November 1999

High Tech House Runs on Copper

Copper Applications in Innovative Technology

By Konrad J. A. Kundig

It's late, but you can't resist doing a little Web-surfing on your new super-fast, multi-gigahertz laptop. The entertainment system in your family room is sending a little night music to your SOHO, sensors linked to your home's intelligent control circuitry having told the system to find you there. Your 24-hour central air-conditioning system keeps you comfortable, even in your garage, where your electric car is quietly charging in preparation for tomorrow's travels.

Mozart follows you outside as you walk through the patio surrounding your heated swimming pool and spa. Pathway lights automatically turn on and dim again as you pass. You pause to admire the twinkling international space station as it orbits overhead. You observe that the station is particularly easy to spot tonight, what with the city experiencing a massive power failure. Despite the summer breeze, though, the 90-degree heat soon sends you back indoors...

Wait a minute. What massive power failure?

Without electricity, what's running the computer, the pool heater, the patio lights, the air conditioner, the stereo and the electric vehicle charger?

Well, in a very real way, copper is. In fact, everything in this scenario runs on copper. And it keeps running regardless of little annoyances like power failures. How? The answer lies in the fact that, today, copper is used in a number of leading-edge ways that even Thomas Edison never thought of.

To understand how it all works, let's go back about 12 hours, when the sun was just starting to turn up the heat that overloaded the city's electric generating stations and caused the power outage we happily ignored.

The Power Supply

Up on our high-tech copper house's roof, an array of photovoltaic (PV) cells began to generate electricity as soon as the sun's light reached them. Sure, PV solar cells have been around for decades, but they've never really been practical for home use.

They are now, because our solar cells are based on a complex alloy of copper and the relatively uncommon elements indium and selenium, with maybe a little gallium and sulfur added. Strictly speaking, the "alloy" is what's called an intermetallic compound. Scientists have known about intermetallic compounds for years, but it's only been since the mid-1950s that the interesting properties of some of these compounds have become known.

The intermetallic compound in our solar cells, which has the formula Cu(In, Ga)(Se, S)2, has the special property of converting light into electricity, much like conventional solar cells made from silicon. The important difference is that the new copper-base cells will potentially be lower in cost than conventional cells. In addition, the new panels are more efficient than any other low-cost thin-film solar cell.

Solar cells are a very wonderful way to generate electricity. They're quiet (no moving parts), they require no maintenance, no fuel, and they're environmentally friendly. No need for power plants or concerns about greenhouse gases, just sunlight being converted efficiently into electric power. The new copper-based solar cells promise to finally bring solar power into the mainstream.

Back to that summer breeze. It's spinning a wind turbine, which in turn is driving a high-efficiency copper-wound generator. The wind turbine helps supply power at night, and it and the solar panels provide more electricity than the house needs. Any unused power is sold to the utility, helping to defray the system's cost. (When is the last time your electric utility paid you?)

But what happens at night? The breeze might stop, and even copper-based solar panels don't run on moonlight.

Here's how it works: Our high-tech house uses the solar panels and wind turbine to charge a bank of batteries that can store enough electricity to carry us through the night. They can even ride out a few cloudy days. (Backup batteries are common in industrial settings, and they're now also found in the uninterruptible power supplies (UPS) that we use to protect our home computers from power outages.) Solar panels and wind turbine generate direct-current (DC) electricity. Unfortunately, DC power isn't very useful in our homes since most appliance motors require alternating current (AC) power. We get around this problem by sending the output from the batteries to a device known as an inverter before it is sent to the wall outlets. Inverters convert DC power to AC. There are several of them in your automobile, where they provide AC power to things like your dashboard lights. There's also an inverter in the UPS that protects your computer.

Copper for Efficient Power Use

So much for the power supply. Since we're generating our own power, we want to make certain we're applying it in the most efficient manner possible. Here again, we rely on new ways to use copper.

First, there's that new super-fast computer. The microchip that drives it is unlike its ancestors, which were built with thin aluminum conductor paths. Our new computer uses microchips built with even thinner conductor of copper. Copper's thermal and electrical conductivities are higher than any other metal except silver, and its electrical conductivity is more than 50% higher than that of the purest aluminum. Using copper instead of aluminum enables the computer to run faster and cooler, using less electrical power for the same amount of computing power. Lower power consumption means the computer can use a smaller, lighter battery, too.

Next, there's the air conditioner. Actually, it's a heat pump that acts as both an air conditioner and a heater, depending on the season. Heat pumps are very common nowadays; most new homes have had them for years. Heat pumps work very much like ordinary refrigerators, compressing a gas to a liquid, then expanding the liquid to a gas again, exchanging heat in the process. Heat pumps are an efficient way to convert electricity into heat or cold, but the ones found in most homes aren't nearly as efficient as they could be.

Our heat pump is better. It's known as a direct exchange, ground-coupled heat pump. It's simpler than a conventional air-coupled unit and it is much more efficient. And, of course, it relies on copper.

Rather than using the atmosphere as the heat source or sink, our heat pump uses the earth. Running under our house's back yard are lengths of copper tubing that transmit heat to or from the earth depending on whether the heat pump is cooling or heating the house. As we said earlier, copper has very high thermal conductivity, and that property enables the copper tubing to transmit heat in the system's working fluid to or from the earth very efficiently.

The direct exchange feature of our heat pump means that our unit only uses one working fluid for both refrigeration/heating and heat transfer to and from the earth. This arrangement simplifies the system (it eliminates one heat exchanger) and improves the equipment's efficiency.

Copper Motor Rotors

The efficiency star of the system, however, is the all-copper electric motors that drive it. Aren't all electric motors made from copper, you might ask? Actually, no. Most electric motors contain copper windings in the stator (the stationary outer part of the motor surrounding the rotor). The rotor itself is made from a stack of round magnetic steel sheets, in and around which is a squirrel-cage-like die casting made from pure aluminum. The aluminum cage acts as the electrical conductor to generate the magnetic field that spins the motor. Until now, all such motors were made from die-cast aluminum. Aluminum made them light and cheap, but left them neither as efficient nor as small as they could be.

Our motor is better. In place of the die-cast aluminum "squirrel cage", it has a die-cast copper rotor. That may not sound like much of a change, but it's a truly major technical breakthrough, particularly when you consider what it involves.

The process called die-casting involves squirting liquid metal into a water-cooled steel mold cavity that has the shape of the casting we want to make. The cool die causes the metal to freeze rapidly, creating a precise, complex-shaped casting. The process is relatively easy when used with low-melting-point metals like aluminum, zinc or magnesium.

To make die-cast motor rotors, for example, liquid aluminum is squirted into a cylindrical-shaped mold cavity, inside which is a stack of circular steel sheets, all of which contain precise slots and holes. The aluminum flows into these slots and holes, and when it freezes the rotor becomes a single solid block.

Aluminum melts at 660 degrees Celsius (1220 degrees Fahrenheit), but copper melts at 1085C (1985F). In order to die-cast copper, it was necessary to find a die material able to withstand the high temperature and rapid cycling between hot and cold. Ordinary die steels just didn't work. After cycling between hot and cold a few times, they began to crack, soon taking on the appearance of fine-grained alligator hide. The surface of castings made in such dies was clearly unacceptable.

The development of new die materials able to endure the severe thermal cycles took several years. Research continues. Die cast copper motor rotors may revolutionize the way we use electric motors. And, because of their high efficiency, they'll let us use more motors—or use motors more—without having to build more power plants. It may sound like an odd combination, but in this case using more copper actually reduces greenhouse gas emissions. This is just one example of how copper's favorable engineering properties benefit the environment.

It's Not Only About Saving Energy

There are a few other features of our high-tech copper house that should be mentioned.

  • The swimming pool and spa are heated with copper solar collectors, of course. That sort of technology is well known, but it is becoming increasingly utilized as the public becomes more concerned over energy conservation and the desire to reduce greenhouse gases. And, because of copper's well-known biostatic properties, copper solar systems are inherently sanitary.
  • The high-tech house is fitted with what is known as structured wiring, all made from copper, of course. Structured wiring (picture it as a star-burst, with all wiring circuits emanating from a single point) makes it easy to introduce such features as networked home computers, computer control of appliances, entertainment, security and fire alarm systems - and motion sensors to turn on the lights when needed. Structured wiring systems can also take advantage of new types of cable, in which electric power, telephone, data and control functions can all readily be accommodated. Structured wiring systems also maximize the house's flexibility. Computers, telephones, thermostats and alarm sensors can be placed in any room or moved at will to suit changing needs.
  • Remember that electric vehicle (EV) we left charging in the garage? The EV uses high-efficiency all-copper electric motors, some of which double as generators when the car is braking. The car contains all the convenience features we're used to, but its compact all-copper motors and ribbon-like multiplexed wiring harnesses keep it light.

That leaves the orbiting space station. Even it wouldn't be there if it weren't for copper. The rocket engines on the space shuttle that carried the station's components into space (and all other large rockets, for that matter) are made from a special composite material based on copper.

Ordinarily, the term "composites" brings to mind such high-tech materials as the graphite-fiber-epoxy structures used in fighter aircraft, golf clubs and tennis rackets. There is also a relatively new class of similar materials known as metal-matrix composites, and these materials can be designed to have very interesting properties. The copper-refractory metal composites used in rocket motors are just one example. Their combination of ultra-high strength and copper's high thermal conductivity is what makes the rocket engine possible. Engineers are currently looking for other ways to apply these useful materials.

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