## 3 Conventional BEMS Techniques
### Energy Efficiency
#### 3.3.2 Active Methods  
Active energy-saving technology has also been widely employed to lower the energy consumption of buildings. Active measures include enhancing HVAC systems, energy-efficient appliances, lighting systems, and the use of renewable energy, as well as distributing energy as efficiently as possible while ensuring occupant comfort [67]. The active systems were also designed to take advantage of various renewable energy sources, such as solar thermal, free cooling with night air, or geothermal heat, by utilizing thermal energy storage systems to shift heating and cooling loads [71].  
To improve a building's energy efficiency, a variety of active approaches have been adopted. Heat pumps and boilers, for example, are active tactics in traditional air-conditioning. Furthermore, advancements in compressor technology and hybrid systems help to increase heat pump efficiency [72]. Active techniques, on the other hand, have some insurmountable restrictions. A heat pump's coefficient of performance (COP) is rarely greater than six, whereas a boiler's combustion efficiency is far less than 100%. Heat pumps and boilers are used in HVAC systems to overcome a significant temperature difference between indoor and outside temperatures (e.g., 10C in summer and 20C in winter). Inevitably, this would result in the system's overall energy efficiency being low [73].  
Substantial energy savings can also be achieved in existing mechanical HVAC systems. Air conditioners that are properly maintained can reduce peak demand, conserve energy, and save running expenses. Correcting low air-flow rates, rectifying refrigerant overcharging and undercharging, and addressing duct leaks are just a few examples of individual repair techniques. Heat pumps and other high-efficiency mechanical systems can be usedto replace older technology equipment in applications where air-conditioning is required, resulting in significant energy savings [24].  
After HVAC systems, lighting uses a significant amount of electrical energy in buildings. Lighting efficiency is the most cost-effective active technique, and air conditioning efficiency was the second [74]. A well-designed lighting system can conserve energy while also providing the maximum visual comfort to building occupants [75]. Standard groups such as the European Committee for Standardization have developed standards to guide and provide specifications and requirements for technological systems in order to achieve lighting system energy efficiency in buildings. The European Standard EN12464-1 (i.e., for interior design) [76], the European Standard EN12464-2 (i.e., for external design) [77], and the European Standard EN15193 (i.e., for lighting system) [78], and the European Standard EN15193 (i.e., for performance evaluation) [79]. LED is a prominent energy-efficient lamp, used in lieu of conventional lamps, with a greater photometric performance (e.g., luminous flux, color rendering index, and luminous efficiency) and easier to regulate when compared to other lamps [75]. The regulation of a lighting system is the most important factor to increase energy savings in buildings, and it has received a lot of attention from researchers in the previous decade. Lastly, it is recommended that traditional appliances should be replaced with five-star energy-efficient appliances [80]. Many other energy-saving strategies are also employed in a low-energy-consumption structure. Included are energy-saving air conditioning designs, equipment energy-saving, behavior energy-saving, and energy-saving by operation adjustment, among other things.  
In summary, active and passive techniques each have their own set of benefits and drawbacks, and neither can be considered a replacement for the other [81]. To take advantage of the benefits of both tactics, a growing number of passive strategies are being coupled with active strategies or used actively. As a result, a combination of passive and active technology is both promising for energy savings and good interior environment assurance.