Process configurations involving the use of waste produced in different farms as feedstock for a centralized AD plant were assessed too. The results show that the levelized cost of electricity are lower for large-scale plants due to the use of more efficient conversion devices and their lower capital cost per unit of electricity produced. The levelized cost of electricity was estimated to be 4.3 p/kWh e for AD plants processing the waste of 125 dairy cow sized herds compared to 1.9 p/kWh e for AD plants processing waste of 1000 dairy cow sized herds. The techno-economic feasibility of the installation of CO 2 capture units in centralized AD-CHP plants was also undertaken. The conducted research demonstrated that negative CO 2 emission AD power generation plants could be economically viable with currently paid feed-in tariffs in the UK.
Abstract:This paper reports on the development, experimental validation and application of a semiempirical model for the simulation of the phase change process in phase change materials (PCM). PCMs are now increasingly being used in various building materials such as plasterboard, concrete or panels to improve thermal control in buildings and accurate modelling of their behaviour is important to effectively capture the effects of storage on indoor thermal conditions. Unlike many commercial simulation packages that assume very similar melting and freezing behaviour for the PCM and no hysteresis, the methodology employed treats the melting and freezing processes separately and this allows the inclusion of the effect of hysteresis in the modelling. As demonstrated by the results in this paper, this approach provides a more accurate prediction of the temperature and heat flow in the material, which is of particular importance in providing accurate representation of indoor thermal conditions during thermal cycling. The difference in the prediction accuracy of the two methods is a function of the properties of the PCM. The smaller the hysteresis of the PCM, the lower will be the prediction error of the conventional approach, and solution time will become the determining factor in selecting the simulation approach in practical applications.
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