Hot water heating is one of the simplest applications of solar energy. The sun s energy is used to heat water for residential or commercial use. The technology is intended to substitute for gas or electricity-powered hot water heaters. In many areas of the world, especially in tropical climates, solar hot water heating is used extensively. Figure 3.1 shows a house in Golden, Colorado equipped with solar collectors.

Figure 3.1: Rooftop solar collectors on a house in Golden, Colorado.

For certain applications, the solar heating system focuses solar radiation onto a vessel or pipe containing a heat transfer fluid. Solar power towers using liquid sodium as the working fluid can generate temperatures high enough for industrial process heat applications. Figure 3.2 shows a parabolic trough solar collector used for hot water heating at a prison in Colorado.

Figure 3.2: Parabolic trough solar collector for hot water heating at a prison in Adams County, Colorado.

The Sun s Energy

The sun behaves as a black body radiator at a temperature of about 6,000 degrees Kelvin. The Planck distribution law describes the frequency distribution of radiation emitted by a black body at absolute temperature T:

where ? is the frequency, h is Planck s constant, c is the speed of light, T is the temperature, and k is Boltzmann s constant. The maximum of the energy distribution occurs at a frequency ? _max, given by:

where ? _max is the wavelength at the maximum...