Direct Solar Energy
consumption compared to the insulated building without phase-change material.
Passive solar system applications are mainly of the direct-gain type, but they can be further subdivided into the following main application categories: multi-story residential buildings and two-story detached or semi-detached solar homes (see Figure 3.2, left panel), designed to have a large equatorial-facing façade to provide the potential for a large solar capture area (Athienitis, 2008). Perimeter zones and their fenestration systems in office buildings are designed primarily based on daylighting performance. In this application, the emphasis is usually on reducing cooling loads, but passive heat gains may be desirable as well during the heating season (see Figure 3.2, right panel, for a schematic of shad- ing devices).
In addition, residential or commercial buildings may be designed to use natural or hybrid ventilation systems and techniques for cooling or fresh air supply, in conjunction with designs for using daylight throughout the year and direct solar gains during the heating season. These build- ings may profit from low summer night temperatures by using night hybrid ventilation techniques that utilize both mechanical and natural ventilation processes (Santamouris and Asimakopoulos, 1996; Voss et al., 2007).
In 2010, passive technologies played a prominent role in the design of net-zero-energy solar homes—homes that produce as much elec- trical and thermal energy as they consume in an average year. These houses are primarily demonstration projects in several countries cur- rently collaborating in the International Energy Agency (IEA) Task 40 of the Solar Heating and Cooling (SHC) Programme (IEA, 2009b)—Energy Conservation in Buildings and Community Systems Annex 52—which focuses on net-zero-energy solar buildings. Passive technologies are essential in developing affordable net-zero-energy homes. Passive solar gains in homes based on the Passive House Standard are expected to reduce the heating load by about 40%. By extension, systematic pas- sive solar design of highly insulated buildings at a community scale, with optimal orientation and form of housing, should easily result in a similar energy saving of 40%. In Europe, according to the Energy Performance of Buildings Directive recast, Directive 2010/31/EC (The European Parliament and the Council of the European Union, 2010), all new buildings must be nearly zero-energy buildings by 31 December 2020, while EU member states should set intermediate targets for 2015. New buildings occupied and owned by public authorities have to be nearly zero-energy buildings after 31 December 2018. The nearly zero or very low amount of energy required should to a very significant level be covered by RE sources, including onsite energy production using combined heat and power generation or district heating and cooling, to satisfy most of their demand. Measures should also be taken to stimu- late building refurbishments into nearly zero-energy buildings.
Low-energy buildings are known under different names. A survey car- ried out by Concerted Action Energy Performance of Buildings (EPBD) identified 17 different terms to describe such buildings across Europe,
including: low-energy house, high-performance house, passive house (‘Passivhaus’), zero-carbon house, zero-energy house, energy-savings house, energy-positive house and 3-litre house. Concepts that take into account more parameters than energy demand again use special terms such as eco-building or green building.
Another IEA Annex—Energy Conservation through Energy Storage Implementing Agreement (ECES IA) Annex 23—was initiated in November 2009 (IEA ECES, 2004). The general objective of the Annex is to ensure that energy storage techniques are properly applied in ultra- low-energy buildings and communities.The proper application of energy storage is expected to increase the likelihood of sustainable building technologies.
Another passive solar application is natural drying. Grains and many other agricultural products have to be dried before being stored so that insects and fungi do not render them unusable. Examples include wheat, rice, coffee, copra (coconut flesh), certain fruits and timber (Twidell and Weir, 2006). Solar energy dryers vary mainly as to the use of the solar heat and the arrangement of their major components. Solar dryers constructed from wood, metal and glass sheets have been evaluated extensively and used quite widely to dry a full range of tropical crops (Imre, 2007).
Active solar heating and cooling
Active solar heating and cooling technologies use the Sun and mechani- cal elements to provide either heating or cooling; various technologies are discussed here, as well as thermal storage.
In a solar heating system, the solar collector transforms solar irra- diance into heat and uses a carrier fluid (e.g., water, air) to transfer that heat to a well-insulated storage tank, where it can be used when needed. The two most important factors in choosing the correct type of collector are the following: 1) the service to be provided by the solar collector, and 2) the related desired range of temperature of the heat-carrier fluid. An uncovered absorber, also known as an unglazed collector, is likely to be limited to low-temperature heat production (Duffie and Beckman, 2006).
A solar collector can incorporate many different materials and be man- ufactured using a variety of techniques. Its design is influenced by the system in which it will operate and by the climatic conditions of the installation location.
Flat-plate collectors are the most widely used solar thermal collectors for residential solar water- and space-heating systems. They are also used in air-heating systems. A typical flat-plate collector consists of an absorber, a header and riser tube arrangement or a single serpentine