One figure that keeps coming back to me was part of a lecture on our first day of class: the figure for rejected energy in the United States. A simple definition of rejected energy would be energy that is produced but not ultimately consumed; it’s simply released into the environment, mostly in the form of heat. It’s energy that’s wasted: some of it unavoidably so, but wasted nonetheless.
Just under a hundred quads of energy are produced commercially for use in the United States. That’s about a fifth of all the energy produced in the world. A quad is equal to a quadrillion British thermal units (BTUs); one BTU is the amount of energy needed to heat or cool a pound of water by one degree Fahrenheit. Put another way, one BTU is the energy generated by burning a four-inch wooden match; one quadrillion BTUs, the energy generated by detonating 252 thousand kilotons of high explosives.
The total rejected energy in the US last year was 59.4 quads, or roughly 15 million kilotons. The numbers were comparable the year before, and the year before that. That means the total amount of wasted energy in the US each year is equal to about a million times the force of the bomb that destroyed Hiroshima.
That’s the figure that stays with me: a million Hiroshimas, every year.
As I mentioned, some waste is unavoidable. To be useful, energy generally has to be converted from one form to another. The energy stored in the chemical bonds of petroleum, for example, is converted into heat, which is then used in various machines to do work: but not all the heat winds up being put to use. No vehicle can convert 100% of its energy to motion; no turbine can convert 100% of its motion to electricity. There are calculable limits to how much energy any machine can convert from one form to another.
But it is possible to swap less efficient machines for more efficient ones. For example: according to the US Energy Information Administration, about 412 billion kilowatt hours of electricity were used last year to light American homes and businesses. That’s about 1.4 quads – over 2,300 Hiroshimas. Incandescent bulbs, which are being phased out in commerce in the US, use considerably more electricity to create the same amount of light as CFL or LED bulbs; in fact, an LED might need as little as 10-20% as much power.
There are other trade-offs to be considered, but in general, using less energy to light our homes means that more is available for other purposes – or it can mean that we don’t have to generate as much energy to begin with and can pocket the savings.
In addition to improving efficiency, it’s also possible to look for ways to put rejected energy (sometimes called “waste heat”) to work. This chart, based on the work of Icelandic chemical engineer Baldur Lindal, shows various uses for waste heat in a variety of settings:
In an earlier blog entry here, we looked at how high temperature (300-320o C; 575-600o F) geothermal fluid is used to drive high-pressure turbines producing hundreds of megawatts of electricity in Iceland’s largest geothermal plants. The steam gives up much of its thermal energy in the process; but because it’s not possible for all the heat to be converted to electricity, there’s still a lot of thermal energy remaining … just not enough to drive the high-pressure turbines.
Still: lower-temperature fluid can generate electricity using a process called binary power generation. In general, it involves using cooler fluids to drive lower-pressure turbines. At the Svartsengi power station in southwestern Iceland, six power plants handle fluids of varying temperatures to generate electricity and hot water.
One of the six plants is dedicated to taking spent fluids from the others – fluids that are still hot but not enough to turn to steam – and transferring that heat to other fluids that boil at lower temperatures. These fluids do turn to steam, and that steam is used to turn seven low-pressure turbines generating an additional 8.5 MW of electricity.
Even after the fluids are no longer useful for electrical production, they still contain at least some energy, and Icelanders have taken Lindal’s ideas to heart by finding ways to put that energy to use. Some is used to heat homes; some, to heat the vast spa called the Blue Lagoon, one of Iceland’s best known tourist attractions. The Blue Lagoon is also home to research labs developing cosmetics and health care products based on the minerals in the spent geothermal fluid and the algae that the fluids can nourish.
Still more heat is used in industrial processes, like drying fish for export. More than 3000 tons of dried fish heads, for example, are exported to Nigeria every year, relying on heat that would otherwise have had to have been generated by other means.
Icelanders also use their geothermal waste heat for other purposes, such as warming greenhouses to grow foodstuffs like bananas – not exactly the sort of thing you’d expect to find flourishing next to the Arctic Circle.
In fact, some Icelandic greenhouse-grown products, like tomatoes, cucumbers, and peppers, actually fall within the European definition of industrial products because the greenhouses use CO2 captured from geothermal fluid. Being classified as industrial products entitles these foods to more favorable international tax treatment than conventionally-grown produce.*
There are projects around the world to utilize some of the rejected energy that otherwise goes to waste. In the US, for example, automakers are working with university researchers and engineers to develop thermoelectric generators that would use waste heat from auto engines to produce electricity, saving the fuel that would otherwise have been used for things like powering an air conditioning unit. An energy-recycling company operates a power generation plant in East Chicago using waste heat from a nearby steel refinery to generate 90 MW of electricity.
None of these projects are themselves enough to make a significant dent in the amount of wasted energy. But to the extent they help spur new approaches seeking to squeeze as much work as possible out of every BTU of energy, they’re a start.
*I imagine that somewhere in Iceland, a very clever tax lawyer is rightfully proud of that.