The laws of thermodynamics might be paraphrased as (1) you can’t win, (2) you can’t break even, and (3) you can’t get out of the game. The second law is the frustrating truth that all engineers face. Whenever you change energy from one form to another, you always lose some as heat. Eventually all energy winds up as heat. Some say the universe as a whole will one day end up as one great expanse of uniform heat – the heat death of the universe. A cheerful thought, that.
To an engineer, heat is useless. It is loss. It is the price grudgingly paid for getting work done. The lower the price paid, the greater the success. You might say that the mission of the engineer is to squeeze the very most usable work out of a given unit of fuel before it inevitably becomes worthless, unusable heat.
In this context, the idea of burning fuel for warmth is absurd – all that fuel being turned into heat, with zero work done. And yet we do it all the time. Guelph’s Community Energy Initiative document of 2007 pegged the annual city-wide consumption of natural gas at 231 million cubic metres. Virtually all of that was (and still is) being burned in boilers or furnaces to create heat – heating our buildings, heating our domestic hot water, even heating for cooling (a feat performed by absorption chillers).
There’s another way.
If you burn that fuel in an engine – the same kind of internal combustion engine in your car – you get torque and heat. That torque can turn a generator to produce electricity, while the leftover heat can be used for any of the purposes mentioned above. Using the same fuel to provide both electricity and heat is called Combined Heat and Power, or CHP (also called cogeneration). It gets you a second squeal from the pig. Or, viewed another way (see diagram at right, from building.co.uk), it gives you 260 units of valuable output from 325 units of fuel; conventional methods require 465 units of fuel (43% more) to produce the same output. If you rely on grid electricity and 80% efficient boilers, CHP is an attractive alternative.
There are variations on the engine idea. Large-scale CHP uses not gas engines but gas turbines – similar to what you find under the wing of a Boeing 737- to produce energy in the range of megawatts rather than kilowatts. At the other end of the scale, there’s a micro-CHP device called BlueGEN that doesn’t burn the fuel at all, but rather runs it through a ceramic fuel cell to convert it to electricity – enough to supply a typical single-family home, with sufficient leftover heat to satisfy the family’s domestic hot water needs.
In areas where the utility grid uses carbon-intensive brown coal for much of its power generation, like the Australian state of Victoria, a product like BlueGEN offers significant savings in carbon emissions compared to grid electricity – a welcome benefit over and above the financial savings. This is partly because coal-fired power plants produce the most carbon of any option on the energy generation menu. It is also because the grid electricity comes with a substantial drag in the form of unusable heat – the coal plants produce steam from high-grade heat, but they can’t use the low-grade heat left over so they discharge it as a waste product. When the fuel is consumed right at the point of use, this waste heat can be put to productive use.
Where the grid has a low carbon intensity, like here in Ontario, one might think that household-scale CHP would be a non-starter. It’s true that the average amount of carbon produced per unit of energy is a lot lower than Down Under in the state of Victoria. However, the average is meaningless in this situation. There are several forms of power generation in the Ontario mix – nuclear for base load, followed by hydroelectric, wind (when it’s available), and finally natural gas. These are dispatched consecutively as demand rises, and shut down in the reverse order as demand drops. As demand falls off, the first source to be shut down is natural gas.
When a household takes itself off the grid by producing its own electricity, that demand comes right off the top of the generation stack. If you’re going to compare the emissions of a product like BlueGEN, you have to compare its emissions against those of the marginal grid generation source – natural gas. During the period from midnight on January 14th, 2015 to 1:00 PM on January 19th, the average emissions factor for the Ontario grid was 73g of CO2 per kilowatt hour (for current data, click here). However, the marginal emissions factor was seven times that figure: 512g CO2/kWh. By comparison, a BlueGEN unit with waste heat captured for domestic hot water comes in with an emissions factor of 240g CO2/kWh – less than half of the marginal grid emissions intensity.
What would happen if all of Guelph adopted CHP?
Earlier I mentioned that the 2006 city-wide natural gas consumption was 231 million cubic metres. If CHP units (with the efficiencies shown in the diagram) were used to produce the same amount of heat as our furnaces currently do, it would produce 1,402 GWh of electricity. The entire electricity demand of Guelph as of 2006 was 1,630 GWh. So, if we met our heating requirements with CHP units instead of furnaces, we could meet 86% of our city-wide electricity needs at the same time.
We mustn’t forget thermodynamics rule #1 – you can’t win, or rather you can’t get something for nothing. This scenario would double our natural gas consumption. However, even more of that “extra” natural gas – 121 million cubic metres, to be exact – is being burned in a peaker plant anyway to supply our electricity. The reason CHP looks so much better is that peaker plants dissipate their waste heat, rather than getting that all-important second squeal from the pig.
Deployed broadly in Guelph, CHP could save 121 million cubic metres of natural gas per year. That corresponds to 228,000 tonnes of carbon, not to mention $23 million in annual cash savings. It would also mean we produce 86% of our electricity right here within the city limits – a big plus for resiliency in an era when freak ice storms can tear down high voltage transmission lines and leave thousands without power, as happened in Quebec in late 1998.
Let’s get that pig squealing.