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Abstract Energy storage is needed for the renewable energy power, and it is highly promising to convert CO2 to fuels utilizing the surplus renewable energy. Solid oxide cell (SOC) is an efficient energy conversation device, and it can realize energy storage by reversible operation. However, the CO2 electrolysis process is limited by the intensive energy input and low efficiency. To improve the electrolysis efficiency and realize syngas production, in this work, nickel-yttria stabilized zirconia (Ni/YSZ) supported solid oxide cell has been used for CH4 assisted CO2 electrolysis (CH4-SOEC). Moreover, a novel energy storage concept by cycling operating the cell in CH4-SOFC mode (discharge) and CH4-SOEC mode (charge) has been proposed, called CH4 assisted solid oxide cell (CH4-SOC). Besides energy storage, the CH4-SOC technology can realize syngas production as well. In the experiments, CH4 is continuously supplied to the Gd-doped ceria (GDC) modified Ni/YSZ anode, while air/CO2 are supplied to the YSZ-La0.6Sr0.4FeO3−δ (LSF)/GDC cathode in discharge/charge cycles, respectively. Experiments demonstrate that the cell presents well electrochemical performance and durability for 400 h under different operation modes. Using distribution of relaxation times (DRT) method, the rate limiting steps of the fundamental electrochemical processes in the electrodes have been identified. Moreover, thermodynamic calculations have been performed to evaluate the cell efficiency. Results suggest that the cell could realize an energy conversion efficiency of 84% in case of fuel utilization of 0.9, where the ratio of the discharge electric energy to the charge electric energy is about 0.95. Both experiments and calculations demonstrate the feasibility of technology, and using a conventional SOC is much important for the real application of it.

This paper aims to develop a green building meta-model for a representative passively designed high-rise residential building in Hong Kong. Modelling experiments are conducted with EnergyPlus to explore a Monte Carlo regression approach, which intends to interpret the relationship between input parameters and output indices of a generic building model and provide reliable building performance predictions. Input parameters are selected from different passive design strategies including the building layout, envelop thermophysics, building geometry and infiltration & air-tightness, while output indices are corresponding indoor environmental indices of the daylight, natural ventilation and thermal comfort to fulfil current green building requirements. The variation of sampling size, application of response transformation and bootstrap method, as well as different statistical regression models are tested and validated through separate modelling datasets. A sampling size of 100 per regression coefficient is determined from the variation of sensitivity coefficients, coefficients of determination and prediction uncertainties. The rank transformation of responses can calibrate sensitivity coefficients of a non-linear model, by considering their variation obtained from sufficient bootstrapping replications. Furthermore, the acquired meta-model with MARS (Multivariate Adaptive Regression Splines) is proved to have better model fitting and predicting performances. This research can accurately identify important architectural design factors and make robust building performance predictions associated with the green building assessment. Sensitivity analysis results and obtained meta-models can improve the efficiency of future optimization studies by pruning the problem space and shorten the computation time.

Abstract Low-temperature heat is abundant, accessible through solar collectors or as waste heat from a large variety of sources. Thermoacoustic engines convert heat to acoustic work, and are simple, robust devices, potentially containing no moving parts. Currently, such devices generally require high temperatures to operate efficiently and with high power densities. Here, we present a thermoacoustic engine that converts heat to acoustic work at temperature gradients as low as ∼4–5 K/cm, corresponding with a hot-side temperature of ∼50 °C. The system is based on a typical standing-wave design, but the working cycle is modified to include mass transfer, via evaporation and condensation, from a solid surface to the gas mixture sustaining the acoustic field. This introduces a mode of isothermal heat transfer with the potential of providing increased efficiencies – experiments demonstrate a significant reduction in the operating temperature difference, which may be as low as 30 K, and increased output – this ‘wet’ system produces up to 8 times more power than its dry equivalent. Furthermore, a simplified model is formulated and corresponds quite well with experimental observations and offering insight into the underlying mechanism as well as projections for the potential performance of other mixtures. Our results illustrate the potential of such devices for harvesting energy from low-temperature heat sources. The acoustic power may be converted to electricity or, in a reverse cycle, produce cooling – providing a potential path towards solar heat-driven air conditioners.

Abstract The conversion of solar energy into chemical fuels is becoming more attractive and can be used for flexibly generating power; however, the poor annual solar-to-fuel efficiency severely hinders the application of the solar fuel process in the power system. Here, we study a mid-temperature solar fuel process having synergetic dual thermochemical reactions, in which solar heat at around 300 °C provides heat to drive methanol steam reforming and decomposition successively with a variation in the solar irradiance. At solar irradiance below 400 W/m2, the low-grade solar heat is used to drive methanol steam reforming, at solar irradiance above 400 W/m2, the solar heat is combined with methanol decomposition. The solar thermochemical performances of the proposed solar fuel process were analyzed for typical days and a full year. In contrast to the individual solar-driven methanol decomposition process which mainly utilize solar irradiance above 400 W/m2, the annual average solar-to-fuel efficiency in the synergetic process, which can utilize wider solar irradiance from 100 to 1000 W/m2, is improved to 60%, and the annual average solar-to-electricity efficiency is improved to 21%. The promising results can offer a possibility of greatly improving the annual average performance of the solar fuel process, especially for enlarging the effective utilization of lower solar irradiance.

Abstract The time-varying nature of the heating loads of public buildings creates scope for exploring strategies to improve the energy system efficiency and to reduce the energy consumption and system operating costs. A well-researched and refined energy system operation strategy based on time-varying heating load demands is proposed in this paper. The proposed strategy is more effective and efficient than the existing experience-based operation strategies used to run energy systems. With full consideration of the factors affecting building heating loads under various scenarios, a multi-objective particle swarm optimisation algorithm combined with a scenario analysis is presented in this paper. The system efficiency and operation cost are set as two basic objectives to generate a Pareto frontier, and the occupant thermal comfort level is the dominant consideration while selecting an optimal state point for the final operation strategy. Using this simplified decision-making process, this approach can simultaneously calculate both the starting sequence and parameter settings for an optimised operation of the heat supply units. An energy plant on a university campus in Tianjin was selected to implement and evaluate this optimisation strategy. The case study results show that, without compromising the requirements of the thermal comfort of the building occupants, the energy system operating cost can be reduced by 38.9%, with an increase by a factor of 2.24 in the system coefficient of performance when compared with the current experience-based operation strategies.

This study presents the clockspeed analysis of a peer group comprising six major integrated US energy companies with substantial US interstate natural gas pipeline business activities: El Paso, Williams, NiSource, Kinder Morgan, MidAmerican and CMS Energy. For this peer group, the three clockspeed accelerators have been benchmarked at both corporate level and gas transmission business level, using time-series analysis and cross-sectional analysis over a 6-year period (2002–2007). The results are visualized in so-called clockspeed radargraphs. Overall corporate clockspeed winners – over the performance period studied – are: Williams, El Paso and Kinder Morgan; MidAmerican is a close follower. Corporate clockspeed laggards are: CMS Energy and NiSource. The peer group ranking for the natural gas transmission business segment shows similar clockspeed winners, but with different ranking in the following order: Kinder Morgan, MidAmerican and El Paso; Williams is a close follower. Clockspeed laggards for the natural gas transmission segments coincide with the corporate clockspeed laggards of the peer group: CMS Energy and NiSource (over the performance period studied); laggards of the past may become clockspeed leaders of the future if adjustments are made. Practical recommendations are formulated for achieving competitive clockspeed optimization in the US gas transmission industry as a whole. Recommendations for clockspeed acceleration at individual companies are also given. Although the US natural gas market is subject to specific regulations and its own geographical dynamics, this study also provides hints for improving the competitive clockspeed performance of gas transmission companies elsewhere, in other world regions.

Building-Integrated Concentrated Photovoltaics (BICPV) is based on Photovoltaic (PV) technology which experience a loss in their electrical efficiency with an increase in temperature that may also lead to their permanent degradation over time. With a global PV installed capacity of 303 GW, a nominal 10 °C decrease in their average temperature could theoretically lead to 15 GW increase in electricity production worldwide. Currently, there is a gap in the research knowledge concerning the effectiveness of the available passive thermal regulation techniques for BICPV, both individually and working in tandem. This paper presents a novel combined passive cooling solution for BICPV incorporating micro-fins, Phase Change Material (PCM) and Nanomaterial Enhanced PCM (n-PCM). This work was undertaken with the aim to assess the unreported to date benefits of introducing these solutions into BICPV systems and to quantify their individual as well as combined effectiveness. The thermal performance of an un-finned metallic plate was first compared to a micro-finned plate under naturally convective conditions and then compared with applied PCM and n-PCM. A designed and fabricated, scaled-down thermal system was attached to the electrical heaters to mimic the temperature profile of the BICPV. The results showed that the average temperature in the centre of the system was reduced by 10.7 °C using micro-fins with PCM and 12.5 °C using micro-fins with n-PCM as compared to using the micro-fins only. Similarly, the effect of using PCM and n-PCM with the un-finned surface demonstrated a temperature reduction of 9.6 °C and 11.2 °C respectively as compared to the case of natural convection. Further, the innovative 3-D printed PCM containment, with no joined or screwed parts, showed significant improvements in leakage control. The important thermophysical properties of the PCM and the n-PCM were analysed and compared using a Differential Scanning Calorimeter. This research can contribute to bridging the existing gaps in research and development of thermal regulation of BICPV and it is envisaged that the realised incremental improvement can be a potential solution to (a) their performance improvement and (b) longer life, thereby contributing to the environmental benefits.

A volume transmission phase holographic element was designed and constructed to perform as a building integrated photovoltaic concentrator. The holographic lens diffracts light in the spectral bandwidth to which the cell presents the highest sensitivity with a concentration factor of 3.6X. In this way, the cell is protected from overheating because the infrared for which the solar cell is not sensitive is not concentrated. In addition, based on the asymmetric angular selectivity of the volume hologram and based on the linear concentration, only single-axis tracking is needed. The use of the holographic element increases the efficiency of the PV cell by 3% and the fill factor by 8%.

Abstract Rice straw was subjected to fungal pretreatment using Pleurotus ostreatus and Trichoderma reesei to improve its biodegradability and methane production via solid-state anaerobic digestion (SS-AD). Effects of moisture content (65%, 75% and 85%), and incubation time (10, 20 and 30 d) on lignin, cellulose, and hemicellulose degradation during fungal pretreatment and methane yield during anaerobic digestion were assessed via comparison to untreated rice straw. Pretreatment with P. ostreatus was most effective at 75% moisture content and 20 d incubation resulting in 33.4% lignin removal and a lignin/cellulose removal ratio (selectivity) of 4.2. In comparison Trichoderma reesei was most effective at 75% moisture content and 20 d incubation resulting in 23.6% lignin removal and a lignin/cellulose removal ratio (selectivity) of 2.88. The corresponding methane yields were 263 and 214 L/kg volatile solids (VS), which were 120% and 78.3% higher than for the untreated rice straw, respectively.