Solar Gain The ultimate free lunch!
Some Basics • Why do we need to heat our homes? – Living rooms – Bedrooms – Staircases & halls 21 o C 18 o C 16 o C
Heat Flow • 1 st Law of Thermodynamics – conservation of energy – heat = work = energy • 2 nd Law of Thermodynamics – Heat flows from a hotter body to a colder body (unless work is expended) – Entropy increases
Energy Flow – U Values • Q f = UA(T 1 ­T 2 ) – Q t = Rate of heat transfer in Watts – A = Area in m 2 – T 1 ­T 2 = Internal temp – external temp – U = Overall thermal transmittance co­efficient in W/m 2 o C U = 1 å R åR =
L L 1 L 1 + + R a + L n + R n k 1 k 1 k n L = thickness in m k = thermal conductivity W/m o C R = resistance of surface in m 2o C/W
U Values • Walls – Cavity wall – air void 1.5 W/m 2 o C – 200mm cast concrete 3.1 W/m 2 o C – 150mm cast concrete 3.5 W/m 2 o C • Windows – Single glazed – Double glazed 6mm – Double glazed 12mm 5.6 W/m 2 o C 3.4 W/m 2 o C 3.0 W/m 2 o C
Typical Insulated Masonry wall types to achieve a U­value of 0.27W/m 2 degK or better.
Wall type 1 Up to 150mm cavity, partial­fill cavity insulation External Internal Render (external) Outer Leaf Airspace in cavity wall Cavity insulation Inner leaf (100mm Block) Render (internal) Maximum cavity width is 150mm in accordance with I.S.325 hence with a 40mm airspace, the maximum insulation thickness is 110mm 95mm Aerobord Platinum will give a U­value of 0.27 W/m 2 degK when used in this wall type.
Wall type 2 100mm cavity partial­fill insulation with internal insulation External Internal Render (external) Outer Leaf Airspace in cavity wall Cavity insulation Inner leaf (100mm Block) Air space between masonry and internal insulation Internal insulation Vapour barrier Plasterboard With a 40mm cavity, the maximum insulation thickness in the cavity is 60mm, the balance of the insulation fixed internally 65mm Aerobord Platinum in the cavity and 25mm Aerobord Platinum internally will give a U­value of 0.27 W/m 2 degK. If the internal insulation is increased to 65mm the U­value will be reduced to 0.21 W/m 2 degK.
Wall type 3 Cavity wall with 50mm cavity and internal insulation only External Internal Render (external) Outer Leaf Airspace in cavity wall Inner leaf (100mm Block) Air space between masonry and internal insulation Internal insulation Vapour barrier Plasterboard 65mm Ecotherm PIR (Polyisocyanurate) Board fixed internally will give a U­value of 0.27 W/m 2 degK.
Wall type 4 Full fill cavity insulation External Internal Render (external) Outer Leaf Full fill Cavity insulation Inner leaf (100mm Block) Internal Render Maximum cavity width is 150mm in accordance with I.S.325
95mm Aerobord Platinum cavity fill insulation will give a U­value of 0.27 W/m 2 degK when used in this wall type. 105mm Blown Aerobord Platinum bead will give a U­value of 0.27 W/m 2 degK when used in this wall type. When cavity insulation is increase to 150mm a figure if 0.20 W/Mm 2 degK is achieved. Wall type 2 100mm cavity partial­fill insulation with internal insulation External Internal Render (external) Hollow Block Air space between Masonry and internal insulation Internal insulation Vapour barrier Plasterboard The internal insulation is fixed to the masonry wall either on plaster dabs or on battens/metal furring. This gives rise to a small airspace between insulation and wall. 65mm Ecotherm PIR (Polyisocyanurate) Board fixed internally will give a U­value of 0.27 W/m 2 degK.
Thermal loss and gain is not steady state process
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Solar Energy • Solar radiation – 0.9 kW/m 2 – Varies with • Latitude • Time of day and year • Atmospheric clarity • Orientation of surface – Direct, Diffuse or Reflected
Passive Solar Design • Heating, Lighting and ventilation! Not just heating • In a typical office 40% of energy usage can be due to lighting, in other cases 40% can be used by air conditioning
Catching some rays! • Photovoltaic cells – Expensive – but developing technology, watch this space! • Water based collectors – matt black pipes in a shallow glazed box • Passive collection
Problems? • The sun doesn’t shine at night! • When the sun isn’t shining it’s cold! • When the sun is at its hottest you don’t want to heat your house
Greenhouse Effect • How does a greenhouse work? – Think of a car on a hot day Radiated heat is allowed in and warms the contents of the greenhouse, in hot weather the rate of gain is higher than the rate of loss, hence the temperature increases
Heat Transfer ­ Mechanisms • Conduction • Convection • Radiation
Windows
Window Design ­ Climate dependant • U Values • Low­emissivity (Low­E) coatings – control heat transfer through windows with insulated glazing – Low­E coatings typically cost about 10%–15% more than regular windows, but they reduce energy loss by as much as 30%–50%. • Solar Heat Gain Coefficient – Low SHGC, less solar heat transmitted, good shading – High SHGC, good for collecting solar heat gain during the winter. • Visible Transmittance – how much light is let in • Spectrally selective coatings – filter out 40%–70% of the heat normally transmitted through insulated window glass or glazing, while allowing the full amount of light to be transmitted. • Light­to­solar gain (LSG) – ratio between the SHGC and VT. The higher the number, the more light transmitted without adding excessive amounts of heat.
Direct Solar Gain
Thermal Mass
Indirect solar gain
Thermosiphon
Overhangs & Shading & Shelter • Overhangs – Can reduce effect of summer sun • Want to let winter rays in • Shelter can help reduce heat loss
Overhangs & Shading
Example
Orientation
Example 1
Example II
Example III
Southfacing
Passive Solar Design is location dependent!!! • Previous examples were Californian • Glazing ­ dilemma – Potential gain in radiated heat – Potential loss through lower thermal performance
Sunrooms
Importance of Shelter Deciduous trees’ bare branches allow sunlight to filter through in winter
Potential Energy Savings – Estate in UK
Principal glazed elevation facing south
Latitude
Overshading – minimum spacing
House spacing – Latitude
Site topology is a factor
Avoid self­shading
Be aware of potential shading due to unnecessarily steep pitch
Avoid Garages on the south side
Minimising overshading
Vegitation – privacy and light effects
Typical Characteristics of Passive Solar houses
Preferable house layouts
The truth • Reduce area of North, East and West facing glazing – heat loss is always greater than loss of solar gain (even with low emissivity glass)– keep at 15% of floor area to give adequate lighting • In sites with good solar exposure – areas of south facing double glazing are thermally neutral. Slightly negative in colder areas and positive in more southerly sections of the UK. • If low emissivity glass is used then south facing glazing will be thermally positive over most areas of the UK • A passive solar house typically has between 60% and 75% of its glazing on its southern aspect