a waste heat

February 25, 2016 | Jim Lane

Whoops! Those inefficient processes and engines are blowing off more energy than we can spare.

For many people, shower time is thinking time, and tomorrow morning a good topic is the idea of exergy.

Exergy is the useful portion of energy. All energy conversion — such as a water heater for the shower — has some inefficiency in it. So, there is the part we waste, and the part we use, and the second part is known as exergy.

Waste energy can take many forms — for one, a CO2 emission carries some energy into the atmosphere. But then, so does wasted heat that goes out the window to mix with the cooler air outside. Or, the heat that bleeds from an internal combustion engine instead of providing propulsion.

About that shower you’ll take

So, for example, consider the morning shower. Energy is converted by the sun into a fossil fuel, such as coal or natural gas. Then, fossil energy was converted into heat, to drive a steam turbine. Then, heat energy was converted into electricity to deliver to the home. Then, electric energy was converted into heat energy, to produce hot water for the shower.

That’s four conversion steps, and each step waste energy was expended, and exergy was reduced. By contrast, taking a dip in a geothermally-heated pool involves just one — the heat of the earth’s radiation converted into water heat.

Over at Business Green, writer Tom Burke made an astute observation:

Overall, only about 40% of the primary energy from fossil fuels ends up delivering useful energy services to consumers. The actual amount of energy services that renewables need to deliver to eliminate fossil fuels is…some 5.2 billion tonnes of oil equivalent.

As Burke points out, we use 13 btoe of oil, gas and coal, and one of the reasons people despair over the prospect of scaling up renewables is the difficulty of producing 13 btoe of renewable energy. But, it’s the 5.2 btoe we use. The rest goes out the metaphysical window as waste heat.

Getting the waste out of transportation engines

One of the most important efforts to reduce waste heat is in transportation engines.

As LanzaTech’s Robert Conrado observed:

“Vehicles require 2.4 TW of fuel for only 0.24 TW of propulsion but aircrafts require 0.35 TW for 0.10 TW of propulsion – aircrafts require 15% as much fuel for 40% as much propulsion. By the same token, if no energy was lost to waste heat, we would need to replace as little as 12% of our fuel demand for vehicle and airplanes. From this perspective – there is absolutely a path to completely supply our energy needs without fossil fuels.”

In transportation engine efficiency, there’s nothing more ambitious on the agenda than the US effort to raise corporate averaged fuel economy (CAFE) standards. The US set a goal of 35.5 miles per gallon for 2016 (up from 25 mpg), and 54.5 miles per gallon for cars and light-duty trucks by 2025, in an agreement with 13 major automakers.

One of the most important opportunities to increase fuel efficiency is in increasing engine compression ratios. Under high compression, more energy becomes exergy — that is, the engine is more efficient, more fuel produces work instead of waste heat.

As J Szybist et al observed in a paper for the Society of Automotive Engineers, “ethanol offers significant potential for increasing the compression ratio of SI engines resulting from its high octane number and high latent heat of vaporization.” Interestingly, they also observed that “high compression ratios can increase the efficiency of ethanol fuel blends, and as a result, the fuel economy penalty associated with the lower energy content of E85 can be reduced by about twenty percent.”

Or, as Costa and Sodr´found in this study, “Higher compression ratios improved engine performance for both fuels throughout all the speed range investigated, with major effects being observed when hydrous ethanol was used.”

Reducing waste heat

When US Undersecretary of Energy Franklin Orr was at Stanford heading the Precourt Institute for Energy and its Global Climate and Energy Project, he gave a definitive 2011 lecture on Exergy which is detailed here. He noted that “Road and rail accounts for largest destruction of exergy in the global transportation system and this transformation occurs with the lowest efficiency, and road and rail also accounts for greatest CO2 releases in the transportation sector.” But he critiqued “very low conversion efficiencies” as well in heating, lighting and cooking. For example, the exergy efficiency of lighting is around 5 percent, and indoor air heating is around 4 percent.

So, we have to dig up 20 tons of oil equivalent to get 1 ton’s worth of indoor air heat. Orr noted that “there is no shortage of energy – the issue is conversions at a price we are willing to pay.”

The solutions, in his view, to exergy destruction? Orr pointed to five.

1. Stop wasting energy (turn stuff off when not in use!)

2. Increase the efficiency of energy conversions

3. Switch to primary energy resources that have no or lower carbon emissions: Wind, solar, nuclear, geothermal, Biomass (with minimal fertilizer, water pumping, …), CH4 instead of coal (~60% CO2 per unit energy in the fuel, and higher efficiency in the power plant).

4. Reduce the number of energy conversions (fuel to heat to electricity to heat; sunlight to sugars, to ethanol, to mechanical work in a vehicle) – each conversion has an efficiency, typically much less than 100%

5. Capture some of the CO2 and store it somewhere other than the atmosphere.