Saturday, March 10, 2007

Electricity Inefficient for Heating

Most people think of all energy as equivalent. In reality, there are higher and lower orders of energy, and understanding the difference is key to making the best alternative energy decisions. Energy in the form of work, such as pushing your car or running an electric motor is a higher order of energy than for heating purposes.

As a result, engines which create the work from heat, whether they be turbines, steam,, gasoline or diesel engines operate at low efficiencies. Generally, these engines work at maximum theoretical efficiencies of around 40%. Ultimately, after friction and other mechanical inefficiencies are considered, the efficiencies of this type equipment are generally much lower.

This makes sense when you think about it. Taking your car as an example... you know that a significant amount of the energy in your fuel is dumped to atmosphere as heat from your radiator and exhaust. In fact, about one third of the energy in the fuel is discharged in each of these areas.

So what does this have to do with electricity? Your electricity is generated by some type of engine. So, the efficiency is necessarily less than 40%. In fact, when mechanical and electrical losses are considered, efficiencies are likely in the 30% range. This is ok when it is used for higher order work, since you would have these inefficiencies even if you generated the power on site. But for direct heat, your efficiencies are well under half what they would be if you generated the heat, a lower order energy, directly with the fuel. This is the reason an electrical water heater is, by definition, considerably more expensive to operate than a gas or oil fired heater. The same applies to your clothes drier.

So, can anything be done about it? The answer is no, concerning the basic comcept. This is a fact resulting from thermodynamic laws. However there are some ways to avoid the negative effect, which may help you in thinking about your energy usage:
  • Use fuels or heat sources directly whenever possible for heating. Gas, oil or solar are all preferable to electricity if available. This concept is the reason heating from solar is much more efficient than using photovolataics or electricity for the same purpose.
  • Use a heat pump. Heat pumps, as the name implies, do not create heat directly, they pump the heat from a lower temperature to a higher temperature. As long as the temperature differences are not to high, the efficiency of the heat pump makes up for the inefficiencies inherent in generating electricity. And, heat pumps, by virtue of the pump effect, create cooling in the location from which the energy is pumped, as well as warmth on the warm side, so they can create both cooling and heat in the same process. In this era of high energy costs, increased use of heat pumps for water heating can significantly decrease energy use over direct electrical heating.
  • Cogeneration. Where there is a need for both power and heat at the same location, they can be connected to significantly increase efficiencies. Cogeneration is being increasingly used in the process industry. If a plant, or nearby plants, have a need for heat, they can use the waste heat from the engine while generating power or electricity, thereby substantially improving efficiency from below 40% to over 90%. Cogeneration is well established in process plants, so why can't it be used at home? In theory, it can. Homes require both heat and work. The problems lie in economies of scale and the lack of low cost, reliable, efficient small engines. And, begging for some good use for their waste heat...your automobile.

So, hopefully, you have a better understanding of energy use and efficiency basics that will help with identifying and selecting energy investments.

1 comment:

Ethan said...

What are your thoughts on biomass? I am thinking about a pellet stove for our duplex, and I wonder what the environmental and cost implications of this choice are.