MEAN RADIANT TEMPERATURE
(MRT): The combined effects of surface temperatures and angles of exposure at one point in space
EFFECT OF CLOTHING
(CLO): The measurement of the thermal resistance of clothing from the skin to the outer layer of clothing ranging from 0 to 5:
1 CLO : 0.155 m2/v 2°F
Passive solar architecture is a symbiotic response to the context in which it occurs. Although the following pages look at different aspects, the relation to each other and the building as a whole is equally as important as specific data.
One of the tenets of modern design was that with industrial progress, architecture could be freed of the constraints of the site. Passive solar design reverses that assumption, proclaiming that the site is a major design consideration not only compositionally, but in regard to thermal sources and sinks the site offers to serve the building’s energy needs. [2]
The unique characteristics of site can be expressed in the concept of microclimate. While it is generally recognized that steep coastal mountains and island biomes can have great micro-climatic variations, great variation can also exist elsewhere. The classic study of micro-climate done in the Neotoma Valley, Ohio, USA, using 109 weather stations in a 0.65 km2 area illustrates how even in mid-continent, micro-climates can greatly vary. [1]
Difference in highest temperature ......... 21 °C [38°F]
Difference in Jan lowest temperature....... 22° C [40°F]
Difference in the time of the last spring frost ....73 days
Difference in the time of the first fall frost .... 64 days
Difference in the number of frost free days ....... 152 days
Solar radiation in and night sky radiation out are the most important determinants of micro-climate. These are effected by the sun’s path, the topography, the landscape and type of land cover. It is the interaction of site elements and climatic elements that determine specific micro-climates.
Hour by hour data is needed for the simulation models described on pages 53–54. Digital monitoring devices have made it easy to collect micro-climate data directly at the site. [95] This allows weather tapes from distant locations to be checked and adjusted.
Climate data is available from a wide range of sites see: Http://apps1.eere.energy.gov/buildings/energyplus/cfm/weather_data.cfm For USA http://www.ncdc.noaa.gov/oa/climate/regionalclimatecenters.html.
The choice of building site should take into account micro-climate and other specific site characteristics. Shown below are many of these site aspects for choosing a site for residential scale construction.
Historic hill-towns of Southern Europe illustrate the many advantages of using a ‘military crest’ building site.
Psychrometrics chart allows the plotting of specific climatic data in relation to standard comfort zone and appropriate passive design strategies.
Recent research has shown how strongly comfort is influenced by mean radiant temperatures and expectations.
| In areas of strong sun & high cooling loads, reflected radiation (mostly from the ground) can be an important aspect of heat gain in the cooling season. Reflected radiation can be utilized for heating when snow conditions exist. | Direct solar radiation has the spectral characteristics shown below. Recent advances in glazing have given us the ability to be selective in regard to the short wave spectrum. | In areas of low irect sun strength and dominant overcast skies, diffuse radiation reflected from the sky and clouds can be major components of the radiation a building receives. | In areas with clear night sky and low humidity, night sky radiation can be up to one third of daily direct insolation and a substantial cooling resource. |
Equally important is how a passive solar building transfers energy internally, one unique characteristic is that heat is transfered by radiation rather than convection using hot or cool air. This means the mean radiant temperature and the interior surfaces of the building are similar. The result is a greater feeling of comfort and well being.
This type of control is very straight forward for the seasonal extremes, but becomes more complex in spring and fall. This is because the thermal mass of the earth tilts the seasons while the sun angles are the same; early spring is usually a heating season and early fall is usually a cooling season. This seasonal variation is likely to become less predictable with changes in weather patterns, so the trend should be an increase in the demand for adjustable solar control devices for overhangs and intermittent use of shade cloths.
For calculation process, see page 47.
Passive considerations need to be part of each phase of work starting with programming. Leaving passive considerations out of the early phases of work, and attempting to graft them on later as illustrated below, results in less cost effective integration. Trying to add passive solar and sustainability elements at the end of a project’s design or during construction often hinders success.
Passive Solar Considerations & Relative Cost ($, ∈) per Design Phase
The myth that passive solar buildings cost more to build has been fueled by this lack of commitment to passive strategies throughout the design and construction process. In reality, passive solar buildings can be similar in cost to conventional buildings and often less because they often reduce the need for expensive mechanical systems which can often be ⅓ the cost of a building. Reducing mechanical systems to the status of back-up systems saves in up-front capital costs as well as operating costs.
The importance of the programming phase of work as a prerequisite to the later phases cannot be overestimated as it greatly affects each successive decision as illustrated on the following page.
The following example is for multi-story moderate-to-high density housing. It illustrates the relationship between programming and planning for passive solar architecture.
In many locations solar access and night ventilation with some thermal mass can provide most of the heating, cooling and day-lighting needs of this building type because the heat gain/loss coefficient is small for compact units of this nature.
To be equitable and effective, all units need direct solar access and night ventilation capability while maintaining acoustical privacy. Unless this is part of the program statement however, the double loaded corridor circulation decision is likely because it is assumed to be simple and inexpensive. Unfortunately, it also negates equitable and efficient passive conditioning.
Another approach that would fulfill the passive solar program goal would be clustered units around a vertical circulation element.
A demographic look at the history of world population growth reveals patterns that repeat though three successive cultural eras.
Each era has a different resource and energy basis described by their names.
These patterns also reveal three different times of stress, the third of which we are in at the moment. We are also in the process of evolving a new cultural era based on information and sustainability.
Continuity and sustaining human society depends on making this transition.
Passive solar architecture is a key part of this transition.
The transition implied on page 27 will require changes in the way we use energy and resources but also will affect our perceptions and goals.
Architecture is a cultural and social artifact as much as a technical planning and construction process and therefore historically has carried a message of the times expressed in space, order, form and materials as well as functions served.
Similarly, passive solar architecture will have this power as it becomes more common and robust. The attitudes on the right side of this chart need to become part of our architectural vocabulary.
