Toward a more rational distribution of electrical generation

comCHP graph world capacityThe best way to maximize overall national energy efficiency is to decentralize the electric power production and locate it very close to heat users. Since quite a lot of our electric power is actually used to generate heat (e.g. in electric furnaces, or electrically heated buildings) it is entirely possible that a more rational distribution of electrical generation would actually cut electric power demand, while simultaneously cutting energy (exergy) use and carbon dioxide emissions.

How To Get Less From More (And More From Less)

–  Robert U. Ayres. Los Angeles, CA.  11 November 2014

Speaking of efficiency (exergy efficiency, that is) it’s what the electric utilities do every day. This paradox starts from the fact that the electric utilities are not supposed to sell heat, never mind such by-products as sulfuric acid or coal ash). They were designed – and they are regulated – to maximize their output of electric power per unit of fuel (mostly coal) input.

Of course most of the customers for electricity are in cities, so the first power plants were there too. Edison’s first plant was at Pearl St. in lower Manhattan, mainly to minimize the length of copper wire needed by the system. But, let’s face it, coal is dirty, coal smoke – especially before the earliest smoke control devices – is unhealthy and dirty. In fact breathing that tuff over a period of years, even after the first smoke controls were first introduced in the 1920s, can kill you. Even since the devices to remove sulfur and nitrogen oxides, starting in the 1970s and 1980s, people don’t want to breathe it or live near it.

So the coal-fired power plants moved out of the cities. Once they were far enough away to minimize the smoke nuisance, they were too far to even think of selling excess heat to anybody. The “rule of sevens” takes care of that.[1]  So both the utilities and the regulators back in the 1920s concentrated on selling electricity, rather than electricity plus heat. To maximize the output of electricity from a ton of coal (or gas) from a coal fired Rankine-cycle steam turbine, the trick is to maximize the temperature difference between the steam input and the condenser.

Since the upper limit is fixed by the strength of the available materials (carbon steel) at about 900 degrees F (480 deg. Celsius) the traditional game is to make the condenser as cool as possible. To do this, some power plants actually bring cold water from the bottom of a lake. At any rate, the maximum possible energy efficiency for such a power plant is about 44%, not counting transmission losses, and most are several degrees less if they depend on air from a “cooling tower”. Realistically, the average is closer to 33%, taking transmission losses into account, and it hasn’t changed since the later 1960s. Two thirds of the energy (exergy) input is then useless, i.e. wasted.

All clear? OK, here’s what happens if the power plant is small, burns gas instead of coal, and is located near a market for some heat at, say 200 deg. F. Now there is no need for a condenser or any coolant. The bottom of the Rankine cycle. The bottom of the Rankine cycle can be set high enough — say at 200 deg. F. — so that the heat content is no longer wasted.  It can be used for various chemical processes, for laundries, for cooking, or even for running small stationary motors.  Not only that, some of these processes can be regulated so that the heat rejected is still worm enough for heating a room or a greenhouse (for instance).

Those of you who understand these calculations can easily verify that the overall efficiency of the system – as a system – can now be higher than the efficiency of the electric generating component per se. In fact, it can be higher than the most efficient possible electric power system (with a condenser). This is the “miracle” of combined heat and power (CHP).

The best way to maximize overall national energy efficiency is to decentralize the electric power production and locate it very close to heat users. Since quite a lot of our electric power is actually used to generate heat (e.g. in electric furnaces, or electrically heated buildings) it is entirely possible that a more rational distribution of electrical generation would actually cut electric power demand, while simultaneously cutting energy (exergy) use and carbon dioxide emissions.

 

[1] It is 7 X more expensive to send energy over a wire than to send it in the form of fuel. It is another 7 times more expensive to ship thermal energy through pipes than it is to send electric energy by wire. Approximately, of course.

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bout the author:

ayres bw smallRobert Ayres’s career has focused on the application of physical ideas, especially the laws of thermodynamics, to economics; a long-standing pioneering interest in material flows and transformations (Industrial Ecology or Industrial Metabolism); and most recently to challenging held ideas on the economic theory of growth.

* More at http://ruayres.wordpress.com/on-ayres/206-2/

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