Open-Loop Systems are More Efficient
Ground-source heat pumps with closed loops circulate a nonfreezing liquid through tubing placed in horizontal trenches or in vertical wells. In contract, open-loop systems make use of water from a stream, well, or lake that passes through the system only once before it is returned to its source or jettisoned elsewhere.
Of the two, the best open-loop systems available are slightly more efficient. According to the U.S. Department of Energy, open-loop heat pumps can operate with a COP of 5.5, vs 4.9 for the best closed-loop system.
Ground-source heat pumps, often called geo-thermal heat pumps, tap into the earth or a source of water for residual heat. There are all kinds of ways to connect the heat pump with the ground. In most cases, loops of plastic tubing filled with a nonfreezing fluid are buried in trenches that may stretch hundreds of feet around a building site. Tubing can also be dropped into one or more vertical walls, a technique that takes a lot less room but can be quite costly. Tubing can be placed at the bottom of a nearby pond, or water from a well can be the source of latent heat.
Types of Ground-Source Heat Pumps
Horizontal Ground-Linked Heat Pump
Tubing filled with antifreeze absorbs heat from the ground and carries it to a ground-source heat pump. Even in winter, underground temperatures are high enough to allow the system to work, but hundreds of feet of tubing must be buried and excavation can be extensive.
Open-Loop System Where Water is Abundant
With an open-loop system, water is pumped from a well through the heat pump and returned to earth. It passes through the system only once. If the well produces enough water, it can be used for both domestic supplies and the heat pump.
Closed-Loop Vertical System Recirculates Same Fluid
Tubing can be placed in one or more vertical wells if there isn't enough room for horizontal trenches. The number and depth of the wells depends on a variety of factors. A closed loop of nonfreezing liquid absorbs heat from the earth and moves it to the heat pump inside, where it is concentrated and distributed around the house.
Comparing Heat Pump Performance
The highest performing ground-source heat pumps on the market offer excellent energy savings over their lifetime-at least on paper. In one simulation from the U.S. Department of Energy, the most efficient heat pump was expected to use only about one third of the energy for heating as a standard air-source heat pump, 10,900kWh vs. 29,800kWh. Savings for cooling were not quite as dramatic, but overall the simulation estimated that annual energy savings were better than 2 to 1 with the ground-source heat pump.
However, anecdotal evidence suggests that ground-source heat pumps can be finicky and disappointing. Efficiency is described as a coefficient of performance (COP) which is the heating capacity of the device in Btu divided by the electrical input.
A heat pump with a COP of 4 produces 4 units of energy for every 1 unit of electrical energy it consumes.
But keep in mind, COP refers to the energy consumed by the heat pump itself, not the electric pump that move the transfer fluid through all the tubing in the ground.
Ground-source heat pumps also lower the temperature of the soil around the tubing during the winter. The colder the soil, the less efficient the heat pump, and the nonfreezing glycol that must be used where freezing is a threat takes more energy to pump while offering a lower heat-transfer capacity. Ground-source heat pumps are more efficient than conventional electric-resistance heat and air-source heat pumps. But they are not magic. Efficiency can be lower (and thus the consumption of electricity higher) than their COPs would suggest.