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Heat scavenging

According to the law of conservation of energy, energy won’t disappear, it merely transform from one form to another. If we can harvest excess heat in the atmosphere and use it to generate electricity, we can solve the global warming and energy crisis problem at the same time.

Mar 4th 2010, The Economist
The idea of recycling paper, glass, metal and plastics has become commonplace. New technologies allow heat to be recycled, too

“WATER, water everywhere, nor any drop to drink,” lamented the becalmed Ancient Mariner. Oddly, the same is true of energy. As with the water that surrounds a desert island, there is abundant energy right under people’s noses, in the form of wind, sun, tides and heat. The trouble is that, like saltwater, none of these sources is easily tapped. Wind turbines, solar panels and devices that extract energy from wave and tide have become more common in recent years. But technologists have been slower to exploit the vast amounts of ambient heat available in the atmosphere, or produced by machinery.

True, some simple forms of heat recycling have been around for a while: using heated waste water to warm flooring and melt ice on driveways, for example. Such systems can also reduce the need to heat water in a home. By running pipes that carry outgoing waste hot water alongside those carrying incoming fresh cold water, it is possible to warm the inlet stream and thus reduce the amount of energy needed to heat it up. “Combined heat and power” stations produce electricity while also warming nearby homes using their waste heat. In industrial settings, waste heat from boilers and large refrigeration units is sometimes recycled to reduce heating costs elsewhere. And some green-minded householders are fitting “air-source heat pumps” to keep their homes cosy using heat extracted from outdoor air.

Yet, with the spread of computers, which generate vast amounts of heat and need to be kept cool, tactics for recycling are getting ever more creative. Power and cooling demands grow in tandem and, as machines get more powerful, the world is paying dearly to keep them cool enough to run properly. From 2006 to 2011 the cost of powering and cooling servers in America alone is expected to grow from $4.5 billion to $7.4 billion, according to the country’s Environmental Protection Agency.

Frustrated by seeing ice on his roof at the same time as being told that his computer simulations could not run faster because cooling costs were too high, Paul Brenner, a computer scientist at the University of Notre Dame, in South Bend, Indiana, decided to take action. Dr Brenner and his colleagues explored the idea of using waste heat from computers as part of a thermostat-controlled indoor heating system. In theory, such a system would save on heating and cooling bills by exposing computers to low-temperature offices while exposing office workers to the warmth of the computers.

To try the idea out, the team secured an office and plonked some servers in it, linked to the Notre Dame campus computer pool used for big calculations. A thermometer in the office detected when it was getting too cold and sent a signal to the network requesting that calculations be diverted to the servers in the office. Once the temperature had risen, a second signal suspended work on the servers.

Having proved that their idea worked on campus, Dr Brenner and his team tried it out in the wider world—specifically, at the South Bend Botanical Conservatories and Greenhouse, a botanical garden that was spending $115,000 a year running boilers and propane heaters to keep temperatures high enough for its plants to survive.

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.
More than just hot stuff

Dr Brenner’s system, then, is helpful when heat itself is a valuable commodity, and it is simply a question of delivering it where it is useful. The most transportable form of energy, though, is electricity, so systems that convert waste heat into electricity are also desirable. The usual way to do this is with a thermocouple—a sandwich of two metals that produce a current when one side of the sandwich is hotter than the other. But Steven Novack, Dale Kotter and their colleagues at the Idaho National Laboratory are working on a new approach, using devices called nanoscopic antennae (nantennae), which are built out of gold or a nickel-chromium alloy.

Hot objects emit infrared radiation, and the electrons in these metals vibrate when exposed to such radiation. This vibration creates an alternating current that can be tapped. Though the current in each nantenna is small, an array of them can produce a useful amount of electricity. The nantennae themselves are built with a large-scale stamp that is used to emboss their underlying structure onto slightly heated, thin sheets of plastic. Once these structures are in place, the trenches made by the stamp are filled with metal. Only small amounts of metal are needed, though, so the end result is a cheap and flexible material.

The main problem is that the alternating current created is very high frequency. Mains electricity in America is supplied as alternating current at 60 hertz. The nantennae produce about 30 terahertz, or some 500 billion times that frequency. To make use of such a current, the nantennae require a device called a rectifier that reduces the frequency to something manageable. The trouble is that commercially available rectifiers can only cope with frequencies of up to 100 gigahertz. The frequency of the current from the nantennae is around 300 times higher.

To deal with this problem, Dr Novack and his collaborators are trying to embed a nanoscopic diode into the nantennae. A diode is a device which only allows current to flow in one direction. This would turn the rapid alternating current into a direct current, which is easier to handle. The researchers hope this addition will turn their heat-scavenging technology into a practical reality.
Getting closer

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.

Bob DiMatteo of MTPV, a start-up based in Boston, is working on a way around this problem. He and his colleagues have discovered that the conditions change if a photovoltaic cell is placed a few hundred nanometres (billionths of a metre) from a hot surface made of silicon carbide alloy. When the width of the gap is smaller than the wavelength of the infrared radiation coming off the alloy, the photons are not internally reflected, but continue to travel into the cell. This approach, which is being called micron-gap thermal photovoltaics (hence MTPV), is capable of generating 5-10 watts per square centimetre, a massive increase over what has been possible before. The technology looks most promising for use in industrial facilities near exceedingly hot objects, like the machinery used to manufacture glass, or to reclaim waste heat in power stations.

Peter Hagelstein, an electrical engineer at the Massachusetts Institute of Technology, is pursuing a related approach that also involves very small gaps between hot objects and photovoltaic cells. As an object heats up it radiates heat, but it also generates an electric field near its surface as a result of the random thermal motion of its atoms. Although this electrical field is exceedingly small, Dr Hagelstein and his colleagues theorised that if they could expose the electrons in a photovoltaic cell to it, those electrons would be knocked free from their atoms just as if they had been struck by photons.

But even the tiny gap involved in Mr DiMatteo’s approach proved to be too wide: the electric field of the hot object was still too far away to interact with the electrons in the photovoltaic cell. The researchers found that they had to close the distance to just 5-20 nanometres. At such close proximity, the electrons in the photovoltaic cell were being liberated by the electric field generated by the hot object, but the usual process of generating a current was going awry. Normally, in a photovoltaic cell, electrons knocked free by photons are then carried away by an electric field within the photovoltaic material to a contact wire. The electrons knocked free at the surface of the cell, however, were not being carried away in this manner. Instead, they were merely shuffling between different atoms at the surface.

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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