Passive Solar Design: Sustainable Energy for Air Conditioning and Heating
Passive solar design is a method to collect solar energy using windows, walls, floors etc and distribute it to the room during winters to maintain considerably high temperature than outside or reject it to maintain inside temperature considerably cool during summer. The system does not require any mechanical or electrical devices which make it best suited for building design. Passive design takes advantage of climate in best possible way to maintain the comfortable temperature range in a room.
Passive solar design eliminates the use of any auxiliary items for heating or cooling systems, which account for 35% or more reduction of energy use, saving substantial electricity bill. Buildings are properly oriented and windows are designed in such a way that they carefully balance energy requirements without adding any mechanical or electrical devices and with minimal maintenance cost.
Passive Heating and Cooling
Masonry walls properly oriented towards south with allowance of 30 degrees, absorb heat energy when sunlight strike the wall surface. Heat absorbed is transferred inside. Once heat is inside the building, there are several technologies to spread it uniformly in a room using conduction, convection or radiation, in some homes small fans or blowers help to distribute it uniformly. Water filled containers are also used to store heat energy, as it is more effective and store twice as much heat as masonry walls per cubic meter, but water thermal storage tanks require carefully designed structural support. If building can support its weight, it can be installed in an existing building.
For passive solar heating approximately eight percent window to floor area is required for south walls. High performance windows with insulated frames, multiple glazing, low e-coating, insulating glass spacers and inert gas fills reduce heat loss by approx. 50% – 75%. For passive cooling, the focus is on things which reduce heat energy, minimize equipment in room or building to reduce heat generation and proper dissipation in environment. Shading devices such as eaves, window awnings, shutters, trellises, glazing windows and plantation can effectively reduce solar gains upto 90%. External heat gains can also be minimized by reducing window size, using reflective materials in roof and walls. If building is under construction, special emphasis should be given to cross-ventilation and direction of winds, the most effective source of cooling during night using wind breezes.
Thermal Mass (Trombe Wall)
Trombe wall consist of an 8 inch -16 inch thick masonry wall oriented toward south. A masonry wall with dark heat absorbing material on exterior surface and with single or double layers of glass. Glasses are placed with provision of some air space of 0.75 inch to 2 inch to provide heat transfer from glass to wall using conduction. High transmission glass maximizes solar gain to the wall. Heat from sunlight, pass through glass, stored in the wall and slowly conducted inward. Heat travels through a masonry wall at an average rate of one inch per hour, i.e. if heat absorbed to the wall at 12 noon, 8 inch think wall will radiate inward at 8 pm.
Proper overhang roof design is very important for Trombe walls, improper designing can prove very uncomfortable during weather changes. Overhang roofs should be designed accordingly to reduce the sunlight striking the wall during summer and maximize during winter. Shading the Trombe wall can prevent the wall from getting hot during the time of the year when the heat is not needed.
Because of extremely large size of chillers and huge power consumption, reduction of energy consumption is one of the major focus of research. Major issue is, available material such as silica gels are used which can absorb heat but at very low space. To cool considerable amount space, it need very huge storage for silica gel.
Some of the recent technologies developed by Dr Raya al Dadah at the University of Birmingham, at Heinrich Heine University in Dusseldorf, and at MIT in the US, will adsorb more than four times as much water as silica gel. As the name implies, these materials are made of various metals, such as zirconium or chromium, bound together with a loose web of carbon atoms, so that there is a much greater surface area with sites where water molecules can be bound. But the work of the chemists and engineers is not done. Many materials will work well for a few cycles of adsorption and its reverse, desorption. But a commercial adsorption chiller needs to keep up this performance for thousands of cycles without requiring a change of the MOF.
These materials are undergoing such tests, and if they succeed we might soon see commercial air conditioning using 10% of the energy required for today’s units.