Research on Operation Optimization of Heating System Based on Electric Storage Coupled Solar Energy and Air Source Heat Pump

For heating systems based on electricity storage coupled with solar energy and an air source heat pump(ECSA),choosing the appropriate combination of heat sources according to local conditions is the key to improving economic efficiency.In this paper,four cities in three climatic regions in China were selected,namely Nanjing in the hot summer and cold winter region,Tianjin in the cold region,Shenyang and Harbin in the severe cold winter region.The levelized cost of heat(LCOH)was used as the economic evaluation index,and the energy consumption and emissions of different pollutants were analyzed.TRNSYS software was used to simulate and analyze the system performance.The Hooke-Jeeves optimization algorithm and GenOpt software were used to optimize the system parameters.The results showed that ECSA systemhad an excellent operation effect in cold region and hot summer and cold winter region.Compared with ECS system,the systemenergy consumption,and the emission of different pollutants of ECSA system can be reduced by a maximum of 1.37 times.In cold region,the initial investment in an air source heat pump is higher due to the lower ambient temperature,resulting in an increase in the LOCH value of ECSA system.After the LOCH value of ECSA system in each region was optimized,the heating cost of the system was reduced,but also resulted in an increase in energy consumption and the emission of different pollutant gases.

Operating performance of novel reverse-cycle defrosting method based on thermal energy storage for air source heat pump

To solve the fundamental problem of insufficient heat available during defrosting while ensuring the efficient and safe system operation for air-source heat pumps (ASHPs). A novel reverse-cycle defrosting (NRCD) method based on thermal energy storage to eliminate frost off the outdoor coil surface was developed. Comparative experiments using both the stand reverse cycle defrosting (SRCD) method and the NRCD method were carried out on an experimental ASHP unit with a nominal 2.5 kW heating capacity. The results indicate that during defrosting operation, using the NRCD method improves discharge and suction pressures by 0.24 MPa and 0.19 MPa, respectively, shortens defrosting duration by 60%, and reduces the defrosting energy consumption by 48.1% in the experimental environment, compared with those by the use of SRCD method. Therefore, using the NRCD method can shorten the defrosting duration, improve the indoor thermal comfort, and reduce the defrosting energy consumption in defrosting.

Research on Operation of Electrothermal Integrated Energy System Including Heat Pump and Thermal Storage Units Based on Capacity Planning

In view of the Three North areas existing wind power absorption and environment pollution problems,the previous scholars have improved the wind abandon problem by adding electrothermal coupling equipment or optimizing power grid operation.In this paper,an electrothermal integrated energy system including heat pump and thermal storage units was proposed.The scheduling model was based on the load data and the output characteristics of power units,each power unit capacity was programmed without constraints,and the proposed scheduling model was compared with the traditional combined heat and power scheduling model.Results showed that the investment and pollutant discharge of the system was reduced respectively.Wind power was fully absorbed.Compared with the traditional thermal power unit,the proportion of the output was significantly decreased by the proposed model.The proposed system could provide a new prospect for wind power absorption and environment protection.

Heat transfer and parametric studies of an encapsulated phase change material based cool thermal energy storage system

This work investigates the transient behaviour of a phase change material based cool thermal energy storage (CTES) system comprised of a cylindrical storage tank filled with encapsulated phase change materials (PCMs) in spherical container integrated with an ethylene glycol chiller plant. A simulation program was developed to evaluate the temperature histories of the heat transfer fluid (HTF) and the phase change material at any axial location during the charging period. The results of the model were validated by comparison with experimental results of temperature profiles of HTF and PCM. The model was also used to investigate the effect of porosity, Stanton number, Stefan number and Peclet number on CTES system performance. The results showed that increase in porosity contributes to a higher rate of energy storage. However, for a given geometry and heat transfer coefficient, the mass of PCM charged in the unit decreases as the increase in porosity. The St number as well as the Ste number is also influential in the performance of the unit. The model is a convenient and more suitable method to determine the heat transfer characteristics of CTES system. The results reported are much useful for designing CTES system.

Numerical Assessment on Fin Design Parameters Employed for Augmentation of Natural Convection and Fluid Flow in a Horizontal Latent Heat Thermal Energy Storage Unit

The present work focus on the thermal performance of a horizontal concentric heat exchanger, which is numerically investigated to evaluate the heat transfer enhancement process by adding fins with different configurations. As a part of this investigation, the melting process is simulated from the onset of phase change to the offset involving physics of natural convection in PCM fluid pool. The investigation is carried out by ANSYS Fluent code, which is an efficient numerical analysis tool for investigating fluid flow and convective heat transfer phenomena during PCM melting process. The attention is mainly focused on the extension of contact area between the PCM body and cylindrical capsule to enhance heat transfer rates to PCM bodies during the melting process by employing longitudinal fins in the enclosed capsule. Two commercial PCMs: RT50 and C58, are introduced in a 2D cylindrical pipe with their thermo-physical properties as input for modelling. The selected modelling approach is validated against experimental result with respect to the total enthalpy changes that qualify our model to run in the proceeding calculation. It is ensured that an isothermal boundary condition (373 K) is applied to the inner pipe throughout the series of simulation cases and the corresponding Rayleigh number (Ra) ranges from 104 - 105 and Prandtl number (Pr) 0.05 - 0.07. Finally, parametric study is carried out to evaluate the effect of length, thickness and number of longitudinal fins on the thermal performance of PCM-LHTES (Latent Heat Thermal Energy Storage) system associated with the physics of natural convection process during PCM melting.

Thermal comfort assessment and energy consumption analysis of ground-source heat pump system combined with radiant heating/cooling

A new ground source heat pump system combined with radiant heating/cooling is proposed, and the principles and the advantages of the system are analyzed. A demonstration of the system is applied to a rebuilt building： Xijindu exhibition hall, which is located in Zhenjiang city in China. Numerical studies on the thermal comfort and energy consumption of the system are carded out by using TRNSYS software. The results indicate that the system with the radiant floor method or the radiant ceiling method shows good thermal comfort without mechanical ventilation in winter. However, the system with either of the methods should add mechanical ventilation to ensure good comfort in summer. At the same level of thermal comfort, it can also be found that the annual energy consumption of the radiant ceiling system is less than that of the radiant floor system.

Thermal energy storage inside the chamber with a brick wall using the phase change process of paraffinic materials:A numerical simulation

Phase change materials are one of the potential resources to replace fossil fuels in regards of supplying the energy of buildings.Basically,these materials absorb or release heat energy with the help of their latent heat.Phase change materials have low thermal conductivity and this makes it possible to use the physical properties of these materials in the tropical regions where the solar radiation is more direct and concentrated over a smaller area.In this theoretical work,an attempt has been made to study the melting process of these materials by applying constant heat flux and temperature.It was found that by increasing the thickness of phase change materials’layers,due to the melting,more thermal energy is stored.Simultaneously it reduces the penetration of excessive heat into the chamber,so that by increasing the thickness of paraffin materials up to 20 mm,the rate of temperature reduction reaches more than 18%.It was also recognized that increasing the values of constant input heat flux increases buoyancy effects.Increasing the Stefan number from 0.1 to 0.3,increases the temperature by 6%.

Influence of the Fin Design on the Melting and Solidification Process of NANO3 in a Thermal Energy Storage System

The melting and solidification process of sodium nitrate, which is used as energy storage material, is studied in a vertical arranged energy storage device with two different bimetal finned tube designs （with and without additional lateral fins） for enhancing the heat transfer. The finned tube design consists of a plain steel tube while the material for the longitudinal （axial） fins is aluminum. The investigation analyses the influence of the lateral fins on the charging and discharging process. Three-dimensional transient numerical simulations are performed using the ANSYS Fluent 14.5 software. The results show that, every obstruction given by lateral fins reduces the melting and solidification velocity in direction to the outer shell.

Advances in thermal energy storage development at the German Aerospace Center(DLR)

Thermal energy storage(TES)is a key technology for renewable energy utilization and the improvement of the energy efficiency of heat processes.Sectors include industrial process heat and conventional and renewable power generation.TES systems correct the mismatch between supply and demand of thermal energy.In the medium to high temperature range(100~1000℃),only limited storage technology is commercially available and a strong effort is needed to develop a range of storage technologies which are efficient and economical for the very specific requirements of the different application sectors.At the DLR's Institute of Technical Thermodynamics,the complete spectrum of high temperature storage technologies,from various types of sensible over latent heat to thermochemical heat storages are being developed.Different concepts are proposed depending on the heat transfer fluid(synthetic oil,water/steam,molten salt,air)and the required temperature range.The aim is the development of cost effective,efficient and reliable thermal storage systems.Research focuses on characterization of storage materials,enhancement of internal heat transfer,design of innovative storage concepts and modelling of storage components and systems.Demonstration of the storage technology takes place from laboratory scale to field testing(5 kW^1 MW).The paper gives an overview on DLR's current developments.

UTES （Underground Thermal Energy Storage）---Applications and Market Development in Sweden

The market for shallow geothermal solutions has been continuously growing in Sweden and is recognized as a cost effective and environmental sound way for space heating. In later years, UTES （underground thermal energy storage） systems have become fTequently installed for combined heating and cooling of commercial and institutional buildings. After 20 years, operational experiences of these systems are proved to be energy efficient, technically safe and profitable. In this paper, the current statistics of UTES applications are given as well as market trends and technical development. The goal is to encourage designers and installers in other counties to use this promising technology.

Thermal energy storage as a way to improve transcritical CO_(2) heat pump performance by means of heat recovery cycles

Electrification of the heating sector is a major target of energy transition towards a more sustainable,efficient,and less polluted future.Heat pumps are considered more suitable than electrical heaters or fossil-fueled boilers;however,common refrigerants cause ozone layer depletion,which exacerbates the greenhouse effect.Natural refrigerants,such as CO_(2),perform comparably and even better than hydrofluorocarbons while minimizing the negative aspects.This study presents a model of a water-heater CO_(2) transcritical heat pump in a configuration that increases the overall coefficient of performance(COP)by introducing thermal energy storage(TES).The thermodynamic cycle was divided into two separate phases.After heating the TES(charging),warm water was used as the heating fluid in the evaporator to increase the evaporation temperature and pressure of CO_(2),which reduced the work of the compressor.As the water temperature decreased progressively,the discharge cycles improved the total COP.The case study focuses on dairy processes and suggests a more straightforward and cheaper method to improve cycle efficiency than the current processes,such as ejector-expansion systems or double compression.

Recent Progress on Thermal Energy Storage for Coal-Fired Power Plant

With countries proposing the goal of carbon neutrality,the clean transformation of energy structure has become a hot and trendy issue internationally.Renewable energy generation will account for the main proportion,but it also leads to the problem of unstable electricity supply.At present,large-scale energy storage technology is not yet mature.Improving the flexibility of coal-fired power plants to suppress the instability of renewable energy generation is a feasible path.Thermal energy storage is a feasible technology to improve the flexibility of coal-fired power plants.This article provides a review of the research on the flexibility transformation of coal-fired power plants based on heat storage technology,mainly including medium to low-temperature heat storage based on hot water tanks and high-temperature heat storage based on molten salt.The current technical difficulties are summarized,and future development prospects are presented.The combination of the thermal energy storage system and coal-fired power generation system is the foundation,and the control of the inclined temperature layer and the selection and development of molten salt are key issues.The authors hope that the research in this article can provide a reference for the flexibility transformation research of coal-fired power plants,and promote the application of heat storage foundation in specific coal-fired power plant transformation projects.

Seasonal thermal energy storage using natural structures:GIS-based potential assessment for northern China

Seasonal thermal energy storage(STES)allows storing heat for long-term and thus promotes the shifting of waste heat resources from summer to winter to decarbonize the district heating(DH)systems.Despite being a promising solution for sustainable energy system,large-scale STES for urban regions is lacking due to the relatively high initial investment and extensive land use.To close the gap,this study assesses the potentials of using two naturally available structures for STES,namely valley and ground pit sites.Based on geographical information system(GIS)methods,the available locations are searched from digital elevation model and selected considering several criteria from land uses and construction difficulties.The costs of dams to impound the reservoir and the yielded storage capacities are then quantified to guide the choice of suitable sites.The assessment is conducted for the northern China where DH systems and significant seasonal differences of energy demand exist.In total,2,273 valley sites and 75 ground pit sites are finally identified with the energy storage capacity of 15.2 billion GJ,which is much larger than the existing DH demand in northern China.The results also prove that 682 valley sites can be achieved with a dam cost lower than 20 CNY/m^(3).By conducting sensitivity analysis on the design dam wall height and elevations,the choices of available natural structures are expanded but practical issues about water pressures and constructions are also found.Furthermore,the identified sites are geographically mapped with nearest urban regions to reveal their roles in the DH systems.In general,560 urban regions are found with potential STES units and most of them have STES storage capacities larger than their own DH demand.The novel planning methodology of this study and publicly available datasets create possibilities for the implementations of large-scale STES in urban DH systems.

Optimized design and integration of energy storage in Solar-Assisted Ground-Source Heat Pump systems

The integrated use of multiple renewable energy sources to increase the efficiency of heat pump systems,such as in Solar Assisted Geothermal Heat Pumps(SAGHP),may lead to significant benefits in terms of increased efficiency and overall system performance especially in extreme climate contexts,but requires careful integrated optimization of the different system components.In particular,thermal storages take a fundamental role in optimizing the integration of renewable energy sources and the system operation.This work investigates the potential design optimization of a SAGHP system in a mountain site by exploring many different alternatives to optimize the mutual relationship between the solar field,the geothermal field and the water thermal storages.This is done through an original simulation-based multi-objective optimization framework considering energy efficiency and economic feasibility,which allows appraising the impact of the different design alternatives on the overall system performance and on the dynamics of the different system components.Results identify a set of optimized system configurations that optimize the integrated exploitation of the different thermal sources showing a potential increase of the overall system performance leading to 34%lower global cost compared to the initial design.High robustness of the optimal design solutions is reported with respect to the current context of high economic uncertainty.

Possibilities of Reducing Energy Consumption by Optimization of Ground Source Heat Pump Systems in Babylon, Iraq

Iraq is located in the Middle East with an area that reaches 437,072 km2 and a population of about 36 million. This country is suffering from severe electricity shortage problems which are expected to increase with time. In this research, an attempt is made to minimize this problem by combining the borehole thermal energy storage (BTES) with a heat pump, the indoor temperature of a residential building or other facility may be increased or reduced beyond the temperature interval of the heat carrier fluid. Due to the relatively high ground temperature in Middle Eastern countries, the seasonal thermal energy storages (STES) and ground source heat pump (GSHP) systems have a remarkable potential, partly because the reduced thermal losses from the underground storage and the expected high COP (ratio of thermal energy gain to required driving energy (electricity)) of a heat pump, partly because of the potential for using STES directly for heating and cooling. In this research, groundwater conditions of Babylon city in Iraq were investigated to evaluate the possibility of using GSHP to reduce energy consumption. It is believed that such system will reduce consumed energy by about 60%.

Simulation and experiment of a photovoltaic—air source heat pump system with thermal energy storage for heating and domestic hot water supply

For China,the development of low-energy buildings is one of the necessary routes for achieving carbon neutrality.Combining photovoltaic(PV)with air source heat pump(ASHP)yields a great potential in providing heating and domestic hot water(DHW)supply in non-central heating areas.However,the diurnal and seasonal inconsistencies between solar availability and building heat load can severely affect the efficacy of solar energy systems.This study creates and numerically simulates a PV-ASHP system with thermal energy storage(TES)in transient system simulation software,TRNSYS.Experimental studies are conducted to validate the simulation model.The system’s yearly operational characteristics are simulated to reveal the energy conversion relationship between the system’s thermoelectric storage and heating and DHW demand.The results show that the synergy between heating and DHW simultaneously improves the direct utilization of solar energy compared to single heating.The yearly self-consumption and self-satisfaction rates of PV and the COP of the ASHP increase by 131.25%,10.53%and 9.56%,respectively.Solar energy contributes 55.54%to the system,with a PV capacity of 82 W per square meter of building area.This study provides fresh approaches to developing flexible building-integrated PV-ASHP technologies and balance of the energy exchange among the PV,building load and TES.

Comparative study on heat transfer enhancement of metal foam and fins in a shell-and-tube latent heat thermal energy storage unit

Metal foam and fins are two popular structures that are employed to enhance the heat transfer of phase change materials in shell-and-tube heat storage units.However,it remains unclear which structure is better in terms of energy storage performance.In this study,the heat transfer enhancement performances of metal foam and fins are compared to provide guidance on the optimal structure to be chosen for practical applications.Three fin structures(four fins,two vertical fins,and two horizontal fins)are considered.Under the full configuration(volume fraction of metal=3%),the unit with four fins was found to have a faster melting rate than those with vertical or horizontal fins.In other words,increasing the number of fins helps to accelerate the melting process.Nevertheless,the unit with metal foam enhancement has the highest melting rate.Under the half configuration(volume fraction of metal=1.5%),the melting rate of the unit enhanced by metal foam is significantly decreased,whereas there is no remarkable changes in the units enhanced by fins.However,metal foam is still shown to be the best thermal enhancer.The energy storage rate of the unit enhanced by metal foam can be up to 10 times higher than that of the unit enhanced by fins.

Sensitivity analysis of borehole thermal energy storage: examining key factors for system optimization

Borehole thermal energy storage(BTES)systems have garnered significant attention owing to their efficacy in storing thermal energy for heating and cooling applications.Accurate modeling is paramount for ensuring the precise design and operation of BTES systems.This study conducts a sensitivity analysis of BTES modeling by employing a comparative investigation of five distinct parameters on a wedge-shaped model,with implications extendable to a cylindrical configuration.The parameters examined included two design factors(well spacing and grout thermal conductivity),two operational variables(charging and discharging rates),and one geological attribute(soil thermal conductivity).Finite element simulations were carried out for the sensitivity analysis to evaluate the round-trip efficiency,both on a per-cycle basis and cumulatively over three years of operation,serving as performance metrics.The results showed varying degrees of sensitivity across different models to changes in these parameters.In particular,the round-trip efficiency exhibited a greater sensitivity to changes in spacing and volumetric flow rate.Furthermore,this study underscores the importance of considering the impact of the soil and grout-material thermal conductivities on the BTES-system performance over time.An optimized scenario is modelled and compared with the base case,over a comparative assessment based on a 10-year simulation.The analysis revealed that,at the end of the 10-year period,the optimized BTES model achieved a cycle efficiency of 83.4%.This sensitivity analysis provides valuable insights into the merits and constraints of diverse BTES modeling methodologies,aiding in the selection of appropriate modeling tools for BTES system design and operation.

Synthesis and characterization of salt-impregnated anodic aluminum oxide composites for low-grade heat storage

Thermochemical heat storage(THS)systems have recently attracted a lot of attention in research and development.In this study,an anodic aluminum oxide(AAO)template,fabricated by a two-step anodization method,was used for the first time as the matrix material for a THS system.Different salts were studied as thermochemical materials for their suitability in low-grade heat storage application driven by solar energy for an open system.Compositions were prepared by absorbing CaCl2,MgCl2,LiCl,LiNO3 and mixtures of these salts under a vacuum in an AAO matrix.Field Emission Scanning Electron Microscopy was used to examine the morphology of the produced AAO composites.Thermal energy storage capacities of the composites were characterized using a differential scanning calorimeter.Characterization analysis showed that anodized Al plates were suitable matrix materials for THS systems,and composite sorbent prepared with a 1:1 ratio LiCl/LiNO3 salt mixture had the highest energy value among all composites,with an energy density of 468.1 k J·kg-1.

Simulation Study of the Control Strategy of a DC Inverter Heat Pump Using a DC Distribution Network

Photovoltaics,energy storage,direct current and flexibility(PEDF)are important pillars of achievement on the path to manufacturing nearly zero energy buildings(NZEBs).HVAC systems,which are an important part of public buildings,play a key role in adapting to PDEF systems.This research studied the basic principles and operational control strategies of a DC inverter heat pump using a DC distribution network with the aim of contributing to the development and application of small DC distribution systems.Along with the characteristics of a DC distribution network and different operating conditions,a DC inverter heat pump has the ability to adapt to changes in the DC bus voltage and adds flexibility to the system.Theoretical models of the DC inverter heat pump integrated with an ice storage unit were developed.The control strategies of the DC inverter heat pump system considered the influence of both room temperature and varied bus voltage.A simulation study was conducted using MATLAB&Simulink software with simulation results validated by experimental data.The results showed that:(1)The bus fluctuation under the rated working voltage had little effect on the operation of the unit;(2)When the bus voltage was fluctuating from 80%-90%or 105%-107%,the heat pump could still operate normally by reducing the frequency;(3)When the bus voltage was less than 80%or more than 107%,the unit needed to be shut down for the sake of equipment safety,so that the energy storage device could adjust to the sharp decrease or rise of voltage.