Conference Agenda

Heat pumps for space heating and domestic hot water (DHW) supply have great potential for decarbonizing the building sector. In the European Union, the F-Gas Regulation on fluorinated greenhouse gases aims to reduce the use of HFC refrigerants. Natural refrigerants, especially R-290, offer great potential for efficient operation, especially for heat pump applications. Heat pumps are available in a wide range for single-family houses (SFH) and small multi-family houses (MFH), therefore houses in urban areas are the focus of developments. In these areas the use of an air/water heat pump is not always possible due to the often dense population, the noise restrictions or the limited available space. For this reason, Fraunhofer ISE is investigating the combination of heat pumps and Photovoltaic thermal hybrid solar (PVT) collectors as a heat source.

The focus of this article is the heat pump system with the refrigerant cycle, the water storage tank, the heating, and the DHW circuit. An alternative concept is presented in comparison to the standard heat pump systems for heating and DHW. The goal of this system is to produce the DHW during normal heating mode without a special DHW mode. Particular attention is paid to the condenser of the heat pump, which can be designed as an immersed or mantle heat exchanger. The use of R-290 makes the charge of refrigerant in the heat pumps a design criterion. Therefore, a detailed thermodynamic model of the heat pump is developed with the program language Modelica. The compressor and evaporator are fitted with data from the manufacturers to achieve realistic simulation results.

The simulation results are used to design the condensers and to compare the two types with each other. The design parameters are the coefficient of performance (COP), the achievable DHW temperature in the upper part of the storage tank, and the refrigerant mass. The results are compared with state-of-the-art R-290 heat pumps.

Complete electrification of heating and cooling systems in the United States would overload the existing electrical grid. To this end, recent research efforts have focused on energy storage as one way to reduce ever-increasing electricity demands. One such technology is heat pump integrated thermal-energy storage (HP-TES) systems, which can enable load-shifting and reduce demand during peak hours. In this work, a Modelica model of a 4-ton, dual-mode (heating and cooling) HP-TES system using a room-temperature phase-change material (PCM) integrated through a secondary hydronic loop was developed and simulated for a typical meteorological year in Riverside, CA, (ASHRAE Climate Zone 2B) and Chicago, IL, (ASHRAE Climate Zone 5A) for a DOE small office prototype building. The COP improvement and demand reduction were analyzed during the time-of-use peak hours along with the required recharge cycle energy demand. A typical summer in Riverside, CA resulted in model-predicted peak COP increases of 50 – 100% compared to the baseline heat pump, especially at higher outdoor temperatures, coupled with maximum peak energy reductions of 22%. During shoulder and winter season simulations, heating energy savings were higher as outdoor temperatures dropped below the PCM phase change temperature. For Chicago, IL, backup heating was necessary to meet the required system capacity for very cold ambient conditions, resulting in winter peak energy savings between 16 – 60% when accounting for backup heating requirements. The recharge energy was found to be mostly below the baseline HP energy during the peak. With improved component selections and controls, the recharge energy may be reduced. The outcomes from the simulations can provide first-order estimates for potential peak energy savings, challenges, and required controls in any location before conducting any field testing.

Buildings, as primary electricity consumers, have significant potential to provide demand flexibility through their heating, ventilation, and air-conditioning (HVAC) systems, augmented by thermal energy storage (TES). Currently, most residential, and light commercial HVAC systems use heat pumps (HP) in either direct expansion or water loop/fan coil configurations. The potential of these systems to contribute to the demand flexibility (e.g., load shifting) depends on both the design of the integrated heat pump-thermal energy storage (HP-TES) system and the operational control of such a system. In this work, the optimal control of conventional HP-TES system was studied. A Modelica-based simulation framework is developed for the HP-TES system performance evaluation. The building and HP-TES systems were then simulated in the Spawn of EnergyPlus environment, addressing buildings with various load profiles under different climatic conditions.

As a case study, we implemented an economic model predictive control (MPC) strategy for Phase Change Material (PCM) based HP-TES system, aiming to minimize operation costs and power fluctuations while maintaining comfortable indoor temperatures. The simulation results show that the HP-TES system based on PCM thermal storage has significant potential in terms of effectiveness in power system signal response capability and reduced operating cost. When the prediction step size is less than 12 hours, the load transfer effect increases with the increase of the step size, thereby reducing the system operating cost; when the prediction step size is greater than 12 hours, the solver cannot always guarantee the reduction of the operating cost due to the increase of the computational complexity. Based on these findings, we provide tailored recommendations for advanced control strategies applicable to conventional HP-TES systems. This simulation framework provides an avenue for future extension, including the exploration of state-of-art TES materials and the investigation of aggregated control strategies for distributed HP-TES systems, with the objective of improving the study grid-to-building integration.

Thermal energy storage (TES) has been considered for integration into residential heating systems with air source heat pumps (ASHP-TES) due to the benefits it may provide to users and the grid. Several simulation tools are available to determine if adopting TES is economically attractive. However, these tools are not accessible to all stakeholders in the energy market, which may hinder the deployment of such systems. This paper proposes a new analytical method for estimating the economic performance of residential ASHP-TES systems. The method uses known parameters of the heat pump, dwelling, and storage device, and employs a simplified calculation procedure that can be easily implemented in spreadsheets to estimate the levelized cost of electricity and storage for specific ASHP-TES projects. The method was tested in five distinct locations in the United States. Compared to the simulation results for the levelized cost of energy (LCOE), the relative deviations ranged from 0.5 % to 3 %. Additionally, the method’s error for the calculation of the levelized cost of storage (LCOS) was ten times smaller than the viability criterion ASHP- TES systems. This indicates that the method can be used to generate initial estimations of the system’s performance and viability. Because the method dispenses the use of sophisticated tools, it may assist more engineers, architects, contractors, and clients in quickly deciding whether further exploration of an ASHP-TES project is worthwhile. While the results are encouraging, the impact of the simplifications adopted in the method must be further explored.

Heat pumps can be combined with thermal storage to enable the heat pump to operate at the time of day desired by the operator. Moving the heat pump operation to a different time of day can reduce the power draw during the grid peak time and can affect the efficiency by resulting in heat pump operation at different ambient temperatures. This work explores the performance limits of HP-TES systems. Fundamental limits are explored (assuming Carnot heat pump operation), and realistic limits are explored (using realistic vapor compression cycle performance). The performance is evaluated with respect to efficiency and power demand during peak grid times. This manuscript investigates the potential for both energy and demand reduction offered by heat pumps (HPs) integrated with thermal energy storage (TES) systems, employing both analytical and numerical modeling techniques for HPs. Simulation analysis explores the conceivable temperature configurations in HP-TES systems, including the application temperature, TES temperature, and ambient temperature. The findings suggest that maximum energy savings are predominantly achieved when the temperature of the TES closely aligns with that of the application. Conversely, a substantial temperature differential between the TES and the application yields the greatest reduction in peak demand. Furthermore, the potential for energy conservation increases with the increase in the amplitude of ambient temperature fluctuations.

The existing ground source heat pump (GSHP) systems are widely utilizing R410A as the primary refrigerant. As a replacement for R22, which was phased out due to its ozone-depleting potential, R410A can operate at a higher pressure and deliver improved energy efficiency in heat pump systems. However, the urgent objective of achieving carbon neutrality in the coming decades necessitates a continuous reduction in greenhouse gas emissions. This can be realized by incorporating more low-global warming potential (low-GWP) refrigerants into GSHP systems. Simultaneously, it is crucial to recognize that the subsurface environment may affect the thermal efficiency of the refrigeration system, which has not been examined in documented studies. In this study, a typical residential building operating under cold climate is first created and dynamically modeled in TRACE3D Plus. Furthermore, two most potentially applicable alternatives—R454B and R32 are explored as the working fluids in a GSHP model in TRNSYS. To evaluate the long-term impacts of different refrigerants on the subsurface overcooling and the degradation of GSHP efficiency, a lifespan operation of GSHP combining the building load profile is simulated. In the meantime, to assess the responsiveness of different refrigerants to environmental fluctuations, sets of heterogeneous geological units, comprised of varied ground thermal conductivities, layered thickness, and ground heat capacities, are integrated into the model as inputs. The varied thermal behaviors exhibited by refrigerants are carefully examined to analyze their influence on the heat pump operation. The results showed non-negligible differences when obtaining the annual coefficient of performance (COPs) and excessive ground cooling resulting from different combinations of refrigerants and diverse geological contexts. This study may provide insights for future optimization of GSHP systems.