Greenhouse Heating-Climate Battery Design

[illahe]

 I'm posting up a project that may be of interest to those that have been participating in the Greenhouse Heating discussions. 

I recently began construction on a Climate Battery Design to heat a greenhouse that will be used mostly for propagation, but if all works well to help overwinter a collection of tender bulbs as well. You can read a lot more about the project and see pictures of the project underway at the blog: illahe climate battery greenhouse construction

I'll summarize the design here, often called 'Poor Mans Geothermal' the concept relies on buried pipes to move air into the relatively stable soil temperatures underneath the greenhouse. Then to recirculate the air into the greenhouse to either cool or heat it depending on the ambient air temperature. These designs have been widely researched and implemented in the American mid-west where they are often used to grow vegetables year around despite often frigid winter temperatures. I can't find much on the construction or use in the Pacific Northwest, but if anyone on here has any first hand experience I would love to hear it, I may be breaking new ground (no pun intended). 

The initial cost is in the cost of the pipe and the installation equipment involving a lot of digging, but the yearly operating costs are just the electricity to run the fans and thermostats that recirculate the air. I have seen some equations relating this cost of operation to that of the yearly cost to operate a household refrigerator. With the rising costs of gas heating for winter and the water scarcity issues that plague the often used evaporative cooling for summer, climate batteries may provide an eco-friendly and cost effective solution.

In the initial digging phase of this project, the original design was to place the lateral pipes (4" perforated ADS running between 12" ADS manifolds with riser pipes at either end of the greenhouse) down about 4' but we ran into groundwater so had to raise them up to just above it at around 3.5' deep. Measurements showed the soil temperature to be 51 degrees at that depth despite the air temps in the high 20's to low 30's. Some research showed that in Western Oregon, the soil temps usually lag about 3 months behind the air temps. So the soil temps of late December are reflective of the warmer temps of Autumn. Pumping this 20 degree temperature difference through the greenhouse has the potential to raise the temperature significantly. Most of the bulbs I grow are quite hardy and the goal is not to grow vegetables year around but maintain a greenhouse just above freezing if possible with as little energy inputs as possible. The benefit in the summer is the cooler soil temperatures can then be used to cool the greenhouse and since I grow a lot of alpine plants as well as bulbs I'm hoping this offset to be particularly useful. 

Climate batteries are probably not the easiest design to retrofit into an existing greenhouse, but if you are building a new one it's a design to consider if your soil/conditions allow for it. I'm also doing a lot of research into the new modern phase change liquids and tiles that are coming onto the market, Hoping to implement these in addition to the climate battery design.  PCM tiles allow the ability to store massive amounts of a days heat in the greenhouse and then release it back slowly into the greenhouse as temperatures cool at night. You can read about those here: Phase Change tiles for greenhouse heating/cooling

Attached are a couple of pics, but in addition to the blog mentioned above where you can follow this project, i'm also posting updates on the instagram at illahe_rare_plants.

Mark

[Ron]

I have read about these, it is an interesting concept.  Curious to hear what you think after it's completed and running awhile.

[illahe]

Hi Ron, 

I will definitely post up the the updates and the review when it's completed and in operation. 

Mark

[OrchardB]

50 + years ago excavating beneath a greenhouse, loosely filling it with stacked bricks, and circulating air down through them, during the day to store heat and at night to recover was suggested. Electric house storage heaters, using a steel box full of high density bricks, and cheaper night time electricity was common. My bricks were used as paving later when we went over to gas. Heat stores/recovery systems, in boxes, or snaking through the garden, are still being promoted of course. We adapt, slowly, to whatever we feel is the most economic at any one time.
Many years ago I visited a contact to see his Nerine collection. One greenhouse had a rain-water tank/pond the full length of the house under the bench(s). It was populated with very many noisy frogs. Don't know if they were a good heat store ;-)

[CG100]

Heat stores/recovery systems, in boxes, or snaking through the garden, are still being promoted of course.

They are called air-source and ground-source heat pumps.

Air-source start to struggle as temperatures drop. Ground-source heat-exchange piping is usually installed at a depth of around 1.5m (5 feet) - it needs to be that deep to remain at a reasonably constant temperature when in use (in the UK - 10C)

[illahe]

To Orchard B, that design with the bricks sounds very interesting, would love to see a diagram if you ever stumble across a picture? I love to study the old designs, it seems to me often enough that we left behind some great innovations in the past. 

To CG100, I think 5' would be ideal here as well, but high groundwater where I'm at makes that an impossibility, fortunately enough we never get that cold here and from some studies I read from Oregon State University the soil remains fairly stable here once you get below 3'. One of the best studies I read, reported that as shallow as 12" had a temperature lag of  3 months behind the current. As mentioned above most of the American research into these ground source air geo thermal is done in the Midwest where they talk of depths of 6-9' to get below frost lines and into stable temps. 

It's an experiment for sure, hope to get the greenhouse up in the next couple of months and we will monitor over the next few years to see how it does. 

Mark

[Uli]

This is a very interesting discussion.
In Portugal where I live winter temperatures do not get below freezing but the greenhouse gets quite cold at night. To buffer this, I use a lot of 5-7 litre plastic water bottles which are used by many people for drinking water. I get them for free. In autumn these bottles are filled with rain water and placed inside the greenhouse on sun exposed sides. They warm up quite well on sunny days and release the warmth at night. It does not work on dull days but it's the bright sunny weather that has the cold nights. In spring I use the water for the plants in the greenhouse and store the empty bottles because otherwise the greenhouse would become too hot in summer. I have not measured the effects but I think it will reduce temperature fluctuations inside the greenhouse to some extent. The more bottles there are the better.
Very simple at no cost.

[CG100]

I think 5' would be ideal here as well, but high groundwater where I'm at makes that an impossibility,

One of the limitations to ground source heat pumps is ground water/soil moisture - they are better conductors of heat than dry soil.

I forget which one, but at least one very large country house in the UK uses their large lake as their heat source. If nothing else it is a hell of a lot easier to locate the exchange array compared to digging miles of trench.

[Arnold]

Uli

Paint the water bottles black and you'll do much better with stored heat.

[CG100]

Uli

Paint the water bottles black and you'll do much better with stored heat.

Possibly, possibly not as the water cannot get hotter than the greenhouse, but the loss rate when the temperature falls will certainly be faster to some degree.

What is a better absorber is, by equal measure, a better emitter.

[Uli]

Very good idea, indeed. Thank you! I will paint some at first and compare. Not sure if it is right that the water cannot become warmer than the air in the greenhouse. Radiation behind a glass pane is very powerful. Black pots and aluminum become very hot.

[CG100]

Very good idea, indeed. Thank you! I will paint some at first and compare. Not sure if it is right that the water cannot become warmer than the air in the greenhouse. Radiation behind a glass pane is very powerful. Black pots and aluminum become very hot

Radiation behind glass is the same as on the outside of the glass, unless you have some unusual glass. (Actually, it is quite a bit less as the glass absorbs far more than you imagine or can perceive with the eye.)

The reason that a greenhouse gets warm/hot is that there is no change of air, or not much compared to outside, even in still weather, not least, unless you have an entire roof that opens, there is no convection (rising of warm air).

The SHC of water is 4.2kJ/litre/degree C.
Assuming that the water gets 20C above what you require as a minimum, that would be a contribution of 84kJ/litre, although you culd never get all of it emitted as the loss rate from the water decreases as the temperature drops.
One kWhr is 3600kJ, so you would need all the heat from 43 litres of water warmed by 20C to store 1kWhr of heat. It will also lose this heat as the greenhouse cools rather than when it reaches your minimum temperature.

The water is having essentially no effect, although where you are located it probably doesn't matter much, if at all.

[David Pilling]

CG100 - good to see actual calculations. In the old days I used to put buckets of hot water in the greenhouse at night. Now I see I was probably wasting my time.

My problem is slightly different, I am only trying to keep temperatures above freezing. I like to kid myself that what matters is the latent heat of fusion not the specific heat capacity (SHC). In other words a lot of energy changes hands when water freezes. Latent hear 333.2 kJ/kg against SHC 4.2 kJ/kg.

So by packing my greenhouse with plants (soil, water) I can postpone freezing, I only have to do that for a few hours.

We've all heard of the 'greenhouse effect' - radiation goes in, but can't at a different wavelength get out. That's why it is so hot.

Similar placebo effect, I run an oil lamp, I read that there is 10kWh of energy per litre of paraffin/kerosene (CG100 you said 12 for diesel). However I burn a lot less than 1 litre per night - a quarter maybe.

I always wanted to set up a fan to move air between the top and the bottom of the greenhouse. In the day time the top is hot and the bottom cold. At night the situation reverses. Damage from cold will start at the top and work its way down - put the tenderest plants on the lowest shelves.

Possibly not all in my imagination, since I monitor the temperatures in heated and unheated greenhouses and the lamp does make a difference.

[CG100]

We've all heard of the 'greenhouse effect' - radiation goes in, but can't at a different wavelength get out. That's why it is so hot.

I very strongly suspect that that is very largely or entirely urban myth in terms of why greenhouses get hot/warm.
The last (scientific) consideration that I read was as I mention above. Otherwise, that was my belief/explanation until I read the article.

Lots of common liquid fuels are going to have very very similar heats of combustion - they are all various mixtures of hydrocarbons at the end of the day. The one uncommon one being alcohol, which is not a hydrocarbon. Where they differ markedly are in vapour pressure and flash point.

If you are adding heat, no matter how modest the source, the temperature cannot help but be raised, it is just a question of how much. Lots or people used to use a 60W or 100W indandescent lamp for frost protection and/or to discourage condensation, in unheated buildings, such as garages. As kids, my father grew a LOT of potatoes, and the store for winter was a large wooden trunk, wrapped with layers of old carpet and hessian sacks, in a very small wooden shed that had a suspended floor. When frost threatened, he used to burn a small hurricane lamp in the shed. Every once in a while, they were taken out to check for any rotten ones, and although it must have happened, I do not recall any frost-damaged ones, so it wasn't common - in Colchester, so in one of the winter-chillier parts of England, short of heading for the hills.

As for the temperature "inversion" day-night in a greenhouse, quite possibly an effect of heat being given off by the floor at night. If so, the effect should change quite a bit between either end of a series of several days and more, of low temperatures day and night (which we/I are/am headed for this coming week - "normal" winter temperatures - nothing above 5C and plenty of time around 0C or slightly under). If the greenhouse doesn't warm appreciably during the day, the floor may even be an overall heat-sink at night (concrete has a SHC around 1 and earth/soil a little less).

If anyone wanted to make best use of the bottled water idea, they'd be near the glass while there was any sun, and in the middle of the greenhouse otherwise. Leaving them near the glass just ensures that "some" of the heat is lost to no effect.

I did once view a house - 35ish years ago - where the owners and their son were tinkerers with modern technology - they had a small wind turbine and a solar panel. The solar panel was not generating electricity - it was a long way before that idea was developed significantly - the panel was a serpentine wound clear plastic pipe sandwiched between glass and black-painted (probably) plywood and it stored energy as heat in a very large underground water tank. It was a long while ago, but the panel wasn't small - 6-8 feet square, maybe more.

Latent heat of change of state is usually huge, far in excess of SHC, which is why regrigeration and related plants, and much else, uses change of state rather than simple change of temperature. (One of the great whte hopes that is being persued for storage of solar energy is common salt - using the molten state, stored underground.)