Passive Solar Design and Concepts

Transcription

1 Passive Solar Design and Concepts Daylighting 1

2 Passive Solar Heating Good architecture? The judicious use of south glazing coupled with appropriate shading and thermal mass. Summer Winter Passive solar Direct (or indirect) gain of solar energy through windows or in attached sun-spaces for space heating high performance fenestration and/or transparent insulation application of thermal mass for storage and to reduce overheating can include natural ventilation design for maximization of natural Daylighting Apply design principles to increase heat gain and reduce cooling loads Good architecture and energy conservation! 2

3 Seasonal Variation in sun-altitude Sun Position and Chart Solar Radiation Definitions G G = G dir + G dif + G ref Global solar irradiance and its components The radiation from the sun that meets the earth without any change in direction is called direct or beam radiation, G dir. The radiation from the sun after its direction has been changed by scattering in the atmosphere is called diffuse radiation, G dif. The radiation from the sun after it is reflected on the ground is called the ground reflected radiation, G ref. The sum of the beam, diffuse and reflected solar radiation on a surface is called the global solar irradiance, G G. From Planning and Installing Solar Thermal Systems, James & James/Earthscan, London, UK 3

4 Solar Spectrum Solar irradiance outside atmosphere Direct solar irradiance at sea level Sun spectrum AM 0 in space and AM 1.5 on the earth with a sun elevation of 41.8 o From Planning and Installing Solar Thermal Systems, James & James/Earthscan, London, UK Solar Radiation Global solar irradiance and its components with different sky conditions From Planning and Installing Solar Thermal Systems, James & James/Earthscan, London, UK 4

5 ipreciado.wordpress.com 5

6 Direct or Indirect Passive solar Gain Direct gain Indirect gain 6

7 Traditional Passive Solar Design (Direct Gain) Direct Solar Gain South Glazing (clearstory windows) 7

8 Passive solar design at York University Passive Solar Heating Mass walls and (transparent) Insulation 8

9 Passive Solar Heating/Cooling Fenestration The location and operation of shading Exterior Shade Interior Shade Passive Solar Heating on Residence, France Attached Sunspaces Photo Credit: Pamm McFadden (NREL Pix) Attached Sun Space 9

10 Indirect Passive Solar Concepts Mass Wall Trombe Wall (Indirect Passive Solar) 10

11 Storage Walls A storage wall (e.g. Trombe wall) is a sun-facing wall built from material that can act as a thermal mass (such as stone, concrete, adobe or water tanks), combined with an air space, insulated glazing and vents to form a large solar thermal collector. During the day, sunlight would shine through the glazing and warm the surface of the thermal mass. At night, if the glazing insulates well enough, and outdoor temperatures are not too low, the average temperature of the thermal mass will be significantly higher than room temperature, and heat will flow into the house interior. From Solar Engineering of Thermal Processes, Duffie & Beckman Energy flows through direct-gain vs collector storage wall as a function of time of day 11

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13 Passive Solar Examples Involves the direct use of sunlight for daylighting and space heating 13

14 Window Performance Passive Solar Heating Advanced fenestration 14

15 Canadian Energy Rating System Canadian Energy Rating System was designed to show the average heating season thermal performance of windows. The Energy Rating procedure is incorporated in the Canadian Standard A , Energy Performance of Windows and Other Fenestration Systems. The Energy Rating (ER) combines the effects of U-value, SHGC and air leakage characteristics of windows. Where, ER = solar heat gains conductive heat losses air leakage heat losses ER = 0.8 * 72.2 * SHGC w 21.9 * U w 0.54 * (L75 / A w ) ER is Energy Rating, W SHGCw Solar heat gain coefficient of a window The Solar Heat Gain Coefficient (SHGC) is the percent of solar energy incident on the glass that is transferred indoors both directly and indirectly through the glass. The direct gain portion is the solar energy transmittance, while the indirect is the fraction of solar energy incident on the glass that is absorbed and re-radiated or transmitted through convection indoors. For example, 1/8" (3.1 mm) uncoated clear glass has an SHGC of approximately 0.86, of which 0.84 is direct gain (solar transmittance) and 0.02 is indirect gain (convection / re-radiation). (See more at: U w Overall heat loss coefficient, W/(m 2 oc) 72.2 represent the average solar radiation on a vertical window during the heating season, (W/m 2 ) 0.8 factor is to account for exterior shadings on windows represents average temperature difference over the heating season. Canada has a variety of weather patterns ranging from mild climate in southern BC to very cold northern regions. Therefore, there have been a number of situations arising in certain mild regions as well as with window designs in which the ER values do not seem to reflect the use of better fenestration technologies (such as low-e, argon-filled, insulated spacer and so on). This has been identified as a major obstacle in acceptance of the Energy Rating system. Again, the issue is not the technical soundness of the ER equation but the proportional contribution of the solar effects and insulating effects. Manufacturers can voluntarily rate energy performance of windows using the services of Canadian Standards Association. See also Passive or Active? Mass Wall OOPS! No storage 15

16 Simple Example Model Polystyrene Insulation Double Pane Vertical Window Concrete Patio Stone Floor (painted black) Why Store Energy? solar energy is a time-dependent energy resource load does not match available energy cost consideration (avoid peak use) short term or long term storage A solar energy process with storage. (a) Incident solar energy, G T, collector useful gain, Q U, and loads, L, as a function of time for a 3 day period. From Solar Engineering of Thermal Processes, Duffie & Beckman 16

17 Air Based Thermal Storage An air based thermal storage (e.g. Solarwall, InSpire Wall) pre-heats the outside air before it enters the building to provide fresh air changes and natural humidification. Source: Source: Packed-bed Storage A packed bed is a large insulated container filled with loosely packed rocks a few centimeters in diameter. Circulation of air through the void of the packed bed rocks results in natural or forced convection between the air and the rocks. Direction of flow From Solar Engineering of Thermal Processes, Duffie & Beckman 17

18 Modes of Operation Mode 1 Charging Mode When the sun is shining but there is no space heating demand, hot air from the collector enters the top of the storage unit and heats up the rock bed. As the air flows downward, heat transfer between the air and the rocks results in a stratified temperature distribution of the rock bed, being the hottest at the top and the coolest at the bottom. The cool air then returns to the collector to be heated. From Solar Energy Engineering, Jui Sheng Hsieh Charging Mode High stratification due to high heat transfer coefficient-area product, UA. From Solar Engineering of Thermal Processes, Duffie & Beckman 18

19 Modes of Operation Mode 2 Discharging Mode When no solar energy can be collected but there is a heating demand, hot air is drawn from the top of the rock bed into the house and cooler air from the house is returned to the bottom of the bed, causing the bed to release its stored energy. (Note: Charging and discharging a pack-bed storage cannot be executed at the same time! This is in contrast to water storage systems.) From Solar Energy Engineering, Jui Sheng Hsieh Modes of Operation Mode 3 Auxiliary Mode When there is sunshine and at the same time load demand, hot air from the collector is led directly into the house and cooler air from the house is led directly into the collector, both bypassing the storage unit. The auxiliary heater shown in the figure can be used to remedy the energy deficiency of the collector or the storage to meet the loads. Through the by-pass route, the auxiliary heater alone can be called upon to meet the entire energy demand. 100% Auxiliary From Solar Energy Engineering, Jui Sheng Hsieh 19

20 Horizontal Flow Rock Bed Baffles (used to increase flow path) From Solar Energy Program, A Guide to Rock Bed Storage Units, Enermodal Engineering Limited Sensible Heat Storage Materials * Water has three times the heat capacity of rock on a volume basis, meaning that rock requires three time more volume than water to store the same amount of sensible heat! From Solar Energy Engineering, Jui Sheng Hsieh 20

21 Energy Calculations Energy Equation: Energy needed to heat hot water is Q Q = Vol x Density x Specific Heat x Temperature Rise = kj Or Units Check Q = (L) x kg/l x kj/kg C x C = kj The (constant pressure) specific heat of water or Cp is the amount of energy (KJ) required to heat one Kilogram of water 1 degree Celcius or (Kelvin). This value is not constant but varies slightly with temperature, e.g., Specifc heat and density of water 1005 Properties of Water 4.25 Range 4.24 Density, kg/m kg/m kj/kg o C Specific Heat (kj/kg o C) Temperature, o C For our purposes, over the temperature range considered, we can assume the value of the specific heat and density of water is effectively fixed at the average values given above. 21

22 Example Cal c. Example: What is the energy required to heat a 270 L tank from 15 C to 55 C For this example the following is assumed to be true: The density of water is 0.993, Cp = (1 litre of water is equal to kg) The price of electricity is $ 0.35 kwh 1 Joule is equal to a Watt second (i.e., J = Ws) T = 40 Q = 270 L x kg/l x kj/kg C x 40 C = 44,838.7 kj or 44.8 MJ In kilowatt hours this much energy is: (Note that one Joule of energy is a Watt of power operating for one second or a Ws) Therefore Q= 44,838.7 kj = 44,838.7 kws = 44,838.7 kws x( 1 hr/3600 s) = 44,838.7/3600 = kwh At an electrical energy cost of $035/kWh, this energy costs: Cost = $035/kWh x kwh = $

23 Enthalpy Phase Change Storage When a substance undergoes a solid-liquid phase transition, it usually involves a large amount of latent heat with a small volume change. A phase change storage would be a space-saver if it satisfies the following conditions: 1) the phase transition must occur at a temperature compatible with the heating and cooling load requirement 2) the process must be reversible over a large number of cycles without degradation 3) the material must be inexpensive and can be used safely A few salt hydrates (salts bonded to water molecules) possess the desired qualities to serve as phase-change materials (PCMs). For Phase Change v Dh Temperature 23

24 Storage Media From Thermal Energy Storage for Solar and Low Energy Buildings, IEA Solar Heating and Cooling Task 32 24