Pond-based Geothermal


An Intro to Pond-based Geothermal Systems

There is a vast quantity of free, clean, and renewable energy in our ponds, oceans, and the waters that surround the Vineyard. Through the use of geothermal heat pumps, we can collect and concentrate this low grade stored solar energy into high grade energy to heat and cool our buildings. This energy is available 24 hours a day all year long.

Because the Sun is storing this energy for free, a geothermal heat pump can typically collect 2 to 4 units of free energy for every unit of energy put into the heat pump. The end result is an efficiency ranging from 300 to 500%! In comparison, boilers and furnaces are fast approaching their limit of efficiency (currently some of them have 98% efficiency) of 100%.

Because geothermal systems are 3 to 5 times more efficient than the best fossil fuel systems, their operating costs are about half. In other words, the annual cost for electricity to run a geothermal system is half the annual cost of propane or fuel oil to run a furnace or boiler.

We proposed our ocean based geothermal system in November 2006 and our pond based approach in September 2007. The following sections will have additional details about the ocean and pond approaches as well as an introduction to some ecological and permitting issues.

Pond-based Geothermal System


32-ton Slim Jim Heat Exchanger

We wish to describe a geothermal system that will use the stored solar energy in a pond (or any sufficiently sized body of water) to heat and cool a building – we call this system “pond based geothermal”.

This system will consist of a plate type heat exchanger (SlimJim brand) submerged in a pond (held in a vertical position and mounted to a skid resting on the bottom), a geothermal heat pump and circulator pump located inside the structure, and supply and return pipes buried at a 4 foot depth connecting the heat exchanger in the Pond and the heat pump.

A First for The Vineyard:

We believe that there have been no geothermal installations on the Vineyard that have used ponds (or other bodies of water) for their source of stored solar energy. While this pond based geothermal will be the first use of this geothermal concept on the Vineyard, there are many successful installations of this type around the world.

In Chicago, the Coast Guard Marine Safety Station uses a SlimJim heat exchanger under their pier to provide all of their heating and cooling. Even in the middle of last February with two feet of ice on Lake Michigan, the Coast Guard station was warm and toasty with heating provided solely by the stored solar energy in the 37 F lake water.

The largest manufacturer of geothermal heat pumps in the world, WaterFurnace, heats and cools their entire factory from the stored solar energy in a man made pond in front of their headquarters in Indiana.

The Concept: Pond Based Geothermal:

The geothermal process uses energy from the Sun that is stored in the Earth, the oceans, or the bodies of water on the surface of the Earth. As we receive more energy in 10 seconds from the Sun than mankind uses in an entire day, this is an essentially limitless supply. The geothermal process connects this abundant yet low grade energy to a heat pump which is able to concentrate the energy and raise it in temperature to a more useable form. Through the use of a heat pump we will be able to use the solar energy stored in a pond to heat a building.

How Does This Geothermal Process Affect a Pond?

This geothermal concept will use a small fraction of the available solar energy stored in the pond’s water. Sizing software will allows us to project

the amount of solar energy available in this pond. The available energy is basically a function of the pond’s area and its volume of water.


The amount of energy we wish to “remove” from the pond to heat a residence will be minute compared to the constant input of solar energy into the pond – our analysis indicates that for a properly sized installation, there will be no measurable effect on the pond’s temperature.

The Installation: Plate Type Heat Exchanger

The heat exchanger (herein called the SlimJim) is a stainless steel or titanium welded plate type device used to exchange heat between the pond water and a closed geothermal loop. The SlimJim is formed by welding and pressing two plates of stainless steel or titanium together – each assembly is pressure tested and fitted with inlet and outlet pipe connections. Nominal dimensions of a 5 ton capacity (60,000 Btu/hr) unit are 4’ in height, 8’ in length, and ½” thick with a weight of 100 pounds. This heat exchanger plate may then be suspended under a pier, suspended below a float, or mounted on skids for pond-bottom locations. There is an absolute barrier between the pond water and the closed geothermal loop – the system is designed to be leak free over its entire life expectancy of 50 years (or more).

A typical installation would involve placing the SlimJim in the deepest part of the pond – it is recommended that at least two feet of unfrozen pond water be above the SlimJim at all times.

The Installation: Geothermal Heat Pump

The geothermal heat pump will be located in a mechanical room and will be connected to the buildings heating and cooling system. The advantage of a geothermal system is that it replaces direct use of fossil fuels and has an efficiency on the order of 400% – for every unit of electricity put in the system outputs 4 units (because the sun has given us three units for free into the pond). This means that typical operating costs are up to 50% of a gas or oil system. Once a geothermal system is coupled with electricity generated by a renewable source, as wind or solar PV, the carbon footprint of the project shrinks to zero.

The Installation: Supply and Return Pipes

The pipes that connect the SlimJim to the heat pump with be black polyvinyl (HDPE) well water tubing of 2” in diameter. They will be buried at a 4 foot depth from the house to the SlimJim. The pipes can be installed with a trenching machine to reduce disturbance of foliage to a minimum.