Solar heating panels consist of liquid-filled pipes in a glass box. The glass allows light and heat in, which is absorbed by the dark-coloured pipes and this, in turn, heats up the liquid moving through them.
Since the exterior surface of the pipes is warmer than the water flowing through them, heat moves from the pipes to the water. Through this constant transfer of heat energy, the water is heated and is brought throughout the building, allowing for a constant circulation of hot water as the Sun
shines. This method is often referred to as passive heating because it does not necessarily require pumps or electrical systems. This method depends on strong sunshine and only heats the water.
FIG. 11.13 Solar panels can heat the water throughout a building
Another type of solar energy uses photovoltaic cells. These are explained further in Chapter 30 on the electron and the photoelectric effect. This method allows for the conversion of light into electricity but these cells are significantly more expensive to buy and install.
House insulation
The U-value of a substance indicates how good the substance is at transmitting heat across it. The higher the U-value, the more heat energy can be transmitted:
• high U-value = good conductor • low U-value = good insulator. U-values are measured in W m–2 K–1.
The U-value of a material is the amount of heat energy that can be transmitted across 1 m2 of its surface every second, so long as there is a temperature difference of 1 K each side of the material.
QUESTIONS AND ANSWERS
4. A house has a square solar panel of side 4 m. If the Sun is shining perpendicularly on it for 6 hours uninterrupted, how much energy is incident if the solar constant is 1.35 kW m−2?
