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Comment by aero_code | Hacker News Reader

I don't think the numbers are accurate in the quantity of gas. Since kWh and BTU are both units of energy, finding the cf of gas is unnecessary (assuming the efficiency numbers are correct).

1 kWh = 3.6 megajoules and 1 BTU = 1055 joules

The 6.6 kWh of the heat pump is 23.76 MJ which is 22,521 BTU of energy. Assuming that the power plant and distribution are 60%, it would take 37,535 BTU of gas to produce (22,521/60%).

Instead, using that 37,535 BTU of gas in an 80% efficient furnace would only produce 30,028 BTU of heat, which is worse than the 50,000 BTU from the heat pump.

I'm pretty sure even a poor heat pump will be more efficient than heating directly with gas. (Of course, they have drawbacks, like they can leak their refrigerant that causes more of a greenhouse effect than CO2.)

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> "(Of course, they have drawbacks, like they can leak their refrigerant that causes more of a greenhouse effect than CO2.)"

My heat pump contains 2.1 kg of R32 refrigerant. R32 has a GWP of 675, so that 2.1 kg is the equivalent of 1417 kgs of CO2. (older refrigerants were much worse!)

Heat pumps should never leak their refrigerant during their lifetime, and installers will remove and recycle the refrigerant when servicing or decommissioning systems. But of course, accidents happen, so let's pessimistically assume that 50% of systems installed will eventually leak. In the real world it's hopefully far less than that, but that would mean on average 708 kg CO2e in refrigerant is emitted per system over its lifetime.

On the other hand, heating a typical US home with natural gas emits 2900 kgs of CO2 per year.

I think it's safe to say that the climate impact of refrigerant leaks in modern heat pump systems is minuscule compared to that of the CO2 emitted from natural gas heating.

You kinda do need to figure that out - EIA says that it takes 7.42cf of gas to make 1kWh of energy.

https://www.eia.gov/tools/faqs/faq.php?id=667&t=8

I dont know where EIA gets those numbers, but that was the basis of my calculation. Maybe I shouldn't have multiplied that by the efficiency of the plant, but rather just taken of distribution losses.

They are averaging the efficiency from the current fleet of gas turbines after subtracting the useful heat output and coming up with 44.4%.

However, it’s a misleading number in multiple ways because the fleet is made up of a mix of low and high efficiency turbines. Grid operators use a mix of turbine types as a cost optimization, a far cheaper and far less efficient turbine that’s only used 1% of the time it worth it. The average number of kWh per cf of gas is therefore heavily in favor of high efficiency turbines.

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