Direct Solar Energy
possible phase change) while providing increased thermal resistance in the opaque state.
Increasingly larger window areas become possible and affordable with the drop in prices of highly efficient double-glazed and triple-glazed low- emissivity argon-filled windows (see Sections 3.4.1 and 3.4.2). These increased window areas make systematic solar gain control essential in mild and moderate climatic conditions, but also in continental areas that tend to be cold in winter and hot in summer. Solar gain control techniques may increasingly rely on active systems such as automati- cally controlled blinds/shades or electrochromic, thermochromic and gasochromic coatings to admit the solar gains when they are desirable or keep them out when overheating in the living space is detected or anticipated. Solar gain control, thermal storage design and heating/ cooling system control are three strongly linked aspects of passive solar design and control.
Advances in thermal storage integrated in the interior of direct-gain zones are still possible, such as phase-change materials integrated in gypsum board, bricks, or tiles and concrete. The target is to maximize energy storage per unit volume/mass of material so that such materi- als can be integrated in lightweight wood-framed homes common in cold-climate areas. The challenge for such materials is to ensure that they continue to store and release heat effectively after 10,000 cycles or more while meeting other performance requirements such as fire resistance. Phase-change materials may also be used systematically in plasters to reduce high indoor temperatures in summer.
Considering cooling-load reduction in solar buildings, advances are pos- sible in areas such as the following: 1) cool-roof technologies involving materials with high solar reflectivity and emissivity; 2) more system- atic use of heat-dissipation techniques such as using the ground and water as a heat sink; 3) advanced pavements and outdoor structures to improve the microclimate around the buildings and decrease urban ambient temperatures; and 4) advanced solar control devices allowing penetration of daylight, but not thermal energy.
In any solar building, there are normally some direct-gain zones that receive high solar gains and other zones behind that are generally colder in winter. Therefore, it is beneficial to circulate air between the direct- gain zones and back zones in a solar home, even when heating is not required. With forced-air systems commonly used in North America, this is increasingly possible and the system fan may be run at a low flow rate when heating is not required, thus helping to redistribute absorbed direct solar gains to the whole house (Athienitis, 2008).
During the summer period, hybrid ventilation systems and techniques may be used to provide fresh air and reduce indoor temperatures (Heiselberg, 2002). Various types of hybrid ventilation systems have been designed, tested and applied in many types of buildings. Performance tests have found that although natural ventilation cannot maintain appropriate
summer comfort conditions, the use of a hybrid system is the best choice— using at least 20% less energy than any purely mechanical system.
Finally, design tools are expected to be developed that will facilitate the simultaneous consideration of passive design, daylighting, active solar gain control, heating, ventilation and air-conditioning (HVAC) sys- tem control, and hybrid ventilation at different stages of the design of a solar building. Indeed, systematically adopting these technologies and their optimal integration is essential to move towards the goal of cost- effective solar buildings with net-zero annual energy consumption (IEA, 2009b). Optimal integration of passive with active technologies requires smart buildings with optimized energy generation and use (Candanedo and Athienitis, 2010). A smart solar house would rely on predictions of the weather to optimally control solar gains and their storage, ensure good thermal comfort, and optimize its interaction with the electricity grid, applying a mixture of inexpensive and effective communications systems and technologies (see Section 8.2.1).
Active solar heating and cooling
Improved designs for solar heating and cooling systems are expected to address longer lifetimes, lower installed costs and increased tempera- tures. The following are some design options: 1) the use of plastics in residential solar water-heating systems; 2) powering air-conditioning systems using solar energy systems, especially focusing on compound parabolic concentrating collectors; 3) the use of flat-plate collectors for residential and commercial hot water; and 4) concentrating and evacuated-tube collectors for industrial-grade hot water and thermally activated cooling (see Section 3.3.4).
Heat storage represents a key technological challenge, because the wide deployment of active solar buildings, covering 100% of their demand for heating (and cooling, if any) with solar energy, largely depends on developing cost-effective and practical solutions for seasonal heat storage (Hadorn, 2005; Dincer and Rosen, 2010). The European Solar Thermal Technology Platform vision assumes that by 2030, heat storage systems will be available that allow for seasonal heat storage with an energy density eight times higher than water (ESTTP, 2006).
In the future, active solar systems—such as thermal collectors, PV pan- els, and PV-thermal systems—will be the obvious components of roof and façades, and will be integrated into the construction process at the earliest stages of building planning. The walls will function as a com- ponent of the active heating and cooling systems, supporting thermal energy storage by applying advanced materials (e.g., phase-change materials). One central control system will lead to optimal regulation of the whole HVAC system, maximizing the use of solar energy within the comfort parameters set by users. Heat- and cold-storage systems will play an increasingly important role in reaching maximum solar thermal contributions to cover the thermal requirements in buildings.