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 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 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.
•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.
•Novel modelling approach for thermal energy storage with immersed coil heat exchangers.•Discretizing the storage tank into only three fully-mixed control volumes.•Experimental validation of the ...numerical model using a full-scale testing facility.•Numerical model validated for storage tanks with and without phase change materials.•Numerical model captures different thermal behaviors of thermal storage tanks.•Satisfactory agreement between the experimental and numerical results.
A novel modelling approach is presented for a thermal energy storage system with immersed coil heat exchangers. The energy store consists of a water tank in which rectangular phase change material (PCM) modules are submerged. The immersed coil heat exchangers are placed at the bottom and top sections of the tank for charging and discharging, respectively. This design promotes thermal mixing inside the tank when charged from the bottom coil and discharged from the top coil. In addition, the location of the coil heat exchangers favorably limits the heat transfer when charged through the top coil or discharged through the bottom coil giving rise to the thermal diode effect. PCMs can offer large storage capacity since they store large amounts of thermal energy in the form of latent heat of phase change (solid-liquid). This paper proposes a simplified physics-based numerical model of the heat transfer and phase change in the thermal storage tank. The model discretizes the storage tank one-dimensionally into three fully-mixed control volumes; two control volumes surrounding the immersed coil heat exchangers and one control volume in the middle of the tank including the PCM modules. All the heat transfer dynamics between the control volumes, the heat exchangers and the PCM modules are considered. Experimental validations, using a full-scale test facility, are carried out for systems with and without PCMs under various operation scenarios and the results are found to agree well within the experimental uncertainties. Such model potentially provides high computational efficiency that is desirable in simulating the performance of storage systems under long-term operations.
The effects of appendages on flow characteristics and scalar mixing in gap-connected twin-subchannel geometries has been investigated. Mixing is assessed for a symmetric, rectangular compound channel ...geometry connected by a single rectangular gap using computational fluid dynamics (CFD). Detailed numerical models, characterizing the full test section from a reference experimental study, are generated and validated against measurements. Time varying details of the gap induced periodic structures and appendage induced vortices are captured through calculations in an unsteady Reynolds Averaged Navier-Stokes (URANS) framework coupled with the Spalart-Allmaras (SA) turbulence model closing the RANS equations. Companion simulations are performed at each of two Reynolds numbers (2690 and 7500), one with and one without a gap-centered appendage. The appendage effects on flow characteristics and mixing are isolated through comparison of the associated simulations. In the absence of appendages, fluid exchange between subchannels is dominated by quasi-periodic flow pulsations through the gap (gap vortex streets) formed due to flow instability in the near gap region. Without a gap-centered appendage, the magnitude, frequency, and structure length of the gap pulsations are well predicted by the model at both Reynolds numbers. Mixing between subchannels is reasonably well predicted for Re = 2690 (within approximately 17% of the experimental value). The model fails to capture the measured increase in scalar transfer through the gap with increased Reynolds number, underpredicting scalar mixing by 55% at Re = 7500. An argument is presented that the use of an isotropic turbulence model in the subchannels (SA), which precludes the development of subchannel secondary flows, is the source of the discrepancy between modelled and measured mixing at Re = 7500. Appendages, such as those introduced by end plates or bearing pads in CANDU fuel bundles, augment the exchange process between subchannels. With an appendage representative of a CANDU fuel bundle end plate web introduced into the gap region, cross flow velocity and frequency are predicted to increase immediately downstream of the appendage due to flow diversion and vortex shedding. The higher local frequency is shown to be consistent with the vortex shedding frequency calculated for a stationary rectangular cylinder at the gap conditions. Further downstream, gap induced instabilities begin to re-establish as the dominant contributor to cross flow pulsations although they are not fully recovered by the test section exit. Mixing is enhanced by the appendage with increasing Reynolds number for the conditions examined.
•The pumping effect enhances the heat transfer inside the hybrid energy storage system.•Thermal resistance on the PCM modules is dominated by its total surface area.•High PCM volume fractions hinder ...the pumping effect.•Charging rate of the hybrid storage system is limited by the design of the coil.•Interference of thermal boundary layers between PCM modules drops the performance.
Charging of modular thermal energy storage tanks containing water with submerged Phase Change Materials (PCMs) using a constant temperature coil heat exchanger was numerically investigated. Under appropriate operating conditions, the energy density of this hybrid system can be significantly increased (two to five times) relative to a system containing water only (sensible energy storage system). However, due to the low thermal conductivity of PCMs, the geometry and configuration of the PCM encapsulation requires optimization to meet typical charging and discharging cycles. In the current study rectangular PCM modules in a modular storage tank (∼200L) were studied to determine the effect of PCM volume fraction and spacing between the modules on the heat transfer characteristics as well as the charging rate of the storage system. Two-dimensional Computational Fluid Dynamics (CFD) simulations using ANSYS CFX 14.0 showed an 85% increase of the charge rate of the system by increasing the PCM volume fraction from 0.025 to 0.15. The charge rate was not affected when the PCM volume fraction was increased beyond 0.15. For a fixed coil heat exchanger design, further increase of the PCM volume fraction caused the thermal resistance on the PCM side to be significantly lower than that on the coil side. As a result, further increase of the charge rate necessitates increasing the surface area of the heat exchanger. Simulations showed that the gap spacing between the PCM modules had a negligible impact on the heat transfer characteristics for the studied cases, where the gap spacing was larger than the thermal boundary layer thickness.
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 novel approach to extracting thermal energy from abandoned oil wells is proposed.•Simulations indicate that 200 kW of power could be produced from the base case.•Increasing the geothermal gradient ...had the greatest positive effect on power production.
Geothermal energy is an ideal renewable resource for power production since it is not limited by the intermittency issues associated with other renewables like wind and solar. Despite its ability to provide a steady supply of electricity on demand, the adoption of geothermal is not widespread, largely due to the high cost of drilling. One way to mitigate this cost is to use abandoned oil and gas wells, which are abundant (with an estimated 2.3 million in the United States) but pose a potential hazard to the environment. Studies in the literature have proposed installing heat exchangers in vertical wells to solve this problem. Vertical designs, however, would undergo high friction and temperature losses due to the use of internal piping. Such designs also do not enable the use of the L-shaped directionally drilled wells which have greater contact with the ground at high temperatures. This paper proposes an approach for extracting geothermal energy from abandoned oil and gas wells by using directionally drilled wells, thereby improving upon the existing designs by eliminating internal piping and maximizing contact with high ground temperatures. Through additional drilling, two adjacent wells can be connected to create a continuous loop from which thermal energy can be collected for use in a power cycle. The proposed geometry was studied numerically to determine the outlet temperatures and heat extraction rates from the system for the prediction of thermal energy extraction and electrical power production. The results show that a system with 4000 m deep vertical wells and a 4800 m horizontal section could produce approximately 2 MW of thermal energy and 200 kW of power using an organic Rankine cycle. Moreover, these wells are capable of providing a heat source for electricity generation for several decades.
•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.
