Activity Overview: Site energy is one subset of primary energy, which refers to the subset of primary fuels used on-site, such as natural gas burned for heating, cooking, or water heating in homes. Importantly, efficiency and system losses related to energy consumption differ greatly between off-site primary energy and site energy. The conversion of site energy by a boiler within a home incurs few losses. However, the conversion of off-site primary energy and subsequent electricity distribution incurs many more losses. Tracking energy losses is necessary to understand the full story of energy consumption in the built environment.

Looking specifically at heating, this activity looks at calculating the end-to-end system efficiency of two different types of water heaters. The first uses electricity produced offsite, distributed to a home, and then converted into heat by an electric boiler. The second uses natural gas in a combustion-based boiler. As with all energy conversions, both experience losses through the energy life cycle but in different ways and with varying effects. The total end-to-end efficiency is a product of all the component efficiencies.

Procedure:

The energy cycles of the two types of water heater are illustrated in the images on this page. One pair of images contains the answer to this activity. Provide the efficiency for each stage of the energy journey from source to hot water or project or print the student version of the images below.

Before you begin, explain that the end-to-end efficiency of an energy system is the product of the efficiency of the component conversions. The efficiency of a primary source is 1 because no conversion is taking place. The efficiency of a conversion in which all energy is lost is 0.0. The efficiency of every conversion falls somewhere between due to the second law of thermodynamics.

Students should use what they know about fractions and decimals to calculate the overall efficiency of the two different water heaters by multiplying the value of each of the component efficiencies together.