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The industrial sector is one of the biggest oil consumers in Cyprus, corresponding to 20% of the fuel consumption. A parabolic trough collector (PTC) system seems the best option to produce clean thermal energy at higher temperatures than those that can be achieved from the already widely used flat plate collectors on the island. This paper presents a simulation model built to investigate the performance of the first industrial PTC system in Cyprus, installed at the Cyprus biggest soft drinks factory. The simulation model is built in TRNSYS and is validated using data from the real PTC system installed at the factory. The results show a very good fitting between the operating parameters and the power output of the Solar Field (SF), Concrete Thermal Storage System (CTES), and the Steam Generator (SG). The average percentage relative error of the system's contribution to the process is less than 6.32% for the daily steam production and during a week did not exceed 6.45%. The novelty of this study is the development of a design tool that can be used by potential interested industries to identify the suitable system that fits their needs. All data are provided in the form of graphs and allow anyone to use as input data the thermal energy demand and required steam temperature of the industry to retrieve information about the size of a suitable system which satisfies these requirements depending on each case. The payback period for all cases examined varies from 2 to 6 years, depending on the size of the system.

Sensible thermal energy storage is the most common solution for small-to-medium solar process heat systems. Hence, it is vital to enhance the thermal stratification and energy utilization of stratified tanks. In this study, four process heat systems are dynamically simulated: 1) S0 with a single outlet port to the load, 2) S1 with a single port and a thermostatic mixing valve, 3) M0 with multiple ports, and 4) M1 with multiple ports and a mixing valve. Two novel models are proposed for the compound parabolic collectors and the stratified tanks. System M1, with annual system efficiency of 47.3% and solar fraction of 0.57, was found to be the best-performing one, followed by S1, M0, and S0. System M1 also reduced the annual energy consumption of the backup heater in S0 by 16.45%. The thermostatic mixing valve reduced the thermocline thickness. The incorporation of both mixing valve and multiple ports delayed the startup of the heater, reduced the system's thermal losses, and slightly improved the collectors' efficiencies. The proposed tank model can be a viable tool for solar energy designers to optimize the number and relative positions of the inlet and outlet ports for maximized system efficiency and solar coverage.

Solar tower plants (STPs) can foster reduce toxic emissions and a cleaner environment. Here, to step forward in this advance, this system is employed as a heat source for an alkali metal thermoelectric converter (AMTEC). Thermal energy storage (TES) is applied to stabilize the performance of the hybrid system along with solar irradiation fluctuations. The considered system is investigated on three different days for the humid subtropical climate of Bandar Abbas, Iran. This study's primary outcome and contribution is a novel simulation framework via TRNSYS to predict the performance of the system under fluctuations of solar irradiation at this site. Both TES and STP systems are defined in TRNSYS software as a new component type via utilizing the Fortran program. The effects of various parameters, including the total energy absorbed by the central receiver, the thermal capacity of TES, and the heliostats field area, are investigated on crucial output parameters of the AMTEC. The mass flow rate of the heat transfer fluid at several areas is then studied in different operating conditions. The optimum area for the heliostats (14,500 m 2 ), leading to the highest AMTEC efficiency (18%) is determined, which requests a TES tank volume of 28.13 m 3 . This system is noticed to have its most remarkable performance in June and July.

Recently there have been many reports on the application of phase change materials (PCM) in solar water heating system (SWHS). However, there are some doubts and different views in this field, since the sensitivity of the system to the PCM design parameters and external conditions. In this study, a typical SWHS with a PCM storage tank is proposed to overcome the existing problems. The mathematical model is established and validated against experimental data. Annual simulations have been done under a typical user load and different PCM design parameters (phase change temperature, PCM usage amount) among six cities which have different meteorological conditions. The results indicate that the application of PCM in SWHS can reduce the power consumption of electric auxiliary heater (EAH) and the heat loss, but the benefits vary widely by regions and PCM design parameters. The highest power saving rate is up to 31% in Lhasa which has extremely abundant solar energy, while for the heat loss saving rate the highest value is 9.9% in Guangzhou which is located in the hot summer and warm winter zone. Besides, a parametric study has been conducted to explore the power saving potentials with different PCM design parameters in different regions, as well as to discuss the optimal phase change temperature and the PCM usage amount. This study aims to provide theoretical references and suggestions for the design of the PCM storage tank in SWHS.

In cold climatic regions such as those located across Canada, it is necessary to implement heating system technology that is ultra-efficient and that has near-zero rates of emissions. Such systems would satisfy consumers’ energy needs and also comply with environmental standards, especially because the systems would account for more than 80% of residential energy use. This paper investigates two complementary efficient systems that can support these heating systems, ground-source heat pumps (GSHPs) and organic Rankine cycle systems (ORCs). The study proposes to couple these two systems in a parallel configuration. A dynamic simulation model created in TRNSYS platform has been deployed to assess the performance of the combined ORC-GSHP based micro-cogeneration system. This former provides heating to a residential house during the heating mode as needed. It has the capacity to switch to a charging mode, during which the ORC system is directly coupled to the ground heat exchanger (GHE), which works as a thermal energy storage and supplies energy to the GSHP. The feasibility of this combined system arrangement, and its comparison with a conventional GSHP system are examined for use in residential buildings in three cities across the varied climatic regions within Canada, namely Edmonton (AB), Halifax (NS), and Vancouver (BC). Results showed that the proposed micro-cogeneration system recorded less energy use of over 80%. The addition of the ORC system had a definite effect on the performance of the GSHP in that it decreased the operating hours from 11–58% compared to the conventional GSHP case and maintained consistently higher COP values. These results may help to specify viable ORC-GSHP based micro-co/trigeneration systems in cold climatic applications and should be useful for prototype design and development.

With different computational tools, simulations ranging from detailed and rigorous mathematical models to overall process plant of black box models can be carried out. Whereas most of these computational tools cannot practically execute different scales of models at the same time, it becomes relevant to devise strategies in coupling two or more of them for better analysis of processes. In this light, this study proposes Excel as an interactive scale bridge of data exchange to aid the multiscale modeling and dynamic simulation of combined cycle (CC) power plant integration with two-tank thermal energy storage (TES) system using gPROMS and SimCentral. This is relevant to analyze not only the performance of TES, but the feasibility of its integration with CC in augmenting energy production to meet daily power demand. The integrated system modeled in four operational modes of CC increased in power generation by 7.3 MW at an efficiency of 98.30%. The study validated the usefulness of the TES integration of 99.66% efficiency. The research results provide a communication strategy for different computational tools and an approach to effectively increase CC power production to meet varying daily demand.

Concentrated solar power (CSP) plant with thermal energy storage can be operated as a peak load regulation plant. The steam generation system (SGS) is the central hub between the heat transfer fluid and the working fluid, of which the dynamic characteristics need to be further investigated. The SGS of Solar Two power tower plant was selected as the object. The mathematical model with lumped parameter method was developed and verified to analyze its dynamic characteristics. Five simulation tests were carried out under the disturbances that the solar tower power plant may encounter under various solar irradiations and output electrical loads. Both dynamic and static characteristics of SGS were analyzed with the response curves of the system state parameters. The dynamic response and time constants of the working fluids out of SGS was obtained when the step disturbances are imposed. It was indicated that the disturbances imposed to both working fluids lead to heat load reassignment to the preheater, evaporator and superheater. The proposed step-by-step disturbance method could reduce the fluid temperature and pressure fluctuations by 1.5 °C and 0.03 MPa, respectively. The results could be references for control strategies as well as the safe operation of and SGS.

A hybrid ground-coupled heat pump (GCHP) is an efficient and sustainable technology for space heating and cooling. A demonstration house equipped with GCHP with a solar seasonal heat storage (SSHS)...

Modern building complexes have simultaneous heating and cooling demands. Therefore, integrated energy systems with heat pumps and long-term thermal storage are a promising solution. An integrated heating and cooling system for a building complex in Oslo, Norway was analyzed in this study. The main components of the system were heat pumps, solar thermal collectors, storage tanks, ice thermal energy storage, and borehole thermal energy storage. Dynamic simulation models were developed in Modelica with focus on the long-term thermal energy storage. One year measurement data was used to calibrate the system model and two COPs were defined to evaluate system performance. The simulation results showed that more heat had to be extracted from the long-term thermal storage during winter than could be injected during summer. This imbalance led to a decrease in ground temperature (3 °C after 5 years) and decreasing long-term performance of the system: both COPs decreased by 10% within five years. This performance decrease could be avoided by increasing the number of solar collectors from 140 to 830 or by importing more heat from the local district heating system. Both measures led to sustainable operation with a balanced long-term thermal storage.

Solar thermal energy is widely recognized as one of the most important renewable energy resources. However, in high latitudes, due to various climatic and mismatch challenges, such solar district heating networks are difficult to implement. The objective of the paper is to optimize and compare two different design layouts and control strategies for solar district heating systems in Finnish conditions. The two different designs proposed are a centralized and a semi-decentralized solar district heating system. The centralized system consists of two centralized short-term tanks operating at different temperature levels charged by a solar collector and heat pumps. Borehole thermal energy storage is also charged via these two centralized tanks. In contrast, the semi-decentralized system consists of one centralized low temperature tank charged by a solar collector and a borehole thermal energy storage and decentralized high temperature tank charged by an individual heat pump in each house. In this case, borehole thermal energy storage is charged only by the centralized warm tank. These systems are designed using the dynamic simulation software TRNSYS for Finnish conditions. Later on, multi-objective optimization is carried out with a genetic algorithm using the MOBO (Multi-objective building optimizer) optimization tool, where two objectives, i.e. purchased electricity and life cycle costs, are minimized. Various design variables are considered, which included both component sizes and control parameters as inputs to the optimization. The optimization results show that in terms of life cycle cost and purchased electricity, the decentralized system clearly outperforms the centralized system. With a similar energy performance, the reduction in life cycle cost is up to 35% for the decentralized system. Both systems can achieve close to 90% renewable energy fraction. These systems are also sensitive to the prices. Furthermore, the results show that the solar thermal collector area and seasonal storage volume can be reduced in a decentralized system to reduce the cost compared to a centralized system. The losses in the centralized system are 40–12% higher compared to the decentralized system. The results also show that in both systems, high performance is achieved when the borehole storage is wider with less depth, as it allows better direct utilization of seasonally stored heat. The system layout and controls varied the performance and life cycle cost; therefore it is essential to consider these when implementing such systems.

For the successful transition to a renewable energy source powered society, coupling of different energy sectors is inevitable. The extreme case of a future German energy system consisting of power, heat and gas consumers supplied with 100 % renewables is analyzed here. To find the most cost-effective system configuration, different combinations of storage and conversion technologies are compared by performing dynamic simulations and evaluating the average costs over the period of one year. Renewable power production is modeled by using actual power-generation curves and extrapolating the installed power for each technology according to the German energy system framework. Final energy curves for power, heat and gas demand are created as a result of the study. The gas demand only arises from industries using hydrocarbons as a product in processes and for high temperature process heat. The components of the energy system, e.g. storage and conversion technologies are modeled using the equation-based open-source TransiEnt Library based on Modelica®. To obtain the boundaries of the solution scope, the comparison is started by analyzing homogeneous scenarios, e.g. All-Electric or All-Gas with Power-to-Gas with reconversion to power and heat. To find the optimal configuration within this scope, different combinations of power (adiabatic compressed air energy storage (A-CAES), lithium-ion battery, pumped hydro storage), heat storage (hot water storage) and gas storage (underground storage) technologies as well as conversion technologies, i.e. Power-to-Gas (electrolyzer with methanation), Power-to-Heat (electric heat pump, electric boiler), Gas-to-Heat (gas boiler, gas heat pump), and Gas-to-Power (gas turbine, combined cycle gas turbine) are simulated. The results show that a homogeneous energy system configuration where all services are supplied by either power or gas are technically possible but not economic. Due to the limited technical potential of renewables, ecological feasibility of All-Gas systems is limited. A combination of Power-to-Gas with combined cycle gas turbines, electric heat pumps, a lithium-ion battery and pumped hydro storage is the option with the lowest cost. Using an A-CAES instead of the battery or adding an A-CAES to the battery does not lower the cost.

A novel methodology for optimizing thermal storage for biomass boiler heating systems is presented in this paper. Biomass boiler heating systems have gradually gained popularity in residential, commercial, and large-scale district heating plants due to the lower carbon footprint of biomass energy. One unanswered question in this field was the size of the thermal energy storage (TES) tank with respect to different boiler capacities and different types of heat demand profiles. Different standards and recommendations (based only on boiler capacities) have been proposed by various organizations around the world with large deviations on the sizing of the TES tank, but without a clear technical basis for these recommendations. These inconsistencies impede the effective design and deployment of biomass boiler heating systems. This study used a dynamic process simulation system based on field data from a 25-kW wood pellet boiler with radiant floor heating. By running the simulation for different boiler capacities (N) and different types of heat demand profiles (i.e. low, medium, high, intermittent, and variable), the optimum TES tank volume can be estimated by evaluating the TES tank discharge efficiency, boiler on-time, boiler off-time, and maximum boiler output temperature. Linear correlations between optimum TES tank volume and boiler nominal capacity for different types of heat demand profiles were obtained. The results showed that the medium building heat demand (45% N) required the smallest TES tank volume while intermittent building demand requires the largest. High building demand (60% N) generates the highest TES tank discharge efficiency (up to 99%) but also requires large storage volume. From the economic point of view, it is recommended to size the thermal energy storage system such that the average building heat demand is around 45% of boiler nominal capacity.

The present paper simulates and dynamically analyzes a combined heat and power (CHP) system driven by a gas-fueled internal combustion engine. The CHP system uses a heat exchanger to convert the heat loss of the engine to meet heat demand and uses a generator for power generation. Then, the impact of the use or non-use thermal energy storage (TES) is examined on power and heat generation. Given the different demands of different seasons, two smart control strategies are developed concerning time to attain higher efficiency in different seasons. A TES is included in the control strategy for cold and hot seasons, and analyses are performed for a typical week. The gas engine of the CHP system uses time and temperature variables simultaneously to operate smartly by two strategies for a year instead of one single strategy. The results show that when a control strategy with variable partial loads based on temperature and time is employed, the efficiency of the CHP system’s gas engine is improved. Using the results of the new and smart control strategy, the CHP system exhibits an efficiency of 84.2% in the hot season and an efficiency of 87.0% in the cold seasons for a typical week.

Phase Change Materials have been acknowledged for their potential to be used as passive strategy for improving energy efficiency and occupants' thermal comfort in buildings. However, their performance still needs to be enhanced to have them effectively used. In this view, this study investigates the potential improvement of PCMs performance for passive cooling application by efficient natural ventilation in residential building stock. Therefore, coupled dynamic simulation and optimization analysis is performed to explore the optimum melting temperature of PCM integrated in the external building envelope to minimize cooling loads in different Italian climate zones. Moreover, various natural ventilation control strategies are implemented to assess their influence on the process of PCM charge-discharge cycle. Results show that PCM inclusion in the building envelope provides significant cooling savings, up to about 300 kWh/year in mild climates. Furthermore, both night and temperature controlled natural ventilation are able to enhance the efficiency of PCMs thermal energy storage charge-discharge cycle. However, the optimum performance is obtained by coupling PCMs with natural ventilation controlled by indoor/outdoor temperature difference in all considered climate contexts. Accordingly, considerable building cooling energy need reduction is achievable through the optimum combination of PCMs and natural ventilation control, especially in milder climates. GREiA is certified agent TECNIO in the category of technology developers from the Government of Catalonia. The work was partially funded by the Ministerio de Ciencia, Innovación y Universidades de España (RTI2018-093849-B-C31 - MCIU/AEI/FEDER, UE). The work was partially funded by the Ministerio de Ciencia, Innovación y Universidades - Agencia Estatal de Investigación (AEI) (RED2018-102431-T). The authors at the University of Lleida would like to thank the Catalan Government for the quality accreditation given to their research group (2017 SGR 1537). GREiA is certified agent TECNIO in the category of technology developers from the Government of Catalonia. This work is partially supported by ICREA under the ICREA Academia programme. The authors at University of Perugia thank the Italian Ministry of research and university for supporting the NEXT.COM project under the framework of PRIN 2017 call and the IEA EBC Annex 79 “Occupant-centric building design and operation” for inspiring human-centric research.

Renewable energy (RE) penetration growth without energy storage system (ESS) is restricted due to its poor dispatchability. Compressed air energy storage system (CAES) is one of the promising ESS technologies. But, low energy density and high mechanical inertia confined its application as a bulk ESS. In this paper, a novel hybrid thermal-compressed supercritical carbon dioxide energy storage (T-CCES) system is proposed to address CAES shortcomings. In this system, a stratified water thermal energy storage (TES) is employed to store both released heat of compression process and high-frequency reluctant power of a wind farm (which cannot be followed by CCES) using electrical heaters. A dynamic simulation is performed to evaluate system characteristics in steady wind generation condition and in a real wind generation condition. Stochastic simulation of the proposed system integrated with a real wind farm is also fulfilled to assess its characteristics and performance by considering uncertainties using sparse polynomial chaos expansion. Additionally, global sensitivity analysis via Sobol’ indices is conducted to identify the most important parameters of the system. Results show that the system can store wind power during off-peak hours and smoothly releases it into the grid with 57.55% round-trip efficiency (RTE) and 84.1 kWh/m3 energy density. Uncertainty quantification results indicate that the system can properly work under uncertainties with standard deviation of 55.72–58.16% RTE and 83.7–86.5 kWh/m3 energy density. Moreover, results show that the random production of wind turbine has a negligible effect on performance characteristics of the T-CCES which makes it a promising solution for integration with wind farms.

The operational flexibility of a thermal power plant has played an essential and promising role in accommodating the increment of variability in the supply and demand sides in China. The rapid activation of thermal storage in entire coal-fired power plant lies at the heart for the strategy. Understanding the thermodynamic characteristics of the power unit in the transient process quantitatively remains a challenge. In this study, dynamic simulations of an entire 660 MW supercritical coal-fired power plant were conducted via GSE software, and the models were validated in the steady state and transient processes. Then, five different measures were introduced to regulate the extraction steam of high-pressure heaters for operational flexibility. The dynamic characteristics of the main thermodynamic parameters and output power were described and compared. Moreover, the operational flexibility of these measures was discussed. It turns out that: among the five measures, the change degrees of pressure, flowrate, and temperature in main devices increase with the increment in the number of throttled valves and/or the degree of feedwater bypass. The feedwater temperature at steady state reduces by 94.6 °C at most, and the maximum temperature change rate of metal slabs in HP heaters is −44.6 °C min−1. The dynamic process for output power under different measures has two different ramp stages, namely, a rapid stage and a slow stage. Furthermore, compared with other measures, 100% throttling extraction steam of #1, #2, and #3 HP heaters has the best operational flexibility, that is, the maximum average power ramp rate in a minute, power capacity, and energy capacity are 6.19% of rate power per minute, 48.40 MW, 5.58 MW h, respectively. The average power ramp rate, power capacity and energy capacity increase with the increment in the number of throttled valves and/or the degree of feedwater bypass. This work is expected to provide a detailed reference on the use of turbine thermal storage to improve the operational flexibility of coal-fired power plants.

Although, the horizontal thermally stratified storage tanks (TSST) are nowadays the most popular, especially in the domestic solar water heaters, most of the literature is oriented towards the vertical tanks. Recently, a smart adaptive model for TSST was proposed, where the grid segments size and location changes automatically according the different variations in the tank. It showed good agreement with experimental work, however the model and its validation was presented for the vertical configuration only. In this work, the adaptive-grid model is extended to the horizontal cylindrical tank orientation while accounting for the changes in grid segments cross-section during moving in the vertical direction as to be aligned with the thermocline region; the proposed model is validated with experimental data available in the literature. The results show an excellent agreement with the literature and yield a significant reduction in the number of nodes needed for the modeling (∼32.7% reduction) when compared to other horizontal tank models while accounting for the changes tank geometry in the vertical direction.

Thermal energy storage (TES) is a key technology for enabling increased utilization of industrial waste heat in district heating. The ability of TES to equalize offsets in demand and supply depends strongly on the sizing, control and integration in a heating plant. We consider the problem of sizing TES in heating plants utilizing a varying waste-heat source. To this end, we propose a combined dynamic simulation and model predictive control approach that accounts for the dynamics and optimal control of the heating plant with TES. A case study has been carried out on a district-heating plant located in Norway, with 90% of its annual heat production being heat recovered from the off-gas from a ferrosilicon plant. We evaluate the effective peak-heating reduction with different TES sizes and the energy-to-heat-flow-ratio for the TES discharging periods as performance metrics. For the case study, our results suggest that a modest TES tank volume of 1500 m3 is sufficient to achieve a half-year peak-heating reduction of 12% and comparable performance with larger volumes. The proposed methodology constitutes a numerically tractable means of incorporating the impact of model predictive control on the sizing of TES for heating plants with time-varying waste-heat supply and demand. Keywords Thermal energy storageWaste-heat utilizationDistrict heatingModel predictive control

Flexibility enhancement of coal-fired power plants is extremely urgent to accommodate high-penetrated renewable energy and maintain the stability of power grid. Cycling rate, an important aspect of operational flexibility of coal-fired power plants, is mainly dragged by the large boiler thermal inertia. The idea of this study is to enhance the flexibility of the power plant by clarifying and considering the time varying and detailed boiler heat storage characteristics. The dynamic simulation and control models of a double-reheat power plant are developed and validated. The time varying and detailed boiler heat storage characteristics are represented by the principal parameters obtained by using stepwise regression algorithm. The improved control strategy is proposed and its performances are examined. Results show that the improved control strategy can enhance load cycling rate from 9.9 MW min−1 to 23.1 MW min−1, and the accumulative standard coal consumption rate can be reduced by 0.47%. Moreover, the control parameters, including output power and steam temperature, can reach the target quickly and are accurately controlled. The improved control provides a detailed guidance for the trade-off between the flexibility and high-efficiency in power plants during load cycling processes.

In the current study, a building of a Hotel in cold weather conditions is simulated to be powered by a Combined Cooling, Heating and Power (CCHP) system in a transient manner. The system is comprised of photovoltaic thermal panels as renewable technology, coupled with a power block of MGT and cooling devices to produce the needs of the building. To handle the energy flows effectively, the system is able to sense the temperate of ambient with different controllers occupied in the system. Using TRNSYS as a promising and powerful tool to carry out the research dynamically, the thermodynamic, economic, and environmental behavior of the system is investigated. Research results dawned on the fact that the integrated system can generate all the needs of the proposed Hotel. Simulation outcomes indicate that an auxiliary heater plays a critical role since the temperature of 120 °C is constantly required for heating and cooling generation, and the power input to the heater is a challenging value. Although the heater is on and working 85% of the year, the results of the system show that in 65% of the year, the generated power is more than the power demand of the house. Moreover, the annual GHG emission is 0.16 Ton/MWh, in which the minimum of CO2 to the environment happens in July, and the maximum happens in January with 0.085 Ton/MWh and 0.17 Ton/MWh, respectively. Exergy analysis results show that the combustion chamber and PVT are the main sources of exergy destruction in the whole system. The building dynamic simulation results show that the system requires 34 kW cooling demand and 47 kW heating demand.

Dynamic simulation provides an efficient approach for improving the efficiency of parabolic trough power plants and control circuits. In the dynamic simulation, the possibilities and operating conditions of the plant are evaluated regarding materials, processes, emissions, or economics. Several studies related to the dynamic simulation of the parabolic trough technology are summarised and discussed in this work. This study is the first research that presents a thorough description of the advanced control circuits used in the solar field and thermal storage system of a parabolic trough power plant. This power plant was implemented using advanced process simulation software (APROS). The dynamic model was built based on the real specifications of the power plant.

The transition from fossil fuels to green renewable resources presents a key challenge: most renewables are intermittent and unpredictable in their nature. Energy storage has the potential to meet this challenge and enables large scale implementation of renewables. In this paper we investigated the dynamic performance of a specific Adiabatic Compressed Air Energy Storage (A-CAES) plant with packed bed thermal energy storage (TES). We developed for the first time a plant model that blends together algebraic and differential sub-models detailing the transient features of the thermal storage, the cavern, and the compression/expansion stages. The model allows us to link the performance of the components, in particular those of the thermal storage system, with the performance of the whole A-CAES plant. Our results indicate that an A-CAES efficiency in the range 60–70% is achievable when the TES system operates with a storage efficiency above 90%. Moreover, we show how the TES dynamic behaviour induces off-design conditions in the other components of the A-CAES plant. Such device-to-plant link of performance is crucial: only through integration of TES model in the whole A-CAES model is possible to assess the benefits and added value of thermal energy storage. To the authors’ knowledge the present study is the first of this kind for an A-CAES plant.

The transient performance of solar thermal power plants is critical to the system design and optimization. This study numerically investigates the dynamic efficiencies of an open-loop air receiver and a thermal energy storage unit. One-dimensional dynamic models of the air receiver and thermal energy storage were developed using the Modelica language with a graphical user interface and the Dymola solver to provide comprehensive thermal simulation at low computational cost. An air receiver and thermal energy storage experimental platforms were built to validate the simulation models. The simulation results compare well with the experimental data, so the models can be used to predict the variations of air receiver and thermal energy storage efficiencies. The models were then combined into a receiver and thermal energy storage system model with control schemes. The schemes control the air receiver outlet air temperature at relatively stable values while the thermal energy storage automatically switches between charging, discharging and stand-by modes.

This paper analyzes the use of hybrid photovoltaic/thermal (PVT) collectors in nearly zero-energy buildings (NZEBs). We present a design methodology based on the dynamic simulation of the whole energy system, which includes the building energy demand, a reversible heat pump as generator, the thermal storage, the power exchange with the grid, and both thermal and electrical energy production by solar collectors. An exhaustive search of the best equipment sizing and design is performed to minimize both the total costs and the non-renewable primary energy consumption over the system lifetime. The results show that photovoltaic/thermal technology reduces the non-renewable primary energy consumption below the nearly zero-energy threshold value, assumed as 15 kWh/(m2·yr), also reducing the total costs with respect to a non-solar solution (up to 8%). As expected, several possible optimal designs exist, with an opposite trend between energy savings and total costs. In all these optimal configurations, we figure out that photovoltaic/thermal technology favors the production of electrical energy with respect to the thermal one, which mainly occurs during the summer to meet the domestic hot water requirements and lower the temperature of the collectors. Finally, we show that, for a given solar area, photovoltaic/thermal technology leads to a higher reduction of the non-renewable primary energy and to a higher production of solar thermal energy with respect to a traditional separate production employing photovoltaic (PV) modules and solar thermal (ST) collectors.

The combined-cycle gas turbine (CCGT) power plants are often required to provide the essential fast grid balance service between the load demand and power supply with the increase of the intermittent power generation from renewable energy sources. It is extremely challenging to ensure CCGT power plants operating flexibly and also maintaining its efficiency at the same time. This paper presents the feasibility study of a CCGT power plant combined with the cascaded latent heat storage (CLHS) for plant flexible operation. A 420 MW CCGT power plant and a CLHS dynamic models are developed in Aspen Plus based on a novel modelling approach. The plant start-up processes are studied, and large amount of thermal energy can be accumulated by CLHS during the start-up. For load-following operation, extensive dynamic simulation study is conducted and the simulation results show that the extracted exhaust gas can be used for thermal energy storage charging, and the stored heat can be discharged to produce high temperature and high pressure steam fed to the steam turbine. Besides, the stored heat can also be used to maintain the heat recovery steam generator (HRSG) under warm condition to reduce plant restart-up time. The simulation results demonstrate that the integration of CLHS with CCGT power plant is feasible during the start-up, load-following and standstill operations.

Thermal energy storage (TES) plays a crucial role improving the efficiency of solar power utilization. Molten salt (MS) has gained a strong position as a thermal fluid in applications where solar power is stored and used overnight to provide dispatchable energy production. Novel process and operating concepts are being developed for TES systems that require reliable engineering tools. System-wide dynamic simulation provides a virtual test bench and analysis tool for assisting in process and control design and operational issues. Proper characterization of the thermal fluids in simulation tools is critical for successful simulation studies. In this paper, we report the experimental and modelling work related to counter-current heat exchange and free drainage test runs in CIEMAT's multi-purpose MS test loop at Plataforma Solar de Almería in Spain. We present a general method to define MS and non-condensable gas within a homogeneous pressure-flow solver. We present modelling of an indirect MS TES system connected to a thermal oil loop through TEMA type heat exchangers, model calibration with half of the experimental data, and finally, validation simulations against rest of the data. All these experimental data are previously unpublished. The model predicts the system behaviour with good agreement regarding temperatures, pressures, flow rates and liquid levels. The simulations suggest that the heat exchangers’ shell sides suffer from trapped non-condensable gas which significantly affects heat transfer, heat loss to ambient air and hydrodynamic losses. Our results contribute to thermal-hydraulic, system-wide modelling and simulation of MS processes. Furthermore, the results have practical implications for MS TES facilities with respect to system design, analysis and operation.

In recent years, CO2 heat pump water heating systems have been developed, and their performances have been enhanced while their functions have been expanded. In a multi-functional system, used for both hot water supply and bath heating, hot water retrieved from the top of a storage tank is returned to its bottom or side after heat exchange for bath heating, which destroys the stratified temperature distribution in the storage tank and degrades the system performance. In this paper, the performance of a multi-functional CO2 heat pump water heating system has been evaluated by numerical simulation. A system model was created by combining component models for a CO2 heat pump, mixing valves, a storage tank, a heat exchanger, and a bathtub. Partly, perfect mixing by hot water return was assumed in the component model for the storage tank, and its validity was verified through experiments. A performance analysis has been conducted under daily repeated hot water and bath heating demands, and the system performance was evaluated at a periodically steady state. As a result, the system efficiency and the volume of unused hot water in the multi-functional system decreased by 4.9% and 16.3%, respectively, as compared to those in the uni-functional system, when hot water returned to the bottom of the storage tank. When the position for hot water return is heightened, the system efficiency becomes higher than that in the uni-functional system, while the volume of unused hot water decreases furthermore.

Compressed air energy storage systems are often in off-design and unsteady operation under the influence of external factors. A comprehensive dynamic model of supercritical compressed air energy storage system is established and studied for the first time. In this model, important factors, including volume effect and thermal inertia, are considered for system dynamic simulation which used to be ignored in the past. The transient characteristics and control methodology are mainly focused in this work. The exergy efficiency of transient processes is detected to better understand the dynamic process. Specifically, firstly the response characteristics of system power, mass flow rate, thermal storage temperature/outlet water temperature, and exergy efficiency under the step of key regulation parameters are studied in depth. And then a control framework is well built and studied. The result shows that the influence of the volume effect on the system dynamic characteristics is concentrated in the early time, and mainly affecting the mass flow rate and then the power. The influence of thermal inertia on the system dynamic characteristics takes a long time, mainly affecting thermal storage temperature and the outlet temperature. With the new-built controller, during energy charging, under 10% step-down command of load, the power can quickly reach equilibrium for about 10 s, while thermal storage temperature can be controlled in about 8 s. During the energy discharging period, the combination control mode can achieve shorter load equilibrium time and smaller load overshoot.

In this work, the study of a novel solar driven Combined Cooling, Heating and Power (CCHP) system is carried out. In particular, the system is composed of a 60 m2 flat plate solar thermal collectors field, a 10 kWe photovoltaic plant, a 2 m3 Thermal Energy Storage (TES), a 3 kWe micro-Organic Rankine Cycle (micro-ORC) prototype, a 4.4 kWc thermally driven Adsorption Chiller (AC) coupled with a 6 kWc auxiliary Heat Pump (HP). It has been conceived for residential applications and has been integrated with a real bioclimatic nearly zero energy building (NZEB). The building-plant system has been modelled in TRNSYS environment and studied for three Italian locations, spread along the peninsula, with three different climates, Messina, Milano and Rome. An energy, environmental and economic analysis have been carried out. The system has been assessed on hourly basis and the sensitivity analysis has demonstrated that performances are sensitive to location. In particular, the effectiveness of the system is greatly affected by solar radiation and weather condition. The CCHP system works for about 2400 h during the whole year on average with global efficiencies ranging from 32% to 42%. The plant is generally suitable for air conditioning applications in residential sector only with a government’s financial support. A medium value of Pay Back Time of 6 years has been found with a medium Net Present Value of 50 kEUR. In conclusion, this study has highlighted the potential of solar driven micro-CCHP systems based on advances technologies for residential applications.

Packed-bed thermal energy storage (TES) is a cost-effective storage option for high temperature applications. This study aims to accurately model the behavior of a packed-bed TES system during transient operation while maintaining low computation time. This is realized through the development of a novel 1D × 1D model accounting for intra-particle conduction – the first dimension representing the overall device in the axial direction, and the second dimension representing spherical storage particles in the radial direction. An experimental system was constructed to validate the simulation model. Temperatures at the system outlet and in the center of the storage bed were predicted within 3.75% and 3.54% of experimental results, respectively. The validated model was then used to predict the behavior of different materials suitable for large-scale applications, such as stone, concrete, and steel. The potential error due to applying the uniform temperature approximation was investigated on different Biot number configurations. A relative error greater than 55% between surface and particle center temperatures was identified for a system with a Biot number of 3. The novel modeling methodology realized computation times similar to models not accounting for this phenomenon, an 87% improvement upon previous work investigating similar methods, with prediction accuracy remaining unaffected.