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Direct Solar Energy

active heating and cooling, PV electricity generation, CSP electricity generation and solar fuel production. Each section also describes appli- cations of these technologies.


Passive solar and daylighting technologies

Passive solar energy technologies absorb solar energy, store and dis- tribute it in a natural manner (e.g., natural ventilation), without using mechanical elements (e.g., fans) (Hernandez Gonzalvez, 1996).The term ‘passive solar building’ is a qualitative term describing a building that makes significant use of solar gain to reduce heating energy consump- tion based on the natural energy flows of radiation, conduction and convection. The term ‘passive building’ is often employed to emphasize use of passive energy flows in both heating and cooling, including redis- tribution of absorbed direct solar gains and night cooling (Athienitis and Santamouris, 2002).

Daylighting technologies are primarily passive, including windows, sky- lights and shading and reflecting devices.A worldwide trend, particularly in technologically advanced regions, is for an increased mix of passive and active systems, such as a forced-air system that redistributes pas- sive solar gains in a solar house or automatically controlled shades that optimize daylight utilization in an office building (Tzempelikos et al., 2010).

The basic elements of passive solar design are windows, conservatories and other glazed spaces (for solar gain and daylighting), thermal mass, protection elements, and reflectors (Ralegaonkar and Gupta, 2010).With the combination of these basic elements, different systems are obtained: direct-gain systems (e.g., the use of windows in combination with walls able to store energy, solar chimneys, and wind catchers), indirect-gain systems (e.g., Trombe walls), mixed-gain systems (a combination of direct-gain and indirect-gain systems, such as conservatories, sunspaces and greenhouses), and isolated-gain systems. Passive technologies are integrated with the building and may include the following components:

  • Windows with high solar transmittance and a high thermal resis- tance facing towards the Equator as nearly as possible can be employed to maximize the amount of direct solar gains into the liv- ing space while reducing heat losses through the windows in the heating season and heat gains in the cooling season. Skylights are also often used for daylighting in office buildings and in solaria/ sunspaces.

  • Building-integrated thermal storage, commonly referred to as ther- mal mass, may be sensible thermal storage using concrete or brick materials, or latent thermal storage using phase-change materials (Mehling and Cabeza, 2008).The most common type of thermal stor- age is the direct-gain system in which thermal mass is adequately distributed in the living space, absorbing the direct solar gains. Storage is particularly important because it performs two essential functions: storing much of the absorbed direct solar energy for slow


Chapter 3

release, and maintaining satisfactory thermal comfort conditions by limiting the maximum rise in operative (effective) room temperature (ASHRAE, 2009). Alternatively, a collector-storage wall, known as a Trombe wall, may be used, in which the thermal mass is placed directly next to the glazing, with possible air circulation between the cavity of the wall system and the room. However, this system has not gained much acceptance because it limits views to the outdoor environment through the fenestration. Hybrid thermal storage with active charging and passive heat release can also be employed in part of a solar building while direct-gain mass is also used (see, e.g., the EcoTerra demonstration house (Figure 3.2, left panel), which uses solar-heated air from a building-integrated photovoltaic/ther- mal system to heat a ventilated concrete slab). Isolated thermal storage passively coupled to a fenestration system or solarium/sun- space is another option in passive design.

  • Well-insulated opaque envelope appropriate for the climatic condi- tions can be used to reduce heat transfer to and from the outdoor environment. In most climates, this energy efficiency aspect must be integrated with the passive design. A solar technology that may be used with opaque envelopes is transparent insulation (Hollands et al., 2001) combined with thermal mass to store solar gains in a wall, turning it into an energy-positive element.

  • Daylighting technologies and advanced solar control systems, such as automatically controlled shading (internal, external) and fixed shading devices, are particularly suited for daylighting applica- tions in the workplace (Figure 3.2, right panel). These technologies include electrochromic and thermochromic coatings and newer technologies such as transparent photovoltaics, which, in addition to a passive daylight transmission function, also generate electric- ity. Daylighting is a combination of energy conservation and passive solar design. It aims to make the most of the natural daylight that is available. Traditional techniques include: shallow-plan design, allowing daylight to penetrate all rooms and corridors; light wells in the centre of buildings; roof lights; tall windows, which allow light to penetrate deep inside rooms; task lighting directly over the work- place, rather than lighting the whole building interior; and deep windows that reveal and light room surfaces to cut the risk of glare (Everett, 1996).

  • Solariums, also called sunspaces, are a particular case of the direct- gain passive solar system, but with most surfaces transparent, that is, made up of fenestration. Solariums are becoming increasingly attractive both as a retrofit option for existing houses and as an integral part of new buildings (Athienitis and Santamouris, 2002). The major driving force for this growth is the development of new advanced energy-efficient glazing.

Some basic rules for optimizing the use of passive solar heating in build- ings are the following: buildings should be well insulated to reduce overall heat losses; they should have a responsive, efficient heating sys- tem; they should face towards the Equator, that is, the glazing should

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