Thermal Energy Systems Research Articles

Research on the integrated design of solar thermal energy systems and existing multi-storey residential buildings

To save energy and commit to the environmental protection and sustainable development of green ecological buildings, how to integrate solar energy systems with the reconstruction of multi-storey residential buildings has become a key area of growth in the current construction market. Fully integrating solar thermal energy systems with such reconstruction is a major problem. In this paper, the integration of solar thermal energy systems, roof design, and elevation design were studied systematically. Various effective schemes were presented, laying a foundation for the integrated design of existing multi-storey buildings and solar thermal energy systems during reconstruction.

Heat transfer during film condensation inside horizontal tubes in stratified phase flow

In this paper, the experimental study of heat transfer during condensation of freons R22 and R407С in a plain smooth tube with 17 mm inner diameter was carried out at saturated condensing temperature 40 °C, while mass velocity ranged between 6 and 57 kg/(m2s) and vapour quality changed from 0.23 to 0.95. The unique measurements of circumferential heat fluxes and heat transfer coefficients were performed with the thick wall method during the stratified flow of the phases. The authors performed numerical simulation of heat transfer from condensing vapour to cooling water through the thick-walled cylindrical wall. The CFD model was validated by conducting the physical experiment, which indicated the results coincidence with an error from 7 to 20%. The obtained results allowed improving prediction of effective heat transfer coefficients for vapour condensation, which takes into account the influence of condensate flow in the bottom part of the tube on the heat transfer. This method generalizes with sufficient accuracy (error ± 30%) the experimental data on condensation of freons R22, R134a, R123, R125, R32, R410a, propane, isobutene, propylene, dimethyl ether, carbon dioxide and methane under the stratified flow conditions. Using this method for designing heat exchangers, which utilize such types of fluids, will increase the efficiency of thermal energy systems.

Seasonal performance of a hybrid thermal energy system (geo-exchange and solar thermal) at Northern Alberta Institute of Technology (NAIT)-Part 1

Standard energy systems for individual dwellings nowadays are still mostly conventional in North America and other places. Heat generation for a larger part is commonly met by using mostly natural gas while the use of renewable energy systems to replace fossil fuels is starting to become widespread, especially in Alberta, Canada, which is an oil driven region. It is of interest to evaluate the seasonal performance of a hybrid thermal energy system at NAIT, which is being used to meet the heating needs of the lab space in the D building and aid for teaching purposes. Seasonal performance of a hybrid thermal energy system (geo-exchange, solar thermal energy systems and air source heat pumps) at NAIT, will help analyse and describe the concepts and the parameters affecting the design and optimization of a heat source hybrid energy system, identify key parameters to help determine the optimal design of individual component as part of a larger system; highlight year-to-year performance of the system and overall design and optimization. The average yearly and total generation of the evacuated tube solar collector was reported to be 7.84 kWhs and 2268.59 kWhs, respectively compared to 1.59 kWhs and 987.15 kWhs for the geo-exchange system and 0.42 kWhs and 108.76 kWhs, respectively for the flat plate solar collector. The evacuated tube solar collector shows an overall improvement of 94.53%, 95.89% and 95.96% compared to the flat plate solar collector for the year 2015 to 2017, respectively. Comparing the geo-exchange system and the flat plate solar collector, it can be seen that the geo-exchange system outperform the flat plate solar collector with an overall improvement of 42.87%, 61.32% and 67.51, respectively.

Development and thermal properties of a novel sodium acetate trihydrate-Acetamide-micron/nano aluminum nitride composite phase change material

Heat storage phase change materials (PCM) has been widely used in various thermal energy systems, such as solar-heat pump. Harvesting solar energy requires low-cost and efficient PCM with proper melting points and high energy absorbing capacity. This paper proposed a novel Sodium acetate trihydrate (SAT) -Acetamide (AC) -micron/nano Aluminum nitride (AlN) composite phase change material (CPCM) with a melting point of 47.3 °C for solar-heat pump implementation. 10 wt% AC was selected as modifier to lower the melting point of SAT. The nano AlN or micron AlN was added to CPCM to suppress “supercooling” and improve its thermal conductivity. In order to determine the optimal ratio of mixture and examine the composite's thermal properties, DSC, SEM, XRD, FTIR and other characterization methods were implemented in this study. The results showed that 3 μm and 30 nm mixed AlN were the most effective nucleating agent and thermal conductivity filler. Also, the optimal composite with 5 wt% mixed AlN could reduce the “supercooling” degree to 0.27 °C and improve the thermal conductivity to 0.6484 W/m·K. After 100 heat absorbing and releasing cycles, the latent heat of CPCM maintained a respectable value of 226.8 kJ/kg.

A techno-economic comparison between piston steam engines as dispatchable power generation systems for renewable energy with concentrated solar harvesting and thermal storage against solar photovoltaics with battery storage

Given its abundance and distribution, solar electricity is clearly the power source of the future. Solar photovoltaics is currently the favoured electricity production technology but high battery costs limit its use as a truly dispatchable solution. Solar thermal electricity, with higher production costs but lower heat storage costs, is an important alternative. In this work we have compared solar thermal and solar photovoltaic renewable energy systems head to head: using simulations with our own models and the latest information from literature. The solar thermal system uses a Uniflow Piston Steam Engine for electricity production. The analysis incorporates the daily variance in electricity demand in the form of the CAISO duck curve. To supply off grid demand, if natural gas driven engines produce electricity at $0.134/kWh, photovoltaics with battery storage can supply it at 0.159 $/kWh (0.107 $/kWh best case) and solar thermal with piston steam engines can supply at 0.144–0.167 $/kWh at current prices. If solar concentrator prices drop to $75/m2-aperture, as seems promising, solar thermal can supply dispatchable electricity at 0.091–0.103 $/kWh. In view of the declining interest in solar thermal electricity, this is a valuable contribution to the conversation about the relative merits of the two technologies.

Low-Cost High Energy Density Material for Solar Thermal Heat Storage

A low-cost and enhanced thermal properties composite material for sensible heat storage in solar thermal energy storage applications is introduced. The proposed material is produced primarily for small scale solar thermal applications. However, it can be utilized for large scale solar thermal plants. The material has the advantages of high thermal conductivity and large energy storage density. The introduced material is composed of a mixture of cement and cast-iron particles. To obtain an optimal mixture, different samples of the material are prepared with different ratios of the cement-iron weights. The thermal conductivity of the produced samples is measured by using the linear heat conduction method. The specific heat capacity of the produced mixtures is calculated by using the Rule of the mixture. The obtained results show that the introduced material has a significant enhancement in thermal conductivity. Where, thermal conductivity as high as ~6.0 W/m.K and energy storage density as high as ~788 Joule/cm3 are achieved. The estimated volume energy density is ~89% higher than that of water. The produced material has the advantage of high energy volume density, being unhazardous, chemically stable, eco-friendly, easy to fabricate, and integrate with solar thermal energy systems and is a low-cost material.

Thermal Property Enhancement on Water based PCM in Spherical Container for Cold Storage Applications

The purpose of the present work is to study the solidification behaviour of a di-water based PCM for cold energy management applications. Thickening agents such as agar, gelatin and cornstarch are examined at various mass fractions. The experiment is carried out in a low temperature bath of -7°C inside which the PCM sample is kept spherically encapsulated for uniform distribution. The behaviour and effect of addition of the nucleating agents are compared. The analysis showed that the subcooling of di-water was substantially reduced and its crystallisation occurs at a faster rate. It is concluded that faster crystallisation and reduction in subcooling can help increase the efficiency of a cool thermal energy system and can be applied to any commercial system in the society to help alleviate the electrical load.

Evaluation of low-exergy heating and cooling systems and topology optimization for deep energy savings at the urban district level

District energy systems have the potential to achieve deep energy savings by leveraging the density and diversity of loads in urban districts. However, planning and adoption of district thermal energy systems is hindered by the analytical burden and high infrastructure costs. It is hypothesized that network topology optimization would enable wider adoption of advanced (ambient temperature) district thermal energy systems, resulting in energy savings. In this study, energy modeling is used to compare the energy performance of “conventional” and “advanced” district thermal energy systems at the urban district level, and a partial exhaustive search is used to evaluate a heuristic for the topology optimization problem. For the prototypical district considered, advanced district thermal energy systems mated with low-exergy building heating and cooling systems achieved a source energy use intensity that was 49% lower than that of conventional systems. The minimal spanning tree heuristic was demonstrated to be effective for the network topology optimization problem in the context of a prototypical district, and contributes to mitigating the problem’s computational complexity. The work presented in this paper demonstrates the potential of advanced district thermal energy systems to achieve deep energy savings, and advances to addressing barriers to their adoption through topology optimization.

Experimental and numerical investigation of a novel photovoltaic thermal (PV/T) collector with the energy and exergy analysis

The high energy costs and environmental factors in energy systems cause pushed researchers to produce both heat and electricity from a single collector. In this context sustainable solar photovoltaic thermal energy system stand out with cleaner heat and electricity production at the same time. In this study, a new type of finned air fluid photovoltaic-thermal collector was designed, manufactured and tested. Numerical and experimental analysis were performed with different mass flow rates. Numerical analysis of the PV/T design was carried out before the experiments were conducted. ANSYS program was used to predict the surface temperature of PV module based numerical simulations validation ensures good agreement between the numerical and experimental results. In this way, the energy efficiency of the collector and the temperature of the outlet hot air were wanted to be estimated without experiment. The experiments were performed in two different mass flow rates [m˙a=0.031087kg/s and m˙b=0.04553kg/s] under similar meteorological conditions by investigating the effect of cooling on the PV module. Thanks to cooling of PV, 0.42% improvement in electrical efficiency was achieved. Energy and exergy analyses were performed to analyze the thermal and electrical efficiency of the PV/T collector. Thanks to the cooling of the PV panel, electrical efficiency increased by 0.42%. The average thermal and electrical efficiency of PV/T were obtained as for m˙a=0.031087kg/s 37.10% and 13.56% and for m˙b=0.04553kg/s 49.5% and 13.98%, respectively. Energy and exergy analyses were performed to analyze the thermal and electrical efficiency of the collector. The average thermal and electrical efficiency of PV/T were obtained as for m˙a=0.031087kg/s 37.10% and 13.56% and for m˙b=0.04553kg/s 49.5% and 13.98%, respectively. This study offers an efficient solution to numerical and experimental aspects air cooled PV/T for industrial producers with energy and exergy perspective.

Thermal analysis and energy systems

Thermal analysis and energy systems

RETRACTED ARTICLE: Optimization of a building energy performance by a multi-objective optimization, using sustainable energy combinations

Nowadays the application of the sustainable resources (SERs) of the energy and significant role of the building sector due to environmental effects is notable. Therefore, finding the optimum combination of SERs is an important target to be noticed by the designers to achieve sustainable buildings. Regarding the energy consumption for local warm-water, electrical appliances, and to cool and heat the building space, a new method is suggested in this study to design the optimized combination of the sustainable energy systems to unify the building energy demand. Minimization of the two objectives including the initial energy request and the investment expense is considered as the multi-criteria optimization. Also, to find the expense-optimum solution, the universal expense has been studied. Furthermore, a limitation of achieving the SER combination’s minimum amount in line with the Albanian laws is regarded. The Grass Fibrous Root Optimization algorithm is applied as the optimization process using MATLAB® and DesignBuilder simulation software. Because the solutions should adhere to the lowest SER combination’s levels recommended through present regulations, a limitation is studied, too. A regular residential case of Tirana, in Albania, is chosen to be investigated by using the proposed methodology and regarding the present laws related to the efficiency of the energy. SER systems include heat pumps (HPs), PV panels, thermal solar energy systems. It has resulted that, the initial energy demand saves up to 29.5 kWh/m2 a and the universal expense up to 78,850 € as provided by the methodology comparing a building composition not considering the SERs. Aiming at sustainable development, the proposed methodology can be useful regarding two aspects including economic profits for the holders and the environmental advantages.

Energy flexibility potential of a small district connected to a district heating system

The flexibility of thermal energy systems can support the energy system as a whole in integrating a large share of fluctuating renewable energy sources. The current paper investigates the flexible operation potential of a small district with buildings connected to a district heating network. In the study, the thermal mass of the buildings was utilized as heat storage and the investigation was performed for the heating season. The load shifting scenarios used were created based on the available information on heat load and dynamic heat production cost in the Greater Copenhagen district heating system. The results of the study indicate that the applied strategy of load shifting is highly effective. The achieved load shifting in all schedule-based scenarios was between 41% and 51%. The applied scenarios resulted in an increased peak demand and energy use. However, the increased energy use occurs mostly during the low-load periods, when the heat production prices are lower. It was also shown, that the increased peak can be partially mitigated by using the appropriate control strategies.

Multi-Objective Environmental Economic Dispatch of an Electricity System Considering Integrated Natural Gas Units and Variable Renewable Energy Sources

This paper presents a multi-objective economic-environmental dispatch (MOEED) model for integrated thermal, natural gas, and renewable energy systems considering both pollutant emission levels and total fuel or generation cost aspects. Two cases are carried out with the IEEE 30-bus system by replacing thermal generation units into natural gas units to minimize the amount of toxin emission and fuel cost. Equality, inequality like active, reactive powers, prohibited operating zones (POZs) which represents poor operation in the generation cost function, and security constraints are considered as system constraints. Natural gas units (NGUs) are modeled in detail. Therefore, the flow velocity of gas and pressure pipelines are also considered as system constraints. Multi-objective optimization algorithms, namely multi-objective Harris hawks optimization (MOHHO) and multi-objective flower pollination algorithm (MOFPA) are employed to find Pareto optimal solutions of fuel or generation cost and emission together. Furthermore, the technique for order preference by similarity to ideal solution (TOPSIS) is proposed to obtain the best value of Pareto optimal solutions. Three scenarios are investigated to validate the effectiveness of the proposed model applied to the IEEE 30-bus system with the integration of variable renewable energy sources (VRESs) and natural gas units. The results obtained from Scenario III with NGUs installed instead of two thermal units reveal that the economic dispatching approach presented in this work can greatly minimize emission levels as 0.421 t/h and achieve lower fuel cost as 796.35 $/h. Finally, the results obtained show that the MOHHO outperforms the MOFPA in solving the MOEED problem.

Rapid energy flow calculation method for integrated electrical and thermal systems

The specific solution methodologies adopted for solving multi-energy flow problems involving integrated energy systems (IESs), such as integrated electrical and thermal energy systems, suffer from a number of deficiencies, such as the choice of iterative initial values when using Newton–Raphson method, poor numerical stability and computational efficiency issues. While the computational complexity of the district heating system model increases dramatically as the scale of the system increases. As such, the development of a new solution algorithm suitable for application to the complex models of large IESs is urgently required. Therefore, this paper proposes a rapid multi-energy flow calculation method for integrated electrical and thermal energy systems. Firstly, an equivalent function of the pipeline pressure drop equation is derived to avoid the iterative and simultaneous solution of crucial pipeline parameters. Secondly, a linear equation is introduced to decouple and transform a typical ring district heating system into a collection of radial district heating systems, which can be rapidly solved with high-precision. Numerical applications of the proposed approach to the Barry Island system verify that the approach provides accurate and highly efficient multi-energy flow calculations.

Fabrication and characterization of electrospun fatty acid form‐stable phase change materials in the presence of copper nanoparticles

Latent heat storage system using phase change materials (PCMs) has been recognized as one of the most useful technologies for energy conservation. In this study, a novel type of fatty acid eutectic of methyl palmitate (MP) and lauric acid (LA)/polyacrylonitrile (PAN) composite phase change fiber is prepared by single electrospinning method. Additionally, copper nanoparticles (CNPs) with different mass ratio are combined for improving the thermal conductivity of the PCM. The structure and morphology of the fabricated composite PCMs are observed by scanning electron microscopy (SEM), and the thermal properties and performance are also characterized. SEM results show that the liquid fatty acid has been fully stabled by the three-dimensional structure of the fibers. Good compatibility among the components of the composites is also demonstrated. Besides, the addition of nanoparticles leads to an improved thermal conductivity by over 115.2% and a phase transition temperature 21.24 °C as well as a high latent heat of 85.07 J/g. Moreover, excellent thermal reliability of the phase change fiber is confirmed by multiple thermal cycles. Hence, the composite PCM prepared in this study shows a promising potential for thermal energy system such as building insulating and thermal mass regulating textiles.

Lifecycle greenhouse gas emissions of thermal energy storage implemented in a paper mill for wind energy utilization

Power generation by variable renewable energy (VRE) is expected to expand for decarbonization, although it is difficult to ensure stabilization of the energy demand due to fluctuations in the power output from VRE. To accelerate VRE implementation, wind-powered thermal energy systems (WTth-ES), which can provide dispatchable power using wind–thermal energy converter (WECth) and thermal energy storage (TES), have been proposed. In this study, we are tackling the installation of VRE, i.e., wind energy, by TES with WECth compared with wind turbine (WTp) in paper mills to demonstrate the applicability of TES in an industrial sector. Conventional and alternative energy systems in paper mills with TES were mathematically modeled and simulated to analyze the transmittable power output and the greenhouse gas (GHG) emissions by life cycle assessment. With the installation of TES and wind energy, transmittable power could be increased with GHG emissions reduction. However, their excess installation or unbalanced combination resulted in an increase in GHG emissions and a decrease in transmittable power. By comparing the cases, the combination of paper mill, TES, and WECth could convert thermal energy efficiently and mitigate fossil fuel consumption for multiple production of paper and power from the paper mill.

Experimental evaluation of thermal performance of two different finned latent heat storage systems

Enhancing the reliability and acceptability of solar-based thermal energy system requires efficient thermal storage to enable the storage of surplus energy collected during day time for use during non-day light hours. However, most phase change materials (PCMs) used with thermal storages suffer from low thermal conductivity. Fin geometry has a major impact on the heat transfer rates of thermal storage. For this purpose, a comparative thermal performance assessment during charging is achieved for a shell-and-tube-type latent heat storage unit (LHSU) using different fin geometries. An experimental analysis is conducted on three LHSU geometries: non-finned LHSU, longitudinal finned (LF) LHSU (LF-LHSU) and circular finned (CF) LHSU (CF-LHSU). In addition, a visual observation of liquid fraction fronts is applied to confirm the completion of phase change cycles. Experimental results showed that the total charging time reduced by up to 70% and 55% using CF-LHSU and LF-LHSU, respectively. In comparison with the non-finned LHSU, the highest cumulative energy stored enhancement using CF-LHSU was approximately 52%. The experimental comparative assessment suggests that CF-LHSU provides improved charged thermal load operations by a factor of 1.2 as compared with LF-LHSU.

Meta-analysis of concrete as a thermal energy storage medium

Solar energy is a renewable energy source however sunlight is only available during limited hours in the day. Researchers are looking towards an efficient energy storage system to ensure constant energy output. Concrete can be used as a filler material in a solar thermal energy storage system. This meta-study compared the heat capacity and thermal conductivity of concrete to other solid materials and concrete aggregates, allowing for the viability of concrete storage systems to be examined. The heat capacity of concrete was 5-10% higher than the comparative solid materials like brick and sand. Additionally, concrete without cement replacement materials were found to be more thermally conductive than concrete with added fly ash, blast furnace slag or silica fume with conductivity decreasing between 81-87%. However, concrete with the supplementary cementitious materials possess a higher heat capacity than concrete without cement replacement with capacity increasing by 25% at 30% replacement by fly ash with a grain size 300-600µm. When compared to the energy efficiency of other thermal energy systems, a concrete thermocline is shown to be less efficient than a molten salt two-tank energy storage system by less than 5%. Therefore, while concrete is a viable solid filler material in thermal energy storage systems, a molten salt two-tank thermal energy storage system is marginally more efficient. However, a partial cement replacement by supplementary cementitious materials can extend the effectiveness of the concrete thermal storage.

A bilinear robust state estimation method for electric thermal integrated energy system

A bilinear robust state estimation method for electric thermal integrated energy system (IEHS) is proposed. First, the linear weighted least absolute value (WLAV) model of IEHS is constructed by introducing auxiliary variables, and then the state variables are estimated by solving a quadratic programming model with a nonlinear transformation. The advantage of the proposed method is that it has a high calculation efficiency. The effectiveness of the proposed method is verified on the IEHS test system.

A Decision Support Tool for Implementing District Heating in Existing Cities, Focusing on Using a Geothermal Source

In the context of climate change mitigation strategies in urban environments and reducing reliance on carbon-based energy sources, the Netherlands is gradually taking steps towards modification of its thermal energy system. Geothermal energy, widely used in agriculture, has recently emerged as a local, clean, and sustainable energy source able to fulfil the residential demand for space heating and has received growing attention in district energy planning. However, failed attempts in the past and the lack of experience with direct application of this technology in district heating systems has increased uncertainty with respect to the technical, spatial, and socioeconomic barriers to be overcome between supply and demand. This calls for the application of decision support tools in order to remove these barriers and facilitate more appropriate decision making towards the implementation of smart thermal energy grids in local energy communities. This research explores how the network of actors, those who see opportunities in direct use of geothermal energy in the Netherlands, can work on the transformation of the current centralised carbon-based energy systems towards local sustainable energy communities.