This study experimentally investigates an all-season thermal energy storage-integrated heat pump (TES-HP) system developed to enhance building energy efficiency and support grid-interactive operation through load shifting in both cooling and heating. A 4-ton commercial rooftop air-source heat pump was modified by integrating a hydronic thermal energy storage (TES) unit containing a phase change material (PCM) with a melting temperature of 22.0 degrees C. The system employs two reversing valves and three electronic expansion valves to enable six operating modes, including normal, TES charging, and TES discharging in both cooling and heating seasons. A subcooling-controlled electronic expansion valve was implemented to mitigate refrigerant maldistribution caused by unequal internal volumes among the heat exchangers. Compared with a baseline heat pump, the TES-HP reduced power consumption by 30-50% during cooling at an ambient temperature of 40.6 degrees C, and by up to 60% during heating at-15.0 degrees C, while maintaining high performance under cold-climate conditions. In addition to experimental evaluation, a simplified annualized on-peak analysis was conducted to compare the TES-HP with the baseline system, indicating an on-peak electricity reduction of approximately 876 kWhe per unit per year and associated on-peak cost savings under time-of-use pricing. These results demonstrate that the TES-HP effectively decouples HP operation from adverse ambient conditions, improving flexibility, resilience, and energy efficiency. This work advances prior research by experimentally proving the integration of PCM-based TES for a year-round, load-flexible HVAC operation.

Phase change materials (PCMs) can reduce building peak loads and enable demand-responsive thermal energy storage (TES), but their deployment depends on reliable measurement and interpretation of thermal properties across laboratory, intermediate, and application scales. This review systematically examines characterization methods, testing protocols, and recent advances for neat PCMs and PCM composites, emphasizing thermal conductivity, enthalpy-related properties (phase change temperature, latent heat, specific heat), and cycling stability. For thermal conductivity, we compare steady-state and transient techniques and note limitations when phase transition and contact resistance affect measurements. For enthalpy–temperature characterization, we discuss differential scanning calorimetry together with intermediate- and bulk-scale methods, including T-history, heat flow meter testing, and three-layer calorimetry (3LC), to generate application-relevant enthalpy–temperature profiles. Cycling stability is organized into four experimental families: thermoelectric–air, fully thermoelectric, water-bath, and in situ chamber approaches, with attention to separating reversible supercooling from true degradation such as phase segregation. We highlight emerging noncontact diagnostics, including infrared thermography and embedded sensing, for spatially resolved validation and multiscale interpretation. Finally, we review the growing use of AI and machine learning for property prediction, inverse characterization from experimental signals, and real-time state estimation in building-integrated TES. Key needs include harmonized protocols, interlaboratory benchmarking, uncertainty reporting, and metadata-rich datasets to accelerate reproducible PCM qualification for grid-flexible buildings.

This paper proposes and validates a dual-season thermal energy storage-integrated heat pump (TES-HP) system that shifts building electric load in both cooling and heating modes using a single phase-change TES. The design uses the TES as a heat sink for the condenser in summer and a heat source for the evaporator in winter, thereby reducing on-peak electric demand while ensuring compatible with existing air-distribution systems. A 14-kW TES-HP prototype with six operating modes and novel refrigerant charge management strategies was experimentally tested under varied conditions. Subsequently, data-driven polynomial performance curves were developed and validated against experiments, then coupled with a phase change material (PCM) model and a calibrated building model within a rule-based controller. Single-day case studies demonstrated effective on-peak demand reduction while maintaining thermal comfort. During cooling, hourly power was lowered by up to 1.5 kW; in heating, average hourly power decreased from over 3.5 kW to 1.8 kW, compared to an HP-only system, achieving 64.5 % load shifting, and the need for resistance heating was eliminated. Seasonal simulations showed typical on-peak electricity savings of 25-35 % in cooling and 40-65 % in heating, with the largest benefits on the hottest and coldest days. Extended response-surface analysis and nationwide mapping quantified load shifting as functions of ambient and TES temperatures, revealing a seasonal trade-off in TES phase-change temperature. These results demonstrate the TES-HP system as a practical and scalable solution for grid-interactive buildings that reduces on-peak demand and electricity use while maintaining thermal comfort and enhancing grid flexibility and reliability.

Conventional dual fuel heat pumps lack the intelligent control mechanisms to efficiently manage the switch between heat pump and furnace, leading to sub-optimal energy usage and, in some cases, increased operating costs. To resolve this gap, this study applies optimized control on hybrid heat pumps. With a focus on equipment control strategies, we compare the performances of five spacing heating equipment, including a conventional heat pump (HP), a conventional furnace, a dual fuel heat pump (DFHP) with conventional control, a dual fuel heat pump with smart control, and a novel seamlessly fuel flexible heat pump (SFFHP). While DFHP runs on either gas or electricity at any given moment, SFFHP concurrently consumes gas and electricity by continuously optimizing the proportion of each. In this research, a co-simulation framework is developed by integrating a building envelope model with a physics-based heat pump simulation model to analyze the benefits of grid-responsive controls of DFHP and SFFHP. The model-based optimal controls adjust the operation of the heat pump and gas furnace based on utility price signals and marginal grid emission to minimize utility cost and CO2 emissions for multiple climate zones, different utility tariffs, and marginal grid emission scenarios. Case studies in Chicago and Los Angeles demonstrate that SFFHP and DFHP, with model-based optimal control, can deliver significant reductions in peak demand, utility cost, and CO2 emission. In Chicago, SFFHP and smart controlled DFHP yield up to 64.7% and 61.7% utility cost reduction and up to 15.7% and 8.5% CO2 emission reduction compared to the gas furnace. In Los Angeles, SFFHP and smart controlled DFHP achieve up to 43.6% and 40.1% utility cost reduction and up to 13.8% and 14.1% CO2 emission reduction compared to conventional heat pumps. By leveraging the fuel flexibility nature of dual fuel heat pumps, the model-based control optimization approach makes dual fuel heat pump an attractive option for demand response programs.

Residential clothes drying accounts for about 5 % of the total residential-sector energy consumption in the United States. Most dryers use electric resistance heaters to dry clothes and have low efficiencies. Higher-efficiency dryers that use vapor compression heat pumps are expensive and complex and have not gained a large market share in the United States. A novel tumble clothes dryer using a small thermoelectric heat pump with faster airflow than typical dryers is presented in this work. The benchtop performance of the thermoelectric heat pump and high-speed blower are presented, and the development of the prototype dryer is described. The dryer was tested for efficiency and dry time for a range of airflow rates and applied currents to the thermoelectric heat pump. The combined efficiency factor was 5.09-6.29 lbBDW/kWh BDW /kWh (specific moisture extraction rate of 1.23-1.53 kgw/kWh) w /kWh) with 100-138 min dry times for these tests. The measured efficiency was 36 %-68 % greater than the minimum efficiency standard in the United States, and compared with vapor compression heat pump-based clothes dryers, the prototype dryer had less expensive, less complex components and did not use refrigerants. The performance of this small thermoelectric heat pump clothes dryer is also compared with previous iterations of the thermoelectric tumble clothes dryer described in the literature.

The evolution of the energy landscape has been a dynamic process shaped by a complex interplay of technological, economic, and environmental factors. Historically, societies have relied heavily on fossil fuels for their energy needs, but concerns over climate change and energy security have driven a shift towards cleaner, renewable sources of energy. This chapter explores the key drivers and trends in the evolution of the energy landscape.

Globally, cooling thermal loads are continuously increasing. The demand for air conditioning systems is one of the most critical blind spots in the ongoing energy transition. Space cooling demand accounts for nearly 20% of the total electricity used in buildings around the world today. Rising space cooling energy demand is also straining the electric grid systems globally, leading to higher emissions. The global energy demand associated with space conditioning is anticipated to increase significantly for decades to come, driven by increased affordability and global population growth. This presents a significant opportunity to quickly influence the growth of cooling-related energy demand through policies to improve equipment efficiency. Unlike the wide range of heating system options, cooling systems options are limited and can be broadly categorized into ones with and without onsite power generation. Here we explore the environmental impact of cooling technologies with the concept of effective carbon footprint as a metric for evaluating their carbon emissions. It examines the factors influencing the carbon footprint of cooling technologies, including energy efficiency, and thermal load. By comparing the effective carbon footprints of different cooling technologies, the chapter aims to provide insights into their relative environmental impact and inform decision-making for sustainable cooling solutions.

In recent years, thermal energy storage (TES) has gained attention for its role in enhancing renewable energy solutions and sustainable energy consumption. The usage of strontium chloride hexahydrate (SCH), graphene nanoplatelet (GNP), and cellulose nanofibril (CNF) additives were investigated to enhance the performance of calcium chloride hexahydrate (CCH) based on the melting/solidification behavior for TES applications. In this work, we develop a promising phase-change-material (PCM) formulation by introducing these additives that reduce supercooling, improve the thermal conductivity and stabilizing the energy storage capacity of CCH. Rheological characterizations demonstrated that the addition of 1 wt% of CNF into CCH produced the required improvement in viscosity and boosted solid-like rheological behavior. Structural characterizations show a physical mixing of the materials within the PCM composites. Our observations show that the amphiphilicity of CNF enables the surface attachment to GNP via hydrophobic interactions providing effective dispersion of GNP throughout the PCM composite. The addition of a nucleating agent, SCH decreased the degree of supercooling of similar to 20 g of CCH from >20 degrees C to 3 degrees C at a cooling rate of 5 degrees C/min. Thermal characterization showed the resulting PCM composite has a latent heat of melting of 186 Jg(-1), phase change temperature of 32 degrees C, and stable thermal properties after being subjected to 70 melt-freeze cycles. Adding CNF and GNP to pure CCH increased its thermal conductivity by 76 %. The high thermal conductivity of GNP and its effective dispersion by CNF is responsible for this enhancement. The study highlights the use of biodegradable nanocellulose for the preparation of sustainable PCM composites with improved performance. These PCM composites are scalable, they have potential to increase energy efficiency and revolutionalize the heating/cooling applications in buildings and other TES systems.

In the United States, the majority of clothes dryers use electric resistance heaters with a capacity of approximately 4 kW for clothes drying. US dryers typically use a tumble-type drum with a blower to push air and dry clothes. Most existing electric products are electric resistance with once-through airflow, with some condensing dryers using closed-loop airflow. Starting in late 2014, vapor-compression (VC) heat pump dryers have been available. Although they are extensively used in Australia and Europe, they have very low market penetration in the United States. Currently a few heat pump clothes dryers (HPCD) using R134a are available on the market from LG, Whirlpool, and Asko, but they have very high retail prices and relatively long dry times. Two are closed loop, and one uses open-loop airflow. Table 1 provides a summary of the combined energy factor (CEF), drying time, and range of retail prices for existing conventional dryers and existing HPCD products, along with CEF and drying time for HPCD prototypes developed during this project. The major market barriers are seen as the high cost and longer dry times (Denkenberger et al. 2013).

This chapter explores various pathways to achieve decarbonization in the energy sector. It considers a range of scenarios that involve transitioning to low-carbon electricity generation sources, such as renewables and nuclear, as well as reducing carbon emissions from fuel consumption through electrification and alternative fuels. The chapter analyzes the implications of these scenarios on energy systems, carbon emissions, and economic factors. It also discusses the challenges and opportunities associated with each scenario, including technological advancements, policy frameworks, and societal acceptance. By examining different decarbonization pathways, the chapter aims to inform policymakers, researchers, and stakeholders about the complexities of achieving decarbonization goals and the need for integrated strategies across electricity and fuel sectors.

Sodium sulfate decahydrate (SSD) is a low-cost phase-change material (PCM) for thermal energy storage applications that offers substantial melting enthalpy and a suitable temperature range for near-ambient applications. However, SSD's consistent phase separation with decreased melting enthalpy over repeated thermal cycles limits its application as a PCM. Sulfonated polyelectrolytes, such as dextran sulfate sodium (DSS), have shown great effectiveness in preventing phase separation in SSD. However, there is limited understanding of the stabilization mechanism of SSD by DSS at the atomic length and time scales. In this work, we investigate SSD stabilization via DSS using neutron scattering and molecular dynamics (MD) simulations. Neutron scattering and pair distribution function analysis revealed the structural evolution of the PCM samples below and above the phase change temperatures. MD simulations revealed that water from the hydrate structure migrates from the hydrate crystal to the SSD-DSS interfacial region upon melting. The water is stabilized at this interface by aggregation around the hydrophilic sulfonic acid groups attached to the backbone of the polyelectrolyte. This architecture retains water near the dehydrated sodium sulfate, preventing phase separation and, consequently, stabilizing SSD rehydration. This work provides atomistic insight into selecting and designing stable and high-performance PCMs for heating and cooling applications in building technologies.