Conference Agenda

The development of efficient vapor compression systems represents a significant technical opportunity for the decarbonization of the refrigeration, air conditioning, and heat pump sector. An effective construction method for vapor compression cycles should be able to simultaneously optimize the refrigerant selection, cycle structure and cycle parameters simultaneously. In a previous study, the authors proposed the GraPHsep method, which has been demonstrated to overcome the constraints of existing methods. The GraPHsep method is capable of automatically constructing optimal vapor compression cycles with corresponding optimal refrigerants under certain application scenarios. In this paper, the GraPHsep method is applied to construct an efficient air conditioner using the zeotropic refrigerant R32/R1234yf. Based on that, a prototype system is developed. The experimental results demonstrate that the seasonal energy efficiency ratio (SEER) of the new system reaches 5.64, which is 24.7% higher than that of the conventional system using R32.

Powering the world on DC can help the future of renewable energy integration as most distributed energy systems are DC-driven (PV panels, batteries, etc.). Additionally, most residential appliances and devices typically run on DC. Therefore, cutting out conversion losses due to AC-DC rectification has the potential for enhancing system efficiency, increasing reliability, and reducing points of failure. In building applications, space conditioning can sometimes account for up to 40% of the energy consumed. With the electrification of heating becoming more prevalent, it is thus necessary to rethink how these devices can be powered in a more efficient manner. The objective of the present study is to elucidate trends in the steady state performance of a variable speed residential split-system Air Source Heat Pump (ASHP) by comparing its performance on AC versus DC power sources in both heating and cooling test modes as prescribed by AHRI Standard 210/240. This study presents a thermodynamic model and experimental results of the steady state conditions. Findings indicate that by running a variable speed system on DC power, system COP can increase by up to 8% in some cases. Future work will investigate dynamic testing of the DC-driven heat pump in a real home with a DC Nanogrid.

In the pursuit of Germany's 2045 greenhouse gas neutrality goal, one focus is on decarbonizing the building sector, particularly exchanging emission-intensive heating systems. While improving building insulation and adopting heat pumps is energetically ideal, the associated high costs pose accessibility challenges. To address this issue and simultaneously maintain thermal comfort while reducing overall emissions, hybrid systems promise to detach building renovation from investments in new technologies for the entire building. In this context, there is a need to evaluate hybrid systems in existing buildings.

This study proposes a cost-effective alternative for reducing emissions and enhancing comfort in existing buildings. Through dynamic simulations, we assess the integration of a single-split air conditioning unit in the living room of a single-family house, working in tandem with an existing gas condensing boiler. Various systems are modeled, including reference setups with a gas condensing boiler and an air-water heat pump, alongside three hybrid configurations with the air conditioning unit sized at 50%, 75%, and 100% of the living room's heating load. All systems are evaluated in two locations, employing ecological, economic, and comfort criteria.

Results from simulations in Düsseldorf and Ramsau indicate that the combination of a gas condensing boiler and a minimally sized air conditioning unit yields the highest reduction in emissions. Over a period of 15 years, economic analysis shows that hybrid systems are more cost-effective compared to the reference scenarios with stand-alone gas condensing boilers or stand-alone air-water heat pumps. Winter comfort concerns are minimal and can likely be mitigated through parameter optimization, while all hybrid systems significantly enhance summer comfort through active cooling.

In summary, this study highlights the considerable potential of hybrid systems to deliver cost-effective, and comfortable heating and cooling solutions in existing buildings to reduce the emissions. Future research should delve into airflow dynamics, humidity, and additional analyses to optimize and seamlessly integrate this promising hybrid solution.

Comprehensive exploration of heat pump responses to different refrigerant charge levels and compressor speed settings is crucial for devising optimal control and fault detection and diagnosis strategies. However, this aspect remains largely unexplored in the literature. This paper presents an experimental study to investigate the effect of the refrigerant charge level on the energy performance of variable-speed heat pumps based on laboratory testing. The laboratory tests were conducted under the A, E and B indoor and outdoor temperature conditions stipulated by the AHRI Standard 210/240. For each indoor and outdoor temperature condition, the cooling capacity and COP were determined from steady-state measurements at different combinations of refrigerant charge, compressor speed and supply-air flow rate. The results demonstrate that a 38% decrease in refrigerant charge from the manufacturer-suggested nominal charge level results in reductions of up to 23 % and 26 % in COP and cooling capacity, respectively, at nominal compressor/fan speeds. An increase of 28% in charge level results in a decrease in COP of up to 9 %, while the cooling capacity remains relatively constant. Overall, an increase in compressor speed results in a decrease in the COP and an increase in cooling capacity; an increase in supply-air flowrate results in an increase in cooling capacity.

Heat pumps are an increasingly attractive option for residential or commercial space heating applications. In the United States, where air conditioners are a standard for many buildings, an upgrade to air-source heat pumps is comparatively simple. The formation of frost on the outdoor heat exchanger coil surface leads to reduced heat transfer capabilities, compromising the heat pump's efficiency and capacity, a problem, especially for cold and humid regions. Defrosting is required to return the coil back to near original condition. The timing of the defrost initiation as well as the control of the defrost process are linked to the heat pump’s performance in the next heating cycle with many options for component speed paths during the defrost. Developing the necessary algorithms is challenging, and typical on-board platforms are more difficult to program than what can be done in higher level languages such as Python. This paper presents the development of a defrost controller prototyping platform using the Raspberry Pi microcomputer. Leveraging Raspberry Pi’s reliability, ease of debugging Python coding, ease of monitoring internal controller states, and cost-effectiveness, the controller ensures the seamless integration of the defrost function into prototype heat pumps.

Improving the overall efficiency and reliability of air source heat pumps in challenging environmental conditions requires robust defrost algorithms that can handle extreme ambient conditions as well as faulty sensors throughout the lifespan of the equipment. Robust controls contribute to the good reputation of heat pumps and by that to the advancement of sustainable and resilient heating technologies. The Raspberry Pi-based prototyping solution offers a practical and economical approach to control prototyping. Following a successful trial with the prototyping platform, there is potential for its utilization for a wide range of heat pump controls prototyping beyond solely defrosting controls.

This paper presents the lessons learned from a series of frosting and defrosting experiments using a commercial rooftop unit heat pump. In this paper, we focus on the role of psychrometric chamber control and the complications associated with the defrosting cycle. The transient nature of heat pump operation causes instabilities. Here, we explain the importance of careful control over the environment and the system to achieve reliable and repeatable results.
We found that the initial condition of the heat pump coil – dry vs. wet – at the start of the first frosting process changes the unit’s behavior. Starting experiments with a dry coil is a first step towards repeatable behavior. During heat pump operation, stability of temperature and humidity is required. Fluctuations in humidity due to controls or other issues cause non-repeatable behavior and can be caused by the humidifier controls or the unit defrosting cycle.
This study provides insight into the careful investigation of conducting tests on a rooftop unit at a lower ambient temperature, leading to frost on the outdoor coil. The crucial aspects of controlling the psychrometric chamber, humidity control, and managing the conditions during the defrost cycle play fundamental roles in ensuring the reliability and accuracy of the experiments. These investigations expand the understanding of rooftop units in challenging environments, supporting the insights of both the researchers and practitioners in HVAC.