Analysis of Residential Time-of-Use Utility Rate Structures and Economic Implications for Thermal Energy Storage

Thermal energy storage (TES) is an increasingly popular tool to level out the daily electrical demand and add stability to the electrical grid as more intermittent renewable energy sources are installed. TES systems can locally decouple high thermal loads from the operation of a heat pump or reduce the electrical energy demand of the heat pump by providing a more favorable temperature gradient. In addition, many policy makers and utility providers have introduced time-of-use (TOU) rate schedules for residential customers to better reflect the price of electricity generation and demand for specific times. TOU rate schedules price grid-provided electricity differently throughout the day depending on the region’s climate, time of year, and electrical production portfolio. Large differences between on-peak and off-peak electrical prices may create an economic advantage for a residential customer to install a TES system. In this work, the economic and energy savings are calculated for a modeled 223 square foot residential building with water/ice-based TES using a TOU rate structure. The weather data is from Fresno County, CA, ASHRAE climate zone 3B, and a representative residential TOU utility rate structure from a utility provider in California was used. The simulation was carried out for cooling only during a week of extreme hot summer daytime temperature and the results showed that total energy consumption could be reduced by 14.5% with an 87.5% reduction in on-peak energy usage when the TES is installed. The cost of operating this system for space cooling was reduced by nearly 20% using the sample utility rate plan

Review of Inorganic Salt Hydrates with Phase Change Temperature in Range of 5 to 60°C and Material Cost Comparison with Common Waxes

Phase change materials (PCMs) with desirable phase change temperatures can be used to provide a constant temperature source or sink for diverse applications. As such, incorporating PCMs into building materials, equipment, or appliances can shift and/or reduce the energy load. The motivation of this work is to identify low-cost inorganic salt hydrate PCMs that can complement current building systems and designs. Two key challenges to incorporating PCMs into building materials are 1) maintaining desirable thermal properties at large scales, and 2) developing cost-effective systems that are easily incorporated into existing structures and systems. In this work, we present an analysis of inorganic salt hydrates with phase change temperatures in the range of 5-60°C, targeted towards both space heating and cooling. The properties of the salt hydrates are compared with common waxes over the same temperature range. The results showed that salt hydrate systems such as sodium thiosulfate pentahydrate, NaS2O3·5H2O, has a latent heat as high as 201 kJ/kg at a phase change temperature of 48°C which is comparable to some paraffin waxes (213 kJ/kg at 52.5°C). At a density of 1.73 g/cm3, sodium thiosulfate pentahydrate has an energy density of 347 J/cm3 (paraffin waxes, 170 J/cm3). Moreover, it was found that salt hydrates are generally less costly per unit energy in contrast to common waxes with typical salt hydrate costs in the range of 0.001-0.01 $/kJ. This analysis shows the potential of salt hydrate PCMs for developing low-cost heating and cooling thermal energy storage systems to meet a range of applications

Thermal Charging Rate of Composite Wax-Expanded Graphite Phase Change Materials

Phase change materials (PCMs) are valuable for their ability to store heat nearly isothermally around their phase transition temperature. PCMs are at the core of latent heat thermal energy storage (LHTES) systems, which provide the ability to buffer high thermal loads or decouple the time when heating or cooling is needed from when it is produced. Thermal charging and discharging of LHTES systems often employ a constant temperature source, and the rate at which heat can be exchanged is dependent on the thermophysical properties of the PCM. For graphite composite PCMs, the high thermal conductivity of the graphite enables an increased heat transfer rate through the material, but its presence displaces PCM which reduces the effective volumetric latent heat of the composite relative to the pure PCM. This results in a tradeoff between thermal power and volumetric energy storage capacity. The thermal charging rate is the average thermal energy stored in the material for some elapsed time. In this study, composite PCMs of compressed expanded natural graphite (CENG) and n-Octadecane are studied. Samples with varying CENG mass fractions were synthesized and the thermal charging rate was measured under a constant temperature boundary condition. For evaluation of the expanded graphite-PCM composite, one boundary of the material was exposed to a 50°C constant temperature plate, above the 27.5°C melting temperature of the PCM. The melting front progression [mm-s-1] and the thermal charging rate [W-cm-2] of the PCM were determined, and the results were compared with analytical predictions for the 1-D semi-infinite phase change. For CENG addition up to 5.75% mass fraction, a 450% thermal conductivity increase was observed with a only 5% decrease in volumetric energy density as compared to pure octadecane. The average thermal charging rate was increased by over 430% for the melt to penetrate a depth of 22 mm. The experimental results matched analytical predictions, indicating that higher CENG fractions can be evaluated using analytical approaches

Review of Low-Cost Organic and Inorganic Phase Change Materials with Phase Change Temperature between 0°C and 65°C

Phase change materials (PCMs) that undergo a phase transition may be used to provide a nearly isothermal latent heat storage at the phase change temperature. This work reports the energy storage material cost ($/kWh) of various PCMs with phase change between 0 – 65°C. Four PCM classes are analyzed for their potential use in building systems: 1) inorganic salt hydrates, 2) organic fatty acids, 3) organic fatty alcohols, and 4) organic paraffin waxes. Many salt hydrates have low material costs (0.09 – 2.53$/kg), high latent heat of fusion (100 – 290 J/g), and high densities (1.3 – 2.6 g/cm3), leading to favorable volumetric storage density and low energy storage costs, 50 – 130 kWh/m3 and 0.90 – 40 $/kWh, respectively. Some salts are notably more expensive due to their scarcity or pressures from competing industries such as lithium-based salts. Fatty acids have the lowest energy storage cost in the temperature range 8 – 17°C at 6.50 – 40$/kWh. Despite favorable latent heat (125 – 250 J/g) their low density gives (0.9 g/cm3) gives poor volumetric storage capacity, 32 – 80 kWh/m3. Fatty alcohols generally have high material costs 2.50 – 200 $/kg which leads to high energy storage costs, 40 – 3000$/kWh. With latent heat and density similar to fatty acids, fatty alcohols have poor volumetric energy storage, 43 – 55 kWh/m3. Paraffin waxes containing only a single length carbon chain have a higher energy cost (15 – 500 $/kWh) than generic paraffin waxes containing many lengths of carbon chains (7 – 30$/kWh). Pure waxes have a discrete phase change temperature due to their homogeneity. In contrast, a less refined generic wax with several carbon chain lengths is more likely to have a pronounced temperature glide during its phase change. Pure single carbon chain waxes are generally required for applications \u3c45°C as generic paraffin waxes melt between 45 – 70°C. For many waxes, a solid-solid transition occurs at temperatures below the solid-liquid phase change. For pure paraffins with carbon content ≥22 C atoms, these transitions may appear near the same temperature resembling a temperature glide. The challenges with fatty acids, fatty alcohols, and waxes are low thermal conductivity, low density, some flammability concerns, and compatibility issues with some common engineering materials such as polymers. Challenges with salt hydrates are pronounced supercooling (\u3e5°C), incongruent melting, and corrosiveness. All PCMs may degrade if exposed to ambient conditions and therefore require proper sealing

Experiences of Inpatient Bone Marrow Transplantation Nurses and Providers Using Electronic Symptom Reporting

Purpose To investigate the use of electronic patient-reported outcomes (PROs) to assess symptoms and how they can provide opportunities to clinicians to address symptoms in a timely manner to improve clinical care. As part of a larger study to evaluate whether providing standardized symptom reports to the medical team would decrease the time to treatment of reported symptoms in hematopoietic stem-cell transplant recipients, we assessed nurses’ and providers’ perceptions of electronic symptom reporting. Methods Semistructured interviews of RNs, MDs, NPs and PAs were conducted at an academic cancer center in the southeastern United States. Nurses’ and providers’ perceptions of electronic symptom reporting were explored. Interviews were audio-recorded, transcribed, and coded by two investigators to identify major themes. Results Fourteen RNs and seven providers (MDs, PAs, and NPs) participated in the interviews. Three main themes emerged from the interviews: electronic symptom reporting may improve assessment and care, integrating symptom reporting into nurse workflow presents difficulties, and there are barriers for completion of surveys. Conclusion The majority of nurses and providers believed that the inclusion of electronic symptom reporting in bone marrow transplantation inpatient units has the potential to improve care but that barriers to implementation remain