WSHPs Vital Role in Decarbonization Part 3: Site Versus Source Energy Use Intensity (EUI)

Decarbonization and electrification are big buzzwords these days. Intuitively, most understand that decarbonization refers to reducing carbon dioxide resulting from power generation and consumption, and electrification means replacing equipment that runs on fossil fuel with electrically powered equipment. You’ve probably also heard of Energy Use Intensity or “EUI,” generally assumed to be the annual total energy consumed by a building divided by the total floor area of a building. But is it enough to only measure what an operational building consumes within its very walls, otherwise known as site EUI?

The answer is no. Generally speaking, it’s not even close. We must also consider the energy source, which includes embodied energy it takes to make and transfer that energy (fossil fuels, electricity, solar, etc.) to its actual point of use. Omitting source energy from our comparisons of building energy performance can be very misleading.

Let's say a building is totally electric, and all its energy is supplied from the grid. The grid is most likely powered by a combination of fossil fuel, hydropower, nuclear and renewable energies. So, all the energy the building consumes is "secondary" or site energy. Meanwhile, the building down the street uses primary energy (e.g., natural gas for heating and hot water) and secondary energy (electricity for lights, appliances, computers, etc.). If we compare these two buildings' "site" usages, we are not comparing apples to apples because primary energy represents raw fuel while secondary energy represents converted fuel. As such, they are not equivalent.

Recognizing this fact, the EPA has determined that “source energy” rather than “site energy" should be used to evaluate the energy usage of a building. Source energy "incorporates all transmission, delivery, and production losses. By considering all energy use, the score provides a complete assessment of energy efficiency in a building."

The EPA's ENERGY STAR Portfolio Manager, a no-cost, interactive energy management tool, applies site/source ratios to convert site energy units into source energy units for a more equitable and realistic energy evaluation of buildings (Table 1).

Table 1. energy star site/source Ratios

Applying these ratios to building systems yields some eye-popping comparisons regarding heating.

A quick look at Table 2 clarifies why ASHRAE has chosen to focus on heat pump technology as a critical strategy for decarbonization. With a coefficient of performance (COP) of 4.0, an electric, geothermal heat pump consumes 700 MBtu of source energy compared to district steam, the next closest non-heat pump alternative, which consumes 1264 MBtu of source energy.

table 2. Comparison of Alternate Heating Scenarios

This comparison also illustrates why JMP believes water-source heat pumps will become the preferred choice for commercial HVAC and domestic hot water.