1 SOLAR ENERGY Utilization ENGS-44 Sustainable Design Benoit Cushman-Roisin 17 April 2018 Recapitulation 1. We know how much energy the sun provides as a function of - latitude of location - orientation of surface (window, roof) - month of year - hour of day - cloudiness 2. We know the energy need of the building as a function of - R-values of walls, windows, roof, etc. - respective surfaces of walls, roof, etc. - air infiltration - how cold it is outside Solar Heat Gain Factors (SHGFs) Heat Loss (HL) Degree-Days (DD) Cloudiness factor (%) The question now is: How much of the need (part 2) can we meet with the sun (part 1)?
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SOLAR ENERGY
Utilization
ENGS-44 Sustainable Design
Benoit Cushman-Roisin17 April 2018
Recapitulation
1. We know how much energy the sun provides as a function of
- latitude of location- orientation of surface (window, roof)- month of year- hour of day
- cloudiness
2. We know the energy need of the buildingas a function of
- R-values of walls, windows, roof, etc.- respective surfaces of walls, roof, etc.- air infiltration
- how cold it is outside
Solar Heat Gain Factors(SHGFs)
Heat Loss (HL)
Degree-Days (DD)
Cloudiness factor (%)
The question now is: How much of the need (part 2) can we meet with the sun (part 1)?
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In building design, there are basically three passive solar techniques:
1. Direct gain (= let the sun enter through windows)
2. Trombe wall (= enhanced direct gain)
3. Greenhouse (= enhanced trombe wall)
Caution!These techniques, if used at all, need to be used extremely carefully, for it is very easy to focus on cold winter days and then have a building that is uncomfortably warm in summer.
Calculations Recipe for Direct Gain
1. Determine square-feet of glazing (windows) on East (Ae), South (As), West (Aw) and North (AN) sides of the building.
2. Adjust these areas downward for shading by overhangs, vegetation or neighboring structures
3. Select a month and pick the values SHGFe, SHGFs, SHGFw, and SHGFN.
4. Correct the SHGF’s for cloudiness (% sun).
5. Correct for partial reflection by window glass (87% or applicable solar heat gain coefficient (SHGC) depending on window type).
6. Multiply and add for each side of the building:Solar heat gain per day of the month =
SHG = SGHFe x Ae + SHGFs x As + SHGFw x Aw + SHGFN x AN
6. Multiply by number of days in the month.
7. Repeat for other months of the heating season and add the numbers.
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Example: Salt-box house in Lebanon, NH
Near 40oN → SHGFs, in BTUs/(ft2.day), and cloudiness factors, in %
House structure
Need to multiply by 0.87to account for reflectionat window surface
Compare energy demand to solar supply, month after month:
September Demand is HL x Degree-days= (10,880 BTUs/day.oF) x (176 oF.days) = 1.915 x 106 BTUs
Supply is (SHGFeastAeast + …)(0.87 window reflection)(57% cloudiness)= (906 x 64 + 1344 x 162 + 906 x 35 + 238 x 10)(0.87)(0.57)= 153,631 BTUs/day
There are 30 days in September → 153,631 x 30 = 4.609 x 106 BTUs
Good news: Supply is more than enough to cover the demand !
Energy
demand
Solar
supply Difference
September 1.915 4.609 + 2.694
October 5.734 4.873 - 0.861
November 8.835 3.723 - 5.112
December 13.154 3.653 - 9.501
January 15.460 3.914 - 11.546
February 12.947 4.599 - 8.348
March 10.924 4.845 - 6.079
April 6.560 3.814 - 2.746
May 3.101 3.676 +0.575
Values in million BTUsfor each month
In winter, solar energy is rarely enough, but it does make a significant contribution. The danger is to provide too much heat the rest of the year.Shading is essential.
Similar calculations for the remaining heating months of the year. Results are:
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In the winter months, when the solar energy input fails to meet the building demand, additional heat must be supplied from a furnace or other source (solar panels on roof? geothermal heat?)
Alternatively, one can decrease the demand by increasing the insulation of the building, for example, by drawing curtains at night.
or …
one can be clever and get more free energy from the sun !
For example, what happens if one increases the window area by 20% on the southern side of the building?
This does two things, one negative and one positive:
1. It increases the heat loss because the R-value of a window is less thanthat of a wall (R value drops from 21.97 to 1.92):
→ HL increases from 10,822 to 11,192 BTUs/(day . oF)→ October demand increases from 5.703 to 5.898 million BTUs
2. It increases the capture of solar energy:
→ October solar gain increases from 4.873 to 5.633 million BTUs
The October gap is reduced from 0.830 to 0.265 million BTUsa reduction of 68%.
There is a better way to get more sun without more conductive heat loss…
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Except for a small amount of reflection, most of the solar radiation goes through glass because glass is almost perfectly transparent to radiation in the visible spectrum. (We can see through windows!)
This radiation is not absorbed by the air in the room but rather by the opaque surfaces it falls upon, like the floor or walls.
The receiving surface heats up and, in steady state, emits back the same amount of heat, mostly through convection.
Heat is lost through conductive loss through the window (small R-value).
Absorber-storage wall (Trombe wall):
But since glazing creates a relatively large conductive heat loss, consider placing a thick piece of better insulating material just inside
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Improved Trombe wall:
With vent holes through the storage wall to bring some of the heat from the greenhouse into the living space.
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Absorber wall combined with greenhouse:
A variation…
The greenhouse may be stifling during the day and too cold at night for comfort, but it may be just fine to grow plants … and food, too!
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Should interior space get too hot, a passive solution is the
Solar Chimney
A solar chimney — often referred to as a thermal chimney — is a way of improving the natural ventilation of buildings by using convection of air heated by passive solar energy. A simple description of a solar chimney is that of a vertical shaft utilizing solar energy to enhance the natural stack ventilation through a building.
The solar chimney has been in use for centuries, in the Middle East and Near East by the Persians, as well as in Europe by the Romans. (Source: Wikipedia)
Examples of solar chimneys
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Past use closer to home
Waverly Plantation in Columbus, Mississippi
Then, one can think of saving the extra daytime heat for use at night.
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Heat storage:
Heat content = c x M x T↑ ↑ ↑
heat mass temperaturecapacity
(BTUs/lb oF) (lb) (oF)
In buildings, we deal with volumes more than masses:
M = x V ↑ ↑
density volume(lb/ft3) (ft3)
Heat content = c x x V x T = H x V x T
where H = c x = specific heat per volume, in BTUs/(ft3 x oF)
Thermal mass inside a building is adequate for smoothing day-night temperature variations.
For smoothing seasonal temperature fluctuations (i.e., storing summer heat for use in the following winter), one needs to resort to a geothermal system.
Specific heat H of various substances and materialsOn a volume basis: