Introduction to Geothermal Heat Pumps.
There are many choices of heating and cooling systems for your home or business. Fuel source is a huge consideration since the real cost of the choice is often not the installed price, but the cost of operation over time. Oil, propane, natural gas or heat pumps are the most common options for heating, and for cooling there are different grades of air conditioner efficiency. Most new homes these days have air conditioning, and it is a desirable add-on for existing homes without it. Our position is a geothermal heat pump is almost always the best choice for either new construction, or for conversion from a conventional system. Please take the time to review the following, which represents over thirty years of our experience in the heating and cooling arena
What Are They and Why Should You Choose One?
Geothermal heat pumps, also called ground coupled heat pumps, are a variant of common water chillers found for decades in commercial applications. In simple terms, they are systems that move heat from one place to another, as all heat pumps or air conditioners do. Please remember that adding heat and removing heat can be equally beneficial, (heating vs. air conditioning), and each function has its place. Straight air conditioners can’t do both. They are a cooling only system. Heat pumps are virtually the same except for the addition of a reversing valve which enables the system to swap functions, (heat mode or cool mode). Heat pump and air conditioner performance is directly tied to the temperature of the energy source, also called the heat sink. We aren’t concerned with the temperature of the heat sink with fossil fuel heating systems. They simply burn as much fuel as required. However, they require air conditioner condensers, and these components are subject to hot July/August outdoor temperatures, (heat sink). Air source heat pumps are subject to outdoor air temperatures in both winter and summer, and tend to degrade downward in heating performance when the outside temperature dips below 30 degrees. Problems with system performance arise when the outdoor air temperatures swing rather wildly (as expected). Worse, extreme outdoor temperatures and associated declines in performance, coincide with the highest heating and cooling demands. Any heat pump is suited to both tasks due to its reversing nature, but a geothermal heat pump is best suited for the job due to the steady temperature of its heat sink (the earth).
An Overview of System Components and Operations.
A typical geothermal installation has three components, the ground loop, the equipment and the distribution system. Of the three, the distribution system is the component most similar to conventional systems.
For new systems, ducted or radiant systems can be easily designed for geothermal heat allowing for the relatively low output temperatures of the geothermal heat pump, (90-110 degrees). For retrofits, modern conventional duct systems designed for air conditioning are usually compatible with the air flow requirements of an air based geothermal system. Water based distribution systems are more complex. Most existing baseboard systems are not compatible with geothermal because they require higher water temperatures (180 degrees) than a geothermal can produce. Radiant systems can be compatible, but special examination is needed to assure proper performance.
The equipment aspect of a geothermal system comes in several flavors. The vanilla, and most often seen version, is a unitary air based, water to air, machine. This unit looks a lot like a good sized refrigerator and it is set in the basement. Duct, electrical, thermostat and ground loop connections are made, and off you go. No fuel or refrigerant piping is required. Some homes require remote air handlers due to space limitations. This necessitates an air handler someplace outside of the mechanical room, connected to a compressor section usually in the mechanical room. We call these split systems. The equipment is connected with refrigerant pipes. The most flexible, and complex configuration, is the water to water geothermal system. The first water refers to the loop side. The second water refers to the output side. Hence the unit outputs heated or chilled water. This system most often utilizes mass tank(s) and can be combined with radiant heat, water based air handlers (both heat and cool), snow melt, pool heating, towel warmers and most anything you could imagine.
The ground loop is the feature of a geothermal system that sets it apart from convention systems. I like to tell folks to think of the ground loop as their oil tank. Reference to above, the ground loop is the ‘sink’. The loop is sized to carry the home in question through the winter given the prevailing weather conditions. The loop can be laid in trenches horizontally, or inserted into a well vertically, or sometimes, in a body of water (not everyone has one!). The loop is constructed of high density polyethylene plastic pipe. This stuff has been around and proven its durability for decades. Configurations and pipe diameters are variable at the discretion of the designer and are often dictated by the site. Heat of extraction and flow rates are key design criteria. The loop is most often filled with anti-freeze solution. We prefer 20% propylene glycol (non-toxic), but others use methanol or GS-4. Ethylene glycol (Prestone) is frowned upon. In an odd legacy, the loop circulating pump is still sometimes referred to as the ‘brine pump’, harkening back to the days that the antifreeze was a saline solution. The important part to understand about the ground loop is that even if its -5 outside, the heat pump is seeing source temperatures (sink again) generally no less than 32 degrees, well within a heat pump’s efficient operating range. The system maintains its output and efficiency regardless of outdoor air temperatures.
Introduce the Nature of Heat Flow and the Vapor/compression Cycle
I often find myself talking to very earnest yet very confused folks posing the question ‘How do you make that warm?’ They get the loop part, but I think they still have trouble thinking it’s very warm 4’ below grade on a January night. I tend to agree. The best analogy I’ve found is to inquire what they suppose is happening with their refrigerator. Why is it cold in the box, and why is there warm air coming out the bottom? I used to say it’s either magic or voodoo, but in fact, it’s simply the nature of heat flow and the vapor/compression cycle. One simple assumption about heat flow is that heat flows from warmer to colder. This is in line with objects falling from higher to lower, water flowing from higher pressure to lower pressure, and electrons flowing from higher voltage to ground (don’t be in the way). The other concept is that everything (generally) has heat in it. Heat and temperature are often confused. Heat is everywhere, and temperature is simply a measurement of the intensity of heat. If I’m at 98.6 degrees, and I go outside and it’s 50, I’m warm and it’s cold. If I put a block of ice outside, it’s cold and the air is warm. It’s relative. Heat travels from me to the air, or heat travel from the air to the block of ice. All you have to do to make heat energy move is to bring something to close contact with something of a different temperature. Heat will invariably move from warmer to colder.
Now we talk about how to manipulate temperature. I was introduced to geothermal systems by a Swedish gentleman named Sven Ogarksen. He used to say a heat pump is a ‘temperature amplifier’. Basically, just pump the temperature up and let nature let it flow. As you know, there is a relationship between temperature and pressure in regard to gaseous materials. The vapor compression cycle, oft called ‘Carnot’s Cycle”, goes through the entire range of evaporation, compression, condensation and pressure reduction. Let’s start with the evaporator. When a liquid evaporates into a gas, a tremendous amount of heat energy is absorbed, just like a runner trying to dissipate body heat by sweating. For a refrigeration system, it is common for the glycol in the loop (32 degrees or higher) to be exposed to refrigerant on the other side of the heat exchanger that is in a liquid state but at a sufficiently low pressure that it will evaporate at around 18 to 20 degrees. The liquid evaporates into a gas, absorbs a lot of heat, and is piped to the compressor. When you increase pressure, you increase temperature. This is the job of the compressor. Generally, depending on the refrigerant, the compressor pumps up the pressure so that the refrigerant will condense back into a liquid at around 100 degrees. This is more than sufficient to deliver heat to an air stream of 70 degrees. The high temperature/high pressure gas is sent to the condenser, where is gives up its heat and returns into a liquid. This high pressure liquid then passes through a pressure reducing valve, to return to the evaporator and start it all over again. It’s not about creating heat, it’s about moving heat.
Why should you consider one?
This is a short list of the benefits to considering a geothermal heat pump. Each will be treated in more detail in the following weeks
- Payback: Least costly means of heating and cooling you home/business
- Enhanced comfort and indoor air quality: A very gentle heat that does not dry out the air and can be filtered and conditioned
- Environmental benefits: Best reduction in carbon emissions and most potential for remediation efforts
- Energy security: While dependent on electrical grid, studies indicate more stable prices and supplies with electric, as well as better options for self generation.
- Reduced maintenance: Equipment is totally in the building, as opposed to in the elements. Units do not require annual service, although some folks like to.
- Government and utility support: Government support has been strong, in the form of a tax credit, and support is also strong because these systems are hugely beneficial to the power company.