Abstract

This work presents a method for steady state modelling and simulation of 5th Generation District Heating and Cooling (5GDHC) Networks. The method allows to characterise interdependencies between electrical, thermal, and hydraulic parameters for different 5GDHC network configurations. Two case studies, one with free floating network temperature, and the second with active network temperature control are demonstrated. Simulation results show that the electrical power consumption of Water Source Heat Pumps (WSHP) at demand substations are highly sensitive to the balance of heating and cooling loads when the network is operated with free-floating temperatures. In both cases, it was shown that the total electrical power consumption of the network is minimised when the heat injection and extraction applied by the WSHPs is balanced.

Abstract

Rebound effect is recognized as a loss ratio of energy savings or environmental emission reductions. But, this research suggests a positive side of rebound effect to consumers – welfare effect. We define the welfare effect using a dual form to rebound effect, explain its mechanism in the same economic framework with rebound effect, and give its estimation formula using Taylor expansion. Four consumer types are classified according to the sizes of rebound effect and welfare effect, and the determinants of the classification are discussed. We then conduct empirical research on both rebound effect and welfare effect for urban residents in China. We find backfire and large welfare effect, revealing that the urban residents in China are likely to remain ‘the insufficient in demand’ for a long period. Besides, rebound effect tends to decline and welfare effect to increase when energy efficiency measures are enhanced.

Abstract

Extreme cold and hot weather influence the performance of batteries significantly. Vanadium redox flow batteries (VRFB) work efficiently in the temperature range of 10⁰C to 40⁰C. In this work, a physics-based electrochemical model has been developed to calculate the overpotentials and cell voltage at different temperatures. Decrease in temperature hinders the performance of VRFB by increasing the overpotential. The model is validated with the experiment results of 426 cm2 single cell VRFB at 25, 10 and -10⁰C.

Abstract

A biomass-derived hierarchical porous carbon-carbonized rice (CNR) with high porosity (80.9%) and large pore volume (3.45 cm3/g) was prepared using natural and accessible biomass rice as template. Then the CNR was applied to pack polyethylene glycol 2000 (PEG2000) phase change material (PCM) via vacuum impregnation method. The phase change property, thermal storage/release and solar photothermal conversion performance of the materials were explored. The results show that the maximum loading of PEG2000 is 75 wt%. Compared with pure PCM, the supercooling degree of PEG2000/CNR composite material is reduced by 57%, that is, CNR promotes the heterogeneous nucleation of PEG2000. The enthalpy value of PEG2000/CNR composite is up to 137.22 J/g with an encapsulation ratio of 81.6%, showing a better thermal performance than commonly used mesoporous silicon-based, carbon-based and metal-organic framework composite PCMs. The biomass-derived CNR can significantly increase the energy storage/release rate and solar photothermal conversion efficiency (54.7%) than that of pure PCM. This work provides a new idea for the design of phase change composite materials with high heat storage density and stability for thermal and solar energy storage.

Abstract

Decentralized polygeneration systems can provide
multiple energy services for urban districts like
universities and hospitals, with several energetic,
economic and environmental benefits. However,
deciding the right design or optimal capacities for such
integrated multi-energy sources would require
sophisticated modelling and optimization techniques.
Furthermore, several design parameters and timevarying loads and weather conditions influence the
performance of polygeneration systems. This study
investigated the effects of renewables, storage units and
time-varying loads (electricity, heat and cold) on the
performance of a cryo-polygeneration system expected
to be installed in 2022 at the NTU campus located in
Singapore. Diverse design scenarios were analyzed to
study the effect of critical components such as
absorption chiller, cold storage and solar PV units with
energy storage. The optimal design capacities derived
confirms higher efficiency and economic performance
than the reference system, i.e., generating the power
and heat and power separately. The results are of great
interest to academia and industry and contribute
significantly to developing an efficient and cost-effective
energy storage polygeneration system.

Abstract

Carnot batteries represent an emerging thermomechanical energy storage technology based on the
conversion of surplus electricity into medium-low temperature
heat, and subsequent conversion of the heat into electricity. A
promising configuration of the Carnot battery is represented
by the Organic Rankine Cycle Compressed Heat Energy Storage
(ORC-CHEST) that combines a high-temperature heat pump
(charge phase), an Organic Rankine Cycle (ORC) system
(discharge phase) and a thermal energy storage (TES) system.
Indeed, TES is a crucial component in the overall ORC-CHEST
system, since it thermally links the charge and discharge
phases (operating asynchronously) guaranteeing optimal
operation and ensuring significantly high round trip
efficiencies. Most of literature on ORC-CHEST have so far
only focused on preliminary analyses in order to define the
general thermodynamic potential and to identify the limits of
the overall system. Indeed, a detailed analysis of ORC-CHEST
with focus on TES modelling is lacking. This paper presents
such an analysis by developing a dynamic numerical model of
the discharge phase of ORC-CHEST system with a novel packed
bed solution for the TES system. Indeed, we developed for the
first time a plant model in MATLAB that blends together
algebraic and differential sub-models detailing the transient
behaviour of the thermal storage stages and the ORC unit. In
addition, a novel configuration of the TES system design is
proposed utilizing a cascade of multiple phase change
materials (PCMs) in place of the cascade of sensible and single
PCM proposed in literature, enhancing simultaneously both
the TES energy density and the round trip efficiency of the
system. The results are of great interest for academia and
industry and contribute significantly to the development of an
efficient and cost-effective thermal energy storage system,
capable to simultaneously increase the ORC-CHEST round trip
efficiency and energy density by 7 % and 77 %, respectively,
compared to the state-of-the-art solution.

Abstract

For large-scale stationary energy storage applications, flow batteries are gaining attention all over the world. Numerous studies have been done on flow batteries since their invention. Almost all the studies are based on the constant current cycling of flow batteries. In the present work, we explore a different perspective of a flow battery and characterize the power, energy, and efficiency characteristics of a 5-kW scale vanadium redox flow battery system through constant power cycling tests. Different ratios of charge power to discharge power characteristics of solar, wind, and peak shaving applications have been incorporated in the test protocol. It is shown that, over the range of testing, the round-trip energy efficiency and the fractional energy utilization depend linearly on the power at which the battery is charged or discharged

Abstract

As a major profile control method for low permeability reservoirs, nano polymer microspheres (NPM) are used for thousands of wells in Changqing Oilfield every year， and have achieved excellent effect of EOR. Different from the profile control particles optimized based on the theory of matching between particle and pore throat size, the main mechanism of NPM is to increase the flowing resistance after adsorption on the pore throat surface.
However, there is a lack of quantitative research on the adsorption law of NPM, and the double-layer HPAM molecular model, without considering electrical properties, used by scholars cannot reflect the real formation charge properties and the expansion characteristics of NPM, and cannot reveal the adsorption law of NPM, which seriously affects the efficient application in the field.
In this paper, the dynamic adsorption capacity of NPM in sandstone reservoir is studied by QCM-D. Then, a customized SiO2 coupon is used to simulate the cumulative static adsorption capacity of NPM at a certain position of the reservoir. At the same time, above two experimental methods were used to study the effects of different expansion time, different expansion ratio NPM combination and salinity on dynamic and static adsorption capacity. Finally, by changing the mass fraction of H2O and HPAM to simulate NPM with different expansion time, a three-layer molecular model of “NPM + mineralized water + negatively charged SiO2” was constructed to verify the experimental results and reveal the adsorption mechanism of NPM from the molecular scale.
The results show that the maximum static and dynamic adsorption capacities of NPM are 9.26 μg·cm-2 and 0.18 μg·cm-2 respectively. The aggregates in NPM solution will adsorb the monomer on the surface of SiO2 coupon, so there are both adsorption and desorption happened. The maximum cumulative static adsorption capacity reached 150 μg·cm-2 after 3 days of expansion. The combination of different expansion time and new NPM has synergistic effect, which can double the adsorption capacity of expanded NMP. The adsorption capacity of NPM increases with the increase of salinity, and NPM can reduce the mineral adsorption capacity of pore surface.
The research results of this paper quantitatively characterize the adsorption capacity of NPM under different conditions, which lays a foundation for the establishment of adsorption characteristic model in the numerical simulation study of NPM profile control. It also can guide significance for the determination of NPM combination type and the judgment of NPM adsorption position, i.e. flow diversion position, in the field application.

Abstract

Solar hot water systems have developed rapidly in recent years. There are lots of problems in the long term monitoring data onto solar hot water systems, which is difficult to learn the realistic effect of solar hot water systems. This paper studies the temperature monitoring data onto the related projects of solar hot water systems, and set up a set of evaluation system for solar hot water systems in order to evaluate the performance of solar hot water systems in realistic operation. Through comprehensive assessment of annual operation efficiency of solar water heating system, the annual COPs is about 1.5, the COPs of some projects is less than 1.35.

Abstract

Clean and efficient coal utilization technology becomes increasing attention. Supercritical water coal gasification technology has the advantages of low gasification temperature and clean production. In this paper, a novel integrated system of supercritical water coal gasification and power generation is proposed. The innovation of the proposed system is to use the high-temperature flue gas of gas turbine to provide heat for the supercritical water coal gasification process instead of oxidation of coal. The results show that the net power generation efficiency of the proposed system is 53.80% and the exergy efficiency is 52.53%, which are approximately 4.90 and 4.78 percentage points higher than that of the reference system, respectively. Finally, the chemical energy and heat of syngas is converted to power through comprehensive cascade utilization. This work provides a quite promising approach for the clean and highly efficient coal utilization.