What kind of heat losses and gains take place in buildings. How to estimate heat transfer through construction, ventilation, solar radiation, and heat transfer from internal sources. How to derive the overal energy balance in a building using hand calculations.
Always keep in mind where we are in the energy chain (demand side).
Why is energy (heat, cold, electricity) needed in a building? Comfort, cooking, electrical appliances, lighting,
Difference between thermal energy and electrical energy is important.
Space heating and cooling are thermal even though they can be provided by eletricicty.
Appliances are electrical.
Space heating and cooling is responsible for around half of total energy use. Determined by 4 main energy transfer processes:
1) Transmission (temp difference between indoor and outdoor), 2) ventilation and infiltration, 3) Solar gains, 4) Internal heat gains (people and appliances)
Transmission is a mix of conduction and convection. Solar gains are radiation. Internal are radiation and convective.
Conservation of Energy. E_in = E_stored + E_out. We simplify to E_in = E_out (can be a valid assumption).
If unbalanced negatively, need to heat. If unbalanced positively, need to cool.
Cooling is removing heat and shold be counted as E_out (negative).
Energy balance is kWh. Energy flow is kW per second. Temperates are K or deg C.
Transmission losses don't have to do with airflow. Just the temperature difference between inside and outside. There will always be a heat flow from warm side to cold side.
Consider winter conditions (-5 deg C outside). Indoor temp (20 deg C) is higher. So there will be transmission losses.
Summer (40 deg C outside, 20 deg C inside), there will be transmission gains.
Amount of transmission can be estimated quite easily.
e.g. for a wall,
Q_tr = U A (T_o - T_i)
Q_tr: heat flow in W, U: Heat transfer coefficient (W/m^2 k), A: Surface area of wall (m^2),
Heat transfer coefficient (U) is 1/R_c, where R_c is the overall thermal resistance (m^2 K / W).
Thermal resistence depends on the material and thickness of the wall. This is what is indicated on an insulation label.
Thickness / thermal connectivity = thermal resistence.
Things get more complicated when conisdered as composed from multiple materials (more realistic). Need to account for resistence of each layer.
For windows, the principle is exactly the same as a wall. A window indicates the overall U, wall materials mention the R value.
There are also transmission losses occuring at the junctions in a building (e.g. between walls and windows). Thermal bridges. These also cause damange through condensation.
In this course we mainly neglect thermal bridges (but they are important).
New buildings often insulate from the outside, more efficient. Old buildings insulate from the inside, reduces interior area and risk of condensation. Can also fill a cavity with insulation (if it exists), amount of insulation is limited to width of cavity.
Roofs can be insulated from inside or outside. Ground floor can be insulated from crawl space or on top.
Windows can be single, double, triple glazing.
In general, can assume the ground has a constant temperature, equal to the average year temperature of the air.
Flow through the building envelope through air exchange. Dependent on openenings, tightness, indoor and outdoor conditions.
Why do we ventilate? We need fresh air to remain healthy. How much do we need? 25 - 50 m^3/hour per person (a lot). e.g. 7 x 7 m x 1 m box every hour, way more than lung capacity. Big buildings always have giant air shafts.
Ventilation is not the same as infiltration. Infiltration is non controllable, comes through cracks, depends on wind. Ventilation is controllable, does not take infiltration into account (this is the above min 25 m^3/hour per person component).
4 ways to ventilate a building. 1) natural (e.g. open window, grill). works if there is enough wind pressure difference. 2) mechanical ventilation. assist the natural ventilation with e.g. a fan. can be exhaust or supply (3)). 4) mechanical supply and exhaust, with heat recovery.
Heat recovery is a way to reuse waste heat inside the building.
Mechanical supply and exhaust requires a lot of space, and the ventilators make a lot of noise.
Small/old buildings usually depend only on natural ventilation (lots of infiltration). With careful design it can be used in large buildings.
Solar radiation is captured by the facade and roofs, and also penetrates through the windows. The estimation is not as straightforward as transmission or ventilation.
We consider only solar gains through the windows.
Air is not heated by the sun, but your body is. Same with a building. The air in the building is not heated, but surfaces are.
Some solar radiation is blocked by the window (increases temperature of window).The rest arrives in the room, goes through the air, and is absorbed by indoor walls and floors (increase temperature). At some point they are warmer than the indoor air and release heat by convection.
This means steady state calculations like we are doing may not be accurate.
For a light building (little accumulated mass, open floor plan, etc) can assume all solar heat absorbed will be reduced within the same hour.
For heavy buildings (thicker walls, brick, concrete), assume 85% of incoming solar heat will be released within the same hour. The remaining 15% is considered to be lost (not actually true, it will come back later e.g. at night).
The sun is not always shining. Does the mean there is no solar radiation? No. When it is shining, direct radiation. When it is not, diffuse radiation. Overcast sky = only diffuse. Sun shining = diffuse + direct, also called global or total radiation. Measured by meterological institutes.
Also radiation falling on the ground close to the building and being reflected onto the building. We neglect this.
Solar overhangs: block direct, only allow diffuse. Moveable overhangs: only sometimes block. Indoor solar shades (curtains, roller blinds, etc): big disadvantage is they don't prevent radiation from penetrating into the building.
Part of radiation is also reflected back outdoors by the window. Important to know how much is actually transmitted through the glass. Very dependent on type of window.
Q_sol = f g_shade g_glass A_window P_sol
This is very simplified.
The heat generated inside the building from appliances or people.
One person emits approximately 100 W, dependent on activity. Cooking is 180 W / person, dancing is 360 W / person.
Also depends on amount of people. In housing, assume 40m^2/person, in office 20m^2/person, in school 4m^2/person.
Appliances also contribute. Desktop computer ~65 Watt. Fridge ~310 Watt.
Also artificial lighting. Incandescent is 60 W. LED is 5-7 W.
Internal heat gains are less important in houses, more important in offices, and even more significant in schools/theaters.
Because of internal gains, a room will heat up even with no solar radiation and an equal indoor/outdoor temperature.
Energy that comes in is positive, out is negative. Transimssion, ventilation, infiltration can be a gain or loss, solar radiation and internal are always gains.
First principle of thermodynamics: conservation of energy. E_in = E_out.