Towards a better tinyhouse

Inventing to freedom?

Solar plus stirling

leave a comment »

Off grid power supply systems are a frequent topic of discussion. I’m just going to throw this one thing out there, though:

The combination of solar (potentially with a tracking system and sun concentrating mirrors) and a small generator is particularly formidable.

The thing about solar is that you need a very over sized system if you want to be able to have the specified x kwh/day for every day of the year usually the specification is for every day in 5 years except 1. Plus there is usually roughly twice as much electricity from the panels in midsummer summer as in midwinter.

Generators, on the other hand, provide great baseload power. The thing is, they usually require, of course not just fuel, but quite a bit of maintenance. Except. Stirling engines are now available from a commercial standpoint to produce electricity from any sufficiently hot source of heat (since they are external combustion) and the engines themselves require no maintenance whatsoever, because they use air bearings, or the lubricated parts are kept cold. So you could pretty easily burn old vegetable oil, or biomass with them.

They are finally here, on the market, though there is only one that I can find on the consumer market. Good ones, too, that need practically no maintenance, emit very little noise, and have good power output for their size. And they are actually cheaper, even in the case of the whispergen, which I bet they charge a premium on, less than half the price of an equivalent internal combustion engine. And there are several companies out there that really know how to make them, in a variety of sizes and so on. I think this is cogen too, i.e. it puts the waste heat out as hot water so you can use it for something, rather than into the air.

One pre-assembled genset is the whispergen, though the unit as a whole is designed to run on propane or gas or diesel, I bet you could get it to run on old motor oil or vegetable oil easily enough. It is about 850 watts and $6000 USD, though so too big and expensive really. I don’t know how much it can vary it’s output level if any.

Ideally you want to be able to vary the output of the generator so it can match whatever the load is, or as close as possible to it. Otherwise you are faced with discharging (and later recharging) batteries to make up the difference if the load is higher than the generator can put out. Or conversely doing something with the excess power the generator is putting out, using it to charge the batteries only gets you so far before they are full.

So suppose you hand just a generator battery system, no panels, and the generator has a fixed output that cannot change, it can only be turned on/off. If you have a generator that is too small to meet the average load obviously that is inevitably going to lead to a power outage.

If it is too big though that is also less than perfect because you have to turn the generator on and off repeatedly, and when the generator is off you are running on batteries and that costs money because you only get so much charging/discharging out of a battery before it wears out and you have to buy a new one. There is a daily cost, a running cost, in dollars per watt-hour of energy you draw out of/put into the battery. I.e. if you get 1000 charge/discharge cycles of your battery there is a cost per charge/discharge.

Plus you need to buy batteries that can handle the discharge rate involved, as a battery can only handle a certain discharge rate, for example a 100 watt-hour battery may be able to discharge at up to 200 watts. That is called a 2C discharge rate for that battery, 100 watts would be 1C and 50 watts 0.5C. So ideally you want the output of the generator to be as close to the demand as possible.

Interestingly, the honda freewatt (it has several names, being sold in several different countries,) an internal combustion engine, for example, is more than twice the price ($13.5k vs. $6k.) These are cogeneration units, so you get the waste heat for other uses back, which is nothing fancy, but important from a practical standpoint.

There are others that are either are planned or were going to be produced. Actually, it’s pretty annoying, there WERE even more stirling engines per se available, and even more planned, but companies never seem to keep the good stuff on the market for some reason. Stirling technology company, now known as infinia (and putting their stirling engine know how to use in their powerdish) had some great little generators ranging from ~10 watts to 1kw under the RemoteGen brand name. Then they stopped making them! Arg. A company called omachron was going to make really cheap engines out of stamped metal parts, but gave up, apparently. A company called enatec apparently made a residential cogen unit using the remotegen engine-generator widgets. Their website seems to indicate that they are developing a different one now, though.

Another company called microgen was going to make a 1kw cogen unit for about, reportedly, $3k. But they didn’t go the whole hog for some reason, and it wasn’t the engine technology, they seem to have got that pretty much down pat. Anyway, the point is, stirling engines themselves are here, they can be made at a reasonable price if there is the demand for them. I have collected references for what I have said here, or rather recorded the documents I learned some of the above facts from, and I posted them as a .rar file elsewhere.

There is another company called Enatec which has a website now that explains a bit about how their free piston engine works (developed in cooperation with infinia which used to be called Stirling tech of washington, not to be confused with stirling tech in Ohio).

But, here’s the kicker: Even with the exorbitantly high price of the whispergen, it could still turn out to be economical, especially if modified to run off an inexpensive fuel like vegetable oil, vs. a pure solar system. Consider, for example, the system used by these fine individuals:

That’s a very modest 5 kwhr per day system (BTW what you really want to look at is the exact demand curve, these KWhr/day figures are only approximations) for $13.5k. With a $6k whispergen, hypothetically, if it could vary it’s output to a reasonable degree (turning down when demand is light) you could nearly eliminate the batteries, and reduce the required amount of electronics, and reduce the required amount of solar panels by quite a bit (since they no longer need to have so much excess capacity and provide less power on average.) Thereby saving more than the $6000 the whispergen costs. The only purpose of the solar panels really is to save fuel actually, and hypothetically to reduce the needed size of the generator.

And, actually, the whispergen is more power than you really need, so if only there were smaller generators… Anyway, basically the solar panels just save you fuel. By my calculations (41 Mj/kg of vegetable oil and 20% efficiency) you would only need 2.5 liters per day if you got all the electricity from the whispergen. One problem is that if the whispergen produces more power than you need and cannot reduce it’s output, you would have to cycle it on/off, and that means charging and discharging batteries in order to tide you over the off periods. That costs money in itself to do, because you only get so many charge/ discharge cycles out of your batteries. So what a tinyhouse could really use is a smaller version of the whispergen, or several much smaller stirling engines, turning them on and off as needed.

This post is a post from the archives I selected to update and repost. Since few if any of the current readers have read it, I thought it would be just as good as a newly written post.


Written by gregor

April 9, 2011 at 04:04

Posted in Uncategorized

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )


Connecting to %s

%d bloggers like this: