BTU's per dollar

[alpickrel]

In May of 2007 my heating oil cost $2.50 per gallon. In May of 2007 my electricity cost .071 cents per kilowatt. (This number includes all the surcharges and distribution costs state and local consumption costs, etc.)

There are 141,000 BTU's of heat per gallon of fuel oil. My heater is

80% efficient. This means that 20% of the oil I burn goes some place other than into heating the air in my greenhouse. (20% of my oil dollar goes someplace other than into heating the greenhouse air.)

There are 3412 BTU's of heat per kilowatt. A radiant heater without fans is pretty much 100% efficient in converting electricity to heat.

My question is "How much heat can I buy for the air in my greenhouse with $1.00 of oil versus $1.00 of electricity at the price of energy in May 2007?"

My answer is BTU's per oil dollar: 45120 BTU's per electric dollar: 48056

Scary stuff if my calculations are correct. Oil used to be the most cost effective fuel for heating. In October of 1999, for example, my oil cost .96 per gallon and electricity cost .064 cents per kilowatt. The cost of one fuel is going up much faster than the other.

I cannot hope to generate sufficient BTUs with electricity to heat my greenhouse this winter, (all my plugs are too small ;-P and I use an average of 423,000,000 BTUs per winter above what the sun provides) but if the fuel costs have not changed much this summer and my calculations are real then it makes some sense to plug in some electric radiant heaters to help out a bit. And the higher oil goes in comparrison to electricty the more help those radient heaters can offer.

[alpickrel]

Excuse me. As always I must immediately correct myself: There are

3412 BTU's per kilowatt hour.

1 BTU per kilowatt = 1.05505585 seconds.

The problem with all these words is that they are measuring something different.

[alpickrel]

You were smart.

My old hoophouse frame is still standing right next to the greenhouse. It is 15 x 30 (or so). Just the metal frame right now. It has grapes growing on it. I have had the idea in the back of my head to cover it with insualtion and black plastic water circulating tubes and turn it into a giant solar hot water collector/storage unit of some kind that I can then use at night to pump the heat into the greenhouse next to it. The heat sink can't be IN the greenhouse because to be of any real use, whatever goes in there...a pool or whatever..., it has to be able to get hot enough to be of use long after the sun gets off it, so plants won't be able to be grown in there with it. (now I get K Barrett's joke about lying to herself about her barrel of water.)

I was staring at my electrical subpanel for the greenhouse this afternoon and wondering what it would take to run a line large enough to power an electric heater that would make me feel like I was not lying to myself when I plugged it in.

I still think a good option is to find a way to drag a big warm blanket over the greenhouse every night.

I also w> When I put up my last greenhouse I needed a new power service. I had the

[Ray B]

About the least expensive heating method (it's good for cooling, too) I've ever heard of was published in the AOS Bulletin in the late '70's or '80's.

A guy had an extended trench dug 6' into the ground (or some depth well-below his frost line), and made a long duct of large, unglazed clay sewer pipe. One end came up through the floor of the greenhouse, and the other end was enclosed in a structure above ground. The inlet end was covered with a mesh and filter to prevent all but air from entering.

A thermostatically-controlled fan in the GH drew air through the submerged pipe, where the natural subsurface temperature was in the low-'60's F year round, which is certainly plenty warm for nighttime, with solar heating boosting the daytime temp. In the summer, he would switch to a cooling thermostat, drawing that same cool air. That's certainly less maintenance than a swamp cooler!

The article also discussed how the unglazed clay absorbed the condensed humidity in the summer, and moistened the air in the winter, but I don't recall details.

[K Barrett]

Is it 'Project Solar' written by Lee Kuhn from 1978. October 1978, page 891. Kuhn is/was owner of J&L Orchids at the time. The article goes on for 9 pages. IIRC one of J&L's GHs burned down in the late 90s in midwinter. There was an article about that in the Bulletin, too.

K Barrett

[Ray B]

Nope. The one on that page is a solar device with other-building storage. What I was referring to was strictly ground temperature controlled.

[Mark_OK]

Is anyone using this method? Sounds intriguing. Knowing the length of the trench to be effective would be helpful. And what about ducting the hot air from the greenhouse down through the trench? I can see this method working on a small hobby greenhouse.

[Ray B]

How well do you think the flexible, corrugated, black plastic septic-system pipe would work?

It's fairly thin-walled, so the heat transfer would be fairly good I would think, and by being flexible, you could bury a bunch of coils, reducing the amount of property that would have to be dug up.

[alpickrel]

My guess would be that the cooler air down in the underground pipes will not be drawn up into the greenhouse if there is cross venting from inlet to exhaust vents in the greenhouse above that bypasses the pipes unless the pipes have an external inlet. Cool air will settle to the lowest point. A fan to push or pull it through the pipes seems required without this external inlet.

The system is analogous to swamp coolers: in order to be effective the air coming in must be forced through the cooling structure; be it damp crushed clay filled pipes buried in the earth or soaked pads in front of an inlet shutter. The trench of crushed clay pipes needs one open end to be on the outside of the greenhouse and one to be on the inside. Actually, I would guess two inlet openings and one exhaust are needed. One inlet is outside the greenhouse to draw in air for cooling as vents at the top of the greenhouse exhaust the hotter air. A second inlet is needed (and the first one closed) when air inside the greenhouse is circulated though the pipes to warm it to the average mean temp during "heating".

Imagine a semi-passive cooling system where a fan powered by a solar panel forced air through the cooling pipes. For cooling, you only need the fan to come on when the sun is out anyway.

To be ultra effective as a heater makes it bit more complicated. I would need to look into heat exchangers and heat exchange technology because we are talking about a primitive heat pump here. Something is needed to 'concentrate' the BTUs before moving them up into the greenhouse air and releasing them.

I can see the idea of a passive system being of help if I imagine that what the ground pipes are doing is drawing the mean average air temperature held constant by the earth at about the 6 foot depth up into the greenhouse. This way you start your base air heating BTU requirements at 55 degrees or so and not at whatever the current air temperature is outside the greenhouse. In addition, heated air naturally rises. What you are doing with the pipes when you heat is to increase the ground's heat radiating surface. Imaging a long or deep cave with a plastic bubble setting on top of the opening that traps the air heated to the underground mean average temperature.

In a VERY deep tunnel, would the mean average temperature rising up from the bottom be magma?

[Mark_OK]

My thought was a recirculating system. Say an inlet at the top of the greenhouse and an outlet at the bottom of the greenhouse, at opposite ends of the greenhouse. A fan or fans would have to be used to force the air through the duct work. I have a feeling that the amount of underground duct work needed to even make a slight difference will be substantial. It probably would be just as effective if you just sunk the greenhouse into the ground a few feet.

[alpickrel]

Today I am working in the greenhouse and contemplating the frightening oil bill I just got. I thought oil was $2.799 per gallon. But that's the home heating oil price. I would have paid sales tax of 5% on that making it $2.93 per gallon, but last year I learned that as a business I can send the oil vendor a signed certificate to allow me to not be taxed since the fuel is used to produce items for resale. This woke them up to the fact that I am a business and should not be paying "home" heating oil prices. I am now paying "Fuel; commercial tax discount" prices of $3.094 per gallon. (NO tax on that, however) You just can't win. Buying oil sans sales tax will now cost me almost $500 more per season. It never dawned on me to ask if there was a separate pricing structure before I sent the tax form in. I should have known. I looked into electricity prices and learned that as long as Dominion power assumes I am using all this electricity for home use it is less expensive than what businesses pay. So I should not have been caught by surprise here. (Electricty is looking prettier all the time. Of course, if I run a bigger electric line in to power an electric heater for my "commercial" greenhouse, somebody at the electric company will notice.)