Hourly Operating Costs - Climate Battery versus Propane
The Risks of Attempting an Apples to Apples Comparison
Any attempt to compare two systems runs the risk of disadvantaging one over the other, particularly if the person doing the comparison stands to gain from the comparison. In this particular case, we here at Atmos have a vested interest in climate batteries and think they offer significant benefits over the alternative heating method, which is often propane or some other fossil fuel. With all of that out in the open, we’ll be transparent with our numbers so that you can plug in your own assumptions if you’d like.
A Note on Climate Battery Output Versus Propane
A climate battery is not a one-for-one replacement of a rated output propane heater. The capacity of a climate battery greenhouse to heat, like other forms of geothermal, is dependent on the temperature of the surrounding soil. Therefore the heating output (in BTUs or whatever units you prefer) will vary based on the ground temperature and the thermostat set points in the greenhouse.
On the flip side, a propane heater has a rated output that will not vary. It will output that same level of heat hour after hour until the fuel runs out.
Our first climate battery greenhouse after its second winter
The Structure We’re Heating - Heat Loss and Heater Sizing
Since a 30x96 greenhouse is a common size (at least in the Mid-Atlantic and Northeast region of the US where we’re located), and that’s the size of our first climate battery greenhouse, the comparison will be done based on that structure. Our 30x96 is covered with double inflated poly (no sidewall inflation), with polycarbonate end walls. There are more complex factors at play, such as the effect of wind and the film additives, such as IR, on the covering, but we will ignore those for the sake of simplicity.
Using Rimol’s “How to Size a Heating System” article, we use the exposed surface area (rather than the volume of the space being heated as you might assume) as a means to calculate heat loss. Since the climate battery creates about a 20F differential, sometimes up to 24F in extreme cold, we will try to determine its output (and the comparable propane heater) using heat loss calculations.
Step 1 - Calculate Surface Area
Inflated Cover Surface Area (Inflated)
42’ wide * 96’ long = 4,032 sq ft
Two Roll-Up Sidewalls Cover Surface Area (Non-Inflated)
5’ wide * 2 sides * 96’ long = 960 sq ft
Two End Walls Surface Area (Polycarbonate)
10’ average height * 30’ wide * 2 end walls = 600 sq ft
Step 2 - Calculate Heat Loss (Using U Factor)
From the heat loss factors (u factor, the inverse of R factor):
Top - 0.7 u factor - Double poly inflated
Roll-Up Sides - 1.15 u factor - Single poly for the sides (since they are not inflated)
End Walls - 0.63 u factor - Twin Wall Polycarbonate
To Create 20F Differential:
(0.7 * 4,032) + (1.15 * 960) + (0.63 * 600) = 4,304.4 BTUs/degree F
4,304.4 BTUs/degree F * 20F = 86,088 BTUs
Extreme cold output: 4,304.4 BTUs/degree F * 24F = 103,305 BTUs
Step 3 - Choose a Comparable Propane Heater
Propane heaters are not 100% efficient. Some of the most efficient are about 93% efficient, so we will use that figure.
I was able to find a propane heater* rated at 85,000 BTUs, but with efficiency losses it comes out to around 79,050. The step up is the 110,000 BTU model with an output of 102,300, which may be slightly oversized but is still in line with the output we’ve experienced on the coldest nights that the climate battery has experienced (as seen above).
The 110,000 BTU model burns approximately 1.2 gallons of propane per hour of operation since each gallon of propane contains ~91,452 BTUs**, and uses approximately 0.77 kWh of electricity per hour of operation to run the blower fan and associated electronics.
* The brand used for comparison was the Modine Effinity 93. Heaters at 80% efficiency are more common and less expensive but would increase operating costs.
** https://www.eia.gov/energyexplained/units-and-calculators/
Step 4 - Climate Battery Electric Usage
In order for a climate battery to heat a structure, it must first store the heat. This means that to fairly count the true cost of operation, we should count the operation cost twice: once to store (“create”) the heat, and another when it is retrieved (“used”).
This isn’t always true: sometimes the heat comes for “free”: the passive warming of the soil all summer or the buffering up of soil temperatures mid-winter from the surrounding warmer soil. Still, for the most fair comparison, we’ll count the operation cost twice.
Our original 30x96 climate battery uses three fans that draw around 500 watts each for a total hourly consumption of 1.5 kWh per hour of operation. Doubling that, per the reasons above, the climate battery effectively uses 3.0 kWh per hour when heating.
Step 5 - Propane Versus Climate Battery Hourly Cost
To do the final comparison, we have to know what our energy costs us. We will estimate propane at $1.70 per gallon and electric at $0.12 per kWh. Your local rates will vary, so plug in your numbers to get a more accurate picture for your situation.
Propane Heater Hourly Cost - $2.13/hour
(1.2 gallons per hour * $1.70 per gallon) + (0.77 kWh * $0.12 / kWh) = $2.13 per hour of operation for propane
Climate Battery Hourly Cost - $0.36/hour
3.0 kWh per hour * $0.12 / kWh = $0.36 per hour of operation for the climate battery
Is it that simple? Really?
We think that climate battery greenhouses make a lot of sense for many growers in many applications, but they’re not for every application. Remember that a climate battery comes with significant up-front costs for installation, so it’s best viewed as a long-term investment that will continue to pay dividends for years into the future. If your heating needs are very minimal, the up-front cost of a climate battery likely won’t pay back quickly in terms of energy savings.
How much will it save my farm?
To figure out how much you could save, you can start by plugging your own numbers into the calculations above. If you know your annual propane costs, try swapping out your hourly propane costs with that of a climate battery if your propane heater size is similar.
Please know that, as stated above, a climate battery is not a drop-in one-for-one replacement for a propane heater. A climate battery will likely never replace a heater that can create a 50-70F differential.
Still, a climate battery, even an early design referenced above, is very efficient at storing and retrieving heat. If you’re interested in how a climate battery greenhouse can help your farm, learn more through our site, reach out and contact us with basic questions, or schedule an initial consultation to discuss your application.