Direct Gains and Sun Shading - June 2

Hello Everyone,

Let’s talk about sun shading and direct gains.

DIRECT GAINS

Direct gains is the simplest type of passive solar heating. It is called direct gains because the solar energy enters directly into the living spaces. As with most types of passive solar heating, there are three components to the system – south facing glazing, thermal mass and movable insulation.

South Facing Glazing

To discuss glazing orientation we need to cover some basic terms. The sun’s position in the sky is denoted by bearing and azimuth angles. The bearing angle is the horizontal angle off of due south. (If the sun is due east it has a bearing angle of 90 degrees east.) The azimuth angle is the vertical angle off of horizontal. (If the sun is directly overhead it has an azimuth angle of 90 degrees.) The incident angle of the sun is the angle measured between the position of the sun and a line perpendicular to the glazing surface. (If the position of the sun is perpendicular to the glazing it has an incident angle of zero.)

Solar radiation can be collected at all glazing orientations; however, during winters in the northern hemisphere, south facing glazing collects the greatest amount of solar radiation for a fixed orientation.

Winter gains can be increased further by tilting south facing glazing back off of vertical. Unfortunately, it also increase unwanted summer gains. The optimal tilt angle for solar glazing, in terms of the annual cycle, is vertical. Vertical glazing will have greater gains in the winter than in the summer, despite the fact there is more solar energy in the summer than in the winter. The reason why this happens is a function of azimuth and incident angles. In the summer the azimuth angle of the sun is much greater than it is in the winter. (The sun travels high in the sky during the summer, and low on the horizon during the winter.) The percentage of solar radiation falling on a glazing surface that passes through the glazing is a function of the incident angle. It is not a linear function. A high percentage of solar radiation passes through at incident angles of 45 degrees and less. At incident angles of 45 degrees and more the percentage of solar radiation passing though drops off dramatically. When the glazing is vertical, the incident angle in the summer is greater than 45 degrees for more of the time than it is in the winter, which results in less of the radiation passing through the glazing in the summer than in the winter. As a matter of fact, the summer reduction in percent passing is typically greater than the summer increase in available solar radiation, which results in less gain in the summer than in the winter.

One question that routinely comes up is, how close to due south do I need to orient the glazing? Before I answer that question I want to go over a simple relationship. The bearing angle of the sun changes 15 degrees in one hour. (That makes sense if you think about the sun taking 24 hours to travel 360 degrees around the earth. Granted, Nicholaus Copernicus would take exception to that explanation, but you get the point.)

One of the critical concerns with passive solar heating is to minimize daily temperature fluctuations. Passive solar houses tend to be cool in the morning and warm in the afternoon. Since the bearing position of the sun changes 15 degrees with every hour, and solar gains drop off dramatically at incident angles greater than 45 degrees, there is a six hour opportunity to collect solar energy. If the glazing is oriented due south, the opportunity to collect solar energy is from 9:00 in the morning to 3:00 in the afternoon. Shifting the orientation of the glazing 15 degrees east of due south will shift the start time for collecting solar energy to 8:00 in the morning, and shift the cut off time to 2:00 in the afternoon. This shift will help reduce temperature swings by reducing the cool down in the morning and overheating in the afternoon. (These times are solar times, which differ from clock time. Think about it this way. If it is solar noon for only the instant the sun is highest in the sky, then there is only one longitudinal location on the earth corresponding with that instant. Since each time zone covers a wide longitudinal swath there is no way it can be solar noon at clock noon for the entire time zone. Solar time also does not change with daylight saving time, so when it is 10:00 DST it is 9:00 solar time.)

Please keep in mind there are limits to shifting the bearing orientation. The more we shift the orientation to the east the less winter solar radiation we collect. A 15 degree shift reduces gains approximately 4 percent. Shifting the orientation off of due south also increase summer gains by allowing more of the summer solar radiation to strike the surface at an incident angle of 45 degrees or less. Given these limitation, the optimal orientation for vertical glazing is about 15 degrees east of due south.

The only other thing that needs to be discussed at this time is the amount of glazing required. The amount of glazing required for a direct gains solar house in Charlotte is approximately 20 to 25 percent of the floor area being heated. For a house of 1,200 to 1,400 square feet a total of 240 to 350 square feet of glazing is required. This is far more than a couple of large picture windows. You most likely will need to think about designing a glazed curtain wall.

One more point before we move on to thermal mass. Horizontal glazings are not energy efficient in terms of thermal performance. They collect less heat than they loss in the winter, and they are a source of unwanted heat gains in the summer. If you look at the chart titled, Climate Consultant 3 Radiation Range, which located on the college server, you will see what I mean.

Thermal Mass

Answer this question, what happens when all the solar energy needed to heat a house for 24 hours is put into the house during a six hour period? If there is no thermal mass, the house become unbearably hot in the afternoon and unbearably cold just before sunrise. (Solar houses that have little or no thermal mass are called solar tempered houses. They save some energy, but are difficult to regulate.)

Thermal mass acts as a thermal flywheel. It dampens the high and low temperatures. The more thermal mass there is in the house the more it dampens the temperature swings. The most commonly used materials for thermal mass are water and concrete / masonry. Phase change materials (PCM) are sometimes used, but they are most likely beyond our budgets. One of the advantages of using concrete or masonry, verses water, is it can do double duty and be used as the structure for the house.

The three rules of thumb for using concrete or masonry thermal mass are, 1) provide 1 to 1-1/2 cubic feet of thermal mass for every square foot of glazing, 2) expose as much surface area of the mass to direct sunlight and/or high heat as possible; and, 3) distribute the thermal mass through out the house. Ideally, the house should have nine square feet of thermal mass surface area for every one square foot of glazing. For the glazing areas given above a total of 2,160 to 3,150 square feet of thermal mass is needed. This is 1.80 to 2.25 times more surface area than floor area. That is why well designed solar houses have concrete and/or masonry floors and walls. In some passive solar houses most of the interior partitions are masonry and/or concrete.

Another way to provide the surface area needed is to go with a hybrid system. Passive solar systems by definition do not use fans or pumps to move heat. Active solar systems, such as roof mounted flat plate collectors, use pumps and/or fans to move heat from the collectors to storage, like an under slab rock bin. Hybrid systems are a combination of the two systems, but tend to be more like a passive system. The house designed and built in Chicago by the students at IIT is a hybrid house. It uses a concrete floor and a masonry wall for thermal mass. However, more thermal mass was needed, so a rock bin was constructed under the concrete slab to store heat from the top of the atrium. (The atrium serves as a thermal chimney for stack effect cooling in the summer, and as a heat trap for hybrid heating in the winter.) Keep this in mind as you layout your plans.

Movable Insulation

Here is another question. If the glazing on a solar house is collection solar energy for six hours of the day, what is it doing for the other 18 hours? The answer is, it is losing energy. In fact, the amount of energy lost can equal or exceed the amount of energy gained. Using insulating glass units typically does not improve things because the additional layers of glazing reduces the solar gains as much as it reduces the losses.

The different types of movable insulation available are too numerous to go over in detail in this e-mail. There are exterior and interior shutters. There are curtains. There are systems that blow beads of insulation between layers of glazing. And the list goes on. What you need to keep in mind is you will need to include some sort of movable insulation. We will talk about this more when we get to design development.


SUN SHADING

I know this e-mail has gotten long so I will make this short. In order to work properly sun shading needs to be adjustable. The reason is, the geometry of the movement of the sun throughout the year is symmetrical around the summer solstice – June 21st, but annual heating and cooling loads are not. For example, the position of the sun on March 21st at 10:00 in the morning is the same as it is at 10:00 in the morning on September 21st. However, on March 21st we want solar gains; whereas, on September 21st we want sun shading. Fixed sun shading can not do both.

If you plan to use louvers for sun shading, keep in mind horizontal louvers work better on the south elevation, and vertical louvers work better on the east and west elevations.

We will talk about sun shading in greater detail during design development. What you need to know about sun shading is it makes direct gains suitable for Charlotte.

Thanks,
Thomas