Introduction:
On our planet Earth, air, ground and water store close to half the energy striking it’s surface from the sun.
This huge store of renewable solar energy can be tapped using a heat pump to heat our homes in winter.
Also, in the summer, that same heat pump can be reversed to provide air conditioning for our comfort.
The US Environmental Protection Agency (EPA) states that ground source systems can save 30% to 70% of home heating costs compared to natural gas, propane, electricity, oil or coal.
In the summer, cooling costs can be lowered from 20% to 50%.
Type of heat pumps available:
A- Air source heat pump (ASHP) works great in mild to moderate climates and are the most popular heat pump system used in the USA because of its low purchase and installation cost.
They are dependable and durable, as they have been on the market for many decades.
The air source heat pump has an outdoor heat exchanger and compressor, as well as an interior heat exchanger with a forced air distribution system to deliver cooling and heating to the building.
They are noisy when running, so they may need to be located away from living space.
Their operating efficiency drops dramatically as outside air temperature drops below freezing.
B- Ground source heat pump (GSHP) or GeoExchange works fine in all climates, and are the best choice for cold winter areas.
GSHP uses a refrigerant to water base heat exchanger instead of refrigerant to air exchanger for ASHP.
They have a ground water loop located ten feet below the Earth’s surface where the temperature remains fairly constant at around 10 C or 50 F, with negligible seasonal fluctuation.
In winter, the GSHP draws the heat from the ground water loop heat exchanger, and compresses that heat using a vapor compression refrigeration cycle (like our kitchen refrigerator) and releases that heat to the house by forced air, in-floor hydronic heating or baseboard radiators.
In summer the process is reversed for cooling with forced air delivery inside the home. The GSHP can also dehumidify and filter the air circulating in each room.
1- Loop systems:
Source of heat for a GSHP can be the ground, surface water (like a lake or pond) or a drilled well.
Open Loop System:
Where ground water is abundant, this ground water heat pumpsystem is likely the most efficient and least expensive to install due to its simplicity.
Water is drawn from bottom of a well directly to the heat pump heat exchanger, where its heat is extracted (or added). The water is pumped back into the aquifer using a discharge well that is away from source well.
A Surface water heat pump system draws water from a pond, lake or river.
Water must be tested for hardness, acidity and organic matter which can clog or corrode a heat exchanger and pump. Open loop systems have been outlawed in many municipalities because of environmental concerns.
The standing column well is much more friendly to the environment. It is a specialized open loop system using a deep well about 6” in diameter. Water from the bottom of the deep rock well is drawn and circulated through the heat pump heat exchanger and then returned to the top of the well from where it exchanges heat with the surrounding rock as it travels downward back to the bottom of the well which can also be used as a source of potable water for small demand. This system is a great choice for where bedrock is close to the surface, but not any deeper than 200′ because of drilling and pumping costs.
Depending on the size of the heat pump, make sure 5 to 12 gallons of water per minute is steadily available in the heating and cooling season if applicable.
Closed loop system:
Most locations do not have sufficient ground water to operate an open loop system, or this system is banned because of environmental regulations or local codes.
Because of this, closed loop systems are most common for residential application.
The oldest and simplest type of closed loop system is the Direct Exchange geothermal heat pump (DX).
The ground heat transfer is done by circulating refrigerant in a single copper tube loop buried underground and directly connected to the heat pump.
Despite the higher cost of copper, DX will cost less to install compared to other closed loop water systems, as it requires only about 25% of tube length, thus reducing excavation costs.
This is do to the high thermal conductivity in copper compared to the high density polyethylene pipes used by water loop heat pumps.
Also DX is noticeably more efficient as it does not need a water pump nor a water heat exchanger to refrigerant which creates heat losses.
However refrigerant loops are more prone to gas leaks. Expensive brazed copper tubes protected from corrosion in acidic soils is also required.
1- The most common is the closed loop water system
A pump circulates an antifreeze water solution in a plastic pipe buried underground below the local frost line. The heat transfer is achieved by a water heat exchanger to the refrigerant loop at the heat pump (usually located inside the house basement or crawl space).
If adequate land area is available to install the water loop and the ground is easy to dig, then a horizontal ground closed loop is the cheapest solution.
400 to 600 feet of horizontal loop per ton of heating (12 000 btu/h) is required (A 2000 square foot home that is well insulated will need about three tons of heating).
The size of the loop needs to be calculated by an experienced professional, who takes into consideration the average ground temperature, type of soil and its moisture content. Dryer ground loops are less efficient than wet ground loops.
A body of water (lake or pond) is the most efficient way to transfer heat using a sunken closed loop.
Where land for a horizontal loop is not available, or the ground is difficult to dig, then a vertical ground closed loop may be installed by boring holes 100 to 500 feet deep, and then dropping a pair of tubes connected at the bottom of the hole.
To assure best heat transfer, bentonite grout is poured in the bored hole to maximize thermal connection between plastic tubes and surrounding ground.
The cost of installing a vertical loop are generally twice the cost of installing a horizontal loop because of the additional drilling costs.
2- How does a GSHP affect the environment?
About 70% of energy used in the GSHP system comes from free, renewable solar energy stored in the ground or surface water.
The other 30% is the electricity used to run the pumps and compressor.
A heat pump system moves 3 to 4 times more heat energy than it consumes in electricity. It does not burn fossil fuels, so it does not create any pollution locally.
Because the heat pump uses electricity to run, its impact on the environment depends on how that electricity was generated.
In Canada, where electricity production emits lower levels of green house gases, operation of a heat pump has less impact on the environment than any fossil fuel heating system, including high efficiency natural gas furnaces.
In the USA, where electricity production is highly reliant on coal, natural gas furnaces have less impact on the environment.
3- Is a heat pump a sound investment?
GSHP capital costs are much higher than traditional heating and cooling systems, however operational costs are much lower, with realistic savings between 20% to 60% per year.
The average cost of a GSHP system in the US is $10,000. In Canada, it is $20,000, with high end systems reaching above $30,000.
GSHP reliability and efficiency are constantly improving, however the purchase price is also escalating at about 2 to 5 times more compared to a conventional system. Payback averages 12 years compared to natural gas, and about 5 years compared to electricity.
From personal design experience, so far, only a few upscale homes that we designed, have opted to buy a GSHP.
However, with fuel energy costs on the rise, and capital costs for heat pumps likely going down because of increased popularity, attractive government and utility companies subsidies, and many more builders aware of the advantages of GSHP, I believe GSHP will soon be a sound investment for new homeowners in all income brackets.
