Heat exchange system showing insulated water barrel, water chiller, and on-demand water heater.

Overview

Maintaining optimal water temperature is a key part of raising healthy, consistent crops year round. While there are many options for hydroponic temperature management in relatively small systems, the scale of large deep water culture (DWC) ponds presents a challenge.

We now use several temperature control methods in our DWC ponds, with the primary design based around a closed-loop heat exchange to heat or cool the main ponds as necessary. Using these systems, along with some simple best practices for energy management, we are now able to maintain an optimal temperature of 68-72 degrees Fahrenheit in our ponds.

Design Considerations

Progress picture showing construction of the heating/cooling system. Left is the exposed 55-gallon water reservoir, center-left is plumbing for pond circulation, center-right is the water chiller, and right is the on-demand water heater.

In our case, the main considerations driving design of hydroponic temperature management systems were:

  • Space: with our relatively small greenhouses, efficiency of space is at a premium. Many large, commercial-grade systems would simply take up too much valuable production space.
  • Up-front cost: investment cost is always a major consideration for small producers like us. We needed something that wouldn’t break the bank to construct.
  • Operating costs: aside from labor, our highest cost is utilities. We needed systems that wouldn’t cost a fortune in energy.
  • Ease of operation: simpler is always better! We wanted a system that could be pre-set to operate within certain limits, but would also be easy to manually switch on and off. Additionally, we wanted the system to easily change between hot and cold for seasonal changeover.
  • Effectiveness: it needed to work reliably!
Diagram showing heated water flow from heat exchange system to DWC ponds.

System Components

The overall design consists of four components, plus the main DWC pond:

  1. Water Reservoir: The water reservoir holds the water that is either heated or cooled, depending on the seasonal need. It consists of a 55-gallon drum, covered by an insulated blanket.
  2. Heat Exchange Loop: The exchange loop consists of 1/2″ PEX tubing that runs in several loops underneath the pond liner material. A pump in the reservoir circulates the heated/cooled water from the reservoir through the exchange loop. As the water passes through the loop, it transfers heat to the pond water. Alternately, in the summer, the cooled water draws heat out of the pond.
  3. Water Heater and Water Chiller (not shown): In the winter, an on-demand water heater (shown in diagram) heats the water in the reservoir to a specified temperature. Our systems use Airthereal on-demand water heaters. In the summer, we use EcoPlus water chillers to cool the reservoir water instead of heating it.
  4. Temperature monitors & smart plugs: A temperature monitor continuously checks the temperature of the DWC pond water. When configured for heating, this monitor allows the heating system to run during the scheduled timeframe, but automatically turns it off via smart plug when the pond water reaches the upper limit of our desired temperature range. For cooling, the system does the same if the water gets too cold.

Hydroponic Temperature Management Benefits

Top view of water reservoir with assorted plumbing for circulation, heating, and cooling systems.

This current design has proven several benefits for hydroponic temperature management:

  • Increased lettuce yield: as we’ve moved into the colder months of the year, maintaining an optimal temperature has shown a definite increased lettuce yield compared to previous years.
  • Minimized impact on equipment: the main principle behind using a closed-loop system is minimizing wear and tear on the equipment. Specifically, pumping nutrient-rich water from the ponds directly through the heating and cooling systems could lead to faster build up of scaling in those systems. The closed-loop design uses standard tap water in the heating/cooling system to avoid scaling issues.
  • Smaller equipment and operating costs: instead of directly heating or cooling the full 2,000 – 3,500 gallons of the DWC ponds, we instead focus energy into the 55-gallon water reservoir. This smaller volume requires smaller (more affordable) equipment to bring the reservoir temperature up and down as desired. This reduces the run-time and therefore energy cost of the heating/cooling systems.
  • Scheduled run time for optimizing solar panels: using programmable smart plugs, the system is configured to only run during daylight hours when our solar panels are helping offset the energy cost of the heating/cooling systems.
  • Programmable temperature range: combining automated temperature monitors with smart plugs allows for programmable temperature ranges. The system automatically shuts off when the upper limit is reached (for heating), preventing overheating of the pond water. It also only calls for heating when the temperature warrants it – thereby preventing unnecessary energy consumption.
  • Remote, 24/7 monitoring of pond temperatures: using an app to for the temperature monitor, we are able to check our pond temperatures from anywhere with cell service. This also logs data so we can observe changes in pond temperature alongside air temperature, or in response to changes in management practices.

Results

View of exterior pond sidewall showing PEX tubing for heat exchange loop running into the ponds.

Both the heating and cooling systems are proving very effective for hydroponic temperature management in our DWC ponds. Highlights include:

  • Cooling DWC ponds: The first water chiller lowered the temperature of a 2,000-gallon DWC pond from roughly 75 degrees to 68 degrees within seven days. It then effectively maintained that temperature until seasonal temperatures no longer warranted cooling the ponds.
  • Heating DWC ponds: The first water heater raised the water temperature of two DWC ponds (total 3,500 gallons) from 65.5 degrees to 72 degrees within four days.

Maintaining optimal pond temperature has resulted in larger lettuce head sizes compared to previous years for this time of year. After this winter, we will conduct a more in-depth analysis of return on investment comparing the anticipated increase in energy costs against this improved production.

Other Lessons Learned

Relationship of pond temperature to nighttime temperatures: incorporating water and air temperature monitors has provided significant insight into when the season changes for heating vs. cooling requirements. This year had unseasonably high daytime temperatures later in the fall, and we anticipated that the pond temperatures would track accordingly. However, we observed that the pond temperatures trended down with nighttime temperatures, even as daytime temperatures remained relatively high. This showed the need to switch temperature control to heating sooner in the season than predicted.

Role of pond coverage in managing temperature: lettuce in the DWC ponds floats on foam rafts, which doubles as insulation for the pond water. In the past, we’ve experimented with leaving portions of the ponds uncovered at night to reduce pond temperatures; however, the automated water temperature logging showed how significant this management practice can affect pond temperature.

We’ve found that leaving just 5% of the pond surface uncovered overnight showed significantly higher decrease in pond temperatures compared to nights where the ponds were completely covered. This underscores the importance of proper pond coverage and insulation in retaining heat during the winter, but also shows a management practice that can be used to rapidly decrease pond temperatures in the summer if they exceed the optimal range.