This question is a common one that we are asked indirectly rather than directly.
The answer is usually provided in response to the question of whether a traditional (perhaps existing) air conditioning unit or chiller can be connected to a ground heat exchanger.
This is a simple ‘no’ answer, although we do like to provide a comparatively tech free explanation.
Let’s start with the heat pump itself. A heat pump is simply a pump that transfers heat from one place to another. In most instances this is achieved through a vapour-compression refrigeration cycle. The most familiar example to most people is the home refrigerator and it works as follows:
- You place your approximately room temperature groceries (milk, vegetables etc) in the fridge and shut the door;
- The additional heat is sensed and then removed using the refrigeraton cycle by transferring it from your groceries to the ‘black mesh’ (visible in most older units) at the rear of the refrigerator and into the air. This explains the heat at the rear of your refrigerator and why an enclosed refrigerator will typically be less efficient than one where that heat can more readily escape. In the geoexchange context, we would bury that ‘black mesh’ in the ground outside;
- The compressor and refrigeration system will continue working to remove the heat until the pre-determined temperature set point, typically about 4C, is reached; and
- This cycle continues every time you open the door or place an item inside that is warmer than the set point.
This is a very low tech example that hopefully assists in understanding the basic function of a heat pump. (Note: a more detailed technical explanation can be found here).
Heat Pumps and Air Conditioning
Heat pumps can be used wherever the transfer of heat is required. Noting that heat always moves from from hot to cold. So contrary to popular understanding, air conditioning a home actually means removing heat from it and not pumping cold into it.
In the air conditioning sector, the most common types of heat pumps are defined as follows:
- Air source heat pump: Uses the outside air as heat source or heat sink – essentially most reverse cycle air conditioners;
- Water source heat pump: Uses water as a heat source or heat sink and more common in commercial applications. The water temperature is typically controlled by a boiler (heating) or a cooling tower (cooling) to stay within defined temperature limits. This is typically in the 15 to 35C range, although some may extend a little further in each direction; and
- Ground source heat pump: Uses the ground as a heat source or heat sink.
The ground source heat pump definition probably needs a little more explanation here as it is hard to imagine a heat pump that pumps rock and dirt! It is in this element where a ground source heat pump is simply an evolution of the water source heat pump. Where the ground source heat pump uses water being pumped through a Ground Heat Exchanger as a heat source or heat sink.
This evolution occurred in the early days of geoexchange systems. System designers quickly realised that connecting a water source heat pump to a ground heat exchanger resulted in water temperatures within the closed loop system that exceeded operating limits of the water source heat pump.
For example, extracting heat from the ground in heating mode decreases the water temperature within the closed loop ground heat exchanger. As it is a closed loop system, the water temperature will continue to decrease until it reaches the in-built safety cutoff point of the heat pump. A water source heat pump has a boiler (or similar) to maintain a minimum entering water temperature (EWT) into it, something that doesn’t exist with a ground heat exchanger.
It is far more cost effective to modify the EWT range of the heat pump than install a significantly larger ground heat exchanger – thus the ground source heat pump was born!
So in summary, a ground source heat pump is simply an ‘extended range’ water source heat pump. The typical EWT range of a ground source heat pump is -5C to 45C and thus significantly smaller ground heat exchangers can be connected.
Ground source heat pumps come in two main types as follows:
- Water to air ground source heat pump: In heating mode, transfers heat from water within the ground heat exchanger to air inside the building. In the residential context, this is similar to a ducted reverse cycle air conditioner; or
- Water to water ground source heat pump: In heating mode, transfers heat from water within the ground heat exchanger to water inside the building. In the residential context, this is your hydronic heating system for underfloor heating, radiators or even a swimming pool.
All ground source heat pumps are reversible so both cold air and chilled water can also be produced in the above examples. There are also some manufacturers that have developed a combination unit that is capable of producing both hot/cold air for ducted systems and hot water for hydronic heating.
Most residential ground source heat pumps can provide an optional desuperheater for supplementary heating of domestic hot water as described in this post.
Interested in knowing more about ground source heat pumps and what is the best one for your home or building?