Ivica Glavan

Nighttime setback is widely applied in residential heating, yet its net benefit for inverter-driven air-to-air heat pumps in high-thermal-mass buildings remains insufficiently characterised in Mediterranean climates. This study evaluates continuous operation (R1), timer-based setback (R2), and reactive threshold-based control (R3) for a residential building in Zadar, Croatia, using a validated physics-based model calibrated with high-resolution IoT measurements over a 180-day heating season. Simulations cover building heat loss coefficients of 60–120 W/K and both 3.5 kW and 5 kW heat pumps. Timer-based setback increases electricity consumption by 1.8–4.5% (3.5 kW) and 5.4–6.3% (5 kW) relative to continuous operation, driven by a morning recovery penalty. Reactive control eliminates thermal comfort deficits but offers no energetic benefit. Both effects generalise across heat pump capacity, with penalty magnitude scaling with unit size. A formal energy balance shows that, for the timer-based and reactive setback rules investigated here, the recovery term is set by the thermal state at the start of reheating and is not removed by better knowledge of the same weather. The conclusions apply to high-thermal-mass masonry dwellings of the type investigated; they are not intended for lightweight or low-thermal-mass envelopes, where nocturnal decay and the setback energy balance can differ. These findings provide a quantitative basis for assessing conventional setback rules in this building class.

Air-to-air heat pumps are a key technology for improving energy efficiency and reducing carbon emissions in residential buildings, yet their optimal control remains challenging under real-world conditions. This study evaluates the performance of a hybrid physical–LSTM model for controlling an air-to-air heat pump in a residential building in Zadar, Croatia. The hybrid framework integrates a first-order energy balance model of the building envelope with LSTM-based temperature correction using adaptive weighting. The physical model was calibrated and validated against 52,128 real IoT measurements collected during the 2024/2025 heating season, achieving high accuracy (RMSE ≈ 0.076 °C). Rolling one-day and continuous multi-day closed-loop simulations (up to 15 days) show that the hybrid model yields slightly lower RMSE in long-term runs compared to the pure physical model. However, this apparent statistical improvement is accompanied by systematic underestimation of indoor temperature and significantly higher simulated energy consumption. The results indicate that the observed effect originates from an implicit virtual heat flux introduced by the LSTM correction, which affects thermodynamic consistency in closed-loop operation. The findings highlight that short-term error metrics such as RMSE alone are insufficient for evaluating hybrid models intended for model predictive control (MPC). The main contribution of this study is the explicit demonstration and quantification of an implicit virtual heat flux generated by the LSTM correction in closed-loop multi-day operation, which leads to misleading statistical improvements while causing significant thermodynamic inconsistency and energy overconsumption. In 15-day continuous simulations the hybrid model (ω = 0.05–0.10) caused an indoor temperature underestimation of 1.25–1.31 °C and increased simulated electricity consumption by more than 300% (316 kWh vs. 72 kWh) compared to the physical model. These results have direct implications for the development of reliable digital twins and model predictive control strategies in residential HVAC systems.

This paper analyzes the process of energy transition from traditional solid fuel heating to an air-to-air (A2A) heat pump-based heating system. Special emphasis was placed on the implementation of new technologies for improved management of energy systems, aiming to elevate both comfort levels and energy efficiency. This paper explores the use of the open-source software Home Assistant as an integration platform for home automation, designed to manage smart home devices while preserving local control, user privacy, and increasing cybersecurity. The proposed hardware platform includes a Raspberry Pi with appropriate IoT modules, providing a flexible and economically viable solution for household needs.

This paper analyzes the process of energy transition from traditional solid fuel heating to an air-to-air (A2A) heat pump-based heating system. Special emphasis was placed on the implementation of new technologies for improved management of energy systems aiming to elevate both comfort levels and energy efficiency. This paper explores the use of open-source Home Assistant software as an integration platform for home automation, designed to manage smart home devices while preserving local control, user privacy and increasing cybersecurity. The proposed hardware platform includes a Raspberry Pi with appropriate IoT modules, providing a flexible and economically viable solution for household needs.

The paper deals with the topic of refrigerants, their historical evolution, applied legislation and trends in maritime affairs. Regarding the Environmental care paper considers the impact of refrigerants of refrigeration systems, both on stationary plants on land and on plants on board. New regulations of the European Union, and IMO maritime organization, require the reduction and complete abolition of harmful synthetic refrigerants and the introduction of new refrigerants that will have a significant economic and environmental impact. The trend is the introduction of natural refrigerants as a replacement for existing environmentally unacceptable ones. On-board refrigeration systems introduce natural refrigerants such as ammonia and carbon dioxide into applications that require lower refrigeration temperatures. Absorption cooling plants are introduced into air conditioning applications on board the ship. They work with water/lithium bromide (H2O/LiBr) mixtures, thus increasing the efficiency of the plant and reducing the impact on the environment.

The paper discusses the possibility of applying the trigeneration energy concept (cogeneration + absorption cooling) on diesel-powered refrigerated ships, based on systematic analyses of variable energy loads during the estimated life of the ship on a predefined navigation route. From a methodological point of view, mathematical modeling of predictable energy interactions of a ship with a realistic environment yields corresponding models of simultaneously occurring energy loads (propulsion, electrical and thermal), as well as the preferred trigenerational thermal effect (cooling and heating). Special emphasis is placed on the assessment of the upcoming total heat loads (refrigeration and heating) in live cargo air conditioning systems (unfrozen fruits and vegetables) as in ship accommodations. The obtained results indicate beneficiary energy, economic and environmental effects of the application of diesel engine trigeneration systems on ships intended for cargo transport whose storage temperatures range from −25 to 15 °C. Further analysis of trigeneration system application to the passenger ship air conditioning system indicates even greater achievable savings.

Hydraulic system of the fuel oil supply to the electronically controlled main engine resolve the problem of the optimized control of the engine which act beneficially to the specific fuel oil consumption and a reduction of the NOx emission. The assumed numerical example is giving insight to the power consumption for the hydraulic control of the fuel oil injection and exhaust valve operation for the two stroke marine engine. The calculated power is higher for the fuel oil pressure booster operation compared to the exhaust valve actuator for about 88%, which is resulted due to the higher operating pressure of the fuel injection to the cylinder, versus lower operating pressure required for the opening of the exhaust valve.

The validity of the gas turbine unit model largely depends on the accuracy of the flue gas temperature value calculation at the gas turbine inlet (TIT). This temperature is determined by the maximum combustion temperature. In variable running mode, the temperature value is regulated by changing the ratio of air and fuel at the inlet to the combustion chamber. The paper presents a model of a gas turbine combustion chamber using Modelica, an object-oriented language for modeling complex physical systems with the aim of determining the temperature of combustion flue gases, specific heat capacity, enthalpy, and flue gas composition at different gas turbine loads.

This paper presents three approaches for isentropic, energy, and exergy evaluations of a low-duty liquid natural gas (LNG) vapor turbo compressor during exploitation on a conventional LNG carrier. The evaluation was conducted on the measured performance parameters under 22 various turbo compressor operating regimes. The turbo compressor performance was evaluated in the temperature span from −69 to −105 °C and during changes in the rpm of the main propulsion turbine and, consequently, the main boiler load. The results show that the highest measured turbo compressor isentropic efficiency is in agreement with the manufacturer specifications, equaling 75.23% at a main propulsion turbine rpm of 53.5. At the highest measured loads and rpm, the turbo compressor energy and exergy efficiencies reach the highest values of 57.81% and 28.51%, respectively. In each observed operating regime, the influence of the ambient temperature change on the turbo compressor exergy efficiency was investigated. At the lowest and the highest measured loads, turbo compressor energy and exergy flow streams are presented in a Sankey diagram. Techniques for cargo temperature maintenance during the ship voyage are presented, as the results show that low suction gas temperatures influence turbo compressor efficiency.

The system of separate production of heat energy from a boiler and cooling energy from a compression refrigeration unit powered with electrical energy from an external network can be replaced with a CCHP system based on the operation of...