Learn how to choose the right equipment for your geothermal heating systems. Discover which heat pump will work better and serve you longer, how many TONs or BTUs the heat pump should be and what type of geothermal heat exchangers to use.
1. Heat Pump
The heat pump itself is the most important part of any geothermal system and is responsible for extracting or rejecting the hot air to/from the ground. The most commonly used unit in geothermal systems is the single package water to air pump which is the size of the small furnace and has all necessary components pre-installed in it. The typical heat pump consists of refrigerant to water heat exchanger, refrigerant piping and control valve, compressor, air coil that heats air during winter timer and cools it down and dehumidifies it during summer time, blower and electronic controls. The single package design is another advantage (besides efficiency) over the split design that is used in air source heat pumps. Learn how heat pump works.
There is a whole number of great design closed loop geothermal heat pumps on the market, but the following characteristics should be considered while choosing one:
1. manufacturer’s entering liquid temperature’s limits
2. features – such as domestic hot water production capability (so-called desuperheater), insulated water lines, variable speed water pump, electronic controls
3. safety listing (UL, ETL, ARL)
4. performance rating
Manufacturers also offer split systems, dual and multi-speed compressors, water to water heat pumps, rooftop versions of this equipment to suit various needs and applications.
2. Pipes for ground heat exchanger
Piping for the closed loop ground heat exchanger can be divided into two parts: the in-ground piping and the inside piping. The in-ground piping is usually high density polyethylene (HDPE) which in modern days is the only material that is recommended by the heat pump manufacturers. These pipes have 160 psi rating and they come in standard dimensions of which ¾’’, 1’’, 1’’1/4’’ and 2’’ are most common. Use of the standard pipe dimensions and lengths are most economical. Some standard steel pipe sizes like 2.5’’ or 5’’ are not readily available and will be cost prohibitive to purchase. There is a Dimension Ratio index (DR) to determine pipe’s pressure rating. Dimension ratio is the pipe’s outer diameter divided by the wall thickness. As the wall thickness increases the DR decreases and pressure rating increases. In geothermal application pipes with DR = 11 are most commonly used.
Grout is specific low permeability substance used to fill the space between the borehole wall and the pipes in the loops (ground exchanger) to form hydraulic seal and increase thermal conductivity. There are few major types of grout:
1. Bentonite grouts – they have low thermal conductivity and usually cost less, resulting in longest borehole design length to provide the desired design heat transfer rate to or from the ground
2. Cement grouts – also have low thermal conductivity but higher cost – does not reduce borehole design lengths
3. Thermally enhanced grouts – they can be both bentonite based and cement based and have additive (silica sand) added to obtain higher thermal conductivity. They also have the highest cost but reduce overall installation cost by reducing borehole design lengths (reduces drilling, pipe lengths and grout volume) to provide the desired design heat transfer rate to of from the ground
4. Antifreeze for ground heat exchanger circulating fluid
There is a whole variety of antifreeze solutions. It can be salts (calcium chloride or sodium chloride), glycols (propylene glycol, ethylene glycol), alcohols (methyl, ethyl or isopropyl alcohol) and potassium (acetate, carbonate). Not all of these anti=freezes are used our days. Modern antifreezes must meet the following criteria: be safe, non toxic, non corrosive, have good heat transfer, have low cost, be long lasting and have low viscosity. Based on these criteria most common anti-freezes are:
1. propylene glycol – the only antifreeze allowed in geothermal systems with vertical boreholes in many states. It has food grade, meaning it will not harm ground water if leaked. It has higher viscosity below 32 degrees, so it is harder to pump when cold
2. methanol – best when diluted 50/50 with water. Methanol is used primarily in horizontal loops but it might be prohibited to use in some states. It also should be handled with care because of it’s toxicity and flammability but it has low viscosity below 32 degrees so it is easier to pump when cold
3. ethanol – has pumping characteristics similar to propylene glycol but is less flammable and toxic than methanol. Must be denaturized prior to using.