This study investigated the thermal performance and passive cooling effect of an extensive green roof based on an experiment conducted on sunny days in the summer of 2007, for a room with a green roof and a room with a bare roof under air-conditioned and nonair-conditioned states. Three different heat transfer scenarios in terms of average heat flux inside the green roof are defined: (1) heat transfer from indoor space to the green roof when the indoor-outdoor air temperature difference is relatively small, (2) heat transfer from the green roof to the indoor space when the indoor-outdoor air temperature difference is large, (3) ideal thermal equilibrium scenario that heat flux between green roof and indoor space equal to zero. “Cooling flux” is proposed to explain and qualify the passive cooling effect from evapotranspiration on sunny days. A simple method to approximate the “cooling flux” is purposed. The result shows at least 27.4% of the heat flux was absorbed by the passive “cooling flux“ under air conditioned status.
Binary nitrate (NaNO3-KNO3)/expanded graphite (EG)/ nano-SiO2 composite heat storage materials, used as high-temperature thermal energy storage materials, were prepared by mechanical dispersion method, and the effects of EG and nano-SiO2 additives on thermal properties of compound salts were investigated by SEM (scanning electron microscope), STA (Simultaneous Thermal Analysis) and LFA (Laser Flash Apparatus). The results showed that the specific heat capacity of the composites increased first and then decreased with the increase of EG mass fraction. When the mass fraction of EG was 15%, the specific heat of the composite was 3.92J/(g·K), which was 2.58 times higher than that of pure binary nitrate, and the thermal conductivity of the composite was 8.46W/(m·K), which was 16.2 times higher than that of pure binary nitrate. Compared with NaNO3-KNO3/EG(15%wt) composite, the average specific heat capacity and thermal conductivity of the NaNO3-KNO3/EG(15%wt)/nano-SiO2 composite material increased by 15.98% and 42.91% respectively. The preparation process was simple, and the NaNO3- KNO3/EG/nano-SiO2 composite had a broad application prospect in high-temperature energy storage system.
In the current paper, we investigated the phase change material (PCM) volume fraction for three-layers thermocline thermal energy storage (TES) tank system by using spherical capsules filled with high temperature PCM of different thermo-physical properties. A transient one-dimensional dispersion-concentric (D-C) model is modified to calculate the phase change process within capsules so as to determine the temperature distribution. Detailed characteristics of heat transfer between molten salt and the PCMs capsules are discussed, and various numerical results are presented, including the temperature distributions of molten salt and exit temperature. The results show that the volume fraction has a significant impact on the stored/recovered time energy. As the volume fraction of bottom PCM increases, the time required to discharge the thermocline TES tank increases.
With the extensive integration of high penetration renewable energy sources and electric vehicles, frequency regulation service is required to eliminate the impact of uncertainty of load and distributed generation on system frequency. The community integrated energy station (CIES) is an effective way to participate in the service. To realize the optimal configuration of CIES, a method for the planning of CIES for participating in the auxiliary service of the distribution networks is presented. First, the model of frequency regulation and energy station planning is established. Then, the model is solved by the mixed integer linear programing (MILP). Case studies are conducted under different scenarios and results show that participating in the frequency regulation service can effectively reduce the annual total cost of CIES.
A present experimental study demonstrates the visual characteristics of the liquid-solid interface during the melting process in the Latent Heat Thermal Energy Storage System (LHTES) system. The instantaneous morphology of liquid-solid interface and temperature are directly obtained at the different locations in the enclosure when heated at an isothermal temperature condition. The visualization of the phase front and temperature field shows the significance of fins to alleviate the melting phenomenon by the formation of thermal plumes above the top surface of the fins. The role of natural convective transport as a governing mechanism for the phase change process in the partial assisted enclosure is also discussed.
Linear equation solver of real-time simulators for active distribution networks (ADNs) requires significant computation power. Field programmable gate array (FPGA) has many merits such as highly parallel hardware structure and programmable characteristics, which shows great potential for the real-time solving of linear equations. Considering the sparsity of the linear equation, a parallel solver is designed in this paper based on block triangular form (BTF) matrix block algorithm. The design of off-line processing on host PC and online calculation on FPGA are proposed in detail. Simulation results are compared with PSCAD/EMTDC to validate the correctness and effectiveness of the parallel solver design.
The uncertainty of distributed photovoltaic (PV) output brings difficulties to reactive power optimization of power system. A robust reactive power optimization model for power system considering the uncertain output of PV is proposed. The probabilistic model of PV output is used to solve the problem that the robust optimization solution is too conservative. The effectiveness of the proposed model is verified by an example.
Numerical and computational analyses of interface position during inward solidification of composite phase change materials (PCM) in spherical container were explored in this study. The applications of methods such as perturbation, strained coordinates and (improved) quasi-steady solution on spherical inward solidification were investigated and compared with the results of numerical simulation. The solidification positions of porous composite PCM with different porosities solved by strained coordinates and (improved) quasi-steady methods were compared when the Stefan number was 0.1. The complete solidification time was found to be rapidly shortened as the porosity decreased.
Ambitious targets were set in Sweden to increase the share of renewable energy resources and reduce greenhouse gas emissions. Renovating old detached houses can assist in achieving the abovementioned targets, since they make up a great share of the final energy consumption and carbon dioxide emissions in Sweden. Although, several attempts were taken to improve the energy performance of the detached houses, the implementation of energy efficient renovation is yet low due to mainly high investment cost. Former studies evaluated the cost effectiveness of various energy efficient renovations in renovating detached houses in Sweden, but they provided no information how possible climate futures affect the determination and adoption of energy efficiency policies, such as monetary instruments. Accordingly, this study considered three distinct energy renovation packages and analyzed the subsidies required for implementing renovation packages for given interest rates and lifetimes. Furthermore, three different climate scenarios were considered to analyze the effect of possible climate futures on subsidies required. The analyses of results show that increasing the lifetime have greater impact on required subsidies than increasing the interest rate. Furthermore, the results show that variation in future climate conditions changes the required subsidies when implementing energy efficiency renovations. Results can be used as an aid when adopting energy efficiency policies.
Detachment from the national gridline makes the remote mines in the cold climate regions of Canada solely dependent on diesel generators for power generation purposes. Notwithstanding, more than 30% of the consumed diesel by these generators is frittered away as heat through the exhaust. To endure the long harsh winters, these mines also require substantial amounts of heating which is usually provided by burning diesel or propane. In such a scenario, the installation of a diesel exhaust heat recovery system in these remote mines has been considered as a sustainable strategy to preheat the mine intake air. However, this combined heat and power generation strategy cannot provide all the necessary heating due to the daily misbalance between the heating demand and the available heat in the exhaust of the diesel generator. Coupling seasonal thermal energy storage with the waste heat recovery system is a possibility that seeks to resolve such issue. This study investigates the integration of a seasonal thermal energy storage with a diesel exhaust heat recovery system in a remote mine in northern Canada by analyzing several possible alternatives regarding capacity and rates of energy loss. The financial impact of these parameters has been added to show the viability of the proposed strategy