Experimental analysis of R454B charge optimization in a variable-speed brine-to-water heat pump: Performance comparison with R410A

Current energy efficiency regulations have led to the development of heat pumps equipped with variable speed compressors (VSC) and electronic expansion valves (EEV). To reduce the amount of refrigerant charge, heat pump manufacturers are developing units without a liquid receiver. As a result, the amount of refrigerant charge has a direct impact on the unit performance and needs to be optimized. Previous studies have shown the existence of an optimum refrigerant charge that maximizes the COP at a given operating condition. However, with VSC it is not clear if the optimum charge at a given compressor speed is the adequate one at other speed values. This paper analyzes the effects of the refrigerant charge amount on the performance of a brine-to-water heat pump equipped with a VSC and an EEV. For a given compressor speed, the variation of the COP with the charge showed similar trends and an optimum refrigerant charge could be identified. The charge for the maximum COP changes slightly with the compressor speed. The seasonal coefficient of performance (SCOP) is proposed as a better metric for evaluating the optimum charge. Results show that the SCOP is quite stable with the refrigerant charge.

The impact of the COVID pandemic has resulted in many people cultivating a remote working culture and increasing building energy use. A reduction in the energy use of heating, ventilation, and air-conditioning (HVAC) systems is necessary for decreasing the energy use in buildings. The refrigerant charge of a heat pump greatly affects its energy use. However, refrigerant leakage causes a significant increase in the energy use of HVAC systems. The development of refrigerant charge fault detection models is, therefore, important to prevent unwarranted energy consumption and {CO}_{2} emissions in heat pumps. This paper examines refrigerant charge faults and their effect on a variable speed heat pump and the most accurate method between a multiple linear regression and multilayer perceptron model to use in detecting the refrigerant charge fault using the discharge temperature of the compressor, outdoor entering water temperature and compressor speed as inputs, and refrigerant charge as the output. The COP of the heat pump decreased when it was not operating at the optimum refrigerant charge, while an increase in compressor speed compensated for the degradation in the capacity during refrigerant leakage. Furthermore, the multilayer perception was found to have a higher prediction accuracy of the refrigerant charge fault with a mean square error of ± 3.7%, while the multiple linear regression model had a mean square error of ± 4.5%. The study also found that the multilayer perception model requires 7 neurons in the hidden layer to make viable predictions on any subsequent test sets fed into it under similar experimental conditions and parameters of the heat pump used in this study.

Drop-in performance of the low-GWP alternative refrigerants R452B and R454B in an R410A liquid-to-water heat pump

Residential air-source heat pump with refrigerant injection and variable speed compressor: Experimental investigation and compressor modeling

Matts Bäckstöm assessed the selection criteria of heat exchangers for single-speed heat pumps in 1940, and his methodology and conclusions have been kept practically unchanged for almost a century. However, heat pump systems have evolved, especially with the introduction of variable-speed heat pumps. These new systems introduced a new degree of freedom to the heat pump design – the compressor speed – which is not considered by traditional design criteria.This study proposes novel design criteria to optimize the heat exchanger size when the heat pump unit can work under part-load conditions. The proposed method models how heat exchangers and variable-speed compressors perform under different loads and sizes. It considers part-load requirements based on climatic data (which are typically available in the standards) and considers economic factors such as initial investment and operation costs (which are dependent on the components's size). Given the mentioned information, a multivariable optimization algorithm is implemented to find the optimum heat exchanger size that minimizes the total cost. To validate the proposed approach's effectiveness, it was exemplified by determining the optimum size of a coil and a plate heat exchanger for a domestic variable-speed heat pump. The results demonstrate that using traditional criteria could lead to over-dimensioning heat exchangers in variable-speed heat pumps which could incur in an increase of costs of 5% for the studied case.

Effects of simultaneous soft faults in a reversible and variable-speed air-to-water heat pump

Approaching optimum COP by refrigerant charge management in transcritical CO2 heat pump water heater

Refrigerant based heat pump systems are becoming an integral system in electric vehicle architectures due to their high efficiencies in providing heating and cooling to people and components within the car. An important component in heat pump systems that determines optimal efficiency is the amount of refrigerant. As such, the capability to model refrigerant charge helps quantify the health status of the heat pump system, whereby the lack or over abundance of refrigerant in a heat pump refrigerant system leads to various other component failures, e.g., liquid slugging, compressor overheating, material fatigue in heat exchanger, and degraded/stuck expansion valves. In designing a heat pump system, engineers need to perform a set of design of experiments to determine an optimal refrigerant charge based on a set of performance metrics in the presence of certain noise factors. This optimal refrigerant charge provides conditions where the heat pump system operates efficiently in both heating and cooling, in addition to facilitating operational conditions that will not lead to secondary component degradation or damage. The search for optimal refrigerant charge is classified as refrigerant charge determination, whereby engineers incrementally increase the refrigerant in the heat pump system in operation of heating/cooling and collect data about performance metrics. Some of the key performance metrics used to determine efficiency of a heat pump system include i) compressor inlet superheat temperature, ii) condenser outlet subcool temperature, iii) compressor high side pressure, iv) compressor low side pressure, v) condenser outlet pressure, and vi) condenser quality estimate. Furthermore, this process follows design of experiments concepts and is performed for both heating and cooling modes of operation. In this paper, we leverage refrigerant charge determination as a training data source to develop refrigerant charge models, where several performance metrics are health indicators used as model inputs and the amount of refrigerant added to the heat pump system are ground truth refrigerant charges used as model outputs. In this paper we develop regression models to estimate the total refrigerant charge, which is used to classify different health states of refrigerant based on levels of performance degradation corresponding to specific refrigerant charge thresholds. We trained a robust linear regression model using this charge determination data and found that the worst case estimation error was less than 10% with respect to the refrigerant charge grouth truth.

Heat pump drying (HPD) is an energy efficient and environmentally friendly drying method. Heat pumps (HPs) deliver more heat during the drying process than the work input to the compressor. The performance of HP and the dryer is significantly affected by the surrounding conditions. Therefore, the present study investigates the performance of an open and completely closed air duct HPD systems. Also, the influence of refrigerant charge and condenser fan speed was evaluated. The results demonstrated that, the completely closed air duct HPD system had slightly higher coefficient of performance (COP) and heating capacity than the open HPD system because of the heat from the moist exhausted air from the dryer were recovered at the evaporator. However, the drying air temperatures for the closed HPD system were lower than that of open system. This was due to the circulation of cold air from the evaporator. Also, COP increased with the increase of refrigerant charge in both systems. But, at high refrigerant charge the COP values for both systems were decreased because of the decrease in heating capacity which was possible due to the accumulation of refrigerant in the condenser. Similar trend of results were observed at high condenser fan speed, this was due to the increased fan power consumption. Therefore, for the system developed, the optimum refrigerant charge was observed to be 1650 grams and 840 rpm of condenser fan speed (60% of the full speed) and the obtained COP values at this point were 3.85 and 3.75 for the closed and open HPD system, respectively.

Mixed-integer model predictive control of variable-speed heat pumps

Cold climate heat pumps (CCHPs) expand the heat pump market to climates where heating demand is dominant. They can achieve more than 70% energy savings compared with electric resistance heating and operate at lower cost than using tank-stored propane to fuel a furnace. A high-efficiency heat pump with a heating seasonal performance factor (HSPF)—as defined by the Air-Conditioning, Heating, and Refrigeration Institute (AHRI 2008)—greater than 10.0 would be more efficient than gas heating in terms of source energy. However, developing a cost-effective CCHP involves some challenges. A typical single-speed, air-source heat pump (ASHP) with an HSPF of 7.7 Btu/Wh does not work well under cold outdoor temperature conditions typical of cold climate locations for four major reasons: Discharge temperature is too high—The low suction pressure and high compression pressure ratio at low ambient temperatures causes high compressor discharge temperatures in excess of the maximum limit for many of the compressors on the market. Furthermore, system charge of a heat pump is usually optimized in the cooling mode, which leads to overcharge conditions in the heating mode, further increasing the discharge temperature.; Heating capacity is insufficient if sized to meet the building design cooling load— Heating capacity of a single-speed heat pump decreases with ambient temperature. The heating capacity at -13°F (-25°C) typically decreases to 20%–40% of the rated heating capacity at 47°F (8.3°C) (~equivalent to the rated cooling capacity at 95°F [35°C]). Therefore, a single-speed heat pump, typically sized to match the building design cooling load, cannot provide adequate heating capacity to match the building heating load at low ambient temperatures. The capacity deficit is filled by inefficient resistance heat, thus lowering the system efficiency and significantly increasing power demand.; Cyclic loss is significant if sized to meet the building design heating load—If a single-speed heat pump is sized to meet the heating load, it will be significantly oversized relative to the cooling load in many cold climates. This will cause excessive on/off cyclic loss during the cooling and heating operations at moderately low ambient temperatures. Thus, capacity modulation capability (e.g., using a variable-speed or multi-stage compressor) is necessary for a CCHP, which uses its full capacity to meet the peak heating load and partial capacity to meet the cooling and part-load heating loads.; Coefficient of performance (COP) is low—Heating COP degrades significantly at low ambient temperatures owing to the large temperature difference between the heat source and sink. A target CCHP should be sized to meet the building design heating load while minimizing the cyclic loss for the cooling and heating operations at moderate ambient temperatures.

Influence of refrigerant charge and condenser area on direct-expansion solar-assisted heat pump system for radiant floor heating

Performance improvement of heat pumps by optimizing refrigerant charge using novel variable liquid-line length system

Reversible heat pumps are increasingly adopted for meeting the demand for space heating and cooling in buildings. These technologies will play a key role not only in the decarbonization of space air conditioning but also in the development of 100% renewable energy systems. However, to assess the achievable benefits through the adoption of these technologies in novel applications, reliable models are needed, capable of simulating both their steady-state operation and dynamic response at different conditions in terms of heating loads, outdoor temperatures, and so on. The operation of heat pumps is often investigated by highly simplified models, using performance data drawn from catalogs and paying scarce attention to the critical influence of controllers. In this respect, this paper proposed an integrated thermodynamic and control modeling for a reversible air-to-water heat pump. The study considered a heat pump alternatively equipped with variable-speed compressors and constant-speed compressors with sequential control. The developed modeling was then used to investigate the operation of an air-to-water heat pump serving an office building in Italy. Results show that the model provided insights into the transient operation of variable-speed heat pumps (e.g., the settling time). Regarding constant-speed heat pumps, the model provided hints of interest to the control engineer to prevent, in the examined case study, the risk of quick compressors cycling on low-load heating days or when low-temperature heating devices are supplied. Finally, using a control strategy based on a heating curve for the variable-speed heat pump, results show the potential for a sensible increase in the average coefficient of performance, from 17% up to 50%.

Thermodynamic analysis of a novel heat pump water heater with two-stage heating for a great rise of water temperature