Heat pumps have the potential to reduce CO2 emissions due to building heating when compared to fossil-based heating (e.g. natural gas, oil, wood), specifically when used in regions with low-CO2 ...electrical generation. In many regions, emissions from the electric grid tend to peak during peak demand periods due to the dispatching of fossil-based generation. The design of buildings as distributed thermal storage units can act to diminish the peaks in the grid, reduce the overall CO2 emissions from residential heating, increase the utilization of low-CO2 technologies (nuclear, hydro, wind, solar, etc.…), while maintaining the thermal comfort of the occupants.
This study is concerned with how thermal energy storage can be integrated into heat pump systems to improve demand flexibility, and ultimately allow the heating system to remain off during peak periods. Heat pumps tend to operate under a limited temperature range, which limits the energy storage density of water as a thermal storage medium. Phase change materials (PCM) can be used as thermal storage, and they benefit from the ability to maintain a high energy density under limited temperature conditions. The challenge is that PCMs have a relatively low thermal conductivity which can limit the rate of charging and discharging of the stored thermal energy.
In the current state-of-the-art literature, there is no standard methodology to size PCM thermal energy storage units for heat pump systems. This study presents novel results that compare numerical and analytical predictions of a hybrid PCM-water thermal storage tank, and proposes a reduced analytical methodology for sizing PCM thermal storage tanks for heat pumps used for demand side management. System-level numerical simulations, considering the transient complexities of the melting and solidification process in a system environment, are compared against a simplified analytical predictions of thermal storage performance. Storage tanks containing 75% PCM modules of 2 cm thickness were able to reduce storage volume by over three-fold of water-only storage operating under a ΔT=10 °C. Peak periods ranging between 2 and 6 h in a residential household were sustained when the appropriate storage volume is used. Analytical methods for estimating the required volume are presented that ease the storage sizing and discuss the expected benefits and their limitation.
The aim of this paper is providing a detailed review for the applications of phase change materials (PCMs) in residential heating. The study focuses mainly on its use in domestic water heating ...systems. Different studies accounting for structural characterization, research methodology, and long term performance were carefully assessed. The gaps in the literature have been highlighted and recommendations for future studies have been presented. The technical gaps urge the need for research to be directed towards enhancing PCM properties in the domestic system, novel integration of PCM within the system, and optimization of system performance based on real-time accurate weather data. The economic opportunities for such systems were presented in different locations of the world by investigating different energy efficiency measures. The work presented herein identifies potential energy management opportunities that can significantly reduce our reliance on fossil fuels. Consequently, promoting a green future and mitigating greenhouse gas emissions. It is structured to provide guidance for researchers and engineers working in inclusion of PCMs in residential heating applications.
In the effort of mitigating global greenhouse gas emissions, renewables are crucial. Solar energy is a promising resource to be integrated in residential, commercial, and industrial applications. ...Flat plate solar collectors (FPSCs) typically harness solar energy. The goal of latest research trend is to enhance solar collector ability to capture irradiation. Phase change material (PCM) can maximize that due to their high latent storage capacity. However, common PCMs have a low thermal conductivity. Nanocomposites can enhance PCM thermal conductivity and thus improves overall system performance. The current review paper analyzes in details the integration of PCM and nanocomposite PCM in the FPSC. It provides thorough analysis of the system modeling. It also provides quantitative results of FPSC performance enhancement. Carbon based nanocomposites and metal foams have shown the greatest enhancement of PCM thermal conductivity. The optimum position for PCM integration is reported to be beneath the heat transfer fluid tubes. The incorporation of PCM guaranteed a longer time period of hot water supply from the collector. Major research contributions were critically assessed. In addition, the detailed review has highlighted scarce experiments done on nanocomposite PCM integration and a significant gap in the optimization analysis of such systems. Furthermore, there is a gap in the economical analysis that is needed for the widespread applications of the considered systems.
•Phase change materials (PCMs) have a promising energy storage potential.•PCM integration in solar collector studies are critically assessed.•PCMs suitable for low temperature applications have low thermal conductivity.•Nanomaterials are used to enhance PCM thermal conductivity.•A review of nano enhanced PCM integration in solar collector is presented.
The inclusion of thermal storage into a residential heat pump system provides an opportunity for electrical load shifting in order to reduce peak electricity demand. To assess this potential, a ...detailed numerical system model for a ground-source heat pump coupled with thermal storage was developed for residential heating applications. Water-based thermal storage and hybrid storage containing water and phase change materials (PCMs) were considered. The amount of thermal buffering needed to shift the heat pump operation to off peak electricity periods was numerically assessed for a house of 180 m2 floor area in Toronto, Ontario, Canada. A very cold day ambient profile was considered as the extreme case to reach the study conclusions. Results indicated that total electrical load shift to off-peak hours was achieved by a 2.5 m3 water tank or a 1 m3 hybrid tank containing 50% PCM by volume. This implies around 65% reduction in storage volume without compromising the space heating capability. This is attributed to the higher storage capacity offered by the hybrid system when the temperature operating range is limited by heat pump operation. The influence of operating temperature ranges and packing ratio was presented. Lower operating ranges and higher packing ratios lead to better thermal buffering of the hybrid system at smaller storage volumes. However, careful choice of the PCM encapsulation geometry is needed to guarantee complete melting/solidification during the charging/discharging period.
•Hybrid system containing water and phase change materials is simulated.•Solar fraction of hybrid system is compared to water only system.•PCM enhances solar fraction when the tank is undersized for ...the demand.•PCM increases pump run time and reduces collector losses.
Phase change materials (PCM) for thermal energy storage in solar energy systems have been the subject of a great deal of research in the literature. Despite this, the research results pertaining to the efficacy of PCMs in enhancing system solar fraction are mixed. The current paper explores this issue numerically within a systems context. A typical solar domestic hot water system is considered. The PCMs are introduced as vertical cylindrical modules contained within the water tank, thus forming a hybrid PCM/water thermal storage. Water flowing along the length of tank is used as the heat transfer fluid. A model was developed based on the enthalpy-porosity method to solve for the phase change process within the PCM modules. The model was thoroughly validated and verified and predictions were in good agreement (less than 5% deviation) with results from the literature. The hybrid tank model was linked with the collector performance and the system was tested for typical days of Canadian weather with a dispersed demand profile. The solar fraction of the hybrid system was compared to that for an identical system using water-only as the thermal storage medium. The system analysis explores the impact of storage volume on solar fraction for systems with and without PCMs included. The systems approach is critical since it allows for the coupled effects of the thermal storage, solar collector, and household load to be incorporated. The analysis clearly shows that incorporation of PCMs into the thermal storage results in enhanced solar fraction at undersized tank volumes relative to the demand. In contrast, as the tank volume is increased, the benefit of the PCMs diminishes and identical performance is obtained between the two systems at large volumes. An energy balance of the system shows that, despite marginally increased heat losses from the hybrid tank, the benefits of the hybrid storage at small storage volumes are due to the reduction in the collector fluid inlet temperature which increases the pump run time and thus the solar energy collected and reduction of collector losses.
•Hybrid tanks containing water and phase change materials are studied numerically.•Phase change materials with different melting points are placed in the tanks.•The cascaded configuration is studied ...in a solar domestic hot water system context.•A system energy balance reveals the benefit of the hybrid thermal energy storage.•The hybrid system can yield increased solar fraction compared to water-only tanks.
The current paper explores a multi-tank thermal storage system for multi-residential solar domestic hot water applications. The thermal storage system includes phase change materials (PCMs) of different melting temperatures incorporated in the tanks. The PCMs are introduced as vertical cylindrical modules and water flowing along the length of tank is used as the heat transfer fluid. An enthalpy porosity model was developed to solve for the phase change process within the PCM modules. The model was validated and verified with previous work and predictions were in good agreement (less than 5% deviation). The hybrid tank model was linked with the collector performance. Typical Canadian weather data and a dispersed demand profile for a multi-residential building were considered. The performance of the hybrid system was judged based on the maximum possible storage volume reduction compared to the water only system with the same benefit to the end user. PCM maintains cooler water temperature entering the collector which results in a reduction of collector losses and extension of pump activation time. This increases the delivered energy to the load and hence increases the solar fraction. It was found that cascading four 75 L tanks containing PCMs of melting temperatures 54 °C, 42 °C, 32 °C and 16 °C gives a similar solar fraction to that for a 630 L water only tank. The multi-tank hybrid system thus allowed for over 50% reduction in the required storage volume.
A numerical model is developed and validated to simulate the performance of sensible energy storage (water tank) and hybrid energy storage (water tank including phase change material “PCM” modules) ...integrated into solar domestic hot water (DHW) system. Two configurations with direct heat exchange and indirect heat exchange using immersed heat exchangers are explored. A novel comparison is presented between both systems based on the solar fraction, which denotes the solar thermal energy contribution to the load. The effect of the storage volume on the solar fraction of the system is studied during a typical spring day while providing the hot water demands for a single-family residence, assuming a dispersed daily draw profile. The numerical results showed the crucial role of thermal stratification induced in the storage system with direct heat exchange. The storage system with direct heat exchange operates with 18–23% larger solar fraction than that with immersed coil heat exchangers. Adding PCM modules in the water tank with 50% volume fraction can yield around 40% potential reduction in the storage volume. The melting temperature of the PCM must be carefully chosen to maximize the energy storage in the latent form, thus limit the large temperature fluctuations of the system.
•Numerical model for hybrid thermal energy storage with phase change materials is developed.•Experimental validation of the model yields good agreement within the measurements' uncertainty.•Storage with direct heat exchange outperforms that with heat exchanger due to stratification.•Adding phase change materials to storage tank could reduce the storage volume by 40%.•The melting temperature of the phase change material has a critical impact on the performance.
A global effort is directed towards a more sustainable future dominated by renewable and alternative technology. Thermal energy storage is pivotal in those advancements to even out discrepancy ...between supply and demand times. Thermal storage can take different forms: sensible, latent, and thermochemical. Latent storage offers a compact form owing to the high phase change energy. Phase change materials (PCMs) have shown promising performance enhancement in solar systems in particular. The majority of reported studies include single PCM integration in solar systems. It offered augmented storage capacity relative to water based system. It also enhanced the solar fraction. Cascading of PCMs was studied in concentrated solar power context. However, it is scarce in the solar thermal applications. The current review paper provides a detailed analysis of cascaded PCM systems. It compares it to single PCM systems in terms of storage capacity and enhanced efficiency. The review sheds the lights on the challenges of widespread integration of those systems and the predicted future trends.
•Thermal storage can take different forms: sensible, latent, and thermochemical.•Phase change materials (PCMs) have shown promising performance enhancement in domestic applications.•The majority of reported studies include single PCM integration in domestic systems.•Fins and nanometals augment heat transfer in single PCM systems.•Cascading of PCMs was studied in concentrated solar power context. However, it is scarce in the solar thermal applications.
•An analytical method is proposed to size PCM encapsulations for system requirement.•The method was tested on a heat pump system for a demand side management case study.•System-level transient ...numerical models were used to verify analytical results.
Thermal energy storage is essential to the operation of many systems. It plays a vital role in many renewable and CO2-reducing technologies which have a mismatch in time between when thermal energy is available and required. Water is the most commonly used thermal storage medium in many applications, however, for systems with a small temperature operating range, large storage volumes may be required since the energy storage capacity of water is proportional to the temperature range. In contrast, phase change materials (PCMs) can maintain high energy capacity under limited temperature conditions, but they typically have low thermal conductivities which results in slow melting/solidification rates. As such, careful design of the encapsulation geometry is required to take advantage of phase change thermal storage. Systems using phase change materials must ensure complete melting, and the encapsulation thickness must be designed according the system needs.
Models of hybrid storage tanks employing both water and PCM are investigated, with PCM encapsulations embedded in water. This study uses a novel application of analytical formulations of 1-D melting to size rectangular PCM encapsulation to match the requirements of a residential heat pump system. With boundary conditions that reflect the heat pump and storage characteristics, the proposed analytical solution links encapsulation thickness to system requirements. The analytical formulation is derived for encapsulation thickness in terms of heat pump rating, temperature differential, storage material, and storage volume. The solution was verified through system-level numerical simulations using TRNSYS and an in-house enthalpy-porosity modeling tools detailed in the paper. The verifications have shown a good agreement, and the melt thickness predictions were within 4% between both models. The study proposes using the methodology as a standard for designing hybrid water-phase change material thermal storage for heat pumps, which can be expanded to various PCM geometries and thermal systems.
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