In this case the cooling potential of night-time air is optimized by being forced through the building fabric by fans. At the same time it assists in expelling warm air during the day. The system has the advantage that it eliminates the need to have open windows either day or night. It also offers control over the supply and extract air flow.
A variation is described as the 'enhanced surface heat transfer system'. A perforated metal sheet is applied to the underside of the floor slab leaving an air gap. Air is mechanically forced into the gap, creating turbulent air currents which appreciably improve the heat transfer between the air and the slab, maximizing night-time cooling. The external air can be ducted directly to the gap or emitted into the room and then recirculated through the ceiling gap. As with natural ventilation the night-time temperature should be below 20°C. However, the heat gains that can be accommodated can increase to 50 W/m2 (see Fig. 3.9).
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The solar Stirling engine is progressively becoming a viable alternative to solar panels for its higher efficiency. Stirling engines might be the best way to harvest the power provided by the sun. This is an easy-to-understand explanation of how Stirling engines work, the different types, and why they are more efficient than steam engines.