Watta Bargain

A startup called N-Gen technologies is running an interesting crowdfunding campaign for a natural gas-fueled, Stirling engine-powered appliance for generating your own electricity at home. N-Gen doesn’t use the S-word in their fundraising materials but they do in their patent (more below.) They claim it’s going to change the world, cutting out the power company and letting you generate your own power safely and efficiently, immune from power failures.  Could it? Hard to be sure, but they have working prototypes, and it’s nifty technology. The crowdfunding prospectus is worth a look; it reads like the disclaimers for a pharmaceutical ad. Who knew there were so many ways to lose your money?

Before we look at this claim, what the heck is a Stirling engine, anyway?

hot-air-engine
A model Stirling engine kit by howtobuildplans.com

Most of the heat-powered engines we see today are internal combustion engines, which mostly fall into one of two categories. Either fuel is exploded in a closed cylinder to drive a piston,  or it is burned continuously to produce a rapid flow of hot gasses to turn a turbine. Either way, the fuel is burned inside the engine.

That’s not the only way to turn fuel into power, however. There are also external combustion engines. For instance, classic steam engines run on fuel burned in an open firebox to turn water in a closed vessel into pressurized steam that is used to drive one or more pistons. The firebox is outside the engine, open so you can shovel coal into it.

stokers-on-a-german-battle-ship-cphtd5
Stokers of a German battleship

You’ve probably seen scenes of this in the movies where the guys are furiously heaving logs or shoveling coal into the engine’s firebox while the bandits, Indians, German navy, or whoever is chasing their steam-powered vehicle, close in.

The “combustion” in external combustion can be misleading, because strictly speaking, unlike the case of internal combustion engines, the heat source doesn’t necessarily have to be combustion; anything hot will do: geothermal, waste heat from other processes, solar, etc.

Stirling engines are like steam engines in that they don’t burn fuel internally, but unlike steam engines, they are closed systems. In a steam engine, after the live steam drives the piston, it is vented out into the world, which means that a steam engine can run out of water just as it can run out of fuel.

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Stirling diagram from N-Gen’s site

Not so with the Stirling engine. The classic Stirling engine has two cylinders, a hot one and a cold one, and gas cycles back and forth between the two, never leaving the system. Along the way, some of the resulting mechanical energy is siphoned off, classically, as above, in the form of rotary motion, that you can use to do something useful. N-Gen taps mechanical energy from this principle a little differently, but more on that below.

The super-condensed explanation of how vanilla Stirling engines work (see the GIF above) is that externally applied heat warms up and expands cool gas in the first cylinder, which drives the piston outward, cranking a flywheel one-half a turn. This is where most of the work gets done. The momentum of the second half of the flywheel’s turn then pushes the piston back down the cylinder, expelling the used hot gas out to a second cylinder that is attached to the flywheel 90 degrees off.

The crank angles time the cylinder motions so that the second cylinder fills with the used gas while the first empties, and then as the flywheel continues turning, the same gas is returned back into the first cylinder for another cycle.

The same gas thus flushes back and forth endlessly. The magic is that the second cylinder is equipped with a radiator that dumps the heat from the hot gas it receives out into the world. This chills and therefore shrinks the gas that returns to the first cylinder, allowing it to expand again in the next cycle. The chilling between power strokes is the key—if the gas stayed hot, it wouldn’t be able to expand again.

A physics student could tell you exactly how the difference in temperature between the hot part and the cold part affects the power. I think that’s covered in the lesson on the Second Law of Thermodynamics but I dropped out after the First Law. To get power from this style engine, attach either a generator rotor or a belt, gear or other power transmission mechanism to the flywheel and badda-bing—power from heat.

N-Gen’s Improvements

Elegant as the Stirling engine mechanism is, they don’t produce much torque, because they have to heat the gas in the cylinder as it is used rather than developing high pressure steam in a separate reservoir. Therefore, Stirling engines have never been a very effective way to turn large quantities of fuel into usable mechanical power, and have often been relegated to applications where they can squeeze some extra value out of heat that would otherwise go to waste, such as the used steam coming out of a power plant.

N-Gen’s design uses the basic Stirling principle but seems to have made the heat transfer processes more efficient. Evidently, the little fingers are the key innovation, transferring the heat from the hot part (red, in the illustration) to the gas, the expansion of which moves the heat-transfer cylinder (green.) The heat-transfer cylinder has done it’s work when it gets to the fingers on the chilled end of the cylinder, which removes the heat from the heat-transfer cylinder, which in turn can chill and shrink the gas behind it, so that it can return to the starting position against the now lower pressure of the air it has chilled. This design saves a lot energy that would be wasted in moving metal and gas around in the two-cylinder classic design.

I can’t be sure, but the design seems to depend upon coordinating the piston cycle with the amount of time it takes to transmit the heat from component to component, allowing them to do it all with one long cylinder in the cycle time. The design also dispenses with a flywheel by using the reciprocal motion directly by means of what seems to be a linear alternator (if I’m understanding the design.) A linear alternator is very similar to a conventional generator, but instead of inducing current by rotating magnets inside a cylinder of coils, this method moves the magnets back and forth through a doughnut of coils to achieve the same thing. You lose some energy to inertia by having to accelerate the magnets back and forth, but you gain it back by having fewer moving parts and less movement of gas. The magnets are the black cylinder at the end and the brown is the wires in cross section.

N-Gen claims that the energy efficiency beats any other small generator and that with their appliance you will be able to generate your own electricity in your garage more cheaply than the power company can supply it to you. Moreover, the waste heat can be tapped to fuel a hot water heater and/or warm the house when it’s cold.

Beating the Power Company

A quick Google search reveals that the average household in the US uses 10.8 KWh of power per year, which works out to an average usage of 1.25kW per hour round the clock, which is less than their unit produces.

At first glance, that sounds pretty good, but unless you run a marijuana grow farm, your power consumption in real life probably varies wildly throughout the day. I swiped the example illustration below from the Environmental Change Institute Web site. It shows pretty much what you’d think: very little used in the wee hours and peak usage during the day coming in spikes.  So the first objection is that, somewhat like solar, for most people N-Gen, won’t fully replace a connection the power utility because it doesn’t come close to meeting those peak demands. (This is partly ameliorated by a battery in the system, that buffers some power, and it doesn’t run at full power when you’re using less, of course.)

For me, this is not a compelling objection—in fact, it’s a selling point. This is because fully replacing the power utility is inherently wasteful. To see this, don’t think in terms of the height of the spikes, but rather the area in blue, which is the total power consumption.

The tall spikes on the graph below run up to about 6kW but they are very narrow, which means they don’t actually add up to that much of the day’s total power.  The amount of the blue that is above 1.5kW is very small—perhaps 10% to 15% (just judging by eye.) So to get you the full 100% of your day’s electricity you’d need at least four times as much machine as you need to get 90%.

individual-demand
Source: The Environmental Change Institute, University of Oxford (Modified)

Another thing that tends to mitigate the problem is that the spikes are produced by human behavior and to a large extent are easy to adjust with painless behavior changes that would quickly become second nature. Turn on the washer and dryer at night, for instance, so that it doesn’t overlap with the TV.  Avoid running the toaster and the hair dryer at the same time, etc.  Probably half of the power that many houses use is wasted, and merely being conscious of one’s usage can produce a significant reduction. For these reasons, the modest output 1.5kW output doesn’t seem so limiting.

One big advantage is that gas and electricity grids aren’t tightly linked so you will be almost immune to blackouts. At worst, you may have to be a little more careful when the grid is down because there’ll be nothing to cover the spikes.

The Cost

The unit they will start selling this summer generates 1.5 kilowatts and costs USD $3000 for the basic version, and the expected service life is 25 years.  Let’s just assume this is all fairly accurate.

Dividing the Multiplying 10.8KWh by $0.12/kWh and then by 300 month gives $129.50 per month for a typical electric bill.   The cost of the hardware, spread over 25 years, is something like $10/month.

N-Gen does not claim that it generates a kilowatt for less fuel than the utility company’s generators, which would be danged hard to do, in part because industrial rates for fuel are much lower than residential rates, an in part due to economies of scale.

What they claim is that after the power company’s expenses for distribution, transmission losses, paying power-worker salaries and pensions, profit, etc. you can make your own for less than they can sell it for. Sounds plausible, but can it really be true?

After writing this, I discovered that they have a calculator on their Web site, but I’ll step through my own homespun estimates anyway. They’ve thought about this more than I, so take this with a grain of salt.

According to N-Gen, the device produces approximately 8kWH/Therm of natural gas. If kilowatts/cubic-foot divided by the cost-of-gas/kWH-generated, plus the hardware-cost is less than your electric bill, then Bob’s your uncle.

Here in Weehawken NJ, which is two miles from Times Square in NYC, we pay  $0.12/kWh for electricity, and $9.68 for 1000 cubic feet for gas, which is 10 Therms, so our gas price is $0.968/Therm.

The way I work this out, 8kWH/Therm divided at $0.968/Therm works out to $0.121/kWH for the gas.  Our electric bill is usually something like $120/month, so the hardware price of $10 for the unit adds about a $0.01/kWh to the total cost, or $0.131/kWh. Therefore, Bob isn’t actually my uncle, but he’s a pretty close relative.

On the other hand, right across the river in NY, within sight of our house, electricity costs $0.21/kWH, while gas is $14.38/1000 cubic feet. That works out to about $0.18/kWh, plus a penny for the hardware, or $0.19/kWHfor a savings of $0.03/kWH. If we move across the river then one of my parents will get a new brother.

New Jersey gas prices are on the low side for the US, and electricity is on the high side, so in some parts of the US, generating your own power may not pay on the basis of electricity cost alone.  However, if you use the model with the ad-on that gives hot water and heat as a side effect, you could easily be in the ballpark. Winters are cold here, so absent N-Gen, gas heat and hot water are a bigger annual cost than electricity. Generating our own power would obviate all of the cost of heating water and make a significant dent in the heating bills. The heat it produces is roughly equivalent to running an electric heater on high, so it’s not a trivial contribution.

The payback seems to be extremely sensitive to where you live. I asked N-Gen for some details on the payback and got an interesting set of numbers from one of the engineers on his own family’s actual consumption and the local utility the rates where he lives. Here’s a quick summary.

In San Diego, the more electricity you use, the more you pay per kilowatt. They also have a significant rebate on natural gas if you use it for powering your car or generating power. (They don’t do that here, but as California goes, so goes the country, eventually.) His electric bill is $5500/year, and he uses gas heat. Sounds pretty normal. Power is expensive out there; he pays $0.28/kWh and the baseline cost for gas is $1.2/Therm before the rebate.  His family also uses more electricity than we do (you don’t need much AC here) so one N-Gen device would cover about 70% of his power usage. After the price reduction for using less electricity plus the gas rebate, the bill would drop by almost $2000/year, which means the generator would pay for itself in a couple of years.

Something else to consider is that square miles of our area (though not our house) were without electricity for several weeks five years ago in the aftermath of Hurricane Sandy, so being independent of the power grid also has value. The entire East Coast and the Gulf are subject to this kind of disruption, and other areas are vulnerable to winter storm outages and other disasters, so this is hardly a niche risk.

Bottom Line

Will N-Gen get you off the electric grid? Not 100%, unless your needs are modest indeed. What you will do is eliminate most of your electric bill in exchange for a gas bill that will be very dependent upon the prevailing rates for gas in your area.

One thing you do get, regardless of electricity and gas prices, is insurance against losing electric power in a blackout. The fatal flaw of gasoline-powered generators is that they require gasoline. If your area has no power, then your service stations will be out of commission too, and storing enough gas for power generation is dangerous and complicated, as gas has a limited shelf-life. Candles are romantic for about five minutes, right up till you have to charge your cell phone.

If you’re seriously considering N-Gen, you need to factor in what you pay for hot water and heat. This thing produces about a kilowatt of heat, which is significant. The ideal N-Gen sales territory will be where gas is cheap, electricity is expensive, winters are long and hurricanes are frequent.

One thing they don’t mention is the possibility of using this with alternative fuels. As a Stirling engine, it’s not going to care much where the heat comes from. Bio-methane on farms and ranches is one possibility.  One thing I’d love to see is a model that works with wood pellets for those who truly want to be off the grid.

If you’re interested in the investment aspect, don’t wait too long–the campaign is well underway. The link at the top will take you where you need to go.

 

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