• Energy Research

The aim of the paper is to report a comprehensive review into a recently emerging building integrated solar thermal technology, namely, Active Solar Thermal Facades (ASTFs), in terms of concept, classification, standard, performance evaluation, application, as well as research questions. This involves the combined effort of literature review, analysis, extraction, integration, critics, prediction and conclusion. It is indicated that the ASTFs are sort of building envelope elements incorporating the solar collecting devices, thus enabling the dual functions, e.g., space shielding and solar energy collection, to be performed. Based on the function of the building envelopes, the ASTF systems can be generally classified as wall-, window-, balcony-and roof-based types; while the ASTFs could also be classified by the thermal collection typologies, transparency, application, and heat-transfer medium. Currently, existing building and solar collector standards are brought together to evaluate the performance of the ASTFs. The research questions relating to the ASTFs are numerous, but the major points lie in: (1) whole structure and individual components layout, sizing and optimisation; (2) theoretical analysis; (3) experimental measurement; and (4) energy saving, economic and environmental performance assessment. Based on the analysis of the identified research questions, achievements made on each question, and outstanding problems remaining with the ASTFs, further development opportunities on this topic are suggested: (1) development of an integrated database/software enabling both architecture design and engineering performance simulation; (2) real-time measurement of the ASTFs integrated buildings on a long-term scheme; (3) economic and environmental performance assessment and social acceptance analysis; (4) dissemination, marketing and exploitation strategies study. This study helps in identifying the current status, potential problems in existence, future directions in research, development and practical application of the ASTFs technologies in buildings. It will also promote development of renewable energy technology and thus contribute to achieving the UK and international targets in energy saving, renewable energy utilization, and carbon emission reduction in building sector.

Abstract This article proposes a novel application combining the thermal catalytic oxidation with solar-collected wall (SWTCO) in buildings without auxiliary energy. Thermal catalyst MnO x -CeO 2 was prepared by the method of modified coprecipitation. The performance and kinetics of MnO x -CeO 2 for catalytic oxidation of indoor formaldehyde were investigated. Once-through experiments at different concentrations of 300–4300 ppb and temperatures 20–100 °C were conducted. Moreover, formaldehyde conversion experiments in SWTCO system at several typical indoor concentrations (289 ppb, 587 ppb and 1374 ppb) were performed. Furthermore, a simple system model predicting the degradation of indoor formaldehyde versus the time in SWTCO system was built. Results were as follows: (1) The reaction rate expression considering two parameters i.e. concentration and temperature based on modified L-H model fit the experimental data well; (2) System model calculation results showed initial formaldehyde concentration was an important factor to the formaldehyde degradation capability; (3) SWTCO system could realize high pollutant-removal efficiency and low energy costs simultaneously under higher temperature compared with the system using electrical heating; (4) The purification time constant increased with the horizontal solar radiation and the influence of concentration in typical indoor formaldehyde concentration on it could be neglected.

Abstract A portable water generator (7 kg) with two thermoelectric coolers (TECs) was designed and experimentally investigated in this study. Different inlet air relative humidity (RH) and air flow rates were investigated to obtain their impacts on the amount of generated water and condensation rate. The amount of generated water and condensation rate increased with the RH rose. The amount of generated water increased with air flow rates rose but the condensation rate had opposite trend. The maximum amount of generated water was 25.1 g each hour with 0.216 m2 condensation surface and 58.2 W input power. This system had small size and could work at small air flow rates which was suitable for outdoor use. This study had a guiding role in designing and optimizing a high-efficient portable water generator.

Abstract This paper presents an experimental and analytical study on incorporation of thermoelectric modules with glass evacuated-tube heat-pipe solar collectors. The integrated solar heat-pipe/thermoelectric module (SHP-TE) can be used for combined water heating and electricity generation. The experimental prototype unit comprises a glass evacuated-tube, a heat-pipe and a thermoelectric module with its one side attached to the condensation section of the heat-pipe and other side attached to a water channel. The heat-pipe transfers the solar heat absorbed within the glass evacuated-tube to the thermoelectric module. Under the condition of given solar irradiation and water temperature, the current, voltage and power outputs of the thermoelectric module are given for variable external electrical resistance. An analytical model of the prototype unit is presented to relate its thermal and electrical efficiencies with the solar irradiation, ambient temperature, water temperature, areas of the glass evacuated-tube and the thermoelectric module, and the length, cross-section area and number of thermoelements in the thermoelectric module. The analytic model is validated against the experimental data before it is used to optimize the design and operating parameters of the prototype for combined water heating and additional electricity generation.

Abstract Facade-integrated photovoltaic/thermal (BiPV/T) technology is a relatively new concept in improving the overall energy performance of PV installations in buildings. With the use of wall-mounted water-type PV/T collectors, the system not only generates electricity and hot water simultaneously, but also improves the thermal insulation of the building envelope. A numerical model of this hybrid system was developed by modifying the Hottel–Whillier model, which was originally for the thermal analysis of flat-plate solar thermal collectors. Computer simulation was performed to analyze the system performance. The combined effects of the solar cell packing factor and the water mass flow rate on the thermal and electrical efficiencies were investigated. The simulation results indicated that an optimum water mass flow rate existed in the system through which the desirable integrated energy performance can be achieved.

Abstract To address the problem of heat removal by phase change material (PCM) wall at nighttime in the summer season, a new cooling wall that makes use of the high latent heats of PCMs, the high heat conductivities of micro-channel heat pipes (MHPs), and the passive cooling of sky radiative cooling (RC) is introduced, and is named the MHP-RC-PCM wall. In this study, preliminary experiments were first conducted to determine the emissivity of the radiative plate and the properties of PCMs (paraffin, RT28HC). Next, numerical models of the MHP-RC-PCM wall were established to simulate the thermal behavior, and the model was validated with the experimental results. The parameters that affect the thermal behavior of the MHP-RC-PCM wall, including the phase transition temperature, latent heat of the PCM, number of MHPs, and year-round thermal behavior were investigated. The results showed that the phase transition temperature (Tm) of the PCM had a significant influence on the interior surface temperature, liquid fraction and cooling load reduction ratio of the MHP-RC-PCM wall, whereas the PCM latent heat had little effect. The cooling load reduction ratio was approximately 4% for Tm = 31 °C, which was higher than that for Tm = 26 °C. In addition, it was determined that the year-round energy-saving of the MHP-RC-PCM wall were approximately 18.2% greater than that of the Brick wall with the same thickness, and 0.4% higher than that of PCM wall in Guangzhou City, China.

An improved Trombe wall is proposed to adapt to building construction with selective thermo-insulation facades (internal and cavity wall insulation, but not external wall insulation system). The case study is conducted in Xining, Capital city of Qinghai province in China, where the general building facades are mostly selective thermo-insulation facades to fight against the severe cold. A numerical analysis is undertaken to show the effects on the improvement of the building’s thermal environment by comparing the improved Trombe wall system with the classical Trombe wall system. The operating efficiency of the improved Trombe wall can be up to 33.85%, an increase of 56%. The results show that the improved Trombe wall works more effectively than the classical Trombe wall system in utilizing solar energy for the sample building.

Abstract The aim of the paper was to present a dedicated theoretical investigation into the thermal performance of a novel solar loop-heat-pipe facade based heat pump water heating system. This involved thermo-fluid analyses, computer numerical model development, the model running up, modelling result analyses and conclusion. An energy balance network was established on each part and the whole range of the system to address the associated energy conversion and transfer processes. On basis of this, a computer numerical model was developed and run up to predict the thermal performance of such a system at different system configurations, layouts and operational conditions. It was suggested that the loop heat pipes could be filled with either water, R134a, R22 or R600a; of which R600a is the favourite working fluid owing to its relatively larger heat transfer capacity and positive pressure in operation. Variations in the system configuration, i.e., glazing covers, heat exchangers, would lead to identifiable differences in the thermal performance of the system, represented by the thermal efficiency and COP. Furthermore, impact of the external operational parameters, i.e., solar radiation and ambient air temperature, to the system's thermal performance was also investigated. The research was based on an innovative loop-heat-pipe facade and came up with useful results reflecting the thermal performance of the combined system between the facade and heat pump. This would help promote development and market penetration of such an innovative solar heating technology, and thus contribute to achieving the global targets in energy saving and carbon emission reduction.

A dynamic simulation model of a building-integrated photovoltaic and water heating system is introduced in this paper. The numerical model was developed based on the finite difference control volume approach. The integrated use of energy balance and fluid flow analysis allows the prediction of the system dynamic behavior under external excitations such as changes in weather, water consumption and make-up conditions. The validity of the modeling approach was demonstrated by comparing its predicted operating temperature changes and system daily efficiencies with the measured data acquired from an experimental rig at the City University of Hong Kong. The predictions from the model show good compliance with the experimental measurements.