Abstract
We can use natural gas or electricity to heat our homes. And we can generate electricity with a coal fired power plant or a combined cycle gas turbine (CCGT). How does the mix of energy supply and demand impact our net production of CO2?
References
In the spirit of transparency, here are the references. Some of the data is a bit old, but it is consistant and well vetted.
- Table 8.2. Average Tested Heat Rates by Prime Mover and Energy Source, 2007 – 2017, https://www.eia.gov/electricity/annual/
- Carbon Dioxide Emissions Coefficients, https://www.eia.gov/environment/emissions/co2_vol_mass.php
- High efficiency cold climate heat pump, https://www.energy.gov/sites/prod/files/2016/04/f30/32212_Shen_040616-1135.pdf
Source code (Jupyter notebook and PDF file) are available here.
Basis
I will do the calculations based on 10 GJ of heat energy consumed for heating a home (pretty typical from my utility bill for October or November in Calgary).
I will describe the effectiveness of either gas or electricity for providing heat to our homes, and the efficiency of producing electrical power from the two thermal sources.
Heat Sources
| heatSource | energyUtilization | basis | heatProdn | |
|---|---|---|---|---|
| 0 | NatGasFurnace | 0.9 | NatGas | 0.9 |
| 1 | resistanceHeat | 1.0 | Electricity | 1.0 |
| 2 | heat Pump 52F 11C | 4.0 | Electricity | 4.0 |
| 3 | heat Pump 32F 0C | 3.3 | Electricity | 3.3 |
| 4 | heat Pump 0F -18C | 2.6 | Electricity | 2.6 |
The natural gas furnace is assumed to be 90% efficient. Resistance heating is 100% efficient, but we recognize that electricity is a much higher value form of energy than burning fuel.
The Coefficient of Performance for an air source heat pump depends on the ambient temperature (values taken from Shen article). At 11 C (52 F), one unit of electrical energy can supply the house with 4 units of heat. At a low winter temperature of -18 C (0 F) the heat pump would only supply the house with 2.6 units of heat.
Electrical generation sources
| generator | BTU_per_kWhr | fuel | |
|---|---|---|---|
| 0 | BoilerCoal | 10353 | Coal |
| 1 | BoilerNatGas | 10353 | NatGas |
| 2 | simpleGT | 11176 | NatGas |
| 3 | CCGT | 7649 | NatGas |
| 4 | Wind | 0 | Wind |
The efficiency fo a combined cycle gas turbine (50% from this data) over a coal fired boiler (37%) is obvious. This data is a few years old and there have been improvements in turbine efficiency in the last 5 or 10 years.
CO2 intensity
| fuel | kgperMMbtu | |
|---|---|---|
| 0 | NatGas | 53.07 |
| 1 | Coal | 95.35 |
| 2 | Wind | 0.00 |
Energy Cost
| basis | CAD_per_unit | unit | notes | CADperGJ | |
|---|---|---|---|---|---|
| 0 | NatGas | 8.786426 | GJ | from utility bill | 8.786426 |
| 1 | Electricity | 0.166501 | kWhr | from utility bill | 46.250213 |
The cost of electricity is just over five times that of natural gas, on an equivalent energy basis (from my utility bills). Alberta has fairly expensive electricity. The thermal generation mix is approximately 43% coal, 44% simple cycle GT and 13% CCGT.
Energy Demand
Now we calculate the amount of energy (gas or electric) needed to provide 10 GJ of heat to the home.
| heatSource | energy Utilization | basis | heat Prodn | base Heat GJ | heat Consumed GJ | gasGJ | elecGJ | |
|---|---|---|---|---|---|---|---|---|
| 0 | NatGasFurnace | 0.9 | NatGas | 0.9 | 10.0 | 11.111111 | 11.111111 | 0.000000 |
| 1 | resistanceHeat | 1.0 | Electricity | 1.0 | 10.0 | 10.000000 | 0.000000 | 10.000000 |
| 2 | heatPump52F11C | 4.0 | Electricity | 4.0 | 10.0 | 2.500000 | 0.000000 | 2.500000 |
| 3 | heatPump32F0C | 3.3 | Electricity | 3.3 | 10.0 | 3.030303 | 0.000000 | 3.030303 |
| 4 | heatPump0F-18C | 2.6 | Electricity | 2.6 | 10.0 | 3.846154 | 0.000000 | 3.846154 |
In short, supplying 10 GJ to our use requries 11 GJ of natural gas. Or, if we are using a heat pump, between 2.5 — 3.8 GJ of electricity (depending on the outside temperature).
Electrical power generation
Determine the CO2 emissions from each of our thermal power plants.
| generator | BTU per kWhr | fuel | kW per kWhr | specific Consumption | effic | kg per GJ | |
|---|---|---|---|---|---|---|---|
| 0 | BoilerCoal | 10353 | Coal | 10931.42211 | 3.036506 | 0.329326 | 305.706950 |
| 1 | BoilerNatGas | 10353 | NatGas | 10931.42211 | 3.036506 | 0.329326 | 170.150685 |
| 2 | simpleGT | 11176 | NatGas | 11800.40312 | 3.277890 | 0.305074 | 183.676621 |
| 3 | CCGT | 7649 | NatGas | 8076.34963 | 2.243430 | 0.445746 | 125.710672 |
| 4 | Wind | 0 | Wind | 0.00000 | 0.000000 | inf | 0.000000 |
Our coal fired power plant produces the most CO2 for a GJ of electrical energy. A simple cycle gas turbine is slightly less efficient than a coal fired plant, but benifits from the lower CO2 intensity of natural gas compared to coal. The clear winner for emissions is the combined cycle gas turbine.
Supply and Demand
Now we can look at the impact of heating our home with different methods, in terms of CO2 emissions. We need to consider which source of energy ramps up to meet the demand from the additional load provided by our house. If there are different electrical generators on the grid (coal fired, CCGT, wind), it is not obvious if the coal fired plant or the CCGT will increase output when there is an increase in the base load (from an additional home).
I will construct a table for comparison.
| heatSource | BoilerCoal | BoilerNatGas | simpleGT | CCGT | cost | |
|---|---|---|---|---|---|---|
| 0 | NatGasFurnace | 622.611343 | 622.611343 | 622.611343 | 622.611343 | 97.626958 |
| 1 | resistanceHeat | 3057.069498 | 1701.506851 | 1836.766210 | 1257.106723 | 462.502127 |
| 2 | heatPump52F11C | 764.267374 | 425.376713 | 459.191552 | 314.276681 | 115.625532 |
| 3 | heatPump32F0C | 926.384696 | 515.608137 | 556.595821 | 380.941431 | 140.152160 |
| 4 | heatPump0F-18C | 1175.795961 | 654.425712 | 706.448542 | 483.502586 | 177.885433 |
Discussion
First off, electrical resistance heating has the highest CO2 emissions for all of the methods. Consider the case where my electricity were generated by wind. If there is a coal fired power plant on the grid, the planet would be better off if I burned natural gas for my home and used the wind power to back out the coal fired power plant.
If the balance for electrical supply comes from a coal fired power plant, then a natural gas furnace will always result in fewer CO2 emissions than an electrical powered heat pump.
The story changes if the balance for electricaly supply comes from a CCGT. Here, the heat pump is the clear winner, even in fairly cold temperatures of -18 C (0 F). This result surprised me, but the math does not lie. We should be making energy decisions based on statistical cases (not rare extremes): we need to consider the merits of a heat pump with CCGT, particularly in climates with moderate winters.
Another way to assess the results is to look at the heating cost (based on my energy prices). Now, the heat pump is only competitive if the ambient temperature is a very mild 11 C (55 F). Using natural gas to heat your home is less expensive, and it also requires less infrastructure for generating electricity. Less infrastructure means less energy that was consumed to produce the necessary steel and concrete.
But what about those rare winter cases that stretch the power generation capacity to the limit (ie, the Texas cold snap)? One backup would be a natural gas fireplace (70% efficient, no electricity needed) as a heat source that is independant of the power grid. Camping out in the living room is much better than the alternative. A more expensive (but less reliable) backup would be to use a mid-efficiency (80%) gas furnace as the air handler for the heat pump. Home-owners would then be able to heat their homes with natural gas, off-load the power generating facility, and allow electricity to be used where there are not options for energy supply.
