Geothermal Heat Exchanger

There are two major types of geothermal ground heat exchangers: horizontal and vertical. Horizontal ground heat exchanger means that the pipes are buried in the ground in trenches. There can be single or multiple layers of pipe in the trench, depending on their depths, or the pipe can be buried forming so-called slinky. In cases of multiple layers or slinky we have to keep in mind to allow enough soil between layers or otherwise pipes can interfere in terms of thermal conductivity reducing overall system performance. Vertical ground heat exchanger means that pipes are buried in one or more vertical boreholes that can be 100-500 feet deep.

The choice of whether to use vertical or horizontal heat exchanger depends on available land area, local soil characteristics and drilling costs. For residential applications horizontal systems are usually used. Vertical systems are used in large commercial applications or where land area is limited. In some cases it can be the only choice. In any way the selection between vertical or horizontal geothermal ground heat exchanger should be done after the cost study has been made. The cost study should be performed by the certified contractor. The contractor should also pick out the geothermal equipment which they have experience and are comfortable working with.

The geothermal ground heat exchanger can also be either series or parallel. In series ground heat exchanger system fluid cam only flow in one path and in parallel there can be more low paths in the circuit. There is a list of advantages and disadvantages of each system, and the choice depends on what are you aiming for.

Series system advantages:

1. single flow path and pipe size – meaning that the flow path is well defined which makes removing of the trapped air much easier
2. slightly higher performance – since series system requires larger size pipes there is a better thermal conductivity per square foot of pipe

Series system disadvantages:

1. larger pipe size requires larger volume of antifreeze
2. larger size pipes are usually more expensive
3. increased installation cost
4. limited piping lengths due to the pressure drop characteristics

Parallel system advantages:

1. lower pipe cost because of the smaller diameter
2. lower volume of antifreeze is required
3. lower installation cost

Parallel system disadvantages:

1. special attention is required to assure air removal
2. flow balancing between each parallel path to result in equal lengths loops.

There are major components in geothermal ground heat exchangers:

Supply and return headers –

Supply and return headers are the antifreeze supply and return lines from the geothermal heat pump to the ground loops (ground heat exchanger). They carry out the total system flow of the heat pump system. Headers are constructed from the large diameter pipes to minimize antifreeze flow pressure drop down the length of the pipe run.

Loops –

Loops are the pipes extended from the header down to the borehole or to the trench, returned in the same trench or borehole and joined together to the return header.

Reverse return –

Reverse return is a piping arrangement that allows each loop of a parallel system to have the same inlet and outlet pressure. It is used to cancel out the effects of the pressure drop along the header lines.

U-bend –

U-bend is a 180 degree fitting is a loop type heat exchanger used at the bottom of a borehole or at the end of the trench to return the antifreeze.

Additional Information

Geothermal Heat Pump – the ground source heat exchanger and the heat pump need each other to operate, as one will be useless without the other.

Geothermal heating and cooling system is comprised of heat exchanger, heat pump and distribution system – air ducts or radiant heating pipes.

Geothermal energy (also know as ground source, geoexchange or renewable energy) is energy taken from earth, and used to heat or cool buildings depending on season. The basic principle is that in the certain depth in the ground the temperatures are the same throughout the year (about 55 degrees F in New England area) thus giving possibility to retrieve heat during winter time or cold during summer using specific devices called heat pumps. Even though geothermal exchange principle has been known for decades, in residential applications it is relatively new and very fast growing.

Geothermal (or ground source heat pump) system can be installed in any residential structure giving its owner the following benefits:

1. Economic – since the geothermal system uses “free” ground energy, annual saving for heating/cooling season versus regular heating/cooling equipment can reach 75%. Also, geothermal heat pump has prolonged life and requires less maintenance rather than conventional heating/cooling equipment. Usually new geothermal system pays for itself within first couple of years, not to mention that there is a whole variety of rebates and incentives from both federal and state governments and heat pump manufacturers.

2. Environmental – there is no gas or oil burning involved in heat production using geothermal equipment, so there is no CO2 dumping to the earth atmosphere thus minimizing ozone layer destruction. There is no air pollution because closed underground field with environment friendly antifreeze is used.

3. Comfort and safety – geothermal equipment is usually very compact and quiet compared to the same with regular burners. It does not have large noisy blower units, requires less space in your home, there is no open flame or potentially dangerous flammable fuel involved.

There is also a whole lot of other geothermal energy applications rather than just heating and cooling. It can produce hot water for your home, heat pool’s water serve as driveway’s de-icing system during the winter and so on. Geothermal exchange systems is one of the most rapidly growing markets in the united states, and it is becoming more and more efficient as the new equipment and installations techniques are coming to existence.

How Heat Pump Works

The exchange of energy in geothermal system is done by the heat pump – the device that works similar to the refrigerator. The heat pumps, refrigerators and air conditioners all work in the same manner: by pumping the refrigerant through a closed loop (ground loop in geothermal systems), that has two distinct temperature zones – cold and hot.

There a few thermodynamic principles that are used in heat pump’s work. First of all the heat always flows from higher temperature zone to the lower. The heat pump reverses this natural flow making the heat flow from the lower temperature zone to the higher, and by doing that heat pump consumes energy, that is why it is called “pump”. Basically geothermal heat pump is the same refrigerator with a few distinct exceptions: the desired effect for the heat pump is heating and the effect of the heat pump can be reversed to provide either heating or cooling.

When the heat pump heats, antifreeze from the ground loop (loop field) flows by the heat exchanger tubes, containing refrigerant that is cooler than the antifreeze. Since thermodynamics laws say that heat is transferred from the hotter zone to the cooler, refrigerant in the pump’s tubes absorbs the heat from the antifreeze and evaporates within the tubing. The cool refrigerant vapor is than compressed and pumped to the heat pump’s section that has high temperature and often has a refrigerant coil with air blower across it. Since refrigerant gets hotter when compressed it becomes hotter then the relatively cooler air in the space thus giving up heat to it. After refrigerant gives up heat to the air stream it becomes cooler and condenses back to the liquid. The liquid then is pumped through the flow restriction that maintains the pressure difference between cold and hot zones. As the pressure of the liquid drops, it vaporizes becoming cooler and it is ready to go through the refrigerant process again.

If the heat pump works in the cooling mode the process can be reversed – the cold and hot zones then are swapped. With the cold and hot zone reversed the heat pump extracts heat from the indoor air and then rejects it to the ground loop field.

In a geothermal heating and cooling system, heat pump plays an essential role of extracting and delivering ground source thermal energy from a heat exchanger, to the interior space of your home. It is important to choose the correct size heat pump for optimal operation of your geothermal system.

Ground source (geothermal) heat pumps are electrically powered systems that can provide cooling, heating and hot water much more efficiently and less expensively than conventional heating and air-conditioning technologies using ground’s relatively constant temperature. There are two main categories of ground source heat pumps – open loop, and close loop, but major part of all household installations is the closed loop system. The closed loop geothermal heating system circulates the water-based antifreeze solution through a series of pipes buried in the ground, transferring heat to or from the ground, depending on the mode heat pump is working in.

During the summer season heat pump functions as air conditioner extracting the heat from the air inside the building, then transferring it to the antifreeze in the pipes and then dumping it to the relatively cooler earth. During the winter season this process is reversed – the heat pump extracts heat from relatively warmer earth, transfers it to the antifreeze in the pipes and rejects it to the air inside the building. Excessive heat extracted from the air inside the building in the cooling mode during the summer can be used to produce domestic hot water.

For larger residential building or houses a multiple heat pump systems can be used creating a possibility for a multiple heating/cooling zones.

One of the most important characteristics of the geothermal heat pump in terms of the residential heating and cooling is that the efficiency of the heat pump and the energy amount it consumes are directly related to the temperatures between which it operates. The difference between the temperature where the heat is absorbed (the source) and the temperature where the heat is rejected (the sink) is called the lift. The bigger the lift, the more input power is required by the pump. This form the basis of efficiency advantage of the geothermal heat pump (ground source) and the air source heat pump. The air source heat pump must extract the heat from cold outside air during the heating season and reject the heat to the hot outside air during the cooling season. Geothermal ground source heat pump extracts heat from relatively warm ground during winter, and rejects hot air to the relatively cool ground during summer. So the geothermal heat pump works in smaller lift or temperature range than the air source heat pump thus being more efficient and requiring less energy to operate.

Learn more on how geothermal heat pump works.

Geothermal Equipment

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
5. warranty

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.

3. Grout

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.

Basic principles of geothermal heating and cooling system operation:

A geothermal heat pump system uses the ground you already own to heat and cool your home for less. Geothermal works by using the difference in temperature between the outside air and the ground. The ground is like your own solar panel, absorbing fifty percent of the suns energy which keeps the ground temperature a constant fifty five degrees. When the summer sun produces those ninety five degree days, the ground remains at its constant fifty five degree temperature. In the winter, the ground temperature remains around fifty five degrees, even with the snow.

To exploit these temperature differences holes are drilled anywhere from fifty to hundreds of feet deep to house a system of piping known as the loop field. These holes are filled with a grout to improve heat transfer with the earth. The loop field is brought into the home where it connects with the geothermal heat pump. The system circulates a water mixture through the entire loop.

In the winter, the water absorbs the heat in the earth which is then compressed by the heat pump to a warmer temperature and distributed throughout the home as heat. For every unit of electricity used in this process, four units of heat are produced. The most efficient gas furnaces are ninety four percent efficient. Geothermal is four hundred percent efficient.

The operating costs of a geothermal system can be up to seventy percent less than conventional systems. In the summer, the same process is reversed except now the ground acts as a heat sake instead of a heat source. Heat and humidity from the home are pulled out and rejected back into the earth. Cool air is distributed throughout the home providing air conditioning. The geothermal heat pump is also equipped with a desuperheater which is used to preheat hot water saving you even more money!

Geothermal heat pumps operate extremely quietly and often use the same thermostat system. They can be installed as a retrofit or a new construction. Stop paying the propane, heating oil, coal and natural gas companies’ increasing fuel prices. Cut your homes dependence of fossil fuels, your carbon footprint, and combat rising fuel costs. GeothermalEnergy.org encourages you to learn more about why switching to geothermal is not only the right thing to do, it’s the smart thing to do.

Financial benefits of switching to Geothermal:

With average heating bill in the northeast being about $250 per month, and average cooling bill at about $100 per month, you can spend as much as $2100 just for these two items, adding significant amount to your household budget each year. However, with energy prices being so volatile, and usually on the rise, you can expect this amount to increase each year, or at best stay about the same.

Imagine you could save only 70-75% on year heating and cooling bill, and pay $525-630 for your heating and cooling each year. You would save as much as $1500 / year, and would not need to worry much about your energy-related expenses rising rapidly, anytime the OPEC decides to jack the oil prices or the gas prices skyrocketing because the winter is expected to “very cold”.

Residential Geothermal heating and cooling cost:

You may ask – “if everything is so great in Geothermal world, why isn’t every homeowner in the US switching to geothermal?” The answer is rather simple – as with most green and renewable energy technologies out there, the high initial cost of the geothermal heating and cooling system, is prohibitive for most people. However, if you look at the real math, you will see that geothermal systems in not only 400% efficient and is about 70% less expensive to operate than a conventional HVAC systems, it is also cheaper to own.

Let’s say that your new residential geothermal system costs $45000. At the same time a new Gas or Oil heating system will cost about $15000. A central air AC will cost another $4000. Therefore an conventional Heating + Cooling systems costs $19000, and the difference between the geothermal and regular system is $26000.

Now, in the first year, you will get a Renewable energy tax credit of 30% of the total cost of the system – this will be $13500 off your tax bill. So now the difference between the geothermal and conventional heating and cooling systems is $12500.

As we calculated above, a geothermal heating system will save you on average $1500 per year, so it would take about 8 years and 4 months for your geothermal heating and cooling system to pay for itself. But lets be conservative and say that you home is well insulated and your annual saving will be $1250 instead of $1500 per year. In that case it will take you exactly 10 years to break even, after which the savings will be piling on.

In the next article about the residential geothermal systems, we will discuss the financing of the geothermal system with a 5 year home equity loan and compare it to the same loan for a conventional HVAC system.