Capturing waste heat

Insect on pink flower

Comment threads on this blog have previously been rife with discussion about boosting the efficiency of industrial processes through the use of waste heat. It does seem intuitively undesirable to have something like a nuclear power plant venting a significant portion of the total energy being expended from fission in the form of hot air or water being dumped out into the natural environment.

A machine installed at Southern Methodist University demonstrates that there are situations where waste heat can produce a decent amount of electricity (50 kilowatts) at an acceptable cost, and with a payback period of just three or four years. The machine uses an Organic Rankine Cycle, in which a high molecular mass organic fluid is used to convey the waste heat. This is necessary to produce useful work, and eventually electricity, from relatively low temperature sources. As energy prices continue to rise, you can expect to see more such equipment being developed and deployed.

Author: Milan

In the spring of 2005, I graduated from the University of British Columbia with a degree in International Relations and a general focus in the area of environmental politics. In the fall of 2005, I began reading for an M.Phil in IR at Wadham College, Oxford. Outside school, I am very interested in photography, writing, and the outdoors. I am writing this blog to keep in touch with friends and family around the world, provide a more personal view of graduate student life in Oxford, and pass on some lessons I've learned here.

4 thoughts on “Capturing waste heat”

  1. Energy recycling isn’t rocket science… Why don’t we do it more?
    by Tyler on Tue 03 Jun 2008 09:22 PM EDT

    We’ve all heard about the potential — and huge benefits — of recovering waste heat from industrial processes. But for some reason this easy, quick payback approach to reducing energy consumption hasn’t taken off the way it should. It’s perplexing, really. We’re talking about free energy. An Ottawa-based company called Thermal Energy International announced this week it had entered into a heat-recovery agreement with a major North American paper company. The $20 million contract will see Thermal Energy install, own and operate its waste heat recovery system. It receives fixed payments over the life of the contract, which is more than paid for by the savings the paper company gets on its energy bill. The paper mill essentially puchases the recovered heat from Thermal Energy at a discount rate that will result in more than $40 million in total fuel oil savings over the life of the eight-year contract. It could be more as oil prices continue to rise. “This represents the first phase of a multisite agreement involving the deployment of Thermal Energy’s heat recovery technologies across five sites,” Thermal Energy said. “Initial estimates show the project would reduce fuel oil consumption by 105,000 barrels per year with commensurate annual emission reductions of 55,000 tons of CO2, and 625 tons of SO2 — equivalent to permanently removing more than 13,000 automobiles from the road,” said the company, adding that as oil prices continue to rise it’s seeing the scale of its projects get larger. No kidding.

  2. China kiln fact of the day
    By Tyler Cowen on History

    At around the time of the Industrial Revolution:

    “Pottery, for instance, was manufactured in both England and China. The design of the kilns differed greatly, however. English kilns were cheap to build but very fuel inefficient; much of the energy from the burning fuel was lost through the vent hole on the top (Figure 4). The typical Chinese kiln, on the other hand, was more expensive to construct and, indeed, required more labour to operate. Figure 5 shows how heat was drawn into the chamber on the left and then forced out a hole at floor level into a second chamber. The process continued through many chambers until the air, by then denuded of most of its heat, finally exited up a chimney. In England, it was not worth spending a lot of money to build a thermally efficient kiln since energy was so cheap. In China, however, where energy was expensive, it was cost effective to build thermally efficient kilns. The technologies that were used reflected the relative prices of capital, labour, and energy. Since it was costly to invent technology, invention also responded to the same incentives.”

  3. Pingback: Carnot efficiency
  4. Technology Quarterly

    Heat scavenging
    Stealing the heat
    Energy: The idea of recycling paper, glass, metal and plastics has become commonplace. New technologies allow heat to be recycled, too

    Mar 4th 2010 | From The Economist print edition

    “By constructing a computer rack similar to that used in the office test, the researchers were able to provide the greenhouse with badly needed heat. A short while later, the rack was joined by three more racks that today provide the greenhouse with enough heat to cut its gas bills by $15,600 a year—while simultaneously saving Notre Dame $38,000 in cooling costs.

    Another way to recycle heat that is being explored is to capture infrared with photovoltaic cells similar to those used in solar panels. Photovoltaic cells depend on packets of light (photons) knocking electrons free from atoms. They then employ the electrons so liberated to create a current. Photovoltaic cells are usually most responsive to photons in the visible and ultraviolet parts of the spectrum, but they can also respond to high-frequency infrared photons. Objects at a temperature of 1,000-1,500ºC produce plenty of such photons.

    But only those that are travelling at a near-perfect right-angle to the surface of the hot material can escape and travel outwards. Photons travelling at any other angle within the material are reflected back inside when they reach the surface. As a result, photovoltaic cells placed near hot objects have only been able to generate around 0.02 watts per square centimetre. By contrast, photovoltaic cells absorbing sunlight can produce about 20 watts per square centimetre, provided the light is carefully concentrated using mirrors.

    So Dr Hagelstein and his colleagues changed the design of the cell, adding tiny metal wires to the usual sandwich of semiconductor materials in order to pick up the liberated electrons and allow them to be carried off to create an electric current. Although the new device is still at an experimental stage, the team’s calculations, published in a paper in the Journal of Applied Physics in November, suggest that it could convert heat to electricity at a rate of 100 watts per square centimetre. Installed on a laptop, it could recycle heat from the microprocessor and extend running time by around 20%. One way or another, it seems likely that the abundant reservoirs of waste heat are about to be tapped.”

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