Solar fraction

A commonly used solar system performance measure is the solar fraction, which identifies the fractional amount of the building heating energy needed is supplied by the solar thermal system. This Design Guide defines the solar fraction for solar supported heating networks as:

Thus, a building that requires 10.24 MBtu (3000 kWh) per year to generate hot water and obtains 6.83 MBtu/yr (2000 kWh/yr) from its solar system has a solar fraction of 67%. A very similar indicator is the “f-save” ratio. This indicator shows the thermal heating energy saved by the supply of solar energy as compared to the heating energy that would have been used by a non-solar thermal (reference) system for the same purpose. For example, when a non-solar heating system (the reference system) requires 10.24 MBtu/yr (3000 kWh/yr) to provide for a certain hot water use, and the solar thermal system requires 6.8 MBtu/yr (2000 kWh/yr) auxiliary heating, then the f-save of the solar thermal system is 33% (i.e., the fraction of auxiliary energy saved compared to the reference system). The formula for f-save (thermal) is:

For both indicators it must be specified whether direct energy (heating energy demand), primary energy, or secondary energy is used in their calculations. The major difference is what is included in the building heating energy use (the reference system) and the impact of thermal system losses and supporting equipment energy use such as by pumps, etc.

Higher solar fractions mean higher energy and CO2-savings relative to the conventional energy source. However, one must also consider that the higher losses of the solar thermal system negatively affect the solar system efficiency. An economic optimum has to be found.