I've been brainstorming a solar powered greenhouse in my backyard. However, I think I'll need some backup heat. Would running hot tap water into containers be a viable way to do this? I realize that it is non-renewable ( I have a natural gas water heater) but would it work? How much energy would it cost as compared to, say, an electric heater? And there would be the the advantage of thermal mass to cover power outages and not having to deal with electrical shorts in a moist greenhouse. How much heat would be given of by 55 gallons of water as it cools down from 130 degrees to 50 degrees F?

Gilbert Fritz wrote:How much heat would be given of by 55 gallons of water as it cools down from 130 degrees to 50 degrees F?

9,160,712 calories

If we dump the water tank once a day, and refill, that would provide heat equivalent to a 444 watt electric heater running continuously. That would be sufficient to get my small non-insulated greenhouse through a cold night in early spring, but I wouldn't expect it to keep it above freezing during the middle of winter.

By way of comparison, sunlight hitting a one meter square section of the greenhouse is delivering about 1000 watts of energy. My greenhouse has about 9 square meters of exposure to the sun, so receives about 9000 watts of heating when the sun is shining.

Last I checked, heating with natural gas was much less expensive than heating with electricity.

I think they used wood heaters in the past.

I would look into using compost heat; it is constant and low temperature, yet "burns" for a long time, but best of all in the end of the season you will have great compost to use in your greenhouse the next year. Jean Pain was the person who did a lot of research on this and really figured out how to make it work. Maybe something to consider.

Heating the water is expensive.  What temp does the cold water come into your system at?  If you just want a backup system, letting 60 degree water cool to 40 in your greenhouse will keep everything alive, although the eggplant will sulk for a week after.

Another way to move heat in and out of the air is with thermal curtained waterfall. (made up word)  Take a pipe and hang it near the north wall.  The pipe has lots of holes in it.  Hang a sheet of black plastic over the pipe, and run both ends to the ground.  Fuze the side  edges.  Put the bottom edge in a chunk of eavestrough gutter and run to your storage tank.  In operation water comes out of the pipe, and streams between the two layers of plastic, getting warmed by the sun, but not evaporating water into the air.

Note that the usual problem with greenhouses is cooling them, not heating them.

my well water is 6*C

and I have an unlimited supply, just pay for power to move it....

if you have same, you could run it through a radiator, and blow the "warmed" air around....  it would keep it from freezing

How about running your greywater into the greenhouse?
That heat would otherwise be wasted.
Running whatever water through a solar heater on the way to the greenhousemight be a good idea.

Typical household produces something like 300-500 gallons of greywater per day.  This will be a mix of cold and hot water.  

Suppose that the average temp is 90F.  Greenhouse temp is 60 F.  

Thirty degree temp differential.  

500 gallons at 8 lbs/gal = 4000 lbs of water.  30 degrees = 120,000 BTU potential heat.  

***

Now a 2000 square foot green house with 2 layers of poly will be about R2 per square foot.    To make it simple, lets say that the outside temp is 30 F.  So we again have a 30 degree temperature differential.  2000 square feet divided by R2 = 1000 BTU /hour/degree.  

It would take 30,000 BTU/hour to keep the greenhouse warm.  So in this case the grey water could provide 4 hours worth of heat.

So for this scenario, yes, grey water could provide a significant amount of the heat, but unless the greenhouse is small or your winters very mild, it's not sufficient.

You can play with the numbers some with different scenarios.  

What a great analysis!
Thanks for doing the heavy lifting.

Actually, let's take another look at it heating a greenhouse.

The object here is to make the answer more general:

At R2 it takes 1/2 btu/hour/degree F. to heat a square foot of greenhouse.

So for spring and fall when nights roughly equal days we have 12 hours of night, so we need 12 * 1/2 = 6 btu per square foot per degree.

Now no system is perfect, so I will add a third for inefficiencies.  8 pounds.

But that's a gallon

So  1 gallon of water per square foot, will heat a green house if the water is as much warmer as the outside is cooler.

What?

If you want a 60 degree greenhouse, then it takes a gallon of 90 degree water to keep it warm against 30 degree outside temps.

If you are happy with the temp dropping to 50, then your water only needs to be 70.

Multipliers:

If you run into late fall or early spring, you now have a 16 hour night instead of a 12 hour night.  It now takes 4 gallons for every 3 square feet.

If the drop in temp outside is twice as much as the differnce between the desired temp and your source of hot water, you need twice as much water.

If your greenhouse is only R1 instead of R2 you need twice as much water.


So  How much water as thermal mass do you need to keep night time temps at 50F for a 5000 square foot green house.  You want to run all winter, and it gets down to -10F in February.

You don't want greenhouse temps during the day to get over 85 -- plants stop working well.  So the best case is that your water will get to about 80.  That's 30 degrees above your minimum.  

50 to -10 is 60 degrees below your minimum.  That means a multiplier of 2.

So 5000 square feet * 2 gallons/square foot =10,000 gallons of water.  If you did it all in 50 gallon barrels, it would take 200 barrels, which would cover 800 square feet.


However, you now have daily temperature swings of 35 degree.  Most plants get unhappy with this much variation.  Suppose we want to keep the temp at 60 with a max temp of only 80.  

Now we have 70 degrees of heating to do, and only 15 degrees to do it with.  So now the multiplier is almost 5.  Most of the greenhouse is covered in barrels.

Ok.  Can we make water hotter so we need less of it.

Sure.  Don't store it in the green house.

Make a half a greenhouse.  Put up a straw building on the north face.  The top of the greenhouse meets the top of the building.  The face on the inside of the building is painted black, and it is covered with clear plastic held a couple inches away.  A fan at the top of this plenum pulls hot air from the top, and blows it through  the room full of barrels, return cooler air to the bottom of the plenum.  This allows you to use half the light hitting the greenhouse to warm water to a LOT warmer than the greenhouse gets.

More importantly:  By making the room bigger, you can store more heat in a way that doesn't cool off at night much.  (Straw bale walls are about R40)  So you can size the heat store for several days of cloudy weather.

Is it worth it?

Here in Alberta commercial greenhouses are paying about $3/square foot/year to heat with natural gas.  Our 5000 square foot green house costs 15K/year to heat.  A heat battery like I'm mentioning would take up as much room as the greenhouse.  So you get half the greenhouse per acre.  Tyipically the other costs of running a greenhouse, mostly labour, are several times the price of heating.

If land is cheap, it may be worth it.  If gas goes up, it may be worth it.

Not per hour.  Earlier I based on R2, so 1/2 BTU/hour/square foot = 6 btu/sq ft for  for a 12 hour night

Yes, that's glazing area.  If you use a semi-circle roof the area of glazing area to floor area is 1.57 to 1.  This ignores the ends.  This is what is known as a Ball park estimate.

The R2 figure is also an estimate and it's wrong.  It will depend on the amount of moisture inside the green house.  It will depend on how transparent the chosen plastic is to IR radiation.  R2 is a number based on a single air blocker being ~R1, and most poly greenhouses are done with air inflated double poly.


When I lowered the interior temp to 50, the differential dropped from 30 to 20.  Outside temp remained the same.


This calc has ignored air exchange.  In winter use, you want to minimize this during the dark time.  

This calc ignores latent heat conversion as moist air condenses on the inside of the glazing.

The idea here is to get a ballpark figure to see if it is remotely possible.

In MY climate it's not.  The temperature of my well water in winter is about 45 degrees. F


I had a single glazed (glass, old windows) 8x8 greenhouse.  A 1500 watt heater along with a small 100 w fan to keep the air moving could maintain about a 10 C (18F) degree temperature differential between inside and outside.  Useful as a season extender, but not as a 4 season greenhouse.