Air-to-air heat pumps are used in today’s ventilation systems increasingly often as they provide heating and cooling for buildings. The energy transformation modes of these units are subject to constant change due to the varying outdoor air state, including temperature and humidity. When choosing how to operate and control energy transformers, it is important to be able to adapt effectively to the changing outside air conditions. Nowadays, modern commercial heat pumps offer two levels of control flexibility: a compressor with a variable speed and an electronic expansion valve. This combination of control elements has boosted the seasonal energy efficiency of heat pumps. For a long time, cycle control possibilities have been dominated by electronic controls. The authors of this paper aim to present an additional element to the traditional heat pump controls, which provides a third level of control over the cycle. To achieve the objective, experimental investigations of a heat pump integrated into a ventilation unit have been carried out under real-life conditions. The experiments involved varying the operating modes of the unit by adjusting the compressor speed, the position of the expansion valve, and the volume of the system loop. The study examined the performance characteristics of the heat pump and found that the performance of a variable-volume heat pump is comparable to that of a conventionally operated typical constant-volume heat pump system. In addition, the study found that by adding a third level of volume control to the active heating circuit, in combination with conventional controls, the heat pump’s heat output range could be extended by 69.62%. The study determined the variation of the heat pump cycle in the p-h diagram with the variation of the loop volume. The benefits and drawbacks of a heat pump with a variable-volume loop are discussed in this study.

The use of air-to-air heat pumps in modern air handling units is becoming more popular for building heating and cooling supply. However, the constantly changing state of outdoor air (temperature, humidity) means that energy conversion modes of heat pumps need to be flexible and able to efficiently respond to ambient air parameters. The ability to adapt to changes in ambient air conditions in an efficient and rational manner is a crucial factor to consider when selecting the operating mode and control of energy transformers. Modern commercial heat pumps typically have two control degrees of freedom, which include a variable-speed compressor and an electronic expansion valve. This combination of control components has previously helped to improve the seasonal efficiency of heat pumps. However, for a long time, focus was placed mainly on refining electronic control tools, without considering how changes in the thermodynamic cycle of heat pumps are correlated with energy demands for air preparation that correspond to outdoor temperatures. The authors of the article propose introducing an additional component into traditional heat pump control measures to provide a third degree of freedom for the cycle control. The experiments were conducted to assess the impact of the heat pump system volume on its thermodynamic operating cycle and it was found that the system volume parameter has the potential for regulation capabilities in controlling the operation of a heat pump. This technological solution has the potential to increase efficiency indicators of the air preparation process and warrants further development and study.

Heat pumps are becoming increasingly popular and are playing an important role in the heat and cooling supply chain of buildings sector. Although more than 160 years have passed since the manufacture of the first heat pump, this technology, designed to recover low­potential heat energy and its useful use, can be called an innovative and efficient energy transformer. Air­to­water and air­to­air heat pumps are becoming more popular today due to their installation cost compared to ground­to­water heat pumps. It has been noticed that recently air­to­air heat pumps are more often installed in modern air handling units. The main energy transformers in these air handling units are the air­to­air heat pump and recovery heat exchanger. In the scientific literature little attention is paid to the thermodynamic cycle of the combination of the air handling unit and the heat pump, as well as the search for new possibilities to change and control it, this is a potential direction that can increase the efficiency of the whole system. To study the control capabilities of the unit, an experimental stand of the heat pump built into the ventilation unit is needed; it would help validate the results of theoretical studies and investigate the possibilities of expanding the control. Therefore, this paper presents a review of the experimental stands of non­integrated heat pumps and integrated heat pumps in air handling unit. Based on this review, the basic requirements for the installation of an experimental stand of a heat pump built into an air handling unit are formulated and a conceptual scheme of this stand is drawn up.