Fresnel lens greenhouse heating

Im curious, has anyone tried using fresnel lenses, or just magnified solar heating for a greenhouse?

I haven't but it sounds exciting. To me it seems like a fresnel lens might concentrate the heat too much and as the sun moved it would need to be moved to follow.

If you could find a way to track the sun and aim it at a thermal mass that would not melt or Crack then it would be awesome!

Do you have anymore detailed ideas? I would love for something like this to be a break through with northern climates!

Good luck!

I thought quite a bit about this...

I think it would work quite well mostly because it is so simple. No real moving parts, because the salt water brine would just be pumped to static solar collectors that are arranged in the curvature of the suns rotation. As the sun strikes areas of the array, it heats up and then runs via pipe to the floor of the greenhouse where it dispenses heat. As long as the brine was pumped from east to west in the array, as the sun marches across the sky, it would have already dispensed its heat into the floor and thus be cool, then just start picking up heat as it moved towards the west.

We as humans have learned to control heated water so well through HVAC systems that the accuracy is incredible and the cost really cheap. It could also be as simple as a pump, relief valve, and quarter turn ball valve for flow control, or as complex as a plc tied into a variable speed pump, called a metering valve in the HVAC world. None of it is really complicated...

And it should work. You are only taking heat in the array and moving it into the soil or concrete of the greenhouse. It's not rocket science here.

Large Fresnel lenses are often jokingly referred to as "death rays", and for good reason. They can melt stone in seconds. And to make an appreciable difference to heating the volume of a greenhouse, you would need a large one.  The total amount of solar energy collected/concentrated by a 1m^2 fresnel lens is exactly the same as that collected by a simple sheet of glass of the same area.  You could achieve the same net gain with a simple glazed box, some black paint, and a PC fan. There are much less terrifyingly death-inducing options for passive solar heating of greenhouses.

Typically what is needed is also not greater maximum temperatures, but temperature smoothing between day and night. Your freznel lens isn't going to do anything at midnight when you have a hard spring frost. Temperature regulation is usually best achieved by adding thermal mass, and a common choice is a trombe wall made from drums containing water.

I would love to have a big fresnel lens to play with.

Still, I'm a bit foggy on how that provides an advantage over a standard solar water heater. X amount of radiated heat falls on a square foot of Earth. Concentrating it means the collector can be smaller and run hotter, but the fesnel lens area has to be the same as a standard solar collector. Maybe for a niche application?

With a Fresnel lens you could heat a small amount of water to steam (rather than heating a large amount of water several degrees, as with a typical solar hot water collector.)  I don't know if that's useful?

Exactly, and why I mentioned pumping brine through the pipe.

That is also an advantage: a singular pipe. Without a thousand turns in the piping, you get less flow restrictions and much less pipe to buy and deal with. Yo get the proper delta t you just control the flow going through the pipe.

Plus it is simple. If you want to match the solar conditions for the moment, just have a plc match the target temperature to speed up or slow down the metering valve. (Variable speed circulating pump)

Pumps introduce a single point of failure in a system - if it loses power the flow stops. In most solar heating systems that wouldn't be an issue. In a fresnel lens concentrator, it becomes a bomb. You now need to factor in over-pressure/steam release valves.

I'm all for designing new systems, but I think you need a better reason than "because it's possible" to introduce such a complex (and potentially dangerous!) element.

Can you guarantee that your lens will NEVER catch a rogue bit of sunlight and set fire to the surroundings?
Can you design a "fail-safe" system, so that if the power fails it isn't hazardous?
Can you regulate the temperature sufficiently so that you don't cook your plants?

There are many excellent passive solar designs that would be ideal for this situation. I still struggle to see what possible value this might add over those systems.

It is a very simple system because you are just gathering heat from point A and transferring it to point B. I think because of its simplicity people overthink it, or think it could not work, but I am not convinced that is the case. Having tried to work with passive solar for years and failed miserably to control it with any targeted accuracy, I am convinced active solar are the means to the end. We as humans, however, have figured out how to control hot water. It is also 600 times more efficient because it is 600 times more dense than air.

My take on this after a lot of thought is that elements are very simple, like brine, which is not hard to make, we have been stirring up a bag of calcium in water and pumping it into our tractor tires forever.

And there is no danger from an inadvertent fire since the collector is always fixed and pointing at the pipe with the brine running through it.

Overpressure was addressed years ago with the simple, cheap relief valve that has protected hot water heaters in every home now for decades. I just changed the relief valve on my home's boiler last week. It was $21. The same relief valve will protect the system in case of a power failure, just as a solid fuel appliance will protect the appliance if a wood boiler is stoked with wood, and then the power goes out. If that is not to the person's liking, a on-demand back-up generator can be used. Or as our back-up, to our back-up, to our third back-up system at work, a battery bank to keep the pump going.

Steve Zoma wrote:

And there is no danger from an inadvertent fire since the collector is always fixed and pointing at the pipe with the brine running through it.

This also depends on the lens being aligned to the sun as well. If the light intersects the sun at an angle the focal point will not be located where your pipe is. You'll need active sun tracking I think.

Michael Cox wrote:

Steve Zoma wrote:

And there is no danger from an inadvertent fire since the collector is always fixed and pointing at the pipe with the brine running through it.

This also depends on the lens being aligned to the sun as well. If the light intersects the sun at an angle the focal point will not be located where your pipe is. You'll need active sun tracking I think.

I don’t think you do need tracking which made me put a lot of thought into this arrangement.

If you build a convex shaped collector, and it concentrates the light toward a centrally hung pipe off brackets. It will heat the pipe and warm the brine within. To keep it from having moving parts, the concave shape and pipe just needs to arc from east to west as the sun traverses the day.

I would think you could find a happy medium where it would give you pretty good heating and yet have a static structure making everything easier. As an old boss used to say”, if things move, they break”.

It simplifies in one area and perhaps adds complexity in another. But I don't think it would be particularly dangerous.

I like the static mirror idea. I think the lens and heater core would be in a closed housing for efficiency and protection from the elements (hail or strong winds).

I imagine the lens would be slightly defocused, on purpose, as opposed to a pinpoint. The surplus oilfield drill pipe I have around would not be harmed by that even if circulation stopped.

Steve Zoma wrote:I thought quite a bit about this...

I think it would work quite well mostly because it is so simple. No real moving parts, because the salt water brine would just be pumped to static solar collectors that are arranged in the curvature of the suns rotation. As the sun strikes areas of the array, it heats up and then runs via pipe to the floor of the greenhouse where it dispenses heat. As long as the brine was pumped from east to west in the array, as the sun marches across the sky, it would have already dispensed its heat into the floor and thus be cool, then just start picking up heat as it moved towards the west.

We as humans have learned to control heated water so well through HVAC systems that the accuracy is incredible and the cost really cheap. It could also be as simple as a pump, relief valve, and quarter turn ball valve for flow control, or as complex as a plc tied into a variable speed pump, called a metering valve in the HVAC world. None of it is really complicated...

And it should work. You are only taking heat in the array and moving it into the soil or concrete of the greenhouse. It's not rocket science here.

Yeah, I was thinking something like this, using water to store the heat. As far as I know, fresnel lenses can be sanded to change how effective they are, there was a video where a guy showed how to turn a regular tv fresnel lense into a granite melting deathray, part of the process was sanding it a certain way.
I was thinking you could use a system like the one in this video, with no moving parts, where the water is simply moved by heat. https://www.youtube.com/watch?v=B9BQGamXLMk

Michael Cox wrote:
Typically what is needed is also not greater maximum temperatures, but temperature smoothing between day and night. Your freznel lens isn't going to do anything at midnight when you have a hard spring frost. Temperature regulation is usually best achieved by adding thermal mass, and a common choice is a trombe wall made from drums containing water.

If you can increase the day temperatures, you can grow more, and if you can retain that heat at night, same thing. But yes, you dont want your greenhouse at 300degrees

Douglas Alpenstock wrote:I would love to have a big fresnel lens to play with.

Still, I'm a bit foggy on how that provides an advantage over a standard solar water heater. X amount of radiated heat falls on a square foot of Earth. Concentrating it means the collector can be smaller and run hotter, but the fesnel lens area has to be the same as a standard solar collector. Maybe for a niche application?

If you ever see one of those old giant square tvs in the trash, stop and pull the giant fresnel lens out.

I honestly dont know much about it, thats why I asked. maybe a fresnel lens would have more value in sterling engine applications, than greenhouse heating.

Michael Cox wrote:Pumps introduce a single point of failure in a system - if it loses power the flow stops. In most solar heating systems that wouldn't be an issue. In a fresnel lens concentrator, it becomes a bomb. You now need to factor in over-pressure/steam release valves.

I'm all for designing new systems, but I think you need a better reason than "because it's possible" to introduce such a complex (and potentially dangerous!) element.

Can you guarantee that your lens will NEVER catch a rogue bit of sunlight and set fire to the surroundings?
Can you design a "fail-safe" system, so that if the power fails it isn't hazardous?
Can you regulate the temperature sufficiently so that you don't cook your plants?

There are many excellent passive solar designs that would be ideal for this situation. I still struggle to see what possible value this might add over those systems.

Yeah the danger aspect is definitely real with fresnel lenses. I guess it was more of a silly thought experiment.

aurora sev wrote:If you can increase the day temperatures, you can grow more, and if you can retain that heat at night, same thing. But yes, you dont want your greenhouse at 300degrees

Just keep in mind, you would not be hearing the air of the greenhouse. You would be heating the soil beneath the greenhouse in a geothermal/ radiant floor heating way. That will hold its heat better than a less dense air.

Coupled with a simple plc, with a temp sensor in the soil, on the returning loop, and then the ambiant air, it will not overheat or under heat your soil.

Beyond just amazing control, with active solar instead of passive, you could increase your collection size to whatever is needed. Not enough heat? Add more length to your collector. Need less, the pump won’t flow as much. With passive collection, you only got the square surface of your building. That is hard to regulate

If you ever see one of those old giant square tvs in the trash, stop and pull the giant fresnel lens out.

Very good advice!  (Ditto the strong magnets in old microwaves.)

This isn't really related to Fresnel lenses per se, as you could use a parabolic reflector too, but what I want to try using solar energy to heat water to steam (or just vapor) and then direct that up an insulated pipe or gallery to some elevated height and condense it there.  Depending on how long and well insulated the duct is you might need solar heater repeater stations along the way?  Anyway, the idea is that you can get the water up without pumps or moving parts.  You get most of the heat back out when the water condenses, and then you can run it back down to get hydro power or just store it or dribble it out or whatever.

Simon Foreman wrote:

If you ever see one of those old giant square tvs in the trash, stop and pull the giant fresnel lens out.

Very good advice!  (Ditto the strong magnets in old microwaves.)

This isn't really related to Fresnel lenses per se, as you could use a parabolic reflector too, but what I want to try using solar energy to heat water to steam (or just vapor) and then direct that up an insulated pipe or gallery to some elevated height and condense it there.  Depending on how long and well insulated the duct is you might need solar heater repeater stations along the way?  Anyway, the idea is that you can get the water up without pumps or moving parts.  You get most of the heat back out when the water condenses, and then you can run it back down to get hydro power or just store it or dribble it out or whatever.

yes those microwave transformers are pretty handy. I had a few from a welder project.
just dont mess with anything else in the microwave, I think the magnetrons have some super carcinogenic materials in them.

The elevated water storage that would allow water to flow was what I was talking about earlier. I think it would require a very large tank to get much hydro power though.

It would be super dangerous...

The problem with steam that no one considers is, there is always condensate in steam. As the steam travels it is going to touch pipes along the way and even if it is a small portion, some will condense back to condensate. That has to go somewhere, so in a thermosyphon it might take the form of a water or steam hammer. Both are lethal...

Its one reason I mentioned brine very early on. It would never flash to steam making it much safer to work with.

My luck with thermosyphons has been very poor and why I like pumps. You would have to have at least one anyway for make up water unless you were on city water and they provided the make up water under pressure. But I have my high pressure steam license, and worked in boiler houses squeezing 103,000 gallons of water at 750 psi making 31 megawatts. It took a lot of pumps to do that!!

@aurora sev   Thanks for the tip on the carcinogenic materials!  I had no idea.  And yeah, hydropower probably wouldn't be worth it.

@Steve  I'm not thinking of pressurized steam, just getting the water into the air and drafting it up through a tube or vent or gallery.

I'm imagining something like a little plate of metal or stone at the focus of the Fresnel lens or parabolic collector and a drip feed of water falling onto it and sizzling off.  Do that inside a greenhouse for heat and moisture, and then imagine the greenhouse is 3' wide and 100' long and insulated and the other end is up a hill.  At the top end of the greenhouse you have a sort of metal tree and the moisture condenses on the "leaves" and drips into a little pond.

I have no idea if it's practical or not.

Can fresnel lenses be used to melt snow?

Hi Jacob,

Welcome to Permies.

Jacob Kelley wrote:Can fresnel lenses be used to melt snow?

Yes absolutely they can. It's not always the most effective way though. A big, scrounged double pane window on a black background will probably give more volume. But it depends on your locale.

Welcome to Permies, Jacob!

Hello,
I am working on the Fresnel lens based project where we are using PMMA acrylic sheet to manufacture a linear Fresnel lens with different angles and different number of steps... And noted different temperatures with different designs, and we found one great design that gives almost 128.67 WH. And this design I am going to replace it with solar heater panels, to heat the water or the any chemicals to get heated.
Is this is the good startups to do?