Site versus Source Energy

In my previous post, I mentioned that using an electric resistance coil to generate a btu of heat can be expensive relative to burning a fossil fuel on site for the same purpose because of the difference between site and source energy.

If one only considers energy crossing the site boundary and the conversion efficiency of the electric heating coil, electric resistance heat seems like a real winner. With a conversion efficiency of 100%, every kWh that runs through a resistance element provides 3.413 btu of heat to the facility.

In contrast, even the most efficient, current technology condensing boilers available with everything working just right can only deliver 95-98% of the energy that goes into them as heat to the facility. Conventional, non-condensing burners are lucky to achieve 85% efficiency and older equipment near the end of its useful life cycle may only have a combustion efficiency in the mid 60% range! So, why wouldn’t you want to use electric resistance heat and what makes it so expensive?

The answer to that question lies in the difference between site energy and source energy. There are a number of losses that occur between the point where a fossil fuel enters a power plant for conversion to electrical energy and the point where that electrical energy reaches the meter serving a given facility.

One of the most significant losses occurs in converting the fossil fuel to heat, the heat to shaft power and the shaft power to electricity. In general terms these losses are associated with combustion efficiency, losses through insulated piping systems, the irreversibility associated with the thermal cycle of a turbine, and mechanical and electrical losses in the turbine and generator (bearing friction, less than perfect conductors, less than perfect magnetic materials, aerodynamic losses associated with shafts and armatures spinning in air, etc.).

The Energy Information Administration tracks the heat rate for electric generation in a number of publications including their Monthly Energy Review. The graph below plots this data from 1946 through the current year (data for the current year is an estimate).

Note that in general terms, it takes about 3 btus into the generating system to produce 1 btu of electricity.

But wait, there’s more! It takes power to make power. Depending on the exact nature of the facility, the power plant will need to run forced draft fans, induced draft fans, feed water pumps, deaerators, cooling towers, condenser pumps, soot blowers, air compressors, control systems, cooling systems, lighting systems, and life safety systems, to name a few possibilities. All of these systems require power and impact the amount of energy that needs to go into the plant to make the electricity that shows up at your meter.

There are also losses in the transmission and distribution process. Generally, we think of copper and aluminum as being good conductors with virtually no losses associated with the current flowing through them. But if you run current through miles or even hundreds of miles of copper or aluminum wire the over-all resistance adds up.

If you do the math, power is lost as a function of the square of the current multiplied by the resistance it is flowing through. To minimize these “I squared R” losses, we transmit our electrical power at high voltages. But, since the electric and magnetic circuits in our transformers are not perfect, we also experience losses at that point in the distribution process. And since the high voltages used to distribute and transmit power make it very dangerous to be around, we step the voltage back down at the other end of the distribution system, incurring another loss.

The Department of Energy, Office of Energy Management estimates transmission and distribution losses to be in the range of 8%. In a 1995 report titled Measuring Energy Efficiency in the United States’ Economy, factors were developed to take all of these variables into effect on a regional basis. These factors are depicted in the graph below.

A number of reporting agencies like CBECS apply these factors to their site energy consumption figures to assess the source or primary energy required at the power plant to produce the site energy.

To make this graph directly comparable to the heat rate graph, I multiplied the conversion factors by 3,413 btu/kWH.

The bottom line is that only 30-40% of the energy that enters a power plant as a fossil fuel shows up at your electric meter as electricity. Worse yet, short of resistance heaters, very few of the devices that use that electricity once it reaches the meter are 100% efficient.

For instance, a 1,200 gpm chilled water pump with a 25 hp motor might have 8-10% of its incoming power lost in the motor and another 15-20% lost in the pump itself.

If the site energy graph for the lab facility I have been discussing is converted to source energy using the 1992 Western Region conversion factor published in the 1995 report I mentioned, it reveals that while converting to boilers from heat pumps increased the site energy consumption, it decreased the source energy consumption.

So, the planet won, even though the Owner’s pocket book didn’t.  Had the original electric heat been resistance elements instead of heat pumps, the difference would have been even more significant.

The bottom line is that if you are going to burn fossil fuel to make heat, you are frequently a lot better off burning it on site rather than burning it at a power plant to make heat to generate electricity which you then convert back to heat, at least from the standpoint of the environment and conservation of non-renewable resources.

The big hit to your pocketbook typically associated with electric heat only begins to reflect the big hit to the non-renewable resources and environment that electric heat represents when it is served from a fossil fuel burning power plant.

David Sellers
Senior Engineer – Facility Dynamics Engineering

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One Response to Site versus Source Energy

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