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Is Geothermal Really Green?

Is Geothermal Really Green?

By: Cary Weiner, 9/17/17

The City of Denver is considering adopting a target to reduce greenhouse gas emissions by 80% from 2005 levels before 2050. To do so, a task force is recommending that commercial buildings and 200,000 households shift from natural gas to heat sources that do not lead to carbon pollution. Other cities and towns in Colorado with aggressive GHG reduction targets may face similar situations in order to make their goals a reality. So, we wonder, what might be the best way to transition our heating systems?

Geothermal, or ground source, heat pumps (GSHPs) have long been touted as a green solution to heating and cooling. They are highly efficient, generating about three times as much energy as they use by leveraging the relatively constant temperature of the earth as a heat source and sink. And they run off of the electric grid, which can be cleaned up over time. But how does a GSHP stack up against conventional heating and cooling systems using grid electricity as it stands today?

In Xcel service territory, every kilowatt-hour (kWh) consumed emits about 1.3 lbs. of CO2. One therm of natural gas emits 11.7 lbs. To compare apples to apples, kWhs and therms can be converted into BTUs. When this is done, we see that each mBTU of electric energy in Xcel service territory emits 381 lbs. of CO2, whereas each mBTU of natural gas burned emits 117 lbs. of CO2. This is a significant difference!

So in terms of moving from say, a 95% efficient natural gas furnace and conventional air conditioner to a GSHP, we might expect the GHG results to be marginal. And indeed that’s exactly what the math shows. On the heating side of the equation, a high efficiency gas furnace actually emits 3.6% less GHGs than a GSHP using the assumptions below. On the cooling side, because air conditioners pull electricity from the grid but do so less efficiently than GSHPs, they emit 23.4% more than GSHPs. Put together, because we heat more than we cool in Colorado, GSHPs show a mere 2.3% GHG savings over the gas furnace plus air conditioner combo.

So what are the takeaways? First off, there is much more to the decision to convert from conventional heating and cooling to a GSHP: land availability and upfront cost are two big factors. Second, we can agree that going green requires more than using the most efficient technology available – it also requires that we power our electric grid with clean energy such as solar and wind. Third, therefore, continued focus by communities and utilities alike on greening the grid is justified if we are to make a significant dent in GHG emissions.

Assumptions:

  • 589 therms used for heating annually
  • 95% efficient natural gas furnace
  • 0 GSHP coefficient of performance
  • 7 lbs. CO2 per therm
  • 3 lbs. CO2 per kWh (per Xcel’s ‘Energy and Carbon Emissions Reporting 2016 Summary’)
  • 1,500 kWh used for cooling annually
  • 13 SEER air conditioner (12.3 adjusted)
  • 16 EER GSHP
  • =8,859 lbs. CO2 per year from gas furnace + air conditioner
  • =8,659 lbs. CO2 per year from GSHP

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