Sunday, December 13, 2009

Winter Update

It's been awhile since I've posted any updates about the greenhouse. We're now in the blistering cold of winter. This is the first year I've heated the greenhouse.....it may be a costly mistake. Here's a little scientific data to ponder:

From Misc


1) 2:30pm: Sun starts to set (yellow) and the greenhouse has no solar gain and the temperature starts to drop (Orange & green lines)

2) 3:45pm: Temperature goes below 40F (4.5C). Heater is able to turn on/off as it maintains the temp at 40F. Ceiling temp (light green) jumps a bit as the heat rises....

3) 9:15pm: Outside temperature (Red) goes below 20F (-6/7C) and the heater no longer can keep up with the demamd. Temperature slowly declines to around 38F (3.3C). The outside temp is 13.3F (-10.4C)

4) 9:15am: Heater turns off once the sun comes out the next morning. Total run time is roughly 17 hours.

There are 2 electric heaters totaling 2400 watts. After all the fees and taxes, we pay $0.20 per kilowatt:
2400watts & 17 hours = 40.8 kilowatts
40.8kW * 20 cents = $8.16 to keep it heated for the night!

Since we are getting so little sun and it's cold, the lettuce is growing very slowly. There's one tomato plant that I started in the fall still doing OK but needs a lot more light. I had a few cukes growing, but the consistent cold has killed them. I am not generating $8 worth of veggies/day. Perhaps it's time to invest in a propane heater to lessen the heating costs....or just shut it down for the winter..... Jan & Feb will be even colder!

7 comments:

Coop said...

I see you doing the math and I'm just wondering what the output is in the growing season. I'm in Los Angeles so I could go year round. To put it simply, how much food can you get out of a aquaponic greenhouse that size? Is it enough to keep 2 people going? Would you do just as well as a square foot garden?

This looks like great fun but I'm just wondering in your experience what the net food output is.

Thanks,

Coop

Web4Deb said...

With the aquaponics setup, you can get a higer density of plants per square foot than a regular garden. We had plenty of cukes and cherry tomatoes over the summer, but lettuce doesn't like the heat and bolted too fast. During the spring and fall the lettuce grows well in the cooler temps. The strawberries take a lot of room for the amount they produce and I may try them again this spring....if I can't get a better yield, they're coming out!

A greenhouse this size won't grow enough food to sustain 2 people, but it does help offset the amount of produce you need to purchase.

Make sure you check out the video tour on youtube! http://www.youtube.com/watch?v=CU9x_W9X-tM

Dave said...

Hi,
Very nice setup, and the data is interesting. Small greenhouses are hard to justify because the surface to plant ratio is so high and thermal mass is so hard to fit in.

You might want to use a "kill-a-watt" to measure the daily watt/hrs, other than that your data is fantastic.

Also, on the north side of the greenhouse (and any surface which does not recieve significant solar gain) you should add additional insulation such as silver-sided styrofoam sheets. Make sure you have all your building seams taped as well to keep air infiltration to a minimum.

Did you insulate nuder the pavers?

Keep up the good work!

Anonymous said...

Very nice setup!
Have you thought about a little active solar for heating? You already have a large amount of thermal mass (the water), so all you would need to add is a solar water heating panel or two (DIY is pretty easy) outside, a pump, a coil of pipe in a tank, some temp senors, and some updated control software. Use a glycol mix for the transfer fluid.
Such a system would be highly efficient and I bet you could get a significant amount of heat into the water. This would also have the added benefit of increasing veggie production!
I second the comments about insulation!

Anonymous said...

By your numbers you needed about 140k btu for that night.
(40.8kWh * 3412btu/kWh = 139,210 btu)
Electricity is probably the most expensive way to get that heat.
A gallon of propane contains 91,600 btu.
A gallon of fuel oil has 138,200 btu
A vented heater sends ~20% of that heat up the chimney, but even so, they would be cheaper fuel than electricity.
If you go with active solar, your fuel costs would drop much lower.
No matter what you do, I think heating the water would be the way to go, not the air. For a test, you could put an immersion heater in the tank.

Anonymous said...

I'm wondering what purpose the large, 350 gallon tank serves.

Web4Deb said...

The 350 gallon tank is mainly used as a heat sink to help maintain the temp a bit. Also by having more water, it will help to prevent the pH of the water changing too fast.

To respond to an earlier comment, I can't heat the water much. If it gets too much warmer than the air temp, it will start to evaporate and condense then freeze on the poly carb panels. It will also promote mold on the plants with the high humidity.