Climate Battery Energy Consumption During the Cold Season
How Much Energy Does a Climate Battery Use in the Winter? ❄
Energy use in a climate battery greenhouse varies depending on the relatively cloudiness or sunniness of a location, the temperature extremes, and the set points of your greenhouse. Depending on what we’re growing, our thermostat’s lower set point (the point at which heating turns on) is set anywhere from 24°F to just above freezing (32-33°F).
Let me explain a bit further for context. In our oldest “fig” house, we don’t grow any crops over the winter and use the structure to primarily overwinter figs. As a result, once the fig trees are dormant, we turn on the heat only to keep the figs from receiving damage. 24°F as a set-point has worked well for that since it’s just slightly above the average nighttime high temperature in the winter. Since our sunshine to recharge the climate battery is limited in the winter due to our cloudy climate and northern latitude, this lower set-point keeps us from overdrawing on the climate battery and unnecessarily heating our greenhouse when the figs don’t really need it. This not only reduces energy consumption from our climate battery fans, it also keeps us from over-chilling the soil.
In our newer greenhouse, we tend to keep the temperature set a little higher (32-33°F) because we’re trying to overwinter some greens which perform better if the temperatures are kept above freezing. This continued and accelerated growth comes at the expense of running the climate battery fans more and may run the risk of overdrawing on the heat in our climate battery during a really cold spell.
Our Example - Threefold Farm’s Blue House
We’ll use for our example the Blue House at Threefold Farm. This greenhouse now consumes about 1,820 Watts when the fans are on in either heating or cooling mode and those work to heat and cool a 34’x96’ space. That’s similar to the electric usage of a home space heater. We don’t employ backup heat in this greenhouse.
The soil temperature at the point that these measurements were taken was right around 50°F at ~18” below the soil surface.
The high/low set-points for this greenhouse in the winter are 70°F and 33.5°F in order to store heat on those rare sunny winter days and draw from it when the temperature tends toward freezing. In this greenhouse we overwinter fig trees and use the remaining space to grow winter greens, primarily as a cover crop, until planting tomatoes in the early spring.
Climate Battery Usage in Winter - A (Relatively) Sunny Day ☀
Days with full sun are relatively rare here in the winter. I suspect we average 60% cloudy days here in the winter, which means little solar gain, but also means the temperatures are more moderate as the clouds trap the heat. My example will draw from a mid-January day (January 12th, 2021 to be exact). By this time of year, our solar gain is somewhat recovered from the shortest day (the winter solstice), but the sun is far from being strong.
The supplied graph tells the story. It takes until 10:30AM for our greenhouse to reach its cooling set-point, 70°F. At that point, the climate battery fans kick on and begin to pump the warm air underground. On a more cloudy day, the cooling effect that this has would cool down the greenhouse to the point where the fans would cycle on and off, but on most sunny days, the energy from the sun is greater than the systems ability to capture it, leading to a gradual rise in temperature. Around 3PM, even though there are a couple hours until sunset, the temperature in the greenhouse begins to drop and the climate battery fans shut off. This is typical for winter.
Assuming the system ran consistently for 4.5 hours, this would put our consumption at 8.19 kWh (4.5 hours * 1.82 kW). At our approximate electric rate of $0.12/kWh, this puts our daily total around $0.98 for the cooling portion.
The upper set of graphs are the inlet temperature (°F) and humidity, the lower graphs measure the temperature (°F) and humidity of the air exiting the system.
Climate Battery Usage in Winter - A (Relatively) Normal Night 🌙
At night, the greenhouse relies on passive means of warming (radiation and conduction from the soil surface) to heat the greenhouse. This effect appears to be almost negligible at this time of year as the soil temperature (50°F as noted above) is low. The low temperature for the night was 25°F, which consisted of an initial dip to 28°F, a rise, then a fall to 25°F. Not a very cold night, but a good average night.
As the graph notes, the system kicked on around 7pm, then off and on throughout the night as the thermostat calls for heating. The system is able to keep up with heating demand pretty easily due night temperatures only in the mid 20’s.
All in all, there are 7 cycles of heating during the night, each lasting 20-30 minutes. The total runtime worked out to 3 hours and 6 minutes, or 3.1 hours.
This puts our consumption at 5.64 kWh (3.1 hours * 1.82 kW). At our approximate electric rate of $0.12/kWh, this puts our daily total at around $0.68 for the heating portion. Since this night was slightly above average with a warmer portion around midnight, I could see this figure doubling as the system would call for heating more regularly.
Heating begins to kick on around 7:15PM, kicks off for a while during an outside rise in temperature until midnight, then cycles off and on throughout the remainder of the night until around 5:20AM. The upper two graphs are the inlet temperature (°F) & humidity and the lower two graphs are the outlet temperature (°F) and humidity.
Sunny Day Total Cost - $1.66
Taking into account the calculations above, this puts our sunny day total at $0.98 for cooling during the day and $0.68 for heating at night; $1.66 in total.
Imagining a Worst-Case Scenario - Full Day Sun, Very Cold Night Following
Realistically, the heating system runs more frequently on the nights directly following a sunny day. When there are clear skies, the temperature tends to drop drastically at sunset. Even here, if we imagine a worst-case scenario where the system ran consistently overnight for 12 hours without cycling off and on, we would use $2.62 for heating (21.84 kWh * $0.12/kWh).
The example of the sunny day above just about matches our best case sun scenario for January (and the most electric consumption we’d expect during a January day). If we couple this with our worst-case scenario for overnight heating, we would expect to spend $3.60 for the day ($0.98 for cooling + $2.62 for heating). Still not bad. For more detailed calculations and cost comparisons, check out our Costs & ROI page.
Climate Battery Usage in Winter - Benefits and Drawbacks of a Cloudy Day ☁
More often than not, probably 4 days out of 7 each week in the winter, we have mostly cloudy weather. This isn’t great for one’s mood (I much prefer sunny days), and it’s a mixed bag for the climate battery as well.
✅ The Benefits of a Cloudy Day ☁
Cloudy days in our climate typically mean more moderated temperatures: the outside high temperature isn’t vastly different from the low temperature. January 25th was one such day. On this day temperatures in the greenhouse never got above the mid 50’s, and at night, since the outside temperatures were moderated as well, the climate battery heating never kicked on. This helps to preserve any heat stored in the system.
❌ The Drawbacks of a Cloudy Day ☁
The drawback of a cloudy day is that it fails to be an opportunity to store heat into the climate battery. At this time of year, when the best we can hope for is to maintain the soil temperature in the mid to upper 40’s, this isn’t such a big deal. However, as the sun gets stronger heading into spring, a series of cloudy days can disrupt any momentum we’re beginning to build in soil temperature. In other words, it’s a setback.
To Wrap it Up
Climate battery greenhouses have been really cost-effective tools for both winter production and overwintering more sensitive crops, allowing a lower cost means of keeping our greenhouses warmer though the winter. Understanding how they work and being familiar with their limits can help you decide if they’re a good fit for your farm. We’d love to hear from you and your experiences with winter growing!
What do you grow in the winter? How do your costs compare: propane, labor for row covers, etc.? Let us know in the comments below!