## 1 Introduction  
While most countries are implementing policies to increase the energy efficiency of buildings, the average energy consumption per person in the building sector has remained practically unchanged since 1990 [1]. Progress has not been rapid enough to compensate for the growth in the floor area (3% per year) and the increasing energy demand in buildings. Building heating, ventilation and air conditioning (HVAC) systems account for half of the energy consumed in the European Union (EU), much of which is wasted through inefficient use [2]. The heat energy demand of new buildings has significantly decreased with the implementation of heat transfer reduction measures under the Energy Performance of Buildings Directive [3]. At this point, ventilation and air conditioning become the dominant energy use systems in buildings. Additionally, the number of offices is growing both in Lithuaniaand around the world, leading to a rise in energy consumption. About a quarter of energy use in the buildings sector is accounted for by office buildings, where more than 70% of final energy is consumed by HVAC systems [4]. HVAC systems achieve greater energy efficiency with new monitoring and control technologies [5]. Therefore, these systems specifically require new innovative technological solutions to maintain proper indoor air quality and to use energy efficiently. New knowledge is needed to identify opportunities for creating more advanced measures that contribute to the EU and Lithuanian strategic goals of reducing energy demand and global greenhouse gas (GHG) emissions through the integration of renewable energy technologies. The primary energy from renewable sources, such as wind energy and solar energy (photovoltaic solar cells), is typically converted into electricity. Since heat pumps require electricity to operate, they become the potential thermal energy generators in HVAC systems, capable of making a contribution to solving the problem of increasing energy efficiency in the building sector. Regarding [PERSON] et al. [6], the future breakthroughs for integrated renewable energy systems are the use of Machine Learning (ML) with Artificial Intelligence (AI); improved battery technologies to reduce their pollution; a combination of heat pumps and phase change energy storages (PCES); and building design for lower lifecycle emissions. While heat pumps combined with thermal and electrical storages are still under development, further improvements in the efficiency and cost-effectiveness of heat pump technologies are needed. [PERSON] et al. [7] investigated the performance and cost of different compressor types for small-scale heat pumps (under 30 kW). The authors indicated that rotary vane and scroll compressors resulted in higher coefficient of performance (COP) values (around 3 for rotary vane) at an air temperature of +7 \({}^{\circ}\)C, while reciprocating pistons were more competitive at an air temperature of \(-15\)\({}^{\circ}\)C (10% higher than scroll). The cost-effective solution for residential buildings is rotary vane compressors for all locations studied. However, for the commercial buildings scroll compressors were the better choice in warm and mild weather, while reciprocating pistons performed slightly better in colder regions. Experimental studies of [PERSON] et al. [8] highlighted the potential of variable-speed control for single screw compressors in heat pumps. By adjusting the motor speed, the authors achieved a balance between efficiency and heating capacity. The study showed that single-screw compressors perform better at relatively high motor speeds. However, the optimal speed depends on the operating conditions, particularly the evaporation temperature. Therefore, for low evaporation temperatures (colder conditions), a combination of higher speed and liquid refrigerant injection can improve the overall performance of heat pump with a single-screw compressor. [PERSON] et al.