Solar energy is an inexhaustible and sustainable resource with a good potential to power several applications, one of which is water heating. While several kinds of devices are used for harnessing solar energy, flat plate solar collectors are well-developed and generally more commonly used for residential and small commercial water heating applications.

A flat plate solar collector simply converts radiant solar energy from the sun into heat energy, which is then used to heat water. However, while simple in design and operation, there are several components that make these collectors operate desirably and several essential equations that are used for designing them.

Figure 1: Flat plate solar collectors are commonly used for domestic and industrial water heating applications. Source: topten22photo/Adobe Stock

Principle of the flat plate solar collector

Figure 2 shows the basic schematic of a flat plate solar collector. It features a dark-colored metal plate (or absorber plate) that is typically made of copper (or aluminum) with several parallel pipes (also called risers) brazed directly to the plate. These pipes contain the heat transfer fluid (usually water). In addition, the absorber tube is coated with a selective surface that has a high absorptivity, enhancing the absorber plate’s ability to absorb incoming solar radiation and low emissivity that minimizes heat loss by radiation from the surface.

Figure 2: Flat plate collector illustration showing the glazing material, absorber plate, insulation, and flow tubes. Source: EERE/Public domain

The absorber plate and risers are usually contained within an enclosure with a sheet of glazing material. The glazing material, usually made of glass or plastic, allows incoming solar radiation to pass through and creates an insulated air space that reduces heat losses from convection and radiation. In addition, the back and sides of the collector are insulated to prevent heat losses.

So when sunlight is incident on the sheet of glazing material, it passes through and is absorbed by the absorber plate. As a result, the absorber plate heats up and transfers the heat to the water circulating through the pipes. The heated fluid can then be used for various purposes, such as domestic hot water heating or other industrial processes.

Basic calculations for flat plate solar collectors

1. Energy hitting the solar collector

Solar intensity on the Earth’s surface can reach about 1,000 W/m² on a clear day, although this value varies based on geographic location, atmospheric conditions, season and time. For a specific solar intensity, the theoretical total energy input at the solar collector can be calculated using:

Where:

I = solar intensity (W/m²)

A = Collector area (m²)

So, for example, consider a flat plate collector with an area of 2 m². If the solar intensity is 1,000 W/m², then the total energy input on the collector will be 2,000 W. However, it is important to note that real-world conditions may cause this value to vary. Factors such as dust, angle of incidence of solar radiation, shading and weather conditions can reduce the amount of solar radiation reaching the collector.

2. Energy gained by the fluid

Not all energy reaching the collector is captured by the water flowing through the flow tubes; some energy is lost as heat to the surroundings. Therefore, the ratio of energy gained by the working fluid in the absorber tube to the energy hitting the solar collector describes the collector’s efficiency.

The typical efficiencies of flat plate solar collectors range between 40% and 80%, depending on the design, materials, operating conditions and geographic location. Consider the solar collector described in the previous example: if the collector efficiency was specified to be 80% and the energy input on the collector is 2,000 W, then the energy gained by the working fluid will be 1,600 W, as shown below.

Conclusion

Solar water heating systems are paving the way for harnessing renewable energy resources for domestic and industrial applications. Understanding the basic principles of solar collectors and the calculations for estimating energy collection empowers engineers to make more informed decisions when specifying these systems for different applications. While this article presents the basic calculations for flat plate solar collectors, it is advisable to reach out to solar water heaters manufacturers to design and specify these systems to meet real-world application requirements.