Towards a better tinyhouse

Inventing to freedom?

Archive for January 2011

Much cheaper photovoltaics part 2

with one comment

I chose this design because it has a more manageable size, could turn upside down to keep the snow off (though I don’t think you really need to given that the upper reflector sort of shelters it and you could make it a bit closer to the ground if you are willing to dispense with that feature), and should be fairly easy to build, including the curved corner reflectors which are just mylar bent into a cone shape.
Sorry the pictures aren’t very good but I didn’t want to waste time with sketchup:

system has a concentration ratio of 6.65 not including reflectivity, or 6.09 at 0.9 reflectivity (the reflector adds 5.65, so add 1 for the fact that sunlight also hits the panel directly) with the reflectors at an angle of 67.5 degrees( relative to the plane fo the solar panel, which goes in the flat spot in the center) and the reflectors reach 1.68 meters forwards ( to get a spot 1.2 meters wide) for the side reflectors and 2.65 for the top and bottom reflectors. The spot is about 10 cm bigger than the size of the panel, so it is tolerant of some misalignment.

There are 4 flat collectors, and the reflectors at the corners are actually sections of cones. The collection ratio would be reduced when needed with a shadecloth wrapped in a roll above, one above and one below the collector, with numerous holes punched for higher transmission sections, or with 2 layers of shadecloth (the stuff sold for horticlture blocks 60% by default), one above the panel one below, and rolled out when needed, you can have several setting, depending on how much the cloth is covering and it’s transmissibility.

Okay I don’t expect you to grok this right off the bat because it is not a very good explanation, though accurate, and most people can skip it, but for anyone who actually might build this and wants confirmation this will work and wants to see why:
To see why this works, viewing the reflector system from the ground looking upwards, imagine a triangle formed with one angle 112.5 degrees, in the plane which is perpendicular to both the solar panel and the left side reflector (and therefore the right too) with the left side of the triangle along the left reflector, and the 112.5 degree vertex is right at the edge of the reflector where it meets the bright spot area, and one side against the solar panel/the surface of the bright spot area. If a row of sunlight rays hits the left side of the triangle, after reflection they will end up on the right side. So you can see you could rotate the triangle around the rightmost vertex while keeping it in a plane perpendicular to the solar panel, the left side would sweep out a cone, and the light reflected from the cone would hit the area swept out by the side of the triangle which is along the solar panel. You can similarly imagine pivoting the triangle around that vertex with the oblique inner angle, which was against the edge of the reflector, and the reflected light would cover a semicircle shape if the reflector was a suitable cone section. Then remove any unnecessary parts of the cone so the spot of light is the right shape..

The actuator would be a winch, and since you only get 1.5 sq meters or so of panel you would probably need 3 of them to get in the range of 5 kWh per day during the worst month, but they can be controlled with the same winch and control electronics, obviously the winch has plenty of torque. Just wrap a rope around the wind a few times, attach the middle of the rope to the winch drum, and then keep wrapping a few times, and you have both ends of the rope, attach them to the collector for motion, apply tension to prevent motion due to wind. The tension if applied to bothe ends of the rope eaqually balances out and exerts no net torque on the winch drum.

The axis of rotation is sort or around the center of pressure, so wind loading shouldn’t exert much rotational force. To adjust it for the seasons, there is an adjustment that you can see in the video.

If you take a line which is perpendicular to the plane of the solar panel, it draws a cone around the axis of rotation, which when the axis of rotation is parallel to the axis of the earth’s rotation which it is, is perfect 1 axis tracking.

There might be light sensors on the panel so the electronics module knows it has things in focus, with 3 sensors at the edge of the panel, they would all be the same when it was sufficiently in focus. There should also probably be a temp sensor (e.g. thermistor) attached to the back of at least one panel in the array ( for 5 kWh per day minimum there would be 4 collectors like this but they can use the same actuators etc.) (they should all be about the same temp after al on the back of the panel. The electronics module would be an AVR and an h bridge.

Okay, I think it would cost, I can get an 8 foot 2×4 for $2 at home depot I think so I dunno, maybe 8 2x4s $16 and $12 for some door hinges and some screws, then $70 for the atv winch (from google products search “atv winch” list by price) and if you get the electronic module custom made at a sparkfun like place maybe $45 with the light sensors and $3 for a tupperware container to put it all in for water resistance, and $30 for a gearmotor to control the shadecloth with one controlling all the cloths on all collectors (use fishing line?) plus $20 for the mylar.

The performance ratio is 2.43 so with unassisted panels it took 1.73 kWh of solar panel at $1.69 per watt or $2925, so this would be $1203 plus $204+$145 for a total of 4 collectors = 1552 , for a savings of $1373 and a cost per watt of $0.90 per watt. There must be a better reflector system though. Also amorphous panels would help, and I haven’t looked into a cooling fan yet.

Rulgert raises the point about ROI, which I was sort of ignoring, but it looks like, actually, if we reduce the max. operating temperature to 85 degrees, that actually makes almost no difference, giving an improvement ratio of 2.4 instead of 2.43. Reducing it to 65 degrees, which is typical for a rooftop gives 2.24.

Clearly it would be a good idea to at least do the financial calculations, and correctly, which includes the interest on the money you saved, rare but catastrophic possibilities like the possibility that the system will be vandalized, or you will have to sell it at a loss etc due to a change of plans all need to be taken into account too, and these things improve a lot with a lower capital cost.

But there doesn’t seem to be any good information that I can find on how lond panels last at higher temps. So we have a choice: fall for fear, uncertainty and doubt and just do things the easy they are always done (which corporations just love by the way), or we can go for it. Your choice, live in fear or live in hope.

Plus if there are problems with financing and so on, a lower capital cost is more than just a good thing from an ROI standpoint, it could be the difference between able to build it or not.

And finally, even if the panels failed in 7 years instead of 25 or 60 (I’m not saying they would, as I said it is unknown, and they might last for 20 instead), does that even matter so much? In 7 years the price of solar panels and other renewable energy sources is going to be a tiny fraction of what it is now. There are already companies like first solar making panels at 90 cents a watt (they don’t sell them to individuals though darn). People make the decision to use propane instead of solar heat all the time for instance, because what they care most about is getting things up and running and can’t be bothered with longer term results. At least this is a more environmentally friendly way to do so, if it were one.

By the way here are some totally random notes that I don’t want to spend the time to clean up but you migh thave use for to invent your own even better ones. I don’t want to edit themm so they are a total mess but there might be some ideas and info for you to extract:

conc ratio improvement ratio solar panels req for 0.9 kW array total aperture price of panels
3 1.85 4.38 13.14 884.43
4 2.14 3.79 15.14 764.58
6 2.42 3.35 20.08 676.12
8 2.57 3.15 25.21 636.65
12 2.7 3 36 606
25 3.04 2.66 66.61 538.22
50 3.22 2.52 125.78 508.14

winches including atv winches which are available for as low as $70
valve actuator $50 probably no good for main tracking mechanism but maybe good for varying the conc ratio
heavy duty rc servos tend to be expensive though IIRC
drapery actuator No idea what the cost
gearmotor of sufficien size these are a little small but maybe something like them from a similar vendor

maybe even stepper motors http://www.jameco.com/webapp/wcs/stores/servlet/ProductDisplay?langId=-1&storeId=10001&catalogId=10001&pa=238538&productId=238538

Satellite dish actuators are awfully expensive that I have seen, $150 or something each.

I like the idea of using cables to couple the actuator to the main apparatus. Should be easy to do in a do it yourself scenario, you can vary the leverage etc quite easily and so on too. Plus you can connect multiple apparatuses together so they move in synchrony using only one actuator. Also you can use a tupperware container or bucket to put the actuator in if it is not weatherproof by ittself, then run the cables inside. It is also perfect for the winch one. You can have a “balanced” system too to give resistance to wind – suppose you have a winch and there is the connection point on the winch drum somewhere. You could attach one end of the cable/rope to that point and have a winch that can wind in the cable, but suppose you instead attach the middle of the rope to that point. One half of the rope gets wound up clockwise on the left half of the drum, maybe only a few turns but could be many, and the other half get wound up counterclockwise. So now one end of the rope is unravelling while the other is wrapping up, see. Any static force exerted on the cable does not result in a net torque onthe winch which is good.

interesting site with a bunch of info, good starting point for research

coaxial, keeps the snow off the panel too plus shading with “shade cloth” available for greenhouses to reduce conc ratio when needed (shade cloth could be fine for other colllectors)

large Plate reflector at one end of a large parabolic trough collector that is twice or even longer than the solar panels. The panel is located closer to the trough than the focal point so the light is sufficiently diffuse. Bring it closer to reduce conc. ratio. Could be attached to ropes to do so using the cable actuator? Suppose you are looking through a hole in the plate, whan you woulde see when you looked at the trough at an angle is a reflection of the sun in the plate reflector if the rigamarole is oriented right.. You’d see 2 suns right, would sort of approach 2x the concentration of the trough alone. The trough could be vertical, and the flat plate on the ground, or it could be in anothe orientation. In this orientation it might be better to have 1 axis tracking but maybe you could do without.

The plate one, but the areas in between the panels are used by adding a diffuse reflector e.i. like a sheet of paper, conc. ratio is again controlled with shade cloth or by making some of the reelfector panels move so the reflected light form it no longer hits the panels. single axis tracking

like above but at the outer edges of the panels another, slightly curved reflector could be added for more light, sunlight is directed towards solar panel and spreads out due to curve or reflector.

2 plate reflectors could be added to either side of a panel, an then extended a ways beyond the ends of the panel. Then 2 large plate collectors added some ways beyond the ends to direct sunlight towards them, providing maybe 9x, 3 times from the 2 panels (what hits the panel directly counts as 1, add one late is 2, 2 plates is 3) the 3 more from each end plate, single axis tracking?

move the panel rather than moving the reflector might be easier for very harge collectors, could use curtain tracks and drapery actuators but the tracks might rust outside. With an overlength parabolic trough collector just move the panel closer to the mirror when already past the focus for less light, should be fairly uniform I think. Could be used for sun tracking. Problem is that then you loose a lot of sunlight in the morning and evening potentially.

a vertical parabolic trough reflector with a mirror on the ground, which produces a mirror image so is like 2 suns sometimes, vertical axis tracking could be very large for high conc ratios, move panel towards/away from to vary ratio, also add overhang for roof and a bit more reflector

one issue is the need to have the focal point relatively uniform because of the way solar panels work. They are a large number of cells in series or parallel, and if they are in series if a disproportional amount of light hits one cell the extra power is not used optimally, because the maximum power point of voltage-current is not reached. It can’t be because in series the current through all the cells has to be the same, but the ideal voltage-current point changes when the light level changes.
If they are wired in parallel similar idea; the voltages are all the same.

Also when the panel is operating close to it’s maximum acceptable operating temp then you’d have to be sure that excessive light does not get directed to a particular cell or it could be damaged I guess, although I don’t think that 111 is an actual damage threshold, it might be substantially higher. It might only be for purposes of guaranteeing the specified performance parameters/ lifetime of the panel.

After all they are basically giant silicon diodes, and a typical rectifier diode can handle temperatures higher than that I think.

Ideally you would figure out the ideal curves for the shapes of the reflectors, I guess. A set of flat panel reflectors, though, maybe recycled bathroom mirrors or something, actually might be the way to go, as the reflected spot would be automatically uniform.

Another way could be to use a shade cloth material of some sort, with it’s transparency different in different places and just right in the right places so that it subtracts light from the areas that would otherwise be too bright. Maybe use a low density shade cloth and then put multiple layers to dim or punch small holes to brighten, in the right places.

If you have a collection of flat plate reflectors, each directing sunlight to the panel, you could remove several of the plates to reduce the concentration ratio.

problem with the parabolics is that it seems like the center of the spot of light might be brighter I don’t know. Would it be when the spot was outside of the focal plane?

electronics modules:
the red rock ones

programmable logic controllers

can make your own, also if you don’t have the soldering etc. equipment you can get circuit boards custom made relatively easily I think, one company is “sparkfun” but there are probably better ones. Take an AVR and an H-bridge to control the actuator (servos and the valve controller have their own power electronics though) and a suitable sense resistor and voltage divider to measure voltage coming from the panel, measure the amount of power coming from the panel, the microcontroller knows the time of day approximately, then does fine adjustments based on the amount of power coming from the collector, and maybe a ten cent CdS photosensor at the panel surface just for redundancy to protect against excessive irradiance. Put the whole thing in a tupperware container for water resistance. You might need an encoder of some sort to provide feedback about the tracking system’s position.

Another problem is the wind loading during storms. Obviously some of these are pretty big. Still if you can just put it on the ground it’s not any where near as as on a rooftop, where the roof would not be strong enough to stand storm winds.

Also remember amorphous panels would be better, if windspeed is factored i nthat should improve things. also the possibility of a fan is plenty real, the fact that snow might be kept off of the panel could increase real-world gains with a reflector system tha thad taht feature, you save money on batteries too ( I’m pretty sure, have to check how much)
wind speed only went down ot or below 1 meter per second for 24 hours during the entire year so wind is definitely a source of improvement factors that could reduce performance reduce savings that have not been taken into account are:

if life of panel is reduced
potential for damage durig storms but with suitable design should be fine
if system is capable of highg conc ratios potential for malfunction and overheating damaginng panels

Lastly, there must be a substantial amount of money to be made here. I mean think about it for a second – suppose for that hypothetical 5kWh/day minimum system you take a generous $250 in parts to build the collector. How much money did we save again? $2068? And that’s quite a small system, remember, the average American house uses 32kWh per day.

Figure out a good mechanism and you could sell kits, and charge a pretty decent profit on your time, and still leave the customer with huge savings vs. an unassisted array.
Update:
I checked out the amorphous panels, looks like this would give an improvement ratio of 3.03! so pretty good

edit: it occurred to me that if you were concerned about the impact of high temperatures on the panel’s lifetime, you could just turn down the concentration ratio during the summer. That would greatly reduce the time it spends at high temperatue, without sacrificing the net cost of the system, because it still produces just as much during the winter, which is the limiting factor when determining system cost.

edit: I found d a paper about a system that in many regards is similar to this. It has a fixed concentration ration, though of 3. The system has apparently beed running fine for 5 years (notice at the end, he seems to be writing this 5 years after first building the system) :
http://www.math.auckland.ac.nz/%7Eking/Preprints/pdf/WindArt.pdf

Advertisements

Written by gregor

January 30, 2011 at 20:02

Posted in Uncategorized

Much cheaper photovoltaics part 1

with 7 comments

Figure 1:

Okay, what you see above is a nice graph of power output vs. irradiance at the panel surface for a typical polycrystalline panel, for 3 different *ambient* temperatures. Amorphous panels are better, with a temp coefficient of 0.0027 or so, but I couldn’t find any for sale to get the price info, so I didn’t use them, although they are usually cheaper than polycrystalline and everything here should work even better with them if you can get ’em.

Okay, so I previously mentioned the fact that it is clearly wrong that tracking systems and mirrors cannot give you more bang for your buck from a photovoltaic panel. Now I worked out the details reasonably well for Minnesota with a spreadsheet, and it looks like it would greatly reduce the price of the system. Incorporating the stuff below I think you could reduce the cost of the actual panels by a factor of 2.8 or even more if you had a lot of space for the reflector, and there is no way the cost of teh reflector and tracking stuff is going to even start to cancel that out, as I’ll go into a little more in part 2. To start explaining observe the graph below, which shows the “direct normal irradiance”(which is the amount of light hitting a surface perpendicular to the sun’s rays) and “diffuse horizontal irradiance”(which is the amount of radiation cominf from the rest of the sky, after having been scattered by clouds etc.), figure 2:

This is January, and the x axis is days.

Again this is all with typical meteorological year data, which is not perfect but gives a reliable idea of what’s going on. Actually I discovered a while back this great software called “system advisor model” from the National solar radiation database people, which I recommend you google and download if you are thinking of installing a photovoltaic system. It simulates stuff like we are doing here but better, except it can’t simulate the precise type of system that we are talking about, which is a concentrating and sun tracking apparatus that uses commodity solar panels we can actually obtain. It’s where I got the handy equations and some numbers for how to account for the temperature of the solar cells though.

So I did it in calc. The system is corrected for the temperature the PV cells will reach, and this is pretty important because they get much less efficient as the temp goes up (and the temp goes up with the irradiance). This is what causes the curve in figure 1. If the panels had the same efficiency at all temperatures the plot would be a straight line, which would be nice, but…. Also, the windspeed is significant, but was too much work to take into account, so I assumed it is always 1 meter per second, which is lower than usual (average in Minneapolis is 4.6 m/s), which means the simulated performance of the system is only under, not over estimated. So it doesn’t invalidate that this system should work at least as well as laid out here.

Basically I was going to try simulating several systems, but it takes quite a bit of time especially since my computer is so slow, so I just did what would, I think give the very best performance : A variable concentration ratio collector. I have a few ideas on the details of how to do this which I’ll post in part 2 of this, and am sure there are much better ideas in the ether, including in the crowd here. But for now let’s look at the potential performance increase such a system would provide, so we know how much there is to gain.

The fact that it can vary it’s concentration ratio within a certain range allows it to acheive high concentration levels when there is little sunlight (which you can see happens sometimes in figure 2) while at the same time not overheating the panels when the sun comes out in full force.

First we want to know what the maximum operating temperature of the solar cells is. I found on the net somewhere that panels are rated for between -40 and +85 degrees c ambient. They might actually be capable of more than that, I bet there is room for experimentation there. The Datasheet of a typical polycrystalline solar panel, the SUN-SV-T180 tells us that the cells of such a panel operates at 26 degrees above ambient at 800 watts of irradiance and 1 meter per second wind velocity (“open rack” presumably, which means it is not mounted on a roof, just free in the air which improves air circulation). From that we can find that the U value between the actual silicon cell and the great outdoors, 30.7 or so.

If the panels are rated to operate at 85 deg c ambient they must be able to accept 800 watts or so at 85 degrees, so their maximum operating temperature is 85 plus 26 degrees, 111 degrees.

So with this info and the climate data, including hourly ambient temperature and solar irradiance values I can make a spreadsheet that simulates the variable concentration ratio collector, for some reason I decided it should be able to concentrate between 3.6 and 14.7, which is a pretty big collector but actually if you reduce it to 8 that is still good and you can make it even less with performance hits, as mentioned below. You can change it in the spreadsheet easily.


assisted collector, the y axis is in Wh so divide by 1000 for kWh:

I couldn’t get the data out of SAM, so I can’t put them on the same graph, but you can see the difference. Also I would like to put the data for the concentrator on a more detailed graph but calc keeps crashing.

That SUN t180 panel is about the cheapest there is actually obtainable and is $1.69 per watt, $1690 per kW of panels, 12% efficient which translates to $202.8 per square meter.

5 kWh is pretty low useage even for a tinyhouse, and you can see from the graph, the unassisted 0.9 kW (8.1 sq meters) of panels would give you about 78 kWh per month in the limiting month, or 2.6 kWhr per day. So for 5 kW we need 1.92 kW(EDIT: I made a mistake there, tha should read 1.73 kWh arg so the remaining calculations are nor quite right but still same general idea) of panel, or $3244.8, (plus tax and shipping). With the concentrator and tracking system you would get 220 kWh per sq meter of panel in the limiting month for the same size panel so you only need 1/2.83 times as much solar panel (2.86 sq meters).

Also, by the way you can’t see it in the graph, but I checked what would happen with only a max. conc. ratio of 8 and it’s still 1/2.53 the cost. With a solar collector like this you would save (1-1/2.83)*3244.8= $2098. There’s no way a homebrew tracking system and collector made with 2 by 4s, door hinges and $70 actuators and some electronics is going to cost that. Another bonus of this system is that you have a more even distribution of solar input, which would reduce battery cost too. That would be another interesting thing to do with calc. Actually calc is too slow. I can’t believe how much time I spent on this so sorry if I seem like I’m rushing.

I haven’t figured out exactly the best way to make such a thing, but tune in next time for some ideas, and also some actuators and stuff about mirror configurations so maybe y’all can think of something even better.

Improvements in the system could be made by using amorphous panels and maybe and even bigger max. concentration ratio. Also, including the wind speed might give substantial gains. I wonder about using a fan or something, a big, slow moving fan might actually give you a substantial net energy gain after the electricity it uses. As you can see from figure 1, there is not much point in trying to operate the panel at higher irradiance values, though, because you actually end up with *less* power above about 3500 watts/m2 or so due to increased temps.

Here is the spreadsheet:
http://www53.zippyshare.com/v/55013956/file.html

Written by gregor

January 28, 2011 at 17:00

Posted in Uncategorized

VirtualTinyHouseConference #13 summary

with one comment

Rulgert and Dercon (I think?) came, but Rulgert didn’t want to install Vsee and join Dercon and me. We used audio, but there was a thunder-like noise on Dercon’s end. Still I could hear him fine. I was using the headset and everything so I didn’t know what was up. I later determined using sound recorder there is a badly-tuned radio like noise coming from my headset’s microphone and I can’t get rid of it so maybe that was it. Arg. It was working fine when I used it just a few days ago. But I can use the microphone built into the computer+the headphones and the audio is actually pretty good. Dercon has windows 7 and the acoustic echo cancellation works but it’s not perfect. Arg. It says very clearly in the Vsee documentation that it should be fine. But he had headphones so that was solved soon. Still, this is ridiculous. There really should be echo cancellation built into vsee.

But even so we actually talked about a bunch of different stuff for 40 minutes, Dercon is apparently planning to actually to build a totally off grid tinyhouse, and live off grid, even grow a lot of his own food, is actively looking for land and has a goo deal of money saved, so he’s actually going for it. We talked about “solar cans”, photvoltaic systems, heating, sketchup, and the design of the houses. Apparently there is a mistake in the tumbleweed XS house plans that Dercon had spotted with the engineering drawings for the roof, and he seems to be a construction professional of some sort. Was really interesting but would definitely be easier to communicate with audio.

So it sounds like a really interesting project anyway, and I can;t find any information about this “solar can” thing, which sounds like a quonset hut painted black so it warms up in the day… I’ll have to ask him about that if he comes again next time. Also I said to definitely drop me a line if he starts a blog, and I’ll share it with y’all since I think it would make a really interesting project blog.

It occurs to me that several of the people that have showed up are building tinyhouses, but none of them have ever shown up at the same time, excpt eightball and Rulgert, but Rulgert’s tree saucer project is so radical (which is part of why it is so cool though), there’s not that much room for tip-swapping. I just know you guys’d hit it off if you could actually communicate with each other in some way….. oh well. It doesn’t take much effort to keep holding the con except sometimes I feel like I sort of have to make time and make sure I am there, as the host and all. But it’s interesting when people do show up, so I’ll keep holding it for now.

Written by gregor

January 27, 2011 at 12:05

Posted in Uncategorized

VirtualTinyHouseConference #13 is happening now

leave a comment »

Remember: bring speakers and a microphone. If you have them you should use them. I’m hereby declaring it poor etiquette to, if you have it, not use it. Because you would make it difficult for people to hear and talk to you, slowing down the conversation. Still better to come without than not come at all. If you have windows 7 or vista any microphone and speaker will do because these have built in echo cancellation, otherwise headphones or similar are highly recommended, anything so your microphone cant pick up sound from your speakers thereby causing echo.

If you haven’t done so yet, download and run Vsee. There’s no install to go through. When you run the program, it will prompt you to fill out a short form to sign up but you can enter anything you want, and there is no confirmation email or waiting.

I plan to be here, but this post is scheduled to appear automatically so if for some reason I can’t make it, someone else can host it by putting their vsee username in the comments section below. (after someone calls you, click on some of the icons in your video window, and under one of the menus that pop up there will be an option “host meeting” then when someone calls you, somewhere in their video window, you can click “add to meeting” so they can see everyone else).

My Vsee ID is GregorF.

There is also one tinychat room in case Vsee is not working out for some reason, though that has not been a problem so far:
VirtualTinyHouseConference Tinychat room

The VirtualTinyHouseConference is currently being held every Wednesday at 8 pm, Eastern Time. Just come to towardsabettertinyhouse.wordpress.com and there will be a blog post there with easy to follow instructions. Everyone is welcome and anyone in the world with an internet connection could be there.

You can join by video, text, audio, or any combination thereof. If you are not much of a typist it is recommended that you have a microphone. If you have vista or win 7 you’re good with whatever speakers and mic you have because they have built in echo cancellation but if you have Win xp, ear buds (like for an ipod), earphones or a headset are recommended, otherwise you cannot use both the mic and speakers at the same time without causing echo for others (which occurs when they say something, it comes out your speakers and is picked up by your mic and sent back to them).

Some backgrounder : https://towardsabettertinyhouse.wordpress.com/2010/11/14/virtualtinyhousecon-3-is-scheduled-for-nov-20-8-to-9-pm-eastern-time/

https://towardsabettertinyhouse.wordpress.com/2010/11/21/read-this-if-y…tinyhousecon-4/ ‎

Written by gregor

January 26, 2011 at 19:59

Posted in Uncategorized

Reminder: The VirtualTinyHouseConference is today 8 to 9 PM Eastern Time

leave a comment »

Remember: bring speakers and a microphone. If you have them you should use them. I’m hereby declaring it poor etiquette to, if you have it, not use it. Because you would make it difficult for people to hear and talk to you, slowing down the conversation. Still better to come without than not come at all. If you have windows 7 or vista any microphone and speaker will do, otherwise headphones or similar are highly recommended, anything so your microphone can’t pick up sound from your speakers.

Got questions? Got a cool project you are interested in getting feedback on? Want to learn more about solutions for water, building, zoning, locating tinyhouses, or well hey anything else someone else might know or be able to figure out?

You can meet new people! Get some great blogging material! Learn tons of interesting and useful stuff! Share your story! Talk to other awesome people interested in tiny houses!

And it’s free! And you can stay at home and sit in a comfy chair at the same time, what a deal.

Just be here at 8 Pm Eastern (Handy time zone converter)!

Remember, anyone in the world with an internet connection could be here.

Written by gregor

January 26, 2011 at 15:49

Posted in Uncategorized

<$160 year round solar heater for a 250 sq ft tinyhouse

with 3 comments

Okay, remember that gelified water solar collector idea? I decided to take some time to put together a spreadsheet and check out the details. Which took way longer than it should have, but…

Anyway it turns out there is not much point in gellified water, due to the high cost of the gelling agents. Polyvinyl alcohol is $21 per Kg or so apparently, and I checked agar which is triple that, but if you could get the cost of the gelled water to below about $60 per ton it might be cheaper for this application. If you needed higher temperatures then it might make sense. Bizarrely I found a patent that describes the idea here, and you won’t believe me but I found it while looking for the thermal conductance of gelled water. Notice that nowhere does it actually have that information in it – search engines can be so weird. And annoying.

Anyway, so this uses greenhouse plastic. The spreadsheet lets you easily customize it to your own climate, but I chose to use the data for a challenging climate, Minneapolis, Minnesota. This design described below keeps a 250 sq foot, with loft, just built to code, with a little ventilation 0.3 changes per hour, but no heat exchanger, tinyhouse warm all winter in a typical winter. This is using the typical meteorological year data, so it might be a good idea to double check with a more severe winter before you built. A link to a place you can get the data is below, but it costs $20 or so, and I didn’t want to pay that.

If you just want to get your results for the typical meteorological year, you can just cut and paste the information into the spreadsheet after downloading it from the the site linked to below.

I explain how it works below, but the spreadsheet is pretty accurate and factors in solar radiation data, ambient temperature, ventilation rate, the insulation values of the walls and windows, and then you simply enter values for the solar collector and it will return information about system performance, and draw you a graph like the one below. Adjust the parameters, like the size of the collector etc. and you’re all set.

The way the system, and the spreadsheet works is this:

There is a shallow insulated box on the ground. You can set the insulation value. In the specific system I came up with here it is a U value of 0.4 which is r-2.5 so a board of polystyrene foam. the bottom is just foam and rest right on the ground. The sides are probably about 15 cm high and made with 6 by 1 wood boards.

Okay, now there is a plastic liner of greenhouse plastic so this box can now hold water. The box covers 10 square meters. Probably 2 meters wide by 5 meters long. There is some water in the box, a layer a centimeter thick or so doesn’t really matter, and some tubes so the water can flow in an out of the box through them, in along one side and out along the other. The water has antifreeze of some sort in it, probably something harmless and convenient like urea or sugar and some salt or sodium benzoate or something (used to preserve pop) to prevent microbes from growing. This is the water that gets heated up and used to transfer heat around. It could be dyed black, or you could just put some black construction paper on or paint the very bottom of the inside of the box, in contact with this water.

The tubes do not go through the sides of the box, they just go over the edge, so no seals are needed or anything.

Next, directly above this is the insulating transparent layer. You can set it’s light transmission and U value in the spreadsheet. It is set to a u of 4.4 and 0.9 transmission or so right now, which is fine for 2 layers of greenhouse plastic (which is more like u 4). Actually you change the “thickness of gel” cell to change the u value, you might want to change that.

Okay, that’s the actual collector thing. There is also a reflector, and this really helps to reduce the size of the collector and increase output in an economical way. This just something flat with mylar attached. You could just build a frame out of lumber and put the mylar over the frame, but it might flap in the wind. There must be a better reflective material available that is rigid enough to not flap. Otherwise just take some 1/8 or 1/4 inch ply whichever is cheapest and glue the mylar or Al foil or whatever reflective stuff there is to that. The fact that not all the light is reflected from the reflector is accounted for in the spreadsheet and you can set the reflectance.

Also, you’ll notice the reflector is at a certain angle. This is accounted for in the spreadsheet, and you can adjust the angle. You can see it helps to give you a bit more power, and you would probably a little roof extending over the top with a bit more plywood to keep the snow off of the collector, which I forgot to add to the pic, although it will block just a bit of the diffuse light from the sky, it also adds some more direct sunlight. The fact that the reflector also blocks some of the sky is also accounted for, and it still gives you a big net boost.

The reflector is bigger than the actual collector part, probably extends 1.5 meters in either direction to give you a bit more boost in the morning and evening, and a meter longer than the actual collector. It is assumed in the spreadsheet that the spot of light is bigger than the actual collector, so when it moves due to sun movement it is still covering the actual collector. This is not accurate during the early morning or evening but you can see looking at the solar data that almost no sunlight is available then anyway, so this fact doesn’t cause any serious inaccuracies.

Okay, then you have an insulated storage tank, probably just some 2 by 4s, 1/4 inch plywood and lined with some polystyrene foam and some Great Stuff PU foam sealant to seal around the PE foam, and some more greenhouse plastic to line the box with. It is a cube 2 meters by 2 meters by 2 meters. So lots of water, but water’s cheap.

Next, there is some thin walled plastic or metal tubing on the the side and towards the bottom of the inside of the box immersed in the water tank. The heated water flows through this and then back to the collector, by convection.

This is just to allow the water heater (the solar collector) to heat the water in the tank, without just running the water in the tank through the heater. If we just ran the water though the heater we would have to do something about the water pressure that results from the height difference of the water, and also the whole 8 cubic meters of water in the storage tank would have to have antifreeze stuff added to it which would be too expensive and probably kind of messy or damaging to the soil in case of leaks. So there is storage water, and there is transfer water (well transfer fluid since it is more than just water, since it has the antifreeze).

Then, the water from the storage tank is capable of flowing through a radiator inside the house, probably with some sort of simple thermostat control thing of some sort, you’d have to figure that out. The radiator could be a large plastic garbage bag or two coiled up a bit and with some spacers to allow some air to flow between the layers of the coil. It should to have a surface area of more than 2 meters or so. A radiator from a car or from one of those old hot water heat systems might work, just check the surface area. It needs to be able to transfer more than 600 watts with a temperature differential of 20 degrees, which is not that hard. You might be able to make one with, I was thinking, a piece of aluminum flex duct, just flatten it and bend it in a u shape, with the top of the U above the top of the water level in the storage tank, so there are not really any watertight seals to make on it. As you can see there are many options for a cheap radiator if you give it some thought anyway.

Water flows through the radiator again by convection. I still have to check the physics to see what size of and length tubing would be required/okay for adequate convection but we’re talking ten milliliters per second here so it’s not much and shouldn’t be a problem. For the collector side it is hundred and something mls per second peak but I would think not a problem at all with adequate size and short tubing.

Also, when water is heated in the tank by the heating tubing coil it rises to the top of the tank instead of being all evenly mixed and stirred, so it should be more or less a laminar flow tank, like a hot water tank works. This just makes the radiator a bit smaller because the water going in is hotter so the average temp across the surface of the rad is higher. Basically if the whole tank was mixed the energy would still be useable (in this particular scenario) but held as a lot of warm water rather than a smaller amount of hot water. But it doesn’t really make that much difference.

Probably for tubes that enter and exit from the water storage tank, it would be better to have fittings and put them right through the side wall, but you might be able to get away with putting them up and over the side wall thus avoiding the fittings and associated potential leaks and extra construction work.

The plastic would probably cost $60 at most, probably less I think I remember seeing it for $3 a square meter somewhere, then 2 layers. Plywood lumber and tubing and foam etc I would think less than $100.

And that’s it! The system needs no electricity, is dirt cheap to make compared to alternatives, takes no fuel to carry or buy, you could use the water tank to store drinking or cooking water if you make things out of suitable materials and in a typical year it works without leaving a single day without heat! Not bad. Again I would caution that this is for typical not an extreme year. It’s also in a very cold and not very sunny climate, Minneapolis minnesota, too, though, so clearly this sort of system is totally practical.

In the summer you would fold the reflector thing down, over top of the collector, so that the plastic is not exposed to sunlight unnecessarily. Judging from the amount of sunlight exposure this would save the plastic from (shown by the red line in the graph), and that this plastic should last 3-4 years if it were fully exposed year round (I think that’s what they said) this should easily greatly extend the life of the plastic. Another thing that could make it last for more than 10 years, eyeballing it, would be to split the collector into more than one unit, each of which can separately be folded in this way. As you can see from the graph, you only need all 10 meters during the dead of winter, so by folding the ones you don’t need when you don’t need them you could eliminate a lot of sun exposure from them.

One more thing I would like to add if I had time is better simulation of the water tank heat storage, with the capacity to take into account that the tank can store extra heat by increasing the assumed storage temperature during very sunny periods, and that it’s heat can be negative if the water gets colder than the indoor temperature because then energy has to be added to heat it up before it can be used to store useable heat.

Also, this started out as a design that was going to use gelled water, which wa originally why the collector was on the ground (water heavy), but then I switched to greenhouse plastic after finding out the cost of gelling agents, and I think putting the collector on the ground os a pretty good place, you get the protection from snow so it’ll keep working rain or shine, and you can fold it over during summer. You might be able to reduce the cost a bit though with more reflector and less collector but… also you can use glass or whatever if you want, just put the right U value and transmission value into the spreadsheet.

Here’s a quick sketch of it:

A graph, you can see there is plenty of heat in the summer if you wanted to use it for heating hot water for showers etc. or a heat powered air conditioner:

spreadsheet is here : http://www.filefactory.com/file/b61c990/n/solar_heaterrecoveredmorerecent.ods

Polyvinyl alcohol, 20 lbs for $211:

http://www.soapgoods.com/Polyvinyl-Alcohol-p-713.html

example double glazed window: http://www.homedepot.com/Doors-Windows-Windows/h_d1/N-5yc1vZaq1wZ1z11iha/R-202035739/h_d2/ProductDisplay?langId=-1&storeId=10051&catalogId=10053

free climate data for typical year just cut and paste: http://rredc.nrel.gov/solar/old_data/nsrdb/1991-2005/tmy3/by_state_and_city.html

climate data for a variety of years but not free: http://www.ncdc.noaa.gov/oa/ncdc.html

http://www.bwgreenhouse.com/R-valueU-value.html and http://www.greenhousecatalog.com/solexx_faq.php#insulation

Written by gregor

January 22, 2011 at 09:31

Posted in Uncategorized

VirtualTinyHouseConference #12 is happening now

with 5 comments

Remember: bring speakers and a microphone. If you have them you should use them. I’m hereby declaring it poor etiquette to, if you have it, not use it. Because you would make it difficult for people to hear and talk to you, slowing down the conversation. Still better to come without than not come at all. If you have windows 7 or vista any microphone and speaker will do because these have built in echo cancellation, otherwise headphones or similar are highly recommended, anything so your microphone cant pick up sound from your speakers thereby causing echo.

If you haven’t done so yet, download and run Vsee. There’s no install to go through. When you run the program, it will prompt you to fill out a short form to sign up but you can enter anything you want, and there is no confirmation email or waiting.

I plan to be here, but this post is scheduled to appear automatically so if for some reason I can’t make it, someone else can host it by putting their vsee username in the comments section below. (after someone calls you, click on some of the icons in your video window, and under one of the menus that pop up there will be an option “host meeting” then when someone calls you, somewhere in their video window, you can click “add to meeting” so they can see everyone else).

My Vsee ID is GregorF.

There is also one tinychat room in case Vsee is not working out for some reason, though that has not been a problem so far:
VirtualTinyHouseConference Tinychat room

You can try using use Google Wave if you want, by clicking here It is set to allow anyone to edit the wave, but it doesn’t work when I tested it, so you will probably have to sign in to google wave if you want to use it.

The VirtualTinyHouseConference is currently being held every Wednesday at 8 pm, Eastern Time. Just come to towardsabettertinyhouse.wordpress.com and there will be a blog post there with easy to follow instructions. Everyone is welcome and anyone in the world with an internet connection could be there.

You can join by video, text, audio, or any combination thereof. If you are not much of a typist it is recommended that you have a microphone. If you have vista or win 7 you’re good with whatever speakers and mic you have because they have built in echo cancellation but if you have Win xp, ear buds (like for an ipod), earphones or a headset are recommended, otherwise you cannot use both the mic and speakers at the same time without causing echo for others (which occurs when they say something, it comes out your speakers and is picked up by your mic and sent back to them).

Some backgrounder : https://towardsabettertinyhouse.wordpress.com/2010/11/14/virtualtinyhousecon-3-is-scheduled-for-nov-20-8-to-9-pm-eastern-time/

https://towardsabettertinyhouse.wordpress.com/2010/11/21/read-this-if-y…tinyhousecon-4/ ‎

Written by gregor

January 19, 2011 at 19:59

Posted in Uncategorized