Heating Efficiency

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Which is better: gas or electric heat?

Big Ideas: 
  • Calculating or measuring efficiency means tracking the power all the way from source to end-user

Are electric heaters really 100% efficient?

When people hear that electric heaters are 100% efficient it is natural to assume they will be cheaper and less polluting than gas furnaces. However the story isn’t always that simple. To figure out which kind of heat is better, we need to understand what efficiency means, and also where electricity comes from.

Efficiency of Electric and Gas Heaters

Efficiency is broadly defined to be an output to input ratio:

$ \textnormal{Efficiency}, \eta=\dfrac{\textnormal{useful output}}{\textnormal{total input}} $

In the case of a heater:

$ \textnormal{Efficiency}, \eta=\dfrac{\textnormal{heat energy output}}{\textnormal{fuel energy input}} $

A gas furnace produces heat by burning a fuel (e.g. natural gas) and then directing that heat into your home. However, exhaust gases that carry some of the heat from burning the gas are vented to the outside. Because of this heat loss, gas furnaces are never 100% efficient. An old furnace may be as little as 60% efficient, but modern furnaces have an efficiency of 78% - 84%, while new condensing gas furnaces are 90% - 97% efficient 1

By contrast, an electric heater is just a big resistor that converts electrical energy into heat. Because it can convert ALL of the incoming electricity into heat, we would say that it is 100% efficient. However, one cannot make a direct comparison like this; we should consider the fact that this heater needs to be fueled by electricity, and therefore we need to look at the efficiency and pollution associated with where that electricity comes from.

Sources of Electricity in BC

To consider the efficiency and pollution of our electrical system we need to know how that electricity is generated. In British Columbia, around 90% of the electricity that produced within the province (i.e. not imported from elsewhere) comes from hydroelectric dams. 2 The remainder is produced by the Burrard Natural Gas thermal generation plant, other small facilities, and imports from coal-fired power stations outside the province.

For the purposes of illustration, let’s look at the efficiency of our electric heater assuming that ALL of the electricity comes from a natural gas power plant. This allows us to do a clear comparison with a natural gas furnace by starting with the same material: raw natural gas.

Electricity Generation and Distribution in BC

As discussed previously (in the articles on Transmitting electricity and Transformers), electricity is produced at a power plant and then sent to the consumer using a mix of high voltage and medium voltage lines. The high voltage is used for long distance transmission, and then is stepped down to a medium voltage for distribution in the town. Right before it enters your house a third transformer brings the voltage down to the household voltage of 120 and 240 V.

 

To figure out the overall efficiency of turning natural gas into heat in your home we need to know the efficiency of each step of this process.

1. Transformers: Transformers are highly efficient, typically around 99% 3. Having three transformers in our circuit will result in losses of about 3%.

2. High Voltage Transmission: We can estimate these losses using a specific example of transmission from a natural gas power plant to a home in Vancouver. The nearest natural gas generation facility to Vancouver is the Burrard Natural Gas generation plant in Port Moody. This facility has a capacity of 950 MW 4, uses a transmission voltage of 500 kV 5, and is approximately 10 km from the centre of Vancouver. Because this plant is so massive, we’ll assume that its electricity is transmitted with 10 standard high-voltage lines. Using our knowledge of transmission losses discussed in a previous lecture, we can calculate that there will be power loss of approximately 1% in this segment of the transmission

3. Medium Voltage (or “Neighbourhood” transmission): Vancouver uses 25 kV for its neighbourhood distribution voltage6. Based on the size of the city, we can assume that everybody is within 3 km of the closest transforming substation. We will also assume that a single substation only handles 10% of the Burrard station output, and serves 100 medium-voltage transmission lines. Using these assumptions, we estimate that there will be a 2% loss for the distribution segment of the transmission.

4. Household Voltage: Because the distances in the household portion are so small, we will neglect those losses.

5. Generation: The last thing we need to consider is the efficiency of generating the electricity in the first place. The efficiency of the generation plant is defined as:

$ \textnormal{Efficiency}, \eta=\dfrac{\textnormal{electrical energy generated}}{\textnormal{chemical energy of input fuel}} $

The efficiency of natural-gas turbine generators are between 48 – 54% 7,8. If we take the middle of this range that gives us an efficiency of 50% for our generation facility.

Overall Efficiency of Converting Natural Gas to Electric Heat

Because the output of each system is the input of the next system, we can find the overall efficiency by multiplying the efficiencies of each step all together.

We find that the total losses for transmission and distribution are around 5.9%. This matches very closely with the reported mean losses for the U.S., which were 6.1% in 2005 9, so we know we are on the right track..

The overall efficiency for converting natural gas to electricity and then into heat will then be:

<br />
\begin{eqnarray}<br />
{\eta}_{total} &=& {({\eta}_{\textnormal{transformer}})}^{3}({\eta}_{\textnormal{high voltage}})({\eta}_{\textnormal{medium voltage}})({\eta}_{\textnormal{generation}})\nonumber \\<br />
&=& {(99\%)}^{3}(98\%)(99\%)(51\%)\nonumber \\<br />
&=& 49\% \nonumber \\<br />
\end{eqnarray}\\<br />

This is MUCH less efficient than burning natural gas in a modern household furnace.  It may be a surprise but it makes sense… we are comparing the process of turning natural gas directly into heat with the process of turning natural gas into electricity and then into heat. The latter process has more steps and so it makes sense that it’s less efficient.

This result points to the importance of considering the efficiency of source of your energy, and not just the efficiency of the final object that consumes it. With that in mind, let’s re-examine our analysis taking into consideration the fact that in BC only 7.5% of our electrical energy is produced by natural gas.

A More Realistic Comparison of Environmental Impact

Now let’s try to compare a natural gas furnace with a realistic electric heater that is powered by a mixture of electricity generation facilities. However now the comparison becomes harder: the electricity generation facilities don’t all use the same fuel, so we can’t just compare on the basis of efficiency.

Instead, let’s compare these two heaters based on the amount of CO2 they produce. Carbon dioxide is not the only environmental impact of generating electricity, but it is a very important one.

In British Columbia, most of our electricity is produced in hydroelectric power stations that have extremely low greenhouse gas emissions. The average in BC is 7.8 g CO2e/ MJ of energy generated 10. After we take the 6% losses for transmission and distribution into account, we can estimate that in BC, an electric heating has a carbon footprint of 8.3 g CO2e/ MJ.

We might also want to consider electricity generation in Canada more broadly. Because other parts of Canada have a higher proportion of electricity generated by burning fossil fuels, the average greenhouse gas emissions are 48 g CO2e/ MJ of energy generated 11. If we assume the same 6% losses for transmission and distribution, we can estimate that in other parts of Canada, electric heating has a carbon footprint of 54 g CO2e/ MJ.

In comparison, burning natural gas emits 50.4 g CO2 / MJ of heat released 12. Taking the 80% efficiency of a natural gas furnace into account, we can estimate that a natural gas furnace emits 63 g CO2/ MJ of heat, much more than an electric heater in BC, but a little bit less than an electric heater in Canada. Given that the US gets 50% of their electricity from coal, the emissions there will be much higher.

The key point here is that if you’re concerned about GHG emissions, it matters where you get your electricity from. Because BC has an abundance of clean electricity, electric heat emits less CO2 per MJ than natural gas furnaces.

Comments

I would include all the

I would include all the calculations and post it. We should have a very similar article about electric cars. Andrzej.

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