LoopLink RLC Update: Pond Loop GHEX Design

Wednesday, February 22, 2017

Pond loops are closed-loop, surface-water heat exchangers. Regulation permitting, any nearby body of water, such as a lake, stream or pond, can serve as a lower cost heat source/sink for a geothermal heat pump system than conventionally bored or trenched systems. That is of course assuming the pond loop is properly designed and installed.

With our latest update to LoopLink RLC, you now have a simple to use tool to properly design a pond loop.

The RLC Approach

LoopLink RLC uses the calculation methods presented in ASHRAE's Design of Geothermal Systems for Commercial and Institutional Buildings (Kavanaugh and Rafferty, 2014) to perform pond loop design calculations. The calculations account for project location (weather conditions), peak heating and cooling loads, GSHP capacity and efficiency as well as the pond temperature, size and condition (i.e. clean water or muddy).

You have the option of designing pond loops in two configurations:

To make things simple, we assume ASHRAE’s minimum spacing recommmendation of 10 feet for both configurations.

As you work, LoopLink RLC will check that the maximum recommended heating and cooling rates are not exceeded (i.e. the pond is large enough and deep enough to accommodate the load) and will display a warning if necessary. Even with those warnings built in, there are some key things to things to think about when designing a pond loop

Size Matters In Cooling

Rejecting heat to a pond in the summer months even in the warmest of locations is fairly easy due to natural convection and evaporation. The most important thing you need to watch out for is the heat rejection rate.

If the pond (reservoir) is too small, you can change the natural temperature of your pond which is bad for the plants and fish. Plus, you can create excessive amounts of evaporation and run the risk of running your small pond out of water… also bad for the plants and fish.

Size Matters In Heating

Water is pretty amazing. One of the most amazing features of water is its behavior at and near freezing. We won’t get into the physics of water freezing but there are two things you need to know about frozen ponds.

  1. Ice floats
  2. Liquid water is densest at 39.2°F (4­°C)

So, if a lake is frozen at the surface, the temperature of liquid water at the bottom of the lake and it is 39.2°F (4°C). That is of course until we start extracting heat energy out of that water through a long winter.

Size matters in two ways in heating. First and foremost is the fact that if you have too small of a loopfield, you may locally freeze the water around your loops. This will make your loops more buoyant and may lift them off the bottom of the lake causing a host of problems not least of which is the possibility of catastrophic system damage.

The second size issue is the pond itself. If you don’t have a large enough volume of water you may suppress the temperature of the entire body which will again pose a risk to the plants and fish. It is possible to freeze a pond all the way through if the rate of heat extraction is higher than the rate of heat rejection from the soil below the body of water.

ASHRAE Says Size Matters

In any pond loop application, the body of water being used needs to be large enough so that the GSHP system does not alter the natural temperature of the reservoir by more than 1°F. According to ASHRAE, the maximum recommended load for a reservoir is 20 tons/acre in cooling mode and 10 tons/acre in heating mode.

Before designing a pond loop, a detailed study should be performed to ensure that the size of the pond is sufficient given the load and also to find the temperature of the pond in the summer and winter at the installation depth of the loop.