## 6 Future Trends and Issues
### Net Zero Energy Building Concepts  
The primary enabler of a future smart building is the energy performance of buildings, which leads to energy flexibility, generation, and interaction between users. Energy retrofitting for net-zero energy buildings (NZEBs), in conjunction with passive control strategies, energy-efficient technologies, and RER integration, creates a balance between demand and generation while also taking grid integration into account. Smart home energy retrofitting strategies are adapted for the improvement of existing buildings along with key performance indicators for measuring the performance and success of acquiring sustainability in intelligent buildings [188]. A ZEB or NZEB implies the integration of renewable resources if weighted supply and weighted demand are equal to zero, focusing on energy storage systems and materials, energy routers, renewable resources, and plug-and-play interfaces [9; 11]. In addition, an NZEB is a preferred sustainable building style since it can meet its own energy needs while also producing surplus energy to feed into the grid. However, NZEBs are involved in more energy-related systems and grid linkages, and their energy systems are more intricate than in conventional buildings [189]. In [188], the EU adopted long-term goals to reduce 80-95% of carbon by 2050, where existing buildings will require a renovation rate greater than 3% per year in 2050 to achieve decarbonization. In addition, the occupant's behavior variation may lead to a 40% change in energy usage [11]. The UK was the first country to mandate NZEBs on a large scale in 2016 and France followed in 2020. The EU announced plans to initiate NZEBs in January 2021, and the US Department of Energy (DOE) targeted marketable zero-energy homes in 2020, followed by commercial zero-energy buildings in 2025. Other than that, ASHRAE (American Society of Heating, Refrigeration and Air-conditioning Engineers) plans to customize NZEBs in 2030, as shown in Figure 10. Moreover, Denmark has shown 100% renewable energy usage for heating and cooling systems [190]. In new and existing buildings, for example, it is possible to achieve total energy savings of 20%. Significant energy savings for cooling can be achieved by reducing external loads with proper building facade shading, reducing internal loads from lighting with  
Figure 8: Available BIPV systems on the market [185]. Figure 9: (**a**) BIPV facades powered by electricity to assist natural ventilation [186]; (**b**) an entrance with a PV skylight [187]. energy-efficient fluorescent lamps, and using natural cooling techniques such as ground, evaporative, and radiative cooling, as well as night ventilation [24]. In [191], the authors presented a unique yet viable and cost-effective method for advancing the state-of-the-art of smart energy buildings and achieving the real meaning of net-zero energy buildings. The proposed solution consisted of new trigeneration solar collectors without a battery, a highly creative form of heat pump integration that needed minimal size and expense, and the two-way interaction of a building with local energy networks. A zero-energy building is self-sufficient in terms of energy use, producing as much as it consumes. In ZEBs, both passive and active design methods were used to minimize energy consumption, with renewable energy strategies (mostly solar panels) used to meet demand following the reduction [192]. A study was conducted to investigate ZEBs in Southeast Asia, which was adapted from an existing structure and used a variety of passive and active design solutions appropriate for a tropical climate. According to the research findings, active and passive measures should be incorporated into building design to improve energy efficiency. Passive design, in particular, must be used on a broad scale in order to achieve significant energy savings [74]. The combination of active and passive design solutions toward ZEBs are shown in Table 3.  
Figure 10: The aim of countries is to customize NZEBs over the years.