A climate battery is a ground to air heat exchanger, which provides year-round heating and cooling of solar greenhouses and other structures using the thermal mass of the soil underground. Ceres’ patent-pending Ground to Air Heat Transfer (GAHT™) is a high-performance climate battery specialized for year-round greenhouse growers. For more on how a GAHT™ system or climate battery works, please see our GAHT™ page. Below is useful advice for growers who have already installed a GAHT™ system/climate battery. Specifically, we run through how to set up the thermostats properly so the GAHT™ system functions optimally and does not use excess energy. We also discuss how to integrate ventilation fans, back-up heaters, and cooling systems.
Setting Thermostats of a Climate Battery
Every fan in a GAHT™ system or climate battery is operated by two thermostats. One is a turns the fan on and off when the greenhouse gets too cold. It is set at a low set temperature when the greenhouse needs heating (often set at 50F). Because this thermostat will turn the system on to provide heating, we call it the Heating Thermostat for simplicity. A second thermostat turns the same fan on when the greenhouse gets too hot. Called the ‘Cooling Thermostat’ for short, this is set at a high set temperature (often set at 75F) and will turn the fan on when the greenhouse needs cooling (and back off when the greenhouse returns to a cooler temperature).
When the greenhouse is at a suitable temperature in between these two set points, the fan is not running. This prevents unnecessary energy usage when the greenhouse does not need heating or cooling. But it also requires two set temperatures (one high and one low), and thus a need for two thermostats. (There are some advanced control systems in which you can program multiple set temperatures, but these are usually too expensive and complicated for this application.) The fact that a GAHT™ systems or climate battery can involve two set temperatures is often confusing to contractors/electricians, so make sure to explain the intended set-up clearly to the person wiring your greenhouse. Most growers outsource this step of the process and use Ceres’ GAHT™ instructions as a guide for their contractor to wire the system. We strongly recommend labeling the thermostats so that you can operate them going forward.
Integrating Back-Up Heaters, Coolers, and Fans with a Climate Battery
Most greenhouses with a GAHT™ system/climate battery still use exhaust fans to help cool the greenhouse during the day. The set temperatures of exhaust fans should be a few degrees higher than the GAHT™ fan. For example, a GAHT™ fan is set at 70F, the exhaust fans could be set at 85F. This allows the GAHT fan some time to run, storing heat in the soil before the exhaust fan turns on. They may sometimes be on simultaneously—meaning the GAHT™ pumps heat into the ground and the exhaust fan exhausts it outside. This is fine, as long as the GAHT™ fan has some time to run during the day.
Growers in cold climates typically add a back-up heater to supplement the GAHT™ system/climate battery in the winter. This requires a more particular operation. GAHT™ system should be off while the heater is on. GAHT™ systems draw heat down into the soil. That is ideal during the day when the greenhouse provides the heat but results in wasteful energy use if a fossil-fuel-based heater is providing the heat. In other words, you want the heater to heat the greenhouse air, not the soil. Ideally, the two should not be running at the same time.
Thus, for very cold periods of the winter, we recommend disabling the GAHT™ system for a night, or multiple nights if necessary, and allowing the backup heater to provide the greenhouse heating. To disable the heating function of a GAHT™ system, simply turn the heating thermostat (the one with the low set temperature) to a very low temperature (e.g., 30°F [−1°C]). The backup heater should be set at a higher temperature (e.g., 40F [4.4C]). When the greenhouse cools, the back-up heater will engage first and should keep the greenhouse above the GAHT™ fan set point. The GAHT™ fan is still plugged in and can still run during the day to provide cooling, but it cannot run at night because the greenhouse will not reach the low-temperature threshold. For most GAHT™ systems, you will have to do this manually when you anticipate the greenhouse could over-cool.