looking for feedback on feasability of this idea...

[Jbaich]

Hi All, here's the backstory...

I decided to try a little experiment in the garden this year by cutting out the bottoms of plastic milk jugs and placing them over some of my new veggie plants to see if they would be effective at protecting them against late spring frosts. Nigh before last we had a doozey and the survival rate was about 50%, so I'm thinking... damn, if I had been able to keep the temp in the milk jugs like 1/2 a degree to a degree warmer, I probably wouldn't have lost much at all.

So after wracking my brain for a while, I got this idea, and since I'm not well versed in anything related to electronics, I thought I'd throw it up here and let the experts start telling me where the holes are in my plan!

The plan (pilot date next spring) is to convert some of those cheap solar path lights to solar heaters by switching out the photoresistor with a thermistor, so that instead of the LED coming on at dark, it comes on at cold.

I know LED's don't produce much heat, but I don't think much is needed and it'd be great to keep these battery/solar operated so I don't have a million wires running all over the garden. If anyone has any suggestions for replacing the LED with something that may generate more heat, while using a similar amount of power, I'm all ears!

So, does this sound like a realistic idea to anyone? If not, are there any suggestions to achieve a similar result?

Also any suggestions as to what I may need to consider when choosing a thermistor would be very much appreciated as I know exactly nothing

Thanks,
Joe

 

[hevans1944]

You have one thing going for you: conversion of electricity to heat is 100% efficient. You don't need a thermistor. An ordinary resistor will convert whatever electricity is supplied to it directly into heat with 100% efficiency.

Your problem will be to collect and store enough sunlight with a solar cell to provide enough power to keep the plant environment from freezing when it gets cold outside. To do that you must first calculate how much heat is lost to ambient air between sun down (no solar cell energy produced after the sun sets) and sun up (some solar cell energy produced as the sun rises and some more during the day). This number for heat loss is a function of the temperature difference between the plant environment and the outside temperature, as well as the thermal insulation between those two environments. Some basic thermodynamic calculations are necessary. Placing Styrofoam insulation around the plastic milk jugs would help a lot, whether you heated the plant environment or not. Try that next year whether you heat the inside of the milk jug or not. And if there is any wind blowing across the plants inside the milk jugs, set up a wind barrier. Hay bales work great, and you can use them later in the season for protective ground cover while plants grow up through the hay.

Once you know how much heat is lost at night, you can calculate how much energy your solar cell must supply to a rechargeable battery during the day to operate a resistive heater during the night. I think you will find an extension cord from your house connected to heaters (such as are wrapped around water pipes to keep them from freezing) will work a lot better than solar cells. You will need a thermostat to keep the plant environment from getting too warm if house current supplies the heaters.

Or, you could build a greenhouse to keep the plants warm directly with sunshine and store the heat absorbed during the day in rocks or some other material, to be released slowly at night. A small fan is necessary to circulate the air in the greenhouse, and you will have to arrange to vent the greenhouse during the day, lest it become too hot. The Sun puts forth about a kilowatt of power per square meter at high noon. A few solar cells, with a rechargeable battery, can intercept some of that power and convert it to electricity, to be used to operate the fan and control the vent system.

 

[CDRIVE]

You have one thing going for you: conversion of electricity to heat is 100% efficient. You don't need a thermistor. An ordinary resistor will convert whatever electricity is supplied to it directly into heat with 100% efficiency.

Hop, unless I misread his post the Thermistor isn't related to the generation of heat portion of his project. As I read it he wants to replace the Photocell action of an off the shelf LED night light with Thermistor functionality. Thus the LED's turn on when it senses a temp below a set value.

Chris

 

[Alec_t]

I suspect the photocell in a garden light is way too small to provide enough energy during the day to sustain the required heating during the night. And of course if you replaced the cell with a thermistor you'd lose the solar energy harvesting. Either way, an alternative source will be needed IMO.

 

[davenn]

I suspect the photocell in a garden light is way too small to provide enough energy during the day to sustain the required heating during the night. And of course if you replaced the cell with a thermistor you'd lose the solar energy harvesting. Either way, an alternative source will be needed IMO.

you misunderstand the use of the thermistor .... reread the OP's and CDRIVE's posts
he isn't replacing any of the solar panels with the thermistor, just the photo resistor (LDR) ... the day/nite detector


Dave

 

[Bluejets]

I think what alec-t was referring to was the solar panel would at best be rated at maybe 1/2W for example and as such the energy stored in the 300mah battery inside would drive next to nothing at night.

 

[davenn]

I think what alec-t was referring to was the solar panel would at best be rated at maybe 1/2W for example and as such the energy stored in the 300mah battery inside would drive next to nothing at night.

that bit I agree with

 

[CDRIVE]

I'll second that.

Chris

 

[Jbaich]

Hey everyone, thanks for the replies and sorry for the tardiness of this response.

So, to clarify... the thermistor was meant to replace the photoresistor in order to open or close the circuit based on temperature rather than light levels. would adding additional battery capacity help or is the bottleneck at the size and capacity of those small solar PV cells? Keeping in mind that under ideal situations, this won't turn on every night, only the cold ones so there should be several days worth of charging in between. I'm thinking that in the event of a spring (or fall) frost, the killer temp range is probably only in effect for a few hours and I'm hoping even a super small heat source would make a difference? I'm thinking maybe if I swapped out the LED for one of those incandescent mini style Christmas lights (I think they're 2.5V?) and added some battery capacity I might be able to keep one of those little buggers lit for a few hours before the battery is drained?

or just as a little experiment if I could figure out the amount of resistance that could be applied in order to get x hours out of y batteries, I could try a little bit of trial and error and see what temps in the fall were too much for a little set up like this...? kinda just want to have fun with this and use it as a way to learn more about this stuff... got a few other project ideas, but need to learn the basics first

Thanks,
Joe

 

[Alec_t]

he isn't replacing any of the solar panels with the thermistor, just the photo resistor (LDR) ... the day/nite detector

Okay. But in my garden lights the solar cell is the day/night sensor .

 

[shrtrnd]

This is not along the lines of your plan, but I'll submit my 2-cents worth. I used to live in Detroit. Canada's probably got similar issues. I live in Arizona now and I see a lot of people here using methods that work well, but aren't as cost efficient as your plan.
In frost conditions, people use sheets suspended over their plants, with an incandescent bulb underneath.
They don't use plastic sheets because of thermal heat transfer, they use cloth of disparate (no particular) types.
Good luck with your project if you do the electronic thing. The Arizona thing seems to work ok for temperatures
that don't get much lower than 25 degrees F.

 

[Jbaich]

Yeah, sheets are great, however I'm not always home to put them up when the frosty nights arrive, also sometimes the weatherman is wrong and we get frost when it wasn't called for so I thought something a little more automatic might be worth a shot.

 

[Jbaich]

so I found this http://www.christmascentral.com/t-Hints-And-Technical-Amps-Watts-Volts.aspx

If I'm reading it right, I should get around 5 hours of operation of a single bulb off a AA battery?

 

[ChosunOne]

Well, I know this is an Electronics forum, but you may be making it too complicated by using an electronic approach. What you're actually after is a mechanism to store ambient heat for use during transient cool periods.

Have you considered putting a quart-or-so bottle or two of water inside the milk jug with the plant? Water has a decent heat storage capacity and might keep the temp above freezing for a few hours.
It's a trick that has worked for me a few times in a different application.

 

[Bluejets]

Whay

so I found this http://www.christmascentral.com/t-Hints-And-Technical-Amps-Watts-Volts.aspx

If I'm reading it right, I should get around 5 hours of operation of a single bulb off a AA battery?

View attachment 26859

. What bulb and battery size? The amount of energy you require most definitely will not come from a AA cell. When working with solar panels one must remember the useful sunlight charging time is usually around 4to 5 hours depending on you location.

 

[hevans1944]

You will make more progress if you do a little thermodynamic analysis of the situation first. Heat in versus heat lost.

 

[Herschel Peeler]

Hi All, here's the backstory...

I decided to try a little experiment in the garden this year by cutting out the bottoms of plastic milk jugs and placing them over some of my new veggie plants to see if they would be effective at protecting them against late spring frosts. Nigh before last we had a doozey and the survival rate was about 50%, so I'm thinking... damn, if I had been able to keep the temp in the milk jugs like 1/2 a degree to a degree warmer, I probably wouldn't have lost much at all.

So after wracking my brain for a while, I got this idea, and since I'm not well versed in anything related to electronics, I thought I'd throw it up here and let the experts start telling me where the holes are in my plan!

The plan (pilot date next spring) is to convert some of those cheap solar path lights to solar heaters by switching out the photoresistor with a thermistor, so that instead of the LED coming on at dark, it comes on at cold.

I know LED's don't produce much heat, but I don't think much is needed and it'd be great to keep these battery/solar operated so I don't have a million wires running all over the garden. If anyone has any suggestions for replacing the LED with something that may generate more heat, while using a similar amount of power, I'm all ears!

So, does this sound like a realistic idea to anyone? If not, are there any suggestions to achieve a similar result?

Also any suggestions as to what I may need to consider when choosing a thermistor would be very much appreciated as I know exactly nothing

Thanks,
Joe

Available power is the problem. The solar cell only puts out about 0.1 Watts. (3.5 V at 20 mA). What you need is a few watts. A circuit is no problem. A modified night light circuit is attached. The sensor is changed, as suggested. A negative temperature coefficient sensor is used. As temperature goes down resistance goes up. As resistance goes up it turns on a transistor which pulls current through a resistor that gets warm. How much power is required is the question, but here is a starting point, anyway. powered from solar cells instead of line powered? We need to know how much power you need first.
Multiple plants could share one larger power source.

 

[Herschel Peeler]

View attachment 27127

Available power is the problem. The solar cell only puts out about 0.1 Watts. (3.5 V at 20 mA). What you need is a few watts. A circuit is no problem. A modified night light circuit is attached. The sensor is changed, as suggested. A negative temperature coefficient sensor is used. As temperature goes down resistance goes up. As resistance goes up it turns on a transistor which pulls current through a resistor that gets warm. How much power is required is the question, but here is a starting point, anyway. powered from solar cells instead of line powered? We need to know how much power you need first.
Multiple plants could share one larger power source.

For a starting point on solar cells and batteries, here is a solar cell suitable for outside use and a battery pack.
These from Electronic Goldmine. You can google that to get a web site.

 

[Herschel Peeler]

View attachment 27128 View attachment 27129

For a starting point on solar cells and batteries, here is a solar cell suitable for outside use and a battery pack.
These from Electronic Goldmine. You can google that to get a web site.

A 2,300 mA Hr battery can supply 100 mA at 12 V for 23 hours (or equivalent, maybe 1.2 watts per plant to a few plants for a few of the coldest hours per night).
As mentioned, water is a good idea. Heater in water might work. Pure distilled water does not conduct electricity.

 

[Herschel Peeler]

Hi All, here's the backstory...

I decided to try a little experiment in the garden this year by cutting out the bottoms of plastic milk jugs and placing them over some of my new veggie plants to see if they would be effective at protecting them against late spring frosts. Nigh before last we had a doozey and the survival rate was about 50%, so I'm thinking... damn, if I had been able to keep the temp in the milk jugs like 1/2 a degree to a degree warmer, I probably wouldn't have lost much at all.

So after wracking my brain for a while, I got this idea, and since I'm not well versed in anything related to electronics, I thought I'd throw it up here and let the experts start telling me where the holes are in my plan!

The plan (pilot date next spring) is to convert some of those cheap solar path lights to solar heaters by switching out the photoresistor with a thermistor, so that instead of the LED coming on at dark, it comes on at cold.

I know LED's don't produce much heat, but I don't think much is needed and it'd be great to keep these battery/solar operated so I don't have a million wires running all over the garden. If anyone has any suggestions for replacing the LED with something that may generate more heat, while using a similar amount of power, I'm all ears!

So, does this sound like a realistic idea to anyone? If not, are there any suggestions to achieve a similar result?

Also any suggestions as to what I may need to consider when choosing a thermistor would be very much appreciated as I know exactly nothing

Thanks,
Joe

If it doesn't look like a solar cell is going to be cost effective for you a line powered system is simpler. How many plants are we talking about?