Climate Battery Greenhouse Updates

Check in here for 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. Updates will be likely confined to the winter and shoulder seasons as we'll turn the system off in the summers.

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


Winter 2018/2019 Weekly Updates

Week of December 2, 2018 - Outside Low/Hi - 18F/53F

Blue House
Climate Battery Temp - Unk (seems to be mid to low 40s at shallow depths, around 50 deeper)
Thermostat Upper Hi/Lo - 75/37F (3F diff), Thermostat Lower Hi/Lo - 75/37F (3F diff)
Weekly High/Low - 77F/29F

Gray House
Climate Battery Temp - 53→52F
Thermostat Hi/Lo - 75/37F (3F diff)
Weekly High/Low - 84F/33F

Another interesting week. The blue house is showing its overall “colder” state with the battery struggling to keep temperatures above freezing on an 18F night, while the gray house doesn’t seem to have any issues maintaining a ~34F set low temp (when you consider the differential), despite climate battery temperatures continuing to drop. I’m writing this on a Friday, so we have one more day to go, but once again we’ve lacked for sun. We had enough peaks of sun this week to bring the houses up to temp during the day, but not really enough to bank any heat. It will be interesting to see where ground temperatures go from here. I would expect that they’d continue to hover around 50F if we don’t draw on the climate battery too much. The outside soil temps must be in the 40s considering the daytime temperatures. With the overall colder state of the blue house, I also see a lag in how it responds to times of sun. The gray house rises more quickly in temperature and hits temperature marks about a half hour before the blue house, and it also reaches an overall higher temperature during the day. This may be due to the second layer of poly on the gray house providing insulation, though that also has a sunlight-reducing effect.

Later in the week (Thursday morning) I set the thermostats down once again as we get into “winter mode”. The gray house will now kick on at 32.5F while I’m allowing the blue house to dip below freezing. Will reflect those changes in my next update.

Week of November 25, 2018 - Outside Low/Hi - 29F/57F

Blue House
Climate Battery Temp - Unk
Thermostat Upper Hi/Lo - 75/37F (3F diff), Thermostat Lower Hi/Lo - 75/37F (3F diff)
Weekly Low - 74F/33F

Gray House
Climate Battery Temp - 56→53F
Thermostat Hi/Lo - 75/37F (3F diff)
Weekly High/Low - 81F/35F

We’re once again in what seems to be a predominate winter pattern of very little sun (much like last year). Our houses only had two sunny days to capture any warmth but fortunately our night temps barely dipped below freezing. I’m writing this a week late so I won’t spend too much time on details here. As the climate battery drops in temp I will drop both the thermostat high and low set points both to conserve the heat in the soil and make use of the heat that the structure generates. The goal (in my mind) is to generate at least a 15F differential between the air temperature and the ground temperature when “storing” heat.

Week of November 18, 2018 - Outside Low/Hi - 18F/55F

Blue House
Climate Battery Temp - Unk (Approx 50-53F based on random checks of PSU sensors)
Thermostat Upper Hi/Lo - 75/37F (3F diff), Thermostat Lower Hi/Lo - 75/37F (3F diff)
Weekly Low - 29F

Gray House
Climate Battery Temp - 60→56F
Thermostat Hi/Lo - 75/37F (3F diff)
Weekly Low - 34F

We had our first week of significantly cold weather this week, with temperatures for 3 nights dropping down into the low 20s to teens. Our established gray house has been significantly warmer this fall, I believe owing to the remainder of heat stored in the soil. We’re getting into the minimal sun strength portion of the year, so the greenhouse is only running in cooling mode for, at most, 4-5 hours per day. I spent some time hooking up additional acurite sensors in the blue house and will get one to record soil temperatures as well in order to provide better climate battery temperatures. I will likely continue to provide more detailed reporting of thermostat settings, outdoor highs and lows, and perhaps several climate battery temperature readings since I have access to the data now via the sensors installed by PSU (they have several installed at different depths). It’s worth noting here that the figs in the gray house have not yet gone dormant., though the leaves on some look wilt-y. This may be due either to my lack of watering (my bad, I watered thoroughly twice this week after discovering this), or perhaps because they partially rooted into ground outside the greenhouse and perhaps the roots have gone dormant out there due to the ground freezing? In any case, I’m hoping the figs go dormant soon, at least before we have to drop the temperature in the greenhouse to below freezing. My plan currently is to slowly lower the thermostat set points in both houses to an eventual low of 24F and a high setting of around 65F.


2018 Growing Season Summary

 Figs in the tunnel, November 2018. We ripped out the peppers, eggplant, and tomatoes, though they were still going strong.

Figs in the tunnel, November 2018. We ripped out the peppers, eggplant, and tomatoes, though they were still going strong.

Season Summary

The growing season of 2018 was a successful one, despite challenges. Our figs grew well in the tunnels, despite compacted ground, I believe due to their vigorous rooting nature and the addition of compost and a nice wood chip mulch. Some varieties were and are a bit slow to take off, I believe again due to the compacted nature of the backfill soil. We broadforked the tunnel earlier in the spring and will do so again likely this fall and perhaps once again in the spring to provide aeration and to help break up compaction. As for ground temperatures, we maxed out our ground temps in the low 80s in the middle of the summer and they were hovering in the 70s headed into the fall (mid October) and have since dropped to the the low to mid 60s (as of mid November). We’re attempting to manage ground temperatures so the plants slowly go to sleep but also so that we have enough stored thermal energy to weather any cold snaps. Correspondingly, we set our thermostat to kick on at 50F earlier in the fall but have since dropped it to 39F to again signal the approach of winter for our plants.

Thermostat Settings for the Summer

In the summer, we set the original house thermostat to come on when the temperature exceeded 90 or 95F (I can’t remember the exact setting). The idea was to take the edge off of the temperature inside the house (which was fully open) as well as provide some heat to the ground in preparation for moving into the fall. As I’ve written before, there’s really no way to adequately cool the entire structure on a summer day, so unless you have a much greater airflow and an extremely large thermal mass, using a climate battery to cool a greenhouse is best done in the fall, winter, and early spring seasons. Otherwise, the sun is just too strong and the solar gain too great to overcome.

Comparing the Gray House to the Blue House

Earlier this fall (October) we completed work on a second climate battery greenhouse, the blue house. It’s been interesting to watch and compare the difference between the houses heading into fall. With a season of banking heat, and simply being in protected culture, the ground radiates significantly more warmth in the older gray house than in the new blue house that was constructed after the better part of the warm season.

The Benefits of a Climate Battery Greenhouse

On my mind quite a lot has been the benefits of a climate battery greenhouse versus a temporary (or permanent) heater. You could make the argument that a simple propane or diesel heater could be used as a substitute, if what you were trying to do was to keep figs (or other subtropicals or hardy citrus) alive through the winter. I think that a climate battery has some benefits over this approach, though there’s certainly some overlap of benefits:

  1. Elevated Ground Temperature - Because of how heat is stored in the house, the temperature of a climate battery greenhouse would seem to swing faster and be elevated versus a conventional structure. This could lead to earlier planting and quicker take-off of plants in the ground (especially heat-loving ones).

  2. Dehumidification - When the greenhouse is in cooling mode, condensation in the air does seem to drop out. This is reversed on cold nights when the system runs in heating mode, but does not seem to cause any adverse issues.

  3. Reduced Fossil Fuel Use - Yes, this is debatable, since our electric power likely comes through the burning of fossil fuels, but we also don’t have to be concerned about running low and refilling during cold snaps. If we were to generate our own power and store it, our tunnels could, in theory, be completely off grid.

  4. Natural Season Extension - We are beginning to view the climate battery as an additional means of season extension. The ability to store heat gives us a somewhat extended thermal flywheel effect, so that our season extends beyond that of a typical unheated structure.

  5. Shoulder Season Cooling - Rather than vent heat in late September through February, we keep our house mostly to fully shut and take advantage of the cooling we get by storing heat. We’re still figuring out exactly when and how much to vent (since we have to do it manually), but so far this seems to provide enough cooling to keep from damaging our crops while giving us the side effect of enough heat to get through cold nights.

Thoughts on Temperature Control in a Climate Battery Greenhouse

Having had experience with a climate battery for a full year now, I have some preliminary thoughts on how much control over air temperatures our climate battery greenhouse has.

Cooling - The ability to cool the air in the greenhouse is definitely season-dependent. In the late fall and winter, with the sun at its weakest, we can certainly adequately cool the structure for our purposes with little extra ventilation needed. If you’re trying to grow crops that aren’t heat-loving, then additional ventilation may be needed, especially as the ground temperature approaches your desired max temperature in the structure. As the sun regains strength in the spring, additional ventilation will be needed.

Heating - Setting the desired temperature of the structure (the Set Point) needs to factor in the expected minimum outside temperate along with the current temperature of the thermal mass. In general, we expect to heat no more than 20F above the outside temperature if the thermal mass is at least 20F above the set point.

So as a formula:
Minimum Outside Temperature + 20 = Set Point AND Set Point <= Thermal Mass Temperature - 20

So for instance heating into the fall, our Set Point is 50F when our expected nighttime lows are ~28-30F. In order to best accomplish this, the thermal mass needs to be around 70F (or at least in the upper 60s). Later on in the fall, when the thermal mass slowly drops in temperature, our Set Point drops as well.

These are our current guidelines for our Set Point. If we had a smaller or more highly insulated structure, it may be capable of different numbers. Additionally, we could “push” the climate battery further and aim for a higher set temperature, knowing that we’d get close to them most nights. For example, we could aim for a set temperature of 55F with a thermal mass of 65F with night temperatures at 32F. However, this would likely to result in the climate battery system running all night due to the low differential between the set temperature and the thermal mass temperature and the high differential between the set temperature and the outside temperature. This would, in our minds, result in a further chilling of the soil and a premature loss of heat that might otherwise be used to heat the structure during really cold nights.

Could the climate battery provide a 30F differential between the inside and outside? I think that’s within the realm of possibility, but more research will need to be done this winter. Certainly with a better-sealed house and perhaps use of internal covers alongside a well-managed thermal mass temperature we could get close to that goal.


Winter 2017/2018 Summary

Heating Performance

On the coldest of nights, with outside temps of 0F, we could keep the greenhouse around 20F (without an internal cover). In general, we seem to max out at a differential of approximately 20F (inside vs outside temps). Early in the season we set the thermostat above 32F, hoping we could keep the house that warm. We were able to keep it fairly warm most nights, but in doing so we drew down the climate battery temperature immensely and likely hurt our performance later in the season. In future winters we'll likely aim for winter lows of 20-24F as we did later on in the season. This gives us a good buffer from killing temps for our in-ground plants and will likely serve to only draw on the climate battery on nights of extreme cold.

Our fans, based on CFM ratings, and even with losses from running through the tubing, should circulate the air in the house 18-20x per hour, which should be plenty for heating. However, on cold nights we have a hard time drawing enough from the thermal mass in order to properly heat the house. This is likely either a function of conductivity of the soils (they can't replenish heat as fast as we're drawing from them), or CFM loss through the tubing, or some other unknown factor or combination of factors. In talking with some builders of these, they recommend turning down the fan speed at night to maximize the capture of warmth. However, this would also reduce the overall air movement in the house and may not keep pace with the heat losses from the building. There is also the surface of the greenhouse that radiates heat constantly to consider. I would love to have someone run some calculations to determine how the mechanics of this heat transfer works but so far have been unable to find someone to study it.

I certainly understand why folks build these systems with highly-insulated structures. Heat retention seems to be the name of the game in the winter. The downside of a highly insulated building is, of course, cost to build and the addition cost of engineering and design. I would love to see someone come up with an inexpensive kit house, much like you can buy traditional high tunnel structures currently. If anyone knows of an inexpensive manufacturer on the East coast, let me know in the comments.

Cold-Sealing, Wind Breaks, and Internal Covers

It was only later in the season (last week of January) that we ended up completing our rigged internal cover. It was still a fairly clumsy system but it helped to better-manage night temps in the house by essentially halving our heating space. In the future we'll likely keep this covering system, even though it's a bit of a pain to draw over and fully seal on cold nights. Any advice here on a better home-grown solution would be very helpful. We would consider floating row covers over the figs too but might worry about condensation forming under them. We'd like to keep our in-ground plants as dry as possible during the winter.

We have significant plans to better seal the edges of our house through the growing season. We have a considerable amount of scrap polycarbonate that we'll use to form a "skirt" around the house to compensate for any gaps between the ground and the edge of the house. This, along with checking for air leaks, should make for a much more tight (and therefore warm) house next winter.

A long-term solution to help with heating needs will likely come in the form of a windbreak. We have extremely strong winds here in the winter that draw a lot of heat from the house. Even a partial windbreak should help to reduce heating needs over the long term.

Is it worth it?

This is the big question, isn't it? For our needs, this structure, even with its drawbacks (which I've tried to be perfectly frank about), seems to work well. Our in-ground trees should have survived the winter just fine in the house, and our hardier citrus did fine in pots in the house and helped (along with the oat cover crop) to serve as our "canaries in the coalmine" in terms of indicators of hardiness. The house also provides enough season extension (likely 3 months or so) to extend the growing season out to 9 or 10 months of freeze-free temps. We had an extremely cold stretch this winter that is on the order of one of the longest, coldest stretches that have ever happened in this climate, so I am pleased. Additionally, we had what seems to be a very cloudy winter that inhibited the recharging of the system.

We've begun to roughly calculate the "value" of increased soil temperature by comparing it to the cost of raising soil temperature by other means (propane, natural gas). At current prices, the cost of running the system seems to be approximately 10% of propane costs and 25% of natural gas costs (as of March 2018 when I last ran the calculations). Of course, the system does not offer the fine-grained control over temperatures that a natural gas or propane heater would offer, so that must be weighed. However, I think it would function well as a substitute for season-extension similar to how many greenhouse tomatoes are grown. The additional benefit of raising the soil temperatures to promote early growth is important as well. Again, if anyone knows of an engineering firm or university willing to provide feedback or help with ROI calculations, please let me know in the comments below.

Even with the comparison between heating methods it will be hard to calculate the exact "heating" value that the system provides. The benefit of burning fossil fuels is that it only is using energy on-demand. Our system, on the other hand, is using energy to store heat, then often using it again to extract heat. Hopefully by the end of next season we'll know enough of the performance to provide an estimated ROI.

Future System Design

Unknowns in how heat move through the soil and from the soil surface make it difficult to determine how to design a future system. I'll list here some things that I might change in the design in order to perhaps make it more effective.

Our current tubing system sits, for the most part, around 4-5 feet underground, some deeper, with mostly two layers of tubing separated by 1-1.5' of soil. If I were to design another system, I'd likely go with perhaps just as much smaller-diameter tubing but cut up into more "batteries" overall. So instead of our current system operating three "batteries" (three fans, three runs of tubing, each consisting of around 1300-1400' of tubing), I'd up the number of fans to 12 and reduce the length of piping per fan.

This would allow us to use inline fans (probably 6" or 8" inline duct fans) that are perhaps better-suited to pushing air around versus our 20" HAF fans. It would also eliminate the need for large-diameter corrugated piping for risers and manifolds, which were expensive and difficult to work with (though they looked cool). The additional fans would cost more, but would be offset by the elimination of expensive risers and manifolds. This would also allow us more fine-grained control over how many fans came on and so may allow (this is just theory) for staged heating, and perhaps overall lower electric cost.

With the additional fans I would propose two layers of tubing, one at 4' and one at 8'. I'd run the 4' layer on one set of fans (about half), and the 8' layer on another set of fans. It would be great to know if changing the backfill to another type of media (sand, gravel), might allow the heat to flow more quickly away from the piping and into the surrounding soil. Going this deep would effectively double the amount of thermal mass available to draw on and may make the system much more stable.

Before embarking on any additional systems I would certainly like to more formally study this one more, perhaps with the aid of a university or other firm. The system is costly enough, time-consuming enough, and permanent enough that it warrants serious consideration to avoid some of the mishaps of the design and construction of this initial system.

Construction-wise there are several things I'd do differently. For one, I'd certainly attempt to better salvage existing topsoil from the site. We're blessed with great soils here but the construction of the system has us planting in mostly unimproved subsoil at this point. I'm sure that will improve over time but for now it's something I wish were different. I would also construct the system with the backfill process in mind. This was by far the most arduous and time-consuming of all of our tasks and any improvements to it would help immensely. I'm certain there are ways to improve on it. Additionally, I would want to pay particular attention to time spent ensuring the structure we chose to go over the house was as tight as possible and perhaps better insulated. A gutter-connect house may fill those requirements and provide additional growing space to warrant the additional cost of installation of the battery system.

Winter 2017/2018 Updates

Week of April 29, 2018 - Outside Low 39F, Winds Light to 20mph, Climate Battery 68→69F, Inside Low Temp 46F

I'm recording this a few weeks late but spring seems to be here to stay. Nothing much to note in the greenhouse. In the week following this one we had 90 degree days, so there's not too much to note. We turned off the climate battery during the 90 degree days and switched to just running circulation fans in the house. The climate battery has stayed pretty solidly at 69F. Without the battery running we can keep the house approximately 4F above ambient during the nights where we still have cooler temps. With pushing 15 hours of daylight, the system is generating way more heat than is needed, so most of the excess heat is now vented.

This will be the last regular entry until the fall when we start to bank heat for the winter. Our sensors will still capture data through the summer so I'll be sure to note any interesting developments and will continue to monitor the ground temperature. If you've been reading, thanks for doing so! We'd love to hear your thoughts on what would be helpful information-wise (see the comments section below). I'll post a winter season summary after this with some wrap-up thoughts on the system as a whole.

Week of April 22, 2018 - Outside Low 37F, Winds Light to 20mph, Climate Battery 68→68F, Inside Low Temp 46F

Spring seems to be here! We had a pretty temperate week, though we still had some nights in the 30s and needed to run the system on heating mode. Sun was good and we were able to stockpile some heat pretty much every day. On cooler and breezy nights, as mentioned before, we have some trouble maintaining our 50F setpoint. I think the changes we'll be making to tighten up the house should help, and an internal cover at night would help too, but it's just too much bother at this point and the plants we have in there do well without it. Rising night temps should really help as well, and we'll likely set the heating setpoint up to 55F or 60F in the coming week just to speed the growth of our solanaceous crops. After night temps have risen somewhat, maybe by mid-May, we'll probably shut down the system until the fall. At that point we really won't need the system for heating and banking more heat by running the fans would seem to be a waste of energy.

Week of April 15, 2018 - Outside Low 29F, Winds Light to 20mph, Climate Battery 68→65F, Inside Low Temp 43F

Another very cool week for this time of year with temps regularly 15 to 20F below normal. We should normally be in the mid to upper 60s by now but only hit 60 one day this week with lows in the 30s every night. I set the heating thermostat to 50F last weekend and we were able to stay in the 40s despite some cloud weather several days in a row. We really only had 2-3 sunny days this week so most of the time the climate battery was on "heating" mode when it was on at all, as can be evidenced in the ground temperature drop.

What I'm beginning to see is that running the heating several nights in a row without some solar gain really draws on the battery, at least the area immediately surrounding the tubing. The rest of the soil mass may be at 60 or above but the exhaust air when heating starts in the high 50s and drops to the low 50s by the end of the night. The temperature doesn't seem to "recover" (rise) around the tubing enough to match the overall soil temps, so some sun is necessary to provide a temporary recharge of the soil. I'm don't hesitate to keep the night temps set at 50F in the house, even though it causes a draw on the battery, because I'm fairly certain the worst of the cold is over and I'm willing to "spend" some of that heat on keeping the house at a decent night temp to speed growth of the plants in there.

I'm fairly pleased with the system overall and my fig starts that I've put in the house are thriving. Additionally, our sensor in the middle of the greenhouse shows temps a little warmer at night (low of 45F for the week) than our main sensor which is in the northwest corner of the house (the coldest portion), so temps may not be as bad as thought (I was initially disappointed by the inability to keep the house at 50F).

Temps in the forecast finally appear to be on the rise and nighttime temps don't look to dip into the 30s except for the next couple of days.

Week of April 8, 2018 - Outside Low 23F, Winds Light to Moderate, Climate Battery 63→62→66F, Inside Low Temp 33F

A warm end to the week brought increased temps to the soil. We had flurries early in the week and mid-80s by the end. We planted out trees in the house and bumped up the heating thermostat to 40F (we'll bump this up even more next week). Our warm spell will end and next week looks to be much cooler, but the weather very much jump-started spring. Still, we'll struggle to reach the low 60s in the coming week with a lot of clouds again expected. With the climate battery sitting in the mid-60s I think 40F or even 50F is probably achievable at night with lows rarely dipping into the mid-30s. I will move some of our fig starts out early in the week to acclimate them.

This spring and summer our goal is to better seal the house, especially around the perimeter where this is a gap between the baseboard and the soil. We've been using greenhouse plastic to seal this but it was only ever a temporary measure. A better sealed house will allow us to retain heat a lot better and hopefully exclude any critters that might try to make themselves at home in there.

Week of April 1, 2018 - Outside Low 30F, Winds Moderate to 50mph, Climate Battery 62→63F, Inside Low Temp 34F

Still cold! We had a few inches of snow after a beautiful Easter afternoon. No real sun to speak of this week, maybe a total of 1-2 full days amid the clouds and rain. As mentioned before, it doesn't take much sun to cause a spike in temperatures into the 90s, but we just haven't had enough to bank a lot of warmth. Things are very late to bloom this year. Over the next two nights we expect the outdoor temperatures to dip into the mid to upper 20s, but the thermostat in our tunnel is set to kick on at 34, keeping things above freezing. It doesn't look like it will begin to warm up until late next week. With the climate battery temperature in the 60s, it's rather easy to keep the tunnel heated.

We plan to plant in the greenhouse next week!

Week of March 25, 2018 - Outside Low 24F, Winds Calm to moderate, Climate Battery 58→62F, Inside Low Temp 28F

Another odd week! We're still struggling to reach the normal outside temps for this time of year of high 50s to low 60s, and the coming week looks much the same, and mostly cloudy. I'm thinking spring will arrive all at once this year. Still, the grass is greening up and some trees are breaking bud.

We had 3.5 sunnier days this week (it doesn't take much of a break in the clouds to warm up the house). We've had the greenhouse tarped for a little over a week and I'm sure the black surface has helped to increase daytime temps. Also, just last night we upped the heating thermostat to kick on at around 34-35 to keep the entire space above freezing, which worked well (we still let it dip below freezing earlier in the week, therefore the "Inside Low" is below 32F). With the extra heat we've banked this should be pretty easy.

Our potted citrus (mostly hardier varieties) inside the greenhouse, which served as a sort of "canary in the coal mine" for us in terms of temperature, are beginning to leaf out (except for a few we let get too dry).  These may be good indicators too of when other in-ground tropicals or subtropicals may leaf out in the house when we put them in the ground. We'll be planting in the house in just a couple of weeks.

Week of March 18, 2018 - Outside Low 27F, Winds Calm to moderate, Climate Battery 54→58F, Inside Low Temp 29F

What an odd week. We started out with temps in the mid 50s on Sunday and Monday, then received over a foot of snow on Tuesday and Wednesday. We saw climate battery temps shoot up from 54F to 58F those two days, drop a bit over the course of the two snowy days, then recover at the end of the week. The end of the week saw three (mostly) sunny days in a row, making 5 mostly sunny days total for the week, a relative record for the winter. With moderate night temps, sunny days, and the climate battery sitting in the 50s, we seem to be able to bank 1-2F of temperature gain in the climate battery every day.

The snow caused a sort of "leak" in our greenhouse covering by tearing away at a temporary plastic skirt we have around the greenhouse, causing nighttime temps to dip below freezing. We plan to replace the skirt (currently used because of a gap between our baseboard and soil due to an oversight when grading the site) with a permanent solution using leftover polycarbonate strips from the construction once the snow melts and we get into spring. It should help to better seal and insulate the greenhouse as well and perhaps provide a barrier from the infiltration of little pests. At this point in the year we're having to vent the greenhouse partially by leaving a person door open around mid-day, otherwise temps rise above 100F. The climate battery, at our volume of air moved, does provide some cooling but not enough currently with the strength of the sun. We have an additional two days of sun coming, so it'll be interesting to see any climate battery gains from the additional warmth.

Week of March 11, 2018 - Outside Low 21F, Winds 20mph through week, Climate Battery 53→54F, Inside Low Temp 27F

Another wintry week. Really maybe only 3-4 days where the temperature was high enough to bank some heat, though even then not for a whole day. It's been really hard to get sun and as a result the ground temps have been pretty steady. We prepped the greenhouse this week for planting in about a month and spread mulch and eventually will spread a black tarp that should bump temps in the house quite a bit on sunny days. Still allowing the temps to dip below freezing at night though the lows haven't been too bad (low to upper 20s). Outside it remained cold with temps in the upper 30s to low 40s most of the week with a heavy wind most of the time. Next week doesn't look all that much better but perhaps a little warmer. Again, sunny days seem to be the limiting factor right now. Whether this is just a cloudy winter or the norm, I'm not sure.

Week of March 4, 2018 - Outside Low 21F, Winds 20mph through week, Climate Battery 55→56→53F, Inside Low Temp 27F

We took somewhat of a dip back into winter this week with highs only in the low to mid 40 and a fair amount of wind all week. Sunny days are really helpful to the system but we still haven't seen much more than 2-3 sunny days a week. This week we may have had close to 3.5 sunny days. Still allowing the temps to dip below freezing at night. We opened up the house on Friday to begin prepping the soil and spent the day delivering mulch. That may account for the end-of-week drop in temperature of the climate battery, as it was rather windy and cold. Today was (relatively) sunny and warm so we should see that climb again. Once again only 3 sunny days forecast for the next week.

Week of February 25, 2018 - Outside Low 27F, Winds 40-50mph end of week, Climate Battery 55→56F, Inside Low Temp 30F

Very windy end to the week. We had a nor'easter blow in with gusts of 50mph and very strong sustained winds. Tunnel seemed to handle it in stride. Three sunny days meant that we could stockpile some heat but cloudy days seem to mostly offset it. We likely could have easily kept the greenhouse from dipping below 32F but didn't feel the need to. Still haven't put figs in the greenhouse, will likely do that over the next week. The citrus we have in there in pots seem to be fine but haven't moved toward flowering. Our cover of oats is growing like crazy. Over the next week we'll likely roll down the oat cover and mulch over it to prepare for planting sometime in April. Only three days next week are forecast to be sunny, with a stretch of four cloudy days in a row. Not very conducive to further raising the climate battery temperature!

Week of February 18, 2018 - Outside Low 27F, Winds Low to 15mph, Climate Battery 50→55→54F, Inside Low Temp 33F

Another interesting week! It was extremely mild out this week and we had near-record outside temps (70s) two days in a row. This extra heat (and sun) allowed us to bank a lot of extra heat. It's interesting though, that the remainder of the week remained cool and cloudy, so that the inside temp struggle to even reach 50 or 60 despite outside temps in the 40s. This pulled from the daily gains of the very warm days. Cloudiness may be a very limiting factor here in the winter. I'd almost rather have a sunny day in the 30s than a cloudy/rainy day in the 40s or 50s. We're reaching about 11 hours of daylight here, which may also be critical, meaning that the draw on the battery could be about equal with the additions we'd gain through sunny weather. Our cover crop of oats appears to be growing well again, which is a good sign. I'm thinking of placing some plants out in the house in pots to simulate when we'll have actual in-ground plants and determining how/when to best wake them up.

Three sunny days with decent temps in the coming week may help to push the battery even further up. We're sitting around 54F which is warmer than the natural constant temperature of the earth in these parts. Should be interesting to see how quickly it will rise in the spring. Also, I've enabled a comments section below. Feel free to comment or ask questions!

Week of February 11, 2018 - Outside Low 20F, Winds Low to 15mph, Climate Battery 45→50F, Inside Low Temp 25F

Fairly mild week, with only two nighttime lows dipping down into the mid to low 20s. Still not a ton of sun, but even little peaks of sun this time of year cause the temperature to spike. This coming week looks to be very mild with nighttime temps dipping below freezing for a single night. With lots of rain in the forecast I expect the climate battery to climb a little more, perhaps as high as 52F. With more sun this time of year and mild temps I think we could climb even higher, but sun seems to be rare. Corresponding with the rise in the earth temperature, I've upped the cooling thermostat to kick on around 70F so that there's a good differential for extracting heat.

Week of February 4, 2018 - Outside Low 16F, Winds Low to 10mph, Climate Battery 46→45F, Inside Low Temp 25F

Fairly normal week. Outside low only dipped into the teens twice so there wasn't much pull on the battery, but weather remained mostly cloudy so the climate battery was really only able to "store" heat one day out of that period. We've adjusted our heating thermostat to kick on at 24F, since there's really no reason to keep the house above 32F for the types of dormant plants we're growing. At this point I feel confident that we can maintain a Mediterranean climate in the house without supplemental heat, which should be fine for fruits like figs, persimmons, and the more cold-hardy citrus. Earlier in the season we were trying to maintain 32F in the house, just to see if we could remain above freezing. During the two coldest weeks, that thermostat setting of 32F meant that we really drew down our climate battery and set ourselves up for difficult weeks to follow. We probably lagged behind even a conventional unheated house in terms of ability to buffer outside temperatures simply due to the fact that we were actively chilling the soil. Again, it will be interesting to see how next year will pan out with the ability to store heat later into fall and how far that carries us into winter. The setting of 24F should mean that the battery actively runs less for heating, except on really cold snaps, thereby lowering electricity cost.

Week of January 28, 2018 - Outside Low 11F, Winds Low to 10mph, Climate Battery 48→46F, Inside Low Temp 28F

After a warmer week last week, this week was again fairly cold with at least three nights in the teens. We utilized the interior cover and were able to keep the greenhouse at 30F and 28F on the coldest nights (11F and 13F) despite relatively low ground temps.  Wind plays a significant role. This had the effect of drawing down the climate battery, but just a couple degrees. I have been turning off the heating portion of the climate battery on warmer nights, but will begin to switch it such that it kicks on at a lower temperature, perhaps 24F, so that the system isn't taxed as heavily.

The sun is much stronger this time of year versus the winter solstice. On a sunny day, even one that doesn't get above freezing outside, the greenhouse can reach likely close to 80F even with the cooling running. We only seem to get a couple of these days per week this time of year, so they'll be especially valuable in storing heat to the ground. In the future we'll start listing the starting and ending temperatures for the week for the climate battery, and may list in-between temps if there are significant swings. 

Week of January 21, 2018 - Outside Low 18F, Winds Calm to Low, Climate Battery 46F, Inside Low Temp 24F

Had a nice warm-up to end the week last week (and a nice warm-up to this one), so I ended up turning off the heating portion of the climate battery. The battery has climbed to the mid-40s despite not a lot of sunny weather. I would imagine that if left to its own devices, the battery would eventually naturally tend to approach the near-constant temperature of deeper soil. We will keep the battery turned off even through a few nights in the mid-to-low 20s, since these sorts of temps wouldn't hurt the types of plants we want to grow during their dormancy. Plus, it will be interesting to see if there is any warming effect simply from radiant heat from the soil. I've adjusted the climate battery to start its cooling around 60F, just to provide a good delta T to capture some extra warmth.

Without active climate battery heating (only passive radiant heat) and no internal cover, the greenhouse does seem to stay about 4-6F above the ambient temperature, with the soil just sitting at 46F. I imagine this could be greater if we would deploy the internal cover every night.

Week of January 14, 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.

Week of January 7, 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 December 31, 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 December 24, 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.

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.

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 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.

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.