Efficiency is broadly defined to be an output to input ratio:<\/p>\n
$\\textnormal{Efficiency}, \\eta=\\dfrac{\\textnormal{useful output}}{\\textnormal{total input}}\\tag{1}$<\/p>\n
In the case of a heater:<\/p>\n
$\\textnormal{Efficiency}, \\eta=\\dfrac{\\textnormal{heat energy output}}{\\textnormal{fuel energy input}}\\tag{2}$<\/p>\n
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 [note]Choosing The Right Condensing Gas Furnace. (2009, 05 08). Retrieved 10 22, 2012, from Natural Resources Canda: http:\/\/oee.nrcan.gc.ca\/equipment\/manufacturers\/14768<\/a>[\/note]\n 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.<\/p>\n Sources of Electricity in BC<\/strong><\/p>\n 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. [note]Generation System. (2010, 03 25). Retrieved 09 22, 2010, from BC Hydro: http:\/\/www.bchydro.com\/energy_in_bc\/our_system\/generation.html<\/a>[\/note] 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.<\/p>\n For the purposes of illustration, let\u2019s 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.<\/p>\n Electricity Generation and Distribution in BC<\/strong><\/p>\n As discussed previously (in the articles on Transmitting electricity<\/a> and Transformers<\/a>), 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.<\/p>\n Figure 1.<\/strong><\/p>\n 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.<\/p>\n 1. Transformers:<\/strong> Transformers are highly efficient, typically around 99% [note]Buskirk, R. V. (n.d.). Distribution Transformers. Retrieved 09 22, 2010, from Energy Efficiency Standards: http:\/\/ees.ead.lbl.gov\/projects\/current_projects\/distribution_transformers<\/a>[\/note]. Having three transformers in our circuit will result in losses of about 3%.<\/p>\n 2. High Voltage Transmission:<\/strong> 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 [note]Thermal Generation System. (n.d.). Retrieved 09 22, 2010, from BC Hydro: http:\/\/www.bchydro.com\/energy_in_bc\/our_system\/generation\/thermal_generation.html<\/a>[\/note], uses a transmission voltage of 500 kV [note]Private communication with BC Hydro Engineer[\/note], and is approximately 10 km from the centre of Vancouver. Because this plant is so massive, we\u2019ll assume that its electricity is transmitted with 10 standard high-voltage lines. Using our knowledge of transmission losses discussed in a previous lecture<\/a>, we can calculate that there will be power loss of approximately 1% in this segment of the transmission<\/p>\n 3. Medium Voltage (or \u201cNeighbourhood\u201d transmission)<\/strong>: Vancouver uses 25 kV for its neighbourhood distribution voltage[note]Private communication with BC Hydro Engineer[\/note]. 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.<\/p>\n 4. Household Voltage:<\/strong> Because the distances in the household portion are so small, we will neglect those losses.<\/p>\n 5. Generation:<\/strong> 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:<\/p>\n $\\textnormal{Efficiency}, \\eta=\\dfrac{\\textnormal{electrical energy generated}}{\\textnormal{chemical energy of input fuel}} \\tag{3}$<\/p>\n The efficiency of natural-gas turbine generators are between 48 \u2013 54% [note] Bellman, D. K. (2007, 07 18). Power Plant Efficiency Outlook. Working Document of the NPC Global Oil & Gas Study , pp. 1-31.[\/note],[note]Combined Cycle Power Plants. (n.d.). Retrieved 09 22, 2010, from Cogeneration Technologies: http:\/\/cogeneration.net\/combined-cycle-power-plants\/<\/a>[\/note]. If we take the middle of this range that gives us an efficiency of 50% for our generation facility.<\/p>\n Overall Efficiency of Converting Natural Gas to Electric Heat<\/strong><\/p>\n 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.<\/p>\n 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 [note]Fesmire, B. (2007, 07 09). The overall efficiency for converting natural gas to electricity and then into heat will then be:<\/p>\n \\begin{eqnarray} This is MUCH less efficient than burning natural gas in a modern household furnace. \u00a0It may be a surprise but it makes sense\u2026 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\u2019s less efficient.<\/p>\n 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\u2019s re-examine our analysis taking into consideration the fact that in BC only 7.5% of our electrical energy is produced by natural gas.<\/p>\n A More Realistic Comparison of Environmental Impact<\/strong><\/p>\n Now let\u2019s try to compare a natural gas furnace with a realisti<\/em>c<\/em> electric heater that is powered by a mixture of electricity generation facilities. However now the comparison becomes harder: the electricity generation facilities don\u2019t all use the same fuel, so we can\u2019t just compare on the basis of efficiency.<\/p>\n Instead, let\u2019s compare these two heaters based on the amount of CO2<\/sub> they produce. Carbon dioxide is not the only<\/em> environmental impact of generating electricity, but it is a very important one.<\/p>\n 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 CO2<\/sub>e\/ MJ of energy generated [note]BC Hydro. (2010, 3 25). EN16 Greenhouse Gas Intensities. Retrieved 09 22, 2010, from BC Hydro: http:\/\/www.bchydro.com\/about\/accountability_reports\/2011_gri\/f2011_environmental_EN16_2.html<\/a>[\/note]. 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 CO2<\/sub>e\/ MJ.<\/strong><\/p>\n 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 CO2<\/sub>e\/ MJ of energy generated [note]Office of Energy Efficiency. (2010, 08 25). Energy Efficiency Trends in Canada, 1990 to 2009. Retrieved 10 23 2012, from Natural Resources Canada: http:\/\/oee.rncan.gc.ca\/publications\/statistics\/trends11\/chapter3.cfm?attr=0<\/a> [\/note]. If we assume the same 6% losses for transmission and distribution, we can estimate that in other parts of Canada, <\/strong>electric heating has a carbon footprint of\u00a0<\/strong>54 g CO2<\/sub>e\/ MJ.<\/strong><\/p>\n![\"\"](\"https:\/\/c21-wp.phas.ubc.ca\/wp-content\/uploads\/2010\/09\/HE1.png\")
<\/p>\n
\nEnergy Efficiency in the Power Grid. Retrieved 09 22, 2010, from Renewable Energy World: http:\/\/www.renewableenergyworld.com\/rea\/news\/article\/2007\/07\/energy-efficiency-in-the-power-grid-49238<\/a>
\n[\/note], so we know we are on the right track..<\/p>\n
\n{\\eta}_{total} &=& {({\\eta}_{\\textnormal{transformer}})}^{3}({\\eta}_{\\textnormal{high voltage}})({\\eta}_{\\textnormal{medium voltage}})({\\eta}_{\\textnormal{generation}})\\nonumber \\tag{4} \\\\
\n&=& {(99\\%)}^{3}(98\\%)(99\\%)(51\\%)\\nonumber \\\\
\n&=& 49\\% \\nonumber \\\\
\n\\end{eqnarray}<\/p>\n