Towards a better tinyhouse

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Archive for March 2010

Solar plus stirling

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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.

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 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

March 26, 2010 at 20:11

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Hot water heat exchangers

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Using a heat exchanger to save on hot water heat could save a lot of energy, and also open up the possibility of a tankless or nearly tankless, electric hot water system for a tinyhouse, which you can’t otherwise have, because they require too much power. That reduces standby losses associated with a tank, and it also save quite a bit of energy overall.

2 Heat exchangers made to do something like this are:

Those were found by googling “heat exchanger shower.”  There might be more.  Anyway, they are, I assume, counterflow heat exchangers.  The thing is, the diagram on the above mentioned ecodrain page is not the optimal way to use such a heat exchanger. Not at all.   See figure 1 for the optimal config.  sorry it’s a bit messed up.  Click to see the original.

Figure 1, a more efficient hot water system configuration.

Anyway, what you want to do is put the heat exchanger on the drain line, but before, not after, the hot water tank.  Heat exchangers like the “powerpipe” use this configuration, they are connected to the sewage outlet.  The problem is, they have to be designed to allow sewage to pass, which makes the heat exchanger very inefficient.  But, if you are building a tinyhouse from scratch, it’s not hard to separate the sewage drain line, so now the heat exchanger only needs to handle greywater, and you could use one of the ecodrain devices.

There is no cold water line per se.  Only a colder line. All incoming water must flow through the heat exchanger to achieve maximum efficiency.  Another way to do it would be to control the power output of the water heater directly using an electronic control knob as your temp control knob, instead of using water mixing.  More about this later.

Now, how would this change things?  Well, let’s just suppose the heat exchanger ends up being 85% efficient.  It might easily be more than that.  If you have a 1.5 GPM (~6 liters per minute) shower head (the faucet can make use of the system, but the shower has the worst case scenario in terms of water and heat demand so let’s focus on that) and the incoming water is a typical 6 deg c and you want the water to be a comfortable 45 deg c. you would need about 15.6 kw of heating power if you didn’t use the heat exchanger.  So you now have 15.6 kw of heat going down the drain.  The heat exchanger gets back 4*0.85, so you only need to supply 4*0.15, or a mere 2.3 Kw.

That’s doable with a 120v heater on a 30 amp circuit, but it might be better to improve the heat exchanger efficiency.  I’m betting they are probably quite a bit better than that anyway.  It also makes using a solar hot water heater a lot easier because your collector is a lot smaller.

Now, but how would the temperature change with time when you turned on the tap, and when you tried to adjust the temperature? You can see it would not act quite the same as a traditional system.  What you are doing is controlling the amount of energy being added to the system, rather than the temp. of the output at the shower head.  As the temperature goes up, the amount of heat lost through the heat exchanger goes up, and eventually the amount being added and the amount being lost approach an equilibrium. With a totally tankless system, there is also the problem of heating the system up on startup.  The water standing in the supply and drain lines needs to be heated up.

Suppose you had a 2.3 kw heater. Just to estimate how long it would take,  As the water is running, 15% of the heat goes down the drain, so it’s effectively heating the water in the supply and drain lines with 1.95 kw.   Suppose there is maybe 4 meters of supply line with an inner diameter of 1.5 cm, that’s about 700 ml of water in the supply line.  Let’s suppose it’s the same for the drain line.  The water lines are indoors, so let’s suppose it’s at 21 deg c. 1.4 liters total, needs to be heated to 45 deg c., that would take 67 seconds with 1.95 kw.  Hm.  However the plumbing lines could be a lot shorter than that in a tinyhouse.  You could use more narrow lines, too. Too long, really, though, unless you clustered stuff and put the heat exchanger right nearby on purpose. To reduce water wastage and shorten the time a bit, you could put the heater directly after the output of the heat exchanger on the supply side, and turn the water on just the right amount, so that water coming from the heater was the right temp. Because the water is heated before it passes through the heat exchanger even once, you get the full 2.3 kw.

The use of a very small tank to provide that initial input energy might be a good idea.  You could even use a thermos as the tank.  That would be kind of cool.  Just use a regular hot water heating element and thermostat as the heater.  That might be pretty cheap, too.  You have to figure out a way to couple the thermostat to the temp inside the thermos, usually the thermostat measures the temp through the wall of the water heater’s inner wall.  You could drill a hole in the thermos top and put a stainless steel rod in to conduct heat to the thermostat, then put a piece of foam or fiberglass insulation over everything. The thing is, the water in the tank counts as standing water in the system, and you have to figure out how to extract energy from the tank without letting excessively hot water escape from the faucet. Switching the tank per se out of the water flow path might be a good way to do it, see below. But then you can’t use the same heating element to heat the water during the main phase of operation.

Also, you might be wondering how to reduce the temperature, what if you have the water on hot and want to switch it to cold all of a sudden? Well, you need to remove energy from the water flowing through the system.  You could turn off the heater and let it escape through the heat exchanger.  Especially if the heat exchanger is high efficiency, that could actually take a long time.  The other way is to have a third water line leading to the faucet, which skips the heat exchanger entirely on the way in.  If that water us used as a supply line, then drains out through the heat exchanger, it flushes out the hot water that was in the system, losing all that energy.  The heat exchanger has no effect, because there is no water coming in through the exchanger for the outgoing water to heat up.  You then have to put that much energy back in again when you want hot water again, too.   But the whole system would still consume a lot less energy than a normal system.

The law of diminishing returns applies here, so this might not be worth the complexity and cost, but another way is to try to store the energy.   You can use a regenerator that is switched in and out as desired. Or you could just use a small water tank.  This could be done without the use of the third water line I mentioned.  Imagine it located between the shower drain and the heat exchanger.  Water can flow through it in a sort of laminar way, with first in first out i.e. no mixing of the hot and cold water.  The “tank” could be a tube coiled up, for instance.  It only has to store about the 1.4 liters mentioned above.   Water does not normally flow through it.  It is normally filled with water at room temperature, or it could be located outside so the water is kept at the outside temp, or it could be inside a water heater tank, allowing it to be used for system startup, too.  You switch it into the system briefly, just long enough to replace the cold water with the hot water that was in the system, and it simply stores the hot water until you want it back again.  At that point the hot water is displaced with cold water and you have most of your heat back.   If the whole system was computer controlled to take care of everything, so you just tell it the temp you want… The computer could be used to control other tinyhouse systems too, so it wouldn’t be too expensive.

Just one more thing: I haven’t checked the prices of these ecodrain type things, but you could make your own heat exchanger too, just by putting a stainless steel tube inside of a plastic tube.  Stuff might accumulate on the surface of the heat exchanger, though, since you don’t have the anti stick coating (unless maybe you could get some anti stick coated tubing.)  The other problem is the resistance it poses to water flow in the drain line.  These ecodrains are designed to have a much lower resistance to flow on the drain line than a tube in tube heat exchanger would have.  However, if you used a drain pump that would be overcome, and you might need a drain pump anyway because there is very little gravity drop in a tinyhouse.   Google products shows water pumps for <$100, so maybe the price wouldn’t be prohibitive.

This post is a post I selected from the archives to 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

March 20, 2010 at 15:25

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Variable size bathroom

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Using a big one of those folding door things, the kind that have about a a bunch of hinges and panels that make up the door so it can fold up, you could have a sort of variable size bathroom.  The door thing folds out from the wall to form both the wall of the bathroom, and of course the door.  In this way, you can still use the floor space that would otherwise belong to the bathroom, while you aren’t using the bathroom.  The grey lines in the figure indicate the path the door could follow at different times. kitchen counter might not ne the best use for the adjacent space, but you get the idea.

Mind you, I have to admit this seems kind of shabby, if you know what I mean.  It would be a bother to pull the door out, and might seem kind of flimsy.  Once you start going down that road, you start to loose that impression that this is a solid house built to last, ya know? If you could have actual sliding walls, that might be a bit better, like shown in the second figure.  Because of the location of the doorway, you can either use this with the wall slid back in it’s stowed position for a quick piss, or if there is company around or whatever, you can roll it out.  You would probably have some sort of rails on the ceiling, I guess, but wheels could support most of the weight load.

The toilet in the image is shown with what looks like a tank, but it  might be better to use a tankless toilet, takes up a bit less space and look classier, I noticed jay shafer does that.

The space above the toilet is still unused most of the time.  Maybe it would just be better to make the toilet itself fold or roll away when not in use.  You would need a way to do the plumbing, but it’s probably electric flush or whatever anyway, so that’s no big deal, with flexible tubing


This post is a post I selected from the archives to 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

March 19, 2010 at 15:07

Posted in Uncategorized

Tiny kitchens

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I’m just gonna start with the sink: Maybe there’s a good reason to use a bar sink in a tinyhouse, but personally, I think you’ve got to watch out for the same sort of thinking that leads people to put extra small handles on the smaller kitchen knives. I mean, smaller knife, smaller handle, right?  But you hand stays the same size…. granted you need less leverage for a small knife, but a lot of expensive knife brands have realized the error of this way and use more or less the same size handle on the different knives.

The next thing I think is kind of funny about small kitchens is their lack – yes, lack – of appliances.    I stumbled across the Samson Ultrasonic vegetable cleaner recently.  Good idea, a machine that washes vegetables.  I’m not entirely sure ultrasound is the best way to do it, but whatever.  It can also be used for cleaning dishes and all kinds of other stuff too, without so much as the need for detergent.   If I had a tiny kitchen, I think I would want to have an assortment of machines that makes sure I only spend the minimum amount of time in said kitchen (while still eating really healthy). It might not be the usual sort of thing that people think of as a kitchen appliance, but any task that can be automated should be, I say.

Storing food is another potential problem in a tinyhouse.  Obviously this sort of thing is compounded if there is more than one person living in the house.  One thing I’ve found handy personally, is vertical space storage.  If you think about it, storing stuff in a cupboard tends to leave a lot of unused volume.  If, for example, the kitchen counter were designed to flip up, revealing a storage compartment, gravity makes sure stuff gets packed together a bit better in this situation.  I’ve been doing this with relatively tall cardboard boxes to extend the amount of storage space I have, and it works pretty good for large stuff and infrequently used stuff – so bulk foods, then, and some kitchen equipment.  For more frequently required stuff finding stuff is a bit harder.  You want more than one container, because the finding process sort of necessitates taking stuff out of the box and putting it somewhere else (e.g. another container or onto a nearby counter), while you dig deeper into the box to get at stuff below.

I also like the idea of trap door cupboards in the counter top to store appliances.  To use the appliance, instead of taking it out of the cupboard you can just open the trapdoor.   There would be an electrical outlet inside the compartment, with a way to keep the electrical cord away from the (hot) appliance.  You might want to put a safety switch on the door so the outlet is turned off when the door is closed, too, keeping the appliance off. 

Glass cooktops are pretty cheap now, and they can be used as counter space when they are not turned on, so having a full sized cooktop is no stretch.  An oven, though…. Well, it’s just that if you could use the internal space of an oven, e.g. store stuff in the oven, it would take a lot less space when you weren’t using it.  Only how do you prevent the oven from being turned on while the stuff is inside it?  Maybe you could require that the oven door be opened fully for a few seconds every time, before the oven can be turned on.  That would make people look into the oven before turning it on.  You could have a sensor that verifies the oven is empty before it can be turned on (but you can put stuff in after it is on).  The other thing, I figure, is the baked on food in the oven.  An insert box that went into the oven cavity might be a good idea, you take it out and put it on the kitchen counter or nearby before starting the oven.  Have the oven racks fold up against the back of the oven or something so they don’t need to be removed each time.

Another idea might be a foot controlled faucet, maybe not for everyone, but I like the idea of being able to hold onto stuff while I’m controlling the faucet.  It cold reduce water usage, too. There are devices available to do this, but I would want something that’s a bit different than what they are offering, I think having separate temp and volume controls that could be set and then did not need to be held in position would be better.

Heated appliances should all be insulated and use heat exchangers where appropriate, of course, I’ve noticed that a lot of heated appliances like rice cookers seem to be entirely uninsulated.

Oh, and about the fridge and freezer, a deep freeze can be used a fridge, and a small deep freeze would be much more energy efficient and for a lot of models, quieter, than a bar fridge.  Bar fridges are actually terrible for both those things, because they are very poorly insulated, and powerful compressors make a lot of noise, too.  It might make more sense to put an upper level that was fridge, and have the lower level freezer (with insulation in between.)  or actually, what I was thinking for my own purposes, was a freezer with an external cooler, with a convection powered coolant loop so the compressor in the freezer cools the fridge, too.  Basically just take some food grade pvc tubing from the brew your own, put 2 holes in the freezer, probably in the seal for the door, then run the tubing in, around in a coil, then out, and then run it into the cooler, through a coil inside of a plastic bottle filled with water, then out, and close the tubing loop.   Then you need some sort of insulation around the tubing where it runs between the fridge and freezer, to reduce the entry of heat.  The fluid in the tubing would be something that would not freeze to soon, like an alcohol and water mix.  Then, to regulate the temperature inside the cooler, that is what the plastic bottle filled with water is for.  The coolant loop runs through around the inside of the bottle, so when the temp. of the water in the bottle gets to around 0 deg c, it starts to freeze to the coolant tubing.  That impedes the flow of heat, reducing the amount of cooling provided, helping to keep everything at around 0 deg c., just right for a fridge.  The flow of the coolant is entirely convection powered, so the heights of the fridge and freezer have to be appropriate.  The size of the water bottle etc. would have to be sorted out, of course.

edit: There is a company at, and the builder explains why he uses full size sinks in one of the videos.

Written by gregor

March 18, 2010 at 14:58

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