SOLAR ENERGY Utilization

Transcription

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 Solar Heat Gain Factors (SHGFs) Cloudiness factor (%) 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 Heat Loss (HL) Degree-Days (DD) The question now is: How much of the need (part 2) can we meet with the sun (part 1)? 1

2 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 (A e ), South (A s ), West (A w ) and North (A N ) 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 SHGF e, SHGF s, SHGF w, and SHGF N. 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 = SGHF e x A e + SHGF s x A s + SHGF w x A w + SHGF N x A N 6. Multiply by number of days in the month. 7. Repeat for other months of the heating season and add the numbers. 2

3 Example: Salt-box house in Lebanon, NH Near 40 o N SHGFs, in BTUs/(ft 2.day), and cloudiness factors, in % Heating month East South West North Cloudiness September % October % November % December % January % February % March % April % May % House structure East South West North Total R-value U = 1/R Window areas ft External walls 1,898 ft Roof 1,520 ft Add infiltration: I = 4,220 BTUs/(day. o F) HL = 6, ,220 = 10,880 BTUs/(day. o F) Need to multiply by 0.87 to account for reflection at window surface Degree-days for Lebanon, NH September 176 October 527 November 812 December 1,209 January 1,421 February 1,190 March 1,004 April 603 May 285 Compare energy demand to solar supply, month after month: September Demand is HL x Degree-days = (10,880 BTUs/day. o F) x (176 o F.days) = x 10 6 BTUs Supply is (SHGF east A east + )(0.87 window reflection)(57% cloudiness) = (906 x x x x 10)(0.87)(0.57) = 153,631 BTUs/day There are 30 days in September 153,631 x 30 = x 10 6 BTUs Good news: Supply is more than enough to cover the demand! Similar calculations for the remaining heating months of the year. Results are: Energy demand Solar supply Difference September October November December January February March April May Values in million BTUs for 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. 3

4 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 than that of a wall (R value drops from to 1.92): HL increases from 10,822 to 11,192 BTUs/(day. o F) October demand increases from to million BTUs 2. It increases the capture of solar energy: October solar gain increases from to million BTUs The October gap is reduced from to million BTUs a reduction of 68%. There is a better way to get more sun without more conductive heat loss 4

5 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 but rather by the opaque surfaces it falls upon, like the 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). But since glazing creates a relatively large conductive heat loss, consider placing a thick piece of better insulating material just inside Absorber-storage wall (Trombe wall): 5

6 Improved Trombe wall: With vent holes through the storage wall to bring some of the heat from the greenhouse into the living space. 6

7 A variation Absorber wall combined with greenhouse: 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! 7

8 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 8

9 Past use closer to home Waverly Plantation in Columbus, Mississippi Then, one can think of saving the extra daytime heat for use at night. J. Kachadorian The Passive Solar House, 1997, page 39. 9

10 Heat storage: Heat content = c x M x T heat mass temperature capacity (BTUs/lb o F) (lb) ( o F) 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. In buildings, we deal with volumes more than masses: M = x V density volume (lb/ft 3 ) (ft 3 ) Heat content = c x x V x T = H x V x T where H = c x = specific heat per volume, in BTUs/(ft 3 x o F) Specific heat H of various substances and materials On a volume basis: Air extremely low Water very high Concrete 30.1 quite high Concrete block 28.8 Sheetrock 13.0 Plywood 9.86 Particle board 15.5 Asphalt roofing shingle 21.0 values in BTUs/(ft 3 x o F) 10

11 When the sun shines on a wall or : where d dt H V H V T dt dt Q A Q A Q I cos Q I sin V A d for vertical wall for horizontal Heat received from sun: A I sin Heat flowing from to : A U ( T T ) Heat through walls, etc.: HL ( T T outside ) Heat budgets for and air: H H air V V dt dt dt dt A A I sin A U ( T T U ( T ) HL ( T T ) T outside ) 11

12 Heat exchange between and : Warm air created next to rises and convects through the : Q AU T A U ( T T ) (Newton s Law of convection) U due to convection U 0.20 T If the heated surface is a vertical wall: Q AU T A wall U ( Twall T) U T In these expressions, T is in o F and U in BTUs/(ft 2 x hour x o F). Specific heat of air is almost nil, and we can assume steady state for the budget: 0 AU ( T T) HL ( T Toutside) of which the solution is: T A UT A HL T U HL outside = instant adaptation of air temperature to a weighted average between and outside temperatures T T HL T A HL T U HL The heat budget for the then becomes: H V dt dt A AI sin A outside U HL ( T U HL T thermal inertia gain from sun loss to the outside outside ) 12

13 Examples of calculations: 1. Average indoor temperature is adequate but swings too much from day to night. Not enough thermal mass Room temperature 2. Indoor temperature well smoothed between day and night but not high enough in average Not enough solar intake; need to increase glazing Room temperature 13

14 3. About correct balance of solar intake and thermal mass: Room temperature A final remark It is important to keep in mind that in a passive-solar design, the building must accomplish the following three functions simultaneously: 1. Collection of solar energy not too little and not too much with appropriate glazing, overhangs, etc. 2. Storage of energy collected with appropriate amount and placement of thermal mass 3. Distribution of heat with facilitation of natural ventilation into the desired areas. 14