Drinking the Heat Pump Kool-Aid
I just finished reading the piece on heat-pump water heaters in the October issue of EBN [I just don’t get the support for heat pumps. The process is sold as “free heat from the ground” and “simply moving heat from one place to another.” However, the truth is that the compressor used to move all this free heat is an energy hog and uses tons of electricity to do so. It’s a compressor just like the one in your air conditioner—the same kind that your mother and father told you not to run that often back in the ’70s when people cared about energy conservation and saving money! The only difference is that with a heat pump you run it 12 months a year instead of just a couple of weeks in the hot summer months! ].
Also, when you use steam to move a turbine to create electricity at a power plant, 30% of the heat from the fuel creates the steam, but the other 70% of the heat escapes unused into the atmosphere. What that means is that by the time that electricity gets to your house you only get 30% of the heat value. So even if a heat pump has a coefficient of performance of 200%, what you are getting is 200% of 30%—in other words 60%. Compare that to a 95%-efficient natural gas heater/boiler that produces the heat right where it’s needed.
I realize that natural gas will always produce carbon and that, in theory, electricity can be produced using 100% sustainable sources, but with solar and wind producing only 2%–3% of America's electricity today, we are a long way from electrical generation being carbon-free and sustainable.
Somehow heat pumps have become the holy grail (kind of like radiant floor heat), even getting tax credit status under the American Reinvestment and Recovery Act, but in my mind high-efficiency natural gas heating is still a more sustainable and green choice for the near and mid-term future.
ChicagoEditors’ response:Mr. McGrath makes some very good points in his letter—some of them mirroring opinions that EBN has espoused for years. Ground-source heat pumps are commonly oversold; it often makes more sense to put money into improving the building envelope. And it is certainly important to consider primary (source) energy in addition to site energy when selecting heating equipment. Our article addresses heat-pump water heaters, which provide a much higher-efficiency alternative to conventional electric water heaters (which account for approximately 39% of all water heaters in the residential sector—some 43 million). Electric water heating is so prevalent in the U.S. that we believe electric alternatives need to be presented. If electricity is to be used for water heating, it is far better to use those valuable kilowatt-hours in heat-pump models than electric-resistance models. And if the COP of these units exceeds 3.0, as some manufacturers claim, then they are also competitive with natural gas on a carbon-emissions basis. An increasingly relevant benefit of electric water heating (and even electric heating) is that we can generate some—or even all—of our electricity onsite, using renewable sources—while fossil fuels do not offer that option. All that said, we much prefer the solar water heating option.
Published October 30, 2009 Permalink