Water Heating in the Zero Emissions Age

2015, Building Technology

Water Heating in the Zero Emissions Age

Ryan Pollin for Zondits, March 2, 2015

Although standard water heaters come in a few configurations, they have yet to see a significant technology interruption in the same way that HVAC has been swarmed with variable frequency drives, or lighting has been rocked by CFLs and LEDs. Take a look at this energy end-use graphic for residential buildings from the Center for Climate and Energy Solutions.

Residential Building Energy End Use
Residential Buildings Total Energy End Use (2006). Graphic courtesy of Center for Climate and Energy Solutions.

Water heating is 12% of this pie, and it is due for disruption.

In the quest for zero emissions, we might first want to try moving to electric resistance water heaters, and pile on the responsibility of emissions reduction to the power generation side of the equation. An electric water heater has an energy factor (EF), or COP, of 1. That means 100% of its input energy is converted to heat. However, to input that energy, it must endure severe losses in transmission and distribution and power plant efficiency losses. Typically, a direct-fired water heater, which burns natural gas or another fossil fuel at the point of use, wins out on efficiency when compared to the grid. How can we go electric and free ourselves from the dreaded EF of 1? The answer is heat pump water heaters.

With a heat pump, electric energy input powers our old friend the vapor-compression refrigeration cycle. An ENERGY STAR air-air heat pumps for space heating must meet an EER of 12‒that is, 12 Btu of cooling energy out for every watt-hour of electric energy input. A quick unit conversion for that yields an EF of 3.4! Now, today’s ENERGY STAR heat pump water heaters need an EF of 2, and the best get 3.1. The extra heat exchange between the air source to a water primary loop means some losses in efficiency, but the efficiency is climbing. With the water source heat pump, we triple our efficiency over conventional electric water heaters. Not bad!

So the big question is: Do they compete with direct-fired heaters? Well, the best natural gas water heaters are 99% efficient, meaning that if your electricity’s generation and T&D efficiency are anything above 33%, the heat pump water heater wins out. To demonstrate, I’ll calculate for the energy mix in my area:

Note: Efficiency from non-fossil fuel power plants is given as 100% here because efficiency is being used as a proxy for emissions. In reality, nuclear has an efficiency of 33%, hydroelectricity 90%‒95%, and non-hydro renewables vary widely depending on the technology in place.
[i ii iii] Note: Efficiency from non-fossil fuel power plants is given as 100% here because efficiency is being used as a proxy for emissions. In reality, nuclear has an efficiency of 33%, hydroelectricity 90%‒95%, and non-hydro renewables vary widely depending on the technology in place.
So there we have it: heat pump water heaters are already more efficient than direct-fired water heaters, by a factor of almost 2. Moreover, and this is the far more exciting part, they are compatible with a zero emissions energy future.

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i http://oaspub.epa.gov/powpro/ept_pack.charts

ii http://www.eia.gov/electricity/annual/html/epa_08_01.html

iii http://data.worldbank.org/country/united-states

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