There’s more than one way to make good use of the Stirling cycle. Just ask the engineers at Infinia, Kennewick, Wash. They use it in both their PowerDish, a device already on the market that turns sunshine into electricity, and StAC, an innovative air conditioner that has earned development grants from the government. Both exemplify energy efficiency and sustainability.
In the PowerDish, heat from the Sun drives a free-piston Stirling power generator, an external combustion engine. A 161.5-ft2 parabolic dish made of mirrors bonded to curved sheet-molding compound reflects incoming sunlight onto a concentrator at the dish’s focal point. The mirrors are currently made of uncoated high-reflectivity glass and Infinia engineers see no need to add costly coatings at this time. Sunlight gets concentrated in an 800-to-1 ratio, which would raise the temperature at the heat-resistant nickel-alloy concentrator to 2,000°C if the Stirling generator didn’t extract heat from it and keep it at about 650°C, says Tim Talda, Infinia’s director for system electronics and controls.
Inside the generator, the working fluid, high-pressure helium, quickly heats up and expands, sending the displacer on its forward stroke of about 7 in. Meanwhile, the hot helium is shunted to the cold side of the engine where it contracts and quickly loses pressure and temperature. A mix of properly tuned gas and mechanical springs sends the displacer back and helps it maintain smooth, back-and-forth resonant harmonics. The displacer cycles at about 60 Hz and travels at 15 fps.
“We use helium because it has a high thermal coefficient, which lets it absorb a lot of energy,” explains Talda. “In fact, the only gas that can absorb more energy is hydrogen, and it has its own problems with corrosion, being explosive, and even more slippery and hard to contain than helium. And we pressurize the helium to about 500 psi to make it denser so it can trap and transfer even more heat.”
All the helium is contained in a welded metal structure so little leaks out. “We use about 60 grams of helium, or about 12 cubic feet at 60°F and atmospheric pressure, and it should last the 25-year life of the PowerDish,” says Talda.
To get consistent power output, regardless of the local climate, Infinia cools the backside of the Stirling generator, keeping it at 60°C or less, using a closed-loop, liquid-based system. The PowerDish uses an electric pump and fan-cooled radiator to circulate about a gallon of a 50/50 water/glycol mix around the Stirling generator’s cool side. The pump and fan are the most-significant sources of noise for the device, which creates about 65 dBA at 33 ft from the dish. Power to run the pump and fan usually comes from the PowerDish, but if it is not yet making enough power, the PowerDish will pull the needed electricity from the grid. And if properly installed, the PowerDish never needs maintenance for lubrication, replenishing the water or helium, or to replace components.
The displacer’s back-and-forth movement creates a wave of pressure in the helium which moves a linear magnet back and forth inside a coil of wire, but its cyclic motion is about 60° out of phase with the displacer. This generates single-phase ac power on both in and out strokes. The magnet’s motion is damped as it moves past the magnets, and this damping, together with the gas and mechanical springs sets up a balanced system oscillating at a resonant frequency. A rectifier converts the ac power to dc, then an inverter converts it back to grid-quality, three-phase ac electricity. Calculations show that the PowerDish converts 24% of all the incoming sunlight into electricity, making it more efficient than most other commercial solar devices.
“To generate grid-quality ac out of the generator would be costly and complicated,” says Talda. “So accepting nongrid-quality ac gives us lower manufacturing costs on the generators. We also want this to be a global product, so for grid-quality power, we would need 50 and 60-Hz systems, This lets us sell worldwide without having different generators. However, we do lose about 5% of the power going from ac to dc, then back to ac. But photovoltaic systems routinely lose 20% of the power when converting electricity to grid-quality ac.”
One downside of the PowerDish is that it needs direct normal insolation (DNI) or direct sunlight. It can function on stored heat for a while, until a cloud passes, for example, but will not work at all on overcast, foggy, or rainy days. And with the amount of electricity generated a function of the temperature difference between the hot and cold side of the generator, wind can degrade output as well. Wind increases convection losses from the heater head and reduces the heat input. “So we lose about 4% of power for every 15 mph of wind,” notes Talda.
The other downside is that the mirrors must be kept clean for peak efficiency. “But how often they need to be cleaned depends on several variables, including weather, amount of dirt in the environment, and location of the dish,” says Talda. “In fact, we’ve seen storms clean the dishes and also get them dirty. Currently, cleaning is a manual job, but we are looking at ways to automate the process as we scale up.”
But on the positive side, shade cast on the dish from another dish or object degrades the output by the fraction of the dish that it covers. Photovoltaics, on the other hand, can be greatly affected by a little bit of shade. For example, if shade covers 5% of the PowerDish mirrors, power output drops by 5%. But if shade covers 5% of a PV cell, it can drop power output by 40% or more.
At sunup, the dish, mounted on a 15-ft pole, pulls about 50 W of power from the grid to open up and turn the dish toward the Sun. After sunlight reaches 250 W⁄meter2, which is shortly after sunrise, the PowerDish begins generating electricity and tracking the Sun, a task which takes about 5 W. There’s not much material that needs to heat up before it will work, so there is little lag between sunrise and start-up.
The pistons in the Stirling generator are well balanced and sometime need a nudge to get started. “Each dish is programmed to ‘bump’ the displacer to get it moving using power from the grid when the device detects enough sunlight to run the generator. And although generators often self-start, we ‘bump’ them to ensure we get consistent starts on all of them.”
One 1,900-lb PowerDish generates 3.2 kW ac and costs about $10,000, a figure which can changed depending on how many dishes are bought. Infinia estimates it can set up a 1-MW installation of 334 dishes on four acres. A setup with the same number of dishes but on 6 acres would generate slightly more power, due to less shading of one dish on another, but costs for land, taxes, and wiring would increase. So purchasers have to make trade-offs. And currently, purchasers are large users, not individuals. Larger users can better afford to send someone for training in setting up and aligning the dishes. They’re also more likely to be able to afford the engineered foundation needed and to meet codes for attaching the dishes to the grid.
For example, the citizens of Belen, N.M., a site with plenty of direct sunshine, recently spent $300,000 to install three dishes on the roof of its city hall, About $100,000 of that came from the state’s Environmental, Minerals, and Natural Resources Department, and some of the funds went to getting the site certified by the UL. But the building’s monthly power bill went from $1,200 to zero, and on holidays and weekends, the city sells power to the grid at 12 cents/kW-hr.
In the future, Infinia could increase the temperature difference between the hot and cold sides of the Stirling generator. This would let them get more power out, but they might also have to increase the size of the piston. Its engineers will also be looking for ways to reduce conversion or parasitic losses with better fans, cooling pumps, and electronics.
Recently, Infinia engineers decided to run the Stirling cycle backwards, sort of, applying ac power and getting the device to move heat out of rooms or spaces, thus making it a Stirling air conditioner (StAC).
For the StAC, ac power fed into the Stirling motor moves a piston back and forth. When it moves the piston forward, it moves high-pressure helium warmed by the interior toward the exterior where heat exchangers, aided by a fan, dump as much heat as possible from the piston into the outside atmosphere. The piston is then moved back, expanding and cooling the helium, letting it absorb more heat from the interior space with the help of another fan. Flexure bearings and clearance seals eliminate rubbing and wear on parts.
Compared to conventional air conditioners, StAC doesn’t have losses due to throttling or superheating the working fluid, and the Stirling motor is 93% efficient. And unlike traditional air conditioners, which can only be turned on or off to regulate temperature, the StAC can adjust the speed of its fan to modulate cooling and more closely match the thermal load.
Using the same-size Stirling device as the PowerDish, Infinia has built a 1-ton air conditioner with a coefficient of performance greater than 4. The company says it will be able to cost effectively mass produce the device and it will have a longer operational life than traditional vapor-compression systems. It can also operate over a wide range of temperatures and humidities. And if thermal loads are high, more StAC unit can be used to cool the same spaces.
There are some hurdles the Infinia Design team needs to clear, and the U.S. government has granted them some funds from its ARPA-E program to help them. For example, Infinia wants to increase heat transfer into and out of the helium, says Steve Welty, a senior mechanical engineer in Infinia’s R&D Dept. on Government Projects. He envisions them using heat pipes to aid in moving heat. They might also increase the pressure of the working fluid, helium.
Welty also points to some expensive materials used for research, namely nickel-steels. He suggests they could be replaced with lighter, less-expensive injection-molded plastics. “But the pressure vessel would need to remain steel because of permeability issues with helium.”
The government is funding the StAC project through the DoE’s ARPA-E program (Advanced Research Projects Agency—Energy, modeled after the DoD’s successful DARPA program). It sees the device as a way to maintain U.S. technological leadership in cooling technology. ARPA considers StAC’s major advantages to be the ability to modulate its output based on thermal load, the fact it uses no greenhouse gases, and that the device can be made and engineered in the U.S.