More to come, I have just been busy with life.

More to come, I have just been busy with life.

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More to Come, A Simple Solar Box Cooker

I have been busy with a major construction project but came across the solar box cooker I made a while ago. I will post about how I constructed it and how it performed in mid September a number of years ago as well as add some images. I should have that post ready in about 1 week.

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The Solar Furnace

It has been busy in other areas, so this post was delayed, but as with all good things, they are worth waiting for. This post is more of a project that can be utilized in many different ways, from cooking food, boiling or distilling water, melting metal, etc. It is a solar furnace and made in large part from scrap.

Because of the dangerous potential of these if misused, or used carelessly- Should you attempt to construct one, you do so at your own risk and liability is solely with you. This not something to be attempted by idiots or fools.

The simplest description is a base that is made from wood and it supports in this case of my example, the metal frame holding a “Fresnel” lens. The lens is salvaged from a rear projection television and gets it’s name essentially from the style of cut that makes the lens. Most TV’s of this type have a plastic frame that supports the lens to the rest of the cabinet and are not that good for support in this application, but in this rare case, the donor set had one with a metal frame.

All rear projection sets of this type have 3 elements:


The three pieces you find on the face of a rear projection television.

1 is the outer acrylic sheet that afford protection and a neutral gray filter. This is not used here, but it can be used for regular window glazing if you do not mind the tint, or in the event you actually do need the tint.

The second item is usually the polarizer, followed by the lens. Some lenses may just look like they are only a translucent difusion surface, but if you look closely, the circular cuts that make the lens should become evident. You will want the ridges of the lens pointing towards the target area for best results (ie. downward.)


The lens sitting in the frame with the other 2 pieces removed

That second item in the stack is going to be a polarizing sheet, usually a “mechanical” polarizer (lots of tiny holes) or a plastic composite type of polarizer which looks like stripes of alternating black and clear plastic. This piece can be used for some privacy as a window covering or for heat reduction- just cut to fit the glazing (just inside the window frame), and then with a tapeless type adhesive like 3M’s “ATG 323” or “ATG 969” (which has held up many slinky dresses in Hollywood events.). The polarizer is then simply pressed/adhered to the glass. In warmer climes it is probably better to adhere this to the outside of the glass as the polarizer itself will get warm from the sunlight it does absorb. If you already have “Low E glass” in your windows, inside is fine.

The ATG by the roll or the case can usually be ordered through various mail order art supply stores- the real serious ones that cater to industry, such as Dick Blick- only mentioned because I have done business with them in the past. College Bookstores “might” have it. Otherwise if you ask at a store that does picture framing or fine art framing, they might be able to sell you a roll or two. It is not cheap, and the best time to ask them is any time after “New Years day”, better to wait a week or two, through to the first week of November. If you ask at any time after that first week of November, they may not be able to spare any, but if you ask politely they might bend a little bit. Picture framing supply houses- only if they happen to have a “city desk”.

These polarizers reduce light transmission around 70% and are good additions for more temperate climes when the sunlight is not needed for heating. I have a post over in My Farmhouse renovation where I show an example of such an installation.

polarized window

1 window with polarizer on left, one without on right.

I siliconed mine just enough to give some support to the lens but still allow for expansion.
siliconed lens

lens siliconed to frame

The base is very simple- some uprights that the lens frame attaches to and the base to hold the assembly vertical. I made mine so it can be diassembled for storage. I also incorporated an arm that holds the lens in position because the sun does move across the sky and optimal positions will vary as to time of year and time of day. The way the lens is cut will also affect the focal length. In my case, the focal length turned out to be about 4 feet with this lens. If you make your frame out of wood, it is easy to drill new holes and relocate the lens assembly. I will shorten the uprights later. When you use scrap wood, it is sometimes admittedly not the prettiest.


prototype frame

On the day I set it up for the test, it was broken clouds with haze but a light wind.

The sky view

The sky on the day of the testing.

The haze makes a difference in your ultimate heat generated. One thing though- because the lens is focusing the suns rays, you should use at least the equivalent of a gas cutting torch lens. Denser if you plan on looking at the hot spot for extended periods.

It took only a few seconds for this weed to begin to smoke.

defenseless weed 1

defenseless weed 1

Defenseless Weed 2

Defenseless weed 2. Note smoke rising upper center right area.

Defenseless weed 3

Defenseless weed #3- the aftermath.

It only took about 30 seconds for the cat food can lining to darken as it shows in the image. If there were no haze, it should be hot enough to melt the can even at about 43 degrees latitude. If I were using cast iron cookware, there would be no problem bringing water to a boil. If you just set your solar furnace up for cooking, setting it up so the focal point is achieved at the ground level, will allow you put a cooking surface just off the ground and the surface will heat reasonably quickly.


Defenseless cat food can.

If one wants to use this basic setup for purifying water (either distillation or just heat application for evaporation and condensing.) a vessel with a connection at the top and at the bottom with heat resistant tubing or hoses can be constructed so water coming from the bottom of the vessel goes to a metal heating vessel that is placed in the focused beam from the lens. The outlet of that heating vessel connects to the upper hose- and the main vessel should be vented to the atmosphere to handle expansion of the heated water.


Very simplified schematic of a pasteurizing/sterilizing system that has no moving parts.

Due to the nature of heated water being less dense than cooler water, once the heating vessel begins to get adequately hot, the water expands and flows into the top of the main vessel and a thermosiphon is created. Over a certain length of time, the main vessel becomes hot from the heat carried in from the water flowing from the heating vessel
and in this process, at a certain point where the water in the main vessel reaches near boiling point (if insulated) it becomes safer to drink than it was when cold. In theory, the heated water coming directly out of the heating vessel should be hot enough to be reasonably sterilized if it is set up correctly, and potentially hot enough to create steam- so one must keep presence of mind and situational awareness about you because serious burns or steam explosions can result very quickly. And though it should be assumed- one cannot assume anything these days- you need to let the water to cool to ambient air temperature before consuming it- unless it is to be used for cooking.

In fact, while between tests, the wind pushed the lens to the position shown as “At Idle” below, and I instantly noted the proximity of the hot spot to the cross brace, I caught that situation before anything even got singed. This demonstrates that you need to keep your wits about you. Though it was not a problem as nothing got singed, but on an absolutely clear day, it could have lit the frame on fire if I was not paying attention. If you think I am being pedantic- please understand that as handy as these can be in the developed world through third world- they are a tool and need to be respected as such because they can cause harm. I am trying to present educational information, how you choose to use it is out of my control. If you get hurt, injure someone, etc- that burden is upon you.

What about smelting of metals? The day I had it set it up, it might have handled soft solders and lead, but aluminum should be possible on a clear day, which also means zinc may be possible and brass may be possible. Wider lenses (those closer to the 3:4 ratio should work a little bit better than my narrow lens (9:16), but the narrow lens was quite forgiving about sun position in regards to the “hot spot”. In other words, as the sun traversed the sky, the need to move the furnace to keep up was not a real concern. (Unlike when you look at the moon with a telescope with high magnification.) If you are going to smelt metals, gas welding googles are strongly suggested. These are typically a “4” to a “6” for gas wellding by US lens manufacturers. (I am not sure if EU or OZ or other parts fo the world use the same lans ranking system. While I have used #9 and #10 lenses for welding, I have found I much prefer for comfort the gold lenses- those have a protection level comparable to a “13”- which I have used as well, but light transmisibility is a bit closer to a number “10” lens. Most plastic eyeglass lenses filter some UV- but these do not filter Infra-Red, which can cook your retina. Sunglasses are just not good enough for safety with this. Even if you are using a more diffuse focal point for toasting marshmallows, hot dogs, or general cooking- eye protection is a really good idea. Don’t ignore those after images you may see- they are telling you something. The good news is there are some inexpensive “cutting goggles”; that, while not fashion forward; are at least a little more fashionable than 1920’s eyewear.

Now, improvements to the frame design are somewhat obvious, and some not so obvious. This was jsut a prototype assembled just to test a few things. Many things were learned. Such as if the lens is mounted too high, you can find an approximate point of focus by holding something in a manner where you hand is clear of the focal point at all times, such as a wood scrap, an asphalt shingle (you will hear it sizzle when you have it close to the focal point.) and it is merely a matter of finding the “hot spot” first, then moving the target up or down until you have it in a tight focus (the hottest point.). From there you can either make an educated guess on where to drill the new holes, or you can have someone help you measure that distance. This one has obvious loose bolts- they were there at least “there” as assembly pins for the testing I wanted to do. The next itteration will have an investment in “wingnuts”, “carriage bolts” and washers. This includes some tension washers to help hold the lens in position if I decide to keep this frame “as-is”- the wingnuts will allow for easy dis-assembly and space saving when putting the unit away.
You do not want to leave these out where kids, or idiots might get ahold of it- you just never know the depths of ignorance out there.

Some things worth considering on the next itteration of the frame.:
*Re-orienting the lens so it is supported in the long dimension makes sense, especially with a shorter focal length.
*Other possible changes are offsetting the feet and locating the pivot of the lens closer to one side of the lens,
*Offset pivot point will allow for a simple support for the “free” side of the lens to hold position better in mild breezes. Ideally you want to have the lens perpendicular (flat-on) to the sun so the focal point is aligned with the center of the lens, but sometimes this may not be possible. Getting as close as possible to that ideal should then be the goal for best results.

Otherwise, the images above of my prototype show what makes for a pretty reasonable first design.

one point one

The first prototype after height adjustment.

At Idle

Idling silently

The associated blog for above youtube channel
The main blog.
the begining point place to start
The tangential blog.
The passive solar blog- outgrowth from some projects of mine.

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Not to fear, more is yet to come.

I have a Solar Furnace in the works, I have the important parts but it needs to get built and tested before I post images of it in a lengthy post.

Why mention it? Simple- you can use it for cooking in flash, heating water, smelting some metals, and more importantly pasturizing or distilling water.

Posted in Passive Solar Design, Passive technique, projects, solar collection, Solar heat, Solar heat project, Solar steam, Uncategorized | Tagged , , , , , , ,

More to come.

I have sourced new glazing coumpound as the original compound I had used failed so miserably. While the formulation is pretty basic, some companies just feel the need to add something to be able to call it their own. Dap33 failed miserably as a result and whether painted or unpainted it performed the same- badly. While it is probably most common- that does not mean it is the only game in town. nor the best product- it just has good marketing. Could it have been a bad batch? Perhaps. But burned once is all it takes.

I did some research on mixing up my own, and there are some good “recipe’s” on the web for it, but the down side is most take 24 hours to set up. Not unexpected, just not as convenient as most would like.

I will discuss the reglazing and weatherizing of the window sash in the future, which has some connection with passive solar, but some may not consider it as a specific passive solar project.

The associated blog for above youtube channel
The main blog.
the begining point place to start
The tangential blog.
The passive solar blog- outgrowth from some projects of mine.

Posted in insulation, Passive Solar Design, Passive Solar Designs, Passive technique, solar collection, Uncategorized | Tagged , , , , , , , ,

There is more to come,

but at present I have not reliably recovered enough voice to produce video, but I will be trying a few things. A number of videos have been attempted, but they sound worse than the introductory video posts. Most of the near upcoming videos will address some weatherization and window repair, which is important for passive solar designs in many cases. Right now, it is about -20 F outside. The sunlight had the gazebo about 50 degrees warmer than outside during daylight.

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The Heat Project

I already knew how effective my gazebo was at collecting heat in winter, even without insulation, bit the insulation aspect with radiant barrier is still being installed as I write this, the main goal for this entry is the second generation of heat storage in the structure.

The first generation was using used antifreeze in a barrel with sodium bicarbonate buffering and and a Zinc Anode rod. It worked, I just need more capacity.

Therefore I am using 5 gallon containers, with the sunward sides painted black. Mixed results on the paint type as some is flaking off.

The solution discussed in the first entry is to be an approximately 30 percent solution by weight (or mass- whichever is your preference)

With 1 kilo of water being one liter, that makes for a gallon of water to be about 8pounds and a few ounces. So accepting that margin of error of just calling a gallon 8 pounds to simplify calculations: This means for a total weight of 24 pounds of water (3 gallons) in a 5 gallon container with 9 pounds of Calcium Chloride per container.
this is less than a saturated solution, but yet enough to be able to remain liquid to minus 30 F. If I were to be using such heat storage inside a “regular” heated structure that was not expected to get below 60 degrees F, I would add significantly more calcium Chloride to achieve a solution of Calcium Chloride hexahydrate which has much more heat capacity than the saturated solution, but it becomes pretty much a solid at colder temps and you need to be in that phase change region to exploit that characteristic for maximum benefit.

This is something to consider if you have south facing patio doors that are not “Low E” glass.

Low E glass example

Low E glass is to the left

As you can see in the image the degree of reflection in the window at left has, and it has a notable gray tint when beside regular glass, the regular glass is peeking out on the right, and it is easy to spot the lower degree of reflectivity because you can see what is behind it quite clearly.

If you are going passive solar, Low E glass will not help you. While it is okay for north facing “fenestrations”, it is by design- intended to reflect the light wavelengths that could contribute to heat gain inside of structures. This is also a reason why plants fail to thrive with Low E glass in front of them.

Low E Glass has it’s uses, but passive solar designs usually intended to collect and capture heat- not reflect it away. It is this part of energy codes which works against the use of passive solar designs.

Posted in heat storage, liquid heat storage, Low E glass, Passive Solar Design, Passive Solar Designs, Passive technique, Soalr collection, solar collection, solar glass, Solar heat, Solar heat project, Trombe Wall, Uncategorized | Tagged , , , , ,