Greenhouse Temperatures 100 deg!

The heat will radiate away from the pipes but not too fast so the days gains are available overnight,it depends on the soil type,water content etc but probably an inch per hour.
The idea is to balance the gains with the loss so you always have enough capacity for cooling and for heating. Trying to store away heat for months doesn`t work unless you have a really huge mass like an aquifier.
Moist soil holds more heat (due to the water content) but loses it faster and it also travels faster through the soil. Dry soil is a better insulator so its less lossy but takes longer to heat up as the heat travels slower through it.
Variation in the soil temp depends on the depth, how much extra heat you drive into it and how much you take out and how much is lost :wink: Over maybe 5 seasons the average temperature of the soil should rise by a few degrees as its getting more heat input and less loss than it would normally.
The system should be able store 1000w of heat for every watt of electricity used.

 

Hi John,
The flooded floor is basically 3000L of gravel in 5x600L watertight beds, its not hydroponic..its aquaponic.
There will be an 8ft diameter tank complete with most likely rainbow trout.
The gravel beds double as biofilter medium to convert ammonia into nitrate and physical support for the plants. The fish provide the food and the plants provide the fish with clean water.
The beds are flooded to a few inches below the top 15 minutes every hour. The water returns to the fishtank via a buried sump.

The subsoil heat exchanger is also as different to the heatsink used in the INEBG program as the dome is. My exchanger uses 172ft of perforated tubing, 30 tons of clay soil and a 10" duct fan capable of providing 1150m3/hr (40,500ft3/hr). It has a differential thermostat (comparing air/soil temps) and a speed controller to optimise the fan speed to the situation...ie: not running flat out at 3am.

The INEBG heatseak in comparison used one cubic metre of glass in an buried insulated box driven by a pc fan :wink:
Here`s a pic of the bed layout and the fan (the plywood disk is 18" diameter)

 

36C again yesterday, 18C overnight (with 1/4 of the vents left ajar)

Strong wind today, so I expect I'll get more draught in the greenhouse ...

 

And as a Blonde I will never work it out:rotfl::rotfl:

 

Hi John
Trout are ok as long as you can keep the water below about 75F and go overboard with aeration. Summer is the only sticky part of the year and it doesn`t usually last long :wink:
The list of plants that grow in aquaponics is pretty much the same as in soil,even root vegetables, the only thing you really need to add is iron.
The solid waste collects in the bottom of the beds over time so it acts as a slow release fertiliser,it takes a while for the beds to mature but once there they will grow almost anything. All you need do is top up the water and feed the fish..a lazy gardeners dream.
Compost worms thrive in the beds too even though they are periodically flooded, they help break down the solids and take care of decaying roots etc. Don`t forget the bonus crop..fresh rainbow trout.

I didn`t need to import the clay as i have plenty of the stuff, the hard part was digging lots of deep trenches in it using just a spade.
Here`s a pic of a single tomato plant in an 8ft diameter mature gravel bed, total harvested weight was 32kg.

 

The best aquaponic site on the net is http://www.backyardaquaponics.com/forum/portal.php
you`ll find endless amounts of info in the forum, mostly australians but also from around the world. There`s a small commercial setup in Wales too.
I`m just a mad scientist John. Anything out of the ordinary..count me in :wink:

 

Just a bit of an techy update,
I did some testing with the system yesterday to get a rough idea of the performance. I dont have the covering on the greenhouse yet but i`ve included greenhouse airchanges per hour for reference.The greenhouse air volume will be ~900ft3.
I have a sensor for checking the soil temp at the same depth as the lower tubes(~3.5ft), which reported 13C. Each test used a different fan speed, i calculated the cfm by using a handheld ananometer. Outside air temp was the temperature of air going into the plenum, Outlet temp was the temperature of the air leaving the 15 outlets in the plenum.

test 1: 200cfm (~14 airchanges/hr) Outside air: 28C (82.4F) Outlet temp: 13C (55.4F)
test 2: 400cfm (~27 airchanges/hr) Outside air: 30C (86F) Outlet temp: 15C (59F)
test 3: 677cfm (~45 airchanges/hr) Outside air: 30C (86F) Outlet temp: 17C (62.6F)

Conclusion: Although the 200cfm and 400cfm tests looks good from a temperature differential viewpoint, the 677cfm is actually much better.
If the test result temperatures above remained constant for one hour, 200cfm banks 1.72kw of energy, 400cfm banks 3.45kw and 677cfm banks 5kw into the mass.
At 677cfm, the fan takes 170w/hr so the system banks 30w of heat for every watt of electrical power used.
Although the testing was rough and ready it proves the theory is sound. In practice, the humidity levels would be much higher than the tests due to plant transpiration and overhead misting which will increase the performance by a large margin.

 

You'll get a law-of-diminishing-return when you do it for real, with longer continuous runs, though, of course. So you might decide that a higher Delta-T is acceptable, the fan will run slower / less, using less energy, and the heat transfer to store will be similar. The overall temperature in the greenhouse may be a deciding factor. On hot days you may have to run the fan flat out to try to dissipate as much heat as possible, and keep the Delta-T as low as possible, so that the actual air temperature is not unbearable for the plants. OTOH on moderate days the greenhouse temperature may be easily maintained below a reasonable limit for the plants, and thus letting it rise a bit higher won't hurt - so a larger Dealt-T, and less power running the fan.

You into electronics as well Hex? You could spend more time hacking the software than building the greenhouse! (It occurs to me that a differential controller, such as is used for Solar Thermal Systems) might help; they normally have modulation circuits for variable-speed pumps, and alternative programs for "If it gets very hot do this instead")

 

Hi Kristen
Your right about the diminishing returns, smaller temperature differentials lead to higher efficiency but you also need higher flowrates.
With long periods of hot weather the mass will heat up which will reduce its cooling capabilities, thats not to say you can`t use cool night air to dump the days heat to recharge the coolth for the followng days onslaught :wink:
The fan is controlled by a homebrew differential thermostat and speed controller so its flexible. The diff stat senses the temperature difference between the air and soil and if it exceeds the setting (adjustable from 3F-36F) the fan will run. The diff stat also takes air temp into account so if it falls below a preset level in winter (say 10C) the diff stat reverses and decides if the soil temperature is warm enough to add heat to the cold air..if it is it turns on the fan and if not it doesnt :wink:
I`m supplementing the winter heat input with a fence mounted solar air collector which will capture a lot more of the limited sun than the greenhouse can, it`ll use the same fan that drives the rest of the system.

 

You've clearly thought it through

I'll be very interested to know how it performs once its all functional