December 20, 2014

Gas or Electricity?

At eTool, we’re passionate about reducing the carbon intensity of buildings first and foremost. It’s what we live and breathe, so we try to look at every consideration when putting our design recommendations forward…we don’t take it lightly!

Here we give some key considerations to help you make a sound decision when choosing between gas and electricity.

Building-Life-Cycle-Green

Public Perceptions

Industry and government have portrayed electricity as a clean and efficient source of energy, and it is, at the point of use. Perceptions of gas (sorry, “Natural Gas”) are similarly affected by public opinion and government policy that focus on the point of use. This ‘point of use’ perception is reinforced by the way most people interact with electricity and natural gas in their everyday lives, flipping a switch or turning on a burner and paying a monthly bill. They rarely see or understand the generation side of electricity, the power plant, the extraction and transportation of natural gas or the ultimate carbon emissions associated with combustion.

The Life Cycle Story of Electricity

Apart from solar photovoltaic (still a tiny contributor to most electricity grids) all our electricity starts life on the output side of a turbine. These turbines are usually driven by steam, but there are some power plants that use wind or water (Hydro). So where does the steam come from? To make it we need heat, and lots of it. At present we’re getting the overwhelming majority of this heat from coal or gas (the only real exceptions here are reactors that use nuclear power, geothermal plants that use the heat deep under the Earth’s crust, or solar thermal energy).

Coal is the cheapest form of heat for power plants. Unfortunately though it’s also the dirtiest. The furnace exhaust of a coal-fired boiler is full of green house gases and also abrasive fly ash which renders it very unsuitable for heat recovery (it basically wears components out way too quickly to make it viable). In fact, most coal-fired power plants are lucky to capture one third of the heat energy as electricity.

When power plants use gas, there are two advantages. Firstly, burning gas releases less greenhouse gas than coal for equal amounts of heat (it’s still very bad for the planet, but not as bad as coal). Secondly, the waste gases produced from gas-fired turbines can be sent through a heat exchanger which drives a secondary turbine. The combined cycle gas fired turbines capture up to 60% of the heat and turn it into electricity (nearly twice as efficient as normal coal-fired plants).

Once the electricity is produced, we lose more of this energy during transmission and distribution. In Australia, about 7.5% of the electricity generated at the power plants is lost before it gets to the consumer. So when you look at the life cycle of fossil fuel generated electricity, you can see that we’re not getting much bang for buck.

There are also some other minor hidden impacts of electricity generation, although these impacts are very small compared to the CO2 released during combustion. They include:

  • Parasitic loads, which basically means they use some of the electricity generated to run the plant itself.
  • The carbon emissions associated with extracting the fuel source. This can be due to fugitive emissions (for example, methane that escapes from a coal seam as it is mined, crushed and transported, or gas that needs to be flared during processing)
  • The carbon emissions associated with transporting the fuel to the power plant
  • The embodied emissions of the power plant infrastructure

Using electricity in a building for heating water, cooking or space heating is the final step in the chain. The use of electricity at the home doesn’t add any environmental impacts (you don’t release any CO2 in the building, that all happened up stream).

The Life Cycle Story of Gas

Likewise, the natural gas used by a building must be extracted from the ground, processed or refined to remove impurities, and finally transported to the residence. All of these extraction, processing, and transportation steps require energy. And of course there’s the fugitive emissions of gas that escapes at the well, refinery or during transport. In total however, direct use of natural gas in the home is usually far more efficient than sending it to the power station to turn to electricity before returning that electricity to your home. The valid concerns regarding the latest significant source of gas production from “fracking” should not be discounted and may alone be enough to counter any climate change argument for the use of gas.

Of course when you burn gas you emit a lot of carbon dioxide at the building. In most cases however, you can capture a lot more of the heat than is usually captured by your electricity generator when they burn fossil fuels (particularly if they are relying largely on coal which is certainly the case in Australia). For example, good quality gas boilers and instantaneous water heaters now run at 85% efficiency as opposed to coal-fired power plants (33%) and combined cycle gas-fired power plants (55%). So generally you get more “use” out of the gas by burning it on site.

Some Comparisons

Let’s now look at some examples of how electricity compares to gas in terms of local grid networks in different regions. Below we estimate the energy demand of an average house (2.4 people) and look at how that can be delivered with a range of technologies using both electricity and gas. We have chosen a number of regions to demonstrate the importance of local characteristics (largely relating to electricity energy sources).

  • Victoria: Present heavy reliance on black and brown coal for electricity, standard gas supply impacts
  • Tasmania: High renewable hydro content for electricity, standard gas supply impacts
  • Western Australia: The home of eTool, electricity generated largely by gas and coal fired plants with quite low thermal efficiencies, standard gas supply impacts
  • United Kingdom: A fairly typical greenhouse gas profile for Europe (mix of gas, coal and nuclear with small but growing renewables content). Far lower greenhouse gas intensity than the average Australian electricity supply. Standard gas supply impacts
  • Sweden: A very low carbon electricity grid. Gas supply not common, assumed standard distributed gas impacts

Cooking – Cook Tops

Technology

Assumed Technology

Efficiency

(%)

End Use Energy Demand

(MJ)

Greenhouse Gas Emissions

(kgCO2e / Annum)

Victoria

Australia

Tasmania

Australia

Western Australia

United Kingdom

Sweden

Electric Induction

85%

1,273

492

140

343

219

24

Electric Element

75%

1,443

558

159

389

248

27

Gas Ring Burner

40%

2,705

167

163

186

186

186

 

Cooking – Ovens

 

Technology

Assumed Technology

Efficiency (%)

End Use Energy Demand (MJ)

Greenhouse Gas Emissions (kgCO2e / Annum)

Victoria

Australia

Tasmania

Australia

Western Australia

United Kingdom

Sweden

Electric

75%

481

186

53

130

83

9

Gas

60%

555

34

33

38

38

38

 

Water Heating

Please note that water heating load would vary depending on climate. For simplicity in comparison we have assumed a mild climate in the below energy demand calculations. We’ve applied the same demand across all regions so the carbon emissions are comparable between the electricity grids for the same demand profile (despite the fact there’s no mild climates in Sweden!). Similarly we’ve assumed the same solar radiation (an average of Australian capital cities approximately weighted by population). Note that the conclusions drawn below assume standard sized hot water systems with typical efficiencies, tank heat losses and solar collector size (where applicable).

 

Technology

Assumed Technology
Efficiency (%)

End Use Energy Demand (MJ)

Greenhouse Gas Emissions (kgCO2e / Annum)

Victoria
Australia

Tasmania
Australia

Western Australia

United Kingdom

Sweden

Electric Storage

99%

8,159

3,605

1,025

2,514

1,603

173

Gas Storage

85%

9,503

670

654

647

747

747

Gas Instantaneous

85%

7,357

519

506

501

579

579

Heat Pump (COP3)

300%

2,692

1,190

338

830

529

57

Heat Pump (COP5)

500%

1,615

714

203

498

317

34

Solar, Electric Boost

 

8,138

927

372

692

496

189

Solar, Gas Inst. Boost

 

6,489

250

247

246

263

263

 

Space Heating

The following figures assume a conditioned space of 180m2 and a heating energy requirement of 50MJ/m2/year which is the average figure for a new compliant house in Perth. For simplicity in comparison we have assumed the same ambient temperature in different regions to calculate the below technology efficiency.

 

Technology

Assumed Technology
Efficiency (%)

End Use Energy Demand(MJ)

Greenhouse Gas Emissions (kgCO2e / Annum)

Victoria
Australia

Tasmania
Australia

Western Australia

United Kingdom

Sweden

Elec. Air Source Heat Pump (COP4)

400%

2,591

1,002

285

698

445

48

Elec. Air Source Heat Pump (COP5)

500%

2,058

796

226

555

354

38

Gas Heater, Flue, High Efficiency

75%

13,333

822

803

795

917

917

Wood Pellet Heater

95%

10,526

25

25

25

25

25

Is This Enough Information to Make a Decision?

What about the declining carbon intensity of our electricity grids? This is a really valid point that needs to be considered carefully. If the advantage of using gas is marginal, then it’s necessary that the carbon intensity of the electricity grid will drop as the globe responds to climate change, quickly eroding any advantage. Australia is lagging a little in this regard; we’re a very large emitter per capita and have secured some of the easiest targets to obtain in our Kyoto negotiations. That said, the government has committed to reducing total greenhouse gas emissions by 80% on year 2000 levels by 2050. That’s only 37 years away and most buildings we knock up today will still be around at that point. With this in mind there needs to be a very clear advantage in using gas over electricity to justify its use.

There is a slim possibility that existing gas networks may be utilised for distributing renewable gases. This is already happening in some innovative communities where sewer gas is being collected, refined and sent back to apartments for cooking. This may somewhat alleviate any concern building designers may have about encouraging the use of what is essentially a fossil fuel network (natural gas) over electricity grids that can be more easily transformed to renewable sources.

What About Solar Electricity?

Now begins the philosophical debate. If you have a roof top solar PV system large enough to meet all your home energy demands with 100% clean renewable energy, would you use gas or solar electricity for your heating sources?

Electricity seems the obvious choice. Interestingly though, it may not be when you look at the net benefits of choosing each option. Let’s say with your solar PV system, you are energy neutral, that is you use as much as you generate. Of course you need to export into the grid in times of high generation, and import when your usage outpaces your production, but over a whole year you’re energy neutral. If you’re doing this whilst cooking with electricity and then you swap to gas, you’ll be using a little more energy in the building as your gas cooking appliances aren’t as efficient as electricity. That said though, you’ll be exporting more electricity as you’ve displaced some demand with gas. These electricity exports will be reducing the demand for the normal fuel sources used by your generators.

If this is coal, then the net result will be better for the environment as your greenhouse gas “credit” for exporting electricity will be bigger than the impact of using gas.

There are numerous variables that you may want to consider here, the most important of which is probably when you’re exporting and importing and how that relates to the generation of fuels being employed during those periods.

Researched and written by Henrique Mendonca and Richard Haynes

To find out more visit www.etool.net.au

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