Climate Battery Greenhouse Updates
We'll post regular updates here regarding the performance of the climate batteries installed in our high tunnel including lessons learned and other strategies we've employed to make our house functional. I'll only post updates for extraordinary events or findings that seem important to the overall function of the system. If we have a warmer stretch with nights just below freezing they probably won't be noted.
Tentative Future Plans
The winter of 2017/2018 will be spent monitoring the system while we allow our clover/oats cover crop to grow. We'll pay close attention to how the system responds to reduced sunlight (both length and intensity)
We plan at some point to design a more complex control system than the one currently in place. I envision one where several factors are taken into consideration rather than the current system which only takes into consideration the temperature of the greenhouse itself.
Temperature and humidity are monitored in the house via AcuRite temperature sensors, which then communicates with the AcuRite Hub and uploads data to MyAcurite (I promise I don't work for Acurite). This system has been reasonably reliable though some sensors seem to be duds more than others. Take a look at our monitors real-time at device ID 24:C8:6E:08:D2:67 on MyAcurite.
Winter 2017/2018 Updates
November 28, 2017
Sensors in place to monitor greenhouse temperature as well as the temperature of the outlets to determine an approximate climate battery temperature. For those with myAcurite, you can monitor our station at device ID 24:C8:6E:08:D2:67. Some of the sensors go offline at times but the greenhouse and outlet 1 temperatures seem to be very reliable.
December 13, 2017 - Outside Low 19F, Winds 25mph (40-50mph gusts), Climate Battery 42F, Inside Temp Low 29.1F (see below)
No significant sun for the past few days followed by a very windy and cold. Climate battery was drawn down to 42F as the system had to run all night. Heavy winds did not help. Did not employ internal cover as I wanted to see if the battery could heat the entire structure. Greenhouse temperature sensor was placed at plant level at the northwest corner of the structure, so I believe it read a little low. I did not find anything frozen when checking the house in the early morning, so I think the sensor wasn't properly reading the site. Impressive to me that the structure essentially stayed above freezing despite very strong winds (gusts had to be in the 30-40mph range or more). Moved temperature sensor away from the wall of the house and placed it approximately 4' into the structure. Blocked some small cracks that allowed outside air in and repaired our current "skirt" around the greenhouse that blocks outside air due to some gaps. Some of it had come unfastened due to the strong winds. Sunlight doesn't significantly warm the greenhouse until 10am and we begin to see it cool off after around 1pm, I believe due to weak sunlight and a windy day. At this time of year the climate battery seems to be easily able to cool the greenhouse.
December 14, 2017 - Outside Low 23F, Winds Calm, Climate Battery 44F, Inside Temp Low 37F
Despite another cold night, the greenhouse rather easily held 38-39F all night (fans running all night). Still no internal cover. Likely could have conserved some heat with cover. Outlet 1 held steady around 45F all night long, which isn't as high as I'd like to see it but I was glad to see it hold. The day of December 13th (see above) was windy but very sunny (typical for our area after we see a cold front come in), high of 28F but some warmth was, I believe, able to warm the greenhouse and perhaps the climate battery somewhat.
December 22, 2017 - Outside Low 28F, Winds Calm, Climate Battery 48F, Inside Low Temp 36F
After a cold week last week, temps have climbed to more regular levels this week. We had one day this week with temperatures in the upper 50s and a lot of sun which in turn helped the climate battery to recover much of its spent heat. As of yesterday afternoon it had climbed to around 50 degrees, but then was forced to expend some of that heat overnight. Days are very short, and even with higher temperatures the system does not begin charging the climate battery until 10am, and shuts off around 4pm as the sun begins to set. Even with this little bit of charging, the ground temps recover somewhat. We've hit the shortest days of the year, but are now on the upswing. The next month should be challenging with relatively weak sun, short days, and the coldest days of the winter ahead of us. Our average temperatures do not bottom out until late January.
Week of December 25, 2017 - Outside Low 9F, Winds 10mph, Climate Battery 36F, Inside Low Temp 25F
This week is the beginning of an extended cold spell for us. Christmas Day was our last day with a high above freezing and the cold front that came in brought single digits temps and pretty consistently windy conditions. Despite using an internal cover (though not a great one), internal temps dropped significantly below freezing though still 15-16F above the outside temperature. This weather (highs in the 20s, lows in the teens to single digits) will continue for at least until the end of the first week of January. Climate battery has been steadily dropping in temperature but I haven't yet seen it drop below freezing. Having a break, even for an hour or two mid-day seems to help it recover somewhat. We've been able to "bank" some heat but with days this short it's really only running for 3-4 hours. This coming week we'll add extra monitoring to truly measure soil temperatures and work will continue on developing a better internal cover.
Week of January 1, 2018 - Outside Low -1F, Winds 25mph, Climate Battery 32F, Inside Low Temp 19F
This extended cold is really putting a strain on the climate battery. By the time this cold spell ends (expected 1/8), we'll have had nearly two weeks of temps below freezing, many of the days with highs in the teens. Still, it's a good test of the systems and to see how much heat it can actually provide. On one morning we hit nearly -1F outside and the hoop house stayed at 19F, despite outlet temperatures reading around 32F. There's just not much warmth left in the soil and the days are still very short and have been, at times, cloudy.
Despite this cold we haven't seen damage to our cover crop (oats/clover). Oats have a terminal temperature (I'm told) of around 13F, so very similar to the point where figs start experiencing damage. So far they've remained healthy. We continue to add sensors to the greenhouse. This week we'll be adding a true soil temperature sensor and an additional temperature sensor in the house to measure a different area. Our current estimates of climate battery temperature are based off of the exhaust port but that isn't an exact measurement. The actual soil temp appears to be around 40F.
We continue to struggle with creating an internal cover that isn't leaky. We have a new design that may prove successful if we can work out one remaining issue. The internal cover should, in essence, cut our heating needs by about half since we won't heat the upper portion of the house. This should reduce strain on the climate battery and hopefully allow if to retain more heat on these extended cold spells.
Other climate battery implementations include backup heat for extended cold snaps like the one we've been having. I opted against it due to cost and I just didn't like the thought of needing a "backup" source of heat. My current line of thinking is that for any future greenhouses using this system, we may either want to locate all or a significant portion of the tubing further down in the earth. Temperatures are more constant the further you excavate down, and in theory if you go deep enough you'll have a relatively inexhaustible source of heat. This would be very useful for the type of weather we're currently experiencing.
One final note about the fans we're utilizing for our climate batteries. We're utilizing 20" HAF fans from two different manufacturers to push air through the earth tubes. We've been very pleased with the Schaefer VK20 fan for this job, and relatively unimpressed with the others. Both manufacturers are rated to move the same CFM, but the Schaefer pretty clearly is able to push more air through. I've purchased an inexpensive anemometer and should have some stats on airflow shortly. We've liked the Schaefer (model VK20) so much that we have another on order and will likely use all Schaefer fans if this one proves to be as good. The remaining fans will likely be used a simple circulation fans in the greenhouse come spring.
Week of January 8, 2018 - Outside Low 13F, Winds Calm, Climate Battery 38F, Inside Low Temp 24F
We got a break. This Tuesday (1/9) it finally climbed above freezing after two solid weeks of temperatures below freezing. The thaw is short-lived but it comes with some temperatures in the 40s and 50s, perhaps as high as 60(!!). We've been working on a more functional internal cover for the greenhouse and have a design that should work well. It should be in place by the time the next cold snap hits.
We've installed a new reporting sensor approximately 2 feet down in the greenhouse soil to give a better idea of actual climate battery temperatures, so that more accurate figure is reflected in my updates from now on (and is why you see a jump in temperature despite little warming outside). The previous estimate was based off of exhaust port temperatures which don't give a true temperature of the soil, just of the air exiting the exhaust tubing.
Since temperatures tonight and tomorrow will not drop much below 50F, we're opting to keep the circulating fans on in the greenhouse, thinking that the temperature differential between the climate battery and the outside air will help to warm it over the next 24 hours. I'm hoping to raise the ground temps a few degrees to combat this next round of cold coming next week.
Week of January 15, 2018 - Outside Low 7F, Winds Calm, Climate Battery 42F, Inside Low Temp 25F
This has been a mix of a week. We started out very warm due to a warm front and were able to spend a couple days charging the climate battery up to a high of 45F. However, since then we've had a series of cold nights and not-so-warm days. I have noticed a fairly significant difference in the amount and intensity of the sunlight we are receiving, even though we're only about a month out from the shortest day of winter. Instead of only banking heat until 2pm, we seem to be good almost until 3pm. The cold seems to have depleted the battery down to about 42F, but we're headed toward a week of days where nighttime temps don't dip much below freezing. We may fully turn off the "heating" side of the greenhouse and just rely on radiant heat from the ground.
One of the difficulties in designing and utilizing a climate battery are the number of variables involved in designing a system: soil type, latitude, weather, structure to heat, and solar gain. I've been thinking a lot about the house and the climate batteries and have a lot of unanswered questions. If anyone from a university or engineering firm is reading this, some outstanding questions in my mind are: How quickly does heat move through soil? How much heat is transferred from the surface of the soil to the air in the house? For how long can heat be practically stored and how do you improve on its storage?
One of the suggestions that installers of these batteries make is to have some form of backup heat for extended cold snaps. This makes sense, as the draw on the ground as a heating source during these periods would rapidly deplete the store of heat. However, this would seem to greatly tip the scales away from these systems as viable in a commercial setting. In short, what's the point of investing in and using a climate battery if you'll additionally need to invest in a conventional heater? There must be a better way. Enough ranting. I'll provide an update next week as time allows.