Passive solar systems

Passive solar design in all climates consists of arranging the lumped building mass m, the sun-facing area A and the loss resistance R to achieve optimum solar benefit, by structural design. The first step is to insulate the building properly (large R), including draught prevention and, if necessary, controlled ventilation with heat recovery. The orientation, size and position of windows should allow a sufficient product of GA (perpendicular to the glazing) for significant solar heating in winter, with shading preventing overheating in summer. The windows themselves should have an advanced, multi-surface, construction so their resistance to heat transfer, other than short wave solar radiation, is large.

For passive solar buildings at higher latitudes, solar heat gain in winter is possible because the insolation on vertical sun-facing windows and walls is significantly more than on horizontal surfaces. The sun-facing internal mass surfaces should have a dark colour with a>0.8 (Figure (a)6.3), and the building should be designed to have large mass of interior walls and floors (large m) for heat storage within the insulation, thereby limiting the variations in Tr. Overheating can of course be prevented by fitting external shades and shutters, which also provide extra thermal insulation at night.

Example 6.1 Solar heat gain of a house

The Solar Black House shown in Figure 6.3(a) was designed as a demonstration for Washington DC (latitude 38°N), with a large window on the south side and a massive blackened wall on the north. Assuming that the roof and walls are so well insulated that all heat loss is through the window, calculate the solar irradiance required so that direct solar heating alone maintains room temperature 20 °C above ambient.

Solution

If the room temperature is steady, (6.4) reduces to

Figure 6.3 Direct gain passive solar heating. (a) Basic system. (b) Clerestory window (to give direct gain on the back wall of the house). Note the use of massive, dark coloured, rear insulated surfaces to absorb and to store the radiation.

where r is the thermal resistivity from room to outside of a vertical window, single glazed. By the methods of Chapters 3 and 5, r = 0.07 m2 KW—1

Take glass transmittance t = 0.9 and wall absorptance a = 0.8, then 20 °C

This irradiance may be expected on a vertical sun-facing window on a clear day in winter.

Example 6.1 correctly suggests that most of the heating load of a well-designed house can be contributed by solar energy, but the design of practical passive solar systems is more difficult than the above example would suggest. For example, the calculation shows only that the Solar Black House will be adequately heated in the middle of the day. But the heat must also be retained at night and there must be an exchange of air for ventilation.

Example 6.2 Heat loss of a house

The Solar Black House of the previous example measures 2.0 m high by 5.0 m wide by 4.0 m deep. The interior temperature is 20 °C at 4.00 p.m. Calculate the interior temperature at 8.00 a.m. the next day for the following cases:

a Absorbing wall 10 cm thick, single window as before; b Absorbing wall 50 cm thick, thick curtain covering the inside of the window.

Solution

Renewable Energy Eco Friendly

Renewable Energy Eco Friendly

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

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